Simple Solution Plasma Synthesis of Hierarchical Nanoporous MnO2

May 22, 2017 - Solution plasma applications for the synthesis/modification of inorganic nanostructured materials and the treatment of natural polymers...
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Research Article pubs.acs.org/journal/ascecg

Simple Solution Plasma Synthesis of Hierarchical Nanoporous MnO2 for Organic Dye Removal Hyemin Kim,† Anyarat Watthanaphanit,*,‡ and Nagahiro Saito*,†,§ †

Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ‡ Department of Chemistry, Faculty of Science, Mahidol University, 272 Thanon Rama VI, Thung Phaya Thai, Ratchathewi, Bangkok 10400, Thailand § CREST-JST, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan S Supporting Information *

ABSTRACT: We have demonstrated a simple and green approach to synthesize hierarchical nanoporous MnO2 by applying plasma in a liquid precursor; the approach is named the “solution plasma process (SPP).” Three types of sugar, i.e., glucose, fructose, and sucrose, were used as inducers for the nanoporous MnO2 formation (hereafter called G-MnO2, FMnO2, and S-MnO2). These were successfully synthesized within a few minutes (7−19 min) under ambient conditions. It was confirmed that the generated numerous reactive species (e.g., electrons, radicals, and ions) accelerated the reduction of MnO4−. The reaction rate as well as the physical and chemical features of resulting products were found to be related to the type of sugars. Their high surface areas (F-MnO2 (169.1 m2·g−1) > G-MnO2 (141.0 m2·g−1) > S-MnO2 (85.5 m2·g−1)) provided efficient capability for the adsorption of cationic dye molecules, i.e., methylene blue. The dye removal efficiencies of all samples were >99% for an initial dye concentration (C0) of 10 mg·L−1 within 2 min and >82% for C0 = 50 mg·L−1 within 30 min. We expect that the synthesis route presented in this study can be extended to the large-scale production of effective adsorbents and to find practical applications for the industrial and green infrastructure. KEYWORDS: Solution plasma process (SPP), MnO2, Green synthesis, Adsorption, Methylene blue

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INTRODUCTION Water has played a vital role in survival from single-celled organisms to complex plants, animals, and humans. Although there is a huge amount of water on Earth, the rapid growth of population, accompanied by the extension of irrigated agriculture and industrial development, are pushing water shortage and pollution.1 Among the numerous contaminants in water, dye and pesticide organic pollutants are major contributors to pollution due to a great deal of their consumption for foods and materials in textile-derived forms.2 In general, more than half the commercial dyes belong to an azo class, containing −NN− as part of their molecular structures. Many of the pesticides are also amino acid group comprising chemicals in which the N atoms are present. These parts lead to harmful effects to human beings, other living things, and environments.3−6 The removal of dyes and pesticides could be based on the (i) adsorption and (ii) conversion method. For the first factor, as mentioned above, most organic pollutants (as adsorbates) contain a N atom; this part exhibits a positively charged (+) nature after being electronically polarized. The surface of the adsorbent should therefore have oppositely charged behavior, © 2017 American Chemical Society

i.e., negative charge (−), for efficient adsorption. For the second factor, the main conversion method proposed for the conversion of contaminants to nonhazardous substances is chemical treatment employing O3, H2O2, TBHP, and/or an artificial light source (UV).7 Among the available adsorbents, manganese oxides with variable oxidation states (i.e., +II, +III, and +IV) have attracted interest, and the Mn(IV) one, with the formula MnO2, is outstanding owing to its structural flexibility. It can exist in different polymorphic phases including the chain-like tunneltype (α, β, and γ form), the layered type (δ form), and the spinel type (λ form).8,9 Among them, the birnessitea layeredtype (δ form) MnO2 that formed by [MnO6] octahedral sharing edgesis regarded as the most reactive polymorph.10 The presence of vacant sites and/or Mn(III) in the layers can substantially improve its reactivity (e.g., electron transfer efficiency and adsorption capacity).11−13 This also renders the Mn octahedral layers to have a negative charge and thus be the Received: February 22, 2017 Revised: May 7, 2017 Published: May 22, 2017 5842

DOI: 10.1021/acssuschemeng.7b00560 ACS Sustainable Chem. Eng. 2017, 5, 5842−5851

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ACS Sustainable Chemistry & Engineering

purity, Wako Pure Chemical Industries Ltd., Japan) was used as the MnO2 precursor. The following sugars (see Figure 1 for their chemical

effective oxidant and adsorbent for cationic molecules. Furthermore, the birnessite has a greater negative charge density than the others (especially at pH 7), suggesting high affinity for the cationic species binding.14 Although it has such superior properties, the bare MnO2 exhibits a low capacity for practical organic pollutant removal in the absence of an oxidizing agent or artificial light source.15,16 Improvement of the MnO2 physisorption capability based on its own catalytic activities has therefore become an important issue. Methods to synthesize the birnessite-type MnO2 are normally involved in the hydrothermal process,17−19 redox reaction,20 and wet-chemical coprecipitation route accompanied by heating or aging.21,22 These methods typically required a long processing time, a high temperature, and hazardous chemical reagents, such as reducing agents, stabilizers, and strong acids or bases.23−26 As environmental issues have arisen, recent research has been focused on a green route for nanomaterial synthesis accompanied by minimization of various procedures, heating sources, and chemicals.27,28 From this point of view, a type of nonequilibrium cold plasma, named the solution plasma process (SPP), has great potential as a green synthetic method.29 The single-step method, with a short processing time within several minutes, under ambient processing conditions (e.g., room temperature and pressure), and minimization of reducing and oxidizing agents could accordingly be conducted.30,31 In our previous research, we successfully synthesized stable colloids of the birnessite-type MnO2 nanosheets by applying plasma in a potassium permanganate aqueous solution without any additional reagents.32 However, the colloidal form is not well-suited as an adsorbent. Several drawbacks, including liquid−solid phase separation difficulty, low reusability, and ease in aggregation, are encountered. We have further estimated that a functional part for strong binding/entrapping is additionally desired to avoid desorption during dye removal.33,34 Herein, we focus attention on the structures of MnO2 and make the following hypotheses: (i) MnO2 with a porous structure is suitable for the adsorption and entrapment of dye molecules since the desorption of the dyes can be restricted by the wall of holes, and (ii) as an adsorbent, the adsorption− desorption characteristics of the porous MnO2 are affected by its effective surface area and porosity. We already discovered the fact that sugars are quite inert in the solution plasma system. On the other hand, they are capable of reducing metal ions owing to the presence of free aldehyde (−CHO) or ketone (CO) functional groups in their molecular structures.35 On the basis of this fact, we intend to incorporate three types of sugar, (1) glucose, (2) fructose, and (3) sucrose, in the KMnO4 aqueous solution before solution plasma generation. These sugars are supposed to work as an inducer for the formation of hierarchical nanoporous MnO2. They are cheap raw materials compared with other artificial chemical surfactants. The effect of the sugar types on the physical and chemical properties of the resulting nanoporous MnO2 were endorsed. Finally, the removal efficiency of cationic organic dyeexploiting methylene blue (MB) as a cationic dye representativewas investigated to gain a better understanding of the dependence of morphologies and surface areas of the as-synthesized samples on their adsorption performance.

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Figure 1. Chemical structure of sugars, (a) glucose, (b) fructose, and (c) sucrose, employed as inducers for nanoporous MnO2 formation in this study. structures), D-(+)-glucose (C6H12O6, >99.5% purity); D-(−)-fructose (C6H12O6, >98.0% purity), and sucrose (C12H22O11, JIS special grade), employed as inducers for the nanoporous MnO2 formation, were purchased from Wako Pure Chemical Industries Ltd., Japan. , 98.5% purity, Kishida Methylene blue (C16H18ClN3S·nH2O Chemical Co., Ltd., Japan) was selected as a typical organic dye. All aqueous solutions were prepared by using distilled water obtained from an Aquarius water distillation apparatus (RFD250NB, Advantec, Japan) with a resistivity of 18.2 MΩ·cm at 25 °C. Additionally, manganese(IV) oxide powder (85% purity, Kanto Chemical Co., Inc., Japan) was used as a sample for a control test of dye removal ability. Preparation of Nanoporous MnO2. The molar ratio between KMnO4 and each sugar was fixed at 7.6:1. Sample preparation was done as follows. First, 1.2 g of KMnO4 was dissolved in 80 mL of distilled water and stirred until it was homogeneous (which hereafter obtained 0.076 M KMnO4). Meanwhile, 0.01 M of each sugar was prepared. Separately, glucose (0.18 g), fructose (0.18 g), and sucrose (0.34 g) were dissolved in 20 mL of distilled water. The two solutions, i.e., 0.076 M KMnO4 and 0.01 M sugar, were then added into the SPP reactor (100 mL glass beaker) and stirred vigorously for 2 min prior to the plasma generation. A schematic illustration of the SPP experimental setup is shown in Figure 2. The bipolar pulse power supply (Kurita, Japan) was used for generating plasma. Tungsten rods (99.9% purity, Nilaco Corp., Japan) of 1 mm diameter were used as electrodes. The frequency, pulse width, and gap distance between electrodes were controlled at 15 kHz, 2.0 μs, and 0.5 mm, respectively (see Figure S1A). The plasma discharge proceeded at room temperature and pressure conditions in the open reactor. The solution was stirred constantly at 400 rpm during operation. The SPP operation resulted in a rapid, exothermic reaction in which the brown to black precipitates were formed. The synthesized MnO2 samples were obtained by vacuum filtration and then washed with deionized water several times and air oven-dried at 65 °C for 12 h. The products synthesized from the system containing glucose, fructose, and sucrose are designated as G-MnO2, F-MnO2, and S-MnO2, respectively. To investigate the relation between the types of sugar and the states of plasma, the reactive species generated by the plasma were detected with an optical emission spectroscope (Ocean optics Inc., USB4000), operated in the wavelength range from 200 to 850 nm. The intensity was acquired with an integration time of 100 ms and averaged for three scans. The initial stock of the KMnO4 aqueous solution was diluted to decrease the absorbance of SP emission in KMnO4. In addition, a reference experiment, i.e., chemical precipitation via stirring without the SPP, was carried out in order to evaluate the effect of the SPP. Particular periods of the end of chemical reactions between each sugar and MnO4−, compared with that observed in the SPP-assisted route, were investigated. Material Characterizations. The structural features of the assynthesized samples were evaluated with an X-ray diffractometer (XRD, SmartLab, Rigaku Co., Ltd., Japan) with Cu Kα (λ = 1.5418 Å) radiation, operating at 45 kV and 200 mA (Rigaku Corp., Japan). XRD patterns were collected in the range of 2θ = 5−90° with a scanning step size of 0.02° and scanning speed of 2° min−1. The Raman spectra were obtained by using a Raman Microscope (inVia Raman Microscope, Renishaw Co. Ltd., UK) under a 532 nm solid state

EXPERIMENTAL SECTION

Materials. In the present work, all chemicals were used without further purification. Potassium permanganate (KMnO4, > 99.3% 5843

DOI: 10.1021/acssuschemeng.7b00560 ACS Sustainable Chem. Eng. 2017, 5, 5842−5851

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Figure 2. Schematic image of experimental setup of the SPP and photographs of the synthesized products. laser. Fourier transform infrared spectroscopy (FTIR, Nicolet 8700, Thermo Scientific Co, Ltd., US) was performed using the potassium bromide (KBr) pellet method. X-ray photoelectron spectroscopy (XPS) spectra were obtained with a PHI 5000 VersaProbe II with Mg Kα radiation for analyzing the surface elemental composition of materials. The nitrogen adsorption/desorption isotherms were measured with a BELSORP mini II analyzer at 77 K, cooled with liquid nitrogen to analyze the physical properties for effective adsorbents. The specific surface area was determined by Brunauer− Emmett−Teller (BET) method. Total pore volume was obtained from the N2 isotherm at P/P0 = 0.990, and pore size distribution was calculated using the Barrett−Joyner−Halenda (BJH) method. The morphology was observed using field emission scanning electron microscopy (FE-SEM, S-4800, HITACHI High-Technologies Co., Ltd., Japan) at a 5 kV accelerating voltage and transmission electron microscopy (TEM, JEM-2500SE, JEOL, Japan) with an accelerating voltage of 200 kV. Potential Application for Organic Dye Removal. Potentiality of the as-synthesized MnO2 samples as adsorbents for cationic dye removal was determined using methylene blue (MB) as a typical cationic dye. The removal of the dye was carried out using an adsorption process under optimized conditions. Two concentrations of the MB stock solution, i.e., 10 mg·L−1 and 50 mg·L−1, were prepared and used for the test. The 60 mg of G-MnO2, F-MnO2, and S-MnO2 were added into 100 mL of each MB solution (10 and 50 mg· L−1) under constant stirring. The MB removal test was performed under room temperature and uncontrolled natural pH conditions (pH ∼ 6.5). Two reference experiments, (1) the addition of only the pure sugar (i.e., glucose, fructose, and sucrose) and (2) the utilization of the commercial manganese(IV) oxide powder, under the same study conditions, were additionally performed. For the former case, the pure sugar was tested since sugar itself also presented organic dye removal properties.36 All glass beakers were wrapped with aluminum foil during the MB removal test to prevent degradation by light sources from the surrounding environment. The spectrophotometric technique, monitoring by UV−visible spectrophotometry (Shimadzu UV-3600, Japan), was adopted for the estimation of the concentration of dye before and after the adsorption at different time intervals. To avoid influence of the remaining adsorbents, the extracted solution was filtrated using 0.45 μm microsyringe filters before analyzing. The dye removal efficiency (R, eq 1) and adsorption capacity (eqs 2, 3) of the adsorbents were determined using the following equations:

R = (C0 − Ct )/C0 × 100

(1)

qt = (C0 − Ct )V /W

(2)

qe = (C0 − Ce)V /W

(3)

where C0, Ct, and Ce (mg·L−1) are concentrations of the MB solution at initial, contact (with the adsorbent) time t, and equilibrium, respectively; qt and qe (mg·g−1) are the adsorption amount at time t and equilibrium. V (L) indicates the volume of the MB dye solution, and W (mg) is the weight of the adsorbent used for the test.

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RESULTS AND DISCUSSION Solution Plasma Synthesis of MnO2. The competence of the SPP-assisted approach, compared with the non-SPP treated one, was determined by observing a particular period of the end of chemical reactions. We found that the color of the KMnO4/ sugar solution under white light changed from purple to brown, and afterward, the brown-black particles and the colorless transparent solution clearly separated at a certain period of time (see Figure S1A and B). We identified this point as the complete reaction time, which was confirmed by investigating UV−vis spectra of the supernatant as shown in Figure S2. Clearly, characteristic absorption peaks of MnO4− (i.e., 507, 525, 545, and 310 nm) were not observed in the supernatant of all sugar containing systems, indicating that all MnO4− was reduced to MnO2 at this time point. Note that this precipitate occurs owing to the chemical reaction between MnO4− and sugar but not the deformation of the sugar alone under the plasma system. Figure S1C reveals that no precipitate was observed when the plasma was introduced in the pure sugar solution. The complete reaction times vary with the type of sugar precursor and the synthesis route. As can be seen in Table S1, while the time consumption for the complete synthesis of MnO2 by the chemical precipitation is ∼11, ∼34, and ∼43 h with the presence of glucose, fructose, and sucrose, respectively, the respective time consumptions for the SPP-assisted approach are 7, 14, and 19 min. This leads to significant time savings of ∼94, ∼145, and ∼135 times. Considering the effect of the sugar type, the speed of the reaction containing each sugar is in the following order: glucose > fructose > sucrose. This observation is due to the fact that glucose and fructose are reducing sugars, caused by the presence of free aldehyde (−CHO, in glucose) or ketone (CO, in fructose) groups in their molecular structures. Notably, the presence of a hydrogen atom in the aldehyde group of glucose gives rise to its higher capability to 5844

DOI: 10.1021/acssuschemeng.7b00560 ACS Sustainable Chem. Eng. 2017, 5, 5842−5851

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ACS Sustainable Chemistry & Engineering reduce MnO4− to MnO2 than fructose. Since ketones do not have that particular hydrogen atom, they are generally resistant to oxidation under certain conditions.37 However, they could be oxidized to some degree by a powerful oxidizing agent, like KMnO4, through keto−enol tautomerism.38 In opposition to glucose and fructose, sucrose is categorized as a nonreducing sugar. It does not have any free functional group. The aldehyde group of the glucose ring and the ketone group of the fructose ring, in the sucrose structure, are blocked. As a result, the speed of the reaction of the sucrose-containing system is the lowest compared with the others. Optical emission spectroscopy (OES) was additionally performed to elucidate the relation between the types of sugar and the states of plasma. Figure 3 shows OES spectra of

into the interlayer regions.10,38 The first two peaks correspond to the (001) and (002) basal reflections, and the d001 value of ∼7.1 Å (G-MnO2, 7.17 Å; F-MnO2, 7.15 Å; S-MnO2, 7.06 Å) is related to the interlayer spacing. Furthermore, the remaining hk diffraction bands observed on the higher-angle side (i.e., 37°, 66°, and 78°) correspond to the two-dimensional (2D) structure. The layer-by-layer structure is a randomly oriented stacking,38−41 and the peak broadening results from the decrease of ordered 2D structure. Thus, the broadening indicates the presence of atomic-scale random defects and molecular-scale random holes in the plane: nanopores. Figure 4b shows Raman spectra of G-MnO2, F-MnO2, and SMnO2 with a full and narrow spectral range. Results verify that all samples are composed of pure phase MnO2. The predominant three peaks at around 630−645, 570−580, and 500−510 cm−1, marked with ν1, ν2, and ν3, respectively, correspond well with the intrinsic vibrational features of the birnessite.42 Specifically, the Raman band located at ∼625−650 cm−1 is considered the symmetric stretching vibration of the Mn−O bond in the MnO6 octahedral plane. That located at ∼570−585 cm−1 is attributed to the stretching vibration mode of Mn−O in the MnO6 octahedral basal plane.43,44 This finding shows that the MnO2 existing in all products is in the birnessite phase. XPS was carried out to characterize the surface composition and oxidation states of the synthesized MnO2. The survey spectra (Figure 4c) clearly show the existence of manganese (Mn), oxygen (O), and potassium (K) elements. The detected carbon (C) element comes from the applied sugar. The Mn 2p spectrum (Figure 4d) reveals the two sharp peaks corresponding to Mn 2p1/2 and Mn 2p3/2 at binding energies of 653.5 ± 0.1 and 641.8 ± 0.1 eV, respectively, with an energy separation (ΔE) of 11.7 eV. This is in agreement with the previous reports45,46 and indicates the presence of tetravalent Mn, i.e., MnO2. The magnitude of Mn 3s peak splitting was further investigated to diagnose the oxidation state of the Mn. As presented in Figure 4d, the observed ΔE’s of G-MnO2, FMnO2, and S-MnO2 are 5.08, 4.90, and 4.96 eV, respectively. Compared with the previous reported data,47 magnitudes (ΔE) of the Mn 3s peak splitting related to the respective oxidation states of the Mn are as follows: Mn (II), ∼6.0 eV; Mn (III), ≥5.3 eV; Mn (IV), ∼4.7 eV). It should therefore be postulated that all products are probably composed of mixed valences of Mn3+ and Mn4+. We furthermore confirmed this observation by computing the peak area of Mn−O−Mn and Mn−OH components in the deconvoluted high resolution O 1s spectra (see Figure S3). After calculation based on eq S1, the Mn oxidation states could be determined,45,46 and those of GMnO2, F-MnO2, and S-MnO2 are 3.58, 3.78, and 3.67, respectively. Surface Area and Porosity. As the adsorption takes place at the surface boundary, the surface area has a particular importance for the adsorbent. The higher surface area of the adsorbents can provide more active sites to adsorbates, e.g., organic dyes or toxic molecules.48 To prove this property, surface area and porosity were characterized by N2 adsorption/ desorption isotherm analysis. The specific surface area (SSA) and pore size distribution were determined by BET and BJH methods. As shown in Figure 5a, the curves for all samples exhibit type-IV behavior with a hysteresis loop (IUPAC classification).49 This phenomenon reveals that the mesopores (pore size 2−50 nm) exist in all samples. Among them, the hysteresis loop is prominently observed in F-MnO2, indicating

Figure 3. OES spectra of the plasma formed in (a) the KMnO4 aqueous solution and the KMnO4/sugar solutions of different sugar types, (b) glucose, (c) fructose, and (d) sucrose.

the plasma formed in (a) the KMnO4 aqueous solution and (b−d) the KMnO4/sugar solutions of different sugar types. For all OES spectra, Hα (λ = 656 nm), Hβ (λ = 486 nm), Hγ (λ = 434 nm), OH (λ = 309 nm), O I (3p5P → 3s5S2° at λ = 777 nm, 3p3P → 3s3S1° at λ = 845 nm), K I (λ = 766 nm), Mn I (λ = 403 nm), and Mn II (4pz7P2° → 4sa7S3 at λ = 260 nm, 4pz5G2° → 4sa5G3 at λ = 271 nm) lines with similar spectral shapes were observed. This observation suggests that molecules of water and KMnO4 are dissociated into small fragments as the result of high electron and ion temperatures in the plasma zone.32 However, the emission line of atomic carbon was not observed despite the addition of sugar, suggesting that sugar molecules are inert in the solution plasma. The results so far indicate that the synthesis of MnO2 is accelerated by generated reactive species, i.e., ions, radicals, and electrons, in the solution plasma based on the chemistry of the sugar molecules. Also, the SPP shows significant effectiveness for the synthesis porous MnO2. The fascinating rapid, one-pot synthesis could be done in a simple, green synthesis, at room temperature and atmospheric pressure. Structural and Chemical Properties. The XRD patterns of G-MnO2, F-MnO2, and S-MnO2 are shown in Figure 4a. The observed five peaks at around 2θ = 12.6°, 25°, 37°, 66°, and 78° indexed as (001), (002), (20l; 11l), (02l; 31l), and (22l; 40l) correspond to the potassium birnessite-type MnO2 (δ-MnO2: crystal class of prismatic and space group of C2/m), which is a class of layered structures consisting of edge-shared MnO6 octahedral with K+ cations and/or water molecules inserted 5845

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Figure 4. (a) X-ray diffraction patterns, (b) full-range and narrow-range (inset) Raman spectra, (c) XPS survey spectra, and (d) high-resolution Mn 2p and Mn 3s XPS spectra of G-MnO2, F-MnO2, and S-MnO2.

The shapes of G-MnO2 and S-MnO2 are similar. The G-MnO2 has a sphere-like shape with a uniform particle size of approximately 20 to 30 nm. The arrangement of these nanoparticles leads to the occurrence of mesopores (yellow arrow) and macropores (red arrow) with pore diameters of several tens to hundreds of nanometers (Figure 6a-1, a-2, and a3). S-MnO2 also exhibits sphere-like morphology. However, its particle size is relatively larger than that of G-MnO2, i.e., in the range of 50 to 200 nm (Figure 6c-1, c-2, and c-3). Distinguished from the others, F-MnO2 exhibits mixed morphologies of the stacked layers and plate-like particles (Figure 6b-1 and b-2). Both 2D thin layers and nanosized particles are evidenced in its TEM images (Figure 6b-2 and b3). The observed fringe spacing of ∼7.0−7.1 Å (Figure 6a-4, b4, and c-4) is in good agreement with the (001) plane of a typical birnessite MnO2. Furthermore, the broad diffuse rings of the selected area electron diffraction (SAED) for all samples (inset image of Figure 6a-4, b-4, and c-4) agree well with the broadening peaks in XRD patterns, which show that G-MnO2, F-MnO2, and S-MnO2 are poorly crystallized materials. It can be seen in more detail through the intensity profile along a reciprocal of a distance in the SAED patterns (see Figure S5), and these correspond approximately to the XRD data (see also Table S3, d-spacing values). The morphology of the samples synthesized by chemical precipitation via stirring was additionally observed. Results are shown in Figure S6. Clearly, similar morphologies to those synthesized by the solution plasma-assisted route were observed, but with slightly different sizes. We therefore believed that morphology of the obtained products relates mainly to the chemical nature of the sugar but not to the synthesis route. In

the largest number of the mesopores in this sample compared with the others. The BJH pore size distribution was additionally plotted (Figure 5b) to confirm this observation. From Figure 5b, F-MnO2 shows the highest dVp/dDp peak at Dp = 7.98 and 12.24 nm in the distribution range around 1.2−22 nm, which means that it has a hierarchical pore structure consisting of micropores (50 nm) with a small amount of micropores. On the other hand, S-MnO2 presents a slight peak at 18.94 nm, but plateau-like distribution, which indicates low porosity materials. Consequently, the magnitude of SSA is in the following order (see Table 1): F-MnO2 (169.1 m2·g−1) > G-MnO2 (141.0 m2·g−1) > S-MnO2 (85.5 m2·g−1). From these results, we have identified that small pores, i.e., micropores and small mosopores, provide a high surface area. We furthermore compared the surface area of our samples with reported data of MnO2 having a similar crystallographic structure, i.e., birnessite type MnO2, which was synthesized by one-step and/or relatively simple methods under low temperature conditions (see Table S2). Clearly, our products show comparatively high surface area. The observed BET surface areas are greater than most of the data shown in previous reported methods. In addition, this synthesis route is remarkably effective considering its simplicity, speed, being green, and low cost of raw materials. Morphology. Morphology of the G-MnO2, F-MnO2, and SMnO2 were observed by FE-SEM and TEM analysis. As can be seen in the low magnification FE-SEM images (Figure S4), uniform morphologies are observed in the overall area of all samples. However, each sample is different in its shape and size. 5846

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powder, under the same study conditions, are shown in Figures S8 and S9, respectively. In Figure S8, all sugars also exhibit MB removal capacities, but to a too much smaller extent. Although the contact time was prolonged to 4 months, the MB peak and blue color were still observed in all types of sugar. Results observed from the utilization of the Mn(IV) oxide powder (Figure S9) also reveal that the powder received from the company shows much lesser ability than our samples. The remainder of the MB was investigated, although the contact time interval was prolonged to 10 h. Furthermore, the blue-shift at λmax= 665 nm was observed, suggesting the formation of new compounds caused by the reaction between the dye with the adsorbate during the treatment.50 Data indicating physical properties of the commercial Mn(IV) are shown in Figure S10. Briefly, it is a γ-type MnO2 consisting of relatively homogeneous shape of particles with tens-of-nanometer sizes and having a specific surface area of ∼36.6 m2·g−1. The reason rendering the commercial Mn(IV) to exhibit lower MB removal capacity than the as-synthesized one should therefore be the lower specific surface area. The observations additionally support the evidence that the birnessite (δ type) MnO2 is regarded as the most reactive polymorph.10 Regarding the initial concentration and the observed concentration at contact time t of the MB solution, the removal efficiency of G-MnO2, F-MnO2, and S-MnO2 was calculated. Results are shown in Figure 8. For the lowconcentration MB (10 mg·L−1, Figure 8a,b,c), efficiencies of all samples reach >99% because of the contact time of 2 min. For the 50 mg·L−1 MB (Figure 8d,e,f), the removal efficiencies are about 81.2%, 91.1%, and 57.8% within 10 min and reach 99.1%, 99.5%, and 98.2%, after 180 min of contact time. This indicates that nearly all MB is adsorbed. Efficiencies of the raw starting materials, i.e., glucose, fructose, and sucrose, are also shown for comparison (Figure 8g,h,i). Particularly, removal efficiencies of the pure glucose, fructose, and sucrose at a contact time of 60 min are 49.1%, 57.6%, and 54.5%, respectively. However, decolorization no longer progressed despite the passage of about four months (Figure S8). Since pH is an important factor for determining the adsorption, additional adsorption experiments were performed at pH 3 (acid) and pH 11 (base) adjusted by 0.1 M HCl and 0.1 M NaOH, respectively. Results are shown in Figure S11 and Table S4. As can be seen in Figure S11, F-MnO2 shows remarkably higher adsorption capability than the others for all pH conditions, and the capability shows the same trend as the order of surface area (i.e., F-MnO2 > G-MnO2 > S-MnO2). Although the adsorption capability of G-MnO2, F-MnO2, and S-MnO2 changes with an alteration of pH, the differences are not remarkable. It should therefore be postulated that we can use these adsorbents with the dye waste of a broad-range pH. A detailed discussion about the pH effect is shown following Table S4 in the Supporting Information. Furthermore, the adsorption kinetics were studied to gain insight into the adsorption behaviors. Detailed information and discussion are shown in the Supporting Information (Figure S12; Tables S5, S6). Results indicate that the adsorption capacity of G-MnO2, F-MnO2, and S-MnO2 (as quantified by the initial adsorption rate, h, and the rate constant at equilibrium, k2) correlates well with the active surface area and/or pore size of the adsorbents. Through all results so far, all hierarchical nanoporous MnO2 synthesized by the SPP-assisted route exhibit remarkably high

Figure 5. (a) N2 adsorption/desorption isotherms and (b) pore size distribution plots of G-MnO2 (−▲−), F-MnO2 (−●−), and S-MnO2 (−■−) (*Vp = pore volume (cm3·g−1), Dp = pore diameter (nm)).

Table 1. Surface Area, Pore Volume, and Pore Size of the GMnO2, F-MnO2, and S-MnO2 sample GMnO2 FMnO2 SMnO2

BET surface area (m2·g−1)

total pore volume (cm3·g−1)

mean pore size (nm)

141.0

0.76

21.5

169.1

0.39

9.3

85.5

0.32

14.8

other words, the solution plasma plays a vital role in rendering the rapid synthesis of MnO2. Cationic Dye Adsorption Capacity. Figure 7 presents UV−visible spectra of the MB dye solution after contacting GMnO2, F-MnO2, and S-MnO2 at different times. The strong absorption peak at 665 nm represents a characteristic peak of MB (see Figure S7). Herein, the dye removal tests were performed at two different initial concentrations of the MB stock solution: (1) 10 mg·L−1 and (2) 50 mg·L−1. For the lowconcentration MB (10 mg·L−1), adsorption of the MB occurred quickly upon immersing the adsorbent in the solution, i.e., within 2 min, as presented in Figure 7a,b,c. As the concentration of the MB solution was increased 5 times (50 mg·L−1), all samples exhibited a significant MB removal capacity within 10 min, and almost the whole MB was removed after 180 min (Figure 7d,e,f). Results of two additional reference experiments, i.e., the addition of (1) only the pure sugar and (2) the commercially available Mn(IV) oxide 5847

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Figure 6. FE-SEM and TEM images of (a) G-MnO2, (b) F-MnO2, and (c) S-MnO2. That labeled with −1 is the FE-SEM image observed at a magnification of 50 000×. The others labeled with −2, −3, and −4 are the TEM images, along with the corresponding SAED patterns (inset). Scale bar of the inset image of b-3 is 20 nm.

Figure 7. UV−visible spectra, along with the respective photographs, of the MB solution of initial concentrations, (a, b, c) 10 mg·L−1 and 50 mg·L−1, after contact with 60 mg of G-MnO2, F-MnO2, or S-MnO2 in 100 mL or MB solution at different contact time intervals.

with that of the dye, were investigated (see Figure S13). Among all samples, F-MnO2 exhibits the highest adsorption capacity. Their spectra are therefore shown in this section in Figure 9. Result indicates that the spectrum of the as-synthesized FMnO2 (Figure 9a) corresponds well with the birnessite-type MnO2 having water molecules and/or potassium ions inserted in the interlayer. Briefly, the broad peak at 3431 cm−1 and sharp

MB removal performance. Among all samples, F-MnO2 is the most effective one. It is thought that the active surface area is the most important factor for the adsorption capacity in this study. To identify the interaction between the hierarchical nanoporous MnO2 and the MB molecules, FT-IR spectra of the nanoporous MnO2 before and after the dye removal test, along 5848

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MB dye is adsorbed on the nanoporous MnO2. The ζ-potential of MnO2 is negative under pH ∼6 in aqueous solutions.55 The negatively charged (−) nature of the MnO2 surface renders this capability, while the entrapment of the dye can be improved by the presence of holes, which might correspond to the increase of oxygen defects. The greater the surface area, the greater the interactions between the adsorbent and the dye molecules, and thus the adsorption capability.

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CONCLUSION We have developed a prospective green approach for the synthesis of hierarchical nanoporous MnO2 through the solution plasma process using three types of sugar, glucose, fructose, and sucrose, as inducers for the porous MnO2 formation. The numerous reactive species (e.g., e−, radicals and ions etc.) generated by plasma discharging in KMnO4/ sugar solution have led to a significant time-saving effect. The MnO2 existing in the as-synthesized samples is the birnessitetype. Depending on the type of sugar, the complete processing time as well as morphology, surface area, and porosity are different. While the G-MnO2 and the S-MnO2 have a spherelike shape, the F-MnO2 shows combined morphologies of the nanoparticle and the two-dimensional thin layered structures. Though, all samples have high specific surface area with the magnitudes in the following order: F-MnO2 (169.1 m2·g−1) > G-MnO2 (141.0 m2·g−1) > S-MnO2 (85.5 m2·g−1). The removal efficiency of cationic MB dye by the synthesized samples exhibits the same order as the results of surface area analysis. All samples show >99% for C0 = 10 mg·L−1 within 2 min, and >82% for C0 = 50 mg·L−1 within 30 min. We expect that the nanoporous MnO2 synthesized in this study will be applied to remove not only dyestuffs but also organic substances from wastewater. It will be an effective adsorbent having efficient adsorption capacities which complement the relatively insufficient catalytic properties of the MnO2 and, the given synthesis route, will be a powerful means for the production of MnO2 adsorbent industrially.

Figure 8. MB removal efficiency profiles in the presence of 60 mg of each absorbent in 100 mL of MB solution. (a, b, c) G-MnO2, F-MnO2, and S-MnO2 with an initial MB concentration of 10 mg·L−1. (d, e, f) G-MnO2, F-MnO2, and S-MnO2 with an initial MB concentration of 50 mg·L−1 (g, h, i) raw starting materials, i.e., glucose, fructose, and sucrose.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 9. FT-IR spectra of the F-MnO2 (a) before and (b) after the MB removal test, along with (c) the pure MB (solid).

. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00560. Comparison experiments between SPP and chemical precipitation via stirring, high-resolution XPS, FE-SEM, UV−vis spectra of methylene blue removal test and efficiency profiles, effects of pH on MB adsorption, adsorption kinetic, FT-IR for adsorption mechanism, and reference related BET specific surface area (PDF)

peak at 1631 cm−1 are attributed to the O−H stretching and bending vibrations of the adsorbed water molecules, respectively.51 Several absorption peaks observed at around 1527, 1407, and 1054 cm−1 in the midfrequency region are attributed to the vibrations caused by the interaction between Mn and other species (e.g., OH, O, K+, etc.) which are typically characterized in the hydrous MnO2.51,52 In addition, the strong absorption peaks observed in the range of 800−400 cm−1, especially ∼523 and 456 cm−1, are diagnosed with the Mn−O and Mn−O−Mn vibrations.51,52 The spectrum of the MB dye is shown in Figure 9c. After the MB dye adsorption (Figure 9b), peaks attributed to the origin of the MB molecule are observed, i.e., the marked adsorption peaks, at 1598 and 1489 cm−1 for aromatic CC stretching vibrations, at 1395 cm−1 for asymmetrical −CH3 bending vibrations, at 1372 cm−1 for the CS+ stretching vibration, at 1228 cm−1 for C−C stretching vibration in heterocyclic rings, and weak adsorption peaks at 904 and 886 cm−1 for aromatic C−H bending vibration and hydrogen bonding between N and OH stretching vibrations.53,54 These findings confirm that the

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +66-2-441-9817 ext. 1160. Fax: +66-2-354-7165. Email: [email protected]. *Phone: +81-52-789-3259. E-mail: [email protected]. jp. ORCID

Hyemin Kim: 0000-0001-8009-1288 Nagahiro Saito: 0000-0001-8757-3933 Notes

The authors declare no competing financial interest. 5849

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ACKNOWLEDGMENTS We greatly appreciate the financial support from the Core Research for Evolutional Science and Technology (CREST) of Japan Science and Technology (JST) Agency.

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