Fabrication of Fluorapatite NanocrystalâActivated Carbon Composite

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Fabrication of Fluorapatite Nanocrystal–Activated Carbon Composite by Atmospheric Pressure Plasma-Assisted Flux Method Tomohito Sudare, Chikara Mori, Fumitaka Hayashi, and Katsuya Teshima Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01294 • Publication Date (Web): 15 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Crystal Growth & Design

Fabrication of Fluorapatite Nanocrystal– Activated

Carbon

Composite

by

Atmospheric Pressure Plasma-Assisted Flux Method Tomohito Sudare1, Chikara Mori2, Fumitaka Hayashi2, and Katsuya Teshima*,1,2 1

Center for Energy and Environmental Science, Shinshu University, Japan Nagano

380-8553, Japan 2

Department of Materials Chemistry, Graduation School of Shinshu University, Nagano

380-8553, Japan *E-mail address: [email protected]

ABSTRACT: An atmospheric-pressure-plasma is an effective energy source for fabricating inorganic nanocrystal–carbon composite materials in an extremely short time. In this study, nano-sized fluorapatite [Ca5F(PO4)3, FAp] crystals, which are a useful adsorbent for removing metal ion contaminants from water, were directly fabricated

on

the

surface

of

activated

carbon

(AC)

using

an

atmospheric-pressure-plasma-jet with a high energetic density by coupling with the flux 1

ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

method. First, carbonyl and carboxyl groups, nucleation site, were introduced onto the surface of AC by heat treatment with KNO3 powder. Next, FAp crystals were grown on the treated AC using N2 gas-based atmospheric-pressure-plasma-jet with KNO3–LiNO3 flux whose eutectic point is 125 °C. Within 10 s, nanoscale FAp crystals with rod-like shapes were obtained, although the crystals obtained without flux were of indefinite shape. Here, we demonstrated the synergetic effect of plasma and flux, which provide high-density thermal energy and which can be exist as a solvent for crystal growth even within a short duration. Finally, the performance of the fabricated composites in adsorbing various metal ions (Pb, Al, Mn, Cu, Cr, Zn, Cd, and Ni) was investigated.

1. Introduction Various toxic inorganic and organic industrial waste materials are emitted in gas, liquid, and solid states into the environment, causing serious global environmental problems [1-2]. In particular, heavy metal ion contaminants, which are produced by chemical manufacturing, painting and coating, nuclear industries, etc., are found in wastewaters [3] and can be detrimental to living systems. International regulations on water pollution, which are enforced for environmental preservation, aim to reduce the

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emission of heavy metals into water. Therefore, it is essential to develop low-cost and effective technologies to remove heavy metal ions. Many techniques such as chemical precipitation, ion exchange, ion flotation, reverse osmosis, distillation, solvent extraction, electrolytic methods, and adsorption have been used to remove heavy metal ions from wastewaters [4-12]. Adsorption is one of the most effective methods in this regard. Several naturally occurring materials such as biomass, activated carbon (AC), wool, fishbone apatite, polymers, silica, zeolites, and clays have been used as adsorbents [13-24]. Among these materials, apatites have attracted considerable attention owing to their ion-exchange ability, ease of synthesis, and low cost. Ca2+ ions in fluorapatite [Ca5F(PO4)3, FAp] and hydroxyapatite [Ca5(OH)(PO4)3, HAp] can be replaced with other metal ions, depending on the thermodynamic stability of the phosphate compounds [25-34]. When granular FAp particles are used as a practical adsorbent, small particles and high crystallinity are required for a high adsorption capacity and durability, respectively. However, there has been a trade-off relationship between particle size and crystallinity and adsorbent performance. The adsorbent sample with smaller particle sizes exhibits high adsorption capacity and rate. However, critically high water-flow-resistance is encountered especially in case the particle diameter is

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~100 µm, resulting in serious deterioration of the adsorption performance and collapse of the adsorbent device. In the typical crystal growth process, a high temperature is required for highly crystalline particles, but, at the same time, the particle sizes inevitably increase. AC is a known effective adsorbent owing to its unique hierarchical porous structure and surface functional groups and is used for removing toxic organic materials from water [14-18]. When AC is used as a supporting material, these functional groups form covalent bonds with inorganic materials and the typical size (> 100 µm) of the resulting composites is effective for reducing water flow resistance. In this study, we focused on fabricating apatite–AC composites for removing heavy metal contaminants from water. Although various conventional studies for fabricating the apatite–carbon composite materials have been carried out, the resulting apatite crystals have been of low crystallinity [35,36], and their fabrication requires a high temperature and high pressure [36]. In order to obtain the high-crystallinity apatite particles, we focused on the flux method. The flux method is a technique for growing crystals from the liquid phase using a high-temperature molten salt, wherein the driving force for crystal growth is the supersaturation caused by cooling the solution and/or vaporization of the flux [37,38].

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Flux can be used as a high-temperature solvent that can dissolve the crystals with a high thermal stability and reduce the lattice mismatch, resulting in highly crystalline particles with well-developed crystal facets. In our previous study, highly crystalline, facetted FAp crystals with a relatively large length of ~5 µm along the c-axis direction were grown for the first time on a poly(ethylene terephthalate) (PET) substrate at 150 °C by the flux method by using KNO3–LiNO3 flux [39]. This binary flux should be effective for crystallization at low temperatures. However, this procedure has a long (>20 h) duration. A prolonged heating time for nitrate and carbon materials under atmospheric conditions generally destroys the carbon texture. Furthermore, a further smaller size is required for realizing a better adsorption performance. The plasma process can provide high-density thermal energy with an extremely short ignition time and have a high-energy transfer coefficient [40,41]. Although the typical

plasma

process

is

carried

out

under

vacuum

conditions,

the

atmospheric-pressure-plasma process operated under atmospheric conditions has possible application for the flux growth of crystalline FAp particles in a short time. The shortened process duration, i.e., a short ignition time and a high-density energy can not only avoid the destruction of the carbon material but also increase the nucleation density resulting in smaller particles. In this paper, we report the fabrication of

5



high-performance

FAp–AC

composites

Page 6 of 34

by

employing

an

atmospheric-pressure-plasma-jet as the energy source for the flux growth of FAp crystals. The performance of the obtained composite in adsorbing aqueous various metal ions was also investigated.

2. Experimental First, surface modification of AC was carried out. AC (QW8300AL, Futamura Chemical Co., Ltd.) was pre-treated using KNO3 flux at 300 °C to introduce functional groups on the particle surfaces. A reagent-grade KNO3 powder (Wako Pure Chemical Industries, Ltd.) and AC (2 g) were mixed together and put into alumina crucibles with 30 cm3 capacity. After the lids were loosely placed, the crucibles were placed in an electric furnace and heated to 300 °C at a rate of 900 °C·h-1. The crucibles were maintained in the furnace at 300 °C for 5 h and subsequently left to cool. Pre-treated AC was then extracted from the alumina crucibles and washed by distilled water. Before the fabrication procedure of the aimed composite material, the changes in the chemical bonding states of the AC particle surfaces after pre-treatment using KNO3 were analyzed by X-ray photoelectron spectroscopy performed using a JEOL

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JPS-9010MX. The analysis was performed under high-vacuum conditions using a Mg Kα X-ray source that produced photons at hν = 1253.6 eV and FWHM = 0.7 eV. The vacuum in the analysis chamber was always around 1 × 10−7 Pa. The analyzer was operated with a pass energy of 50 eV for wide scans and 10 eV for narrow scans, and with step sizes of 1.0 and 0.05 eV, respectively. After Shirley background removal, the component peaks were separated by the nonlinear least squares program, using symmetrical Gaussian/Lorentzian peak shapes. The adequacy of widths of peak components were confirmed with previous reports on similar materials [40-50]. This peak-fitting procedure was repeated until an acceptable fit was obtained. Next, FAp crystals were grown on the treated AC. Reagent-grade Ca(NO3)2·4H2O, (NH4)2HPO4, KF, KNO3, and LiNO3 (Wako Pure Chemical Industries, Ltd.) were used for FAp crystal growth. Ca(NO3)2·4H2O (2.556 g), (NH4)2HPO4 (0.858 g), and KF (0.126 g) powders was used as a solute. A mixture of KNO3 (12.999 g) and LiNO3 (5.910 g) was chosen as the flux, because the lowest eutectic temperature of the binary flux system (KNO3 : LiNO3 molar ratio = 6:4) was approximately 125 °C. The solute, flux, and pre-treated AC were weighed out and mixed together. To prepare a paste, 1.0 mL of distilled water was added to the mixed powder. The Si substrates were coated with the paste by bar-coating. Then, the paste was irradiated with the plasma at the

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atmospheric pressure for FAp crystal growth (Figure 1). A microwave discharge was used to generate plasma with N2 as the input gas, and the output power was maintained at 150 W. The irradiation time was 10 s. The distance between the plasma jet torch and the substrates was kept constant at 10 mm. After plasma irradiation, the substrates were immersed in distilled water to dissolve the remaining flux, and AC was then removed from the substrates. The final composites were dried in an oven at 100 °C for 5 h. The crystal structures of the composites were investigated using X-ray diffraction (XRD-6000, SHIMADZU) with CuKα radiation (λ = 0.154 nm). The morphology of obtained composites was observed using a field emission scanning electron microscope (FE-SEM, JEOL, JSM-7000F) operated at an accelerating voltage of 15 kV. The performance in adsorbing various metal ions (Pb, Al, Mn, Cu, Cr, Zn, Cd, and Ni) was analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES; Optima 2100 DV, PerkinElmer Inc.). 0.1 g of either AC or the fabricated composite was immersed for 24 h in a series of test solutions containing each of the metal ions at a concentration of 100 µg L-1. The test solution was adjusted using metal standard solutions (Kanto Chemical Co., Inc., Japan). After the removal of AC or the composite by suction filtration from the solution, the concentration of metal ions was measured using ICP-OES.

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3. Results and discussion First, we investigated the effects of the functional groups introduced on the surfaces of AC particles on the formation of FAp crystals. As in the pre-treatment, AC was mixed with KNO3 powder and heated at 300 °C for 5 h. Prolonged treatment causes many cracks to be formed on the AC surfaces because of the strong oxidation ability of the nitrate flux (Figure S1). Figure 2a shows the wide-scan X-ray photoelectron spectra wherein the O1s peaks at around 530 eV after the pre-treatment were higher than those before the pre-treatment, whereas the C1s peak intensity around 280 eV decreased after the pre-treatment. The atomic concentration ratio of carbon/oxygen decreased from 4.56 to 2.57 owing to the pre-treatment, implying that oxidation reaction occurred on the AC surface. In addition, small N1s, K2s, and K2p1/2, 3/2 peaks appeared at around 400, 380, and 293 eV, respectively, but they are with extremely low atomic concentration of 1.21 and 1.34 atom%, respectively. Figure 2b and 2c show the narrow-scan X-ray photoelectron spectra of the C1s peak and O1s peak, illustrating the change in the shape of the peak after the treatment. That indicates the compositional change in chemical components on the surface of AC such as carbon atom- and oxygen atom-relating

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functional groups. A remarkable decrease in the peak intensity at 284.6 eV (FWHM: about 1.3 eV), which originates from the C–C bond, was observed; correspondingly, an increase in the intensities of the other three peaks at 286.1, 287.3, and 288.5 eV (FWHM: about 2.0 eV) which are attributed to the C–O bond in functional groups such as ether, hydroxyl, lactone, and ester group, the C=O bond in carbonyl group, and the O=C–O bond in functional groups such as carboxy, lactone, and ester group, respectively, were observed in Figure 2b [40-50]. The peak around 291 eV might be a shake-up peak of the C–C bond. Further, the ratio of C=O/C–O and O=C–O/C–O was increased from 0.42 to 0.79 and 0.10 to 0.15, respectively, owing to the treatment. In the O1s peaks in Figure 2c, after the treatment, an intense peak around 530.9 eV (FWHM: about 2.0 eV) attributed to the C=O bond in functional groups such as carbonyl, lactone, and carboxy group appeared in addition to that at 532.2, 533.4, and 534.9 eV (FWHM: about 2.0 eV) which are attributed to the aliphatic C–OH bond and/or C=O in epoxy group, the phenolic C–OH and/or C–O–C bond, and chemisorbed water, respectively [40-50]. Among them, the peak at 532.2 eV was relatively decreased. These results indicate that the additional carbonyl and carboxyl groups, potential nucleation sites for the growth of FAp crystals, were successfully introduced on the AC surface, although surface hydroxyl group was slightly reduced. Notably, no substantial structural change

10


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was observed from the XRD profiles (Figure S2). Figure 3a, 3b, and 3c show XRD profiles of the pulverized (a) FAp–AC composites, (b) pre-treated AC, and (c) Ca5F(PO4)3 ICDD PDF. The XRD profile of the composite fabricated without KNO3–LiNO3 flux showed a pattern similar to that of Ca5F(PO4)3 (Figure 3a); however, the characteristic diffraction peak around 32° was not split, which indicated the formation of amorphous FAp. The obtained FAp crystals on the surface were nano-sized particles with poorly developed facets, which tended to form aggregates (Figure 4a and 4b). Figures 5a, 5b, and 5c show XRD profiles of the composite obtained using KNO3– LiNO3 flux with 15, 5, and 1 mol% of solute concentration, respectively. The four characteristic diffraction patterns attributed to FAp were clearly observed in the profiles of the composites obtained with 5 and 15 mol% solute (Figure 5a and 5b), as verified by the Ca5F(PO4)3 ICDD PDF data [51]. The broad diffraction pattern between 18 and 30° was attributed to amorphous-like AC. The characteristic diffraction peak around 32° was clearly split especially at 15 mol%, indicating that crystalline FAp particles were successfully formed. Correspondingly, the surface of AC was found to be densely covered by nano-sized FAp crystals, without any substantial morphological change to the AC itself (Figure 6a, 6c and 6e). In addition, the higher solute concentration get, the

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denser nano-sized FAp crystals grow on the surface (Figure 6b, 6d and 6f), suggesting that the solute concentration strongly affects the nucleation density. In contrast, the XRD profile of the composite fabricated using the original AC showed that FAp crystals hardly grow on the AC surface (Figure S3 and S4). These results indicate that the functional groups introduced by pre-treatment provide the nucleation sites for FAp nanocrystals on the AC, and that the solute concentration determines the number of such nucleation sites. Furthermore, the FAp nanocrystals that were grown at solution concentrations of 5 and 15 mol% had rod-like shapes (Figure 6b and 6d). Importantly, the flux significantly accelerated the FAp crystal growth within only 10 s of plasma irradiation. This indicates that flux is effective for the dissolution and recrystallization of FAp crystals. The growth of FAp crystals on AC using atmospheric-pressure-plasma-assisted flux method is summarized in Figure 7. carbonyl and carboxyl groups on AC serve as inhomogeneous nucleation sites for the growth of FAp crystals because they are negatively charged in the paste and therefore electrostatically attract Ca2+ ions. When the N2 plasma with 150 W is employed, water in the paste is evaporated and a high thermal energy sufficient for melting flux and for the dissolution of the solute to flux facilitates the crystal growth on the surface of AC. Then, immediate rapid cooling by

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stopping the thermal energy supplied by the plasma results in the increment of supersaturation degree in flux, thereby resulting in the formation of nano-sized FAp particles. It is noteworthy that most of the FAp particles lie with the lateral side onto the AC surface. It is known that typical apatite particles have positively charged a-plane because of the relatively large number of Ca sites and negatively charged c-plane because of the relatively large number of PO4 sites [52]. Therefore, the orientation of FAp particles might be determined by the previous adsorption of Ca2+ ions as high-density nucleation sites on the AC surface. Figure 8 shows the concentrations of metal ions in the test solutions after three kinds of adsorbent samples were immersed for 24 h: original AC, FAp–AC composite fabricated with 5 mol% solute concentration, and FAp–AC composite fabricated with 15 mol% solute concentration. The results indicated that the adsorption performance of the composites improved with increasing FAp solute concentrations. In particular, the composite fabricated with 15 mol% of solute concentration exhibited a high adsorption capacity, with about 80% of most of the metal ion types being removed. In short, this result indicates that atmospheric-pressure-plasma-assisted flux method is a promising and important tool for fabricating efficient inorganic nanocrystal–AC composite materials.

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Page 14 of 34

4. Conclusion A remarkably rapid fabrication, within 10 s, of crystalline nanosized FAp–AC composite

adsorbent

was

successfully

carried

out

by

employing

atmospheric-pressure-plasma-assisted flux method using KNO3–LiNO3 flux. Two important observations were made: First, carbonyl and carboxyl groups introduced onto the AC surface by the pre-treatment are necessary for the growth of FAp particles, as they served as inhomogeneous nucleation sites owing to electronegativity. Second, in atmospheric-pressure-plasma-assisted flux method, flux can effectively work as a solvent even in such a short process time, leading to the promotion of crystal growth. Thus, FAp particles with high crystallinity and small size of less than 100 nm with rod-like shape were densely formed on the pre-treated AC. Finally, the performance of the obtained composite in adsorbing various metal ions (Pb, Al, Mn, Cu, Cr, Zn, Cd, and Ni) were investigated. The composites exhibited a high adsorption capacity, with 80% of

most

of

the

ion

types

being

removed.

We

believe

that

atmospheric-pressure-plasma-assisted flux method is a promising new technique for producing high-crystallinity inorganic materials and composite materials within an

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extremely short duration under atmospheric conditions.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. FE-SEM images and XRD profiles of original AC and pre-treated AC at 300 °C for 10 h, AC before and after pre-treatment, and the composites obtained using pre-treated AC without flux.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partly supported by a Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research (A) 25249089.

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Figure 1. (a) Experimental set-up and (b) digital photograph of plasma. The plasma was irradiated to the paste including AC and solute and flux coated on the Si substrates. A microwave discharge was used to generate plasma with N2 as the input gas, and the output power was maintained at 150 W. The irradiation time was 10 s. The distance between the plasma jet torch and the substrates was kept constant at 10 mm.

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Page 27 of 34

(a)

before

Intensity (arb. units)

O1s C1s

O1s

after

K2p1/2.3/2

N1s

C1s K2s

600

500

400

300

200

100

Binding Energy (eV)

(b)

before


C1s

after

292

290

288

286

284

282

Binding Energy (eV)

(c)

before

O1s


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


after

538

536

534

532

530

528

Binding Energy (eV)

Figure 2. (a) Wide-scan and narrow-scan X-ray photoelectron spectra of (b) the C1s peak and (c) the O1s peak of AC before and after the pre-treatment.

27



Figure

3.

XRD

profiles

of

the

composites

Page 28 of 34

fabricated

by

atmospheric-pressure-plasma-assisted flux method using (a) pre-treated AC without flux, (b) pre-treated AC, and (c) Ca5F(PO4)3 ICDD PDF.

28


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Figure 4. (a) Low- and (b) high-magnification FE-SEM images of the composites fabricated by atmospheric-pressure-plasma-assisted flux method, using pre-treated AC without flux.

29



Figure 5. XRD profiles of the fabricated composites using pre-treated AC with flux, with solute concentrations of (a) 15 wt%, (b) 5 wt%, and (c) 1 wt%; and (d) Ca5F(PO4)3 ICDD PDF.

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Figure 6. Low- and high-magnification FE-SEM images of the composites obtained using pre-treated AC with flux with solute concentrations of (a and b) 15 wt%, (c and d) 5 wt%, and (e and f) 1 wt%.

31



Figure 7. Schematic of the formation mechanism of FAp nanocrystal–AC composite by atmospheric-pressure-plasma-assisted flux method, using KNO3–LiNO3 flux.

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Figure 8. Removal performances of the original AC and fabricated FAp nanocrystal– AC composites, with solute concentrations of 5 mol% and 15 mol%.

33



Page 34 of 34

"For Table of Contents Use Only" Fabrication of Fluorapatite Nanocrystal– –Activated Carbon Composite by Atmospheric Pressure Plasma-Assisted Flux Method Tomohito. Sudare, Chikara. Mori, Fumitaka. Hayashi, and Katsuya. Teshima

A remarkably rapid fabrication, within 10 s, of crystalline nanosized FAp crystals–AC composite

adsorbent

was

successfully

carried

out

by

employing

atmospheric-pressure-plasma-assisted flux method using KNO3–LiNO3 flux. FAp particles with high crystallinity and small size of less than 100 nm with rod-like shape were densely formed on AC and exhibit sufficient adsorption performance for various metal ions.

34


Fabrication of Fluorapatite NanocrystalâActivated Carbon Composite

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