Nickel Nanoparticle-Decorated Porous Carbons for Highly Active

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Nickel Nanoparticles Decorated Porous Carbons for Highly Active Catalytic Reduction of Organic Dyes and Sensitive Detection of Hg(II) Ions Pitchaimani Veerakumar, Shen-Ming Chen, Rajesh Madhu, Vediyappan Veeramani, Chin-Te Hung, and Shang-Bin Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07900 • Publication Date (Web): 19 Oct 2015 Downloaded from http://pubs.acs.org on October 20, 2015

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ACS Applied Materials & Interfaces

Schematic illustration of applications of Ni/CPM materials for reduction of organic dyes and detection of toxic heavy metals. 118x115mm (220 x 220 DPI)

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Nickel Nanoparticles Decorated Porous Carbons for Highly Active Catalytic Reduction of Organic Dyes and Sensitive Detection of Hg(II) Ions Pitchaimani Veerakumar,†,⊥ Shen-Ming Chen,*,‡ Rajesh Madhu,‡,⊥ Vediyappan Veeramani,‡ Chin-Te Hung,† and Shang-Bin Liu*,†,§ † ‡

§

Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan Department of Chemistry, National Taiwan Normal University, Taipei 11677, Taiwan

ABSTRACT: High surface area carbon porous materials (CPMs) synthesized by the direct template method via self-assembly of polymerized phloroglucinol-formaldehyde resol around a triblock copolymer template were used as supports for nickel nanoparticles

(Ni

NPs).

The

Ni/CPM

materials

so

fabricated

through

a

microwave-assisted heating procedure have been characterized by various analytical and spectroscopic techniques such as X-ray diffraction (XRD), field emission transmission electron microscopy (FE-TEM), vibrating sample magnetometer (VSM), gas physisorption/chemisorption, thermogravimetric analysis (TGA), and Raman, Fourier-transform infrared (FT-IR), and X-ray photon spectroscopies (XPS). Results obtained from ultraviolet-visible (UV-Vis) spectroscopy demonstrated that the supported Ni/CPM catalysts exhibit superior activity for catalytic reduction of organic dyes, such as methylene blue (MB) and rhodamine B (RhB). Further electrochemical measurements by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) also revealed that the Ni/CPM modified electrodes showed excellent sensitivity (59.6 µA µM−1 cm−2) and relatively low detection limit (2.1 nM) toward the detection of Hg(II) ion. The system is also been successfully applied for detection of mercuric ion in real sea fish samples. The Ni/CPM nanocomposite represents a robust, user-friendly, and highly effective system with prospective practical applications for catalytic reduction of organic dyes as well as trace level detection of heavy metals. KEYWORDS: nickel nanoparticles, porous activated carbon, organic dyes, toxic metal ions, cyclic voltammetry, fish extract

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1. INTRODUCTION Organic dyes, which have been widely exploited in printing, textile, paper, paints, and plastics industries,1 are environmental pollutants that are capable of retarding the photosynthesis cycle in plant metabolism, and may causes mutagenic and carcinogenic diseases to humans and animals.2,3 Thus, the removal and/or reduction of organic dyes from hazardous wastes for example, methylene blue (MB) in waste water, is a demanding task. Various adsorbents, such as activated carbons (ACs), carbon nanotubes (CNTs), graphene hydrogels, metal oxides and so on, have been designed and fabricated for the removal of organic dyes in water4−−7 invoking a variety of different techniques, namely adsorption, photocatalytic degradation, chemical oxidation, membrane filtration, flocculation, and electrooxidation.8,9 In addition, heavy metals, such as cadmium (Cd), mercury (Hg), arsenic (As), and lead (Pb), are also well-known pollutants that are highly toxic, non-biodegradable, and highly carcinogenic to humans and other living organisms, even at relatively low concentrations. Among them, Hg(II) represents the most notorious heavy metal pollutant in the ecosystem since it is not only distributed in contaminated air, water, and soil, but also bio-accumulable in plants, aquatic animals, and other living organisms.10 Moreover, it may leads to serious adverse effects, such as digestive, kidney, liver, cancer, and especially neurological diseases, even at a trace level.11 Hence, it is imperative to develop a simple, rapid, sensitive, and highly selective analytical method for monitoring the level of Hg(II) in our environments. The methods invoked for Hg(II) detection are mostly based on analytical and spectroscopic techniques, including calorimetry,12 cold vapour atomic absorption spectrometry (CV-AAS), inductively coupled plasma with atomic emission or mass spectrometry (ICP-AES or ICP-MS), X-ray fluorescence (XRF) spectrometry, and so on.13−−15 By comparison, electrochemical detection represents a reagent-free and more user-friendly technique owing to its facile procedure, low cost, high sensitivity and selectivity, and fast detection.16,17 Carbon porous materials (CPMs), which possess uniform and tailorable pore structure, high specific surface area, large pore volume, and unique electrochemical properties,18‒20 have drawn considerable R&D attentions and have been widely applied in various fields, e.g., gas separation/adsorption, fuel cells, electrochemical sensors, and energy storage systems.21−−23 In particular, CPMs containing nickel nanoparticles 2



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(Ni NPs) that exhibit magnetic property favorable for separation are desirable materials as catalysts, catalytic supports, or adsorbents.24−−27 It has been demonstrated that CPMs with high surface areas may be easily prepared by a microwave-assisted synthesis route, which is more energy efficient, cost-effective, and time-saving.28−−30 Previously, we have demonstrated a synthesis route to incorporate palladium nanoparticles (Pd NPs) on CPMs via a soft templating method under microwave irradiation.31 A similar synthesis route is adopted herein for the fabrication of the Ni/CPM materials. To the best of our knowledge, this is the first report to utilize the Ni/CPM material as a catalyst for the reduction of methylene blue (MB) and rhodamine B (RhB) dyes as well as selective detection of Hg(II) in real samples, as illustrated in Scheme 1.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Phloroglucinol (ACS, 99.98%, Acros), formaldehyde (37% in water, Acros), HCl (37% Acros), triblock copolymer Pluronic F-127 (EO106PO70EO106, MW = 12,600, Sigma-Aldrich), ethanol (C2H5OH, 99%), Ni(acac)2 (96% Sigma-Aldrich) and dyes (Sigma-Aldrich) were obtained commercially and used without further purification. All other chemicals used were analytical grade and all solutions were prepared by using ultrapure water (Millipore). 2.2. Synthesis of Ni/CPM Catalysts. The Ni/CPMs was prepared according to a modified procedures reported elsewhere.31 A schematic illustration of the synthesis route is shown in Figure S1 of the Supporting Information (hereafter denoted as SI). In brief, a phloroglucinol-formaldehyde (Phl-F) resin was first prepared by the soft-templating method using a triblock copolymer (Pluronic F127; EO106PO70EO106) as the structure directing agent. The Phl-F polymer was dissolved with metal precursor nickel(II) acetylacetonate (Ni(C5H7O2)2; denoted as Ni(acac)2) in 5 mL tetrahydrofuran (THF) solution to obtain a clear orange-green mixture gel. The gel was then loaded on a large Petri dish, dried at room temperature overnight, and subsequently cured at 80 o

C (typically for 1‒2 h) and then subjected to microwave irradiation, followed by

curing at 120 oC overnight. Carbonization treatment was carried out at 900 oC under N2 atmosphere to obtain the Ni/CPM as final product. The catalysts loaded with 0.1 and 0.5 wt% Ni were labeled as Ni/CPM-1 and Ni/CPM-2, respectively.

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Scheme 1. Schematic Illustration of Applications of Ni/CPM Materials for Reduction of Organic Dyes and Detection of Toxic Heavy Metals.

2.3. Characterization Methods. All powdered X-ray diffraction (XRD) experiments were recorded on a PANalytical (X’Pert PRO) diffractometer using CuKα radiation (λ = 0.1541 nm). Nitrogen porosimetry measurements were carried out with a Quantachrome Autosorb-1 volumetric adsorption analyzer at ‒196 oC (77 K). Prior to measurement, the sample was purged with flowing N2 at 150 oC for at least 12 h. The pore size distributions were derived from the adsorption branches of isotherms using the Barrett–Joyner–Halanda (BJH) method. The morphology of the sample was studied by field emission transmission electron microscopy (FE-TEM) at room temperature (25 oC) using an electron microscope (JEOL JEM-2100F) that has a field-emission gun at an acceleration voltage of 200 kV. Elemental composition of various samples was carried out with an energy-dispersive X-ray (EDX) analyzer (equipped with the FE-TEM). X-ray photoelectron spectroscopy (XPS) measurements were performed using a ULVAC-PHI PHI 5000 VersaProb apparatus. Thermogravimetric analysis 4



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(TGA) was performed on a Netzsch TG-209 instrument under air atmosphere. UV-vis absorption spectral measurements were carried out with a SPECORDS100 diode-array spectrophotometer. FT-IR spectra were recorded using a Bruker IFS28 spectrometer in the region of 4000–400 cm−1 with a spectral resolution of 2 cm−1 using dry KBr at room

temperature.

Hydrogen

temperature

programmed

reduction

(H2-TPR)

measurements were performed utilizing a AUTOCHEM-2920 under a flow of 10% H2/Ar gas mixture and a heating rate of 10 oC/min from room temperature to 900 oC. Prior to the TPR analysis, sample was pre-treated by flowing argon at a flow rate of 30 mL/min at 600 oC for 2 h to remove impurities, then, the system was cooled to room temperature. The amount of H2 uptake during the reduction was measured continuously with a thermal conductivity detector (TCD). All Raman spectra were recorded on a Jobin Yvon T64000 Spectrometer equipped with a charge coupled device (CCD) detector cooled with liquid nitrogen. The backscattering signal was collected with a microscope using an Ar+ laser centered at 488 nm as the excitation source. Magnetic properties of the Ni loaded CPM samples were measured by using a vibrating sample magnetometer SQUID VSM (Quantum design, USA; maximum applied continuous field 50,000 G) at room temperature. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) studies were performed by using a CHI 900 electrochemical analyzer (CH instruments). A 0.05 M acetate (HAc + NaAc; pH 5.0) buffer solution was used as the supporting electrolyte during analysis. A conventional three-electrode cell system using modified glassy carbon electrode (GCE) as the working electrode, Ag/AgCl (saturated KCl) as the reference electrode, and a platinum wire as the counter electrode. 2.4. Catalytic Reduction of Organic Dyes. Typically, ca. 2.0 mg of the as-synthesized Ni/CPM catalysts were first added to an aqueous solution of methylene blue (MB; 5.0 mL, 3 × 10−5 M). Subsequently, the above solution was mixed with freshly prepared aqueous NaBH4 (2 mL, 1 × 10−2 M) solution. The reduction reaction was carried out at room temperature under vigorous stirring while the progress was monitored using a UV-Vis spectrophotometer. The disappearance of blue color (MB) into colorless leuco-methylene blue (LMB) indicates the completion of the reaction. Upon completion, the catalyst was separated from the reaction system by means of an external magnet, followed by washing 2‒3 times with ethanol then dried at room temperature for recycling use. The catalytic reduction reaction of rhodamine B (RhB) was carried out using the above procedure for the reduction of MB. 5


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3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. Recently, considerable interests have been focused on metal NPs supported on CPM materials as heterogeneous catalysts by virtue of their versatility in chemical compositions and structural architectures. Figure 1A display the typical X-ray diffraction (XRD) patterns of the highly porous CPM and Ni/CPM materials. The diffraction peaks located at about 2θ = 23.5 and 43.5° of the pristine CPM (Figure 1A(a)) may be attributed to the presences of amorphous (002) and graphitic (100) carbons.28,29 On the other hand, the characteristic peaks at 2θ = 44.5, 51.82, and 76.34° observed in both Figures 1A(b) and 1A(c) are ascribed due to the (111), (200), and (220) facets of the face-centered cubic (fcc) crystalline Ni in the Ni/CPM-1 and Ni/CPM-2 samples with Ni loadings of 0.1 and 0.5 wt%, respectively.30 It is noteworthy that no diffraction peak accountable for NiO was observed. By using the Scherrer equation,32 an average crystallite size of ca. 7 nm was derived for the Ni/CPMs based on features of the most intense (111) peak.

Figure 1. (A) XRD patterns, (B) Raman spectra, (C) N2 adsorption/desorption isotherms, and (D) pore size distributions of (a) pristine CPM, (b) Ni/CPM-1, and (c) Ni/CPM-2 samples.

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Raman spectroscopy is a powerful, non-destructive tool to characterize carbonaceous materials. Figure 1B displays the representative Raman spectra of CPM, Ni/CPM-1, and Ni/CPM-2 composite. As anticipated, the carbon substrates exhibited two prominent features: the peak at ca 1586 cm–1 (G band) arising from the first order scattering of the E2g phonon of sp2 carbon atoms, whilst the peak at ca. 1352 cm–1 (D band) arising from a breathing mode of k-point phonons with A1g symmetry. The latter D band is normally associated with structural defects, amorphous phases, or edges of the carbon that tend to spoil the symmetry and selection rules. Therefore, the intensity ratio of the D vs G band (ID/IG ratio) is usually used as a measure for the disordering of the carbon structure and was found to be ca. 0.98 for all the samples examined (Table 1).33 Figure 1C shows the N2 adsorption/desorption isotherms of various CPM-based samples recorded at 77 K. All samples showed the type IV isotherms (IUPAC classification) with H1 hysteresis loops and notable capillary condensation steps, which are the characteristic of ordered mesoporous materials.18−25 Their pore size distributions are displayed in Figure 1D. Accordingly, their surface areas (SBET), total pore volumes (VTot), and pore diameters (dBJH) were derived and the results are depicted in Table 1. It can be seen that notable decreases in surface area and pore volume of the sample were observed upon increasing the Ni loading. However, not much change in the pore diameter was observed upon loading NiNPs onto the CPM substrate (Figure 1D). These results reveal that the NiNPs are readily reside in the pore channels of the carbon support.24−27 Field emission transmission electron microscopy (FE-TEM) images of the pristine CPM and Ni/CPMs are shown in Figure 2. The crystalline nature of the NiNP is further confirmed by using selected area electron diffraction (SAED) patterns (Insets; Figures 2B and 2C). The high-resolution FE-TEM image of the Ni/CPM-2 catalyst in Figure 2D revealed that the NiNPs encapsulated in the CPM substrate typically has an average size of ca. 6 nm. A complete encapsulation of NiNP in the carbon matrix therefore inhibits agglomeration of the metal particles. Based on the nickel crystal lattice fringes, an interlayer spacing of ca. 0.21 nm may be inferred for the distance between two (011) planes.

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Table 1. Physical Properties of the Pristine CPM and Ni/CPM Materials Ni loading (wt%)

Mp (nm)a

SBET (m2 g−1)b

VTot (cm3 g−1)b

dBJH (nm)c

Ms (emu g−1)d

Hc (Oe)e

Dm (%)f

IG/IDg

pristine CPM

---

---

744

0.53

5.1

---

---

---

0.99

Ni/CPM-1

0.1

6.0 ± 0.4

622

0.50

5.0

1.38

12

0.07

0.98

Ni/CPM-2

0.5

6.0 ± 0.7

554

0.46

5.4

3.80

38

0.25

0.98

sample

a

Average metal particle size determined by FE-TEM analysis. b Brunauer−Emmet−Teller surface area (SBET) and total pore volume (VTot) calculated at P/P0 = 0.99. c Pore diameter derived by the Barrett−Joyner−Halenda (BJH) method using the adsorption branch of the isotherm. d Ms: saturation magnetization. e Hc: coercivity; f Dm: metal dispersion measured by H2 chemisorption at 323 K. g Peak intensity ratio of the G and D bands obtained from Raman spectrum.

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Figure 2. FE-TEM images of (A) pristine CPM, (B) Ni/CPM-1, and (C) Ni/CPM-2 samples.

(D) high-resolution TEM image of Ni/CPM-2 and (E) its corresponding EDX spectrum. Insets in (B), (C) are SAED patterns of the corresponding samples, and inset in (D) is the particle size distribution of Ni NPs on the Ni/CPM-2 catalyst.

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The energy dispersive X-ray (EDX) profile in Figure 2E, which shows characteristic peaks for elements C, O, and Ni, confirmed the presence of nickel with energy bands centered at 7.5 and 8.3 keV (K lines), and 0.8 keV (L lines), respectively. The structural integrity of the Phl-F polymeric resin before and after the carbonization treatment was confirmed by FT-IR experiments, As anticipated, the IR bands responsible for the phenolic –OH (3450 cm−1) stretching and C–H (3000–2800; 950 cm−1), and C–O (1200–950 cm−1) bending/stretching vibrations34,35 arising from the PF-127 vanished after the carbonization treatment at 900 °C in N2, which was described in the Figure S2A (SI). Whereas the bands near the 1600–1400 cm−1 region, which may be accounted for C–C stretching vibrations of tri-substituted aromatic ring structure (from the framework of phenolic resin) retained after the treatment. These results confirm the successful removal of template and formation of graphitic carbons during the carbonization procedure. The reducibility of the catalyst surfaces were further probed by temperature programmed reduction of hydrogen (H2-TPR). Compared to the bulk NiO powder, which showed a broad reduction peak in the temperature range of 250‒400 °C, the Ni/CPM-1 and Ni/CPM-2 catalysts exhibited similar broad (450–800 °C) reduction signal centering at ca. 580 °C (Figure S2B; SI), indicating the absence of NiO species.36,37 By comparison, the H2-TPR profile observed for a sample prepared by incorporating NiO on CPM (denoted as NiO/CPM) revealed a weak reduction peak centering at ca. 330 °C as well as a broad asymmetric peak spanning over a wide temperature range (450–950 °C). The above results therefore reveal that the Ni species are well-dispersed on the surfaces of the CPM support. Moreover, an additional weak reduction peak at ca. 920 °C was also observed for the Ni/CPM catalysts and is most likely due to reduction of NiNPs on the porous activated carbon support. The thermal properties of Ni/CPM catalysts were also monitored by thermogravimetric analysis (TGA) technique. Phloroglucinol showed a descending weight loss within the temperature ranges of 50–150 and 200–900 °C, whereas the Pluronic F127 surfactant exhibited a sharp weight loss at ca. 400 °C, indicating a complete decomposition of the template material as shown in Figure S2C (SI). On the other hand, the TGA profile observed for the Phl-F polymeric resin showed major weight losses at ca. 50–150, 250‒350 and 600 °C, as expected.38,39 Similarly, we also performed TGA analysis for the pristine CPM and Ni/CPM composites as shown in 10



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Figure S2D (SI), which revealed analogous desorption profiles compared to their carbon precursor (Phl-F) as shown in Figure S2C (SI). In particular, the weak DTG peak centering at ca. 100 oC observed for both Ni/CPM-1 and Ni/CPM-2 may be attributed to desorption of physisorbed water and organic solvent, which corresponds to a weight loss of 1.7 and 4.3%, respectively. Whereas the strong DTG peak centering at ca. 600 oC corresponds to a weight-loss of 83.4 and 79.4% for Ni/CPM-1 and Ni/CPM-2, respectively, should be associated with oxidation of the carbon support into gaseous carbon dioxide.40 The surface chemical properties of the catalyst were also examined by X-ray photoelectron spectroscopy (XPS). As an illustration, Figure 3 depicts the XPS spectrum of the Ni/CPM-2 sample, which clearly shows signals corresponding to elements such as Ni, C, and O in the substrate. Although the Auger binding energies (Eb) observed for Ni 2p and Ni 3p photo absorption peaks indicate the presence of metallic Ni, the peaks centered at 852.5, 870.8, 530.5, and 284.8 eV may be attributed to Ni 2p3/2, Ni 2p1/2, O 1s, and C 1s spin-orbits, respectively.41

Figure 3. XPS spectrum of the Ni/CPM-2 catalyst. Insets show the spectra of the Ni 2p3/2 and

the corresponding satellite peaks for O 1s and C 1s.

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The magnetic properties of the Ni/CPM samples were investigated by vibrating sample magnetometer (VSM) at room temperature. As shown in Figure 4, the magnetization curve obtained from the Ni/CPM samples displayed the anticipated hysteresis loops. Accordingly, the saturation magnetization (Ms) value for Ni/CPM-1 and Ni/CPM-2 materials is found to be 1.38 and 3.80 emu g−1, respectively, corresponding to a coercivity (Hc) value of 12 and 38 Oe, respectively, as summarized in Table 1. That the Ms values observed for the Ni/CPM catalysts were much lower than that of bulk Ni (51.3 emu g−1)30 is most likely due to the nanosize effect as well as the confinement of the Ni NPs within the carbon framework.42,43 Moreover, a notable increase in Ms value with increasing Ni loading was found, indicating the capability for magnetic separation, which is desirable for catalyst recovering. This is illustrated in Figure 4c, which clearly shows that the Ni/CPM catalyst (dispersed in ethanol) may be separated effectively when in the presence of an external static magnetic field within a period of only ca. 5−8 s at ambient conditions. Upon removing the magnetic field, the catalyst may be readily re-dispersed in ethanol due to its low remanent magnetization (i.e., remanence) and coercivity. Thus, the Ni/CPM catalysts indeed possess the desirable characteristics for facile magnetic separation, efficient recovery from the reaction mixture, and excellent chemical and structural stabilities favourable for recycling use.

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Figure 4. Magnetization curves of (a) Ni/CPM-1, (b) Ni/CPM-2 samples measured at room temperature. (c) Photographic illustrations of the Ni/CPM catalyst with (right) and without (left) the presence of an external magnetic field.

3.2. Catalytic Reduction of Organic Dyes. As mentioned earlier, Ni is much more cost-effective than precious metals (e.g., Au, Ag, Pd, Pt, Ru, and Rh), which have been extensively exploited as catalysts for organic transformations.44 Organic dyes such as methylene blue (MB) and rhodamine B (RhB) are well-known environmental pollutants that have been widely use in manufacture industries,1 Thus, the development of an efficient and reliable technique for catalytic reduction of organic dyes is a demanding task, to which this work is aiming to resolve. MB is a thiazine cationic dye which is water soluble and mostly presented as ionic form (MB+) in an aqueous medium. The oxidized form of MB normally exhibits UV-Vis absorption peaks in the wavelength (λ) range of ca. 550−750 nm, and typically shows an absorption maxima at λmax = 665 nm, which may be attributed to the n-π* transitions of the MB molecule. Consequently, the progress of the reaction can readily be followed by means of UV-Vis absorption spectrophotometry. Typically, the reduction reaction was carried out in ambient conditions (vide supra), and a complete reduction of MB by NaBH4 may be inferred by the disappearance of intense blue color (MB) to colorless leuco-methylene blue (LMB) within ca. 13 min, as shown in Figure 5A. The Ni/CPM catalysts were also employed for catalytic reduction of RhB. Likewise, the UV-Vis spectra in Figure. 5B also exhibited a consistent decrease in absorption peak intensity (A) at λmax = 552 nm within a time period of 18 min, indicating a successive reduction of RhB to leuco-rhodamine B (LRhB). Figures 5C and 5D display the variations of relative peak intensity, ln(A/A0), vs reaction time during catalytic reduction MB and RhB, respectively. Here, A0 and A represents the initial and final absorption peak intensity of the dye molecule at the reaction time of 0 and t, respectively. A linear correlation between ln(A/A0) and reaction time (t) for the catalytic reduction of MB (Figures 5A and 5C) and RhB (Figures 5B and 5D) dyes were observed over the Ni/CPM catalysts. It is noteworthy that, such correlation was not found for the pristine CPM sample (i.e., without Ni), which revealed a constant (null) ln(A/A0) over time. 13


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The structural transformations of MB and RhB to their corresponding reduced forms of LMB and LRhB are illustrated in Figures 5E and 5F (over the Ni/CPM-1 catalyst), respectively, together with photographs showing their corresponding color changes when the reaction mixtures were respectively placed in an external magnetic field. Accordingly, the oxidized MB (blue color) and RhB (pink color) dyes were effectively transformed to their respective colorless reduced forms (LMB and LRhB) within a short period of time (ca. 5−8 s). Upon removing the magnetic field, the catalysts readily re-dispersed automatically. The excellent activity and stability observed for the Ni/CPMs render practical application of these novel catalysts for reduction of organic dyes by NaBH4, which may be easily monitored by UV-Vis absorption spectroscopy.

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Figure 5. UV-Vis spectra of (A) MB and (B) RhB catalyzed by the Ni/CPM-1 catalyst at various time intervals. Insets: corresponding variations of relative intensity vs reaction time. The corresponding kinetics results for reduction of (C) MB and (D) RhB catalysed by (a) pristine CPM, (b) Ni/CPM-1, and (c) Ni/CPM-2 catalysts are also displayed together with structural and photographic illustrations of transformation of (E) MB and (F) RhB dyes to LMB and LRhB, respectively, over the Ni-CPM-1 catalyst while in the presence of an external magnetic field.

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Furthermore, the reaction mechanism invoked for catalytic reduction of organic dyes over the Ni/CPM catalysts was also proposed based on the earlier reports.45 As illustrated in Figure 6, the reduction process for MB over the Ni/CPM catalyst involves the following steps: Step I, the reducing agent (NaBH4) transfers a hydride to the Ni NP surfaces, leading to formation of covalent Ni–H bonds; Step II, adsorption of MB dye molecules onto the surfaces of the catalyst, which is the rate-determining step provoked by interactions between of adsorbed with the surface-bound hydrogen atoms; Step III, MB molecules tend to capture two electrons from the active surfaces of Ni NPs; Step IV, LMB is formed as a result of the reduction reaction, followed by desorption of the product from the Ni NP surfaces and reactivation of the Ni/CPM catalyst.

Figure 6. Proposed reaction mechanism for reduction of MB by NaBH4 over the Ni/CPM catalysts.

Clearly, the reduction process invoked a two-electron transfer process over the supported Ni NPs, which tends to catalyse the adsorbed MB dye molecules, provoking reduction of double bonds in the heterocyclic rings of MB. This leads to breaking of the conjugated π bonds and shortening of the electron delocalization distance of the dye to favor adsorption of MB, which is effectively reduced by the supported Ni NPs to form LMB.46 Note that such reduction process prevails even under ambient 16



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conditions. The pronounced catalytic activity observed for the synthesized Ni/CPMs during reduction of organic dyes may also be partly attributed to the porous characteristic of the CPM support. The high surface area possessed by the CPM is favorable for dispersion of Ni NPs, which in turn, provokes electron transfer and subsequent reduction reaction on their surfaces. The reduction of MB over the Ni/CPM was further optimized by varying the amount of catalyst used, as exemplified for the Ni/CMP-1 in Figures S3A‒C (SI). As shown in Figure S3A, null reduction of MB was observed when no catalyst was introduced, even under excessive amount of NaBH4. On the contrary, the linear correlation between ln(A/A0) and time observed while in presence of the Ni/CPM catalyst as shown in Figure S3D (SI), implying that the reaction invoked a first-order kinetics.47 By comparing the results in Figures S3B and S3C (SI), it is clear that the reaction kinetics is readily depending on the amount of Ni/CPM catalyst employed. For reaction carried out with fewer amounts (1.0 mg) of Ni/CPM-1 catalyst as shown in Figure S3B (SI), the MB dye failed to reduce completely within 13 min. However, as the catalysts loading was increased to 3.0 mg, a complete reduction of MB was achieved within 8 min (Figure S3C; SI). Moreover, while the intensity of the maximum absorbance peak decreased consistently with time, no apparent shift in its wavelength (λmax = 665 nm) was observed, indicating that oxidative degradation of MB did not occur during formation of LMB.48 The catalytic reduction of MB has been extensively studied by means of metal nanoparticles (MNPs) and supported MNP composites as catalysts and mostly employed NaBH4 as the reduction agent, as summarized in Table 2.49−−59 By comparison, the Ni/CPM catalysts reported herein, which possess a Ni particle size of ca. 6 nm, exhibited super MB reduction performance compared to other catalyst systems reported in literature. A complete reduction of MB may be reached within 13 min at a rate constant of ca. 0.57‒0.58 min‒1 with a moderate catalyst loading of 2 mg. Likewise, our catalysts also show excellent catalytic performance for reduction of RhB, surpassing most of other catalyst systems as shown in Table S1 (SI). A complete reduction of RhB was achieved within 18 min at a rate constant of ca. 0.47 min‒1 over

17


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Table 2. Comparisons of Catalytic Performance for Reduction of MB Over Various Supported Catalyst Systems size of NP (nm) 25

catalyst amount (mg) 5.0

reaction time (min) 22

rate constant k (min−1) 0.430

Pd- tetrahedral nanocrystals (TNCs)/RGO

9

5.0 [µL]

7

0.400

50

copper nanocrystals (CuNCs)

28

0.1 [mL]

5

0.020

51

Au@ polypyrrole (PPy)/Fe3O4

400

2.0

42

0.266

52

Fe3O4@Ag

52

1.6

6

0.410

53

200 ± 15

0.2 [mL]

24

0.041

54

300

10.0

catalyst* Fe3O4@polydopamine (PDA)-Ag NPs

Ir NPs Ag/magnetic Fe3O4@C core-shell NCs

reference 49

10

0.340

55

25

2

1 × 1 cm

10

0.317

56

CoO nanowires

4−16

250 [µL]

81

0.038

57

Cu microsphere

700

5.0

8

0.393

58

> 100

100.0

1

2.220

59

Ni/CPM-1

6.0 ± 0.1

2.0

13

0.583

this work

Ni/CPM-2

6.0 ± 0.7

2.0

13

0.571

this work

silicon nanowire arrays/Cu NPs

Ni nanotube arrays

*

NPs: Nanoparticles; RGO: reduced graphene oxide.

18



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2 mg of catalyst loading. Moreover, these novel Ni/CPM catalysts are also advantaged by easy separation, highly stable, and may be applied in the absence of an irradiation source such as UV or visible light. Additional recycling tests were performed to verify the reusability of the Ni/CPM catalysts. As shown in Figure S4A (SI), after six consecutive catalyst separation and reaction cycles, the rate constant (k) observed for MB reduction over the Ni/CPM-1 catalyst decreased from 0.583 to 0.166. Likewise, for RhB reduction over the same catalyst, the k value declined from 0.471 to 0.143 over the six cyclic runs (Figure S4B; SI). Even through the catalyst may be effectively separated by using a magnet, the gradual decrease in rate constant observed over the repeated cycles is attributed to inevitable loss of catalyst during separation and washing treatments. Upon completion of reduction treatment, the Ni/CPM-1 catalyst was subjected to sonication for about 20−30 mins, then, centrifuged. The reduced sample was extracted in DI water and washed thoroughly with ethanol, followed by drying at 60 oC in a vacuum oven. Subsequently, the degree of MB reduction was checked by UV-vis spectroscopy. As shown in Figure S5 (SI), the primary absorbance peak at 665 nm for MB (before reaction; Figure S5a) diminished after the reduction (Figure S5b), leading to the formation of LMB, which gave rise to a characteristic absorption signal at 255 nm.60 The results clearly indicate that the Ni/CPM-1 material is indeed an active catalyst for efficient removal of organic dyes from the pollutant mixture. 3.3. Interference in Presence of Polluted Water Samples. To further validate the catalytic performance of Ni/CPM materials for reduction of organic dyes, MB blended in two polluted water samples, namely industrial waste water and lake water, obtained from suburb of metropolitan Taipei were examined. The tests were conducted under the same reaction conditions used for the aqueous MB solution (see Figures 5A and 5D) were applied. Again, a linear correlation between ln(A/A0) and reaction time (t) was observed (insets; Figure S6; SI) in both cases, indicating that the presences of these real water samples have little interference with the reduction performance of the Ni/CPM-1 catalyst. The data summarized in Table 2 and Tables S1 and S2 (SI) clearly show that the catalytic properties of Ni/CPM nanocomposite materials outperform majority of other metal-carbon composite catalysts, thus, are highly prospective for practical and efficient removal of organic dyes from polluted water samples.

19


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3.4. Electrochemical Detection of Hg(II) Ions. Taking the advantages of high surface area and surface roughness possessed by the Ni/CPM, which are favorable for dispersion of active sites, we fabricated Ni/CPM-modified glassy carbon electrode (GCEs) as electrochemical sensors for the detection of Hg(II) ion. Their catalytic performances were assessed by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). As displayed in Figure 7A, the CV curves of bare and Ni/CPM-modified GCEs with and without the presences of Hg(II). All curves were recorded in an electrolyte of 0.05 M acetate buffer solution (pH 5). No redox peak was observed at Ni/CPM-modified GCEs when in the absence of Hg(II) (curves b and d). Moreover, a featureless oxidation curve with a weak peak anodic current (Ipa; 0.14 µA) was observed for the bare GCE (curve e) even in the presence of 30 µM Hg(II) in the electrolyte solution. On the other hand, a notable redox peak at peak oxidation potential (Epa) of 0.32 V was observed for the Ni/CPM-1 (curve c) and Ni/CPM-2 (curve a) modified GCEs with an enhanced peak current of 1.5 and 10.5 µA, respectively, corresponding to a respective increase by 11 and 75 folds compared to that observed for the bare GCE. The effects of scan rate on electrochemical performance of Ni/CPM catalysts were also investigated. As illustrated in Figure 7B, a linear increase in Ipa with increasing scan rate was observed for the Ni/CPM-2 modified GCE (Inset; Figure 7B), indicating a typical diffusion-controlled process61 for electrochemical detection of Hg(II). Accordingly, a detection sensitivity of 9.1 µA µM‒1 cm−2 and a lower detection limit (LOD) of 10 nM may be calculated based on the formula: LOD = 3Sb/S, where Sb is the standard deviation of the blank signal and S denotes sensitivity. By comparing the above results with other modified electrodes (Table 3), it is indicative that the Ni/CPM-modified GCEs reported herein indeed exhibit superior performances as highly sensitive sensor for electrochemical detection of Hg(II) ions. 3.5. Selective and Simultaneous Detection of Metal Ions. The selective detection of Hg(II) is highly desirable for real time detection in the presence of interfering metal ions due to possible occurrence of cross reaction, which may lead to formation of multi-metallic compounds among different metal ions.62 To assess the cross reactivity and selectivity of the Ni/CPM modified electrodes toward detection of heavy metal ions, we performed DPV studies on test electrolyte with varied Hg(II)

20



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concentrations while in the presence of other metal ions, viz. Cd(II), Pb(II), and Cu(II), which are somewhat less toxic than the Hg(II) ion.

Figure 7. (A) CV curves of (a) Ni/CPM-2 and (c) Ni/CPM-1 modified GCE, and (e) bare GCE in the presence of 30 µM Hg(II) in 0.05 M acetate buffer solution (pH 5.0). Curves (b) and (d) represent profiles recorded in the absence of Hg(II). CV curves recorded over the Ni/CPM-2 modified GCE at different (B) scan rates (50–500 mV s−1); Inset: plot of anodic peak current Ipa vs square root of scan rate and (C) dosages of Hg(II) (5−35 µM) Inset: plot of Ipa vs. Hg(II)

concentration. (D) DPV curves of the Ni/CPM-2 modified electrode under varied Hg(II) loading (1−74 µM); Inset: plot of oxidation peak current (Ipa) vs Hg(II) concentration.

21


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Table 3. Types of Modified Electrodes and Their Performances as Hg(II) Sensors* detection limit (nM)

sensitivity (µA µM−1 cm−2)

detection method

reference

6.5

58.0

DPV

62

0.034

2.8

SWASV

63

MgO nanosheets

---

15.3

DPV

69

G-DNA

5.0

---

DPV

70

Ni/CPM-2 (fish extract)

2.1

59.6

DPV

this work

electrode SNAC SnO2/RGO

*

SNAC: spherical nanoparticle decorated activated carbon; RGO: reduced graphene oxide; G-DNA: graphene deoxyribonucleic acid; DPV: differential pulse voltammetry; SWASV: square-wave anodic stripping voltammetry.

22



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Figure 8 shows the DPV responses of the Ni/CPM-2 modified GCE in a test electrolyte containing various metal ions while varying the concentration of Hg(II). It is evident that the modified GCE exhibited an excellent selectivity for simultaneous detection of metal ions. More importantly, a linear dependence of the observed peak oxidation current (Ipa) with Hg(II) concentration (within 0−50 µM) was also observed even in the presence of high concentrations of other metal ions (Inset; Figure 8). The linear correlation may be expressed as: Ipa = 0.7492 [Hg(II)] + 1.8643, with R2 = 0.9883.

Figure 8. DPV curves obtained from the Ni/CPM-2 modified GCE in the presence of varied Hg(II) concentrations (0−50 µM) together with 50 µM of Cd(II), Pb(II), and Cu(II) ions in 0.05 M acetate buffer solution (pH 5.0). Inset: plot of peak oxidation current (Ipa) vs Hg(II) concentration.

Based on the above results, a detection limit and sensitivity may be derived as 12 nM and 9.4 µA µM−1 cm‒2, respectively, revealing an excellent electrocatalytic performance of the Ni/CPM-modified GCE for sensitive and selective detection of Hg(II) even in the presence of high concentration of foreign metal species. Interestingly, similar behavior observed for oxidation peaks of the Hg(II) may also be inferred for the Cu(II) ion; a linear dependence between Ipa and [Hg(II)] was observed. It is likely that a thin film of mercury was form on the surface of the modified GCE 23


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prior to the subsequent formation of bimetallic compounds. Some reports are available on the utilization of mercury film modified electrode, for examples, dropping mercury electrode (DME) or hanging mercury drop electrode (HMDE) to enhance the detection sensitivity of foreign metals during electrochemical sensing.63−−65 However, such scheme is highly restricted due to the toxicity of mercury and environmental concerns. From Figure 8, it is indicative that the current observed for the Cu(II) oxidation peak tends level off at high dosage of Hg(II), in accordance with the above notion on formation and surface saturation of Hg film. The sharp responses (i.e., fast electron transfer)63 as well as the desirable peak-to-peak separation (305 mV) therefore afford new possibility to exploit the Ni/CPM modified GCE for selective and sensitive simultaneous detection of Hg(II) and Cu(II) ions in real samples. 3.6. Real Sample Tests. To evaluate and to demonstrate the real time applications of the proposed Hg(II) sensor, sea fishes possibly contaminated with trace level of mercury were collected and tested.66−−70 The collected fish was dried and boiled at 100 °C for 2 h to obtain an oily-like liquid, which was further purified by centrifugation. First, to demonstrate the sensitivity of the Ni/CPM modified GCE for real time detection of Hg(II), real samples with different concentrations of fish extract (pH ∼ 7.0) were prepared, the test electrolytes were adjusted to pH = 5.0 by using diluted H2SO4. In the absence of the fish extract, a featureless DPV curve was observed, as shown in Figure 9 (Inset a; bottom curve). However, in the presence of the fish extract (0‒500 µL), two broad peaks (Inset; Figure 9a), whose intensities both increase with increasing extract concentration, were observed. The peak centered at higher oxidation potential (ca. 0.23 V) should be arising from the Hg(II) in the contaminated fish extract, whereas the other peak at lower potential (ca. 0.17 V) may be attributed to bimetallic compounds (vide supra) in the fish extracts. Moreover, to afford a more accurate derivation of detection limit and sensitivity, test electrolytes doped with varied amount of Hg(II) in fish extract were also prepared. By varying the amounts of fish extract and Hg(II) from 0−21 µM, their corresponding DPV curves revealed the anticipated sharp oxidation peak for Hg(II) together with a weaker shoulder peak accountable for the bimetallic species. A linear correlation between the observed oxidation peak current (Ipa) with the concentration of fish extract and Hg(II) was observed (Inset; Figure 9b). Accordingly, an extraordinary sensitivity of 59.6 µA µM‒1 cm‒2 and a low detection limit of 2.1 nM were derived, 24



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which improve by 6.3 and 5.7 folds compared to the lab sample analysis (vide supra). Thus, it is conclusive that the Ni/CPM modified GCEs, which show superior sensitivity and low detection limit for detection of Hg(II) ions, should render practical applications as an efficient sensors for environmental pollution remediation in various Hg(II) contaminated systems, even in the presence of other metal ions.

Figure 9. DPV curves obtained from the Ni/CPM-2 modified GCE in the presence of varied loading of fish extract and Hg(II) (0−21 µM) in 0.05 M acetate buffer solution (pH 5.0). Insets: (a) DPV curves in the presence of fish extract (0, 100, 200, 300, 400, 500 µL) alone, (b) plot of corresponding oxidation peak current (Ipa) vs fish extract and Hg(II) concentration, and photographs of (c) the collected fish and (d) the purified fish extract.

4. CONCLUSIONS In conclusion, we have developed a facile route to synthesize Ni NPs decorated carbon porous (Ni/CPM) materials via the evaporation-induced self-assembly (EISA) method under microwave-assisted heating. These novel Ni/CPM materials possess high surface areas and mesoporosity desirable for dispersion of Ni NPs, rendering these novel 25


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materials as recyclable heterogeneous catalysts for the reduction of organic dyes and electrochemical detection of heavy metal ions. In particular, the magnetically separable Ni/CPM catalysts were found capable of reducing organic dyes such as MB and RhB with extraordinary reactivity in the presence of NaBH4 as the reducing agent. Typically, a complete reduction of MB and RhB can be reached within 10‒13 min at a rate constant of ca. 0.6 min‒1 even with a catalyst loading of only 2 mg. Moreover, the Ni/CPM-modified GCEs were found to exhibit excellent catalytic activity, selectivity, sensitivity, and low detection limit for the detection of Hg(II) ions, even in real samples and/or simultaneous presence of other metal ions. Thus, these Ni/CPM materials should have potential applications as viable nanocatalysts for reduction of organic dyes in waste-water treatment as well as sensors for quantitative detection and analysis of heavy metal ions in real samples.

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xxxxxxx. Assorted experimental results obtained from FT-IR, TGA, H2-TPR, and UV-Vis studies (PDF)

■ AUTHOR INFORMATION Corresponding Authors *Tel.:

+886-2-2701-7147.

Fax:

+886-2-2702-5238.

E-mail:

[email protected]. *Tel.: +886-2-2366-8230. Fax: +886-2-2362-0200. E-mail: [email protected]. Author Contributions ⊥

P.V. and R.M. contributed equally.

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS 26



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The authors are grateful to the financial supports (NSC 101-2113-M-027-001-MY3 to SMC; NSC 101-2113-M-001-020-MY3 to SBL) from the Ministry of Science and Technology (MOST), Taiwan.

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Table of Content Title: Nickel Nanoparticles Decorated Porous Carbons for Highly Active Catalytic Reduction of Organic Dyes and Sensitive Detection of Hg(II) Ions Authors: Pitchaimani Veerakumar, Rajesh Madhu, Shen-Ming Chen, Vediyappan Veeramani, Chin-Te Hung, and Shang-Bin Liu

Ni nanoparticles decorated carbon porous materials (CPMs) show superior activity and sensitivity for reduction of organic dyes and electrochemical detection of Hg(II) ions.

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Nickel Nanoparticle-Decorated Porous Carbons for Highly Active

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