Nickel Nanoparticle-Decorated Porous Carbons for Highly Active

Research Article www.acsami.org

Nickel Nanoparticle-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 ‡

S Supporting Information *

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 fabricated through a microwave-assisted heating procedure have been characterized by various analytical and spectroscopic techniques, such as X-ray diffraction, field emission transmission electron microscopy, vibrating sample magnetometry, gas physisorption/chemisorption, thermogravimetric analysis, and Raman, Fourier-transform infrared, and X-ray photon spectroscopies. 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 a relatively low detection limit (2.1 nM) toward the detection of Hg(II) ion. The system has also been successfully applied for the 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

1. INTRODUCTION

because it is not only distributed in contaminated air, water, and soil, but also bioaccumulable 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 trace levels.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 environment. The methods invoked for Hg(II) detection are mostly based on analytical and spectroscopic techniques, including calorimetry,12 cold vapor 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 forth.13−15 By comparison, electrochemical detection represents a reagentfree and more user-friendly technique owing to its facile procedure, low cost, high sensitivity and selectivity, and fast detection.16,17

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 cause mutagenic and carcinogenic diseases in humans and animals.2,3 Thus, the removal and/or reduction of organic dyes from hazardous wastes, e.g., methylene blue (MB) in wastewater, is a demanding task. Various adsorbents, such as activated carbons (ACs), carbon nanotubes (CNTs), graphene hydrogels, metal oxides, and so forth, have been designed and fabricated for the removal of organic dyes in water,4−7 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, nonbiodegradable, 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 © 2015 American Chemical Society

Received: August 25, 2015 Accepted: October 19, 2015 Published: October 19, 2015 24810

DOI: 10.1021/acsami.5b07900 ACS Appl. Mater. Interfaces 2015, 7, 24810−24821

Research Article

ACS Applied Materials & Interfaces

nickel(II) acetylacetonate (Ni(C5H7O2)2; denoted as Ni(acac)2) in 5 mL of 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 °C (typically for 1−2 h) and then subjected to microwave irradiation followed by curing at 120 °C overnight. Carbonization treatment was carried out at 900 °C under a N2 atmosphere to obtain Ni/CPM as the final product. The catalysts loaded with 0.1 and 0.5 wt % Ni were labeled as Ni/CPM-1 and Ni/CPM-2, respectively. 2.3. Characterization Methods. All powdered X-ray diffraction (XRD) experiments were recorded on a PANalytical (X’Pert PRO) diffractometer using Cu Kα radiation (λ = 0.1541 nm). Nitrogen porosimetry measurements were carried out with a Quantachrome Autosorb-1 volumetric adsorption analyzer at −196 °C (77 K). Prior to measurements, the sample was purged with flowing N2 at 150 °C 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 °C) using an electron microscope (JEOL JEM2100F) 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 FE-TEM). Xray photoelectron spectroscopy (XPS) measurements were performed using an ULVAC-PHI PHI 5000 VersaProb apparatus. Thermogravimetric analysis (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. Fourier-transform infrared (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 (H2TPR) measurements were performed utilizing an AUTOCHEM2920 under a flow of 10% H2/Ar gas mixture and a heating rate of 10 °C/min from room temperature to 900 °C. Prior to the TPR analysis, the sample was pretreated by flowing argon at a flow rate of 30 mL/ min at 600 °C 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 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 was utilized usinga 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, ∼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, and the progress was monitored using a UV−vis spectrophotometer. The disappearance of blue color (MB) to 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, and 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.

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 attention 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 (Ni NPs) that exhibit magnetic properties 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 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. Scheme 1. Schematic Illustration of Applications of Ni/CPM Materials for Reduction of Organic Dyes and Detection of Toxic Heavy Metals

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 using ultrapure water (Millipore). 2.2. Synthesis of Ni/CPM Catalysts. The Ni/CPMs were prepared according to modified procedures reported elsewhere.31 A schematic illustration of the synthesis route is shown in Figure S1 of the Supporting Information. 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 24811


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3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. Recently, considerable interest has 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

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 ∼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 type IV isotherms (IUPAC classification) with H1 hysteresis loops and notable capillary condensation steps, which are 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 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; Figure 2B and C). 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 have an average size of ∼6 nm. A complete encapsulation of NiNP in the carbon matrix therefore inhibits agglomeration of the metal particles. On the basis of the nickel crystal lattice fringes, an interlayer spacing of ∼0.21 nm may be inferred for the distance between two (011) planes. 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). 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 vibrations,34,35 arising from the PF-127, vanished after the carbonization treatment at 900 °C in N2, which is described in Figure S2A, whereas the bands near the 1600−1400 cm−1 region, which may be accounted for by C−C stretching vibrations of trisubstituted aromatic ring structure (from the framework of phenolic resin), are 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).

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.

highly porous CPM and Ni/CPM materials. The diffraction peaks located at 2θ = approximately 23.5 and 43.5° of the pristine CPM (Figure 1A(a)) may be attributed to the presence of amorphous (002) and graphitic (100) carbons.28,29 Conversely, 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 ∼7 nm was derived for the Ni/CPMs based on features of the most intense (111) peak. Raman spectroscopy is a powerful, nondestructive 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 ∼1586 cm−1 (G band) arising from the first order scattering of the E2g phonon of sp2 carbon atoms, and a peak at ∼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

Table 1. Physical Properties of the Pristine CPM and Ni/CPM Materials sample

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 Ni/CPM-1 Ni/CPM-2

0.1 0.5

6.0 ± 0.4 6.0 ± 0.7

744 622 554

0.53 0.50 0.46

5.1 5.0 5.4

1.38 3.80

12 38

0.07 0.25

0.99 0.98 0.98

a

Average metal particle size determined by FE-TEM analysis. bBrunauer−Emmet−Teller surface area (SBET) and total pore volume (VTot) calculated at P/P0 = 0.99. cPore diameter derived by the Barrett−Joyner−Halenda (BJH) method using the adsorption branch of the isotherm. dMs: saturation magnetization. eHc: coercivity. fDm: metal dispersion measured by H2 chemisorption at 323 K. gPeak intensity ratio of the G and D bands obtained from Raman spectrum. 24812


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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) and (C) are SAED patterns of the corresponding samples, and the inset in (D) is the particle size distribution of Ni NPs on the Ni/CPM-2 catalyst.

species are well-dispersed on the surfaces of the CPM support. Moreover, an additional weak reduction peak at ∼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 the thermogravimetric analysis (TGA) technique. Phloroglucinol showed descending weight loss within the temperature ranges of 50−150 and 200−900 °C, whereas the Pluronic F127 surfactant exhibited sharp weight loss at ∼400 °C, indicating a complete decomposition of the template

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 a similar broad (450−800 °C) reduction signal centering at ∼580 °C (Figure S2B), 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 centered at ∼330 °C as well as a broad asymmetric peak spanning a wide temperature range (450−950 °C). These results therefore reveal that the Ni 24813


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ACS Applied Materials & Interfaces material as shown in Figure S2C. Conversely, the TGA profile observed for the Phl-F polymeric resin showed major weight losses at approximately 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 Figure S2D, which revealed analogous desorption profiles compared to their carbon precursor (Phl-F) as shown in Figure S2C. In particular, the weak DTG peak centered at ∼100 °C 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 ∼600 °C corresponds to a weight-loss of 83.4 and 79.4% for Ni/CPM1 and Ni/CPM-2, respectively, it 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-

Figure 4. Magnetization curves of (a) Ni/CPM-1 and (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.

separated effectively when in the presence of an external static magnetic field within a period of only ∼5−8 s at ambient conditions. Upon removing the magnetic field, the catalyst may be readily redispersed 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 favorable for recycling use. 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 used in manufacturing industries.1 Thus, the development of an efficient and reliable technique for catalytic reduction of organic dyes is a demanding task that this work aims to resolve. MB is a thiazine cationic dye that is water-soluble and mostly presented as an ionic form (MB+) in an aqueous medium. The oxidized form of MB normally exhibits UV−vis absorption peaks in the wavelength (λ) range of ∼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 ∼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). Panels C and D in Figure 5 display the variations of relative peak intensity, ln(A/A0), relative to reaction time during catalytic reduction of MB and RhB, respectively. Here, A0 and A represent 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 (Figure 5A and C) and RhB (Figure 5B and D) dyes were observed over the Ni/CPM catalysts. It is noteworthy that such correlation was not found

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.

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 The magnetic properties of the Ni/CPM samples were investigated by a 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 recovery. This is illustrated in Figure 4c, which clearly shows that the Ni/CPM catalyst (dispersed in ethanol) may be 24814


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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 catalyzed by (a) pristine CPM, (b) Ni/ CPM-1, and (c) Ni/CPM-2 catalysts are also displayed together with structural and photographic illustrations of the 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.

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

in Figure 5E and F (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

for the pristine CPM sample (i.e., without Ni), which revealed a constant (null) ln(A/A0) over time. The structural transformations of MB and RhB to their corresponding reduced forms of LMB and LRhB are illustrated 24815


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Table 2. Comparisons of Catalytic Performance for Reduction of MB Over Various Supported Catalyst Systems

a

catalysta

size of NP (nm)

Fe3O4@polydopamine (PDA)-Ag NPs Pd-tetrahedral nanocrystals (TNCs)/RGO copper nanocrystals (CuNCs) Au@polypyrrole (PPy)/Fe3O4 Fe3O4@Ag Ir NPs Ag/magnetic Fe3O4@C core−shell NCs silicon nanowire arrays/Cu NPs CoO nanowires Cu microsphere Ni nanotube arrays Ni/CPM-1 Ni/CPM-2

25 9 28 400 52 200 ± 15 300 25 4−16 700 >100 6.0 ± 0.1 6.0 ± 0.7

catalyst amount (mg) 5.0 5.0 0.1 2.0 1.6 0.2 10.0 1 × 1 cm2 250 5.0 100.0 2.0 2.0

[μL] [mL]

[mL]

[μL]

reaction time (min)

rate constant k (min−1)

ref

22 7 5 42 6 24 10 10 81 8 1 13 13

0.430 0.400 0.020 0.266 0.410 0.041 0.340 0.317 0.038 0.393 2.220 0.583 0.571

49 50 51 52 53 54 55 56 57 58 59 this work this work

NPs: nanoparticles; RGO: reduced graphene oxide.

invoked first-order kinetics.47 By comparing the results in Figure S3B and C, it is clear that the reaction kinetics are readily depending on the amount of Ni/CPM catalyst employed. For reactions carried out with smaller amounts (1.0 mg) of Ni/CPM-1 catalyst, as shown in Figure S3B, the MB dye failed to reduce completely within 13 min. However, as the catalyst loading was increased to 3.0 mg, a complete reduction of MB was achieved within 8 min (Figure S3C). Moreover, although 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 ∼6 nm, exhibited super MB reduction performance compared to other catalyst systems reported in the literature. A complete reduction of MB may be reached within 13 min at a rate constant of ∼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 the other catalyst systems as shown in Table S1. A complete reduction of RhB was achieved within 18 min at a rate constant of ∼0.47 min−1 over 2 mg of catalyst loading. Moreover, these novel Ni/CPM catalysts also have the advantages of easy separation, being 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, 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). Even though the catalyst may be effectively separated using a magnet, the gradual decrease in rate constant observed over the repeated cycles is attributable 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 approximately 20−30 min and then centrifuged. The reduced sample was extracted in DI water and washed thoroughly with ethanol, followed by

color) and RhB (pink color) dyes were effectively transformed to their respective colorless reduced forms (LMB and LRhB) within a short period of time (∼5−8 s). Upon removing the magnetic field, the catalysts readily redispersed 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. 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 the 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 MB with 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. Clearly, the reduction process invoked a two-electron transfer process over the supported Ni NPs, which tends to catalyze 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 a reduction process prevails even under ambient 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. As shown in Figure S3A, null reduction of MB was observed when no catalyst was introduced, even under an excessive amount of NaBH4. On the contrary, the linear correlation between ln(A/ A0) and time observed while in the presence of the Ni/CPM catalyst is shown in Figure S3D and implies that the reaction 24816


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ACS Applied Materials & Interfaces drying at 60 °C in a vacuum oven. Subsequently, the degree of MB reduction was checked by UV−vis spectroscopy. As shown in Figure S5, 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 the efficient removal of organic dyes from the pollutant mixture. 3.3. Interference in the Presence of Polluted Water Samples. To further validate the catalytic performance of Ni/ CPM materials for reduction of organic dyes, we examined MB blended in two polluted water samples, namely, industrial wastewater and lake water, obtained from a suburb of metropolitan Taipei. The tests were conducted under the same reaction conditions used for the aqueous MB solution (see Figure 5A and D). Again, a linear correlation between ln(A/A0) and reaction time (t) was observed (insets; Figure S6) in both cases, indicating that the presence of these real water samples has little interference with the reduction performance of the Ni/CPM-1 catalyst. The data summarized in Table 2 and Tables S1 and S2 clearly show that the catalytic properties of Ni/CPM nanocomposite materials outperform the majority of other metal−carbon composite catalysts and are thus highly prospective for practical and efficient removal of organic dyes from polluted water samples. 3.4. Electrochemical Detection of Hg(II) Ions. Taking advantage of the 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 electrodes (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). Displayed in Figure 7A are the CV curves of bare and Ni/CPM-modified GCEs with and without the presence 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. Conversely, 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 of 11- and 75-fold 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 a 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

Figure 7. (A) CV curves of (a) Ni/CPM-2- and (c) Ni/CPM-1modified 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.

Table 3. Types of Modified Electrodes and Their Performances as Hg(II) Sensorsa electrode SNAC SnO2/RGO MgO nanosheets G-DNA Ni/CPM-2 (fish extract)

detection limit (nM)

sensitivity (μA μM−1 cm−2)

detection method

6.5 0.034

58.0 2.8 15.3

DPV SWASV DPV

59.6

DPV DPV

5.0 2.1

ref 62 63 69 70 this work

a

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

real time detection in the presence of interfering metal ions due to the possible occurrence of cross reaction, which may lead to the formation of multimetallic compounds among different metal ions.62 To assess the cross reactivity and selectivity of the Ni/CPM-modified electrodes towards the detection of heavy metal ions, we performed DPV studies on test electrolyte with varied Hg(II) concentrations while in the presence of other metal ions, namely, Cd(II), Pb(II), and Cu(II), which are somewhat less toxic than the Hg(II) ion. Figure 8 shows the DPV responses of the Ni/CPM-2modified GCE in a test electrolyte containing various metal ions while varying the concentration of Hg(II). It is evident that the modified GCE exhibited 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 24817


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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.

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.

(inset; Figure 8). The linear correlation may be expressed as Ipa = 0.7492 [Hg(II)] + 1.8643 with R2 = 0.9883. On the basis of 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 a 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 formed on the surface of the modified GCE prior to the subsequent formation of bimetallic compounds. Some reports are available on the utilization of a mercury filmmodified electrode, for example, a 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 a 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 to level off at a high dosage of Hg(II), in accordance with the above notion on the formation and surface saturation of a 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 possibilities 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, we collected and tested sea fish that were possibly contaminated with trace levels of mercury.66−70 The collected fish were dried and boiled at 100 °C for 2 h to obtain an oil-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, and the test electrolytes were adjusted to pH 5.0 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 (∼0.23 V) should arise from the Hg(II) in the contaminated fish extract, whereas the other peak at lower potential (∼0.17 V) may be

attributed to bimetallic compounds (vide supra) in the fish extracts. Moreover, to afford a more accurate derivation of the detection limit and sensitivity, test electrolytes doped with a 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, which improved by 6.3- and 5.7-fold compared to the lab sample analysis (vide supra). Thus, it is conclusive that the Ni/ CPM-modified GCEs, which show superior sensitivity and a low detection limit for the detection of Hg(II) ions, should render practical applications as an efficient sensor for environmental pollution remediation in various Hg(II)contaminated systems, even in the presence of other metal ions.

4. CONCLUSIONS In conclusion, we have developed a facile route to synthesize Ni NP-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 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 ∼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 the simultaneous presence of other metal ions. Thus, these Ni/CPM materials should have potential applications as viable nanocatalysts for the reduction of organic dyes in 24818


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(12) Jayabal, S.; Sathiyamurthi, R.; Ramaraj, R. Selective Sensing of Hg2+ Ions by Optical and Colorimetric Methods Using Gold Nanorods Embedded in a Functionalized Silicate Sol-gel Matrix. J. Mater. Chem. A 2014, 2, 8918−8925. (13) Bi, N.; Chen, Y.; Qi, H.; Zheng, X.; Chen, Y.; Liao, X.; Zhang, H.; Tian, Y. Spectrophotometric Determination of Mercury(II) Ion Using Gold Nanorod as Probe. Sens. Actuators, B 2012, 166−167, 766−771. (14) Shah, A. Q.; Kazi, T. G.; Baig, J. A.; Afridi, H. I.; Kandhro, G. A.; Khan, S.; Kolachi, N. F.; Wadhwa, S. K. Determination of Total Mercury in Chicken Feed, Its Translocation to Different Tissues of Chicken and Their Manure Using Cold Vapour Atomic Absorption Spectrometer. Food Chem. Toxicol. 2010, 48, 1550−1554. (15) Zhao, L.; Zhong, S.; Fang, K.; Qian, Z.; Chen, J. Determination of Cadmium(II), Cobalt(II), Nickel(II), Lead(II), Zinc(II), And Copper(II) in Water Samples Using Dual-cloud Point Extraction and Inductively Coupled Plasma Emission Spectrometry. J. Hazard. Mater. 2012, 239−240, 206−212. (16) Giacomino, A.; Abollino, O.; Malandrino, M.; Mentasti, E. Parameters Affecting the Determination of Mercury by Anodic Stripping Voltammetry Using a Gold Electrode. Talanta 2008, 75, 266−273. (17) Palanisamy, S.; Madhu, R.; Chen, S. M.; Ramaraj, S. K. A Highly Sensitive and Selective Electrochemical Determination of Hg(II) Based on an Electrochemically Activated Graphite Modified Screenprinted Carbon Electrode. Anal. Methods 2014, 6, 8368−8373. (18) Deng, Y.; Wei, J.; Sun, Z.; Zhao, D. Large-pore Ordered Mesoporous Materials Templated from Non-pluronic Amphiphilic Block Copolymers. Chem. Soc. Rev. 2013, 42, 4054−4070. (19) Ryoo, R.; Joo, S. H.; Kruk, M.; Jaroniec, M. Ordered Mesoporous Carbons. Adv. Mater. 2001, 13, 677−681. (20) Liang, C. D.; Dai, S. Synthesis of Mesoporous Carbon Materials via Enhanced Hydrogen-bonding Interaction. J. Am. Chem. Soc. 2006, 128, 5316−5317. (21) Liu, S. H.; Yu, W. Y.; Chen, C. H.; Lo, A. Y.; Hwang, B. J.; Chien, S. H.; Liu, S. B. Fabrication and Characterization of Welldispersed and Highly Stable PtRu Nanoparticles on Carbon Mesoporous Material for Applications in Direct Methanol Fuel Cell. Chem. Mater. 2008, 20, 1622−1628. (22) Zhang, J.; Li, C. M. Nanoporous Metals: Fabrication Strategies and Advanced Electrochemical Applications in Catalysis, Sensing and Energy Systems. Chem. Soc. Rev. 2012, 41, 7016−7031. (23) Madhu, R.; Veeramani, V.; Chen, S. M. Heteroatom-enriched and Renewable Banana-stem-derived Porous Carbon for the Electrochemical Determination of Nitrite in Various Water Samples. Sci. Rep. 2014, 4, 4679. (24) You, H.; Yang, S.; Ding, B.; Yang, H. Synthesis of Colloidal Metal and Metal Alloy Nanoparticles for Electrochemical Energy Applications. Chem. Soc. Rev. 2013, 42, 2880−2904. (25) Zhang, Y.; Xiao, X.; Sun, Y.; Shi, Y.; Dai, H.; Ni, P.; Hu, J.; Li, Z.; Song, Y.; Wang, L. Electrochemical Deposition of Nickel Nanoparticles on Reduced Graphene Oxide Film for Nonenzymatic Glucose Sensing. Electroanalysis 2013, 25, 959−966. (26) Bräuer, B.; Vaynzof, Y.; Zhao, W.; Kahn, A.; Li, W.; Zahn, D. R. T.; Fernández, C. J.; Sangregorio, C.; Salvan, G. Electronic and Magnetic Properties of Ni Nanoparticles Embedded in Various Organic Semiconductor Matrices. J. Phys. Chem. B 2009, 113, 4565−4570. (27) Zhai, Y.; Dou, Y.; Liu, X.; Park, S. S.; Ha, C. S.; Zhao, D. Softtemplate Synthesis of Ordered Mesoporous Carbon/Nanoparticle Nickel Composites with a High Surface Area. Carbon 2011, 49, 545− 555. (28) Menéndez, J. A.; Juárez-Pérez, E. J.; Ruisánchez, E.; Calvo, E. G.; Arenillas, A. A Microwave-based Method for the Synthesis of Carbon Xerogel Spheres. Carbon 2012, 50, 3555−3560. (29) Veerakumar, P.; Madhu, R.; Chen, S. M.; Hung, C. T.; Tang, P. H.; Wang, C. B.; Liu, S. B. Porous Carbon-modified Electrodes as Highly Selective and Sensitive Sensors for Detection of Dopamine. Analyst 2014, 139, 4994−5000.

wastewater treatment as well as sensors for quantitative detection and analysis of heavy metal ions in real samples.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b07900. Assorted experimental results obtained from FT-IR, TGA, H2-TPR, and UV−vis studies (PDF)

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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.

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ACKNOWLEDGMENTS The authors are grateful for the financial support (NSC 1012113-M-027-001-MY3 to S.-M.C.; NSC 101-2113-M-001-020MY3 to S.-B.L.) from the Ministry of Science and Technology (MOST), Taiwan.

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REFERENCES

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Nickel Nanoparticle-Decorated Porous Carbons for Highly Active

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