nickel nanoparticle-decorated porous carbons for highly active catalytic reduction of organic dyes...

Nickel Nanoparticle-Decorated Porous Carbons for Highly ActiveCatalytic Reduction of Organic Dyes and Sensitive Detection of Hg(II)IonsPitchaimani 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 thedirect template method via self-assembly of polymerized phloroglucinol-formaldehyderesol around a triblock copolymer template were used as supports for nickel nanoparticles(Ni NPs). The Ni/CPM materials fabricated through a microwave-assisted heatingprocedure have been characterized by various analytical and spectroscopic techniques,such as X-ray diffraction, field emission transmission electron microscopy, vibratingsample magnetometry, gas physisorption/chemisorption, thermogravimetric analysis, andRaman, Fourier-transform infrared, and X-ray photon spectroscopies. Results obtainedfrom ultraviolet−visible (UV−vis) spectroscopy demonstrated that the supported Ni/CPM catalysts exhibit superior activity for catalytic reduction of organic dyes, such asmethylene blue (MB) and rhodamine B (RhB). Further electrochemical measurementsby cyclic voltammetry (CV) and differential pulse voltammetry (DPV) also revealed thatthe Ni/CPM-modified electrodes showed excellent sensitivity (59.6 μA μM−1 cm−2) anda relatively low detection limit (2.1 nM) toward the detection of Hg(II) ion. The systemhas also been successfully applied for the detection of mercuric ion in real sea fish samples. The Ni/CPM nanocompositerepresents a robust, user-friendly, and highly effective system with prospective practical applications for catalytic reduction oforganic 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

Organic dyes, which have been widely exploited in printing,textile, paper, paints, and plastics industries,1 are environmentalpollutants that are capable of retarding the photosynthesis cyclein plant metabolism and may cause mutagenic and carcinogenicdiseases in humans and animals.2,3 Thus, the removal and/orreduction of organic dyes from hazardous wastes, e.g.,methylene blue (MB) in wastewater, is a demanding task.Various adsorbents, such as activated carbons (ACs), carbonnanotubes (CNTs), graphene hydrogels, metal oxides, and soforth, have been designed and fabricated for the removal oforganic dyes in water,4−7 invoking a variety of differenttechniques, namely, adsorption, photocatalytic degradation,chemical oxidation, membrane filtration, flocculation, andelectrooxidation.8,9

In addition, heavy metals, such as cadmium (Cd), mercury(Hg), arsenic (As), and lead (Pb), are also well-knownpollutants that are highly toxic, nonbiodegradable, and highlycarcinogenic to humans and other living organisms, even atrelatively low concentrations. Among them, Hg(II) representsthe most notorious heavy metal pollutant in the ecosystem

because it is not only distributed in contaminated air, water,and soil, but also bioaccumulable in plants, aquatic animals, andother living organisms.10 Moreover, it may leads to seriousadverse effects, such as digestive, kidney, liver, cancer, andespecially neurological diseases, even at trace levels.11 Hence, itis imperative to develop a simple, rapid, sensitive, and highlyselective analytical method for monitoring the level of Hg(II) inour environment. The methods invoked for Hg(II) detectionare mostly based on analytical and spectroscopic techniques,including calorimetry,12 cold vapor atomic absorption spec-trometry (CV-AAS), inductively coupled plasma with atomicemission or mass spectrometry (ICP-AES or ICP-MS), X-rayfluorescence (XRF) spectrometry, and so forth.13−15 Bycomparison, electrochemical detection represents a reagent-free and more user-friendly technique owing to its facileprocedure, low cost, high sensitivity and selectivity, and fastdetection.16,17

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

Research Article

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© 2015 American Chemical Society 24810 DOI: 10.1021/acsami.5b07900ACS Appl. Mater. Interfaces 2015, 7, 24810−24821

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http://dx.doi.org/10.1021/acsami.5b07900

Carbon porous materials (CPMs), which possess uniformand tailorable pore structure, high specific surface area, largepore volume, and unique electrochemical properties,18−20 havedrawn considerable R&D attention and have been widelyapplied in various fields, e.g., gas separation/adsorption, fuelcells, electrochemical sensors, and energy storage systems.21−23

In particular, CPMs containing nickel nanoparticles (Ni NPs)that exhibit magnetic properties favorable for separation aredesirable materials as catalysts, catalytic supports, or adsorb-ents.24−27 It has been demonstrated that CPMs with highsurface areas may be easily prepared by a microwave-assistedsynthesis route, which is more energy efficient, cost-effective,and time-saving.28−30 Previously, we demonstrated a synthesisroute to incorporate palladium nanoparticles (Pd NPs) onCPMs via a soft templating method under microwaveirradiation.31 A similar synthesis route is adopted herein forthe fabrication of the Ni/CPM materials. To the best of ourknowledge, this is the first report to utilize the Ni/CPMmaterial as a catalyst for the reduction of methylene blue (MB)and rhodamine B (RhB) dyes as well as selective detection ofHg(II) in real samples, as illustrated in Scheme 1.

2. EXPERIMENTAL SECTION2.1. Chemicals. Phloroglucinol (ACS, 99.98%, Acros), form-

aldehyde (37% in water, Acros), HCl (37% Acros), triblock copolymerPluronic 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 furtherpurification. All other chemicals used were analytical grade, and allsolutions 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 Aschematic illustration of the synthesis route is shown in Figure S1 ofthe Supporting Information. In brief, a phloroglucinol-formaldehyde(Phl-F) resin was first prepared by the soft-templating method using atriblock copolymer (Pluronic F127; EO106PO70EO106) as the structuredirecting agent. The Phl-F polymer was dissolved with metal precursor

nickel(II) acetylacetonate (Ni(C5H7O2)2; denoted as Ni(acac)2) in 5mL of tetrahydrofuran (THF) solution to obtain a clear orange-greenmixture gel. The gel was then loaded on a large Petri dish, dried atroom temperature overnight, and subsequently cured at 80 °C(typically for 1−2 h) and then subjected to microwave irradiationfollowed by curing at 120 °C overnight. Carbonization treatment wascarried out at 900 °C under a N2 atmosphere to obtain Ni/CPM as thefinal product. The catalysts loaded with 0.1 and 0.5 wt % Ni werelabeled 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). Nitrogenporosimetry measurements were carried out with a QuantachromeAutosorb-1 volumetric adsorption analyzer at −196 °C (77 K). Priorto measurements, the sample was purged with flowing N2 at 150 °Cfor at least 12 h. The pore size distributions were derived from theadsorption branches of isotherms using the Barrett−Joyner−Halanda(BJH) method. The morphology of the sample was studied by fieldemission transmission electron microscopy (FE-TEM) at roomtemperature (25 °C) using an electron microscope (JEOL JEM-2100F) that has a field-emission gun at an acceleration voltage of 200kV. Elemental composition of various samples was carried out with anenergy-dispersive X-ray (EDX) analyzer (equipped with FE-TEM). X-ray photoelectron spectroscopy (XPS) measurements were performedusing an ULVAC-PHI PHI 5000 VersaProb apparatus. Thermogravi-metric analysis (TGA) was performed on a Netzsch TG-209instrument under air atmosphere. UV−vis absorption spectralmeasurements were carried out with a SPECORDS100 diode-arrayspectrophotometer. Fourier-transform infrared (FT-IR) spectra wererecorded 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 roomtemperature. Hydrogen temperature-programmed reduction (H2-TPR) measurements were performed utilizing an AUTOCHEM-2920 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 wascooled to room temperature. The amount of H2 uptake during thereduction was measured continuously with a thermal conductivitydetector (TCD). All Raman spectra were recorded on a Jobin YvonT64000 Spectrometer equipped with a charge coupled device (CCD)detector cooled with liquid nitrogen. The backscattering signal wascollected with a microscope using an Ar+ laser centered at 488 nm asthe excitation source. Magnetic properties of the Ni-loaded CPMsamples were measured by using a vibrating sample magnetometerSQUID VSM (Quantum design, USA; maximum applied continuousfield 50,000 G) at room temperature. Cyclic voltammetry (CV) anddifferential pulse voltammetry (DPV) studies were performed using aCHI 900 electrochemical analyzer (CH instruments). A 0.05 Macetate (HAc + NaAc; pH 5.0) buffer solution was used as thesupporting electrolyte during analysis. A conventional three-electrodecell system was utilized usinga modified glassy carbon electrode(GCE) as the working electrode, Ag/AgCl (saturated KCl) as thereference electrode, and a platinum wire as the counter electrode.

2.4. Catalytic Reduction of Organic Dyes. Typically, ∼2.0 mgof the as-synthesized Ni/CPM catalysts were first added to an aqueoussolution of methylene blue (MB; 5.0 mL, 3 × 10−5 M). Subsequently,the above solution was mixed with freshly prepared aqueous NaBH4 (2mL, 1 × 10−2 M) solution. The reduction reaction was carried out atroom temperature under vigorous stirring, and the progress wasmonitored using a UV−vis spectrophotometer. The disappearance ofblue color (MB) to colorless leuco-methylene blue (LMB) indicatesthe completion of the reaction. Upon completion, the catalyst wasseparated from the reaction system by means of an external magnet,followed by washing 2−3 times with ethanol, and then dried at roomtemperature for recycling use. The catalytic reduction reaction ofrhodamine B (RhB) was carried out using the above procedure for thereduction of MB.

Scheme 1. Schematic Illustration of Applications of Ni/CPMMaterials for Reduction of Organic Dyes and Detection ofToxic Heavy Metals

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.5b07900ACS Appl. Mater. Interfaces 2015, 7, 24810−24821

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3. RESULTS AND DISCUSSION3.1. Catalyst Characterization. Recently, considerable

interest has been focused on metal NPs supported on CPMmaterials as heterogeneous catalysts by virtue of their versatilityin chemical compositions and structural architectures. Figure1A display the typical X-ray diffraction (XRD) patterns of the

highly porous CPM and Ni/CPM materials. The diffractionpeaks located at 2θ = approximately 23.5 and 43.5° of thepristine CPM (Figure 1A(a)) may be attributed to the presenceof amorphous (002) and graphitic (100) carbons.28,29

Conversely, the characteristic peaks at 2θ = 44.5, 51.82, and76.34° observed in both Figures 1A(b) and 1A(c) are ascribeddue to the (111), (200), and (220) facets of the face-centeredcubic (fcc) crystalline Ni in the Ni/CPM-1 and Ni/CPM-2samples with Ni loadings of 0.1 and 0.5 wt %, respectively.30 Itis noteworthy that no diffraction peak accountable for NiO wasobserved. By using the Scherrer equation,32 an averagecrystallite size of ∼7 nm was derived for the Ni/CPMs basedon features of the most intense (111) peak.Raman spectroscopy is a powerful, nondestructive tool to

characterize carbonaceous materials. Figure 1B displays therepresentative Raman spectra of CPM, Ni/CPM-1, and Ni/CPM-2 composite. As anticipated, the carbon substratesexhibited two prominent features: the peak at ∼1586 cm−1

(G band) arising from the first order scattering of the E2gphonon of sp2 carbon atoms, and a peak at ∼1352 cm−1 (Dband) arising from a breathing mode of k-point phonons withA1g symmetry. The latter D band is normally associated withstructural 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 usuallyused as a measure for the disordering of the carbon structureand was found to be ∼0.98 for all the samples examined (Table1).33

Figure 1C shows the N2 adsorption/desorption isotherms ofvarious CPM-based samples recorded at 77 K. All samplesshowed type IV isotherms (IUPAC classification) with H1hysteresis loops and notable capillary condensation steps, whichare characteristic of ordered mesoporous materials.18−25 Theirpore size distributions are displayed in Figure 1D. Accordingly,their surface areas (SBET), total pore volumes (VTot), and porediameters (dBJH) were derived, and the results are depicted inTable 1. It can be seen that notable decreases in surface areaand pore volume of the sample were observed upon increasingthe Ni loading. However, not much change in the porediameter was observed upon loading NiNPs onto the CPMsubstrate (Figure 1D). These results reveal that the NiNPsreadily 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 Figure2. The crystalline nature of the NiNP is further confirmed byusing selected area electron diffraction (SAED) patterns(insets; Figure 2B and C). The high-resolution FE-TEMimage of the Ni/CPM-2 catalyst in Figure 2D revealed that theNiNPs encapsulated in the CPM substrate typically have anaverage size of ∼6 nm. A complete encapsulation of NiNP inthe carbon matrix therefore inhibits agglomeration of the metalparticles. On the basis of the nickel crystal lattice fringes, aninterlayer spacing of ∼0.21 nm may be inferred for the distancebetween 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 at7.5 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-IRexperiments. As anticipated, the IR bands responsible for thephenolic −OH (3450 cm−1) stretching and C−H (3000−2800;950 cm−1) and C−O (1200−950 cm−1) bending/stretchingvibrations,34,35 arising from the PF-127, vanished after thecarbonization treatment at 900 °C in N2, which is described inFigure S2A, whereas the bands near the 1600−1400 cm−1

region, which may be accounted for by C−C stretchingvibrations of trisubstituted aromatic ring structure (from theframework of phenolic resin), are retained after the treatment.These results confirm the successful removal of template andformation of graphitic carbons during the carbonizationprocedure.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) pristineCPM, (b) Ni/CPM-1, and (c) Ni/CPM-2 samples.

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 (cm

3 g−1)b dBJH (nm)c Ms (emu g−1)d Hc (Oe)e Dm (%)f IG/ID

g

pristine CPM 744 0.53 5.1 0.99Ni/CPM-1 0.1 6.0 ± 0.4 622 0.50 5.0 1.38 12 0.07 0.98Ni/CPM-2 0.5 6.0 ± 0.7 554 0.46 5.4 3.80 38 0.25 0.98

aAverage metal particle size determined by FE-TEM analysis. bBrunauer−Emmet−Teller surface area (SBET) and total pore volume (VTot) calculatedat P/P0 = 0.99. cPore diameter derived by the Barrett−Joyner−Halenda (BJH) method using the adsorption branch of the isotherm. dMs: saturationmagnetization. eHc: coercivity.

fDm: metal dispersion measured by H2 chemisorption at 323 K. gPeak intensity ratio of the G and D bands obtainedfrom Raman spectrum.



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Compared to the bulk NiO powder, which showed a broadreduction peak in the temperature range of 250−400 °C, theNi/CPM-1 and Ni/CPM-2 catalysts exhibited a similar broad(450−800 °C) reduction signal centering at ∼580 °C (FigureS2B), indicating the absence of NiO species.36,37 Bycomparison, the H2-TPR profile observed for a sampleprepared by incorporating NiO on CPM (denoted as NiO/CPM) revealed a weak reduction peak centered at ∼330 °C aswell as a broad asymmetric peak spanning a wide temperaturerange (450−950 °C). These results therefore reveal that the Ni

species are well-dispersed on the surfaces of the CPM support.Moreover, an additional weak reduction peak at ∼920 °C wasalso observed for the Ni/CPM catalysts and is most likely dueto 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 thetemperature ranges of 50−150 and 200−900 °C, whereas thePluronic F127 surfactant exhibited sharp weight loss at ∼400°C, indicating a complete decomposition of the template

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 sizedistribution of Ni NPs on the Ni/CPM-2 catalyst.



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material as shown in Figure S2C. Conversely, the TGA profileobserved for the Phl-F polymeric resin showed major weightlosses at approximately 50−150, 250−350, and 600 °C, asexpected.38,39 Similarly, we also performed TGA analysis for thepristine CPM and Ni/CPM composites as shown in FigureS2D, which revealed analogous desorption profiles compared totheir carbon precursor (Phl-F) as shown in Figure S2C. Inparticular, the weak DTG peak centered at ∼100 °C observedfor both Ni/CPM-1 and Ni/CPM-2 may be attributed todesorption of physisorbed water and organic solvent, whichcorresponds to a weight loss of 1.7 and 4.3%, respectively.Whereas the strong DTG peak centering at ∼600 °Ccorresponds to a weight-loss of 83.4 and 79.4% for Ni/CPM-1 and Ni/CPM-2, respectively, it should be associated withoxidation of the carbon support into gaseous carbon dioxide.40

The surface chemical properties of the catalyst were alsoexamined by X-ray photoelectron spectroscopy (XPS). As anillustration, Figure 3 depicts the XPS spectrum of the Ni/CPM-

2 sample, which clearly shows signals corresponding toelements, such as Ni, C, and O, in the substrate. Althoughthe Auger binding energies (Eb) observed for Ni 2p and Ni 3pphoto absorption peaks indicate the presence of metallic Ni, thepeaks centered at 852.5, 870.8, 530.5, and 284.8 eV may beattributed to Ni 2p3/2, Ni 2p1/2, O 1s, and C 1s spin−orbits,respectively.41

The magnetic properties of the Ni/CPM samples wereinvestigated by a vibrating sample magnetometer (VSM) atroom temperature. As shown in Figure 4, the magnetizationcurve obtained from the Ni/CPM samples displayed theanticipated hysteresis loops. Accordingly, the saturationmagnetization (Ms) value for Ni/CPM-1 and Ni/CPM-2materials 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 valuesobserved for the Ni/CPM catalysts were much lower than thatof bulk Ni (51.3 emu g−1)30 is most likely due to the nanosizeeffect as well as the confinement of the Ni NPs within thecarbon framework.42,43 Moreover, a notable increase in Msvalue with increasing Ni loading was found, indicating thecapability for magnetic separation, which is desirable for catalystrecovery. This is illustrated in Figure 4c, which clearly showsthat the Ni/CPM catalyst (dispersed in ethanol) may be

separated effectively when in the presence of an external staticmagnetic field within a period of only ∼5−8 s at ambientconditions. Upon removing the magnetic field, the catalyst maybe readily redispersed in ethanol due to its low remanentmagnetization (i.e., remanence) and coercivity. Thus, the Ni/CPM catalysts indeed possess the desirable characteristics forfacile magnetic separation, efficient recovery from the reactionmixture, and excellent chemical and structural stabilitiesfavorable for recycling use.

3.2. Catalytic Reduction of Organic Dyes. As mentionedearlier, Ni is much more cost-effective than precious metals(e.g., Au, Ag, Pd, Pt, Ru, and Rh), which have been extensivelyexploited as catalysts for organic transformations.44 Organicdyes such as methylene blue (MB) and rhodamine B (RhB) arewell-known environmental pollutants that have been widelyused in manufacturing industries.1 Thus, the development of anefficient and reliable technique for catalytic reduction of organicdyes is a demanding task that this work aims to resolve. MB is athiazine cationic dye that is water-soluble and mostly presentedas an ionic form (MB+) in an aqueous medium. The oxidizedform of MB normally exhibits UV−vis absorption peaks in thewavelength (λ) range of ∼550−750 nm and typically shows anabsorption maxima at λmax = 665 nm, which may be attributedto the n-π* transitions of the MB molecule. Consequently, theprogress of the reaction can readily be followed by means ofUV−vis absorption spectrophotometry. Typically, the reduc-tion reaction was carried out in ambient conditions (videsupra), and a complete reduction of MB by NaBH4 may beinferred by the disappearance of intense blue color (MB) tocolorless leuco-methylene blue (LMB) within ∼13 min, asshown in Figure 5A. The Ni/CPM catalysts were alsoemployed for catalytic reduction of RhB. Likewise, the UV−vis spectra in Figure 5B also exhibited a consistent decrease inabsorption peak intensity (A) at λmax = 552 nm within a timeperiod of 18 min, indicating a successive reduction of RhB toleuco-rhodamine B (LRhB).Panels C and D in Figure 5 display the variations of relative

peak intensity, ln(A/A0), relative to reaction time duringcatalytic reduction of MB and RhB, respectively. Here, A0 and Arepresent the initial and final absorption peak intensity of thedye molecule at the reaction time of 0 and t, respectively. Alinear correlation between ln(A/A0) and reaction time (t) forthe catalytic reduction of MB (Figure 5A and C) and RhB(Figure 5B and D) dyes were observed over the Ni/CPMcatalysts. It is noteworthy that such correlation was not found

Figure 3. XPS spectrum of the Ni/CPM-2 catalyst. Insets show thespectra of the Ni 2p3/2 and the corresponding satellite peaks for O 1sand C 1s.

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



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for the pristine CPM sample (i.e., without Ni), which revealed aconstant (null) ln(A/A0) over time.The structural transformations of MB and RhB to their

corresponding reduced forms of LMB and LRhB are illustrated

in Figure 5E and F (over the Ni/CPM-1 catalyst), respectively,together with photographs showing their corresponding colorchanges when the reaction mixtures were respectively placed inan external magnetic field. Accordingly, the oxidized MB (blue

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 ofrelative 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.



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color) and RhB (pink color) dyes were effectively transformedto their respective colorless reduced forms (LMB and LRhB)within a short period of time (∼5−8 s). Upon removing themagnetic field, the catalysts readily redispersed automatically.The excellent activity and stability observed for the Ni/CPMsrender practical application of these novel catalysts forreduction of organic dyes by NaBH4, which may be easilymonitored by UV−vis absorption spectroscopy.Furthermore, the reaction mechanism invoked for catalytic

reduction of organic dyes over the Ni/CPM catalysts was alsoproposed based on the earlier reports.45 As illustrated in Figure6, the reduction process for MB over the Ni/CPM catalystinvolves the following steps: Step I, the reducing agent(NaBH4) transfers a hydride to the Ni NP surfaces, leadingto the formation of covalent Ni−H bonds; Step II, adsorptionof MB dye molecules onto the surfaces of the catalyst, which isthe rate-determining step provoked by interactions between ofadsorbed MB with surface-bound hydrogen atoms; Step III,MB molecules tend to capture two electrons from the activesurfaces of Ni NPs; Step IV, LMB is formed as a result of thereduction reaction followed by desorption of the product fromthe 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 tocatalyze the adsorbed MB dye molecules, provoking reductionof double bonds in the heterocyclic rings of MB. This leads tobreaking of the conjugated π bonds and shortening of theelectron delocalization distance of the dye to favor adsorptionof MB, which is effectively reduced by the supported Ni NPs toform LMB.46 Note that such a reduction process prevails evenunder ambient conditions. The pronounced catalytic activityobserved for the synthesized Ni/CPMs during reduction oforganic dyes may also be partly attributed to the porouscharacteristic of the CPM support. The high surface areapossessed by the CPM is favorable for dispersion of Ni NPs,which in turn provokes electron transfer and subsequentreduction reaction on their surfaces.The reduction of MB over the Ni/CPM was further

optimized by varying the amount of catalyst used, asexemplified for the Ni/CMP-1 in Figures S3A−C. As shownin Figure S3A, null reduction of MB was observed when nocatalyst was introduced, even under an excessive amount ofNaBH4. On the contrary, the linear correlation between ln(A/A0) and time observed while in the presence of the Ni/CPMcatalyst is shown in Figure S3D and implies that the reaction

invoked first-order kinetics.47 By comparing the results inFigure S3B and C, it is clear that the reaction kinetics arereadily depending on the amount of Ni/CPM catalystemployed. For reactions carried out with smaller amounts(1.0 mg) of Ni/CPM-1 catalyst, as shown in Figure S3B, theMB dye failed to reduce completely within 13 min. However, asthe catalyst loading was increased to 3.0 mg, a completereduction of MB was achieved within 8 min (Figure S3C).Moreover, although the intensity of the maximum absorbancepeak decreased consistently with time, no apparent shift in itswavelength (λmax = 665 nm) was observed, indicating thatoxidative degradation of MB did not occur during formation ofLMB.48

The catalytic reduction of MB has been extensively studiedby means of metal nanoparticles (MNPs) and supported MNPcomposites as catalysts and mostly employed NaBH4 as thereduction agent, as summarized in Table 2.49−59 Bycomparison, the Ni/CPM catalysts reported herein, whichpossess a Ni particle size of ∼6 nm, exhibited super MBreduction performance compared to other catalyst systemsreported in the literature. A complete reduction of MB may bereached within 13 min at a rate constant of ∼0.57−0.58 min−1

with a moderate catalyst loading of 2 mg. Likewise, our catalystsalso show excellent catalytic performance for reduction of RhB,surpassing most of the other catalyst systems as shown in TableS1. A complete reduction of RhB was achieved within 18 min ata rate constant of ∼0.47 min−1 over 2 mg of catalyst loading.Moreover, these novel Ni/CPM catalysts also have theadvantages of easy separation, being highly stable, and maybe applied in the absence of an irradiation source such as UV orvisible light. Additional recycling tests were performed to verifythe reusability of the Ni/CPM catalysts. As shown in FigureS4A, after six consecutive catalyst separation and reactioncycles, the rate constant (k) observed for MB reduction overthe Ni/CPM-1 catalyst decreased from 0.583 to 0.166.Likewise, for RhB reduction over the same catalyst, the kvalue declined from 0.471 to 0.143 over the six cyclic runs(Figure S4B). Even though the catalyst may be effectivelyseparated using a magnet, the gradual decrease in rate constantobserved over the repeated cycles is attributable to inevitableloss of catalyst during separation and washing treatments.Upon completion of reduction treatment, the Ni/CPM-1

catalyst was subjected to sonication for approximately 20−30min and then centrifuged. The reduced sample was extracted inDI water and washed thoroughly with ethanol, followed by

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

catalysta size of NP (nm) catalyst amount (mg) reaction time (min) rate constant k (min−1) ref

Fe3O4@polydopamine (PDA)-Ag NPs 25 5.0 22 0.430 49Pd-tetrahedral nanocrystals (TNCs)/RGO 9 5.0 [μL] 7 0.400 50copper nanocrystals (CuNCs) 28 0.1 [mL] 5 0.020 51Au@polypyrrole (PPy)/Fe3O4 400 2.0 42 0.266 52Fe3O4@Ag 52 1.6 6 0.410 53Ir NPs 200 ± 15 0.2 [mL] 24 0.041 54Ag/magnetic Fe3O4@C core−shell NCs 300 10.0 10 0.340 55silicon nanowire arrays/Cu NPs 25 1 × 1 cm2 10 0.317 56CoO nanowires 4−16 250 [μL] 81 0.038 57Cu microsphere 700 5.0 8 0.393 58Ni nanotube arrays >100 100.0 1 2.220 59Ni/CPM-1 6.0 ± 0.1 2.0 13 0.583 this workNi/CPM-2 6.0 ± 0.7 2.0 13 0.571 this work

aNPs: nanoparticles; RGO: reduced graphene oxide.



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drying at 60 °C in a vacuum oven. Subsequently, the degree ofMB reduction was checked by UV−vis spectroscopy. As shownin 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 riseto a characteristic absorption signal at 255 nm.60 The resultsclearly indicate that the Ni/CPM-1 material is indeed an activecatalyst for the efficient removal of organic dyes from thepollutant 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 MBblended in two polluted water samples, namely, industrialwastewater and lake water, obtained from a suburb ofmetropolitan Taipei. The tests were conducted under thesame reaction conditions used for the aqueous MB solution(see Figure 5A and D). Again, a linear correlation betweenln(A/A0) and reaction time (t) was observed (insets; FigureS6) in both cases, indicating that the presence of these realwater samples has little interference with the reductionperformance of the Ni/CPM-1 catalyst. The data summarizedin Table 2 and Tables S1 and S2 clearly show that the catalyticproperties of Ni/CPM nanocomposite materials outperformthe majority of other metal−carbon composite catalysts and arethus highly prospective for practical and efficient removal oforganic dyes from polluted water samples.3.4. Electrochemical Detection of Hg(II) Ions. Taking

advantage of the high surface area and surface roughnesspossessed by the Ni/CPM, which are favorable for dispersion ofactive sites, we fabricated Ni/CPM-modified glassy carbonelectrodes (GCEs) as electrochemical sensors for the detectionof Hg(II) ion. Their catalytic performances were assessed bycyclic voltammetry (CV) and differential pulse voltammetry(DPV). Displayed in Figure 7A are the CV curves of bare andNi/CPM-modified GCEs with and without the presence ofHg(II). All curves were recorded in an electrolyte of 0.05 Macetate buffer solution (pH 5). No redox peak was observed atNi/CPM-modified GCEs when in the absence of Hg(II)(curves b and d). Moreover, a featureless oxidation curve with aweak peak anodic current (Ipa; 0.14 μA) was observed for thebare GCE (curve e) even in the presence of 30 μM Hg(II) inthe electrolyte solution. Conversely, a notable redox peak atpeak oxidation potential (Epa) of 0.32 V was observed for theNi/CPM-1 (curve c)- and Ni/CPM-2 (curve a)-modifiedGCEs with an enhanced peak current of 1.5 and 10.5 μA,respectively, corresponding to a respective increase of 11- and75-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 inFigure 7B, a linear increase in Ipa with increasing scan rate wasobserved for the Ni/CPM-2-modified GCE (inset; Figure 7B),indicating a typical diffusion-controlled process61 for electro-chemical detection of Hg(II). Accordingly, a detectionsensitivity 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 blanksignal and S denotes sensitivity. By comparing the above resultswith other modified electrodes (Table 3), it is indicative thatthe Ni/CPM-modified GCEs reported herein indeed exhibitsuperior performances as a highly sensitive sensor forelectrochemical 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 dueto the possible occurrence of cross reaction, which may lead tothe formation of multimetallic compounds among differentmetal ions.62 To assess the cross reactivity and selectivity of theNi/CPM-modified electrodes towards the detection of heavymetal ions, we performed DPV studies on test electrolyte withvaried Hg(II) concentrations while in the presence of othermetal ions, namely, Cd(II), Pb(II), and Cu(II), which aresomewhat less toxic than the Hg(II) ion.Figure 8 shows the DPV responses of the Ni/CPM-2-

modified GCE in a test electrolyte containing various metalions while varying the concentration of Hg(II). It is evident thatthe modified GCE exhibited excellent selectivity for simulta-neous detection of metal ions. More importantly, a lineardependence of the observed peak oxidation current (Ipa) withHg(II) concentration (within 0−50 μM) was also observedeven in the presence of high concentrations of other metal ions

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) in0.05 M acetate buffer solution (pH 5.0). Curves (b) and (d) representprofiles recorded in the absence of Hg(II). CV curves recorded overthe Ni/CPM-2-modified GCE at different (B) scan rates (50−500 mVs−1); (inset) plot of anodic peak current Ipa vs square root of scan rateand (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 electrodeunder varied Hg(II) loading (1−74 μM); (inset) plot of oxidationpeak current (Ipa) vs Hg(II) concentration.

Table 3. Types of Modified Electrodes and TheirPerformances as Hg(II) Sensorsa

electrodedetectionlimit (nM)

sensitivity(μA μM−1 cm−2)

detectionmethod ref

SNAC 6.5 58.0 DPV 62SnO2/RGO 0.034 2.8 SWASV 63MgOnanosheets

15.3 DPV 69

G-DNA 5.0 DPV 70Ni/CPM-2(fish extract)

2.1 59.6 DPV this work

aSNAC, spherical nanoparticle-decorated activated carbon; RGO,reduced graphene oxide; G-DNA, graphene DNA; DPV, differentialpulse voltammetry; SWASV, square-wave anodic stripping voltamme-try.



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(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 performanceof the Ni/CPM-modified GCE for sensitive and selectivedetection of Hg(II) even in the presence of a highconcentration of foreign metal species. Interestingly, similarbehavior observed for oxidation peaks of the Hg(II) may alsobe inferred for the Cu(II) ion; a linear dependence between Ipaand [Hg(II)] was observed. It is likely that a thin film ofmercury was formed on the surface of the modified GCE priorto the subsequent formation of bimetallic compounds. Somereports are available on the utilization of a mercury film-modified electrode, for example, a dropping mercury electrode(DME) or hanging mercury drop electrode (HMDE), toenhance the detection sensitivity of foreign metals duringelectrochemical sensing.63−65 However, such a scheme is highlyrestricted due to the toxicity of mercury and environmentalconcerns. From Figure 8, it is indicative that the currentobserved for the Cu(II) oxidation peak tends to level off at ahigh dosage of Hg(II), in accordance with the above notion onthe formation and surface saturation of a Hg film. The sharpresponses (i.e., fast electron transfer)63 as well as the desirablepeak-to-peak separation (305 mV) therefore afford newpossibilities to exploit the Ni/CPM-modified GCE for selectiveand sensitive simultaneous detection of Hg(II) and Cu(II) ionsin real samples.3.6. Real Sample Tests. To evaluate and to demonstrate

the real time applications of the proposed Hg(II) sensor, wecollected and tested sea fish that were possibly contaminatedwith trace levels of mercury.66−70 The collected fish were driedand boiled at 100 °C for 2 h to obtain an oil-like liquid, whichwas further purified by centrifugation. First, to demonstrate thesensitivity of the Ni/CPM-modified GCE for real timedetection of Hg(II), real samples with different concentrationsof fish extract (pH ∼7.0) were prepared, and the testelectrolytes were adjusted to pH 5.0 using diluted H2SO4. Inthe absence of the fish extract, a featureless DPV curve wasobserved, as shown in Figure 9 (inset a; bottom curve).However, in the presence of the fish extract (0−500 μL), twobroad peaks (inset; Figure 9a), whose intensities both increasewith 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 fishextracts. Moreover, to afford a more accurate derivation of thedetection limit and sensitivity, test electrolytes doped with avaried amount of Hg(II) in fish extract were also prepared. Byvarying the amounts of fish extract and Hg(II) from 0−21 μM,their corresponding DPV curves revealed the anticipated sharpoxidation peak for Hg(II) together with a weaker shoulder peakaccountable for the bimetallic species. A linear correlationbetween the observed oxidation peak current (Ipa) with theconcentration of fish extract and Hg(II) was observed (inset;Figure 9b). Accordingly, an extraordinary sensitivity of 59.6 μAμM−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 labsample analysis (vide supra). Thus, it is conclusive that the Ni/CPM-modified GCEs, which show superior sensitivity and alow detection limit for the detection of Hg(II) ions, shouldrender practical applications as an efficient sensor forenvironmental pollution remediation in various Hg(II)-contaminated systems, even in the presence of other metal ions.

4. CONCLUSIONSIn conclusion, we have developed a facile route to synthesize NiNP-decorated carbon porous (Ni/CPM) materials via theevaporation-induced self-assembly (EISA) method undermicrowave-assisted heating. These novel Ni/CPM materialspossess high surface areas and mesoporosity desirable fordispersion of Ni NPs, rendering these novel materials asrecyclable heterogeneous catalysts for the reduction of organicdyes and electrochemical detection of heavy metal ions. Inparticular, the magnetically separable Ni/CPM catalysts werefound capable of reducing organic dyes, such as MB and RhB,with extraordinary reactivity in the presence of NaBH4 as thereducing agent. Typically, a complete reduction of MB and RhBcan be reached within 10−13 min at a rate constant of ∼0.6min−1 even with a catalyst loading of only 2 mg. Moreover, theNi/CPM-modified GCEs were found to exhibit excellentcatalytic activity, selectivity, sensitivity, and low detectionlimit for the detection of Hg(II) ions, even in real samplesand/or the simultaneous presence of other metal ions. Thus,these Ni/CPM materials should have potential applications asviable nanocatalysts for the reduction of organic dyes in

Figure 8. DPV curves obtained from the Ni/CPM-2-modified GCE inthe presence of varied Hg(II) concentrations (0−50 μM) togetherwith 50 μM of Cd(II), Pb(II), and Cu(II) ions in 0.05 M acetate buffersolution (pH 5.0); (inset) plot of peak oxidation current (Ipa) vsHg(II) concentration.

Figure 9. DPV curves obtained from the Ni/CPM-2-modified GCE inthe presence of varied loading of fish extract and Hg(II) (0−21 μM) in0.05 M acetate buffer solution (pH 5.0). Insets: (a) DPV curves in thepresence of fish extract (0, 100, 200, 300, 400, 500 μL) alone, (b) plotof corresponding oxidation peak current (Ipa) vs fish extract andHg(II) concentration, and photographs of (c) the collected fish and(d) the purified fish extract.



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

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.5b07900.

Assorted experimental results obtained from FT-IR,TGA, H2-TPR, and UV−vis studies (PDF)

■ AUTHOR INFORMATIONCorresponding 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] Contributions⊥P.V. and R.M. contributed equally.NotesThe authors declare no competing financial interest.

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

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nickel nanoparticle-decorated porous carbons for highly active catalytic reduction of organic dyes...

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