NiFe-Layered Double Hydroxide Nanosheet Arrays Supported on

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 6541−6551

www.acsami.org

NiFe-Layered Double Hydroxide Nanosheet Arrays Supported on Carbon Cloth for Highly Sensitive Detection of Nitrite Yue Ma,†,‡ Yongchuang Wang,†,‡ Donghua Xie,†,‡ Yue Gu,†,‡ Haimin Zhang,† Guozhong Wang,† Yunxia Zhang,*,† Huijun Zhao,†,§ and Po Keung Wong∥ †

Key Laboratory of Materials Physics, Centre for Environmental and Energy Nanomaterials, Anhui Key Laboratory of Nanomaterials and Nanotechnology, CAS Centre for Excellence in Nanoscience, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China ‡ University of Science and Technology of China, Hefei 230026, P. R. China § Centre for Clean Environment and Energy, Griffith University, Gold Coast Campus, Nathan, Queensland 4222, Australia ∥ School of Life Sciences, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong 999077, China S Supporting Information *

ABSTRACT: Excessive uptake of nitrite has been proven to be detrimental to the ecological system and human health. Hence, there is a rising requirement for constructing effective electrochemical sensors to precisely monitor the level of nitrite. In this work, NiFe-layered double hydroxide nanosheet arrays (NiFe-LDH NSAs) have been successfully fabricated on a carbon cloth (CC) substrate via a facile one-pot hydrothermal route. By integrating the collective merits of macroporous CC and NiFe-LDH NSAs such as superior electrical conductivity, striking synergistic effect between the dual active components, enlarged electrochemically active surface area, unique three-dimensional hierarchical porous network characteristics, and fast charge transport and ion diffusion, the proposed NiFe-LDH NSAs/CC architecture can be served as a self-supporting sensor toward nitrite detection. As a consequence, the resulting NiFe-LDH NSAs/CC electrode demonstrates superior nitrite sensing characteristics, accompanied by broad linear range (5−1000 μM), quick response rate (ca. 3 s), ultralow detection limit (0.02 μM), and high sensitivity (803.6 μA·mM−1·cm−2). Meanwhile, the electrochemical sensor possesses timeless stability, good reproducibility, and strong anti-interference feature. Importantly, the resulting sensor can determine nitrite level in tap and lake water with high recoveries, suggesting its feasibility for practical applications. These findings show that the obtained NiFe-LDH NSAs/CC electrode holds great prospect in highly sensitive and specific detection of nitrite. KEYWORDS: NiFe-LDH NSAs/CC, self-supporting, nitrite, sensitive

1. INTRODUCTION Nitrite is an alarming pollutant to human beings and the whole ecological systems, which has been widely used as corrosion inhibitors, food antiseptics, and chemical manure. However, long-lasting and excessive accumulation of nitrite in human body will cause a disease commonly called “blue baby syndrome.” In addition, nitrite may also combine with amides/amines to produce harmful N-nitrosamine compounds, resulting in cancer and hypertension.1−3 Therefore, it is of vital importance to exploit a feasible means to detect nitrite from the viewpoint of environmental monitoring and human health concerns. As far as we know, a series of analytical techniques, such as spectrophotometry,4,5 chromatography,6,7 molecular absorption spectrometry,8,9 chemiluminescence,10,11 Raman spectrometry,12,13 spectrofluorimetry,14,15 and electrochemical method,16−20 have been established for the determination of nitrite ions. Notwithstanding these advances, most of them © 2018 American Chemical Society

usually require expensive equipments, tedious treatment procedures, and high cost, greatly hindering their practical applications. In contrast, electrochemical technique has received increasing attention because of its fascinating features such as simple handling, fast response, superior sensitivity, low limit of detection (LOD), and excellent selectivity. Up to now, considerable researchers devote themselves to searching the effective electrode materials with superior catalytic activity and good electrical conductivity for utilization in the environmental and energy fields. Among the developed electrode materials, layered double hydroxides (LDHs), composed of alternate stacking of mixed metal hydroxide layers and intercalated anions, have evoked tremendous Received: October 31, 2017 Accepted: January 30, 2018 Published: January 30, 2018 6541

DOI: 10.1021/acsami.7b16536 ACS Appl. Mater. Interfaces 2018, 10, 6541−6551

Research Article

ACS Applied Materials & Interfaces

as-fabricated 3D NiFe-LDH NSAs/CC architecture are systematically investigated by using various characterization techniques. As a self-standing and binder-free electrode, the designed NiFe-LDH NSAs/CC toward the determination of nitrite is investigated by electrochemical techniques including cyclic voltammetry (CV) and amperometry (i−t) in detail. As a consequence, the fabricated 3D NiFe-LDH NSAs/CC electrode has demonstrated high sensitivity, broad linear range, ultralow LOD, and preferable selectivity toward the determination of nitrite. Furthermore, the stability and reproducibility study as well as real water sample analysis are also carried out to evaluate the feasibility of the proposed selfstanding electrode in practical applications.

research interests because of their attractive features, such as tunable composition and facile exchangeability of intercalated anions together with easy accessibility of metal centers for the adsorption and activation of electroactive species.21−23 Significantly, the unique layered structure of LDH materials promotes free intercalation and transportation of ions between the interlayers, accordingly decreasing the mass-transfer resistance and enhancing the electrocatalytic performance. Particularly, NiFe-LDH can be considered as one of the ideal electroactive materials on account of its superb catalytic activity, even outperforming most of the known transition-metal catalysts for O2 evolution in electrolysis,24−26 accompanied by the additional features in terms of environmental benignity, low cost, and abundant resource. In addition to the composition of electroactive species mentioned above, the morphology of LDHs also plays a vital role in upgrading the electrochemical property associated with the available active surface area, which can be elaborately modulated by engineering their structures. In this regard, twodimensional materials are excellent candidates on account of very thin thickness on the nanosized scale, which can not only achieve full exposure of surface active sites but also sharply shorten the transmission path of ions along the thickness direction, endowing them with better catalytic performances compared with their bulk counterparts.27,28 Unfortunately, the fabrication of LDH nanosheet electrodes still remains a key challenge because they are prone to form irreversible agglomerates, resulting in decreased electroactive areas and limited electrocatalytic capability as a result of inaccessible internal surfaces of the resulting materials. In the meantime, the intrinsic relatively inferior electrical conductivity of LDHs adversely affects the electron-transfer kinetics, resulting in the decrease of electrocatalytic activity; what is more, the poor mechanical stability of powdered LDH nanosheets further restrains their widespread application in electrochemical field. To address these issues, one effective and feasible strategy is to use highly conductive carbon materials as the underlying backbone to support NiFe-LDH nanosheets, in which the three-dimensional (3D) macroporous skeleton can not only guarantee fully exposed electroactive sites but also facilitate efficient mass and charge transfer between CC and the active materials as compared to the conventional dense electrode films; meanwhile, the confinement effect of 3D carbon skeleton prevents the pulverization and aggregation of LDH nanosheets during electrochemical measurements. Thanks to several exceptional advantages including superior electrical conductivity, open macroporous structure, low cost, high thermal and mechanical stability, good corrosion resistance against strong acidic and alkaline media, as well as commercial availability, it is no doubt that CC should be an excellent substrate candidate to anchor LDH nanosheets.29−32 Nevertheless, the conventional electrode fabrication modes with the help of the polymer binder are unable to achieve the optimization of electrocatalytic performance because of the blocking of active sites and possible peeling risk of electroactive materials. Therefore, the welldefined and self-supported electrodes with abundant active sites, high catalytic activity, good electrical conductivity, unique porous architecture, strong mechanical adhesion, and extraordinary chemical stability deserve to be indefatigably pursued. On the basis of the aforementioned considerations, in this work, NiFe-LDH nanosheet arrays have been directly grown on 3D conductive CC using a simple hydrothermal synthetic strategy. The morphological and structural characteristics of the

2. EXPERIMENTAL SECTION 2.1. Preparation of the NiFe-LDH NSAs/CC Composite. Prior to the synthesis of the NiFe-LDH NSAs/CC composite, a piece of CC was washed under the strong ultrasonic treatment. Subsequently, the washed CC was further treated in acid solution for 2 h at 120 °C to improve its hydrophilicity. Finally, the resulting CC substrate was completely cleaned and baked at 60 °C overnight. NiFe-LDH NSAs were directly deposited onto the backbone of CC via a simple one-pot hydrothermal technique. Typically, for the preparation of the NiFe-LDH NSAs/CC composite with Ni/Fe = 3/1, Fe(NO3)3·9H2O (0.5 mmol), Ni(NO3)2·6H2O (1.5 mmol), CO(NH2)2 (10 mmol), and NH4F (5 mmol) were dispersed into deionized (DI) water (35 mL). Afterward, the resultant mixture was decanted to an autoclave with Teflon lining. Then, a slice of acidtreated CC with a dimension of 2 × 3 cm was introduced to the above mixed solution and treated at 120 °C for 8 h. Subsequently, the hybrid substrate was collected and rinsed using ethanol and DI water a couple of times and then dried at 60 °C. In our case, the Ni/Fe mole ratio in the resulting NiFe-LDH NSAs/CC composite is 3:1, unless indicated otherwise. By contrast, a Ni(OH)2/CC or FeOOH/CC composite electrode was fabricated via an analogous protocol using Ni(NO3)2·6H2O or Fe(NO3)3·9H2O as a single metal ion precursor while keeping other conditions invariable, respectively. In addition, the NiFe-LDH/CC composite electrodes with diverse Ni/Fe mole ratios (5/1, 1/1, and 1/ 3) were also fabricated via the analogous procedures.

3. RESULTS AND DISCUSSION 3.1. Morphology and Structural Analyses. The fabrication procedure for the hierarchical NiFe-LDH NSAs on the CC substrate is schematically illustrated in Scheme 1. Herein, the CC is chosen as the substrate for direct growth of NiFe-LDH nanosheets because of its superior electroconductivity and distinctive macroporous characteristics. First of all, bare CC is pretreated using HNO3 (2 M) to increase the number of hydrophilic functional groups and thus guarantees good anchoring of NiFe-LDH on the CC substrate. Then, the Scheme 1. Schematic Illustration of the Synthetic Route of NiFe-LDH NSAs/CC Composites

6542


Research Article


carbon fibers. The high-magnification scanning electron microscopy (SEM) image demonstrates that these carbon nanofibers have an average diameter of about 10 μm with a smooth surface (the inset in Figure 1a). After the hydrothermal treatment, the entire surface of carbon fibers is laden with a dense layer comprising many nanosheets (Figure 1b), indicating that the CC substrate is an ideal matrix to fabricate NiFe-LDH. Finally, the mass loading of NiFe-LDH NSAs on CC is found to be about 1.5 mg/cm2. From the enlarged FESEM image (Figure 1c,d), we notice that the ultrathin sheetlike subunits are homogeneously grafted onto the backbone of carbon fibers, in which the lateral sizes of the intersected nanosheets are 15−20 nm. Noticeably, these NiFeLDH nanosheets are interpenetrated and intertwined together to construct a 3D hierarchical porous architecture. Energydispersive spectrum (EDS) indicates the coexistence of C, O, Ni, and Fe elements in the resulting product (Figure S1), in which iron, nickel, and oxygen are attributed to NiFe-LDH, and carbon originated from CC. Meanwhile, energy-dispersive Xray elemental mapping analysis is further employed to reveal the spatial distribution of every element on carbon fibers. As displayed in Figure 1e,f, the Ni, Fe, and O elements are homogeneously distributed across a carbon nanofiber without any noticeable segregation, indicative of the successful grafting of the NiFe-LDH NSAs on the CC substrate. In addition, no any other element is observed, confirming that the NiFe-LDH NSAs/CC composite is highly pure. The Ni/Fe molar ratio of the as-synthesized sample is calculated based on the inductively coupled plasma-atomic emission spectroscopy measurement to be ∼3:1, consistent with the feeding Ni/Fe ratio in the precursors. To identify the elaborate structure of NiFe-LDH subunits, the nanosheets are peeled from the NiFe-LDH NSAs/ CC film. The representative transmission electron microscopy (TEM) image (Figure 1g) confirms the plateletlike feature of the as-synthesized NiFe-LDH, in which the nearly transparent morphology is indicative of the ultrathin nature, consistent with SEM observations. Meanwhile, discrete spots (the inset in Figure 1g) indicate the well-crystallized characteristic of a single NiFe-LDH nanosheet, in which a clear hexagonal diffraction pattern is discernible. The corresponding high-resolution TEM (HRTEM) image (Figure 1h) reveals that the spacing of the lattice fringes can be identified to be 0.25 nm, matching well with the (012) lattice face of the NiFe-LDH phase. This kind of ultrathin feature of NiFe-LDH nanosheets is expected to offer a great deal of active sites for electrochemical reaction, whereas the open space between the neighboring nanosheets is beneficial for electrolyte infiltration and ion diffusion inside electrodes, thus giving rise to superior electrochemical performance.27,28 Significantly, these hierarchical nanosheets still tightly anchor on the surface of CC scaffold even after strong sonication, enabling its direct utilization as a selfstanding integrated electrode for subsequent nitrite sensing. In addition, the evolution of the morphology and size of the as-prepared NiFe-LDH/CC composites with varying Ni/Fe molar ratios (5/1, 1/1, and 1/3) is also monitored via SEM observations. As shown in Figure S2, the average thickness of NiFe-LDH sheets increases with the decrease of molar ratios between Ni and Fe, which may induce the degradation of active surface areas on the NiFe-LDH/CC hybrid electrodes. Interestingly, when Ni(NO3)2·6H2O is utilized as the single metal ion precursor, a similar sheetlike morphology is also achieved on the CC skeleton (denoted as Ni(OH)2/CC) (Figure S3a and b), whereas the FeOOH/CC composite

pretreated CC is immersed in a mixed solution containing nickel and iron nitrates with urea as the precipitant. After the hydrothermal treatment at 120 °C, a yellow green film is noticed, indicating the formation of NiFe-LDH on the substrate. The possible reactions can be described as follows33 NH4F + H 2O → NH4 + + OH− + HF

(1)

CO(NH 2)2 + 3H 2O → 2OH− + 2NH4 + + CO2

(2)

Ni 2 + + Fe3 + + OH− → NiFe‐LDH

(3)

The hydrothermal reactions consist of gradual hydrolysis of NH4F and urea (eqs 1 and 2). As known, urea may be decomposed into ammonia and CO2 under hydrothermal condition, which is then transformed into CO32− and OH−. Subsequently, the interaction between the metal cations (Ni2+ and Fe3+) and the generated OH− leads to the formation of NiFe-LDH nanosheets onto the CC surface (eq 3), in which carbonate ions are intercalated into the interlayer structure to balance the charges. The surface morphology and microstructure of the synthesized NiFe-LDH NSAs/CC composite are investigated via field emission scanning electron microscopy (FESEM). For the pretreated CC substrate, as displayed in Figure 1a, numerous quasi-parallel aligned carbon fiber bundles are interdigitated together to form the highly porous 3D CC networks with irregular micrometer-scale pores between these

Figure 1. (a) SEM image of bare CC; (b−d) SEM images of NiFeLDH NSAs/CC at different magnifications; (e,f) EDS mapping; (g) TEM (the inset: SAED); and (h) HRTEM images of a single NiFeLDH nanosheet (Ni/Fe mole ratio = 3/1). 6543


Research Article


Figure 2. XRD pattern (a) and Raman spectra (b) of the bare carbon cloth and NiFe-LDH NSAs/CC with a Ni/Fe ratio of 3/1.

Figure 3. XPS spectra of the as-fabricated NiFe-LDH NSAs/CC composite: (a) survey spectrum, (b) C 1s, (c) Ni 2p, and (d) Fe 2p.

by Fe3+ can be balanced by the incorporation of anions in the interlayer, consistent with the reported NiFe-LDH.23 In addition, XRD patterns of the as-obtained products in the absence of Fe(NO3)3·6H2O or Ni(NO3)2·6H2O are demonstrated in Figure S4, confirming the successful fabrication of Ni(OH)2/CC and FeOOH/CC composites. To identify the functional groups and chemical bonding nature of the resulting NiFe-LDH NSAs/CC, Fourier transform infrared (FT-IR) measurement is performed and the corresponding spectrum is displayed in Figure S5. Obviously, a broad and intense absorption peak at about 3420 cm−1 correlates with the OH− stretching vibration. Meanwhile, the band at around 1636 cm−1 may be ascribed to the hydroxyl deformation mode of the absorbed water molecules. In addition to the characteristic bands related to hydroxyl groups, a sharp absorption peak at ca. 1356 cm−1 is also clearly observed, which is associated with the asymmetric stretching mode of CO32−, predominantly originated from the decomposition and transformation of urea. Moreover, two peaks at 786 and 526 cm−1 are attributed to the stretching and bending vibrations of M−O and M−OH bonds (M = Ni or Fe) from the LDH lattice, respectively.35−37 The

displays irregular and unordered aggregation morphology, totally different from Ni(OH)2/CC or NiFe-LDH NSAs/CC (Figure S3c and d), which is extremely adverse for electrochemical sensing. The crystalline phase of the as-fabricated samples is studied via X-ray diffraction (XRD) analysis. The pristine CC shows (curve I in Figure 2a) two peaks at 25.3° and 43.5°, which are attributed to (002) and (101) reflections from graphite-C (JCPDS no. 75-621), respectively. In the case of NiFe-LDH NSAs/CC, several new peaks appear at 11.4°, 23.2°, 33.6°, 34.3°, 38.8°, 46.4°, 59.8°, and 61.2°, corresponding to the diffraction of (003), (006), (101), (012), (015), (018), (110), and (113) planes of the Ni0.75Fe0.25(CO3)0.125(OH)2·0.38H2O phase, respectively (curve II in Figure 2a), confirming the coexistence of carbon and NiFe-LDH in the obtained composites. In addition, no other diffraction peaks from Ni or Fe oxides are observed, revealing high purity of the resultant product. As we all know, Ni2+ inside Ni(OH)2 lattices can be partially replaced by Fe3+ to form a stable NiFe-LDH architecture, which does not cause alteration of the crystal structure.24,34 Meanwhile, the excessive cationic charges caused 6544


Research Article


Figure 4. (a) CVs and (b) EIS spectra of different electrodes in the solution containing 5.0 mM [Fe(CN)6]3−/4− (1:1 molar ratio) and 0.1 M KCl; (c) CVs of NiFe-LDH NSAs/CC in the solution containing 5.0 mM [Fe(CN)6]3−/4− (1:1 molar ratio) and 0.1 M KCl at different scan rates; and (d) linear relationship between the anodic peak currents and the square root of the scan rate.

spectrum (Figure 3c), the strong peaks at 874.1 and 856.3 eV correspond to Ni 2p1/2 and Ni 2p3/2, respectively, whereas the satellite peaks can also be observed around 862.2 and 880.1 eV, indicating the presence of Ni(II). Similarly, the fitting analysis of Fe 2p exhibits two obvious peaks centered at 712.7 and 725.5 eV associated with Fe 2p3/2 and Fe 2p1/2, respectively (Figure 3d), whereas the bonding energies at 718.7 and 732.7 eV can be assigned to satellite peaks, suggesting the presence of Fe3+ in the NiFe-LDH NSAs/CC hybrid.24,25 The abovementioned results further confirm the successful preparation of NiFe-LDH NSAs on the CC surface. 3.2. Electrochemical Activities. The electrochemical behaviors of different samples can be investigated via CV (displayed in Figure 4a). As for CC, the largest peak currents are obtained in comparison with the other three electrodes, which indicate the fast electron-transfer efficiency due to good conductivity of CC. Nevertheless, once FeOOH or α-Ni(OH)2 is deposited onto the CC surface, an obvious decrease in the peak current is observed because of their poor electroconductivity, suggesting slow electron-transfer rate of the resulting electrodes. Compared with Ni(OH) 2 /CC or FeOOH/CC, peak current of NiFe-LDH NSAs/CC displays great enhancement, indicating faster electron-transfer efficiency. Electrochemical impedance spectroscopy experiments on different electrodes are carried out, and the corresponding Nyquist plots are displayed (Figure 4b). As we know, the semicircle part can be correlated with the electron-transfer resistance (Ret), in which the small semicircle is equal to low Ret and fast charge-transfer efficiency.18,19,39 From the inset in Figure 4b, it can be found that the bare CC electrode displays the smallest semicircle diameter, suggesting relatively low Ret. After the in situ growth of different active materials, the semicircle diameters increase at various levels, indicating higher charge-transfer resistance. However, among the three self-

above observation unambiguously reveals that carbonate species may be intercalated into the interlayer spaces of the resulting NiFe-LDH phase, which plays a key role in balancing redundant cationic charges caused by Fe3+. Raman spectra are utilized to explore intrinsic vibrational information in carbonbased materials (Figure 2b). Obviously, two intense peaks around 1585 and 1348 cm−1 can be attributed to G and D bands of CC, respectively, suggesting the existence of graphitized carbon. As for the NiFe-LDH NSAs/CC composite, two fresh bands appear around 550 and 460 cm−1, originated from Fe3+−O−Fe3+ and Fe3+/Ni2+−O−Ni2+ bonds, respectively, whereas the peak at 665 cm−1 can be ascribed to intercalated CO32−, revealing the successful incorporation of carbonate species into the interlayers of NiFe-LDH nanosheets.38 Moreover, it can be found that the D and G bands of the NiFe-LDH NSAs/CC composite show a blue shift (ca. 15 cm−1) compared with pristine CC, consistent with the observations in the previous reports. Specifically, there is no obvious change about intensity ratios (ID/IG) of NiFeLDH NSAs/CC compared to that of CC (1.142 vs 1.113), suggesting that the fabricated NiFe-LDH NSAs/CC composite maintains good electrical conductivity of bare CC.39−41 In an attempt to further investigate the chemical composition of the resulting product and valence state of each element, Xray photoelectron spectroscopy (XPS) characterizations are performed (Figure 3). From the survey spectrum of NiFe-LDH NSAs/CC (Figure 3a), four intense peaks at 855.7, 712.5, 529.5, and 285.1 eV are attributed to Ni 2p, Fe 2p, O 1s, and C 1s, respectively. It is noteworthy that the C 1s spectrum (Figure 3b) can be divided into four peaks associated with O−CO (289.1 eV), C−O (286.2 eV), C−OH (285.4 eV), and C−C (284.7 eV).42 Particularly, the presence of O−CO may be associated with the intercalated CO32− at the NiFe-LDH phase, consistent with FT-IR observations. In the case of Ni 2p 6545


Research Article


Figure 5. (a) CVs of the fabricated NiFe-LDH NSAs/CC, Ni(OH)2/CC, FeOOH/CC, and bare CC electrodes in the presence/absence of 1.0 mM NaNO2 in N2-saturated 0.1 M PBS at a scan rate of 50 mV s−1; (b) CVs of the NiFe-LDH NSAs/CC electrode in N2-saturated 0.1 M PBS containing 0.25−5.0 mM NaNO2 at a scan rate of 50 mV s−1; inset: the linear relationship between Ipa and the concentration of nitrite; (c) CVs of NiFe-LDH NSAs/CC in 0.1 M PBS containing 1 mM nitrite at different scan rates; and (d) linear relationship between Ipa and the square root of the scan rate.

high peak currents and reduced charge-transfer resistance as well as large electrochemical active surface areas prefigures its potential electrochemical detection toward nitrite. 3.3. Electrochemical Detection of Nitrite at the NiFeLDH NSAs/CC Electrode. The electrochemical responses of various electrodes toward nitrite are subsequently evaluated via CVs in the phosphate-buffered saline (PBS) solution. It is worth noting that bare CC, the as-prepared NiFe-LDH NSAs/ CC, Ni(OH)2/CC, and FeOOH/CC composites can all be directly utilized as the self-standing electrodes. As expected, no any electrochemical signal can be detected on the surface of the investigated electrodes without nitrite. Upon injection of nitrite, NiFe-LDH NSAs/CC, Ni(OH)2/CC, and FeOOH/CC electrodes display an obvious electrocatalytic behavior as reflected by the appearance of distinct anodic peaks at about 0.90 V (Figure 5a), indicating that the typical electrocatalytic oxidation of NO2− to NO3− has been achieved on the three electrodes. Compared with Ni(OH)2/CC or FeOOH/CC, the NiFe-LDH NSAs/CC electrode possesses enhanced anodic peak current, highlighting the importance of NiFe-LDH architectures in improving the electrocatalytic performance. Meanwhile, the resulting CV curves are electrochemically irreversible because no corresponding reduction peak is observed. In contrast, the bare CC electrode has almost no electrocatalytic capability toward nitrite (the inset in Figure 5a). Furthermore, the electrocatalytic activity of four NiFe-LDH/ CC electrodes with different Ni/Fe molar ratios is also explored. As illustrated in Figure S8, molar ratios of Ni/Fe have an obvious influence on the electrocatalytic activity of the obtained electrodes, in which the NiFe-LDH NSAs/CC electrode with the 3/1 ratio of Ni/Fe has the largest anodic peak current, which is consistent with the aforementioned

supporting electrodes loaded with various active materials, NiFe-LDH NSAs/CC presents the smallest Ret value, indicating the highest charge-transfer efficiency. The real electrochemical active surface areas of NiFe-LDH NSAs/CC (Figure 4c) are characterized by cyclic voltammetric at various scanning rates. Obviously, the anodic peak current (Ipa) increases with increasing the square root of scanning rate (Figure 4d). The linear relationship can be determined as Ipa (A) = −5.7 × 10−4 + 0.03ν1/2 (R2 = 0.998). Accordingly, the electrochemical surface areas can be calculated according to the Randles−Sevcik equation43,44 Ipa = (2.69 × 105)n3/2ACD1/2ν1/2

(4)

in which Ipa, A, n, D, C, and ν represent the anodic oxidation current (A), the effective area of the working electrode, the electron transport numbers, the diffusion coefficient of K3[Fe(CN)6] in 0.1 M KCl (7.60 × 10−6 cm2 s−1), the concentration of the probe molecules (mol cm−3), and the scanning rate (V s−1), respectively. In contrast, electrochemical active surface areas of Ni(OH)2/CC (Figure S6a,b) and FeOOH/CC (Figure S6c,d) electrodes are also investigated under the identical conditions. As a consequence, the electrochemical active surface areas for NiFe-LDH NSAs/CC, Ni(OH)2/CC, and FeOOH/CC electrodes are 8.0, 7.1, and 4.6 cm2, respectively. In addition, the real electrochemical active surface areas of NiFe-LDH/CC with different Ni/Fe ratios are also estimated to be 7.2 (Ni/Fe = 5/1), 6.1 (Ni/Fe = 1/1), and 5.0 cm2 (Ni/Fe = 1/3) based on the above method (Figure S7). That is, the Ni/Fe ratio less or more than 3/1 in the NiFeLDH/CC hybrid electrodes will depress the electrochemical active surface areas. The excellent electrocatalytic activity of NiFe-LDH NSAs/CC with the 3/1 ratio of Ni/Fe in terms of 6546


Research Article


Figure 6. (a) Amperometric response of the NiFe-LDH NSAs/CC electrode at an operating potential of 0.90 V (vs AgCl) with successive addition of NaNO2 into 0.1 M PBS under continuous stirring and (b) corresponding linear plot between the current response and the concentration of NaNO2.

Table 1. Comparison of the Fabricated NiFe-LDH NSAs/CC with Previously Reported Nitrite Sensors electrode materials

sensitivity (μA mM−1 cm−2)

linear range (μM)

LOD (μM)

Fe3O4/RGO Ag−P(MMA-co-AMPS) MOF-GNRs-50 CoPcF-MWCNTs Hb/Au/GACS CoL/MNSs Fe2O3/RGO α-Fe2O3 NRs NiFe-LDH NSAs

196 104.6 93.8 29.9 150 305.4 204 135.36 803.6

10−2882 1.0−100 000 100−2500 0.096−340 0.05−1000 0.2−30 0.05−780 0.2−5000 5−1000

0.1 0.01 0.015 0.14 0.1 0.75 0.23 0.4 0.02

largest electrochemical active surface area. The above results demonstrate that the NiFe-LDH NSAs/CC electrode with the 3/1 ratio of Ni/Fe displays the highest electrocatalytic capability toward nitrite among the investigated seven electrodes, which is thus selected for the representative in the following nitrite sensing. To investigate the possible application of the NiFe-LDH NSAs/CC electrode, the electrocatalytic oxidation of the resultant NiFe-LDH NSAs/CC electrode toward nitrite is first evaluated by CV curves. Figure 5b depicts electrocatalytic behaviors of the NiFe-LDH NSAs/CC electrode with varying nitrite concentration. Interestingly, with increasing nitrite concentration from 0.25−5.0 mM, the oxidation peak current (Ipa) increased linearly. The transport characteristics of the NiFe-LDH NSAs/CC electrode are also examined via a CV method at diverse scanning rates. As displayed in Figure 5c, the oxidation peaks red shift with increasing scanning rates accompanied by the synergetic increase of peak current. Meanwhile, Ipa can be determined as Ipa (mA) = 0.157ν1/2 + 0.114 (R2 = 0.996) (Figure 5d), revealing that the electron transfer of the NiFe-LDH NSAs/CC electrode is dominated by a diffusion-controlled electrochemical process. Therefore, the electrocatalytic oxidation mechanism of NiFe-LDH NSAs/CC toward nitrite may be summarized as follows: first, the complexing interaction occurs between NO2− and NiFe-LDH NSAs/CC to form [NiFe-LDH NSAs/CC (NO2−)] (eq 5); subsequently, the [NiFe-LDH NSAs/CC (NO2−)] complex loses one electron to produce NO2 (eq 6); afterward, NO2− and NO3− are generated via the disproportionation reaction of NO2 (eq 7); finally, NO3− is the possible product via electrocatalytic oxidation of NO2− (eq 8).41,45

references 17 18 19 41 46 47 48 49

this work

NiFe‐LDH NSAs/CC + NO2− → [NiFe‐LDH NSAs/CC(NO2−)]

(5)

[NiFe‐LDH NSAs/CC(NO2−)] → NiFe‐LDH NSAs/CC + NO2 + e−

(6)

2NO2 + H 2O → NO3− + NO2− + 2H+

(7)

NO2− + H 2O → 2H+ + NO3− + 2e−

(8)

To determine the sensitivity for the fabricated sensor, the typical chronoamperometric (i−t) response of the fabricated NiFe-LDH NSAs/CC electrode is further performed by adding successively nitrite into the electrolyte solution at 0.90 V (Figure 6a). Noteworthy, the electrolyte solution is needed to be constantly stirred throughout the experiments upon the addition of nitrite to guarantee uniform distribution of nitrite in the electrolytic cell. Significantly, the response reaches the maximum steady state within only 3 s, in which the ultrafast sensing response is associated with the remarkable electrocatalytic capability of NiFe-LDH NSAs/CC and rapid electrontransfer process. Particularly, the peak current slightly decreases with time to some extent, which may be resulted from the depletion of nitrite during the electrocatalytic oxidation process, similar to the previous reports.2,18 Additionally, the relationship between oxidation current and nitrite concentration (C) in the range of 0.005−1.0 mM may be described as Ipa = 73.69 + 0.8036C (Figure 6b, R2 = 0.999). As a result, the sensitivity and LOD of nitrite are calculated to be 803.6 μA mM−1 cm−2 and 0.02 μM, respectively, below the threshold value (65 μM) of nitrite recommended by the World Health Organization. Furthermore, the fabricated NiFe-LDH NSAs/CC electrode demonstrates low LOD, broad linear range, and superior 6547


Research Article

ACS Applied Materials & Interfaces sensitivity toward nitrite in comparison with the previous electrodes listed in Table 1, prefiguring the significant potential of the as-fabricated NiFe-LDH NSAs/CC sensor in the quantitative detection of nitrite. All in all, the developed NiFe-LDH NSAs/CC electrode demonstrates the outstanding sensing performance toward nitrite, and the possible reasons can be ascribed to the following collective effects. First, the conductive CC framework is beneficial for enhancing electrical conductivity and charge transfer of the proposed electrode and hence effectively overcomes the weakness of NiFe-LDH in terms of relatively poor conductivity; meanwhile, the separated carbon fibers can effectively prevent NiFe-LDH NSAs from self-aggregation through an anchoring effect.30,31 Second, the resulting NiFeLDH NSAs/CC hybrid electrode exhibits a 3D highly porous hierarchical architecture derived from the interconnected nanosheets, which offers convenient channels for electrolytes to the active sites and is thus responsible for high electrocatalytic activity. Third, the ultrathin feature of the obtained NiFe-LDH nanosheets not only provides abundant catalytically active sites and high electrochemically active surface area but also shortens the diffusion distance of the electrolyte species to the surface of the active components, thus accelerating the electrochemical kinetics.27,28,50 Simultaneously, as compared to single metal hydroxide-based electrodes (Ni(OH)2/CC or FeOOH/CC), the NiFe-LDH NSAs/CC hybrid electrode may possess inherent superiority associated with high peak currents and reduced charge-transfer resistance, as evidenced by CV and EIS analyses. Fourth, the direct growth of NiFe-LDH NSAs on the CC substrate not only greatly simplifies the electrodepreparing procedures but also guarantees good contact between the conductive substrate and the active NiFe-LDH nanosheet, facilitating fast charge transport during electrochemical reactions. Specifically, the integrated electrode effectively avoids the utilization of the additional polymer binders and conductive additives, which might increase the charge-transfer resistance and cause the loss of active sites to some extent.51,52 Moreover, the strong adhesion of NiFe-LDH NSAs on CC is able to ensure good structural stability of the resulting electrode, thus boosting the cycling stability. All of the aforementioned advantageous features will undoubtedly optimize the electrochemical performance of the resulting NiFe-LDH NSAs/CC electrode. 3.4. Specificity, Reproducibility, and Stability of the NiFe-LDH NSAs/CC Electrode. Taking into account the complicacy of the practical waters, besides low LOD and broad linear range, a promising sensor must possess excellent selectivity toward the examined species for the purpose of the reliable sensing data. To evaluate the specificity of the proposed NiFe-LDH NSAs/CC electrochemical sensor toward nitrite, the influence of common interfering species on nitrite sensing is further investigated by the amperometry test. Different species, including CH3COO−, NO3−, SO42−, CO32−, Cl−, K+, Zn2+, Ca2+, Mg2+, IO3−, BrO3−, H2O2, and glucose at a concentration of 50-fold as much as that of nitrite, are utilized for the possible interference test. As illustrated in Figure 7, no appreciable current alteration is found in the successive addition of these interfering ions despite very high concentration, whereas a sharp increase in the amperometric current can be immediately observed upon the further injection of 0.1 mM NO2−. To sum up, the proposed NiFe-LDH NSAs/CC electrochemical sensor possesses an excellent anti-interference

Figure 7. Amperometric response of the NiFe-LDH NSAs/CC electrode upon the successive addition of 0.1 mM nitrite and 5.0 mM interfering species.

capability and thus can be employed to quantitatively determine nitrite in the complicated environment system. The reproducibility of the proposed sensor for the detection of nitrite (1 mM) is also studied at six different electrodes by measuring the changes in the oxidation current (Figure S9). Strikingly, no visible variation about the intensity of oxidation current is found, and the relative standard deviation (RSD) of six parallel experiments is calculated to be 2.83%, indicating the superior reproducibility of the fabricated nitrite sensor due to the consistency of the prepared NiFe-LDH NSAs/CC electrodes. Meanwhile, the stability of the NiFe-LDH NSAs/ CC-based sensor is also examined by recording the change of the oxidation peak current under the conditions of long-term storage. Additionally, the NiFe-LDH NSAs/CC electrode remains 94.6% of its original oxidation current after being stored in air for 30 days (Figure S10), which indicates that the designed sensor possesses an excellent long-term durability. These findings imply that the NiFe-LDH NSAs/CC electrode possesses excellent reproducibility and relatively high stability, guaranteeing the sustainability and reliability of the proposed sensor during cycling operations. 3.5. Practical Applications. In view of remarkable sensing performance and anti-interference capability as well as excellent long-term durability of the proposed NiFe-LDH NSAs/CC electrode, we are encouraged to further explore its feasibility in determining nitrite levels in different water samples. In our case, Dongpu Reservoir (Hefei City, China) and tap water with complex background matrices are selected two representative samples. After the pretreatment through Millipore filters (0.22 μm), two samples mentioned above are spiked with three different concentrations of nitrite and then analyzed quantitatively by amperometry. It should be noted that every experiment is performed five times and mean concentrations accompanied by RSD are listed in Table 2. Interestingly, the concentration values determined based on our designed sensor are good consistent with the spiked concentrations; meanwhile, the obtained RSD values (less than 3%) and recoveries (ranging from 97.4 to 103.8%) are also acceptable. The high reliability and satisfactory recoveries endow the resulting NiFeLDH NSAs/CC electrode with the excellent feasibility toward nitrite sensing in complex environment.

4. CONCLUSIONS In summary, ultrathin NiFe-LDH NSAs have been directly grown on the conductive CC substrate via an effective one-pot hydrothermal route for the determination of nitrite. As a self6548


Research Article


Teams of Chinese Academy of Sciences, Science and Technology Major Project of Anhui Province (15czz04125).

Table 2. Determination of Nitrite in Tap Water and Dongpu Reservoir Water real samples tap water

Dongpu Reservoir water

nitrite spiked (μM)

nitrite found (μM)

recovery (%)

RSD (%, n = 5)

50 100 200 50

48.7 101.4 205.6 51.2

97.4 101.4 102.8 102.4

1.23 1.64 2.16 2.15

100 200

102.7 207.6

102.7 103.8

1.62 2.57

■

(1) Adarsh, N.; Shanmugasundaram, M.; Ramaiah, D. Efficient Reaction Based Colorimetric Probe for Sensitive Detection, Quantification, and on-site Analysis of Nitrite Ions in Natural Water Resources. Anal. Chem. 2013, 85, 10008−10012. (2) Haldorai, Y.; Hwang, S.-K.; Gopalan, A.-I.; Huh, Y. S.; Han, Y.-K.; Voit, W.; Sai-Anand, G.; Lee, K.-P. Direct Electrochemistry of Cytochrome c Immobilized on Titanium Nitride/Multi-walled Carbon Nanotube Composite for Amperometric Nitrite Biosensor. Biosens. Bioelectron. 2016, 79, 543−552. (3) Li, T.; Li, Y.; Zhang, Y.; Dong, C.; Shen, Z.; Wu, A. A Colorimetric Nitrite Detection System with Excellent Selectivity and High Sensitivity Based on Ag@Au Nanoparticles. Analyst 2015, 140, 1076−1081. (4) Filik, H.; Giray, D.; Ceylan, B.; Apak, R. A Novel Fiber Optic Spectrophotometric Determination of Nitrite Using Safranin O and Cloud Point Extraction. Talanta 2011, 85, 1818−1824. (5) Afkhami, A.; Bahram, M.; Gholami, S.; Zand, Z. Micell-mediated Extraction for the Spectrophotometric Determination of Nitrite in Water and Biological Sample Based on Its Reaction with P-nitroaniline in the Presence of Diphenylamine. Anal. Biochem. 2005, 336, 295− 299. (6) Lopez-Moreno, C.; Perez, I. V.; Urbano, A. M. Development and Validation of An Ionic Chromatography Method for the Determination of Nitrate, Nitrite and Chloride in Meat. Food Chem. 2016, 194, 687−694. (7) Croitoru, M. D. Nitrite and Nitrate Can Be Accurately Measured in Samples of Vegetal and Animal Origin Using an HPLC-UV/VIS Technique. J. Chromatogr. B: Anal. Technol. Biomed. Life Sci. 2012, 911, 154−161. (8) Brandao, G. C.; Lima, D. C.; Ferreira, S. L. C. The Chemical Generation of NO for the Determination of Nitrite by High-resolution Continuum Source Molecular Absorption Spectrometry. Talanta 2012, 98, 231−235. (9) Brandao, G. C.; Matos, G. D.; Pereira, R. N.; Ferreira, S. L. C. Development of a Simple Method for the Determination of Nitrite and Nitrate in Groundwater by High-resolution Continuum Source Electrothermal Molecular Absorption Spectrometry. Anal. Chim. Acta 2014, 806, 101−106. (10) Lin, Z.; Dou, X.; Li, H.; Ma, Y.; Lin, J.-M. Nitrite Sensing Based on the Carbon Dots-Enhanced Chemiluminescence from Peroxynitrous Acid and Carbonate. Talanta 2015, 132, 457−462. (11) Dong, S.; Guan, W.; Lu, C. Quantum Dots in Organo-modified Layered Double Hydroxide Framework-improved Peroxynitrous Acid Chemiluminescence for Nitrite Sensing. Sens. Actuators, B 2013, 188, 597−602. (12) Correa-Duarte, M. A.; Perez, N. P.; Guerrini, L.; Giannini, V.; Alvarez-Puebla, R. A. Boosting the Quantitative Inorganic SurfaceEnhanced Raman Scattering Sensing to the Limit: The Case of Nitrite/Nitrate Detection. J. Phys. Chem. Lett. 2015, 6, 868−874. (13) Han, X. X.; Schmidt, A. M.; Marten, G.; Fischer, A.; Weidinger, I. M.; Hildebrandt, P. Magnetic Silver Hybrid Nanoparticles for Surface-Enhanced Resonance Raman Spectroscopic Detection and Decontamination of Small Toxic Molecules. ACS Nano 2013, 7, 3212−3220. (14) Xu, H.; Zhu, H.; Sun, M.; Yu, H.; Li, H.; Ma, F.; Wang, S. Graphene Oxide Supported Gold Nanoclusters for the Sensitive and Selective Detection of Nitrite Ions. Analyst 2015, 140, 1678−1685. (15) Liao, F.; Song, X.; Yang, S.; Hu, C.; He, L.; Yan, S.; Ding, G. Photoinduced Electron Transfer of Poly(o-phenylenediamine)−Rhodamine B Copolymer Dots: Application in Ultrasensitive Detection of Nitrite in Vivo. J. Mater. Chem. A 2015, 3, 7568−7574. (16) Li, X.-R.; Kong, F.-Y.; Liu, J.; Liang, T.-M.; Xu, J.-J.; Chen, H.-Y. Synthesis of Potassium-Modified Graphene and Its Application in Nitrite-Selective Sensing. Adv. Funct. Mater. 2012, 22, 1981−1988.

supporting and binder-free electrode, the resulting NiFe-LDH NSAs/CC exhibits an outstanding electrochemical sensing performance toward nitrite in terms of low LOD, high sensitivity, broad linear range, superior selectivity, and exceptional timeless stability, which are associated with the integrated effects of the unique 3D porous architecture constructed from the interconnected ultrathin nanosheets, sufficient exposure and homogenous distribution of active sites, and highly conductive CC framework. Furthermore, the feasibility of the proposed amperometric nitrite sensor has been exemplified in actual environmental waters. Considering the appealing electrochemical performance together with easy fabrication and cost-effectiveness, the developed NiFe-LDH NSAs/CC electrode will be expected to hold significant prospects for the effective detection of nitrite in practical complicated systems.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b16536. Experimental section; EDS spectrum; SEM images of NiFe-LDH/CC with different Ni/Fe ratios, FeOOH/ CC, and Ni(OH)2/CC; XRD patterns of FeOOH/CC and Ni(OH)2/CC; FT-IR spectrum of NiFe-LDH; cyclic voltammograms of NiFe-LDH/CC with diverse Ni/Fe ratios; CVs of NiFe-LDH/CC electrodes with diverse Ni/Fe mole ratios; cyclic voltammograms of six different NiFe-LDH NSAs/CC electrodes; cyclic voltammograms of a NiFe-LDH NSAs/CC electrode before and after one month of storage (PDF)

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Haimin Zhang: 0000-0003-4443-6888 Guozhong Wang: 0000-0002-2900-3577 Yunxia Zhang: 0000-0002-5312-6411 Huijun Zhao: 0000-0003-3794-4497 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This study was financially supported by National Natural Science Foundation of China (grants 51572263, 51772299, and 51472246), Strategic Priority Research Program of the Chinese Academy of Sciences (grant XDA09030200), and CAS/SAFEA International Partnership Program for Creative Research 6549


Research Article


Double Hydroxides Intercalated with Polysulfides. Chem. Mater. 2014, 26, 7114−7123. (36) Wen, T.; Wu, X.; Tan, X.; Wang, X.; Xu, A. One-Pot Synthesis of Water-Swellable Mg-Al Layered Double Hydroxides and Graphene Oxide Nanocomposites for Efficient Removal of As(V) From Aqueous Solutions. ACS Appl. Mater. Interfaces 2013, 5, 3304−3311. (37) Dou, L.; Zhang, H. Facile Assembly of Nanosheet Array-Like CuMgAl-Layered Double Hydroxide/rGO Nanohybrids for Highly Efficient Reduction of 4-Nitrophenol. J. Mater. Chem. A 2016, 4, 18990−19002. (38) Tang, C.; Wang, H.-S.; Wang, H.-F.; Zhang, Q.; Tian, G.-L.; Nie, J.-Q.; Wei, F. Spatially Confined Hybridization of NanometerSized NiFe Hydroxides into Nitrogen-Doped Graphene Frameworks Leading to Superior Oxygen Evolution Reactivity. Adv. Mater. 2015, 27, 4516−4522. (39) Ma, Y.; Song, X.; Ge, X.; Zhang, H.; Wang, G.; Zhang, Y.; Zhao, H. In Situ Growth of α-Fe2O3 Nanorod Arrays on 3D Carbon Foam as an Efficient Binder-Free Electrode for Highly Sensitive and Specific Determination of Nitrite. J. Mater. Chem. A 2017, 5, 4726−4736. (40) Liu, J.; Ge, X.; Ye, X.; Wang, G.; Zhang, H.; Zhou, H.; Zhang, Y.; Zhao, H. 3D Graphene/δ-MnO2 Aerogels for Highly Efficient and Reversible Removal of Heavy Metal Ions. J. Mater. Chem. A 2016, 4, 1970−1979. (41) Li, P.; Ding, Y.; Wang, A.; Zhou, L.; Wei, S.; Zhou, Y.; Tang, Y.; Chen, Y.; Cai, C.; Lu, T. Self-Assembly of Tetrakis (3-trifluoromethylphenoxy) Phthalocyaninato Cobalt(II) on Multiwalled Carbon Nanotubes and Their Amperometric Sensing Application for Nitrite. ACS Appl. Mater. Interfaces 2013, 5, 2255−2260. (42) Hou, Y.; Lohe, M. R.; Zhang, J.; Liu, S.; Zhuang, X.; Feng, X. Vertically Oriented Cobalt Selenide/NiFe Layered-Double-Hydroxide Nanosheets Supported on Exfoliated Graphene Foil: An Efficient 3D Electrode for Overall Water Splitting. Energy Environ. Sci. 2016, 9, 478−483. (43) Yang, J.; Gunasekaran, S. Electrochemically Reduced Graphene Oxide Sheets for Use in High Performance Supercapacitors. Carbon 2013, 51, 36−44. (44) Silva, T. A.; Zanin, H.; Saito, E.; Medeiros, R. A.; Vicentini, F. C.; Corat, E. J.; Fatibello-Filho, O. Electrochemical Behaviour of Vertically Aligned Carbon Nanotubes and Graphene Oxide Nanocomposite as Electrode Material. Electrochim. Acta 2014, 119, 114− 119. (45) Fu, L.; Yu, S.; Thompson, L.; Yu, A. Development of A Novel Nitrite Electrochemical Sensor by Stepwise In Situ Formation of Palladium and Reduced Graphene Oxide Nanocomposites. RSC Adv. 2015, 5, 40111−40116. (46) Jiang, J.; Fan, W.; Du, X. Nitrite Electrochemical Biosensing Based on Coupled Graphene and Gold Nanoparticles. Biosens. Bioelectron. 2014, 51, 343−348. (47) Parsaei, M.; Asadi, Z.; Khodadoust, S. A Sensitive Electrochemical Sensor for Rapid and Selective Determination of Nitrite Ion in Water Samples Using Modified Carbon Paste Electrode with a Newly Synthesized Cobalt(II)-Schiff Base Complex and Magnetite Nanospheres. Sens. Actuators, B 2015, 220, 1131−1138. (48) Radhakrishnan, S.; Krishnamoorthy, K.; Sekar, C.; Wilson, J.; Kim, S. J. A Highly Sensitive Electrochemical Sensor for Nitrite Detection Based on Fe2O3 Nanoparticles Decorated Reduced Graphene Oxide Nanosheets. Appl. Catal., B 2014, 148−149, 22−28. (49) Liu, X.; Liu, J.; Chang, Z.; Luo, L.; Lei, X.; Sun, X. α-Fe2O3 Nanorod Arrays for Bioanalytical Applications: Nitrite and Hydrogen Peroxide Detection. RSC Adv. 2013, 3, 8489−8494. (50) Zhu, W.; Lu, Z. Y.; Zhang, G.; Lei, X.; Chang, Z.; Liu, J.; Sun, X. Hierarchical Ni0.25Co0.75(OH)2 Nanoarrays for a High-Performance Supercapacitor Electrode Prepared by an In Situ Conversion Process. J. Mater. Chem. A 2013, 1, 8327−8331. (51) Yang, Q.; Lu, Z.; Li, T.; Sun, X.; Liu, J. Hierarchical Construction of Core−shell Metal Oxide Nanoarrays with Ultrahigh Areal Capacitance. Nano Energy 2014, 7, 170−178. (52) Meng, G.; Yang, Q.; Wu, X.; Wan, P.; Li, Y.; Lei, X.; Sun, X.; Liu, J. Hierarchical Mesoporous NiO Nanoarrays with Ultrahigh

(17) Bharath, G.; Madhu, R.; Chen, S.-M.; Veeramani, V.; Mangalaraj, D.; Ponpandian, N. Solvent-Free Mechanochemical Synthesis of Graphene Oxide and Fe3O4−Reduced Graphene Oxide Nanocomposites for Sensitive Detection of Nitrite. J. Mater. Chem. A 2015, 3, 15529−15539. (18) Rastogi, P. K.; Ganesan, V.; Krishnamoorthi, S. A Promising Electrochemical Sensing Platform Based on a Silver Nanoparticles Decorated Copolymer for Sensitive Nitrite Determination. J. Mater. Chem. A 2014, 2, 933−943. (19) Kung, C.-W.; Li, Y.-S.; Lee, M.-H.; Wang, S.-Y.; Chiang, W.-H.; Ho, K.-C. In Situ Growth of Porphyrinic Metal−Organic Framework Nanocrystals on Graphene Nanoribbons for the Electrocatalytic Oxidation of Nitrite. J. Mater. Chem. A 2016, 4, 10673−10682. (20) Saraf, M.; Rajak, R.; Mobin, S. M. A Fascinating Multitasking Cu-MOF/rGO Hybrid for High Performance Supercapacitors and Highly Sensitive and Selective Electrochemical Nitrite Sensors. J. Mater. Chem. A 2016, 4, 16432−16445. (21) Li, C.; Wei, M.; Evans, D. G.; Duan, X. Layered Double Hydroxide-Based Nanomaterials as Highly Efficient Catalysts and Adsorbents. Small 2014, 10, 4469−4486. (22) Fan, G.; Li, F.; Evans, D. G.; Duan, X. Catalytic Applications of Layered Double Hydroxides: Recent Advances and Perspectives. Chem. Soc. Rev. 2014, 43, 7040−7066. (23) Wang, Q.; O’Hare, D. Recent Advances in the Synthesis and Application of Layered Double Hydroxide (LDH) Nanosheets. Chem. Rev. 2012, 112, 4124−4155. (24) Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.; Zhou, J.; Wang, J.; Regier, T.; Wei, F.; Dai, H. An Advanced Ni-Fe Layered DoubleHydroxid Electrocatalyst for Water Oxidation. J. Am. Chem. Soc. 2013, 135, 8452−8455. (25) Yu, L.; Zhou, H.; Sun, J.; Qin, F.; Yu, F.; Bao, J.; Yu, Y.; Chen, S.; Ren, Z. Cu nanowires Shelled with NiFe Layered Double Hydroxide Nanosheets as Bifunctional Electrocatalysts for Overall Water Splitting. Energy Environ. Sci. 2017, 10, 1820−1827. (26) Lu, X.; Zhao, C. Electrodeposition of Hierarchically Structured Three-Dimensional Nickel-Iron Electrodes for Efficient Oxygen Evolution at High Current Densities. Nat. Commun. 2015, 6, 6616. (27) Mendoza-Sanchez, B.; Gogotsi, Y. Synthesis of Two-Dimensional Materials for Capacitive Energy Storage. Adv. Mater. 2016, 28, 6104−6135. (28) Xu, M.; Liang, T.; Shi, M.; Chen, H. Graphene-Like TwoDimensional Materials. Chem. Rev. 2013, 113, 3766−3798. (29) Zheng, X.; Quan, H.; Li, X.; He, H.; Ye, Q.; Xu, X.; Wang, F. In Situ Fabrication of Ni-Co (oxy)hydroxide Nanowire-Supported Nanoflake Arrays and Their Application in Supercapacitors. Nanoscale 2016, 8, 17055−17063. (30) Tian, J.; Liu, Q.; Liang, Y.; Xing, Z.; Asiri, A. M.; Sun, X. FeP Nanoparticles Film Grown on Carbon Cloth: An Ultrahighly Active 3D Hydrogen Evolution Cathode in Both Acidic and Neutral Solutions. ACS Appl. Mater. Interfaces 2014, 6, 20579−20584. (31) Tian, J.; Liu, Q.; Asiri, A. M.; Sun, X. Self-Supported Nanoporous Cobalt Phosphide Nanowire Arrays: An Efficient 3D Hydrogen-Evolving Cathode Over the Wide Range of pH 0-14. J. Am. Chem. Soc. 2014, 136, 7587−7590. (32) Pu, Z.; Liu, Q.; Asiri, A. M.; Sun, X. Tungsten Phosphide Nanorod Arrays Directly Grown on Carbon Cloth: A Highly Efficient and Stable Hydrogen Evolution Cathode at All pH Values. ACS Appl. Mater. Interfaces 2014, 6, 21874−21879. (33) Ganiyu, S. O.; Le, T. X. H.; Bechelany, M.; Esposito, G.; van Hullebusch, E. D.; Oturan, M. A.; Cretin, M. A Hierarchical CoFeLayered Double Hydroxide Modified Carbon-Felt Cathode for Heterogeneous Electro-Fenton Process. J. Mater. Chem. A 2017, 5, 3655−3666. (34) Zhao, X.; Xu, S.; Wang, L.; Duan, X.; Zhang, F. Exchange-Biased NiFe2O4/NiO Nanocomposites Derived From NiFe-Layered Double Hydroxides as a Single Precursor. Nano Res. 2010, 3, 200−210. (35) Ma, S.; Islam, S. M.; Shim, Y.; Gu, Q.; Wang, P.; Li, H.; Sun, G.; Yang, X.; Kanatzidis, M. G. Highly Efficient Iodine Capture by Layered 6550


Research Article

ACS Applied Materials & Interfaces Capacitance for Aqueous Hybrid Supercapacitor. Nano Energy 2016, 30, 831−839.

6551


NiFe-Layered Double Hydroxide Nanosheet Arrays Supported on

Recommend Documents