NiFe-Layered Double Hydroxide Nanosheet Arrays Supported on

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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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16536 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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ACS Applied Materials & Interfaces

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, Queensland 4222, Australia. ＆

School of Life Sciences, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, China.

ABSTRACT: Excessive uptake of nitrite has been proven to be detrimental to 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 onepot hydrothermal route. By integrating the collective merits of macroporous CC and NiFe-LDH NSAs such as superior electrical conductivity, striking synergistic effect between dual active components, enlarged electrochemically active surface area, unique three−dimensional (3D) hierarchical porous network characteristics, and fast charge transport and ion diffusion, the proposed NiFe-LDH NSAs/CC

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architectures can be served as the 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), ultra-low 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 the practical application. 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 spectrofluorimetry14,15 and electrochemical method16−20 have been established for determination of nitrite ions. Notwithstanding these advances, most of them usually require expensive equipments, tedious treatment procedures and high cost, greatly hindering their practical application. In contrast, electrochemical technique has received increasing attention due to their fascinating features


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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 (LDH), composed of alternate stacking of mixed metal hydroxide layers and intercalated anions, have evoked tremendous research interests due to 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 the free intercalation and transportation of ions between the interlayers, accordingly decreasing the mass transfer resistance and enhancing electrocatalytic performance. Particularly, NiFe-LDH can be considered as one of the ideal electroactive materials on account of 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 electrochemical property associated with the available active surface area, which can be elaborately modulated by engineering their structures. In this regard, twodimensional materials are of 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 transmission path of ions along 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



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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’s more, 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 3D macroporous skeleton can not only guarantee fully exposed electroactive sites but also facilitate efficient mass and charge transfer between CC and active materials as compared to 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 carbon cloth should be an excellent substrate candidate to anchor LDH nanosheets.29−32 Nevertheless, the conventional electrodes 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. Based on 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 as-fabricated 3D NiFe-LDH NSAs/CC architecture are


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systematically investigated by using various characterization techniques. As a self-standing and binderfree 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 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 self-standing electrode in practical application.

2. EXPERIMENTAL SECTION Preparation of NiFe-LDH NSAs/CC Composite. Prior to synthesis of NiFe-LDH NSAs/CC composite, a piece of a 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 was directly deposited onto the backbone of CC via the simple one-pot hydrothermal technique. Typically, for the preparation of 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 water (35 mL). Afterwards, the resultant mixture was decanted to an autoclave with Teflon lining. Then, a slice of acid-treated 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 for 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, 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 single metal ion precursor while keeping other



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conditions invariable, respectively. In addition, the NiFe-LDH/CC composite electrodes with diverse Ni/Fe mole ratios (5/1, 1/1, 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 NiFeLDH NSAs on CC substrate is schematically illustrated in Scheme 1. Herein, CC is chosen as the substrate for direct growth of NiFe–LDH nanosheets due to 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 guarantee the good anchoring of NiFe-LDH on the CC substrate. Then, the pretreated CC is immersed in a mixed solution containing nickel and iron nitrates with urea as the precipitant. After hydrothermal treatment at 120 oC, a yellow green film is noticed, indicating the formation of NiFe-LDH on the substrate. The possible reactions can be described as follows:33 －

NH4F + H2O → NH4+ + OH + HF

(1)

CO(NH2)2 + 3H2O → 2OH－+ 2NH4+ + CO2

(2)

Ni2+ + Fe3+ + OH－→ NiFe–LDH

(3)

The hydrothermal reactions consist of gradual hydrolysis of NH4F and urea (eqn (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 metal cations (Ni2+ and Fe3+) and generated OH－ leads to the formation of NiFe-LDH nanosheets onto CC surface (eqn (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 is investigated via FESEM. For the pretreated CC substrate, as displayed in Figure 1a, numerous quasiparallel aligned carbon fiber bundles are interdigitated together to form the highly porous 3D CC


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networks with irregular micrometer-scale pores between these carbon fibers. The high magnification 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 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 FE-SEM image (Figure 1c and d), we notice that the ultrathin sheet-like 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 NiFe-LDH nanosheets are interpenetrated and intertwined together to construct a 3D hierarchical porous architecture. 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, EDX elemental mapping analysis is further employed to reveal the spatial distribution of every element on carbon fibers. As displayed in Figure 1e and 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 CC substrate. In addition, no any other element is observed, confirming that NiFe-LDH NSAs/CC composite is highly pure. The Ni/Fe molar ratio of assynthesized sample is calculated based on ICP-AES 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 NiFe-LDH NSAs/CC film. Representative transmission electron microscopy (TEM) image (Figure 1g) confirms the platelet-like 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 well-crystallized characteristic of a single NiFe-LDH nanosheet, in which a clear hexagonal diffraction pattern is discernible. The corresponding



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high-resolution TEM image (Figure 1h) reveals that the spacing of the lattice fringes can be identified to 0.25 nm, matching well with (012) lattice face of 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; while the open space between 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 self-standing integrated electrode for the subsequent nitrite sensing. In addition, the evolution of morphology and size of the as-prepared NiFe-LDH/CC composites with varying Ni/Fe molar ratios (5/1, 1/1, 1/3) are 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 sheet-like morphology is also achieved on CC skeleton (denoted as Ni(OH)2/CC) (Figure S3a and b); whereas FeOOH/CC composite 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 the electrochemical sensing. The crystalline phase of the as-fabricated samples is studied via XRD analysis. The pristine CC shows (Curve I in Figure 2a) two peaks at 25.3o and 43.5o, 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 11.4o, 23.2o, 33.6o, 34.3o, 38.8o, 46.4o, 59.8o and 61.2o, 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


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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 stable NiFe-LDH architecture, which does not cause the alteration of crystal structure.24, 34 Meanwhile, the excessive cationic charge caused 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. In order to identify the functional groups and chemical bonding nature of the resulting NiFe-LDH NSAs/CC, 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 OH- stretching vibration. Meanwhile, the band at around 1636 cm-1 may be ascribed to 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 LDH lattice.35−37 The 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 carbon-based 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 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



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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 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 NiFe-LDH NSAs/CC compared to that of CC (1.142 vs. 1.113), suggesting 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, 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 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 spectrum (Figure 3c), the strong peaks at 874.1 and 856.3 eV correspond to Ni 2p1/2 and Ni 2p3/2, respectively; while the satellite peaks can be also 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 centred at 712.7 and 725.5 eV associated with Fe 2p3/2 and Fe 2p1/2 (Figure 3d); while the bonding energy at 718.7 and 732.7eV can be assigned to satellite peaks, suggesting the presence of Fe3+ in the NiFe-LDH NSAs/CC hybrid.24,25 The abovementioned results further confirming the successful preparation of NiFe-LDH NSAs on CC surface. 3.2. Electrochemical Activities. The electrochemical behaviors of different samples can be investigated via cyclic voltammetry (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


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electron transfer efficiency due to good conductivity of CC. Nevertheless, once FeOOH or αNi(OH)2 is deposited onto CC surface, an obvious decrease in the peak currents 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 electron-transfer resistance (Ret), in which 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, i.e. 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 three self-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 + 0.03 ν1/2 (R2 = 0.998). Accordingly, the electrochemical surface areas can be calculated according to the Randles-Sevcik equation:43,44 Ipa = (2.69 × 105) n3/2ACD1/2ν1/2

(4)

in which Ipa, A, n, D, C and ν represent anodic oxidation current (A), effective area of the working electrode, electron transport numbers, diffusion coefficient of K3[Fe(CN)6] in 0.1 M KCl (7.60 × 10−6 cm2 s-1), concentration of the probe molecules (mol cm−3), scanning rate (V s−1), respectively. In contrast, electrochemical active surface areas of Ni(OH)2/CC (Figure S6a and b)



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and FeOOH/CC (Figure S6c and 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), respectively, based on the above method (Figure S7). That is, less or more Ni/Fe than 3/1 in NiFe-LDH/CC hybrid electrodes will depress the electrochemical active surface areas. The excellent electrocatalytic activity of NiFe-LDH NSAs/CC with 3/1 of Ni/Fe ratio in terms of 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 NiFe-LDH NSAs/CC Electrode. The electrochemical responses of various electrodes towards nitrite are subsequently evaluated via CVs in PBS solution. It's worth noting that bare CC, the as-prepared NiFe-LDH NSAs/CC, Ni(OH)2/CC and FeOOH/CC composites can be all 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, NiFe-LDH NSAs/CC electrode possesses enhanced anodic peak current, highlighting the importance of NiFe-LDH architectures in improving electrocatalytic performance. Meanwhile, the resulting CV curves are electrochemically irreversible since no corresponding reduction peak is observed. In contrast, the bare CC electrode has almost no electrocatalytic capability towards nitrite (the inset in Figure 5a). Furthermore, the electrocatalytic


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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 electrocatalytic activity of the obtained electrodes, in which the NiFe-LDH NSAs/CC electrode with 3/1 of Ni/Fe ratio has the largest anodic peak current, which is consistent with the aforementioned largest electrochemical active surface area. The above results demonstrate that NiFe-LDH NSAs/CC electrode with 3/1 of Ni/Fe ratio displays the highest electrocatalytic capability towards nitrite among the investigated seven electrodes, which is thus selected for the representative in the following nitrite sensing. To investigate the possible application of 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 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 NiFe-LDH NSAs/CC electrode are also examined via CV method at diverse scanning rates. As displayed in Figure 5c, the oxidation peaks redshift with increasing scanning rates accompanied by the synergetic increase of peak current. Meanwhile, Ipa can be determined as: Ipa (mA) = 0.157 ν1/2 + 0.114 (R2 = 0.996) (Figure 5d), revealing that the electron transfer of 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: firstly, the complexing interaction occurs between NO2− and NiFe-LDH NSAs/CC to form [NiFe-LDH NSAs/CC (NO2−)] (eqn (5)); subsequently, [NiFe-LDH NSAs/CC (NO2−)] complex loses one electron to produce NO2 (eqn (6)); afterwards, NO2− and NO3− are generated via the disproportionation reaction of NO2 (eqn (7)); finally, NO3− is the possible product via electrocatalytic oxidation of NO2− (eqn (8)).41, 45 NiFe-LDH NSAs/CC + NO2− → [NiFe-LDH NSAs/CC (NO2−)]


(5)


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

(6)

2NO2 + H2O → NO3− + NO2− + 2H+

(7)

NO2− + H2O → 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 electrolyte solution at 0.90 V (Figure 6a). Noteworthily, 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 remarkable electrocatalytic capability of NiFe-LDH NSAs/CC and rapid electron transfer 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 + 0.8036 C (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 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


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of relatively poor conductivity; meanwhile, the separated carbon fibers can effectively prevent NiFeLDH NSAs from self-aggregation through an anchoring effect.30,31 Second, the resulting NiFe-LDH 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 analysis. Fourth, the direct growth of NiFe-LDH NSAs on CC substrate not only greatly simplify the electrode preparing procedures but also guarantees good contact between conductive substrate and 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 NiFeLDH NSAs on CC is able to ensure good structural stability of the resulting electrode, thus boosting the cycling stability. All 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 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



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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 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 the excellent anti-interference capability and thus can be employed to quantitatively determine nitrite in complicated environment system. The reproducibility of the proposed sensor for 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 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 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 Application. 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


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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 added by three different concentration of nitrite and then analyzed quantitatively by amperometry. It should be noted that every experiment is performed for 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 (range from 97.4 to 103.8) are also acceptable. The high reliability and satisfactory recoveries endow the resulting NiFe-LDH 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 self-supporting and binderfree electrode, the resulting NiFe-LDH NSAs/CC exhibit the 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 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 costeffectiveness, the developed NiFe-LDH NSAs/CC electrode will be expected to hold significant prospects for the effective detection of nitrite in practical complicated systems.



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ASSOCIATED CONTENTS Supporting Information. 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 NiFeLDH; 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.

AUTHOR INFORMATION Corresponding Author ∗Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This study was financially supported by National Natural Science Foundation of China (Grants 51572263, 51772299, 51472246), Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDA09030200), CAS/SAFEA International Partnership Program for Creative Research Teams of Chinese Academy of Sciences, Science and Technology Major Project of Anhui Province (15czz04125).

REFERENCES


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Scheme 1. Schematic illustration of the synthetic route of NiFe-LDH NSAs/CC composites.



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


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Figure 2. XRD pattern (a) and Raman spectra (b) of the bare carbon cloth and NiFe-LDH NSAs/CC with 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; (d) Fe 2p.



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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; (d) the linear relationship between the anodic peak currents and the square root of scan rate.


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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 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 concentration of nitrite; (c) CVs of NiFe-LDH NSAs/CC in 0.1 M PBS containing 1 mM nitrite at different scan rates; (d) the linear relationship between Ipa and the square root of scan rate.



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Figure 6. (a) Amperometric response of 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; (b) corresponding linear plot between the current response and 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)

References

Fe3O4/RGO

196

10−2882

0.1

17

Ag–P(MMA-coAMPS)

104.6

1.0–100000

0.01

18

MOF-GNRs-50

93.8

100–2500

0.015

19

CoPcFMWCNTs

29.9

0.096−340

0.14

41

Hb/Au/GACS

150

0.05−1000

0.1

46

CoL/MNSs

305.4

0.2−30

0.75

47

Fe2O3/RGO

204

0.05−780

0.23

48

α-Fe2O3 NRs

135.36

0.2−5000

0.4

49

NiFe-LDH NSAs

803.6

5−1000

0.02

This work


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

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

48.7

97.4

1.23

100

101.4

101.4

1.64

200

205.6

102.8

2.16

50

51.2

102.4

2.15

100

102.7

102.7

1.62

200

207.6

103.8

2.57



Table of Contents Graphic


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NiFe-Layered Double Hydroxide Nanosheet Arrays Supported on

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