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metal electrode and its application
Diamond & Related Materials 104 (2020) 107746
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Spark plasma sintering compaction of hybrid nanodiamond/carbon nanotubes/metal electrode and its application ⁎
T
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Shi Sua,b, ,1, Lei Hana,1, , Lili Gaoa,c, Qingying Rend, Meng Niea, Zhihong Zhua, Hao Wana, Hui Liua, Nan Wua a
Key Lab of MEMS of Ministry of Education, Southeast University, Nanjing 210096, PR China Southeast University-Monash University Joint Research Institute, Suzhou 215123, PR China c College of Electronic Science and Technology, Shenzhen University, Shenzhen 518060, PR China d College of Electronic and Optical Engineering & College of Microelectronics, Nanjing University of Posts and Telecommunications, Nanjing 210023, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanodiamond Spark plasma sintering Heavy metal detection Stripping voltammetry Electrochemistry
Nanodiamond has varies of mechanical, chemical and biomedical advantages, but its nano-size and discrete particle properties hinder its wide applications. In this work, we have fabricated a nanodiamond electrode by spark plasma sintering technology and applied it as a heavy metal detection sensor. The detonation nanodiamond was pressed by using spark plasma sintering compaction at 900 K under 0.2 GPa pressure. Then, its structure and composition were analyzed by many characteristic techniques, while the porous electrode having a density of 1.45 g/cm3 with hardness > 0.17 GPa. Comparing with traditional carbon-based electrodes, the spark plasma sintering nanodiamond electrode is inexpensive and environment friendly. The analytical performance was characterized and optimized to be 1.52–2.82 μA·μg·L−1 for vary types of heavy metal ions in acetate buffering solution. Ultimately, with stable, accurate and precise electrochemical spectroscopy, this nanodiamond sensor permits rapid detection of Cd2+, Ni2+, Mn2+, Co2+ as well as Pb2+, and could potentially be coupled with point-of-use portable measurement equipment in order to substitute the noble electrodes.
1. Introduction Recently, the nanoscale carbon family, includes fullerenes, carbon onions, graphene and nanodiamond (ND), etc., has attracted great attention during the past ten years. Ultra-dispersed ND powder with particle diameter ranging from 4 to 6 nm, which is synthesized by detonation 2,4,6-Trinitrotoleune (TNT) and RDX (Hexahydro-1,3,5trintro-1,3,5-triazine, Hexogen) in chamber, has attracted great interest in many fields. The ND powder exhibits excellent properties in hardness, Young's modulus, chemical inertness, thermal conductivity as well as electrical properties [1]. Therefore, it has been utilized in lubrication addictive, biological labelling, NV− center quantum dots and drug delivery, etc. in addition, the average price of ND has been reduced to ~$1 per gram based on latest commercial production and purification techniques [2]. However, the extremely tiny particles with discrete distribution has impeded its applications in macroscopic fields. Thus, a compact bulk of ND is preferably desirable for improving its further applications.
On the other hand, the quick detection of heavy metal trace in aqueous environment has been of great concerns in many fields such as chemistry, biology and environment [3]. Laboratory quantitative examination techniques on heavy metal ions include inductively coupled plasma (ICP), atomic/optical emission spectroscopy (AES/OES) and mass spectroscopy (MS), atomic absorption spectroscopy (AAS), or gas chromatography (GC) [4]. All these techniques require complicated equipment, which hindered its further applications in real-time detections. In order to overcome this shortage, differential pulse stripping voltammetry (DPSV) technique was developed to measure the heavy metal ions based on examining the electrochemical faradic current accumulated by working electrode. Thus, mercury-contained electrodes, noble metal electrodes, glassy carbon as well as carbon-based electrodes have been utilized as the working electrode [5]. However, the mercury film prohibits its wide applications for the secondary mercury pollution during the measurement and also for the disposal toxicity [6]. Besides, the diamond material has favorable chemical inertness, low background current noises, and also wide potential range
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Corresponding authors at: Key Lab of MEMS of Ministry of Education, School of Electronic Science and Engineering, Southeast University, Nanjing 210096, PR China. E-mail addresses: [email protected] (S. Su), [email protected] (L. Han). 1 The authors contribute equally to this article. https://doi.org/10.1016/j.diamond.2020.107746 Received 27 August 2019; Received in revised form 21 January 2020; Accepted 5 February 2020 Available online 05 February 2020 0925-9635/ © 2020 Elsevier B.V. All rights reserved.
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3. Characterization
on redox evolutions. But the cost of diamond electrode is unaffordable for its further application [7]. Some previous reported introduced the diamond-like carbon films for DPSV test, but it also involved extremely complicated deposition process and expensive synthesis equipment [8]. To overcome these shortages, we balanced the cost and electrochemistry performance and then utilized detonation ND as the starting material. This novel electrode platform has shown its potential to be a sensitive, selective and repeatable electrochemical sensor for determination of tracing heavy metals and also provide insights into the preparation of sensing hazardous metal ions.
The ND pellet sample was acquired by polishing and collected. Then, the powder was smashed and grinded in an agate container. Subsequently, the ND powder was dispersed in deionized water and dipped onto a Cu grid for high-resolution transmission electron microscopy (HRTEM) observation. The HRTEM analysis was introduced to verify the status of the SPSed ND electrode, as shown in Fig. 1g and h. During the SPS process, the continuous discharge between the ND particles instantaneously creates uncountable local plasma zones, which result in temperature increase and phase transitions. The phase transition is associated with the volume expansion contributed by both the inner pressure and the outer pressure from the SPS machine. With the applied pressure and temperature increases, the carbon atoms on the ND surface will be activated at a higher energy level. Consequently, the surface carbon atoms migrate towards to neighbor carbon onions structures. Meanwhile, the outer graphitic layers are becoming disordered and stack densely to aggregate together, as confirmed by HRTEM [12]. Amorphous materials including non-diamond species was also observed on the surface of the ND shells. In addition, a certain considerably large amount of disordered content was found in both asproduced NDs and SPSed NDs. It was possibly a partial sp3-sp2 transformation took place during the high-temperature high-pressure sintering process. It is shown that the MWCNTs remain their tubular structures together with the ND particles aggregated around. The select area electron diffraction (SAED) pattern in Fig. 1g inset figure confirmed the existence of cubic diamond structure in the SPSed NDs sample. The SAED (111), (220) and (311) rings agree well with the diffractions in bulk diamond with lattice spacing d(111) = 2.06 Å, d(220) = 1.26 Å and d(311) = 1.08 Å, respectively. Although the TEM images indicated that the presence of amorphous carbon, it is still obvious that the unconverted NDs in the matrix with recognizable crystal lattice (Fig. 1h, green outline highlighted). The distance between the lattice planes is 2.06 Å, which belongs to diamond (111) planes. From the SEM and TEM characterization, it can be estimated that many of the NDs content experienced a graphitization process, while the residual NDs remain their formats without conversion, which enable its potential of being utilized in electrochemical field. The addictive heavy metals enhance the matrix informality, but also increase its electrochemical conductivities and stabilities. The characterization techniques involved Raman spectra, Thermogravimetry analysis/Differential scanning calorimetry (TGA/ DSC), X-ray diffraction analysis (XRD), Fourier-transformation infrared spectra (FTIR), X-ray photoelectron spectrum (XPS) as well as nanoindentation measurement. The Raman spectra (Fig. 2a) show two significant peaks at 1317 cm−1 and 1582 cm−1, respectively. The peak located at 1317 cm−1 can be assigned as the diamond characteristic peak, which is derived from the carbon sp3 hybridization T2g vibration [13]. Due to the nano-size phonon confinement, the diamond characteristic peak will be broadened (FWHM > 30 cm−1) and down-shift (~1317 cm−1) comparing with bulk diamond [14]. The other wide shoulder located at 1582 cm−1 is assigned as G band of graphitization carbon. Fig. 2b recorded the thermogravimetric curve (TGA) and differential scanning calorimeter (DSC) data of SPSed ND electrode heating in Argon up to 1270 K. From the TGA results, the SPSed ND electrode remained stable at 473 K and started to oxidize weakly at 500 K. Afterwards, the sample oxidized significantly at 790 K and completely finished at 960 K, which is lower than the bulk diamond film (1070 K). It is obviously documented by DSC curve and an endothermic peak around 870 K corresponding to the initial oxidization process [15]. All the thermodynamic test proved that the SPSed ND electrode is really stable in open air, which ensured its electrochemical applications in complex situation. According to the X-ray Diffraction (XRD) spectra, the diamond Fd3m crystal structure was exhibited significantly and labeled as blue square in the figure. The diamond characteristic peaks of (111), (022) and (311) are located at d = 44°, 76°,
2. Fabrication In this work, we aimed to produce a bulk electrode of compacted ND by spark plasma sintering (SPS) technique. Detonation ND was preliminarily purified with 250 mL Aqua Regia solution at 333 K to remove the metal residuals. Subsequently, it was washed and merged in piranha solution for 30 min. After the deep-purified procedures, the ND powder was carefully washed several times with deionized water and dried at 393 K in oven. Approximately 4 g 4–6 nm deep-purified ND was purchased from Sigma Aldrich Ltd. with a purity > 97%, which had a specific surface area of 200–450 m2/g and a bulk density of 0.2–0.7 g/ mol. Then, the ND powder was thoroughly mixing with 0.4–1 g copper (purity > 98%, 10–25 μm diameter)/tungsten (purity > 99.99%, 10 μm diameter)/molybdenum powder (purity > 99.99%, 1–5 μm diameter) and multi-wall carbon nanotubes (MWCNTs, purity > 98%, 10 μm average length/12 nm average diameter) powder were all purchased from Sigma Aldrich Ltd. and grinded with a weight ratio of 1:1:1:5 in an agate mortar. Subsequently, the fully mixed powder was pressed into a graphite die in order to form a disk-shape electrode sample with 20 mm diameter and 5 mm thickness. The tiny metal elements, as well as MWCNTs will enhance the final mechanical properties of the SPSed ND electrode, but also serve as concrescence media between ND particles [9]. Take in consideration of both the conductivity and mechanical property, the concentration of ND in total weight was mediated from 90% to 98%. The SPS experiment was performed by Model-SPS 1050, Sumitomo Co. Ltd. system, with an axial pressure of 0.2 GPa in vacuum. The heating rate was set to 100 K/min to reach 900 K, while the typical holding time was 10 min. The applied current in SPS was approximate 1000 A with a voltage under 5 V (pulse duration: 12 ms, interval: 2 ms), meanwhile the real-time temperature was monitored by a pyrosis infrared detector. After the sintering process, the SPSed ND electrode was carefully polished and entirely cleaned, as shown in Fig. 1a. The scanning electron microscopy (SEM) images of SPSed-ND electrode show the morphology and contents in the SPSed-ND electrodes we synthesized, as illustrated in Fig. 1b, and c. After the SPS treatment, most of the ND has been graphitized on their surface significantly, while intertwined with MWCNTs as well as the other metal residual particles. The metal particles are spherical shape with average diameter around 60 nm. The ND without graphitization still exists and distributed on the surface of metal as well as amorphous carbons [10]. It is obvious that the MWCNTs serve as the reinforce matrix between NDs and metal particles, meanwhile the ND remains it original formation. In Fig. 1c, a detailed SEM image was purchased to find out the structure of sintered material. The inset electron dispersive spectrum (EDS, Fig. 1d) shows the carbon main content in together with the addictive metals, including tungsten, copper and molybdenum. A detailed element analysis will be introduced with XPS characterization afterwards. Fig. 1e, and f provide the hybrid structure of the as-prepared electrode. The MWCNTs served as the links between the NDs and other graphitized carbons (e.g. carbon onions, as described in Fig. 1g and h). The formation between NDs and carbon onions has detailed discussed and explained previously [11].
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Fig. 1. Microscope image showing the top-view morphology of SPS ND sensor: (a) as-prepared SPSed ND electrode sensor; (b) SPSed ND electrode sensor after 1000 EIS cyclic voltammetry cycles in buffer solution; (c) detailed SEM with higher magnification of aggregated NDs after sintering, together with (d) the corresponding EDS of NDs; (e), (f) SEM images of SPSed ND electrode; (g), (h) HRTEM images of SPSed ND, inset shows the SAED patterns of ND particles.
91°/2.06, 1.26, 1.08 Å, correspondingly. The main content of the SPSed ND was graphitization composition, which occupied the obvious peak at 26°, assigned as graphite (002) planes. The other peaks are assigned to the metal compositions, as labeled in Fig. 2c. The FTIR spectra peaks at 1632, 1382 and 1256 cm−1 correspond to the acyclic C]C, nitro‑carbon bonds NeC, and ethers bonds vibrations in Fig. 2d, respectively [16]. The elements in SPSed ND electrode were analyzed by XPS, and the survey spectra were shown in Fig. 2e. The main composition of the SPSed ND electrode was carbon, which took > 90.56% of the total elements in survey spectra. Other metal was examined through corelevel high resolution peaks, including tungsten, molybdenum, and copper, which were in according with the narrow scanning peaks detected in the sample, as shown in Fig. 2f. The main C1s peak can be deconvoluted at 284.6 eV(C]C), 285.8 eV(CeC, sp3), 287.3 eV(CeO), 288.7 eV(C]O, including hydroxyl and epoxy groups) and 290.7 eV (OeC]O, including carbonyl, carbolic acid as well as ester groups). The original C]C sp2 contents share a certain percentage in ND for the outer-shell non-diamond structures as well as the amorphous compositions in the residual products. After the SPS treatment, it is reasonable to predict that a certain part of CeC sp3 structures transferred to sp2 hybridizations. According to the results, the SPS process removed quite a certain percentage of functional groups. Especially for the nitrogen (seen from the XPS results), most of the nitrogen-contained functional groups has been removed (most of them might be nitro- or eCeN groups). Additionally, the surface carboxyl groups have been restrained and annealed by the SPS process. Meanwhile, what more important, is the transformation from sp3 to sp2 hybridization, and sintering them together. Finally, its mechanical properties were measured through nanoindentation, and the load-displacement curve was obtained. The hardness and elastic modulus were found to be 0.17 GPa and 4.1 GPa for the optimized procedure [17]. Table 1 lists the SPSed ND electrodes pressed by SPS techniques with different ND concentration. Other sintering temperature and pressure was attempted, but the results were unfavorable. For lower sintering temperature (800–900 K) and pressure (80–100 MPa), the SPSed ND electrodes were loose with more porous structures, which
resulted in a relative lower hardness for mechanical stability. If the sintering temperature is higher (1000−1200 K), the SPSed ND electrodes will shrink significantly together with a slightly weight loss. The reason behind the elimination is due to the pellet consolidation at high temperature, when the metals are softened and then facilitated the densifications. At the same time, a volume expansion associated with the phase transition of diamond particles to graphite onions also contributes to this phenomenon, which, however, offsets part of the former mechanism induced shrinkage [11]. Thus, the optimized condition for SPSed ND sintering was set to 900 K/0.2 GPa in our experiment. All the samples show elasto-plastic behavior, which is typical for porous structure. The porous structure of SPSed ND electrode was evaluated by nitrogen absorption-desorption analysis and the overall feature of the nitrogen isotherm was in agreement with previous deduction. It is abnormal to find out that the conductivity of ND pellet increased as the weight percentage increased. A reasonable explanation is that the sp2 contents after sintering process will rise up, together with the crosssection area of the pellet. Consequently, the electric conductivity will increase as well. The specific surface area of SPSed ND electrode was determined throughout BET test and its pore-size distribution was analyzed by classic non-local functional theory (NLDFT), implying that the structure has a relative narrow pore size in a range of 1–1.5 nm.
4. Electrochemistry and stripping voltammetry The SPSed ND electrode was assembled as working electrode in electrochemistry experiment with an active surface area of 1 cm2 (defined by O-ring seal). The working electrodes are critical for DPSV measurement for it will be selectively sensitive to different kinds of heavy metal ions. To realize the heavy metal detection, DPSV technique was applied for its remarkable low detection limits. It shares the advantages of parallel multi-elements detection channels with spectroscopic windows, portable and compact structure for instruments, as well as extremely inexpensive cost [18–20]. A standard three-electrodes electrochemistry system was introduced to electrochemical measurement, where a platinum plate was applied as 3
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Fig. 2. (a) Raman spectroscopy, the diamond characteristic peak was spotted at 1317 cm−1 for T2g vibration. The G peak located at 1582 cm−1, indicating the graphitization of the SPSed ND electrode; (b) TGA/DSC results; (c) XRD of SPSed ND electrode, all the corresponding diffraction angles have been labeled respectively; (d) FTIR spectra; (e) XPS survey spectra; insets: narrow element scanning of N1s, O1s, as well as C1s peaks; (f) load-displacement curves of nanoindentation measurement on SPSed ND electrodes: stiffness tests on different ND concentration, varies from 90% to 98%.
(Fig. 3, inset) and sealed with a rubber O-ring. Electrochemical impedance (1 Hz–10 MHz, 0.1 V) and cyclic voltammetry (scan rate = 50 mV/s, −1.00–+1.00 V) of the modified SPSed ND electrode was measured in a 0.1 M acetate buffering solution containing 5 mM
the counter electrode and an Ag/AgCl electrode as the reference electrode. The SPSed ND electrode was carefully polished and then ultrasonicated by deionized water for 30 min. Subsequently, the SPSed ND electrode was assembled in the custom Teflon working electrode holder
Table 1 Phase composition of the ND electrode produced by SPS pressing determined by the characterization techniques. Consolidation conditions
ND contents (wt%)
Apparent density (g·cm−3)
Specific surface area (m2·g−1)
Conductivity (mS)
Hardness/Young's modulus (GPa)
SPS SPS SPS SPS SPS
98 96 94 92 90
1.98 2.04 2.15 2.20 2.37
243 315 356 327 398
378 350 347 239 190
0.156/3.82 0.167/3.95 0.174/4.14 0.163/4.10 0.157/4.04
900 900 900 900 900
K/0.2 K/0.2 K/0.2 K/0.2 K/0.2
GPa GPa GPa GPa GPa
Bold number indicates the highest value compared with other attempts. 4
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Fig. 3. Left: Electrochemistry impedance spectra (EIS) of SPSed ND sensor, and the equivalent circuit extracted to fit the standard Warburg diffusion model; inset figure: Bode impedance spectra (red) and phase spectra (blue); right: cyclic voltammetry spectra of SPSed ND sensor; inset figure: assembled SPSed ND working electrode in EIS measurement. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
[Fe(CN)6]3−/4− [21]. The reduction and oxidation peaks were not detected in the window, which indicated that the SPSed ND electrode is stable for the heavy metal ions detection applications. The electrochemical properties were measured throughout impedance spectroscopy and cyclic voltammetry, as plotted in Fig. 3. This technique measures the impedance over a certain range of frequency and records the dissipation properties induced by dipole moment or ionic relaxation [22]. The Nyquist plot shows that the SPSed ND electrode has a semicircular region in high frequency together with a straight line in low frequency range. The equivalent circuit was extracted by the impedance spectroscopy that models the diffusion process to characterize its permittivity. The Warburg element and Constant Phase Element (CPE) in the model are expressed by
ZW = RW +
ZCPE =
time-controlled electrochemical deposition process as well as a positively-applied-potential square-wave stripping scanning. The parameters are listed as follows: Edep = 150 s, teq = 20 s, Ebegin = −1.2 V, Eend = 5 mV, Eamp = 25 mV, and f = 25 Hz. Before the next cycle, a 30 s clean step at −1.0 V was applied to remove the residuals, while the acetate buffer solution was stirred during the whole cleaning and electrodeposition steps [24].
5. Discussion Deducing from Fig. 5, the detection sensitivity and detection sensitivity of SPSed ND electrode calculated from Fig. 4 are 2.5–12.5 μg·L−1 and 2.82(Cd2+), 2.72(Mn2+), 2.28(Co2+), 1.52(Ni2+), 1.52(Pb2+) μA/ppm [20]. Meanwhile, the linear fitting exhibited that the SPSed ND electrode had an excellent linearity in the detection range. Following the optimized procedures, the stripping peak current response was replotted versus to a function of concentration of the heavy metals in Fig. 5. The precontraction parameters, including both deposition potential and deposition time, have been optimized in the measurement to meet the requirement of detecting heavy metal ions in aqueous solution. A linear current peaks height is dependence with the ion concentration as well as deposition time (120 s at −1.2 V vs. Ag/ AgCl), simultaneously. The electrochemical behavior of heavy metal ions varies with different electrolytes, such as HCl, H2SO4, HNO3, acetate (NaAc-HAc), and phosphate (Na2HPO4-NaH2PO4). Among these electrolytes, the stripping voltammetry of Pb2+ and Cd2+ ions are optimized because of the occurrence of well-defined peaks together with the highest peak current [25]. The SPSed ND electrode sensing performance demonstrates relative excellent linearity in measuring range. By using DPSV technique, the electrode exhibits an excellent electrical conductivity for a fraction of C sp2 hybridization forming with metal additives. The obtained results prove that kinetics of absorption varies with heavy metal ions and involve extra intermediate diffusions. Future exploration of higher EIS electron transfer performance needs to be investigated, and meanwhile many types of surface modifications, such as hydrogen-termination or other modification, are desired. The minor shifts of the stripping peak (< 0.1 V) of Mn2+ and Ni2+ suggested the influence on both the working and reference electrodes [19]. This EIS performance demonstrates that the repeatability is comparable with mercury electrode and
1 iW CW
1 − π ni e 2 Q0 ω n
The Warburg impedance is associated with a charge-transfer complex resistance together with a double-layer capacitance. The CPE is connected with the electrode roughness or porous phenomenon, which is caused by double layer capacitance [23]. Thus, the results was fitted into a general electrochemical circuit cell (Randles Model), as shown in Fig. 3. Thus, we have performed a series of DPSV test in aqueous environment, and selected several metal ions including Cb2+, Mn2+, Co2+, Ni2+, and Pb2+ to evaluate its EIS response. For each 0.1 M sodium acetate (NaAc) electrolyte, it contains 0.01 M cadmium chloride, nickel chloride, cobaltous chloride, manganese chloride and 1 mM lead acetate (all purchased from Sigma Aldrich Ltd.), was applied as the measured targets. The baseline subtraction approach ensures accurate measurement on each heavy metal ion with different curves [18]. DPSV was performed in a conventional three-electrodes cell with Ag/AgCl reference electrode as well as Pt counter electrode. The measurement was carried out with submicromolar ion concentrations by holding the potential at −1.2 V (vs. Ag/AgCl). This situation is linked with the hydroxide formation during the process in the basic media. As the pH value in the solution is lower than 3.0, the influence on the selective stripping sensitivity is not as pronounced as the basic buffering solution [22]. The sensing principle for the heavy metal ions includes a 5
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Fig. 4. DPSV response curves for different heavy metals ions dissolved in 0.1 M NaAc buffer solution. The lowest DPSV curves are baseline signals, while the upper ones are measured with the heavy metal ions added (+50 nM each measurement), respectively. The spectra were optimized with Savitzky-Golay smoothing to reduce the background noise.
sintering lead to an increase in total sp3 composition, as well as hardness and Young's modulus. This SPSed ND electrode can be also utilized as a mercury-free heavy metal detector with favorable commercial advantages. The unique EIS response of SPSed ND heavy metal sensor shows an extremely stability and background noise current, which endows it a more promising selective in DPSV analysis. This suggested process could be utilized as the first step for manufacturing ND/composite device and future applications of this electrode might involve a promising and preferable online monitoring in both civil and industrial fields.
glassy carbon electrodes. To improve the detection sensitivity, the SPSed ND electrode need to be polished and its thickness should be reduced to enhance its conductivity. Meanwhile, rotate disk electrode or rotating ring-disk electrode techniques should be introduced during the EIS measurement. A comparison of carbon-based electrode for heavy metal ions detection was summarized in Fig. 6 [22–32]. The reproducibility of SPSed ND electrode for heavy metal ions detection was verified > 10 time with independently optimized experiment setup. The relative standard deviations (RSD) of peaks heights for > 10 stripping cycles of Cd2+, Mn2+, Co2+, Ni2+ and Pb2+ are measured as 3.6%, 5.5%, 3.3%, 6.3% and 4.6%, respectively. Such reasonable error was confined under a relative low level and implicated that the ions might not easily escaped from the electrode due to the high cationexchange capacity and strong absorption of SPSed ND electrode.
7. Experimental The HRTEM images were collected by FEI TITAN G2, operated under an accelerate voltage of 300 kV. The SEM images were taken by FEI Apreo with a field emission gun of 1.3 nm resolution at 1 kV. Argon absorption was carried out using Quantachrome Autosorb and the BET test was analyzed by NETZSCH STA449F3. The Raman spectra were performed with Horiba Jobin Yvon, LabRAM, with a He-Cd excitation laser wavelength of 325 nm. Its thermal performance was characterized in vacuum environment by TGA/DSC technique with TGA 5500 from TA instrument Ltd. The FTIR spectra were purchased from Nicolet 6700 and the stiffness of the SPSec ND electrode was measured by a
6. Conclusion The main focus of this study is to demonstrate the possibility of fabricating ND compaction bulks with SPS technology. We have produced a ND electrode by using SPS technology at 900 K with a compaction pressure of 0.2 GPa. The sintering process graphitized the surface species of ND but maintained the core sp3 content, which provided a hardness over 0.17 GPa. The isothermal heating and plasma
Fig. 5. DPSV current response plotted versus heavy metal ions concentration in linear relationship. Unit: Sens = sensitivity (μA/ppm). 6
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Fig. 6. Comparison of carbon-based electrodes in heavy metal ions detection [22–32]. The price, detection range and sensitivity are selected and listed in the figure. Due to the differences of the detection target heavy metal ions, typical sensitivities and detection ranges are concluded to enlarge the zone in the figure.
Acknowledgements
nanoindentation (Hysitron, TI-750L). The XPS spectra were recorded by Thermo Scientific ESCALAB Xi+ with monochromatography Al X-ray source. The X-ray diffraction signals were acquired by Rigaku D/ max2500V with Cu Kα lines from 5 to 100°. All the electrochemistry impedance, cyclic voltammetry, and anodic stripping voltammetry experiment was conducted by Autolab electrochemistry station PGSTAT302N. The nitrogen adsorption/desorption isotherms and Brunauer-Emmett-Teller (BET) surface areas were determined by an Autosorb-IQ-MP gas sorption analyzer.
The research was supported by the National Natural Science Foundation of China (Grant No: 11327901, 11525415, 51420105003 and 61274114) and Natural Science Foundation of Jiangsu Province (Grant No. BK20160702). The author would like to thank Shiyanjia Lab for the assistance and supports of the characteristic experiment. The authors are grateful to Prof. Xiaodong Huang, Mr. Fei Zhang and Ms. Xinyi He for the sincere assistance in experiment. References
Prime novelty and statement
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This research provided a feasible and cutting-edge technique to fabricate a hybrid nanodiamond/metal/CNTs electrode for heavy metal ion detection. Ultimately, with stable, accurate and precise electrochemical spectroscopy, this ND sensor permits rapid detection of heavy metal ions, which could be introduced in real-time electrochemical online detector in both civil and industrial pollution monitoring. CRediT authorship contribution statement Shi Su: Conceptualization, Writing - review & editing, Supervision, Project administration. Lei Han: Funding acquisition, Resources. Lili Gao: Software. Qingying Ren: Methodology. Meng Nie: Investigation. Zhihong Zhu: Investigation, Validation, Formal analysis, Writing original draft, Visualization. Hao Wan: Investigation, Validation, Formal analysis, Writing - original draft, Visualization. Hui Liu: Investigation, Validation, Formal analysis, Writing - original draft, Visualization. Nan Wu: Investigation, Validation, Formal analysis, Writing - original draft, Visualization. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 7
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Spark plasma sintering compaction of hybrid nanodiamond/carbon nanotubes/metal electrode and its application ⁎
T
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Shi Sua,b, ,1, Lei Hana,1, , Lili Gaoa,c, Qingying Rend, Meng Niea, Zhihong Zhua, Hao Wana, Hui Liua, Nan Wua a
Key Lab of MEMS of Ministry of Education, Southeast University, Nanjing 210096, PR China Southeast University-Monash University Joint Research Institute, Suzhou 215123, PR China c College of Electronic Science and Technology, Shenzhen University, Shenzhen 518060, PR China d College of Electronic and Optical Engineering & College of Microelectronics, Nanjing University of Posts and Telecommunications, Nanjing 210023, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanodiamond Spark plasma sintering Heavy metal detection Stripping voltammetry Electrochemistry
Nanodiamond has varies of mechanical, chemical and biomedical advantages, but its nano-size and discrete particle properties hinder its wide applications. In this work, we have fabricated a nanodiamond electrode by spark plasma sintering technology and applied it as a heavy metal detection sensor. The detonation nanodiamond was pressed by using spark plasma sintering compaction at 900 K under 0.2 GPa pressure. Then, its structure and composition were analyzed by many characteristic techniques, while the porous electrode having a density of 1.45 g/cm3 with hardness > 0.17 GPa. Comparing with traditional carbon-based electrodes, the spark plasma sintering nanodiamond electrode is inexpensive and environment friendly. The analytical performance was characterized and optimized to be 1.52–2.82 μA·μg·L−1 for vary types of heavy metal ions in acetate buffering solution. Ultimately, with stable, accurate and precise electrochemical spectroscopy, this nanodiamond sensor permits rapid detection of Cd2+, Ni2+, Mn2+, Co2+ as well as Pb2+, and could potentially be coupled with point-of-use portable measurement equipment in order to substitute the noble electrodes.
1. Introduction Recently, the nanoscale carbon family, includes fullerenes, carbon onions, graphene and nanodiamond (ND), etc., has attracted great attention during the past ten years. Ultra-dispersed ND powder with particle diameter ranging from 4 to 6 nm, which is synthesized by detonation 2,4,6-Trinitrotoleune (TNT) and RDX (Hexahydro-1,3,5trintro-1,3,5-triazine, Hexogen) in chamber, has attracted great interest in many fields. The ND powder exhibits excellent properties in hardness, Young's modulus, chemical inertness, thermal conductivity as well as electrical properties [1]. Therefore, it has been utilized in lubrication addictive, biological labelling, NV− center quantum dots and drug delivery, etc. in addition, the average price of ND has been reduced to ~$1 per gram based on latest commercial production and purification techniques [2]. However, the extremely tiny particles with discrete distribution has impeded its applications in macroscopic fields. Thus, a compact bulk of ND is preferably desirable for improving its further applications.
On the other hand, the quick detection of heavy metal trace in aqueous environment has been of great concerns in many fields such as chemistry, biology and environment [3]. Laboratory quantitative examination techniques on heavy metal ions include inductively coupled plasma (ICP), atomic/optical emission spectroscopy (AES/OES) and mass spectroscopy (MS), atomic absorption spectroscopy (AAS), or gas chromatography (GC) [4]. All these techniques require complicated equipment, which hindered its further applications in real-time detections. In order to overcome this shortage, differential pulse stripping voltammetry (DPSV) technique was developed to measure the heavy metal ions based on examining the electrochemical faradic current accumulated by working electrode. Thus, mercury-contained electrodes, noble metal electrodes, glassy carbon as well as carbon-based electrodes have been utilized as the working electrode [5]. However, the mercury film prohibits its wide applications for the secondary mercury pollution during the measurement and also for the disposal toxicity [6]. Besides, the diamond material has favorable chemical inertness, low background current noises, and also wide potential range
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Corresponding authors at: Key Lab of MEMS of Ministry of Education, School of Electronic Science and Engineering, Southeast University, Nanjing 210096, PR China. E-mail addresses: [email protected] (S. Su), [email protected] (L. Han). 1 The authors contribute equally to this article. https://doi.org/10.1016/j.diamond.2020.107746 Received 27 August 2019; Received in revised form 21 January 2020; Accepted 5 February 2020 Available online 05 February 2020 0925-9635/ © 2020 Elsevier B.V. All rights reserved.
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3. Characterization
on redox evolutions. But the cost of diamond electrode is unaffordable for its further application [7]. Some previous reported introduced the diamond-like carbon films for DPSV test, but it also involved extremely complicated deposition process and expensive synthesis equipment [8]. To overcome these shortages, we balanced the cost and electrochemistry performance and then utilized detonation ND as the starting material. This novel electrode platform has shown its potential to be a sensitive, selective and repeatable electrochemical sensor for determination of tracing heavy metals and also provide insights into the preparation of sensing hazardous metal ions.
The ND pellet sample was acquired by polishing and collected. Then, the powder was smashed and grinded in an agate container. Subsequently, the ND powder was dispersed in deionized water and dipped onto a Cu grid for high-resolution transmission electron microscopy (HRTEM) observation. The HRTEM analysis was introduced to verify the status of the SPSed ND electrode, as shown in Fig. 1g and h. During the SPS process, the continuous discharge between the ND particles instantaneously creates uncountable local plasma zones, which result in temperature increase and phase transitions. The phase transition is associated with the volume expansion contributed by both the inner pressure and the outer pressure from the SPS machine. With the applied pressure and temperature increases, the carbon atoms on the ND surface will be activated at a higher energy level. Consequently, the surface carbon atoms migrate towards to neighbor carbon onions structures. Meanwhile, the outer graphitic layers are becoming disordered and stack densely to aggregate together, as confirmed by HRTEM [12]. Amorphous materials including non-diamond species was also observed on the surface of the ND shells. In addition, a certain considerably large amount of disordered content was found in both asproduced NDs and SPSed NDs. It was possibly a partial sp3-sp2 transformation took place during the high-temperature high-pressure sintering process. It is shown that the MWCNTs remain their tubular structures together with the ND particles aggregated around. The select area electron diffraction (SAED) pattern in Fig. 1g inset figure confirmed the existence of cubic diamond structure in the SPSed NDs sample. The SAED (111), (220) and (311) rings agree well with the diffractions in bulk diamond with lattice spacing d(111) = 2.06 Å, d(220) = 1.26 Å and d(311) = 1.08 Å, respectively. Although the TEM images indicated that the presence of amorphous carbon, it is still obvious that the unconverted NDs in the matrix with recognizable crystal lattice (Fig. 1h, green outline highlighted). The distance between the lattice planes is 2.06 Å, which belongs to diamond (111) planes. From the SEM and TEM characterization, it can be estimated that many of the NDs content experienced a graphitization process, while the residual NDs remain their formats without conversion, which enable its potential of being utilized in electrochemical field. The addictive heavy metals enhance the matrix informality, but also increase its electrochemical conductivities and stabilities. The characterization techniques involved Raman spectra, Thermogravimetry analysis/Differential scanning calorimetry (TGA/ DSC), X-ray diffraction analysis (XRD), Fourier-transformation infrared spectra (FTIR), X-ray photoelectron spectrum (XPS) as well as nanoindentation measurement. The Raman spectra (Fig. 2a) show two significant peaks at 1317 cm−1 and 1582 cm−1, respectively. The peak located at 1317 cm−1 can be assigned as the diamond characteristic peak, which is derived from the carbon sp3 hybridization T2g vibration [13]. Due to the nano-size phonon confinement, the diamond characteristic peak will be broadened (FWHM > 30 cm−1) and down-shift (~1317 cm−1) comparing with bulk diamond [14]. The other wide shoulder located at 1582 cm−1 is assigned as G band of graphitization carbon. Fig. 2b recorded the thermogravimetric curve (TGA) and differential scanning calorimeter (DSC) data of SPSed ND electrode heating in Argon up to 1270 K. From the TGA results, the SPSed ND electrode remained stable at 473 K and started to oxidize weakly at 500 K. Afterwards, the sample oxidized significantly at 790 K and completely finished at 960 K, which is lower than the bulk diamond film (1070 K). It is obviously documented by DSC curve and an endothermic peak around 870 K corresponding to the initial oxidization process [15]. All the thermodynamic test proved that the SPSed ND electrode is really stable in open air, which ensured its electrochemical applications in complex situation. According to the X-ray Diffraction (XRD) spectra, the diamond Fd3m crystal structure was exhibited significantly and labeled as blue square in the figure. The diamond characteristic peaks of (111), (022) and (311) are located at d = 44°, 76°,
2. Fabrication In this work, we aimed to produce a bulk electrode of compacted ND by spark plasma sintering (SPS) technique. Detonation ND was preliminarily purified with 250 mL Aqua Regia solution at 333 K to remove the metal residuals. Subsequently, it was washed and merged in piranha solution for 30 min. After the deep-purified procedures, the ND powder was carefully washed several times with deionized water and dried at 393 K in oven. Approximately 4 g 4–6 nm deep-purified ND was purchased from Sigma Aldrich Ltd. with a purity > 97%, which had a specific surface area of 200–450 m2/g and a bulk density of 0.2–0.7 g/ mol. Then, the ND powder was thoroughly mixing with 0.4–1 g copper (purity > 98%, 10–25 μm diameter)/tungsten (purity > 99.99%, 10 μm diameter)/molybdenum powder (purity > 99.99%, 1–5 μm diameter) and multi-wall carbon nanotubes (MWCNTs, purity > 98%, 10 μm average length/12 nm average diameter) powder were all purchased from Sigma Aldrich Ltd. and grinded with a weight ratio of 1:1:1:5 in an agate mortar. Subsequently, the fully mixed powder was pressed into a graphite die in order to form a disk-shape electrode sample with 20 mm diameter and 5 mm thickness. The tiny metal elements, as well as MWCNTs will enhance the final mechanical properties of the SPSed ND electrode, but also serve as concrescence media between ND particles [9]. Take in consideration of both the conductivity and mechanical property, the concentration of ND in total weight was mediated from 90% to 98%. The SPS experiment was performed by Model-SPS 1050, Sumitomo Co. Ltd. system, with an axial pressure of 0.2 GPa in vacuum. The heating rate was set to 100 K/min to reach 900 K, while the typical holding time was 10 min. The applied current in SPS was approximate 1000 A with a voltage under 5 V (pulse duration: 12 ms, interval: 2 ms), meanwhile the real-time temperature was monitored by a pyrosis infrared detector. After the sintering process, the SPSed ND electrode was carefully polished and entirely cleaned, as shown in Fig. 1a. The scanning electron microscopy (SEM) images of SPSed-ND electrode show the morphology and contents in the SPSed-ND electrodes we synthesized, as illustrated in Fig. 1b, and c. After the SPS treatment, most of the ND has been graphitized on their surface significantly, while intertwined with MWCNTs as well as the other metal residual particles. The metal particles are spherical shape with average diameter around 60 nm. The ND without graphitization still exists and distributed on the surface of metal as well as amorphous carbons [10]. It is obvious that the MWCNTs serve as the reinforce matrix between NDs and metal particles, meanwhile the ND remains it original formation. In Fig. 1c, a detailed SEM image was purchased to find out the structure of sintered material. The inset electron dispersive spectrum (EDS, Fig. 1d) shows the carbon main content in together with the addictive metals, including tungsten, copper and molybdenum. A detailed element analysis will be introduced with XPS characterization afterwards. Fig. 1e, and f provide the hybrid structure of the as-prepared electrode. The MWCNTs served as the links between the NDs and other graphitized carbons (e.g. carbon onions, as described in Fig. 1g and h). The formation between NDs and carbon onions has detailed discussed and explained previously [11].
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Fig. 1. Microscope image showing the top-view morphology of SPS ND sensor: (a) as-prepared SPSed ND electrode sensor; (b) SPSed ND electrode sensor after 1000 EIS cyclic voltammetry cycles in buffer solution; (c) detailed SEM with higher magnification of aggregated NDs after sintering, together with (d) the corresponding EDS of NDs; (e), (f) SEM images of SPSed ND electrode; (g), (h) HRTEM images of SPSed ND, inset shows the SAED patterns of ND particles.
91°/2.06, 1.26, 1.08 Å, correspondingly. The main content of the SPSed ND was graphitization composition, which occupied the obvious peak at 26°, assigned as graphite (002) planes. The other peaks are assigned to the metal compositions, as labeled in Fig. 2c. The FTIR spectra peaks at 1632, 1382 and 1256 cm−1 correspond to the acyclic C]C, nitro‑carbon bonds NeC, and ethers bonds vibrations in Fig. 2d, respectively [16]. The elements in SPSed ND electrode were analyzed by XPS, and the survey spectra were shown in Fig. 2e. The main composition of the SPSed ND electrode was carbon, which took > 90.56% of the total elements in survey spectra. Other metal was examined through corelevel high resolution peaks, including tungsten, molybdenum, and copper, which were in according with the narrow scanning peaks detected in the sample, as shown in Fig. 2f. The main C1s peak can be deconvoluted at 284.6 eV(C]C), 285.8 eV(CeC, sp3), 287.3 eV(CeO), 288.7 eV(C]O, including hydroxyl and epoxy groups) and 290.7 eV (OeC]O, including carbonyl, carbolic acid as well as ester groups). The original C]C sp2 contents share a certain percentage in ND for the outer-shell non-diamond structures as well as the amorphous compositions in the residual products. After the SPS treatment, it is reasonable to predict that a certain part of CeC sp3 structures transferred to sp2 hybridizations. According to the results, the SPS process removed quite a certain percentage of functional groups. Especially for the nitrogen (seen from the XPS results), most of the nitrogen-contained functional groups has been removed (most of them might be nitro- or eCeN groups). Additionally, the surface carboxyl groups have been restrained and annealed by the SPS process. Meanwhile, what more important, is the transformation from sp3 to sp2 hybridization, and sintering them together. Finally, its mechanical properties were measured through nanoindentation, and the load-displacement curve was obtained. The hardness and elastic modulus were found to be 0.17 GPa and 4.1 GPa for the optimized procedure [17]. Table 1 lists the SPSed ND electrodes pressed by SPS techniques with different ND concentration. Other sintering temperature and pressure was attempted, but the results were unfavorable. For lower sintering temperature (800–900 K) and pressure (80–100 MPa), the SPSed ND electrodes were loose with more porous structures, which
resulted in a relative lower hardness for mechanical stability. If the sintering temperature is higher (1000−1200 K), the SPSed ND electrodes will shrink significantly together with a slightly weight loss. The reason behind the elimination is due to the pellet consolidation at high temperature, when the metals are softened and then facilitated the densifications. At the same time, a volume expansion associated with the phase transition of diamond particles to graphite onions also contributes to this phenomenon, which, however, offsets part of the former mechanism induced shrinkage [11]. Thus, the optimized condition for SPSed ND sintering was set to 900 K/0.2 GPa in our experiment. All the samples show elasto-plastic behavior, which is typical for porous structure. The porous structure of SPSed ND electrode was evaluated by nitrogen absorption-desorption analysis and the overall feature of the nitrogen isotherm was in agreement with previous deduction. It is abnormal to find out that the conductivity of ND pellet increased as the weight percentage increased. A reasonable explanation is that the sp2 contents after sintering process will rise up, together with the crosssection area of the pellet. Consequently, the electric conductivity will increase as well. The specific surface area of SPSed ND electrode was determined throughout BET test and its pore-size distribution was analyzed by classic non-local functional theory (NLDFT), implying that the structure has a relative narrow pore size in a range of 1–1.5 nm.
4. Electrochemistry and stripping voltammetry The SPSed ND electrode was assembled as working electrode in electrochemistry experiment with an active surface area of 1 cm2 (defined by O-ring seal). The working electrodes are critical for DPSV measurement for it will be selectively sensitive to different kinds of heavy metal ions. To realize the heavy metal detection, DPSV technique was applied for its remarkable low detection limits. It shares the advantages of parallel multi-elements detection channels with spectroscopic windows, portable and compact structure for instruments, as well as extremely inexpensive cost [18–20]. A standard three-electrodes electrochemistry system was introduced to electrochemical measurement, where a platinum plate was applied as 3
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Fig. 2. (a) Raman spectroscopy, the diamond characteristic peak was spotted at 1317 cm−1 for T2g vibration. The G peak located at 1582 cm−1, indicating the graphitization of the SPSed ND electrode; (b) TGA/DSC results; (c) XRD of SPSed ND electrode, all the corresponding diffraction angles have been labeled respectively; (d) FTIR spectra; (e) XPS survey spectra; insets: narrow element scanning of N1s, O1s, as well as C1s peaks; (f) load-displacement curves of nanoindentation measurement on SPSed ND electrodes: stiffness tests on different ND concentration, varies from 90% to 98%.
(Fig. 3, inset) and sealed with a rubber O-ring. Electrochemical impedance (1 Hz–10 MHz, 0.1 V) and cyclic voltammetry (scan rate = 50 mV/s, −1.00–+1.00 V) of the modified SPSed ND electrode was measured in a 0.1 M acetate buffering solution containing 5 mM
the counter electrode and an Ag/AgCl electrode as the reference electrode. The SPSed ND electrode was carefully polished and then ultrasonicated by deionized water for 30 min. Subsequently, the SPSed ND electrode was assembled in the custom Teflon working electrode holder
Table 1 Phase composition of the ND electrode produced by SPS pressing determined by the characterization techniques. Consolidation conditions
ND contents (wt%)
Apparent density (g·cm−3)
Specific surface area (m2·g−1)
Conductivity (mS)
Hardness/Young's modulus (GPa)
SPS SPS SPS SPS SPS
98 96 94 92 90
1.98 2.04 2.15 2.20 2.37
243 315 356 327 398
378 350 347 239 190
0.156/3.82 0.167/3.95 0.174/4.14 0.163/4.10 0.157/4.04
900 900 900 900 900
K/0.2 K/0.2 K/0.2 K/0.2 K/0.2
GPa GPa GPa GPa GPa
Bold number indicates the highest value compared with other attempts. 4
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Fig. 3. Left: Electrochemistry impedance spectra (EIS) of SPSed ND sensor, and the equivalent circuit extracted to fit the standard Warburg diffusion model; inset figure: Bode impedance spectra (red) and phase spectra (blue); right: cyclic voltammetry spectra of SPSed ND sensor; inset figure: assembled SPSed ND working electrode in EIS measurement. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
[Fe(CN)6]3−/4− [21]. The reduction and oxidation peaks were not detected in the window, which indicated that the SPSed ND electrode is stable for the heavy metal ions detection applications. The electrochemical properties were measured throughout impedance spectroscopy and cyclic voltammetry, as plotted in Fig. 3. This technique measures the impedance over a certain range of frequency and records the dissipation properties induced by dipole moment or ionic relaxation [22]. The Nyquist plot shows that the SPSed ND electrode has a semicircular region in high frequency together with a straight line in low frequency range. The equivalent circuit was extracted by the impedance spectroscopy that models the diffusion process to characterize its permittivity. The Warburg element and Constant Phase Element (CPE) in the model are expressed by
ZW = RW +
ZCPE =
time-controlled electrochemical deposition process as well as a positively-applied-potential square-wave stripping scanning. The parameters are listed as follows: Edep = 150 s, teq = 20 s, Ebegin = −1.2 V, Eend = 5 mV, Eamp = 25 mV, and f = 25 Hz. Before the next cycle, a 30 s clean step at −1.0 V was applied to remove the residuals, while the acetate buffer solution was stirred during the whole cleaning and electrodeposition steps [24].
5. Discussion Deducing from Fig. 5, the detection sensitivity and detection sensitivity of SPSed ND electrode calculated from Fig. 4 are 2.5–12.5 μg·L−1 and 2.82(Cd2+), 2.72(Mn2+), 2.28(Co2+), 1.52(Ni2+), 1.52(Pb2+) μA/ppm [20]. Meanwhile, the linear fitting exhibited that the SPSed ND electrode had an excellent linearity in the detection range. Following the optimized procedures, the stripping peak current response was replotted versus to a function of concentration of the heavy metals in Fig. 5. The precontraction parameters, including both deposition potential and deposition time, have been optimized in the measurement to meet the requirement of detecting heavy metal ions in aqueous solution. A linear current peaks height is dependence with the ion concentration as well as deposition time (120 s at −1.2 V vs. Ag/ AgCl), simultaneously. The electrochemical behavior of heavy metal ions varies with different electrolytes, such as HCl, H2SO4, HNO3, acetate (NaAc-HAc), and phosphate (Na2HPO4-NaH2PO4). Among these electrolytes, the stripping voltammetry of Pb2+ and Cd2+ ions are optimized because of the occurrence of well-defined peaks together with the highest peak current [25]. The SPSed ND electrode sensing performance demonstrates relative excellent linearity in measuring range. By using DPSV technique, the electrode exhibits an excellent electrical conductivity for a fraction of C sp2 hybridization forming with metal additives. The obtained results prove that kinetics of absorption varies with heavy metal ions and involve extra intermediate diffusions. Future exploration of higher EIS electron transfer performance needs to be investigated, and meanwhile many types of surface modifications, such as hydrogen-termination or other modification, are desired. The minor shifts of the stripping peak (< 0.1 V) of Mn2+ and Ni2+ suggested the influence on both the working and reference electrodes [19]. This EIS performance demonstrates that the repeatability is comparable with mercury electrode and
1 iW CW
1 − π ni e 2 Q0 ω n
The Warburg impedance is associated with a charge-transfer complex resistance together with a double-layer capacitance. The CPE is connected with the electrode roughness or porous phenomenon, which is caused by double layer capacitance [23]. Thus, the results was fitted into a general electrochemical circuit cell (Randles Model), as shown in Fig. 3. Thus, we have performed a series of DPSV test in aqueous environment, and selected several metal ions including Cb2+, Mn2+, Co2+, Ni2+, and Pb2+ to evaluate its EIS response. For each 0.1 M sodium acetate (NaAc) electrolyte, it contains 0.01 M cadmium chloride, nickel chloride, cobaltous chloride, manganese chloride and 1 mM lead acetate (all purchased from Sigma Aldrich Ltd.), was applied as the measured targets. The baseline subtraction approach ensures accurate measurement on each heavy metal ion with different curves [18]. DPSV was performed in a conventional three-electrodes cell with Ag/AgCl reference electrode as well as Pt counter electrode. The measurement was carried out with submicromolar ion concentrations by holding the potential at −1.2 V (vs. Ag/AgCl). This situation is linked with the hydroxide formation during the process in the basic media. As the pH value in the solution is lower than 3.0, the influence on the selective stripping sensitivity is not as pronounced as the basic buffering solution [22]. The sensing principle for the heavy metal ions includes a 5
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Fig. 4. DPSV response curves for different heavy metals ions dissolved in 0.1 M NaAc buffer solution. The lowest DPSV curves are baseline signals, while the upper ones are measured with the heavy metal ions added (+50 nM each measurement), respectively. The spectra were optimized with Savitzky-Golay smoothing to reduce the background noise.
sintering lead to an increase in total sp3 composition, as well as hardness and Young's modulus. This SPSed ND electrode can be also utilized as a mercury-free heavy metal detector with favorable commercial advantages. The unique EIS response of SPSed ND heavy metal sensor shows an extremely stability and background noise current, which endows it a more promising selective in DPSV analysis. This suggested process could be utilized as the first step for manufacturing ND/composite device and future applications of this electrode might involve a promising and preferable online monitoring in both civil and industrial fields.
glassy carbon electrodes. To improve the detection sensitivity, the SPSed ND electrode need to be polished and its thickness should be reduced to enhance its conductivity. Meanwhile, rotate disk electrode or rotating ring-disk electrode techniques should be introduced during the EIS measurement. A comparison of carbon-based electrode for heavy metal ions detection was summarized in Fig. 6 [22–32]. The reproducibility of SPSed ND electrode for heavy metal ions detection was verified > 10 time with independently optimized experiment setup. The relative standard deviations (RSD) of peaks heights for > 10 stripping cycles of Cd2+, Mn2+, Co2+, Ni2+ and Pb2+ are measured as 3.6%, 5.5%, 3.3%, 6.3% and 4.6%, respectively. Such reasonable error was confined under a relative low level and implicated that the ions might not easily escaped from the electrode due to the high cationexchange capacity and strong absorption of SPSed ND electrode.
7. Experimental The HRTEM images were collected by FEI TITAN G2, operated under an accelerate voltage of 300 kV. The SEM images were taken by FEI Apreo with a field emission gun of 1.3 nm resolution at 1 kV. Argon absorption was carried out using Quantachrome Autosorb and the BET test was analyzed by NETZSCH STA449F3. The Raman spectra were performed with Horiba Jobin Yvon, LabRAM, with a He-Cd excitation laser wavelength of 325 nm. Its thermal performance was characterized in vacuum environment by TGA/DSC technique with TGA 5500 from TA instrument Ltd. The FTIR spectra were purchased from Nicolet 6700 and the stiffness of the SPSec ND electrode was measured by a
6. Conclusion The main focus of this study is to demonstrate the possibility of fabricating ND compaction bulks with SPS technology. We have produced a ND electrode by using SPS technology at 900 K with a compaction pressure of 0.2 GPa. The sintering process graphitized the surface species of ND but maintained the core sp3 content, which provided a hardness over 0.17 GPa. The isothermal heating and plasma
Fig. 5. DPSV current response plotted versus heavy metal ions concentration in linear relationship. Unit: Sens = sensitivity (μA/ppm). 6
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Fig. 6. Comparison of carbon-based electrodes in heavy metal ions detection [22–32]. The price, detection range and sensitivity are selected and listed in the figure. Due to the differences of the detection target heavy metal ions, typical sensitivities and detection ranges are concluded to enlarge the zone in the figure.
Acknowledgements
nanoindentation (Hysitron, TI-750L). The XPS spectra were recorded by Thermo Scientific ESCALAB Xi+ with monochromatography Al X-ray source. The X-ray diffraction signals were acquired by Rigaku D/ max2500V with Cu Kα lines from 5 to 100°. All the electrochemistry impedance, cyclic voltammetry, and anodic stripping voltammetry experiment was conducted by Autolab electrochemistry station PGSTAT302N. The nitrogen adsorption/desorption isotherms and Brunauer-Emmett-Teller (BET) surface areas were determined by an Autosorb-IQ-MP gas sorption analyzer.
The research was supported by the National Natural Science Foundation of China (Grant No: 11327901, 11525415, 51420105003 and 61274114) and Natural Science Foundation of Jiangsu Province (Grant No. BK20160702). The author would like to thank Shiyanjia Lab for the assistance and supports of the characteristic experiment. The authors are grateful to Prof. Xiaodong Huang, Mr. Fei Zhang and Ms. Xinyi He for the sincere assistance in experiment. References
Prime novelty and statement
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This research provided a feasible and cutting-edge technique to fabricate a hybrid nanodiamond/metal/CNTs electrode for heavy metal ion detection. Ultimately, with stable, accurate and precise electrochemical spectroscopy, this ND sensor permits rapid detection of heavy metal ions, which could be introduced in real-time electrochemical online detector in both civil and industrial pollution monitoring. CRediT authorship contribution statement Shi Su: Conceptualization, Writing - review & editing, Supervision, Project administration. Lei Han: Funding acquisition, Resources. Lili Gao: Software. Qingying Ren: Methodology. Meng Nie: Investigation. Zhihong Zhu: Investigation, Validation, Formal analysis, Writing original draft, Visualization. Hao Wan: Investigation, Validation, Formal analysis, Writing - original draft, Visualization. Hui Liu: Investigation, Validation, Formal analysis, Writing - original draft, Visualization. Nan Wu: Investigation, Validation, Formal analysis, Writing - original draft, Visualization. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 7
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