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Nanostructured boron-doped diamond electrode for seawater salinity detection
Journal Pre-proofs Full Length Article Nanostructured Boron-Doped Diamond Electrode for Seawater Salinity Detection Dan Shi, Nan Huang, Lusheng Liu, Bing Yang, Zhaofeng Zhai, Yibao Wang, Ziyao Yuan, Hong Li, Zhigang Gai, Xin Jiang PII: DOI: Reference:
S0169-4332(20)30408-6 https://doi.org/10.1016/j.apsusc.2020.145652 APSUSC 145652
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
16 October 2019 20 January 2020 3 February 2020
Please cite this article as: D. Shi, N. Huang, L. Liu, B. Yang, Z. Zhai, Y. Wang, Z. Yuan, H. Li, Z. Gai, X. Jiang, Nanostructured Boron-Doped Diamond Electrode for Seawater Salinity Detection, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145652
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© 2020 Published by Elsevier B.V.
Nanostructured Boron-Doped Diamond Electrode for Seawater Salinity Detection
Dan Shia,b, Nan Huanga, *, Lusheng Liua, Bing Yanga, Zhaofeng Zhaia,b, Yibao Wangc, Ziyao Yuana,b, Hong Lid, Zhigang Gaic, *, Xin Jianga, * aShenyang
National Laboratory for Materials Science, Institute of Metal Research,
Chinese Academy of Sciences, No.72 Wenhua Road, Shenyang 110016, China. bSchool
of Materials Science and Engineering, University of Science and Technology
of China, No.72 Wenhua Road, Shenyang 110016, China. cInstitute
of Oceanographic Instrumentation, Qilu University of Technology
(Shandong Academy of Sciences), No.37 Miaoling Road, Qingdao 266001, China. dGuangdong
Institute of New Materials, Changxing Road 363, Tianhe District,
Guangzhou 510650, China
Author information *Corresponding
author: [email protected],[email protected],[email protected]
Notes 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.
Abstract Electrodes with high response and stability are urgently required to reach a steady and accurate measurement of seawater salinity. Herein, boron-doped diamond (BDD) is for the first time considered as a promising alternative to commercial platinum black in the seawater salinity detection due to their considerable conductivity and outstanding stability. However, the intrinsically low double layer capacitance (DLC) of planar BDD film (BDDF) electrodes affect their response in the detection. To solve these problems, boron-doped diamond nanorod forest (BDDNF) electrodes are fabricated via hot filament chemical vapor deposition. Noted, four orders of magnitude larger of DLC (2820 μF/cm2) than BDDF (0.35 μF/cm2) is achieved on BDDNF due to a much enhanced electroactive surface area which is calculated to be 1.47 times larger than BDDF. Benefited from the enhanced DLC and accelerating diffusion resulted by the nanostructure, the response of BDDNF exhibits an enhancement by a factor of 1.5 compared to BDDF at the salinity of 40. Moreover, in striking contrast to commercial platinum black electrode, the obtained BDDNF electrodes exhibit outstanding stability thanks to the increased amount of carbon oxygen functional groups on BDDNF surface. This work greatly promises the potential applications of BDDNF electrodes in the seawater salinity detection.
Keywords: boron-doped diamond, double-layer capacitor, nanorod forest, salinity, seawater, conductivity electrode
1. Introduction Salinity, as a vital parameter in oceanography [1], is generally considered to evaluate the ocean-atmosphere fluxes of heat and water which could provide information for the research on global warming [2, 3], ocean energy development [4-6], and marine ecosystems [7-9]. Therefore, the measurement of seawater salinity from 2 to 42 psu (practical salinity units) with high accuracy and repeatability is crucial. Currently, various optical fiber techniques, such as refractive index [10] and fiber Bragg grating [11, 12], are being utilized for seawater salinity detection with high accuracy. However, various drawbacks, such as optical mirror fouling, inferior mechanical performance and poor long-term stability have severely limited the commercialization of optical fiber techniques [13]. Consequently, most of the salinity commercial sensors currently are based on conductivity measurement [10] according to the practical salinity scale 1978 which is based on an equation relating to the ratio of the electrical conductivity of seawater at 15 °C to that of a standard potassium chloride solution (KCl) [14-17]. Therefore, the essence of salinity measurement is conductivity measurement.
Conductivity electrode, as a core component of conductivity measurement system, directly affects the accuracy and reproducibility of the measurement. Significant progress has been made in the development of conductivity electrode based on noble metal, inorganic and organic materials such as platinum black (Pt black), Iridium oxide and conducting polymers [18-20]. Pt black, fabricated by electrodepositing a
platinum porous layer of on the platinum foil, is the main commercial electrode for seawater salinity detection due to the lower polarization impedance and desirable electrical properties [20, 21]. However, the fragility of the deposited Pt black layer, poor reproducibility of the deposition protocols and high cost have limited its application in conductivity measurement. Iridium oxide electrode exhibits time-dependent characteristic which could influence the performance in the long-term monitoring [20]. In addition, conducting polymers electrode is as fragile as Pt black due to the poor mechanical durability, and it must be kept in a humid environment to avoid delaminating [20]. Hence, novel electrode with high reproducibility is still highly required in the long-term monitoring of solution conductivity.
Compared with other electrode materials, boron-doped diamond (BDD) electrode is especially promising for conductivity sensing because of its extremely chemical and physical stability, high corrosion resistance and excellent mechanical robustness[22, 23]. It has been employed for solution conductivity sensing accurately range from 101∼105 μS/cm [24]. However, due to the intrinsically low double layer capacitance (DLC) of the planar BDD film (BDDF) [25], this narrow test range hardly satisfies the measurement for seawater conductivity ranging from 10 to 100 S/cm [26].Therefore, the enlargement of the conductivity range is very urgent for BDD in seawater salinity detection. Meanwhile, high response and reproducibility are still required to reach an accurate and steady measurement.
Herein, the boron-doped diamond nanorod forest (BDDNF) electrodes were well designed through template growth on silicon nanowires substrate using hot filament chemical vapor deposition (HFCVD), and they demonstrated enhanced response as well as excellent long-term reproducibility compared with Pt black in seawater salinity detection for the first time. The effect of enhanced DLC on the response and the carbon-oxygen functional group on the reproducibility are discussed in detail. The key results herein verify that the BDDNF electrode holds great potentials for efficient and stable sensing in seawater salinity application.
2. Experiment methods 2.1 Preparation and characterization of BDDNF electrode The synthesis process of BDDNF is illustrated in Scheme 1. As the substrate for growing BDDNF, silicon nanowires were fabricated by a two-step reaction from heavily doped n-type silicon wafers through electroless metal deposition [27]. Firstly, the silicon wafers were cleaned with acetone and ethanol to remove organic grease. The clean silicon wafers then immediately immersed into a solution containing 4.8 M HF and 0.005 M AgNO3 for 1 min at room temperature to deposit Ag nanoparticles, featured with the color of the silicon changed from dark to colorful. Secondly, the Ag-deposited silicon wafers were sufficiently rinsed with deionized water to remove extra silver ions and then immediately soaked into the etching solution for 45 min. Here, the etching solution contains 4.8 M HF and 0.3 M H2O2. After the etching process, the silicon wafers were immersed into nitric acid for approximately 12 hours
to remove the remaining Ag from the as-obtained nanowires. Subsequently, the obtained silicon nanowires were rinsed with deionized water and blown dry under a stream of nitrogen. The morphology of the prepared Si nanowire is illustrated in Fig. S1.
The BDDNF electrodes were fabricated on the prepared silicon nanowires after seeding pre-treatment by HFCVD technique. The power of 3.3 kW and the chamber pressure of 3.5 KPa were employed in the deposition process. Boron was doped from an additional H2 flow containing 1% trimethylborane (TMB). During the growth time (7.5 h), the gas flow rate of hydrogen, methane and TMB were controlled at 800 sccm, 8 sccm and 9 sccm to obtain the desired B/C ratio of 10000 ppm. And the substrates temperature was estimated to be ∼820 °C monitored by an infrared thermometer. A BDD film (BDDF) on planar silicon substrate was also deposited under the same conditions for comparison.
The morphology of BDDNF and BDDF was characterized by field-emission scanning electron microscopy (FESEM, SU-70). The phase compositions and orientation of boron-doped diamond films were examined by X-ray diffraction characterization (XRD, Rigaku RINT 2000). The doping level and the crystalline quality of the films were estimated by Raman spectroscopy (λ = 532 nm, Labram HR Evolution, Horiba). Surface chemical bonds was measured by X-ray photoelectron spectroscopy (XPS, Thermal VG/ESCALAB250) using Al Kα (hν = 1486.6 eV).
2.2 Electrochemical behavior investigation Electrochemical
experiments
were
conducted
on
an
Autolab
workstation
(PGSTAT302N). A classical three-electrode system was employed with the conductive BDDF or BDDNF as working electrode, platinum net as counter electrode, Ag/AgCl (3 M KCl) electrode as reference electrodes. The geometric area of the working electrode is 0.053 cm2. The electroactive surface areas (EASA) of BDDF and BDDNF were estimated by the CV in [Fe (CN)6]3-/4-. DLC values of BDDF and BDDNF electrode were measured with cyclic voltammetry (CV) in 0.1 M H2SO4 at scan rates of 0.1 V/s. Electrochemical impedance of BDDF and BDDNF electrode were measured via electrochemical impedance spectra in 0.1 M H2SO4 between 0.1 Hz and 0.1 MHz with amplitude of 10 mV at open circuit potential. All chemical solutions were prepared using Milli-Q (Millipore Direct-Q 8 system) water (R ≥ 18.2 MΩ·cm).
2.3 Salinity detection Herein, seawater salinity detections were conducted using an Autolab workstation (PGSTAT302N) with a two-electrode system using BDDF or BDDNF (geometric surface area = 0.053 cm2) as the working electrode and a platinum net as counter electrode, as illustrated in Scheme 2a.The circuit diagram is as shown in Scheme 2b, E is the excitation signal. Rc is the equivalent resistance between the electrodes while the current gets measured and Rl is the internal resistance of E. As long as the voltage drop Vc and loop current I get measured, the electrical conductivity of seawater γ can
be determined by the following formula [28, 29]: γ = (I VC) K
(1)
(K is the cell constant which depends on the cell geometry and is determined during cell calibration)
To avoid the effects of the electrodes polarization and make the measurements more sensitive to seawater conductivity, square wave voltammetry (SWV) with amplitude of 0.33V and frequency up to 105 Hz was applied to the electrodes. Five Standard Seawater (SSW) samples (S = 5, 20, 30, 35, 40 psu) manufactured by National Center of Ocean Standards and Metrology (NCOSM) in China were primarily detected to obtain the standard plot. Five real seawater samples were prepared by diluting real seawater (obtained from the Yellow Sea of China) into different concentrations. By fitting the current density into the standard plot, their test salinity values were obtained. Meanwhile, their reference values were obtained by the commercial salinometer (Ruosull, RCB100). Their test values and reference values were compared to evaluate feasibility of BDDNF electrode in practical seawater salinity detection. Furthermore, long-term reproducibility of BDDNF were evaluated by weekly measuring the responses towards standard seawater after storing in standard seawater (S=40) for different time. Each measurement was repeated at least for five times.
The accuracy of BDDNF electrode and the commercial platinum black electrode was
tested in the diluted standard seawater are S=2.500, 9.998, 15.001, 17.499, 20.003 (‰) respectively, which called reference value here. Test salinity values are obtained by substituting the measured current density into the standard plot. The absolute values of the differences between the reference values and test values can reflect the accuracy of BDDNF electrode and the commercial platinum black electrode.
3. Results and Discussion 3.1. Morphology and compositional characterization Fig. 1a illustrates the SEM images of BDD film grown on Si substrate (BDDF). The as-obtained BDD film is compact. The grain size of the planar BDD films are smaller than 1 μm with predominant (111) orientation. In addition, ridges and steps oriented parallel to one other are observed, suggesting twinning bands within the grains intersecting on the surface, which is the dominant feature of boron doped diamond [30]. The SEM cross-section image of BDDF is typical micron diamond columnar crystal morphology, as exhibited in Fig. 1b. On the other hand, the morphology of the BDDNF deposited under the same conditions is shown in Fig. 1c. Plenty of boron-doped diamond nanorods with tip diameter of 2 μm uniformly cover on the whole substrate surface. These nanorods composed of various grains which possess rough surface. The crystal size is about 500 nm on BDDNF, which is much smaller than the crystal size of BDD films as observed in Fig. 1a. From the SEM cross-section images of BDDNF as shown in Fig. 1d, a large amount of boron-doped diamond nanorods standing vertically on a silicon wafer. Moreover, the coverage of the
nanocrystalline diamond film is compact and continuous, as depicted in the inset graph of Fig. 1d.
To identify the crystalline quality of BDDF and BDDNF films, Raman and X-ray diffraction measurements were carried out. Fig. 1e depicts the Raman spectra of BDDF and BDDNF with B/C of 10000ppm. Raman spectra of BDDF and BDDNF are characterized by (i) downshifts of 500 cm-1 and 1220 cm-1 which are normally observed in BDD with boron atoms 1020 <[B]< 1022 cm-3 [31] (500 cm-1 shfits to 448 cm-1 for BDDF and BDDNF; 1220 cm-1 shfits to 1196 cm-1 for BDDF, to 1198 cm-1 for BDDNF); (ii) downshifts of shoulder appears at approximately 1332 cm-1 which corresponds to the one-phonon sp3 carbon.( 1332 cm-1 shfits to 1292 cm-1 for BDDF, to 1287 cm-1 for BDDNF). These shifts are due to quantum interference between discrete zone-center phonon and the continuum of electronic transitions generated by the acceptor atoms, which called Fano resonance [32, 33], typically seen at [B]> 1020 boron atoms cm-3 [34, 35]. Meanwhile, negligible sp2-bonded carbon exists in the film, as evidenced by the completely disappeared G band in BDDF and BDDNF.
In addition, the composition is verified by XRD results. Fig. 1f exhibits the XRD patterns of BDDF and BDDNF. Characteristic diffraction peaks of (111), (220) and (311), locating at 43.91°, 75.27°, and 91.46° correspond to diamond diffraction peaks (PDF# 65-0675) [36, 37], indicating the formation of diamond both in BDDF and BDDNF. The preferential orientation of BDDF and BDDNF films along the [111]
direction can also be confirmed, which agrees well with the SEM characterization. All the above results verify that BDDNF maintains phase composition of diamond.
3.2 Electrochemical characterization The effective EASA of electrodes, which has significant effect on electrochemical activity [38, 39], was evaluated by analyzing the CV curves in [Fe(CN)6]3-/4- using the Randles-Sevcik equation (Eq. (2)), exhibited in Fig. 2a [40]. Ip=2.69
×
105
·
n3/2
·
A
·
D1/2
·
C
v1/2
(2) where Ip (A) is the peak current, n is the electron transfer number, D (cm2/s) is the diffusion coefficient, C (mol/cm3) is the bulk concentration, v (V/s) is the scan rate, and A (cm2) is the EASA. The redox reaction of [Fe(CN)6]3-/4- involves one-electron transfer (n =1), and the diffusion coefficient ( D ) is 6.30 × 10
−6
(cm2/s) [38]. The
EASA of BDDNF is estimated to be 0.25 cm2, 1.47 times larger than that of BDDF (0.17 cm2). The larger electroactive area derived from the three-dimensional forest-like structure could increase the electrochemical active sites on BDDNF electrodes, thus improving the performance in seawater salinity detection.
The DLC values were then assessed by cyclic voltammetry in 0.1 M H2SO4 at scan rate of 0.1 V/s, as depicted in Fig. 2b. Firstly, it can be noticed that although both electrodes show the typical of interfacial double-layer charging in a non-faradaic process [41] their CV curves is quite different from others. The CV curve of BDDF is almost a straight line which exhibits the typical capacitive behavior of diamond
electrodes, whereas the CV of BDDNF have a ‘‘quasi-rectangular’’ shape owing to the higher DLC resulted from the increased EASA [42, 43]. Furthermore, the DLC values were assessed by the enclosed areas of CV curves according to Eq. (3) [44]: C = ∫I(V)dV 2∆VνA
(3)
where I is the capacitive current obtained from CV, ν is the scan rate, ΔV is the scanned potential range and A is the geometric area of the electrode. As expected, the calculated DLCs are found to be 0.35 μF/cm2 for BDDF and 2820 μF/cm2 for BDDNF, which confirms that DLC is successfully enhanced by 4 orders of magnitude by using silicon nanowires as template substrate.
To evaluate the impedance of the BDDF and BDDNF electrodes, the electrochemical impedance spectroscopy (EIS) measurements are conducted at open circuit potential in 0.1 M H2SO4. Fig. 3a demonstrates the Nyquist plots of BDDF and BDDNF electrodes, which clearly show the expected capacitive behavior of the electrode– electrolyte interface without faradaic process. In addition, as exhibited in the inset of Fig. 3a, the EIS data of BDDF and BDDNF could be well fitted using classical Randles equivalent circuit, in which the pure capacitance element is substituted by a constant phase element (CPE) on accounting of the frequency dispersion of the DLC. The admittance Y0 of Q is given by Q = Y0 (jω) n. In the case of the exponent n = 1, the CPE element (Q) is pure capacitive indicating an ideal capacitive interface [45-47]. Besides, the exponent n of BDDF (n=0.999) and BDDNF (n=0.994) are all approximately to 1, therefore CPE (Q) of both electrodes are almost pure capacitive in
0.1 M H2SO4. Moreover, it is noteworthy that Y0 of BDDNF (Y0=36 μMho) is three orders of magnitude larger than that of BDD (Y0=10.9 nMho). Meanwhile, the Bode plots (Fig. 3b) shows that the impedance markedly decreases from BDDF to BDDNF. Typically, the impedance modulus of the BDDNF is found to be almost 150 times lower than that of the BDDF electrode in the region of low frequencies (0.1–10 Hz) which corresponds to the capacitive regimes of the electrodes [48], in well agreement with the larger DLC results as discussed above. This clearly indicates the forest-like structure in the BDDNF plays a vital role in enlarging the EASA, thus leading to a remarkable enhancement of DLC.
3.3 Salinity detection 3.3.1. Response studies The response of the commercial platinum black, BDDNF and BDDF electrode was examined, as shown in Fig. 4. It can be seen that the response of BDDNF is less than platinum black but 1.5 times higher than that of BDDF at the salinity of 40. To explain this issue, equivalent circuit diagram model corresponding to solution electrical conductivity measurement is studied, as shown in the inset of 4a. Under the excitation signal of alternating voltage, there are three current paths existing among the measuring electrodes [24]: (1) through the capacitance of the electrochemical DLC (Cdl) and the solution resistance Rs; (2) through the capacitance associated with the electrode pair that make up the cell (Cc); (3) through other parasitic capacitances unavoidably present in the measurement electronics and wiring, collectively termed
(Cp). As shown in the inset of 4a, Cc and Cp are in parallel, so they can be sum up get a value, Cex. The characteristic impedances (Zdl, Zp and Zc) of three capacitors (Cdl, Cp and Cc) have been defined in Eq.(4) by each capacitance (C) and the frequency (f) [24]: Z = 1 2πfC
(4 )
The impedance of the external factors (Zex) is defined as the sum of Zp and Zc. Only on the condition that Rs≫∣Zdl∣and Rs≪∣Zex∣ would the conductivity sensor able to measure solution conductivity accurately [24]. Consequently, the conductivity measurement range, also the salinity measurement range, is controlled by Cdl (Zdl) and Cex (Zex). Thus, in order to detect the seawater conductivity ranging from 10~100 S/cm, Cex should be minimized to extend the range to low conductivity, and Cdl should be maximized to extend the range to high conductivity, [24, 49]. Cdl, as the total DLC of the electrode/electrolyte interfaces, has contributions from the Helmholtz capacitance (CH), the diffuse layer capacitance (Cdiff) and the capacitance of the space charge region (CSC),expressed in Eq. (5) [50-53]. Cdl-1= CH-1+ Cdiff-1+ CSC-1
(5)
In concentrated electrolytes like seawater, Cdiff≫CH and therefore Cdiff-1 in Eq. (5) can be neglected [51]. CSC is determined by the local density of states (LDOS, D(Ef)) at the Fermi level as indicated in Eq. (6) [51, 54]. CSC= 𝑒0𝜀𝜀0𝐷(𝐸𝑓)
(6)
where e0 is the electronic charge, ε is the dielectric constant and ε0 is the vacuum
permittivity. Assuming a typical value of CH ≈ 20 μF/cm2 [50, 51], and the CSC is determined by D(Ef). Metals, such as Platinum, have D(Ef) ≈ 1023/ cm3 ⋅ eV [55], while the LDOS for the sufficiently boron doped diamond films is estimated to be 6.3(±2.0)×1021 / cm3 ⋅eV [50]. A larger D(Ef) results into a greater CSC, leading to a greater Cdl to trigger more sensitive measurement of the salinity. Therefore, current response of platinum black is slightly higher than BDDNF electrode. Meanwhile, Cdl is proportional to the electrode area [56]. Therefore, the higher response of BDDNF compared with BDDF attributes to the enlarged EASA come from the forest-like structure, which is verified by the cyclic voltammetry as mentioned above. Hence, for BDDNF, the lower response than platinum black could be explained by lower LDOS and the higher response than BDD could be explained by higher electrode area.
Additionally, the nanostructure electrode could reduce the diffusion length and realize ion diffusion in multiple directions [57]. The accelerating diffusion also contributes to the higher conductivity response of BDDNF.
The accuracy of the BDDNF electrode and the commercial platinum black electrode is determined using the diluted standard seawater whose salinity values are S=2.500, 9.998, 15.001, 17.499, 20.003 (‰) respectively, which called reference value here. Test salinity values are obtained by substituting the measured current density into the standard plot (Fig. 4d).It can be seen from the differences as shown in the Tab. S6 and Tab. S7, which are the absolute values of the difference between the reference
values and test values, the accuracy of the proposed BDDNF electrode and the commercial platinum black electrode is 0.1 unit of salinity (‰).In our future work, we will strive to improve the test accuracy from the aspects of electrode growth design, electrode surface treatment, electrode packaging technology, circuit design and test software.
3.3.2 Stability studies To verify the stability of BDDNF electrode in seawater salinity detection, current response towards different salinity were measured weekly, as shown in Fig. 4b-d. Fig. 4b demonstrates that the current response of platinum black electrode slightly changed over time which is ascribed to the poor mechanical stability of platinum black [19]. Fig. 4c demonstrates the BDDF electrode also shows poor stability of current response. Impressively, negligible changes of the current response could be observed on BDDNF electrode within four weeks immersing, as illustrated in Fig. 4d, indicating the excellent long-term stability of BDDNF electrode.
In order to address this issue, XPS were performed on BDDF and BDDNF. As shown in the Fig. 5a, the oxygen amount increases from 5.19% for BDDF to 10.53% for BDDNF. This surface oxygen can be considered to exist in the form of carbon-oxygen surface groups. After fitted by Lorentzian−Gaussian peaks with a Shirley background subtraction, the XPS data of the C 1s spectra regions are shown in Fig.5b and Fig.5c. Five peaks deconvoluted from the C1s spectra were assigned to the following
components: 283.0 eV (sp2 C−C), 284.1 eV (C−H bond), 284.7 eV (sp3 C−C), 285.4 eV (C−O bond), and 286.5 eV (C=O bond) [58, 59]. The determined relative fractions of components are summarized in Table 1. Compared to planar BDDF, the BDDNF shows an increased amount of the C−O bonds and a decreased amount of the C−H bonds. Besides, along with the decrease of sp3 C−C bonds, sp2 C−C bonds increase for BDDNF. Although no obvious evidence of sp2 was seen in the Raman spectra of BDD and BDDNF, XPS is a much more sensitive surface technique. For BDDNF, high levels of sp2 carbon may present at the grain boundaries, due to the more grain boundaries caused by the smaller crystal size, which is consistent with the SEM characterization.
Significant differences in stability between BDDF and BDDNF electrodes are mainly attributed to the electrostatic repulsive interaction of the surface dipole. Surface hydrogen of diamond surface gives rise to form a dipole layer induced by C-–H+ heteropolar bonds of surface atoms and the dipole moment essentially decreases when the oxidized surface was obtained. Therefore, hydrogen-terminated surfaces are hydrophobic, whereas oxygen-terminated ones are hydrophilic. Dipoles of water are found to be mainly oriented parallel to the hydrophobic surface while normal to the hydrophilic surface, which results in a weak interaction of water molecules with the hydrogenated film surface and a strong interaction with the oxidized surface [60, 61]. Moreover, the repulsive interaction disappears as increasing of oxygen-containing functional groups [62]. Hence, the interaction between seawater and BDDNF
electrode is stronger than BDDF. In summary, the stability of BDDNF is dramatically improved compared with BDDF ， due to the increased amount of surface oxygen presenting in the form of C-O functional and decreased amount of the C−H bonds .
Meanwhile, BDDNF electrode were storing in standard seawater (S=40) and measured the responses towards salinity weekly and negligible changes of the current response could be observed, indicating the excellent anti-pollution performance of BDDNF electrode. The excellent anti-fouling performance of boron doped diamond electrode has been proven [63, 64] and there are several factors contribute to the strong resistance to fouling of boron doped diamond electrode in electrochemical processes [65]. The low capacitance of the boron-doped diamond electrode may diminish the electrostatic interaction at the interface toward an applied potential sweep. The lacking of interaction with dissolved reactive intermediate species can diminish the adsorption of surface-active agents [66]. The minimal number of oxygen functional functionalities on the boron-doped diamond electrodes may also be responsible for their minimal tendency towards surface-active agents’ adsorption [67]. Consequently, the low capacitance, the lack of adsorption sites, the minimal number of carbon-oxygen functional groups in addition to the low surface energy may all contribute to the innate strong resistance to fouling of boron doped diamond electrode. The excellent anti-fouling performance combined with the high stability and sensitivity encourages us to apply it in salinity measurement and the results of the experiment confirmed the excellent performance exactly.
3.3.3 Real sample analysis In view of the excellent performance including the high stability and sensitivity of BDDNF electrode in standard seawater, the performance of BDDNF electrode needs to be verified in practical application. Therefore, BDDNF electrode was applied to real seawater salinity measurement in order to verify the feasibility in practical application, as shown in Fig.6. And it was accomplished by comparing the reference salinity value obtained by commercial salinometer and test salinity value obtained on BDDNF electrode by substituting the measured current densities into the standard plot in Fig. 4(d). Due to the very small standard deviation whose magnitude is 10-4, while the magnitude of unit of y axis in Fig. 6 is 10-2, so the error bars are basically invisible as well as Fig. 4. The standard deviations of all measurements in Fig. 6 are all below 9.46×10-5 A/cm2, which listed in Tab.S5.The differences between the test values and reference values of the five real seawater samples are listed in the insert table of Fig. 6. For the five samples, all differences were all less than 0.68. From the relative error listed in the insert table, we can find that the relative errors are less than 10% in the normal seawater salinity ranging from 5‰ to 34 ‰. In the next stage, we will strive to reduce the relative difference from the aspects of electrode packaging technology, circuit design and test software.
4. Conclusions
In summary, BDDNF electrodes which were synthesized using silicon nanowires as template substrate by HFCVD, show excellent response and ultra-high long-term stability in seawater salinity detection. It overcomes the limitation of low DLC of the planar electrode. Its DLC value is as high as 2820 μF/cm2 which is four orders of magnitude larger than BDDF (0.35 μF/cm2) owing to the higher EASA (0.25 cm2, 1.47 times larger than BDDF), which derived from the forest-like structure. Benefited from this improvement, it shows enhanced conductivity response towards seawater salinity. Moreover, due to the increased amount of C-O functional group on electrode surface and the outstanding intrinsic stability of diamond, the long-term stability of BDDNF is also dramatically improved and even better than the commercial platinum black electrode in seawater salinity detection. Also, the accuracy of the proposed BDDNF electrode is 0.1 unit of salinity (‰), comparable to the commercial platinum black electrode. Significantly, small deviation is verified between test results and reference results in the salinity detection of real seawater, demonstrating the feasibility of BDDNF electrodes in practical application. The present studies suggest that the BDDNF electrode has great potential to be an alternate in the practical seawater salinity detection.
Acknowledgments We sincerely acknowledge financial support from the Joint Research Fund Liaoning-Shenyang National Laboratory for Materials Science (No.2018510009) and GDAS'
Special
Project
of
Science
and
Technology
Development
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Figure and table captions Scheme 1.Schematic illustration of the synthesis process for BDDNF electrode Scheme 2.(a) Test diagram, and (b) circuit diagram of conductivity salinity detection Fig. 1.SEM images, Raman spectra and XRD patterns of BDDF and BDDNF with B/C atomic ratio of 10000 ppm (a) Surface and (b) cross-section SEM images of BDDF (c) Surface and (d) cross-section SEM images of BDDNF (e) Raman spectra of BDDF and BDDNF (f) XRD patterns of BDDF and BDDNF Fig. 2.Cyclic voltammetry curves in (a) 1 M KCl containing 1 mM [Fe(CN)6]3-/4- and (b) 0.1 M H2SO4 based on BDDF and BDDNF. Scan rate=0.1 V/s. Fig. 3.(a) Nyquist plots and (b) Bode representations of electrochemical impedance spectra of BDDF and BDDNF in 0.1 M H2SO4 Fig. 4.(a) Current response towards different salinity using BDDF, BDDNF and Pt black electrodes and stability evaluation of current response towards different salinity for
(b)
Pt
Black,
(c)
BDDF
and
(d)
BDDNF
electrodes
weekly.
(The standard deviations are all below 2.388×10-4 A/cm2 (n=5)) Fig. 5.(a) XPS spectra of BDDF and BDDNF and deconvoluted C1s XPS spectra of (b) BDDF and (c) BDDNF electrodes Fig. 6.Current response towards different salinity using BDDNF in the standard seawater and real seawater (The standard deviations are all below 9.46×10-4 A/cm2 (n=5))
Table 1 The relative abundance of the components obtained from deconvolution of the C 1s spectra for BDDF and BDDNF
Samples
C-sp2
C-H
C-sp3
C-O
C=O
BDDF
0.3%
8.89%
80.54%
7.46%
2.81%
BDDNF
3.05%
3.05%
78.66%
9.15%
6.10%
Scheme 1
Scheme 2
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Author contributions Dan Shi: Conceptualization, Investigation, Methodology, Writing - Original Draft, Writing - Review & Editing, Formal analysis Nan Huang: Validation, Supervision, Project administration, Writing - Review & Editing, Funding acquisition Lusheng Liu: Project administration, Supervision Bing Yang: Writing - Review & Editing, Supervision Zhaofeng Zhai: Methodology, Writing - Review & Editing, Formal analysis Yibao Wang: Methodology, Writing: Review & Editing, Formal analysis Ziyao Yuan: Methodology, Writing: Review & Editing, Formal analysis Hong Li: Writing: Review & Editing Zhigang Gai: Writing: Review & Editing Xin Jiang: Conceptualization, Resources, Funding acquisition, Review & Editing
Declaration of interests ☒ 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.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Highlights:
Boron-doped nanorod forest (BDDNF) electrode fabricated using HFCVD technique is considered as a promising alternative to commercial platinum black electrode for the seawater salinity detection due to their considerable conductivity and outstanding stability for the first time.
Four orders of magnitude larger of double layer capacitance (2820 μF/cm2) than planar BDD film (BDDF) electrode (0.35 μF/cm2) is achieved on BDDNF due to a much enhanced electroactive surface area (EASA) which is calculated to be 1.47 times larger than BDDF.
Benefited from the enhanced double layer capacitance and accelerating diffusion resulted by the nanostructure, the response of BDDNF exhibits an enhancement by a factor of 1.5 compared to BDDF at the salinity of 40.
BDDNF electrodes exhibit outstanding stability thanks to the increased amount of carbon oxygen functional groups on BDDNF surface.
BDDNF electrode with excellent response and stability has great potential to be an alternate in the practical seawater salinity detection.
S0169-4332(20)30408-6 https://doi.org/10.1016/j.apsusc.2020.145652 APSUSC 145652
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
16 October 2019 20 January 2020 3 February 2020
Please cite this article as: D. Shi, N. Huang, L. Liu, B. Yang, Z. Zhai, Y. Wang, Z. Yuan, H. Li, Z. Gai, X. Jiang, Nanostructured Boron-Doped Diamond Electrode for Seawater Salinity Detection, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145652
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© 2020 Published by Elsevier B.V.
Nanostructured Boron-Doped Diamond Electrode for Seawater Salinity Detection
Dan Shia,b, Nan Huanga, *, Lusheng Liua, Bing Yanga, Zhaofeng Zhaia,b, Yibao Wangc, Ziyao Yuana,b, Hong Lid, Zhigang Gaic, *, Xin Jianga, * aShenyang
National Laboratory for Materials Science, Institute of Metal Research,
Chinese Academy of Sciences, No.72 Wenhua Road, Shenyang 110016, China. bSchool
of Materials Science and Engineering, University of Science and Technology
of China, No.72 Wenhua Road, Shenyang 110016, China. cInstitute
of Oceanographic Instrumentation, Qilu University of Technology
(Shandong Academy of Sciences), No.37 Miaoling Road, Qingdao 266001, China. dGuangdong
Institute of New Materials, Changxing Road 363, Tianhe District,
Guangzhou 510650, China
Author information *Corresponding
author: [email protected],[email protected],[email protected]
Notes 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.
Abstract Electrodes with high response and stability are urgently required to reach a steady and accurate measurement of seawater salinity. Herein, boron-doped diamond (BDD) is for the first time considered as a promising alternative to commercial platinum black in the seawater salinity detection due to their considerable conductivity and outstanding stability. However, the intrinsically low double layer capacitance (DLC) of planar BDD film (BDDF) electrodes affect their response in the detection. To solve these problems, boron-doped diamond nanorod forest (BDDNF) electrodes are fabricated via hot filament chemical vapor deposition. Noted, four orders of magnitude larger of DLC (2820 μF/cm2) than BDDF (0.35 μF/cm2) is achieved on BDDNF due to a much enhanced electroactive surface area which is calculated to be 1.47 times larger than BDDF. Benefited from the enhanced DLC and accelerating diffusion resulted by the nanostructure, the response of BDDNF exhibits an enhancement by a factor of 1.5 compared to BDDF at the salinity of 40. Moreover, in striking contrast to commercial platinum black electrode, the obtained BDDNF electrodes exhibit outstanding stability thanks to the increased amount of carbon oxygen functional groups on BDDNF surface. This work greatly promises the potential applications of BDDNF electrodes in the seawater salinity detection.
Keywords: boron-doped diamond, double-layer capacitor, nanorod forest, salinity, seawater, conductivity electrode
1. Introduction Salinity, as a vital parameter in oceanography [1], is generally considered to evaluate the ocean-atmosphere fluxes of heat and water which could provide information for the research on global warming [2, 3], ocean energy development [4-6], and marine ecosystems [7-9]. Therefore, the measurement of seawater salinity from 2 to 42 psu (practical salinity units) with high accuracy and repeatability is crucial. Currently, various optical fiber techniques, such as refractive index [10] and fiber Bragg grating [11, 12], are being utilized for seawater salinity detection with high accuracy. However, various drawbacks, such as optical mirror fouling, inferior mechanical performance and poor long-term stability have severely limited the commercialization of optical fiber techniques [13]. Consequently, most of the salinity commercial sensors currently are based on conductivity measurement [10] according to the practical salinity scale 1978 which is based on an equation relating to the ratio of the electrical conductivity of seawater at 15 °C to that of a standard potassium chloride solution (KCl) [14-17]. Therefore, the essence of salinity measurement is conductivity measurement.
Conductivity electrode, as a core component of conductivity measurement system, directly affects the accuracy and reproducibility of the measurement. Significant progress has been made in the development of conductivity electrode based on noble metal, inorganic and organic materials such as platinum black (Pt black), Iridium oxide and conducting polymers [18-20]. Pt black, fabricated by electrodepositing a
platinum porous layer of on the platinum foil, is the main commercial electrode for seawater salinity detection due to the lower polarization impedance and desirable electrical properties [20, 21]. However, the fragility of the deposited Pt black layer, poor reproducibility of the deposition protocols and high cost have limited its application in conductivity measurement. Iridium oxide electrode exhibits time-dependent characteristic which could influence the performance in the long-term monitoring [20]. In addition, conducting polymers electrode is as fragile as Pt black due to the poor mechanical durability, and it must be kept in a humid environment to avoid delaminating [20]. Hence, novel electrode with high reproducibility is still highly required in the long-term monitoring of solution conductivity.
Compared with other electrode materials, boron-doped diamond (BDD) electrode is especially promising for conductivity sensing because of its extremely chemical and physical stability, high corrosion resistance and excellent mechanical robustness[22, 23]. It has been employed for solution conductivity sensing accurately range from 101∼105 μS/cm [24]. However, due to the intrinsically low double layer capacitance (DLC) of the planar BDD film (BDDF) [25], this narrow test range hardly satisfies the measurement for seawater conductivity ranging from 10 to 100 S/cm [26].Therefore, the enlargement of the conductivity range is very urgent for BDD in seawater salinity detection. Meanwhile, high response and reproducibility are still required to reach an accurate and steady measurement.
Herein, the boron-doped diamond nanorod forest (BDDNF) electrodes were well designed through template growth on silicon nanowires substrate using hot filament chemical vapor deposition (HFCVD), and they demonstrated enhanced response as well as excellent long-term reproducibility compared with Pt black in seawater salinity detection for the first time. The effect of enhanced DLC on the response and the carbon-oxygen functional group on the reproducibility are discussed in detail. The key results herein verify that the BDDNF electrode holds great potentials for efficient and stable sensing in seawater salinity application.
2. Experiment methods 2.1 Preparation and characterization of BDDNF electrode The synthesis process of BDDNF is illustrated in Scheme 1. As the substrate for growing BDDNF, silicon nanowires were fabricated by a two-step reaction from heavily doped n-type silicon wafers through electroless metal deposition [27]. Firstly, the silicon wafers were cleaned with acetone and ethanol to remove organic grease. The clean silicon wafers then immediately immersed into a solution containing 4.8 M HF and 0.005 M AgNO3 for 1 min at room temperature to deposit Ag nanoparticles, featured with the color of the silicon changed from dark to colorful. Secondly, the Ag-deposited silicon wafers were sufficiently rinsed with deionized water to remove extra silver ions and then immediately soaked into the etching solution for 45 min. Here, the etching solution contains 4.8 M HF and 0.3 M H2O2. After the etching process, the silicon wafers were immersed into nitric acid for approximately 12 hours
to remove the remaining Ag from the as-obtained nanowires. Subsequently, the obtained silicon nanowires were rinsed with deionized water and blown dry under a stream of nitrogen. The morphology of the prepared Si nanowire is illustrated in Fig. S1.
The BDDNF electrodes were fabricated on the prepared silicon nanowires after seeding pre-treatment by HFCVD technique. The power of 3.3 kW and the chamber pressure of 3.5 KPa were employed in the deposition process. Boron was doped from an additional H2 flow containing 1% trimethylborane (TMB). During the growth time (7.5 h), the gas flow rate of hydrogen, methane and TMB were controlled at 800 sccm, 8 sccm and 9 sccm to obtain the desired B/C ratio of 10000 ppm. And the substrates temperature was estimated to be ∼820 °C monitored by an infrared thermometer. A BDD film (BDDF) on planar silicon substrate was also deposited under the same conditions for comparison.
The morphology of BDDNF and BDDF was characterized by field-emission scanning electron microscopy (FESEM, SU-70). The phase compositions and orientation of boron-doped diamond films were examined by X-ray diffraction characterization (XRD, Rigaku RINT 2000). The doping level and the crystalline quality of the films were estimated by Raman spectroscopy (λ = 532 nm, Labram HR Evolution, Horiba). Surface chemical bonds was measured by X-ray photoelectron spectroscopy (XPS, Thermal VG/ESCALAB250) using Al Kα (hν = 1486.6 eV).
2.2 Electrochemical behavior investigation Electrochemical
experiments
were
conducted
on
an
Autolab
workstation
(PGSTAT302N). A classical three-electrode system was employed with the conductive BDDF or BDDNF as working electrode, platinum net as counter electrode, Ag/AgCl (3 M KCl) electrode as reference electrodes. The geometric area of the working electrode is 0.053 cm2. The electroactive surface areas (EASA) of BDDF and BDDNF were estimated by the CV in [Fe (CN)6]3-/4-. DLC values of BDDF and BDDNF electrode were measured with cyclic voltammetry (CV) in 0.1 M H2SO4 at scan rates of 0.1 V/s. Electrochemical impedance of BDDF and BDDNF electrode were measured via electrochemical impedance spectra in 0.1 M H2SO4 between 0.1 Hz and 0.1 MHz with amplitude of 10 mV at open circuit potential. All chemical solutions were prepared using Milli-Q (Millipore Direct-Q 8 system) water (R ≥ 18.2 MΩ·cm).
2.3 Salinity detection Herein, seawater salinity detections were conducted using an Autolab workstation (PGSTAT302N) with a two-electrode system using BDDF or BDDNF (geometric surface area = 0.053 cm2) as the working electrode and a platinum net as counter electrode, as illustrated in Scheme 2a.The circuit diagram is as shown in Scheme 2b, E is the excitation signal. Rc is the equivalent resistance between the electrodes while the current gets measured and Rl is the internal resistance of E. As long as the voltage drop Vc and loop current I get measured, the electrical conductivity of seawater γ can
be determined by the following formula [28, 29]: γ = (I VC) K
(1)
(K is the cell constant which depends on the cell geometry and is determined during cell calibration)
To avoid the effects of the electrodes polarization and make the measurements more sensitive to seawater conductivity, square wave voltammetry (SWV) with amplitude of 0.33V and frequency up to 105 Hz was applied to the electrodes. Five Standard Seawater (SSW) samples (S = 5, 20, 30, 35, 40 psu) manufactured by National Center of Ocean Standards and Metrology (NCOSM) in China were primarily detected to obtain the standard plot. Five real seawater samples were prepared by diluting real seawater (obtained from the Yellow Sea of China) into different concentrations. By fitting the current density into the standard plot, their test salinity values were obtained. Meanwhile, their reference values were obtained by the commercial salinometer (Ruosull, RCB100). Their test values and reference values were compared to evaluate feasibility of BDDNF electrode in practical seawater salinity detection. Furthermore, long-term reproducibility of BDDNF were evaluated by weekly measuring the responses towards standard seawater after storing in standard seawater (S=40) for different time. Each measurement was repeated at least for five times.
The accuracy of BDDNF electrode and the commercial platinum black electrode was
tested in the diluted standard seawater are S=2.500, 9.998, 15.001, 17.499, 20.003 (‰) respectively, which called reference value here. Test salinity values are obtained by substituting the measured current density into the standard plot. The absolute values of the differences between the reference values and test values can reflect the accuracy of BDDNF electrode and the commercial platinum black electrode.
3. Results and Discussion 3.1. Morphology and compositional characterization Fig. 1a illustrates the SEM images of BDD film grown on Si substrate (BDDF). The as-obtained BDD film is compact. The grain size of the planar BDD films are smaller than 1 μm with predominant (111) orientation. In addition, ridges and steps oriented parallel to one other are observed, suggesting twinning bands within the grains intersecting on the surface, which is the dominant feature of boron doped diamond [30]. The SEM cross-section image of BDDF is typical micron diamond columnar crystal morphology, as exhibited in Fig. 1b. On the other hand, the morphology of the BDDNF deposited under the same conditions is shown in Fig. 1c. Plenty of boron-doped diamond nanorods with tip diameter of 2 μm uniformly cover on the whole substrate surface. These nanorods composed of various grains which possess rough surface. The crystal size is about 500 nm on BDDNF, which is much smaller than the crystal size of BDD films as observed in Fig. 1a. From the SEM cross-section images of BDDNF as shown in Fig. 1d, a large amount of boron-doped diamond nanorods standing vertically on a silicon wafer. Moreover, the coverage of the
nanocrystalline diamond film is compact and continuous, as depicted in the inset graph of Fig. 1d.
To identify the crystalline quality of BDDF and BDDNF films, Raman and X-ray diffraction measurements were carried out. Fig. 1e depicts the Raman spectra of BDDF and BDDNF with B/C of 10000ppm. Raman spectra of BDDF and BDDNF are characterized by (i) downshifts of 500 cm-1 and 1220 cm-1 which are normally observed in BDD with boron atoms 1020 <[B]< 1022 cm-3 [31] (500 cm-1 shfits to 448 cm-1 for BDDF and BDDNF; 1220 cm-1 shfits to 1196 cm-1 for BDDF, to 1198 cm-1 for BDDNF); (ii) downshifts of shoulder appears at approximately 1332 cm-1 which corresponds to the one-phonon sp3 carbon.( 1332 cm-1 shfits to 1292 cm-1 for BDDF, to 1287 cm-1 for BDDNF). These shifts are due to quantum interference between discrete zone-center phonon and the continuum of electronic transitions generated by the acceptor atoms, which called Fano resonance [32, 33], typically seen at [B]> 1020 boron atoms cm-3 [34, 35]. Meanwhile, negligible sp2-bonded carbon exists in the film, as evidenced by the completely disappeared G band in BDDF and BDDNF.
In addition, the composition is verified by XRD results. Fig. 1f exhibits the XRD patterns of BDDF and BDDNF. Characteristic diffraction peaks of (111), (220) and (311), locating at 43.91°, 75.27°, and 91.46° correspond to diamond diffraction peaks (PDF# 65-0675) [36, 37], indicating the formation of diamond both in BDDF and BDDNF. The preferential orientation of BDDF and BDDNF films along the [111]
direction can also be confirmed, which agrees well with the SEM characterization. All the above results verify that BDDNF maintains phase composition of diamond.
3.2 Electrochemical characterization The effective EASA of electrodes, which has significant effect on electrochemical activity [38, 39], was evaluated by analyzing the CV curves in [Fe(CN)6]3-/4- using the Randles-Sevcik equation (Eq. (2)), exhibited in Fig. 2a [40]. Ip=2.69
×
105
·
n3/2
·
A
·
D1/2
·
C
v1/2
(2) where Ip (A) is the peak current, n is the electron transfer number, D (cm2/s) is the diffusion coefficient, C (mol/cm3) is the bulk concentration, v (V/s) is the scan rate, and A (cm2) is the EASA. The redox reaction of [Fe(CN)6]3-/4- involves one-electron transfer (n =1), and the diffusion coefficient ( D ) is 6.30 × 10
−6
(cm2/s) [38]. The
EASA of BDDNF is estimated to be 0.25 cm2, 1.47 times larger than that of BDDF (0.17 cm2). The larger electroactive area derived from the three-dimensional forest-like structure could increase the electrochemical active sites on BDDNF electrodes, thus improving the performance in seawater salinity detection.
The DLC values were then assessed by cyclic voltammetry in 0.1 M H2SO4 at scan rate of 0.1 V/s, as depicted in Fig. 2b. Firstly, it can be noticed that although both electrodes show the typical of interfacial double-layer charging in a non-faradaic process [41] their CV curves is quite different from others. The CV curve of BDDF is almost a straight line which exhibits the typical capacitive behavior of diamond
electrodes, whereas the CV of BDDNF have a ‘‘quasi-rectangular’’ shape owing to the higher DLC resulted from the increased EASA [42, 43]. Furthermore, the DLC values were assessed by the enclosed areas of CV curves according to Eq. (3) [44]: C = ∫I(V)dV 2∆VνA
(3)
where I is the capacitive current obtained from CV, ν is the scan rate, ΔV is the scanned potential range and A is the geometric area of the electrode. As expected, the calculated DLCs are found to be 0.35 μF/cm2 for BDDF and 2820 μF/cm2 for BDDNF, which confirms that DLC is successfully enhanced by 4 orders of magnitude by using silicon nanowires as template substrate.
To evaluate the impedance of the BDDF and BDDNF electrodes, the electrochemical impedance spectroscopy (EIS) measurements are conducted at open circuit potential in 0.1 M H2SO4. Fig. 3a demonstrates the Nyquist plots of BDDF and BDDNF electrodes, which clearly show the expected capacitive behavior of the electrode– electrolyte interface without faradaic process. In addition, as exhibited in the inset of Fig. 3a, the EIS data of BDDF and BDDNF could be well fitted using classical Randles equivalent circuit, in which the pure capacitance element is substituted by a constant phase element (CPE) on accounting of the frequency dispersion of the DLC. The admittance Y0 of Q is given by Q = Y0 (jω) n. In the case of the exponent n = 1, the CPE element (Q) is pure capacitive indicating an ideal capacitive interface [45-47]. Besides, the exponent n of BDDF (n=0.999) and BDDNF (n=0.994) are all approximately to 1, therefore CPE (Q) of both electrodes are almost pure capacitive in
0.1 M H2SO4. Moreover, it is noteworthy that Y0 of BDDNF (Y0=36 μMho) is three orders of magnitude larger than that of BDD (Y0=10.9 nMho). Meanwhile, the Bode plots (Fig. 3b) shows that the impedance markedly decreases from BDDF to BDDNF. Typically, the impedance modulus of the BDDNF is found to be almost 150 times lower than that of the BDDF electrode in the region of low frequencies (0.1–10 Hz) which corresponds to the capacitive regimes of the electrodes [48], in well agreement with the larger DLC results as discussed above. This clearly indicates the forest-like structure in the BDDNF plays a vital role in enlarging the EASA, thus leading to a remarkable enhancement of DLC.
3.3 Salinity detection 3.3.1. Response studies The response of the commercial platinum black, BDDNF and BDDF electrode was examined, as shown in Fig. 4. It can be seen that the response of BDDNF is less than platinum black but 1.5 times higher than that of BDDF at the salinity of 40. To explain this issue, equivalent circuit diagram model corresponding to solution electrical conductivity measurement is studied, as shown in the inset of 4a. Under the excitation signal of alternating voltage, there are three current paths existing among the measuring electrodes [24]: (1) through the capacitance of the electrochemical DLC (Cdl) and the solution resistance Rs; (2) through the capacitance associated with the electrode pair that make up the cell (Cc); (3) through other parasitic capacitances unavoidably present in the measurement electronics and wiring, collectively termed
(Cp). As shown in the inset of 4a, Cc and Cp are in parallel, so they can be sum up get a value, Cex. The characteristic impedances (Zdl, Zp and Zc) of three capacitors (Cdl, Cp and Cc) have been defined in Eq.(4) by each capacitance (C) and the frequency (f) [24]: Z = 1 2πfC
(4 )
The impedance of the external factors (Zex) is defined as the sum of Zp and Zc. Only on the condition that Rs≫∣Zdl∣and Rs≪∣Zex∣ would the conductivity sensor able to measure solution conductivity accurately [24]. Consequently, the conductivity measurement range, also the salinity measurement range, is controlled by Cdl (Zdl) and Cex (Zex). Thus, in order to detect the seawater conductivity ranging from 10~100 S/cm, Cex should be minimized to extend the range to low conductivity, and Cdl should be maximized to extend the range to high conductivity, [24, 49]. Cdl, as the total DLC of the electrode/electrolyte interfaces, has contributions from the Helmholtz capacitance (CH), the diffuse layer capacitance (Cdiff) and the capacitance of the space charge region (CSC),expressed in Eq. (5) [50-53]. Cdl-1= CH-1+ Cdiff-1+ CSC-1
(5)
In concentrated electrolytes like seawater, Cdiff≫CH and therefore Cdiff-1 in Eq. (5) can be neglected [51]. CSC is determined by the local density of states (LDOS, D(Ef)) at the Fermi level as indicated in Eq. (6) [51, 54]. CSC= 𝑒0𝜀𝜀0𝐷(𝐸𝑓)
(6)
where e0 is the electronic charge, ε is the dielectric constant and ε0 is the vacuum
permittivity. Assuming a typical value of CH ≈ 20 μF/cm2 [50, 51], and the CSC is determined by D(Ef). Metals, such as Platinum, have D(Ef) ≈ 1023/ cm3 ⋅ eV [55], while the LDOS for the sufficiently boron doped diamond films is estimated to be 6.3(±2.0)×1021 / cm3 ⋅eV [50]. A larger D(Ef) results into a greater CSC, leading to a greater Cdl to trigger more sensitive measurement of the salinity. Therefore, current response of platinum black is slightly higher than BDDNF electrode. Meanwhile, Cdl is proportional to the electrode area [56]. Therefore, the higher response of BDDNF compared with BDDF attributes to the enlarged EASA come from the forest-like structure, which is verified by the cyclic voltammetry as mentioned above. Hence, for BDDNF, the lower response than platinum black could be explained by lower LDOS and the higher response than BDD could be explained by higher electrode area.
Additionally, the nanostructure electrode could reduce the diffusion length and realize ion diffusion in multiple directions [57]. The accelerating diffusion also contributes to the higher conductivity response of BDDNF.
The accuracy of the BDDNF electrode and the commercial platinum black electrode is determined using the diluted standard seawater whose salinity values are S=2.500, 9.998, 15.001, 17.499, 20.003 (‰) respectively, which called reference value here. Test salinity values are obtained by substituting the measured current density into the standard plot (Fig. 4d).It can be seen from the differences as shown in the Tab. S6 and Tab. S7, which are the absolute values of the difference between the reference
values and test values, the accuracy of the proposed BDDNF electrode and the commercial platinum black electrode is 0.1 unit of salinity (‰).In our future work, we will strive to improve the test accuracy from the aspects of electrode growth design, electrode surface treatment, electrode packaging technology, circuit design and test software.
3.3.2 Stability studies To verify the stability of BDDNF electrode in seawater salinity detection, current response towards different salinity were measured weekly, as shown in Fig. 4b-d. Fig. 4b demonstrates that the current response of platinum black electrode slightly changed over time which is ascribed to the poor mechanical stability of platinum black [19]. Fig. 4c demonstrates the BDDF electrode also shows poor stability of current response. Impressively, negligible changes of the current response could be observed on BDDNF electrode within four weeks immersing, as illustrated in Fig. 4d, indicating the excellent long-term stability of BDDNF electrode.
In order to address this issue, XPS were performed on BDDF and BDDNF. As shown in the Fig. 5a, the oxygen amount increases from 5.19% for BDDF to 10.53% for BDDNF. This surface oxygen can be considered to exist in the form of carbon-oxygen surface groups. After fitted by Lorentzian−Gaussian peaks with a Shirley background subtraction, the XPS data of the C 1s spectra regions are shown in Fig.5b and Fig.5c. Five peaks deconvoluted from the C1s spectra were assigned to the following
components: 283.0 eV (sp2 C−C), 284.1 eV (C−H bond), 284.7 eV (sp3 C−C), 285.4 eV (C−O bond), and 286.5 eV (C=O bond) [58, 59]. The determined relative fractions of components are summarized in Table 1. Compared to planar BDDF, the BDDNF shows an increased amount of the C−O bonds and a decreased amount of the C−H bonds. Besides, along with the decrease of sp3 C−C bonds, sp2 C−C bonds increase for BDDNF. Although no obvious evidence of sp2 was seen in the Raman spectra of BDD and BDDNF, XPS is a much more sensitive surface technique. For BDDNF, high levels of sp2 carbon may present at the grain boundaries, due to the more grain boundaries caused by the smaller crystal size, which is consistent with the SEM characterization.
Significant differences in stability between BDDF and BDDNF electrodes are mainly attributed to the electrostatic repulsive interaction of the surface dipole. Surface hydrogen of diamond surface gives rise to form a dipole layer induced by C-–H+ heteropolar bonds of surface atoms and the dipole moment essentially decreases when the oxidized surface was obtained. Therefore, hydrogen-terminated surfaces are hydrophobic, whereas oxygen-terminated ones are hydrophilic. Dipoles of water are found to be mainly oriented parallel to the hydrophobic surface while normal to the hydrophilic surface, which results in a weak interaction of water molecules with the hydrogenated film surface and a strong interaction with the oxidized surface [60, 61]. Moreover, the repulsive interaction disappears as increasing of oxygen-containing functional groups [62]. Hence, the interaction between seawater and BDDNF
electrode is stronger than BDDF. In summary, the stability of BDDNF is dramatically improved compared with BDDF ， due to the increased amount of surface oxygen presenting in the form of C-O functional and decreased amount of the C−H bonds .
Meanwhile, BDDNF electrode were storing in standard seawater (S=40) and measured the responses towards salinity weekly and negligible changes of the current response could be observed, indicating the excellent anti-pollution performance of BDDNF electrode. The excellent anti-fouling performance of boron doped diamond electrode has been proven [63, 64] and there are several factors contribute to the strong resistance to fouling of boron doped diamond electrode in electrochemical processes [65]. The low capacitance of the boron-doped diamond electrode may diminish the electrostatic interaction at the interface toward an applied potential sweep. The lacking of interaction with dissolved reactive intermediate species can diminish the adsorption of surface-active agents [66]. The minimal number of oxygen functional functionalities on the boron-doped diamond electrodes may also be responsible for their minimal tendency towards surface-active agents’ adsorption [67]. Consequently, the low capacitance, the lack of adsorption sites, the minimal number of carbon-oxygen functional groups in addition to the low surface energy may all contribute to the innate strong resistance to fouling of boron doped diamond electrode. The excellent anti-fouling performance combined with the high stability and sensitivity encourages us to apply it in salinity measurement and the results of the experiment confirmed the excellent performance exactly.
3.3.3 Real sample analysis In view of the excellent performance including the high stability and sensitivity of BDDNF electrode in standard seawater, the performance of BDDNF electrode needs to be verified in practical application. Therefore, BDDNF electrode was applied to real seawater salinity measurement in order to verify the feasibility in practical application, as shown in Fig.6. And it was accomplished by comparing the reference salinity value obtained by commercial salinometer and test salinity value obtained on BDDNF electrode by substituting the measured current densities into the standard plot in Fig. 4(d). Due to the very small standard deviation whose magnitude is 10-4, while the magnitude of unit of y axis in Fig. 6 is 10-2, so the error bars are basically invisible as well as Fig. 4. The standard deviations of all measurements in Fig. 6 are all below 9.46×10-5 A/cm2, which listed in Tab.S5.The differences between the test values and reference values of the five real seawater samples are listed in the insert table of Fig. 6. For the five samples, all differences were all less than 0.68. From the relative error listed in the insert table, we can find that the relative errors are less than 10% in the normal seawater salinity ranging from 5‰ to 34 ‰. In the next stage, we will strive to reduce the relative difference from the aspects of electrode packaging technology, circuit design and test software.
4. Conclusions
In summary, BDDNF electrodes which were synthesized using silicon nanowires as template substrate by HFCVD, show excellent response and ultra-high long-term stability in seawater salinity detection. It overcomes the limitation of low DLC of the planar electrode. Its DLC value is as high as 2820 μF/cm2 which is four orders of magnitude larger than BDDF (0.35 μF/cm2) owing to the higher EASA (0.25 cm2, 1.47 times larger than BDDF), which derived from the forest-like structure. Benefited from this improvement, it shows enhanced conductivity response towards seawater salinity. Moreover, due to the increased amount of C-O functional group on electrode surface and the outstanding intrinsic stability of diamond, the long-term stability of BDDNF is also dramatically improved and even better than the commercial platinum black electrode in seawater salinity detection. Also, the accuracy of the proposed BDDNF electrode is 0.1 unit of salinity (‰), comparable to the commercial platinum black electrode. Significantly, small deviation is verified between test results and reference results in the salinity detection of real seawater, demonstrating the feasibility of BDDNF electrodes in practical application. The present studies suggest that the BDDNF electrode has great potential to be an alternate in the practical seawater salinity detection.
Acknowledgments We sincerely acknowledge financial support from the Joint Research Fund Liaoning-Shenyang National Laboratory for Materials Science (No.2018510009) and GDAS'
Special
Project
of
Science
and
Technology
Development
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Figure and table captions Scheme 1.Schematic illustration of the synthesis process for BDDNF electrode Scheme 2.(a) Test diagram, and (b) circuit diagram of conductivity salinity detection Fig. 1.SEM images, Raman spectra and XRD patterns of BDDF and BDDNF with B/C atomic ratio of 10000 ppm (a) Surface and (b) cross-section SEM images of BDDF (c) Surface and (d) cross-section SEM images of BDDNF (e) Raman spectra of BDDF and BDDNF (f) XRD patterns of BDDF and BDDNF Fig. 2.Cyclic voltammetry curves in (a) 1 M KCl containing 1 mM [Fe(CN)6]3-/4- and (b) 0.1 M H2SO4 based on BDDF and BDDNF. Scan rate=0.1 V/s. Fig. 3.(a) Nyquist plots and (b) Bode representations of electrochemical impedance spectra of BDDF and BDDNF in 0.1 M H2SO4 Fig. 4.(a) Current response towards different salinity using BDDF, BDDNF and Pt black electrodes and stability evaluation of current response towards different salinity for
(b)
Pt
Black,
(c)
BDDF
and
(d)
BDDNF
electrodes
weekly.
(The standard deviations are all below 2.388×10-4 A/cm2 (n=5)) Fig. 5.(a) XPS spectra of BDDF and BDDNF and deconvoluted C1s XPS spectra of (b) BDDF and (c) BDDNF electrodes Fig. 6.Current response towards different salinity using BDDNF in the standard seawater and real seawater (The standard deviations are all below 9.46×10-4 A/cm2 (n=5))
Table 1 The relative abundance of the components obtained from deconvolution of the C 1s spectra for BDDF and BDDNF
Samples
C-sp2
C-H
C-sp3
C-O
C=O
BDDF
0.3%
8.89%
80.54%
7.46%
2.81%
BDDNF
3.05%
3.05%
78.66%
9.15%
6.10%
Scheme 1
Scheme 2
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Author contributions Dan Shi: Conceptualization, Investigation, Methodology, Writing - Original Draft, Writing - Review & Editing, Formal analysis Nan Huang: Validation, Supervision, Project administration, Writing - Review & Editing, Funding acquisition Lusheng Liu: Project administration, Supervision Bing Yang: Writing - Review & Editing, Supervision Zhaofeng Zhai: Methodology, Writing - Review & Editing, Formal analysis Yibao Wang: Methodology, Writing: Review & Editing, Formal analysis Ziyao Yuan: Methodology, Writing: Review & Editing, Formal analysis Hong Li: Writing: Review & Editing Zhigang Gai: Writing: Review & Editing Xin Jiang: Conceptualization, Resources, Funding acquisition, Review & Editing
Declaration of interests ☒ 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.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Highlights:
Boron-doped nanorod forest (BDDNF) electrode fabricated using HFCVD technique is considered as a promising alternative to commercial platinum black electrode for the seawater salinity detection due to their considerable conductivity and outstanding stability for the first time.
Four orders of magnitude larger of double layer capacitance (2820 μF/cm2) than planar BDD film (BDDF) electrode (0.35 μF/cm2) is achieved on BDDNF due to a much enhanced electroactive surface area (EASA) which is calculated to be 1.47 times larger than BDDF.
Benefited from the enhanced double layer capacitance and accelerating diffusion resulted by the nanostructure, the response of BDDNF exhibits an enhancement by a factor of 1.5 compared to BDDF at the salinity of 40.
BDDNF electrodes exhibit outstanding stability thanks to the increased amount of carbon oxygen functional groups on BDDNF surface.
BDDNF electrode with excellent response and stability has great potential to be an alternate in the practical seawater salinity detection.