Insight into the Effect of Core-Shell Microstructure on the

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Insight into the Effect of the Core−Shell Microstructure on the Electrochemical Properties of Undoped 3D-Networked Conductive Diamond/Graphite Zhaofeng Zhai,†,‡ Nan Huang,*,† Bing Yang,† Chun Wang,† Lusheng Liu,† Jianhang Qiu,† Dan Shi,†,‡ Ziyao Yuan,†,‡ Zhigang Lu,†,‡ Haozhe Song,† Meiqi Zhou,†,‡ Bin Chen,†,‡ and Xin Jiang*,†,§

J. Phys. Chem. C Downloaded from pubs.acs.org by WASHINGTON UNIV on 03/06/19. For personal use only.

†

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, No. 72 Wenhua Road, Shenyang 110016, China ‡ School of Materials Science and Engineering, University of Science and Technology of China, No. 72 Wenhua Road, Shenyang 110016, China § Institute of Materials Engineering, University of Siegen, No. 9-11 Paul-Bonatz-Str., Siegen 57076, Germany S Supporting Information *

ABSTRACT: Microstructure engineering has aroused tremendous interest to tailor the electrochemical properties of an sp3/sp2-bonded carbon composite in the chemical sensing field. In this work, the undoped diamond/graphite (D/G) nanoplatelet is controllably synthesized without nitrogen/ boron incorporation using microwave plasma chemical vapor deposition. Assisted with high-resolution transmission electron microscopy and conductive atomic force microscopy, it is revealed that the D/G composite is composed of an insulate diamond nanoplatelet stem encapsulated in highly conductive graphite shells. The three-dimensional (3D) conductive graphite edges possess high electrochemical activity, whereas the adjacent inactive diamond core could influence the adsorption of the reactant onto the graphite edges; thus, tunable electrochemical properties from the boron-doped diamond feature to the graphite feature are verified with the thickening of the surrounding graphite shells and the thinning of the diamond stem. Impressively, it is noteworthy that the undoped 3Dnetworked D/G-8% nanoplatelet film, with a thick diamond stem encased into thin graphite shells (∼4 nm), demonstrates improved electrochemical activity while retaining the advantages of a wide potential window (3.18 V) and low background currents (127.6 μF cm−2) as much as possible, holding great promise in electrochemical sensing fields. The D/G hybridized methodology herein paves a novel route toward designing a nanocarbon electrode with excellent electrochemical properties.

1. INTRODUCTION Carbon nanostructured materials currently arouse much attention in electrochemical field owing to the abundant allotropes (e.g., fullerenes, carbon nanotubes, graphene, and nanodiamond) as well as outstanding chemical/physical properties.1−5 The covalent bonds of carbon atoms, derived from the hybridization of s and p orbitals, could be classified into sp2 and sp3 hybrids, forming π and σ bonds. Depending on the typical hybridized form, their properties vary. sp2-Bonded carbon nanostructured materials usually possess high electrical conductivity (e.g., σ = 2 × 103 S cm−1 for graphene sheets4) and superior electrochemical activity. However, the narrow potential window has impeded the application at higher or lower potentials;6 besides, the high background current could give a low ratio of signal-to-noise (S/N), thus leading to poor performance in the detection of analytes.7,8 On the other hand, sp3-bonded diamond materials exhibit significant advantages, such as good hardness, high thermal conductivity, good chemical stability, and inherent biocompatibility,9,10 over conventional materials, showing good performances in wear© XXXX American Chemical Society

resistant coating, heat dissipators, infrared lenses, and electrofield emitters.11−13 Especially, via doping boron,14−16 diamond materials with high conductivity have demonstrated a wide potential window and a low background current, exhibiting great performances in electrochemical applications.3,17 Nevertheless, some key limitations arise when considering the practical applications based on boron-doped diamond (BDD), owing to the inferior electrochemical activity and the hazardous dopant source (e.g., diborane and trimethylborane).18,19 To mitigate such a problem, the hybridization of sp2bonded carbon and sp3-bonded diamond could induce a synergistic effect of the individual chemical/physical properties and hence have great potentials in electrochemical sensing applications. The inclusion of sp2-bonded carbon into the sp3-bonded BDD film has been proposed to enhance the electrochemical Received: December 10, 2018 Revised: February 2, 2019 Published: February 21, 2019 A

DOI: 10.1021/acs.jpcc.8b11865 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Table 1. Deposition Parameters and Characteristic Properties of D/G Films film type

MW power (kW)

CH4 flow (sccm)

gas pressure (mbar)

Ts (°C)

surface roughness (Rq, nm)

D/G-7% D/G-8% D/G-9% D/G-12%

10

28 32 36 48

70

1007 1018 1030 1035

40.2 44.2 53.4 70.1

conductivity (S cm−1) 0.05 13.03 34.06 114.28

± ± ± ±

0.02 1.46 4.05 9.28

background current, and good electrochemical activity, demonstrating great analytical performance in both individual and simultaneous determination of trace Zn(II), Cd(II), Pb(II), and Cu(II). To deeply understand why the undoped D/G nanoplatelet film nearly exhibits a BDD-featured electrochemical property, more investigations are required on the correlation between microstructures and electrochemical properties. Besides, the impacts of sp2-bonded graphite and sp3-bonded diamond on the electrochemical properties of the hybrid electrode will be clearly clarified based on such undoped D/G electrodes, despite the possible effects coming from nitrogen or boron dopants. In this work, the effect of tunable microstructures and constituents on the tailoring of electrochemical properties is elaborately discussed based on high-resolution transmission electron microscopy (HRTEM), conductive atomic force microscopy (C-AFM), field-emission scanning electron microscopy (FE-SEM), Raman spectroscopy, X-ray diffractometer (XRD), and X-ray photoelectron spectroscopy (XPS). The key results herein verify that the D/G hybridized methodology is an effective undoped route toward designing a nanocarbon electrode with good electrical conductivity and high electrochemical activity as well as a wide potential window, a low background current, robust mechanical properties, and good chemical stability, holding great promise in the electrochemical sensing field.

activity while maintaining as many of the great properties of BDD as possible.20−22 A voltammetric pH sensor, fabricated by the controlled incorporation of activated sp2 carbon into a BDD electrode using laser micromachining, exhibited excellent Nernstian linearity, good sensitivity, and high stability because of the improved electrocatalytic activity while retaining the advantages of BDD such as a wide solvent window, low background currents, and catalytically hindered oxygen reduction reaction (ORR) response as much as possible.21 The enhanced electrochemical performance was also verified in the simultaneous detection of guanine and adenine based on the hybrid multilayer graphene/BDD nanowall electrode.23,24 In addition, many investigations22,25−28 have been conducted to clarify the effect of sp2-bonded carbon on the electrochemical performance of the hybrid electrode, however, controversy still exists owing to the fact that both sp2-bonded carbon and the boron-doped level could greatly influence the electrochemical properties.22 Recently, hybrid sp3-bonded diamond/sp2-bonded graphite (D/G) nanowires composed of a diamond core encapsulated into graphitic shells have been fabricated by the microwave plasma chemical vapor deposition (MPCVD) setup with nitrogen incorporation.29 Lin and coauthors have done much research to investigate the effect of N2 incorporation in the Ar/ CH4 admixture on the growth of the hybrid D/G nanocomposite.30−32 CN species were considered as the essential key for the nanocomposite formation because they would preferentially attach on the tip of the nanowire, promoting the elongation of the diamond grains.30 Owing to the large apparent surface area and high electrocatalytic activity, great performances in the determination of adenine dinucleotide, dopamine (DA), and urea have been achieved in the electrochemical sensing field.33−35 Besides, Raina et al. proposed a nitrogen-incorporated D/G film with a unique “ridge” morphology and demonstrated great sensitivity in the detection of DA36 and glucose.37 Very recently, such nitrogenincorporated D/G nanoplatelet films have been used as biocompatible platforms for electrically assisted proliferation and differentiation because of the cooperation of D/G inherent properties (i.e., excellent biocompatibility, great electrical conductivity, and the unique microstructure with a ridge− trough morphology).5 Because nitrogen was essential for preparing the hybrid D/G nanocomposite films in prior studies,12,13,29,30,38 much focus was given to tuning the properties of the D/G film via controlling the nitrogen content.5,31,39,40 However, through varying the sp3/sp2 content, the effect of tunable microstructures on the electrochemical properties of D/G nanoplatelet films is still not completely understood, particularly, in the nitrogen-free case. In our early work, without nitrogen or boron incorporation, we have developed undoped D/G nanoplatelet films by the MPCVD system in the CH4/H2 admixture.41,42 High growth temperature and high methane concentration are crucial for the synthesis of D/G nanoplatelets.43 The undoped D/G nanoplatelet film possessed a wide potential window, a low

2. EXPERIMENTAL SECTION 2.1. Preparation of D/G Films. The preparation of a D/G film was accomplished by the MPCVD technique in a 915 MHz reactor (Iplas, Cyrannus) powered by a microwave of power 10 kW. Prior to growth of films, the cleaned, (100) oriented single-crystal silicon was ultrasonically seeded in the diamond suspension to enhance diamond nucleation. During the deposition process, the gas pressure in the chamber was fixed at 70 mbar, and the H2 flow rate was kept at 400 sccm, whereas the flow rate of CH4 was varied to modulate the ratio of diamond and graphite contents in the film. The detailed deposition conditions for the D/G films are listed in Table 1, and the films grown with varied CH4 concentrations are designated as D/G-x% (i.e., 7% CH4 as D/G-7%, 8% CH4 as D/G-8%, 9% CH4 as D/G-9%, and 12% CH4 as D/G-12%). BDD film was prepared by MPCVD in a 2.45 GHz reactor (ASTeX, A5000), as a comparison, using the following deposition conditions: microwave power of 1800 W, deposition pressure of 60 mbar, and CH4 and trimethylborane (1.15% diluted in H2) concentrations fixed at 1 and 1.25% with a total gas flow rate of 400 sccm. The FE-SEM image (see Figure S1) implies that the microcrystalline BDD is formed in the film, and the electrical conductivity is 441 S cm−1. Meanwhile, the graphite electrode was purchased from Gaoss Union Technology Co., Ltd (Wuhan, China). 2.2. Characterization of D/G Films. The morphological characterization was carried out by FE-SEM (Zeiss Supra 55) operating at 20 kV. The HRTEM was performed with a FEI B


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The Journal of Physical Chemistry C Tecnai (G2 F20) transmission electron microscope at 200 kV. To investigate the conductivity variations corresponding to the surface morphologies, C-AFM was performed with a Dimension ICON system (Bruker). Typically, C-AFM was conducted in the contact mode by employing a conductive probe tip (PFTUNA, Bruker); hence, a topography image could be normally obtained, and meanwhile, the current mapping image was generated through recording the current passing from the sample to the tip when a dc bias was applied. Besides, the C-AFM measurement was carried out using the same conductive probe tip, and negligible signal saltation was observed in the mapping process. Thus, the possible variety in the AFM tip quality herein has little impact on the current mapping results. Raman spectroscopy was conducted by LabRAM HR Evolution (Horiba) microscope using an 1800 line/mm grating for the visible line (532 nm) and a 2400 line/ mm grating for the near-ultraviolet (UV) line (325 nm). The phase characterization of D/G films was identified by XRD (Rigaku RINT 2000) based on Cu Kα radiation. Surface chemical bonds were recorded by XPS (Thermal VG/ ESCALAB 250) using Al Kα (hν = 1486.6 eV) as the X-ray source. Before the measurement, the samples were cleaned in place for 50 s by argon ions first to remove the contamination from atmospheric air. The electrical conductivity of the films was measured utilizing the Hall effect measurement system (Ecopia, HMS-5000). 2.3. Electrochemical Measurements. Electrochemical characterization was carried out on an Autolab workstation (PGSTAT302N). A conventional three-electrode system was used with Ag/AgCl (3 M KCl) as the aqueous reference electrode, Ag/Ag+ (0.01 M) as the nonaqueous reference electrode, platinum as the counter electrode, and the different D/G films, BDD, or graphite as the working electrodes. The electrochemical windows were measured in 0.1 M H2SO4 solution, whereas the electrochemical properties were investigated at a successive scan rate in the solution of 0.1 M KCl containing 1 mM [Ru(NH3)6]3+/2+ or 1 mM [Fe(CN)6]3−/4− and CH3CN containing 0.1 M TBABF4 and 1 mM ferrocene. For application in the determination of heavy metal ions, differential pulse anodic stripping voltammetry (DPASV) measurements were performed in an electrochemical cell containing 0.1 M acetate buffer (pH = 5.5) and target metals by the following process: a negative potential of −1.6 V was applied for 270 s in a preconcentration step; then, differential pulse voltammetry was conducted to strip the metals. Standard solutions of Pb2+, Cd2+, and Zn2+ were prepared with metals basis Pb(NO3)2, Cd(NO3)2·4H2O, and Zn(NO3)2·6H2O powders. All above metals basis chemicals as well as K3Fe(CN)6, K4Fe(CN)6·3H2O, KCl, and CH3CN were obtained from Aladdin. Ru(NH3)6Cl3 and tetrabutylammonium tetrafluoroborate (TBABF4) were purchased from SigmaAldrich. H2SO4 and ferrocene were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used as received without further purification. Milli-Q water from Millipore (R ≥ 18.2 MΩ·cm) was used for all experiments.

Figure 1. FE-SEM morphology of D/G films grown in the xCH4/H2 source gas mixture: x = 7% (a), x = 8% (b), x = 9% (c), and x = 12% (d). The arrows in the insets indicate nanoplatelet-like grains.

transition occurs when the CH4 content increases to 8%. As shown in Figure 1b, it is observed that some nanoplatelet-like grains sprout in the film; besides, abundant nanocrystals (∼35 nm) adhere on the nanoplatelet grains. Figure 1c,d exhibits that both the length and density of the nanoplatelet-like grains in the film increase with increasing CH4 content, leading to a much rougher surface. For example, the D/G-12% film shows a surface roughness of 70.1 nm because of plenty of nanoplatelet-like grains mounting in the films, as shown in the inset of Figure 1d. The cross-sectional SEM images of D/G films are illustrated in Figure S2. Figure S2a depicts the D/G-7% film consisting of nanocrystal diamonds from bottom to up. With the CH4 content increasing to 8%, some nanoplatelet-like grains are observed in the cross-sectional image (see Figure S2b). Figure S2c,d exhibits the nanoplatelet-like structure, which becomes more distinguishable on further increasing the CH4 content. Besides, the thickness of D/G films slowly increases as a function of the CH4 concentration, suggesting that the CH4 content has a slight effect on the growth rate of the film. To identify the carbon phases in the film, multiwavelength Raman and X-ray diffraction measurements were carried out. After linear background removal and normalization to 1580 cm−1 G band, Figure S3 shows the Raman spectra of D/G films using 532 and 325 nm lines as the excitation sources. As depicted in Figure S3a, the trans-polyacetylene (TPA, 1140 cm−1), D-band (1345 cm−1), and G-band (1580 cm−1) are clearly observable in the D/G-8−12% nanoplatelet films. The ID/IG ratios monotonously decrease with increasing CH4 concentration, suggesting an increase of the graphite content. Compared with the apparent T2g diamond peak in the D/G7% nanocrystal film, the diamond peaks could be convoluted into high D-band peaks in the D/G-8−12% nanoplatelet films, especially when the amount of diamond phase becomes smaller at higher methane content because of the lower sensitivity to sp3-bonds than to sp2-bonds as well as the strong fluorescence background utilizing a visible laser.44 Thus, Raman spectroscopy employing UV radiation (325 nm) is a

3. RESULTS AND DISCUSSION 3.1. Material Characterization. FE-SEM micrograph in Figure 1a indicates that the D/G-7% film contains much of nanocrystals. The crystal size in the film is smaller than 100 nm. Investigated by AFM over a 5 × 5 μm2 area, the rootmean-square (Rq) roughness of the film is 40.2 nm (as summarized in Table 1). However, a dramatic morphology C


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The Journal of Physical Chemistry C

Figure 2. High-resolution C 1s XPS spectra of D/G films grown in the xCH4/H2 source gas mixture: x = 7% (a), x = 8% (b), x = 9% (c), and x = 12% (d).

the methane concentration increases, which is in line with the Raman and XRD results (see Figures S3 and S4). It is noteworthy that the controllable transformation from nanodiamond to D/G nanoplatelet could be achieved by only adjusting the methane concentration at a high power and a high pressure using the MPCVD setup. The D/G nanoplatelet structure herein is reproducibly synthesized without nitrogen (as shown in Figure S5, negligible peaks can be observed in the high-resolution N 1s XPS spectrum) or boron incorporation, therefore, the formation of D/G nanoplatelets could not be attributed to the nitrogen/boron-related species such as CN−, CNH−, or BHx, which were proposed to play a key role in the growth process in the prior works.23,30 The cross-sectional TEM image in our previous work demonstrated that the D/G nanoplatelets originated from the nanodiamond region and grew through the D/G region to the surface of the film during growth.43 We herein ascribe that at a high methane concentration, the increased graphite content preferentially forms on the grain boundary of the diamond, thus leading to the growth of diamond along a certain direction. A detailed explanation for the growth of D/G nanoplatelet film is beyond the scope of the present work but will be discussed elsewhere. In the following, we focus on the effect of such tunable microstructures on the electrochemical properties of undoped D/G films. 3.2. Electrochemical Characterization of Undoped D/ G Films. Electrical conductivity is an important characteristic for considering the performance of electrodes in the electrochemical sensing field.48 Hall effect measurement system was utilized to evaluate the electrical conductivity of D/G films, and the results are listed in Table 1. The nanocrystal diamond film grown with 7% methane shows a low conductivity of 0.05 S cm−1. However, the conductivity increases by 261 times when the CH4 content increases to 8%. The conductivity of undoped D/G films linearly increases as a function of CH4 concentration, reaching 114.28 S cm−1 for D/G-12% nanoplatelet film. Therefore, the conductive D/G-x% (x = 8−12)

critical characterization tool for the D/G material. Figure S3b shows the obtained UV (325 nm) Raman spectra. Distinct T2g diamond signatures at 1332 cm−1 are contained in all films. This peak and the G-band peak confirm that the films consist of diamond and graphite phases.45,46 The broader T2g-band line and the sharper G-band line explain the poorer crystallinity of diamond and the better crystallinity of graphite at higher methane content. Besides, the Idiamond/IG and ID/IG ratios decrease with increasing methane content, indicating that the relative amount and crystallinity of graphite generally increase in the films, which is in agreement with the result of Raman spectra using the 532 nm line. Note that the TPA peak and the D-band peak shift to higher wavenumbers than those in visible Raman spectroscopy (Figure S3a), which is attributed to the linear dispersive nature.46 In addition, the composition is also verified by XRD results. Figure S4 shows the XRD pattern of D/G films grown with various methane contents. Characteristic peaks of (111), (220), and (311), located at 44.0°, 75.5°, and 91.5°, respectively, indicate the formation of diamond in all films. Besides, the spectrum of films grown with 8−12% CH4 has observable peaks with 2θ values of 54.8° and 77.7° that are characteristic of graphite, which further confirm that the nanoplatelet-like films are composed of diamond and graphite phases. To evaluate the sp3-bond/sp2-bond ratios of the D/G films formed with different CH4 contents, the XPS spectra were examined. As shown in Figure 2, the XPS spectra are deconvoluted into two components with binding energy located at 285.5 and 284.5 eV, which are assigned to tetrahedral sp3-bonded carbon and plane triangle sp2-bonded carbon.19,47 As summarized in Table S1, D/G-7% and D/G-8% films are mainly composed of sp3-bonded carbon, typically, 73.96% for D/G-7% and 63.93% for D/G-8%. Besides, it is clearly observed that the sp3/sp2 ratios monotonously decrease with increasing CH4 content, for example, the sp3 content reduces to 39.81% when 12% CH4 is employed, suggesting that much more graphite is formed in the film. The XPS spectra results imply an increasing tendency of the graphite phase as D


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Figure 3. CVs for BDD, graphite, and D/G films grown in the xCH4/H2 source gas mixture (x = 7, 8, 9, and 12%) at a scan rate of 100 mV/s in the solution of (a) 0.1 M H2SO4 (for the estimation of potential windows), (b) 0.1 M H2SO4 (fine scans for the estimation of background capacitive currents), and (c) 0.1 M KCl containing 1 mM [Ru(NH3)6]2+/3+. (d) Corresponding peak current density of different D/G films as a function of scan rates in [Ru(NH3)6]2+/3+.

nanoplatelet films are suitable to be used for electrochemical applications.49 The working potential windows and background capacitive currents are crucial when considering a material as an electrochemical electrode.6 To examine the oxygen/hydrogen evolution behaviors, the cyclic voltammetry (CV) scans were performed on D/G films, BDD, and graphite in 0.1 M H2SO4 solution. As illustrated in Figure 3a, the i−E curve is nearly a straight line when utilizing the D/G-7% film as an electrode, which can be attributed to the low conductivity and a limited number of electrochemical active sites on the electrode. When the methane concentration increases to 8%, large oxygen/ hydrogen evolution currents arise, and the electrode exhibits a wide working potential window of 3.17 V (threshold current density of ±1 mA cm−2 is applied), which is on par with BDD’s performance. As listed in Table 2, notably, the working potential range of undoped D/G-8% film (−1.21 to 1.97 V) is more negative than that of the BDD electrode (−0.80 to 2.15 V), which might lead to a good performance in the detection of trace heavy-metal ions with more negative equilibrium potential, such as Zn2+ (−1.14 V).14 With increasing CH4

content, the overpotential for oxygen/hydrogen evolution monotonously reduces, eventually leading to a decrease of the working potential window. For example, the undoped D/G12% film exhibits a narrow potential window from −0.88 to 1.17 V, which is nearly like the potential window range of the graphite electrode. In addition, two noteworthy peaks, located at ∼1.5 and ∼−0.5 V can be seen in the i−E curves of D/G films grown in the xCH4/H2 plasma (x = 8−12%). The anodic peak is correlated with the partial oxidation of non-diamond carbon phases, and the cathodic peak is ascribed to the ORR or reduction of carbon/oxygen functionalities.50,51 To further estimate the double-layer capacitances (Cdl) of D/G films, fine scans were conducted in 0.1 M H2SO4, as illustrated in Figure 3b. Cdl has been calculated using the following eq 149 Cdl = jav /ν

where jav is the average background current at a typical potential and ν is the scan rate. As summed in Table 2, Cdl of undoped D/G-8% nanoplatelet film is 127.6 μF cm−2, which is 1/14 of that for the graphite electrode. Cdl increases monotonously with increasing methane content. Cdl of D/G12% film is more than 3 times high as that of D/G-8% film, and hence a better S/N ratio is foreseen based on the D/G-8% nanoplatelet electrode in the electrochemical sensing field. The electrochemical properties of D/G films were further investigated by utilizing the redox probes of [Ru(NH3)6]2+/3+ and [Fe(CN)6]3−/4− in aqueous solution as well as ferrocene in nonaqueous solution. Figure 3c shows the CV i−E curves of D/G films in aqueous solution (0.1 M KCl containing 1 mM [Ru(NH3)6]2+/3+). Well-characteristic redox peaks were obtained for the D/G-x% (x = 8−12) nanoplatelet films. From the results of Figure 3c, the electrochemical properties are statistically summarized in Table 3. The D/G-7% film has the largest ΔEp of 415.04 mV, indicating a sluggish electrontransfer (ET) rate. As the methane concentration increases, ΔEp dramatically reduces to 80.57 mV for D/G-8% film, and thereafter the ΔEp continues to decrease and reaches 75.68 mV

Table 2. Potential Windows (±1 mA cm−2) and DoubleLayer Capacitances (Cdl, Calculated at 0.2 V) of BDD, Graphite, and Various D/G Films film type D/G-7% D/G-8% D/G-9% D/G-12% BDD graphitea

potential windows range (V) −1.21 −1.07 −0.88 −0.80 −0.71

to to to to to

1.97 1.44 1.17 2.15 1.34

potential windows (V)

Cdl (μF cm−2)

3.18 2.51 2.05 2.95 2.05

10.7 127.6 335.2 570.7 9.0 1747.5

(1)

a

Denotes that the potential window of graphite is defined with a threshold of ±2.5 mA cm−2. E


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electrochemical electrodes in both aqueous and nonaqueous solutions. As discussed above, through varying the methane content, the morphology and phase constituents of D/G films could be engineered, eventually tailoring the electrochemical properties of decreasing the potential windows, increasing the capacitances, and enhancing the ET kinetics. Notably, intrinsic diamond is a wide band gap semiconductor (∼5.4 eV), with the valence band position higher than the formal potential (Eo′ ) of redox couples, thus suppressing the ET rates during electrochemical reactions. 31,52 The midgap density of electronic states mainly derive from the doping atoms,50 whereas the grain boundaries, crystal defects, and nondiamond carbon phases in the polycrystalline films are considered to contribute less to the midgap density of electronic states.50 Under no dopant conditions, it is not possible that the sp2bonded carbon shifts the Fermi level of diamond to trigger a high electrochemical activity. Therefore, herein, we could speculate that the conductive graphite phase triggers good electrochemical properties of the composite films. However, as explained above, the electrochemical properties of the undoped D/G-8% film nearly exhibit a BDD feature but not a graphite feature. Meanwhile, the undoped D/G-12% film shows a narrow potential window and a high background capacitive current, much like the graphite electrode. Such tunable electrochemical properties could be ascribed to the unique diamond and graphite hybridized microstructure. In the following, the effect of the microstructural and compositional transformation on the electrochemical properties will be further discussed. 3.3. Dependence of Tunable Electrochemical Properties on the Microstructure of Undoped D/G Films. Transmission electron microscopy (TEM) was performed on the D/G-8% and D/G-12% nanoplatelet films to unravel the unique microstructure. Figure 4a,b shows that both D/G-8% and D/G-12% films are composed of nanoplatelet-like grains. The ring-shaped diffraction patterns in the selected area electron diffraction (SAED) insets verify that the films consist of diamond and graphite, however, the brighter diffraction ring of (002)G implies that more graphite phase is contained at the methane content of 12%. Besides, nanoplatelet-like grains are

Table 3. Electrochemical Properties of Different Redox Probes Based on BDD, Graphite, and Various D/G Films film type

redox probes

ΔEp (mV)

D/G-7%

[Ru(NH3)6]2+/3+ [Fe(CN)6]3−/4− ferrocene [Ru(NH3)6]2+/3+ [Fe(CN)6]3−/4− ferrocene [Ru(NH3)6]2+/3+ [Fe(CN)6]3−/4− ferrocene [Ru(NH3)6]2+/3+ [Fe(CN)6]3−/4− ferrocene [Ru(NH3)6]2+/3+ [Fe(CN)6]3−/4− [Ru(NH3)6]2+/3+ [Fe(CN)6]3−/4−

415.04 490.72 634.76 80.57 85.45 92.77 78.13 75.68 85.45 75.68 73.24 80.56 76.91 80.56 79.34 90.33

D/G-8%

D/G-9%

D/G-12%

BDD graphite

ipa/ipc

linearity of ip with ν1/2

1.24 1.35 1.06 1.04 0.98 0.99 1.34 1.00 0.99 0.96 1.02 0.98

no no no yes yes yes yes yes yes yes yes yes

for the D/G-12% film, suggesting that the ET kinetics is enhanced in the redox reaction. Besides, Table 3 demonstrates that undoped D/G-x% (x = 8−12) nanoplatelet films possess comparable ΔEp values with BDD and graphite electrode, implying the great electrochemical properties of undoped D/G nanoplatelet films. In addition, Table 3 shows that the ipa/ipc ratios are close to unity for D/G-x% (x = 8−12) films. The low ΔEp and equal peak current densities indicate that the electrochemical reaction is a quasi-reversible process. Figure 3d exhibits the dependence of peak current density (ip) on the square root of the scan rate (ν1/2). ip is observed to be linear to ν1/2 except for the D/G-7% film, indicating that the electrochemical process is controlled by the diffusion step based on the D/G nanoplatelet electrodes. Furthermore, as depicted in Figure S6, electrochemical performance was examined in [Fe(CN)6]3−/4− in aqueous solution and in ferrocene in nonaqueous solution. Table 3 reveals that the electrochemical properties in both the redox probes have the same tendency as that in the [Ru(NH3)6]2+/3+ redox probe, confirming the feasibility of D/G nanoplatelet films as

Figure 4. Low-magnification (a,b), high-magnification (c,d), and high-resolution (e,f) TEM images of the D/G nanoplatelet films grown in the xCH4/H2 source gas mixture: x = 8% (a,c,e) and x = 12% (b,d,f). The insets in the images (a,b) show the corresponding SAED patterns. Right panels exhibit magnified HRTEM images corresponding to the dashed rectangles in the left parts. F


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Figure 5. Surface topographies (a−d) and current mapping (e−h) of the D/G films grown in the xCH4/H2 source gas mixture: x = 7% (a,e), x = 8% (b,f), x = 9% (c,g), and x = 12% (d,h), which were obtained utilizing C-AFM derived from the contact mode. The size of the AFM images is 2 × 2 μm2, and the bias on the sample is 1 V.

Figure 6. (a) 3D superimposition and reconstruction for corresponding surface topography and current mapping of D/G-12% nanoplatelet film shown in Figure S9. (b) Cross-sectional analysis corresponding to the dashed lines in (a) and Figure S9. The bias on the sample is 1 V.

more distinctly observed in Figure 4b. To exhibit such discrepancy, a statistical survey of D/G nanoplatelet lengths and thicknesses is carried out. As shown in Figure S7, the nanoplatelet-like grains in D/G-12% films are longer, being ∼600 nm in length, compared with ∼200 nm for the D/G-8% film. Analogous results could be observed in the thickness, that is, ∼25 nm thick for D/G-12% film in contrary to ∼15 nm for the D/G-8% film, which suggests that both the length and thickness of the D/G nanoplatelet become larger as the methane content increases. In addition, high-magnification TEM images in Figure 4c,d demonstrate that the nanoplateletlike grains are composed of a core−shell microstructure. More detailed investigations are conducted using HRTEM, as shown in Figure 4e,f. Figure 4e indicates that the nanoplatelet-like diamond grain is surrounded by graphite layers of ∼4 nm thickness, which could be identified by the interplanar distance of 0.21 and 0.38 nm [see the insets 1 and 2 (Figure 4e)], well in agreement with the theoretical values of (111) diamond facets and (002) graphite facets. Figure 4f shows the HRTEM image of the D/G film grown in 12% CH4/H2 gas mixture. The insets 1 and 2 of Figure 4f also verify that the core−shell microstructure consists of diamond/graphite phases. Significantly, it is clearly observed that the thickness of the diamond stem and graphite shells vary for different methane contents, namely, the thickness of graphite shells becomes larger, approaching ∼10 nm, whereas it reduces to ∼5 nm for the diamond stem when the methane concentration increases to 12%. Such investigations indicate that the content ratios of diamond to graphite in the D/G nanoplatelet films get small with increasing methane content, which is in line with the XPS

and Raman results (see Figures 2 and S3). Meanwhile, as shown in the HRTEM images (Figure S8), it is revealed that both 8 and 12% D/G nanoplatelet films also contain nanodiamond granules and amorphous carbon (a-C). Considering these TEM investigations, the D/G nanoplatelet film is composed of nanoplatelet-like diamond encapsulated in several graphite layers, diamond nanoclusters, and a-C, respectively, in great convergence with the Raman results (Figure S3). To spatially distinguish the electrical variations on the film surface in a microscopic viewpoint, C-AFM measurements were carried out on all films grown in 7−12% CH4/H2 plasma. As shown in Figure 5a−d, the evolutions in film surface topographies are clearly distinguished. For the D/G-7% film (Figure 5a), only nanocrystal grains could be seen in the AFM topography. As the methane content increases, the nanoplatelet-like grains become distinct, which is in agreement with the FE-SEM morphology, as shown in Figure 1. More importantly, the current mapping corresponding to the AFM topography could be recorded as depicted in Figure 5e−h. All current images are arranged with the same color bar from −1.5 to 6.0 nA, and the brighter region represents a higher current at the sample surface. There is no significant current signal until the CH4 concentration increases to 8%; besides, the current grows higher as well as the area of high current region becomes larger when continuing to increase the methane content, as inferred by the brightness and the acreage of the bright region in the images (Figure 5f−h). Figure 5 shows that the shapes of the bright region in the current mapping images are similar to those in topography images. To scrutinize the interdependence between the surface G


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electrolyte interface. However, the overall electrochemical reaction process could be hindered if one involving step was slow. As reported previously, the absorption and desorption of Hads species take a key role in the hydrogen evolution reaction process because either too weak or too strong binding between Hads and the electrode would result in difficulty either in poor adsorption of the reactant or in releasing the product, thus hindering the overall electrochemical reaction.59,60 In our case, the undoped D/G nanoplatelet film is composed of nanoplatelet-like diamond stem encapsulated in several graphite layers, in which graphite edges possess excellent electrochemical activity,58 whereas the inactive diamond core could impede the adsorption of protons or hydronium onto the diamond and the graphite edges.61 Therefore, despite the 3D conductive graphite shells triggering the electrochemical reactions, they are also electronically influenced by the adjacent diamond stem on a nanoscale. 28,62 Such a phenomenon could be especially distinct at the undoped D/ G-8% nanoplatelet film owing to the thick diamond stem encapsulated into thin graphite shells (∼4 nm), as shown in Figure 4, thus leading to a wide cathodic potential window (see Figure 3). The analogous mechanism could also explain the reason for the wide anodic potential window. Finally, the undoped D/G-8% nanoplatelet exhibits a wide potential window on par with the BDD electrode. With increasing graphite content, the electrochemical reaction on the graphite edges is slightly influenced by the diamond because thinner diamond stem and thicker graphite shells (∼10 nm) are formed in the nanoplatelet, as depicted in Figure 4, thus leading to a monotonous reduction of the overpotential in the oxygen/hydrogen evolution process, and eventually the electrochemical properties of the undoped D/G-12% film resemble the graphite feature. Niwa et al. also developed a hybrid amorphous sp3/sp2 nanocarbon film electrode (the sp3/ sp2 ratios varied from 0.702 to 0.190) by using electron cyclotron resonance.61 The hybrid carbon film consisted of parallel graphite sheets and fine-closed carbon structures vertically oriented to the film surface, which is completely different from the D/G nanoplatelet crystal structure herein. However, a similar covalent carbon bond arrangement, that is, sp3-bonded and sp2-bonded carbons hybridizing on a nanometer scale, leads to an analogous reduction tendency of the potential windows, as the sp3/sp2 ratio decreases in the carbon film.19,61,63 Nevertheless, more significant verifications are needed to clarify the effect of inactive diamond on the electrochemical reaction occurring at the adjacent active graphite edges. In our future work, calculations based on density functional theory and investigations based on scanning electrochemical microscopy will be employed to investigate such issue on the nanoscale. In addition, as shown in Table 2, the hybrid D/G-8% nanoplatelet film shows an inferior capacitance value compared with the BDD electrode. Cdl of the hybrid amorphous sp3/sp2 nanocarbon film was 1−2 orders of magnitude higher than that of BDD, which was attributed to the graphite sheets of high electrochemical activity in the previous work.8 Besides, as shown in Figures 1 and S1, the D/G-8% film possesses a rougher surface than BDD, also contributing to an increasing background capacitance. The higher background capacitance could lead to a worse S/N ratio in the electrochemical sensing field. Nevertheless, Cdl of undoped D/G-8% nanoplatelet film is more than an order of magnitude lower than that of the graphite electrode. With the increase of CH4 content, the

structure and electrical properties, Figure S9 and Figure 6 displays the high-magnification surface topography and current mapping image of the D/G-12% nanoplatelet film. The surface topography (Figure S9a) and the current mapping result (Figure S9b) are constructed and superimposed, as shown in Figure 6a, and a stereo three-dimensional (3D) image of the nanoplatelet is depicted. The dark regions have a platelike shape, and they are mostly surrounded by the bright regions. Furthermore, Figure 6b shows the height and current profiles corresponding to the dashed lines in Figures 6a and S9. The current profile is smooth and continuous at the nanoplatelet boundary, and the current value is ∼5 nA at the height trough, in distinct comparison with 0 nA at the top of the height crest, which greatly differs from the hackly fluctuation of the current due to the “geometrical effect.”53 Thus it is confirmed that the D/G nanoplatelet structure consists of an insulate core encased in the highly conductive shells. The current mapping images in Figures 5 and 6 demonstrate that the graphite shells work as electron channels sweeping across the entire D/G films, leading to establish a 3D electrical network. Impressively, the 3D-networked conductive D/G films demonstrate three characteristics: (a) No graphite possibly coats on the top edge of ṭhe diamond platelet because the current variation shows zero current response at the crest of the height profile. Besides, our previous work43 verified that the D/G nanoplatelet could further grow and lengthen by prolonging the deposition time, which further precludes the possibility that the top edge of the diamond platelet was coated by graphite. (b) Because of nanoplatelet-like diamond encapsulated in several graphite layers, the electron conducts along both sides of the diamond nanoplatelet. (c) As the methane concentration increases, the amount of graphite increases, and the length and thickness of the entire nanoplatelet become larger, together with a monotonous increase of the outer graphite thickness, which induces much faster and denser conducting channels, thus resulting in an enhancement of conductivity. More significantly, such a unique conductive pattern as well as the microstructural transformation contribute to the tailoring of the electrochemical properties as the methane content varies. Ascribing to the sp3-bonded carbon pattern, the adsorption of the reactive intermediate onto the diamond electrode surface is very low during the hydrogen or oxygen evolution process, thus leading to a wide potential window.18,54 Meanwhile, such feeble adsorption of electrochemically active species, together with the low charge carrier densities and few carbon/oxygen functionalities on the surface, would also contribute to an inferior ET rate and a very low Cdl.18,28,54 By contrast, the sp2-bonded carbonaceous electrodes (e.g., glassy carbon and graphite) exhibit a narrow potential window and high capacitance because of the high electrochemical activity and great adsorption rate toward the reactants.19,22 In the present work, the undoped 3D-networked D/G nanoplatelet is composed of inactive diamond surrounded by active graphite edges, and the electrochemical properties would be greatly imparted by the unique core−shell microstructure and both D/G constituents. Owing to the fact that the ET rate constant for the edge-plane reaction is about 7 orders of magnitude higher than for the basal-plane reaction,55−58 the edge planes of graphite shells shown in Figure 4 have effective electrochemical activity in comparison with the basal planes. So it is facile to trigger the electrochemical reaction on the shells of the nanoplatelet at the D/G nanoplatelet film/ H


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Figure 7. Differential pulse anodic stripping voltammograms (a) for the determination of 500 μg L−1 Pb2+ (under nonstirring condition) using D/ G nanoplatelet electrodes grown in the xCH4/H2 gas mixture (x = 8, 9, and 12%) and (b) for the determination of 250 μg L−1 Pb2+, Cd2+, and Zn2+ (under stirring condition) using the D/G-8% nanoplatelet film electrode.

nanoplatelet film has great capability in the detection of heavy metal ions, and the great analytical performance (the limit of detection, linear range, reproducibility, stability, and comparison with the literature data) in both individual and simultaneous trace determinations has been thoroughly investigated based on the D/G-8% nanoplatelet film in our recent work.41

amount of graphite phase increases, thus boosting the electron conduction between the electrode and the current collector. Meanwhile, the length and thickness of the entire nanoplatelet become larger, especially with an increase of the outer graphite thickness, leading to greater specific surface area. As a result, enhancing capacitive currents are verified with increasing methane content, even though the undoped D/G-12% nanoplatelet film shows much lower capacitance than the graphite electrode, which could be attributed to the hybrid diamond/graphite microstructure with limited number of active sites, as shown in Figure 6. On the other hand, owing to the enhancement of conductivity and the thickening of graphite edge planes, the ET rate is accelerated at the electrode/electrolyte interface as the methane content increases, thus leading to small ΔEp values in the electrochemical reactions. All of the above results clearly verify that because of the unique 3D-networked conductive diamond/graphite hybridized core−shell arrangement as well as microstructural evolution, the electrochemical property tailoring of undoped D/G nanoplatelet films from the BDD feature to the graphite feature is achieved by varying the methane content. Impressively, the undoped D/G-8% nanoplatelet film exhibits improved electrochemical activity while retaining the advantages of a wide potential window and a low background capacitance as much as possible. Combining these electrochemical properties with easy fabrication, green initiative, and good mechanical stability, the undoped 3D-networked conductive D/G-8% nanoplatelet film holds great potential in the electrochemical sensing field. 3.4. Application of Undoped D/G Film on Electrochemical Sensing. As shown in Figure 7a, DPASV measurements were performed in 0.1 M acetate buffer containing 500 μg L−1 Pb2+ based on different nanoplatelet film electrodes. The significant differences in the background currents and stripping peak intensities are clearly observable. Attributing to the lower background currents (see Figure 3b), the D/G-8% nanoplatelet film shows an enhanced S/N ratio compared with other D/G films. Meanwhile, considering the widest working potential window as well, it is optimal to employ the D/G-8% nanoplatelet film electrode for the determination of Pb2+, Cd2+, and Zn2+. As illustrated in Figure 7b, well-defined peaks are noticed in the stripping voltammetry, and the stripping peaks at the potentials of −548, −785, and −1100 mV [vs Ag/AgCl (3 M KCl)] can be ascribed to the oxidation of Pb, Cd, and Zn deposits, respectively. The above results verify that the D/G-8%

4. CONCLUSIONS Through manipulating the methane concentrations using the MPCVD setup, the microstructures and phase constituents could be engineered, and eventually, electrochemical property tailoring of undoped 3D-networked conductive D/G nanoplatelet films from the BDD feature to the graphite feature is achieved. CV measurements verify a general tendency of decreasing working potential windows and increasing doublelayer capacitances with increasing methane content; besides, the peak-to-peak separation (ΔEp) reduces in both aqueous and nonaqueous solutions as well. Such electrochemical properties have been thoroughly explained to correlate with the microstructural evolution of D/G nanoplatelet films. TEM investigations and current mapping results reveal that the D/G nanoplatelet is composed of an insulate diamond nanoplatelet stem encapsulated in highly conductive graphite shells, and the surrounding graphite shells work as fast electron conducting channels to establish a 3D electrical network. The amount of graphite in the film monotonously increases with the methane content, and the length and thickness of the nanoplatelet synchronously become larger, especially with an increase of the outer graphite thickness, which induces much faster and denser conducting channels, thus resulting in an enhancement of conductivity. Importantly, attributing to the surrounding graphite edges with high electrochemical activity, it is facile to trigger electrochemical reactions on the 3D graphite network, however, which would be greatly influenced by the adjacent diamond stem. Therefore, the undoped D/G-8% nanoplatelet film, with a thick diamond stem encapsulated into thin graphite layers (∼4 nm), demonstrates improved electrochemical activity while maintaining a wide potential window (3.18 V) and a minimum increase in the background capacitance (127.6 μF cm−2) on par with those of the BDD electrode. Meanwhile, the ET kinetics is boosted arising from the thickening graphite shells, thus leading to lower overpotential for oxygen/hydrogen evolution, higher background capacitances, and small ΔEp values, as the methane content increases. Finally, the electrochemical properties of the undoped D/G-12% nanoplatelet film resemble the graphite I


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(7) Kurita, R.; Nakamoto, K.; Sato, Y.; Kamata, T.; Ueda, A.; Kato, D.; Hirono, S.; Niwa, O. An Sp2 and Sp3 Hybrid Nanocrystalline Carbon Film Electrode for Anodic Stripping Voltammetry and Its Application for Electrochemical Immunoassay. Anal. Sci. 2012, 28, 13−20. (8) Yanagisawa, H.; Kurita, R.; Kamata, T.; Yoshioka, K.; Kato, D.; Iwasawa, A.; Nakazato, T.; Torimura, M.; Niwa, O. Effect of the Sp2/ Sp3 Ratio in a Hybrid Nanocarbon Thin Film Electrode for Anodic Stripping Voltammetry Fabricated by Unbalanced Magnetron Sputtering Equipment. Anal. Sci. 2015, 31, 635−641. (9) Š vorc, L.; Borovská, K.; Cinková, K.; Stanković, D. M.; Planková, A. Advanced Electrochemical Platform for Determination of Cytostatic Drug Flutamide in Various Matrices Using a BoronDoped Diamond Electrode. Electrochim. Acta 2017, 251, 621−630. (10) Cheng, H.-Y.; Yang, C.-Y.; Yang, L.-C.; Peng, K.-C.; Chia, C.T.; Liu, S.-J.; Lin, I.-N.; Lin, K.-H. Effective Thermal and Mechanical Properties of Polycrystalline Diamond Films. J. Appl. Phys. 2018, 123, 165105. (11) Wang, T.; Xiang, L.; Shi, W.; Jiang, X. Deposition of diamond/ β-SiC/cobalt silicide composite interlayers to improve adhesion of diamond coating on WC-Co substrates by DC-Plasma Assisted HFCVD. Surf. Coat. Technol. 2011, 205, 3027−3034. (12) Santos, N. F.; Holz, T.; Santos, T.; Fernandes, A. J. S.; Vasconcelos, T. L.; Gouvea, C. P.; Archanjo, B. S.; Achete, C. A.; Silva, R. F.; Costa, F. M. Heat Dissipation Interfaces Based on Vertically Aligned Diamond/Graphite Nanoplatelets. ACS Appl. Mater. Interfaces 2015, 7, 24772−24777. (13) Sankaran, K. J.; Lin, Y.-F.; Jian, W.-B.; Chen, H.-C.; Panda, K.; Sundaravel, B.; Dong, C.-L.; Tai, N.-H.; Lin, I.-N. Structural and Electrical Properties of Conducting Diamond Nanowires. ACS Appl. Mater. Interfaces 2013, 5, 1294−1301. (14) McGaw, E. A.; Swain, G. M. A Comparison of Boron-Doped Diamond Thin-Film and Hg-Coated Glassy Carbon Electrodes for Anodic Stripping Voltammetric Determination of Heavy Metal Ions in Aqueous Media. Anal. Chim. Acta 2006, 575, 180−189. (15) Petrák, V.; Vlčková Ž ivcová, Z.; Krýsová, H.; Frank, O.; Zukal, A.; Klimša, L.; Kopeček, J.; Taylor, A.; Kavan, L.; Mortet, V. Fabrication of Porous Boron-Doped Diamond on SiO2 Fiber Templates. Carbon 2017, 114, 457−464. (16) Ogose, T.; Kasahara, S.; Ikemiya, N.; Hoshi, N.; Einaga, Y.; Nakamura, M. In Situ ATR-IR Observation of the Electrochemical Oxidation of a Polycrystalline Boron-Doped Diamond Electrode in Acidic Solutions. J. Phys. Chem. C 2018, 122, 27456−27461. (17) Ma, Y.; Liu, J.; Li, H. Diamond-Based Electrochemical Aptasensor Realizing a Femtomolar Detection Limit of Bisphenol A. Biosens. Bioelectron. 2017, 92, 21−25. (18) Zhi, J. F.; Tian, R. H. Electrochemistry of Diamond Thin Film. Prog. Chem. 2005, 17, 55−63. (19) Kamata, T.; Kato, D.; Ida, H.; Niwa, O. Structure and Electrochemical Characterization of Carbon Films Formed by Unbalanced Magnetron (UBM) Sputtering Method. Diamond Relat. Mater. 2014, 49, 25−32. (20) Cobb, S. J.; Ayres, Z. J.; Macpherson, J. V. Boron Doped Diamond: A Designer Electrode Material for the Twenty-First Century. Annu. Rev. Anal. Chem. 2018, 11, 463−484. (21) Ayres, Z. J.; Borrill, A. J.; Newland, J. C.; Newton, M. E.; Macpherson, J. V. Controlled Sp2 Functionalization of Boron Doped Diamond as a Route for the Fabrication of Robust and Nernstian PH Electrodes. Anal. Chem. 2016, 88, 974−980. (22) Watanabe, T.; Honda, Y.; Kanda, K.; Einaga, Y. Tailored Design of Boron-Doped Diamond Electrodes for Various Electrochemical Applications with Boron-Doping Level and Sp2-Bonded Carbon Impurities. Phys. Status Solidi A 2014, 211, 2709−2717. (23) Siuzdak, K.; Ficek, M.; Sobaszek, M.; Ryl, J.; Gnyba, M.; Niedziałkowski, P.; Malinowska, N.; Karczewski, J.; Bogdanowicz, R. Boron-Enhanced Growth of Micron-Scale Carbon-Based Nanowalls: A Route toward High Rates of Electrochemical Biosensing. ACS Appl. Mater. Interfaces 2017, 9, 12982−12992.

feature. More impressively, the undoped 3D-networked conductive D/G-8% nanoplatelet film shows well-defined peaks in the anodic stripping voltammetric determination of Pb2+, Cd2+, and Zn2+, holding great promise in electrochemical applications. The D/G hybridized methodology herein paves a novel route toward designing a nanocarbon electrode with excellent electrochemical properties.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b11865. FE-SEM image of BDD; cross-sectional FE-SEM images, Raman spectra, and XRD patterns of D/G films; XPS spectrum of the D/G nanoplatelet film; CV curves for D/G films in the solution of [Fe(CN)6]3−/4− in aqueous solution and ferrocene in nonaqueous solution; histograms of the length and thickness of nanoplatelets in D/ G films; HRTEM images of D/G nanoplatelet films; high-magnification surface topography and current mapping of the D/G nanoplatelet film; and the ratios for carbon chemical species estimated from the deconvoluted C 1s peak of D/G films (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-024-83970109 (N.H.). *E-mail: [email protected] (X.J.). ORCID

Nan Huang: 0000-0003-0871-158X Jianhang Qiu: 0000-0002-7029-1523 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We sincerely acknowledge the financial support from the National Natural Science Foundation of China (grant no. 51202257) and the Shenyang Two-hundreds Project (Z18-0025&Z17-7-027).

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REFERENCES

(1) De Volder, M. F. L.; Tawfick, S. H.; Baughman, R. H.; Hart, A. J. Carbon Nanotubes: Present and Future Commercial Applications. Science 2013, 339, 535−539. (2) Chee, W. K.; Lim, H. N.; Zainal, Z.; Huang, N. M.; Harrison, I.; Andou, Y. Flexible Graphene-Based Supercapacitors: A Review. J. Phys. Chem. C 2016, 120, 4153−4172. (3) Yang, N.; Yu, S.; Macpherson, J. V.; Einaga, Y.; Zhao, H.; Zhao, G.; Swain, G. M.; Jiang, X. Conductive Diamond: Synthesis, Properties, and Electrochemical Applications. Chem. Soc. Rev. 2019, 48, 157−204. (4) Wu, Z.-S.; Ren, W.; Gao, L.; Zhao, J.; Chen, Z.; Liu, B.; Tang, D.; Yu, B.; Jiang, C.; Cheng, H.-M. Synthesis of Graphene Sheets with High Electrical Conductivity and Good Thermal Stability by Hydrogen Arc Discharge Exfoliation. ACS Nano 2009, 3, 411−417. (5) Santos, N. F.; Cicuéndez, M.; Holz, T.; Silva, V. S.; Fernandes, A. J. S.; Vila, M.; Costa, F. M. Diamond-Graphite Nanoplatelet Surfaces as Conductive Substrates for the Electrical Stimulation of Cell Functions. ACS Appl. Mater. Interfaces 2017, 9, 1331−1342. (6) Nebel, C. E.; Rezek, B.; Shin, D.; Uetsuka, H.; Yang, N. Diamond for Bio-Sensor Applications. J. Phys. D: Appl. Phys. 2007, 40, 6443−6466. J


Article

The Journal of Physical Chemistry C (24) Sankaran, K. J.; Ficek, M.; Kunuku, S.; Panda, K.; Yeh, C.-J.; Park, J. Y.; Sawczak, M.; Michałowski, P. P.; Leou, K.-C.; Bogdanowicz, R.; et al. Self-Organized Multi-Layered GrapheneBoron-Doped Diamond Hybrid Nanowalls for High-Performance Electron Emission Devices. Nanoscale 2018, 10, 1345−1355. (25) Show, Y.; Swope, V. M.; Swain, G. M. The Effect of the CH4 Level on the Morphology, Microstructure, Phase Purity and Electrochemical Properties of Carbon Films Deposited by Microwave-Assisted CVD from Ar-Rich Source Gas Mixtures. Diamond Relat. Mater. 2009, 18, 1426−1434. (26) Garcia-Segura, S.; Vieira dos Santos, E.; Martínez-Huitle, C. A. Role of Sp3/Sp2 Ratio on the Electrocatalytic Properties of BoronDoped Diamond Electrodes: A Mini Review. Electrochem. Commun. 2015, 59, 52−55. (27) Duo, I.; Fujishima, A.; Comninellis, C. Electron Transfer Kinetics on Composite Diamond (Sp3)−Graphite (Sp2) Electrodes. Electrochem. Commun. 2003, 5, 695−700. (28) Alehashem, S.; Chambers, F.; Strojek, J. W.; Swain, G. M.; Ramesham, R. Cyclic Voltammetric Studies of Charge Transfer Reactions at Highly Boron-Doped Polycrystalline Diamond ThinFilm Electrodes. Anal. Chem. 1995, 67, 2812−2821. (29) Vlasov, I.; Lebedev, O. I.; Ralchenko, V. G.; Goovaerts, E.; Bertoni, G.; Van Tendeloo, G.; Konov, V. I. Hybrid DiamondGraphite Nanowires Produced by Microwave Plasma Chemical Vapor Deposition. Adv. Mater. 2007, 19, 4058−4062. (30) Sankaran, K. J.; Kurian, J.; Chen, H. C.; Dong, C. L.; Lee, C. Y.; Tai, N. H.; Lin, I. N. Origin of a Needle-Like Granular Structure for Ultrananocrystalline Diamond Films Grown in a N2/CH4 Plasma. J. Phys. D: Appl. Phys. 2012, 45, 365303. (31) Shalini, J.; Lin, Y.-C.; Chang, T.-H.; Sankaran, K. J.; Chen, H.C.; Lin, I.-N.; Lee, C.-Y.; Tai, N.-H. Ultra-Nanocrystalline Diamond Nanowires with Enhanced Electrochemical Properties. Electrochim. Acta 2013, 92, 9−19. (32) Rani, R.; Panda, K.; Kumar, N.; Sankaran, K. J.; Pandian, R.; Ficek, M.; Bogdanowicz, R.; Haenen, K.; Lin, I.-N. Triboenvironment Dependent Chemical Modification of Sliding Interfaces in Ultrananocrystalline Diamond Nanowall Film: Correlation with Friction and Wear. J. Phys. Chem. C 2017, 122, 945−956. (33) Shalini, J.; Sankaran, K. J.; Chen, H.-C.; Lee, C.-Y.; Tai, N.-H.; Lin, I.-N. Mediatorless N2 Incorporated Diamond Nanowire Electrode for Selective Detection of NADH at Stable Low Oxidation Potential. Analyst 2014, 139, 778−785. (34) Shalini, J.; Sankaran, K. J.; Dong, C.-L.; Lee, C.-Y.; Tai, N.-H.; Lin, I.-N. In Situ Detection of Dopamine Using Nitrogen Incorporated Diamond Nanowire Electrode. Nanoscale 2013, 5, 1159−1167. (35) Shalini, J.; Sankaran, K. J.; Lee, C.-Y.; Tai, N.-H.; Lin, I.-N. An Amperometric Urea Bisosensor Based on Covalent Immobilization of Urease on N2 Incorporated Diamond Nanowire Electrode. Biosens. Bioelectron. 2014, 56, 64−70. (36) Raina, S.; Kang, W. P.; Davidson, J. L. Fabrication of NitrogenIncorporated Nanodiamond Ultra-Microelectrode Array for Dopamine Detection. Diamond Relat. Mater. 2010, 19, 256−259. (37) Ko, C.-Y.; Huang, J.-H.; Raina, S.; Kang, W. P. A High Performance Non-Enzymatic Glucose Sensor Based on Nickel Hydroxide Modified Nitrogen-Incorporated Nanodiamonds. Analyst 2013, 138, 3201−3208. (38) Peng, X.; Yuan, W.; Zou, J.; Wang, B.; Hu, W.; Xiong, Y. Nitrogen-Incorporated Ultrananocrystalline Diamond/Multilayer Graphene Composite Carbon Films: Synthesis and Electrochemical Performances. Electrochim. Acta 2017, 257, 504−509. (39) Raina, S.; Kang, W. P.; Davidson, J. L. Optimizing Nitrogen Incorporation in Nanodiamond Film for Bio-Analyte Sensing. Diamond Relat. Mater. 2009, 18, 718−721. (40) Rakha, S. A.; Xintai, Z.; Zhu, D.; Guojun, Y. Effects of N2 Addition on Nanocrystalline Diamond Films by HFCVD in Ar/CH4 Gas Mixture. Curr. Appl. Phys. 2010, 10, 171−175. (41) Zhai, Z.; Huang, N.; Zhuang, H.; Liu, L.; Yang, B.; Wang, C.; Gai, Z.; Guo, F.; Li, Z.; Jiang, X. A Diamond/Graphite Nanoplatelets

Electrode for Anodic Stripping Voltammetric Trace Determination of Zn(II), Cd(II), Pb(II) and Cu(II). Appl. Surf. Sci. 2018, 457, 1192− 1201. (42) Guo, Y.; Huang, N.; Yang, B.; Wang, C.; Zhuang, H.; Tian, Q.; Zhai, Z.; Liu, L.; Jiang, X. Hybrid Diamond/Graphite Films as Electrodes for Anodic Stripping Voltammetry of Trace Ag+ and Cu2+. Sens. Actuators, B 2016, 231, 194−202. (43) Tian, Q.; Yang, B.; Zhuang, H.; Guo, Y.; Wang, C.; Zhai, Z.; Liu, L.; Huang, N.; Jiang, X. Hybrid Diamond/Graphite Films: Morphological Evolution, Microstructure and Tribological Properties. Diamond Relat. Mater. 2016, 70, 179−185. (44) Williams, O. A. Nanocrystalline Diamond. Diamond Relat. Mater. 2011, 20, 621−640. (45) Arenal, R.; Montagnac, G.; Bruno, P.; Gruen, D. M. Multiwavelength Raman Spectroscopy of Diamond Nanowires Present in N-Type Ultrananocrystalline Films. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 245316. (46) Klauser, F.; Steinmüller-Nethl, D.; Kaindl, R.; Bertel, E.; Memmel, N. Raman Studies of Nano- and Ultra-Nanocrystalline Diamond Films Grown by Hot-Filament CVD. Chem. Vap. Deposition 2010, 16, 127−135. (47) Hu, X.; Chen, C.; Lu, S. High Mobility N-Type Conductive Ultrananocrystalline Diamond and Graphene Nanoribbon Hybridized Carbon Films. Carbon 2016, 98, 671−680. (48) Dai, L.; Xue, Y.; Qu, L.; Choi, H.-J.; Baek, J.-B. Metal-Free Catalysts for Oxygen Reduction Reaction. Chem. Rev. 2015, 115, 4823−4892. (49) Zhuang, H.; Yang, N.; Zhang, L.; Fuchs, R.; Jiang, X. Electrochemical Properties and Applications of Nanocrystalline, Microcrystalline, and Epitaxial Cubic Silicon Carbide Films. ACS Appl. Mater. Interfaces 2015, 7, 10886−10895. (50) Granger, M. C.; Witek, M.; Xu, J.; Wang, J.; Hupert, M.; Hanks, A.; Koppang, M. D.; Butler, J. E.; Lucazeau, G.; Mermoux, M.; et al. Standard Electrochemical Behavior of High-Quality, Boron-Doped Polycrystalline Diamond Thin-Film Electrodes. Anal. Chem. 2000, 72, 3793−3804. (51) Sobaszek, M.; Siuzdak, K.; Ryl, J.; Sawczak, M.; Gupta, S.; Carrizosa, S. B.; Ficek, M.; Dec, B.; Darowicki, K.; Bogdanowicz, R. Diamond Phase (Sp3-C) Rich Boron-Doped Carbon Nanowalls (Sp2C): Physicochemical and Electrochemical Properties. J. Phys. Chem. C 2017, 121, 20821−20833. (52) Macpherson, J. V. A Practical Guide to Using Boron Doped Diamond in Electrochemical Research. Phys. Chem. Chem. Phys. 2015, 17, 2935−2949. (53) Rezek, B.; Stuchlík, J.; Fejfar, A.; Kočka, J. Microcrystalline Silicon Thin Films Studied by Atomic Force Microscopy with Electrical Current Detection. J. Appl. Phys. 2002, 92, 587−593. (54) Martin, H. B.; Argoitia, A.; Landau, U.; Anderson, A. B.; Angus, J. C. Hydrogen and Oxygen Evolution on Boron-Doped Diamond Electrodes. J. Electrochem. Soc. 1996, 143, L133−L136. (55) Davies, T. J.; Hyde, M. E.; Compton, R. G. Nanotrench Arrays Reveal Insight into Graphite Electrochemistry. Angew. Chem., Int. Ed. 2005, 44, 5121−5126. (56) Banks, C. E.; Davies, T. J.; Wildgoose, G. G.; Compton, R. G. Electrocatalysis at Graphite and Carbon Nanotube Modified Electrodes: Edge-Plane Sites and Tube Ends Are the Reactive Sites. Chem. Commun. 2005, 829−841. (57) Banks, C. E.; Moore, R. R.; Davies, T. J.; Compton, R. G. Investigation of modified basal plane pyrolytic graphite electrodes: definitive evidence for the electrocatalytic properties of the ends of carbon nanotubes. Chem. Commun. 2004, 1804−1805. (58) Tang, C.; Zhang, Q. Nanocarbon for Oxygen Reduction Electrocatalysis: Dopants, Edges, and Defects. Adv. Mater. 2017, 29, 1604103. (59) Zou, X.; Zhang, Y. Noble Metal-Free Hydrogen Evolution Catalysts for Water Splitting. Chem. Soc. Rev. 2015, 44, 5148−5180. (60) Chen, W.-F.; Muckerman, J. T.; Fujita, E. Recent Developments in Transition Metal Carbides and Nitrides as Hydrogen Evolution Electrocatalysts. Chem. Commun. 2013, 49, 8896−8909. K


Article

The Journal of Physical Chemistry C (61) Jia, J.; Kato, D.; Kurita, R.; Sato, Y.; Maruyama, K.; Suzuki, K.; Hirono, S.; Ando, T.; Niwa, O. Structure and Electrochemical Properties of Carbon Films Prepared by a Electron Cyclotron Resonance Sputtering Method. Anal. Chem. 2007, 79, 98−105. (62) Sekioka, N.; Kato, D.; Ueda, A.; Kamata, T.; Kurita, R.; Umemura, S.; Hirono, S.; Niwa, O. Controllable Electrode Activities of Nano-Carbon Films While Maintaining Surface Flatness by Electrochemical Pretreatment. Carbon 2008, 46, 1918−1926. (63) Kamata, T.; Kato, D.; Hirono, S.; Niwa, O. Structure and Electrochemical Performance of Nitrogen-Doped Carbon Film Formed by Electron Cyclotron Resonance Sputtering. Anal. Chem. 2013, 85, 9845−9851.

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