Enhanced electrochemical properties of the DLC films with an arc interlayer, nitrogen doping and annealing

Accepted Manuscript Enhanced electrochemical properties of the DLC films with an arc interlayer, nitrogen doping and annealing

Myoung Jun Son, Teng Fei Zhang, Yeong Ju Jo, Kwang Ho Kim PII: DOI: Reference:

S0257-8972(17)30911-8 doi: 10.1016/j.surfcoat.2017.09.025 SCT 22663

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

23 June 2017 8 September 2017 11 September 2017

Please cite this article as: Myoung Jun Son, Teng Fei Zhang, Yeong Ju Jo, Kwang Ho Kim , Enhanced electrochemical properties of the DLC films with an arc interlayer, nitrogen doping and annealing, Surface & Coatings Technology (2017), doi: 10.1016/ j.surfcoat.2017.09.025

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Enhanced electrochemical properties of the DLC films with an arc interlayer, nitrogen doping and annealing

School of Materials Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea

Global Frontier R&D Center for Hybrid Interface Materials, Pusan National University, Busan 609-735,

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Myoung Jun Son a,b,1, Teng Fei Zhangb,c,1, Yeong Ju Jo a,b, Kwang Ho Kima,b,c*

Republic of Korea

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National Core Research Center for Hybrid Materials solution, Pusan National University, Busan 609-735, South Korea

These two authors contributed equally to this work

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Abstract: The low electrical conductivity of DLC film and its poor adhesion to metallic substrates are the main drawbacks of DLC film for electrode applications such as waste-water treatment.

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In this study, various kinds of electrodes, the DLC films on metal Ti plates were synthesized by the PECVD technique. The effects of the arc interlayer, nitrogen doping, and post-

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annealing process on the adhesion force, electrical, and electrochemical properties of the

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electrodes were systemically investigated in this study. The introduction of multi-layer of

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Ti/TiC by an arc ion plating (AIP) technique between the DLC film and the Ti substrate enhanced the adhesion force of the DLC film to the Ti substrate, resulting in a large increase

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of the electrode lifetime. In addition, arc droplets produced during the AIP process enlarged the surface area of the DLC films and thus enhanced the electrochemical activity of the

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electrodes. With a small amount of nitrogen (3.4 at. %) doping into the DLC film, the resistivity of DLC the films was significantly reduced and the electrochemical activity was

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enhanced. The effects of the post-annealing temperature on the N-DLC/Ti electrode were

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also studied with respect to the corresponding electrical and electrochemical properties.

Keywords:

Nitrogen-doped DLC films, adhesion force to metal Ti, resistivity, electrochemical property, PECVD *

Corresponding author: E-mail: [email protected]

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1. Introduction Recently, DLC films have attracted a remarkable amount of attention for electrochemical applications. Many previous studies have shown that DLC films have high chemical inertness and a relatively wide electrochemical potential window, making them promising candidates

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for electrochemical electrode materials, especially for waste-water treatment applications [1,

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2]. As compared to typical boron-doped diamond (BDD) film electrodes, DLC films possess

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great advantages in that they are easily synthesized on various substrate materials at low

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temperatures (even at room temperature), which is advantageous with regard to large-scale and low-cost industrial production [3].

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However, there remain several drawbacks which prevent the use of DLC films as electrode materials, such as their high electrical resistivity, high internal stress, and poor adhesion onto

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metallic substrates. In this study, we considered three factors to solve these problems: (1) Appling an interlayer to DLC films can improve the adhesion between the substrate and the

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DLC films [4-7]. There are several deposition methods which can be used to synthesize interlayers. Especially, the arc ion plating (AIP) technique has a high ionization rate and high

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ion energy, resulting in an excellent adhesive layer between the film and the substrate [8, 9]. The droplets generated during the AIP process are commonly considered as a detrimental effect of hard coating applications, for which a smooth surface is typically the goal [10]; however, such a coating can play a positive role in improving the adhesion and electrochemical properties of the film electrodes in this work. (2) Doping various elements into the DLC films will be one of the most effective solutions to reduce the film resistivity

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[11-13]. Especially, nitrogen incorporation in the DLC films favors the formation of sp 2 bonds and acts as a carrier donor source, which can lead to a significant increase in the conductivity [14, 15]. (3) The resistivity of DLC films can also be easily affected by the post-

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annealing temperature, which determines the relative concentration of sp2 and sp3 bonded C species in DLC films [16]. Therefore, in order to fully understand and overcome the

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limitations of DLC films in electrochemical electrode applications, in this study we

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systematically investigate the synthesis parameters associated with the arc interlayer, nitrogen

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doping, and post-annealing temperature with reference to the surface morphology, microstructure, and electrical and electrochemical properties of DLC film electrodes. For the

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deposition of DLC films, the DC-PECVD technique was adopted by simply applying a negative DC bias voltage to the substrates in a gas mixture of C2H2, Ar and N2 to generate

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glow discharge plasma, which is a promising method for the large-area deposition of DLC film electrodes and for mass production in industrial applications.

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2. Experimental details

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2.1. Fabrication of the nitrogen-doped DLC films Nitrogen-doped DLC (N-DLC) film was synthesized by the DC-PECVD on Si and Ti substrates which were cleaned using acetone and ethanol. Si substrate was used for characterizing of the N-DLC films and Ti substrate was used for characterizing of the adhesion force and electrochemical properties of the N-DLC film electrodes (N-DLC/Ti electrodes). Before the deposition process, contamination on the substrate surfaces was cleaned by Ar+ ion bombardment with applying a negative bias for 10 min. The synthesis process of the N-DLC film was as follows: The Ti interlayer was initially deposited on the 4

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substrate by AIP, followed by deposition of a TiC interlayer by AIP. Subsequently, on the surface prepared as described above, the N-DLC layer was deposited by DC-PECVD at a deposition temperature of 300 C, using the same deposition conditions with our previous work. To observe the nitrogen doping effect, the N2 gas flow rate was controlled at 0, 2, 4 and

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8 sccm, respectively, and the working pressure was fixed at 5.0×10-2 by adjusting the throttle valve on the device. The film thicknesses were fixed at approximately 1.2 μm. The finally

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prepared N-DLC/Ti electrodes (with arc interlayer, N2 flow rate of 2sccm) underwent a post-

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annealing process at temperatures ranging from 500 C to 800 C for two hours in a quartz

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tube furnace in an Ar atmosphere. The deposition conditions of N-DLC/Ti electrodes and

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detail information of typical film samples are summarized in Table 1 and Table 2, respectively.

2.2. Characterizations of the N-DLC films

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To observe the surface morphology and thickness of the deposited films, planer-view and

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cross-section images were obtained using a scanning electron microscope (SEM, Hitachi, S4800, accelerating voltage = 15 kV). Raman spectroscopy (Horiba Jobin Yvon, France) was

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utilized to identify the hybridization of the carbon atoms in the N-DLC films. The test range

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of Raman spectra was set as 1000 cm-1 to 2350 cm-1 to obtain the characteristic peaks of NDLC films at around 1360 cm-1 to 1650 cm-1. An Ar+ laser with a 514.5 nm line was adopted as the excitation source. The Raman-scattered light signal in a backscattering geometry was accumulated by a ×100 microscope objective lens. A beam size with a diameter of approximately 1µm was used in the Raman excitation process. X-ray photoelectron spectroscopy (XPS, Thermo Fischer Scientific) with a monochromatic Al Kα source (hv = 1486.6 eV) using a low-energy electron flood gun as the excitation source at a spot size of 5

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400 μm (in diameter) was used to characterize the chemical bonds of the films. To remove the contamination layer, an Ar+ ion beam (4 keV) was applied to etch the film surface for 180 s. All spectra were calibrated through offsetting with an adventitious carbon (C1s) reference

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core-level peak at 284.6 eV. A Hall effect measurement system (HMS 3000, Ecopia) was used to measure the resistivity of the N-DLC films. The ohmic contacts were worked at each

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side of the films using indium metal. The average resistivity was determined by ten

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measurements for each sample. AFM (MFP-3D, Asylum Research) was used to determine

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the typical three-dimensional (3D) surface morphology of the N-DLC films. To measure the adhesion force of the N-DLC films, scratch tester (JLST022, J&L Tech Co., Ltd.) was

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utilized using a Rockwell C indenter with a spherical diamond tip. The scratch tests were

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conducted with a 10 mm scratch length under a gradually increasing load from 0 N to 20 N.

2.3. Appraisal of the electrochemical properties of the N-DLC/Ti electrodes

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The electrochemical properties of the N-DLC/Ti electrodes were analyzed by cyclic voltammograms (CV) at a 20 mV/s scan rate by a potentiostat (ZIVE SP2, WonATech Co.,

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Ltd.) with a saturated reference electrode (KCl-Ag/AgCl) and a counter electrode (platinum). CV tests for 10 cycles at three different locations were performed to get reliable data. The potential window was measured using a 0.5 M Na2SO4 solution as an electrolyte and the electrochemical activity was measured using an electrolyte consisting of a 50 mM K3Fe(CN)6/K4Fe(CN)6 mixture in 0.5 M Na2SO4.

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3.1 Effect of the arc interlayer on the microstructure and properties of N-DLC/Ti electrodes

To observe the effect of an arc interlayer on the morphology of the deposited films, the

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surface of multi-layer of Ti/TiC for interlayer, N-DLC films (2 sccm) with and without interlayer were observed by SEM and AFM. Fig. 1a and b show planer-view and cross-

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sectional SEM images of N-DLC films without an interlayer, respectively. A dense, uniform

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and glassy-like structure is clearly observable. The surface and cross-sectional morphology of

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the multi-layer of Ti/TiC are shown in Fig. 1c and d, where obvious arc droplets evaporated from the metallic target are shown to be deposited on the Ti substrate. Fig. 1e and f show

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SEM images of the planer-view and cross-sections of the N-DLC films on the Ti/TiC interlayer, respectively, where good step coverage of the N-DLC layer was observed,

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covering the rough surface of the droplet-containing multi-layer of Ti/TiC continuously and uniformly. To investigate the influence of the arc interlayer on the roughness of the N-DLC

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films quantitatively, AFM analyses with three-dimensional (3D) views of the N-DLC films with and without interlayers were conducted, as shown in Fig. 1g and h respectively. The

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surfaces of the N-DLC film samples without an interlayer were very smooth with a low rootmean square (RMS) roughness value of 3.0 nm. However, after adding the interlayer, the RMS roughness of the N-DLC film increased significantly to 181.3 nm due to the arc droplets. Traditionally, arc droplets have negative effects on the performance of such films because they degrade the surface quality of the films, leading to high surface roughness and a high friction coefficient [17]. However, in this study, it was considered that the arc droplets

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play a positive role in that they enhanced the surface roughness and surface area, possibly enhancing the response current density as an electrode material and improving the adhesion force [10].

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Peeling off is one of the main reasons for the failure of film electrodes during electrochemical wastewater treatments; therefore, the adhesion force is one of the important

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criteria pertaining to DLC electrodes. The addition of an arc interlayer was done to improve

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the adhesion force between the N-DLC film and the Ti substrates. The adhesion force of the

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DLC films was investigated by a typical scratch test. The results in Table 2 showed that higher critical load exhibited N-DLC film with an arc interlayer than that without an

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interlayer. This result indicates that the adhesion force of N-DLC films onto the substrate is enhanced by introducing an arc interlayer. Meanwhile, it was considered that the relatively

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rough interface between the DLC and the arc interlayer stemming from the droplets also provided additional mechanical-lock effects to improve the adhesion force. Fig. 2a and b

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show cyclic voltammograms (CV) of the N-DLC/Ti electrodes with and without an interlayer in a 0.5 M Na2SO4 solution and in 50 mM ferri/ferro cyanide in a 0.5 M Na2SO4 solution,

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respectively. An arc interlayer does not have an obvious influence on the overpotentials of hydrogen and oxygen evolution of the samples. It can be seen that the N-DLC film electrode with an interlayer showed higher oxidation (Fe(CN)63−) and a reduction (Fe(CN)64−) peak current density compared to that of the film electrode without an interlayer, which indicates that N-DLC /Ti electrodes were electrochemically activated for Fe(CN)63−/4− by the addition of the arc interlayer due to the surface area, which was enlarged by the arc droplets [10, 18].

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3.2 Effect of nitrogen doping on the microstructure and properties of DLC film electrodes Resistivity of DLC films can be significantly decreased by incorporating dopants. After

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applying the arc interlayer, N-DLC films were fabricated with various N2 flow rates to observe the effect of N doping of the DLC film electrodes on the microstructure and

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properties. As shown in Fig. 3a, by introducing a small amount of nitrogen gas (2 sccm), the

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deposition rate exhibited an obvious increase, while a further increase of the nitrogen flow

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rate to 8 sccm led to a slight increase of the deposition rate. As shown in Fig. 3b, the resistivity of the DLC films decreased significantly from 2.0 × 102 Ω∙cm to 1.2 × 10-2 Ω∙cm

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under a low nitrogen flow rate of 2 sccm, and a further increase in the nitrogen flow rate to 8 sccm resulted in a slight decrease of the film resistivity. Two main reasons are suggested for

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the decreased resistivity: (1) The increased sp2 content, namely graphitization by N doping in DLC films, led to a decrease in the electrical resistivity; (2) as mentioned above, the

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incorporated N reduced the band gap [19] and also acting as a good electron donor in the DLC films to reduce the resistivity, which was also reported in the literature [20]. On the

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other hand, the nitrogen can form essentially three possible aromatic configurations with carbon. The first is substitution into a six-fold ring, resulting in three σ bonds to three carbon atoms. This N possesses one π electron and one electron which can be doped into the solid film. The second configuration is that the nitrogen has only two σ bonds to its two carbon neighbors, where the two electrons in a nonbonding ΠX orbital and one electron in a delocalized ΠZ orbital with a non-doping configuration. The third configuration is a five-fold

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ring like in pyrrole, where the nitrogen has three σ bonds to its three carbon neighbors and two electrons in a delocalized ΠZ orbital, the arrangement of which is the non-doping type [21]. The effective doping of nitrogen is only the first incorporation mechanism for reducing

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the resistivity of DLC films. Moreover, in DLC films, nitrogen can exist in the form of molecular N2, which is retained during the deposition of multiple bonds, such as the N≡C

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bond, which does not contribute to the conductivity [22, 23]. Therefore, in this study,

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increasing the N2 gas flow rate further to 8 sccm only led to a slight decrease of the resistivity,

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which indicates that the effective N doping started to become saturated after the flow rate exceeded 2 sccm, and the doping efficiency obviously decreased with further increases in the

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N2 gas flow rate. N-DLC film with a N2 flow rate of 2 sccm was selected for further XPS characterization and electrochemical tests. Fig. 3c shows the XPS survey spectra of pure

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DLC and N-DLC films (2 sccm). Peaks corresponding to C1s and O1s were observed for each sample, while the N1s peak appeared only in the N-DLC film. The N content was 3.4 at.

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% at a N2 gas flow rate of 2 sccm. After the subtraction of the inelastic background, Voigt function fitting was conducted in the XPS spectra of the C1s and N1s peaks to observe the

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bonding state of the pure DLC and N-DLC films. In Fig. 3d, the C1s and N1s peaks can be deconvoluted into several sub-peaks. The C1s spectrum of the pure DLC film was deconvoluted at 284.5, 285.3 and 288.4 eV, which corresponded to the sp 2 C-C, sp3 C-C and C-O bonds, respectively, and the C1s spectrum of the N-DLC film was deconvoluted at 284.5, 285.3, 288.4 eV and one additional peak at 286.9 eV, which corresponded to the C-N bond [24-26]. In the XPS C1s spectra, the ratio of sp2 and sp3 (sp2/sp3) in the films reflected by the

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area ratio of the two peaks at 284.5 eV and 285.3 eV [26, 27]. The sp2/sp3 ratio of the DLC films was increased from 0.9 to 1.4 after nitrogen doping. In other words, the doped nitrogen apparently promoted the sp2 sites in the films and led to an increase in the amount of sp2 bonding. It has been reported that N incorporation encourages the formation of C=C sp2

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bonds because the cross-linking H is removed from the C-H sp3 network. Once the local C-N

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sp2 bond is formed, it continues to grow as a sp2-rich cluster at a larger size according to the

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sub-implantation growth model. Nitrogen also acts as abridge for sp2 domains and results in

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an enlargement of the sp2-rich cluster size [28-31]. The XPS N1s spectra of the N-DLC film were deconvoluted into three sub-peaks. The peaks located at approximately 398.6, 400.1 and

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402.1 eV corresponded to N atoms bonded to sp3-coordinated C atoms (N-C sp3), N atoms bonded to sp2-coordinated C atoms (N-C sp2) and N-O bonds, respectively [32]. Fig. 3e

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shows the cyclic voltammograms (CV) of the DLC/Ti and N-DLC/Ti electrodes in the 0.5 M Na2SO4 solution. The over-potential of pure DLC/Ti and N-DLC/Ti electrodes for hydrogen

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evolution were similar. However, after the nitrogen doping to the DLC film, the oxygen evolution potential shifted slightly toward the negative direction. The electrochemical

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potential window of the DLC/Ti and N-DLC/Ti electrodes was approximately 3.2 V and 3.1 V, respectively. The reduction of the sp3 contents in the DLC films is attributed to the slight decrease of the potential window of the DLC film after N doping. Fig. 3f shows the CV variation of the DLC/Ti and N-DLC/Ti electrodes in 50 mM K3Fe(CN)6 in the 0.5 M Na2SO4 solution.

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After nitrogen doping, Fe(CN)63− oxidation and the Fe(CN)64− reduction peak of the films shifted to a smaller anodic peak potential for cathodic peak potential separation (ΔEp). This result indicates that on the surface of the N-DLC/Ti electrode, there was a greater reversible electrode reaction of Fe(CN)63−/Fe(CN)64− than that on the pure DLC/Ti electrode. The

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intensity of the peaks was also increased in the N-DLC/Ti electrode, which indicated that the

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electrochemical activity and catalytic ability for the redox of Fe(CN)63−/Fe(CN)64− were

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greatly enhanced by the doping process. Two main reasons were considered to be behind

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these results. First, by doping nitrogen, the resistivity of the DLC films was largely reduced, which led to a higher response current density. Second, nitrogen in the DLC/Ti electrodes

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activated the surface, and the active C-N bonds enhanced the electrochemical reaction [3234]. In summary, with a small amount nitrogen doping (3.4 at. %), the electrical conductivity,

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the electrochemical activity and the catalytic ability of the DLC film electrodes was increased

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dramatically without greatly sacrificing the electrochemical potential window.

3.3 Effect of post-annealing on the microstructure and properties of N-DLC/Ti electrodes

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High-temperature post-annealing is another effective method to enhance the electrical conductivity and electrochemical properties of DLC film electrodes [20]. To investigate the post-annealing effect on the microstructure and electrochemical properties of the N-DLC film electrodes, further characterizations were carried out on N-DLC/Ti electrodes deposited with an arc interlayer at a N2 flow rate of 2 sccm, after which they were post-annealed at temperatures from 500 C to 800 C.

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Fig. 4a shows the Raman spectra of the as-deposited and annealed N-DLC films in a test range of 1000 cm-1 to 2350 cm-1, which were also analyzed by a Gaussian curve fitting method and deconvolved into the D peak and the G peak. The D peak, which appears at approximately 1360 cm−1, is related to the closed sp2 structures (aromatics) and the G peak,

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which appears at approximately 1650 cm−1, is associated either with closed or open sp2

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structures [35]. With an increase in the post-annealing temperature, the G peak position shifts

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mildly toward a higher frequency (from 1564.9 to 1590.8 cm-1) and the ID/IG ratio increases

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(from 0.86 to 1.16), as shown in Fig. 4b. The increase in the ID/IG ratio together with the shift of the G peak position towards higher wavenumbers will usually be interpreted in terms of

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the increase in the graphitic domains in amorphous carbon films, either in number or in size [36]. An XPS analysis was conducted to investigate the change in the bonding structure of the

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N-DLC films further after the post-annealing step. Fig. 4c shows the XPS survey spectra of

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the as-deposited and post-annealed (800 C) N-DLC films. C1s, N1s and O1s peaks were observed in both samples. The N content dropped slightly from 3.4 to 3.0 at. % after post-

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annealing at 800 C, which was considered to be attributed to the weakly bonded nitrogen

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atoms, which were annealed out during the high-temperature treatment. After the subtraction of the inelastic background, Voigt function fitting was conducted in the XPS spectra of the C1s and N1s peaks to observe the bonding state of the N-DLC films. Fig. 4d shows that the C1s spectra of both the as-deposited and annealed N-DLC films were deconvoluted at 284.5, 285.3, 286.9 and 288.4 eV, which correspond to the sp2 C-C, sp3 C-C, C-N and C-O bonds, respectively [20, 25, 37]. The sp2/sp3 ratio was increased from 1.4 to 1.9 after post-annealing

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at 800 ℃. The N1s spectra were deconvoluted at 398.6, 400.1 and 402.1 eV, which correspond to N-C sp3, N-C sp2 and N-O bonds, respectively [32, 38]. The N-C sp2/N-C sp3 ratio was increased after annealing at 800 C, which suggests that N-C sp2 bonds are

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preferentially formed at a high post-annealing temperature. The XPS results were in good agreement with the Raman analysis outcome and further confirmed the graphitization of the

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N-DLC films after the post-annealing process.

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Fig. 5a shows resistivity of the N-DLC films depending on post-annealing temperature.

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The resistivity of the films decreased significantly from 1.2 × 10-2 cm (as-deposited) to 8.8 × 10-5  after post-annealing at 800 C, which was much lower than that of previous works

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[30]. The increase in the sp2-bonded carbon content, namely graphitization, led to a decrease in the resistivity of the DLC films, as proved and explained earlier in this paper. Fig. 5b

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shows the CV curves of the as-deposited and annealed N-DLC/Ti electrodes in the 0.5M Na2SO4 solution. It can be seen that the hydrogen evolution potential shifted toward the

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positive direction while the oxygen evolution potential shifted toward the negative direction of the N-DLC/Ti electrode as the post-annealing temperature was increased. Correspondingly,

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the potential window of the N-DLC/Ti electrodes decreased from about 3.3 V (as-deposited) to approximately 2.2 V (annealed at 800 C) as the post-annealing temperature was increased. The potential window is basically determined by the sp3 content in the DLC electrode material, which means higher sp3 content led to larger potential window. In this study, decrease of sp3 contents in the DLC films caused by high-temperature annealing led to the decrease of potential window. The post-annealing process also resulted in an increase of the

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background current density of the N-DLC/Ti electrodes. Fig. 5c shows the CV variation of the N-DLC/Ti electrodes in 50 mM K3Fe(CN)6 in the 0.5M Na2SO4 solution depending on the post-annealing temperature. The Fe(CN)63− oxidation peak and the Fe(CN)64− reduction peak of the films showed obvious shifting, and the intensity of the peak increased

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dramatically as the post-annealing temperature was increased. The reduced ΔEp (306 mV to

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89 mV) at a higher post-annealing temperature suggested a more reversible reaction and these

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values were much lower than those of previously reported results [32]. The increased current

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density at a higher post-annealing temperature indicated that the electrochemical activity and catalytic abilities of the N-DLC/Ti electrodes for the redox of Fe(CN)63-/Fe(CN)64- were

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remarkably enhanced by post-annealing at high temperatures owing to the remarkable reduction of the film resistivity by the post-annealing process. In summary, with post-

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annealing at a high temperature, the electrical conductivity, electrochemical activity and catalytic ability dramatically increased, though this also slightly weakened the

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electrochemical potential window of the N-DLC/Ti electrodes. 3.4 Lifetime performance of the N-DLC/Ti electrodes

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Lifetime performance is another important factor for film electrodes in the practical wastewater treatment applications. However, the lifetime stability of the carbon film electrodes has been rarely reported. Therefore, in this study, we further investigated the influence of the adhesion force and sp2/sp3 ratio on the lifetime performance of the N-DLC/Ti electrode by conducting CV scan in the range of 0 to 1.8 V in 0.5 M Na2SO4 solution for 200 cycles on the N-DLC/Ti electrodes surface with and without arc interlayer and the samples

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before and after post-annealing treatment [39]. After the cycling test, we examined changes in the surface morphologies and chemical compositions of the films through energy dispersive x-ray spectroscopy (EDX) analyses. Fig. 6a and b show planer-view SEM images

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of N-DLC/Ti electrodes without an interlayer before and after the cycling test, respectively. It can easily be observed that the carbon film was delaminated from the substrate and severely

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worn and that the Ti substrate appeared after the 200-cycle test (Fig. 6b). These results were

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also confirmed by the corresponding EDX results. It was considered that the N-DLC/Ti

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electrode with high inter-stress and poor adhesion force was hard to withstand the high potential applied onto the electrode surface, leading to delamination. After adding the Ti+TiC

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arc interlayer, no obvious delamination or corrosion failure could be observed on the surfaces of the N-DLC/Ti electrodes after the cycling test, as shown in Fig. 6c and d, which indicated

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that the enhanced adhesion force led to greater electrochemical cycling stability. Fig. 6e and f show the surface morphology of the annealed N-DLC/Ti electrode before and after the

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cycling test, respectively. Unlike the film without an interlayer (Fig. 6b), more uniform corrosion and reduction of the film thickness were observed on the surface of the annealed N-

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DLC/Ti electrode after the cycling test, without direct exposure of the Ti substrate. After high-temperature post-annealing, graphitization of the N-DLC film occurred and the number of sp3 carbon bonds in the film decreased, indicating greater chemical stability than that of sp2 carbon bonds. Therefore, the increased sp2/sp3 ratio of the DLC film led to a decrease of the cycling performance.

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4. Conclusion DLC/Ti electrodes were synthesized by a PECVD technique. An arc interlayer, nitrogen doping, and a post-annealing process were adopted to enhance the electrochemical properties

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of the DLC/Ti electrodes. The results indicated that the introduction of the arc interlayer enhanced the adhesion force of DLC film on Ti substrates and lifetime cycling of the DLC/Ti

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electrodes. The arc droplets enlarged the surface area of the DLC films, which enhanced the

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electrochemical activity of the DLC/Ti electrodes. By doping with nitrogen, the sp2/sp3 ratio

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of the DLC films exhibited an obvious increase; thus, the electrical conductivity and electrochemical activity of the N-DLC/Ti electrodes were dramatically increased. After high-

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temperature annealing, graphitization of the N-DLC film occurred and the sp2/sp3 ratio of NDLC film increased considerably, leading to significantly decreased resistivity and enhanced

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electrochemical activity of the N-DLC/Ti electrodes, whereas the potential window and lifetime cycling of the N-DLC/Ti electrodes were weakened. There is a conflict that higher sp2 content benefits for electrical conductivity and electrochemical activity, which also

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weakens the lifetime performance, while higher sp3 content benefits for the stability and

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lifetime performance, but weakens the electrical conductivity and electrochemical activity. Therefore, optimized doping parameter and post-annealing temperature are required to achieve good electrical and electrochemical properties without sacrificing too much lifetime performance.

Acknowledgement

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This work was supported by the Global Frontier R&D Program on Center for Hybrid

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Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning.

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References [1] C. Maddi, F. Bourquard, T. Tite, A.S. Loir, C. Donnet, F. Garrelie, V. Barnier, K. Wolski, P. Fortgang, N. Zehani, M. Braiek, F. Lagarde, C. Chaix, N. Jaffrezic-Renault, T.C. Rojas, J.C.

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Sanchez-Lopez, Structure, electrochemical properties and functionalization of amorphous CN films deposited by femtosecond pulsed laser ablation, Diam. Relat. Mater. 65 (2016) 17-25.

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[2] A.P. Zeng, V.F. Neto, J.J. Gracio, Q.H. Fan, Diamond-like carbon (DLC) films as

SC

electrochemical electrodes, Diam. Relat. Mater. 43 (2014) 12-22.

NU

[3] K. Bewilogua, D. Hofmann, History of diamond-like carbon films - From first experiments to worldwide applications, Surf. Coat. Techol. 242 (2014) 214-225.

MA

[4] J. Vetter, 60 years of DLC coatings: Historical highlights and technical review of cathodic arc processes to synthesize various DLC types, and their evolution for industrial applications,

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D

Surf. Coat. Technol. 257 (2014) 213-240.

[5] S. Kang, H.P. Lim, K. Lee, Effects of TiCN interlayer on bonding characteristics and

13-16.

CE

mechanical properties of DLC-coated Ti-6Al-4V ELI alloy, Int. J. Refract. Met. H. 53 (2015)

AC

[6] F. Cemin, L.T. Bim, C.M. Menezes, M.E.H.M. da Costa, I.J.R. Baumvol, F. Alvarez, C.A. Figueroa, The influence of different silicon adhesion interlayers on the tribological behavior of DLC thin films deposited on steel by EC-PECVD, Surf. Coat. Technol. 283 (2015) 115121. [7] G.S. Wu, L.L. Sun, W. Dai, L.X. Song, A.Y. Wang, Influence of interlayers on corrosion resistance of diamond-like carbon coating on magnesium alloy, Surf. Coat. Technol. 204

19

ACCEPTED MANUSCRIPT

(2010) 2193-2196. [8] M.S. Li, F.H. Wang, Effects of nitrogen partial pressure and pulse bias voltage on (Ti,AI)N coatings by arc ion plating, Surf. Coat. Technol. 167 (2003) 197-202.

PT

[9] S. Creasey, D.B. Lewis, I.J. Smith, W.D. Munz, SEM image analysis of droplet formation during metal ion etching by a steered are discharge, Surf. Coat. Technol. 97 (1997) 163-175.

RI

[10] F.L. Jia, C.F. Yu, Z.H. Ai, L.Z. Zhang, Fabrication of nanoporous gold film electrodes

SC

with ultrahigh surface area and electrochemical activity, Chem. Mater. 19 (2007) 3648-3653.

NU

[11] B. Pandey, D. Das, A.K. Kar, Electrical and magnetic properties of electrodeposited nickel incorporated diamond-like carbon thin films, Appl. Surf. Sci. 337 (2015) 195-207.

MA

[12] J.K. Lee, J.H. Lee, B.K. Kim, W.Y. Yoon, Electrochemical Characteristics of DiamondLike Carbon/Cr Double-Layer Coating on Silicon Monoxide-Graphite Composite Anode for

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Li-Ion Batteries, Electrochim. Acta 127 (2014) 1-6. [13] J.C. Pu, S.F. Wang, C.L. Lin, J.C. Sung, Characterization of boron-doped diamond-like

CE

carbon prepared by radio frequency sputtering, Thin Solid Films 519 (2010) 521-526. [14] M. Guerino, M. Massi, H.S. Maciel, C. Otani, R.D. Mansano, The effects of the nitrogen

AC

on the electrical and structural properties of the diamond-like carbon (DLC) films, Microelectr. J. 34 (2003) 639-641. [15] S.R.P. Silva, J. Robertson, G.A.J. Amaratunga, B. Rafferty, L.M. Brown, J. Schwan, D.F. Franceschini, G. Mariotto, Nitrogen modification of hydrogenated amorphous carbon films, J. Appl. Phys. 81 (1997) 2626-2634. [16] C.H. Su, C.R. Lin, C.Y. Chang, H.C. Hung, T.Y. Lin, Mechanical and optical properties

20

ACCEPTED MANUSCRIPT

of diamond-like carbon thin films deposited by low temperature process, Thin Solid Films 498 (2006) 220-223. [17] H. Takikawa, H. Tanoue, Review of cathodic arc deposition for preparing droplet-free

PT

thin films, Ieee T. Plasma. Sci. 35 (2007) 992-999. [18] H. Chhina, S. Campbell, O. Kesler, High surface area synthesis, electrochemical activity,

SC

membrane fuel cells, J. Power Sources 179 (2008) 50-59.

RI

and stability of tungsten carbide supported Pt during oxygen reduction in proton exchange

NU

[19] J. Robertson, Diamond-like amorphous carbon, Mat. Sci. Eng. R 37 (2002) 129-281. [20] T.F. Zhang, K.W. Kim, K.H. Kim, Nitrogen-Incorporated Hydrogenated Amorphous

MA

Carbon Film Electrodes on Ti Substrates by Hybrid Deposition Technique and Annealing, Journal of the Electrochem. Soc. 163 (2016) E54-E61.

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[21] S. Waidmann, M. Knupfer, J. Fink, B. Kleinsorge, J. Robertson, Electronic structure studies of undoped and nitrogen-doped tetrahedral amorphous carbon using high-resolution

CE

electron energy-loss spectroscopy, J. App.l Phys. 89 (2001) 3783-3792. [22] Y.L. Pei, Y. Luan, Surface modification of NiTi alloys using nitrogen doped diamond-

AC

like carbon coating fabricated by plasma immersion ion implantation and deposition, J. Alloy Compd. 581 (2013) 873-876. [23] K. Honda, H. Naragino, Y. Shimai, Control of Electric Conductivity and Electrochemical Activity of Hydrogenated Amorphous Carbon by Incorporating Boron Atoms, J. Electrochem. Soc. 161 (2014) B207-B215. [24] A.G. Shard, J.D. Whittle, A.J. Beck, P.N. Brookes, N.A. Bullett, R.A. Talib, A. Mistry, D.

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ACCEPTED MANUSCRIPT

Barton, S.L. McArthur, A NEXAFS examination of unsaturation in plasma polymers of allylamine and propylamine, J. Phys. Chem. B, 108 (2004) 12472-12480. [25] J. Wang, C.H. Wang, Y.S. Liu, L.F. Cheng, W.N. Li, Q. Zhang, X.J. Yang, Microstructure

PT

and chemical bond evolution of diamond-like carbon films machined by femtosecond laser, Appl. Surf. Sci. 340 (2015) 49-55.

SC

and applications, J. Mater. chem. 21 (2011) 599-614.

RI

[26] M. Ahmad, J. Zhu, ZnO based advanced functional nanostructures: synthesis, properties

NU

[27] P. Merel, M. Tabbal, M. Chaker, S. Moisa, J. Margot, Direct evaluation of the sp3 content in diamond-like-carbon films by XPS, Appl. Surf. Sci. 136 (1998) 105-110.

MA

[28] D. Passeri, M. Rossi, J.J. Vlassak, On the tip calibration for accurate modulus measurement by contact resonance atomic force microscopy, Ultramicroscopy 128 (2013) 32-

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D

41.

[29] S.F. Yoon, X. Rusli, J. Ahn, Q. Zhang, C.Y. Yang, H. Yang, F. Watt, Deposition of

CE

polymeric nitrogenated amorphous carbon films (a-C : H : N) using electron cyclotron resonance CVD, Thin Solid Films 340 (1999) 62-67.

AC

[30] M.K. Fung, W.C. Chan, Z.Q. Gao, I. Bello, C.S. Lee, S.T. Lee, Effect of nitrogen incorporation into diamond-like carbon films by ECR-CVD, Diam. Relat. Mater. 8 (1999) 472-476. [31] S.E. Rodil, N.A. Morrison, J. Robertson, W.I. Milne, Nitrogen incorporation into tetrahedral hydrogenated amorphous carbon, Phys. Stat. Sol. A, 174 (1999) 25-37. [32] Q.K. Zhou, P.L. Ke, X.W. Li, Y.S. Zou, A.Y. Wang, Microstructure and electrochemical

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properties of nitrogen-doped DLC films deposited by PECVD technique, Appl. Surf. Sci. 329 (2015) 281-286. [33] L.X. Liu, E. Liu, Nitrogenated diamond-like carbon films for metal tracing, Surf. Coat.

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Technol. 198 (2005) 189-193. [34] E. Liu, H.W. Kwek, Electrochemical performance of diamond-like carbon thin films,

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Thin Solid Films 516 (2008) 5201-5205.

SC

[35] J.H. Kaufman, S. Metin, D.D. Saperstein, Symmetry-Breaking in Nitrogen-Doped

NU

Amorphous-Carbon - Infrared Observation of the Raman-Active G-Bands and D-Bands, Phys. Rev. B 39 (1989) 13053-13060.

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[36] A.C. Ferrari, Determination of bonding in diamond-like carbon by Raman spectroscopy, Diam. Relat. Mater. 11 (2002) 1053-1061.

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[37] X.B. Yan, T. Xu, G. Chen, S.R. Yang, H.W. Liu, Study of structure, tribological properties and growth mechanism of DLC and nitrogen-doped DLC films deposited by

CE

electrochemical technique, Appl. Surf. Sci. 236 (2004) 328-335. [38] M. Tsuchiya, K. Murakami, K. Magara, K. Nakamura, H. Ohashi, K. Tokuda, T. Takami,

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H. Ogasawara, Y. Enta, Y. Suzuki, S. Ando, H. Nakazawa, Structural and electrical properties and current-voltage characteristics of nitrogen-doped diamond-like carbon films on Si substrates by plasma-enhanced chemical vapor deposition, Jpn. J. Appl. Phys. 55 (2016). [39] A.M. Polcaro, A. Vacca, M. Mascia, S. Palmas, J.R. Ruiz, Electrochemical treatment of waters with BDD anodes: kinetics of the reactions involving chlorides, J. Appl. Electrochem. 39 (2009) 2083-2092.

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Table captions: Table 1. Deposition parameters of the N-DLC/Ti electrodes. Table 2. Thickness, nitrogen content and adhesion strength of the arc-interlayer and DLC

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films.

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Figure captions: Figure 1. Surface and cross-sectional images of (a), (b) DLC film without an interlayer, (c), (d) Ti/TiC multilayers, and (e), (f) DLC films with an interlayer. AFM images of (g) DLC

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film without an interlayer and (h) DLC films with an interlayer. Figure 2. (a) CV in a 0.5 M Na2SO4 solution and (b) CV in a 50 mM Fe(CN)64−/Fe(CN)63−

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redox couple in a 0.5 M Na2SO4 solution of DLC/Ti electrodes without and with an interlayer

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(with droplets), respectively.

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Figure 3. (a) Deposition rate of DLC films as a function of the nitrogen flow rate. (b)

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Resistivity of DLC films as a function of the nitrogen flow rate. (c) XPS survey spectra of pure DLC and N-DLC films. (d) The deconvolved C1s or N1s XPS spectra of pure DLC and

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electrodes, respectively.

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Fe(CN)64−/Fe(CN)63− redox couple in a 0.5 M Na2SO4 solution of DLC/Ti and N-DLC/Ti

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Figure 4. (a) Raman spectra of the as-deposited and annealed N-DLC films. (b) The integral

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ID/IG ratio and G peak position of the films as a function of the annealing temperature. (c) XPS survey spectra of as-deposited and annealed N-DLC films. (d) The deconvolved C1s or N1s XPS spectra of as-deposited and annealed N-DLC films, respectively. Figure 5. (a) Resistivity of the N-DLC films as a function of the post-annealing temperature. (b) CV in a 0.5 M Na2SO4 solution and (c) CV in a 50 mM Fe(CN)64−/Fe(CN)63− redox

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couple in a 0.5 M Na2SO4 solution of the N-DLC/Ti electrodes as a function of the postannealing temperature. Figure 6. Planer-view SEM images of N-DLC/Ti electrodes before (a, c, e) and after (b, d, f)

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lifetime tests of 200 cycles: (a), (b) N-DLC/Ti electrode without an arc interlayer, (c), (d) NDLC/Ti electrode with an arc interlayer and (e), (f) N-DLC/Ti electrode with an arc interlayer

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after annealing at 800 C, with the corresponding elemental contents analyzed by EDX.

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Fig. 1

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Fig. 2

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Fig. 3

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Table 1 Ti

TiC

N-DLC

Power source

Arc

Arc

DC

DC Bias Voltage (V)

- 50

- 50

- 700

Ar:C2H2:N2 gas flow (sccm)

95 : 0 : 0

95 : 15 : 0

2 : 16 : 0 ~ 8

Working pressure (torr)

8.0 × 10-3

8.0 × 10-3

5.0 × 10-2

Deposition time (min)

10

10

180

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Parameters/Films

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Table 2 Ti/TiC arc-interlayer -

Pure DLC film

N-DLC film

N-DLC film

No

No

Yes

Thickness (μm)

0.63

1.04

1.08

1.24

Nitrogen content (at. %)

0

0

3.4

3.4

Adhesion strength (N)

10.4

2.8

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Parameters/ films Ti/TiC arc-interlayer

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Highlights

N-DLC/Ti electrodes with arc interlayer were synthesized by a PECVD technique.

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Arc interlayer lead to enhanced adhesion and electrochemical property of DLC.

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N doping lead to reduced resistivity and improved electrochemical activity of DLC.

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Annealing lead to significant improved conductivity and electrochemical activity.

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Lifetime cycling of N-DLC/Ti electrodes was improved by introducing arc-interlayer.

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