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PECVD process
Accepted Manuscript Title: Synthesis and Electrochemical Properties of Ti-doped DLC Films by a Hybrid PVD/PECVD Process Authors: Yeong Ju Jo, Teng Fei Zhang, Myoung Jun Son, Kwang Ho Kim PII: DOI: Reference:
S0169-4332(17)33102-1 https://doi.org/10.1016/j.apsusc.2017.10.151 APSUSC 37494
To appear in:
APSUSC
Received date: Revised date: Accepted date:
8-8-2017 12-10-2017 22-10-2017
Please cite this article as: Yeong Ju Jo, Teng Fei Zhang, Myoung Jun Son, Kwang Ho Kim, Synthesis and Electrochemical Properties of Tidoped DLC Films by a Hybrid PVD/PECVD Process, Applied Surface Science https://doi.org/10.1016/j.apsusc.2017.10.151 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and Electrochemical Properties of Ti-doped DLC Films by a Hybrid PVD/PECVD Process
Yeong Ju Jo1,2,†, Teng Fei Zhang2,3,†, Myoung Jun Son1,2 and Kwang Ho Kim1, 2, 3,*
1
School of Materials Science and Engineering, Pusan National University,
Busan 609-735, South Korea 2
Global Frontier R&D Center for Hybrid Interface Materials, Pusan National University,
Busan 609-735, South Korea 3
National Core Research Center for Hybrid Materials Solution, Pusan National University, Busan 609-735,
South Korea †
These two authors contributed equally to this work
*Corresponding author: E-mail: [email protected] Graphical abstract
1
Highlights
Ti-DLC films with various Ti content were synthesized a hybrid PVD/PECVD process.
Ti doping led to increased surface roughness and sp2/sp3 ratio of the DLC films.
Ti atoms existed amorphous-phase Ti carbide at low Ti doping ratio
Electrical and electrochemical properties of the films were improved by Ti doping.
The adhesion strength of the DLC films was also improved by Ti doping.
Abstract: Low electrical conductivity and poor adhesion to metallic substrates are the main drawbacks of diamond-like carbon (DLC) films when used in electrode applications. In this study, Ti-doped DLC films with various Ti contents were synthesized on metal Ti substrates by a hybrid PVD/PECVD process, where PECVD was used for deposition of DLC films and PVD was used for Ti doping. The effects of the Ti doping ratio on the microstructure, adhesion strength, and electrical and electrochemical properties of the DLC films were systematically investigated. An increase in the Ti content led to increased surface roughness and a higher sp2/sp3 ratio of the Ti-DLC films. Ti atoms existed as amorphous-phase Ti carbide when the Ti doping ratio was less than 2.8 at. %, while the nanocrystalline TiC phase was formed in DLC films when the Ti doping ratio was exceeded 4.0 at. %. The adhesion strength, electrical resistivity, electrochemical activity and reversibility of the DLC films were greatly improved by Ti doping. The influence of Ti doping ratio on the electrical and electrochemical properties of the DLC films were also investigated and the best performance was obtained at a Ti content 2
of 2.8 at. %. Keywords: Ti-doped DLC; electrical resistivity; electrochemical property; TEM; hybrid PVD/PECVD
1. Introduction In recent years, the electrochemistry of diamond-like carbon (DLC) films has drawn a remarkable amount of attention due to the unique properties of these films, such as high chemical inertness and a relatively wide electrochemical potential window, which make them promising candidates as new electrode materials for environmental and electroanalytical applications, especially in relation to waste-water treatment [1-3]. As compared to traditional boron-doped diamond (BDD) film electrodes, one great merit of DLC films is that it is readily synthesizable on various substrate materials at low temperatures (even at room temperature), which is beneficial for large-scale and low-cost industrial production processes [4]. However, there remain drawbacks to resolve before DLC films can be used as electrode materials, such as their high electrical resistivity, high internal stress, and poor adhesion on metallic substrates [5-7]. Doping various elements such as N and B into DLC films is an effective means of resolving these limitations, as these techniques can significantly improve the electrical conductivity and adhesion strength of DLC films. Thus far, a considerable amount of research has focused on N-doped and B-doped DLC film electrodes, showing that greatly enhanced electrode performance can be achieved via the doping of N and B atoms [8, 9], however, the electrochemical properties of metal-doped DLC films are rarely reported. On the other hand, it is very important to choose a proper substrate on which the carbon 3
films are coated for electrochemical electrodes in the real industrial applications. Ti is a desirable candidate of substrate because it is inexpensive, highly conductive and much more mechanically strong, as compared to the conductive Si wafer, which is not ideal for industrial applications due to its fragility [10]. Therefore, Ti plates were chosen to be used as the substrates for DLC film deposition in this study. Meanwhile, Ti element was also selected as the dopant in DLC films to improve the electrical conductivity and poor adhesion to Ti substrates. In this study, Ti-DLC films with various Ti contents were synthesized on Ti substrates. The effects of the Ti doping content on the microstructure, adhesion strength, and electrical and electrochemical properties of the DLC films were systematically investigated. A hybrid PVD/PECVD technique was utilized to deposit the Ti-DLC films, where PECVD was used for deposition of DLC films and PVD was used for Ti doping. This hybrid process facilitates both the advantages of the PECVD technique, such as a low deposition temperature and uniform and large-scale deposition, and those of the PVD technique, in this case various options of doping sources from solid targets, making it promising technique for the industrial mass production of doped DLC film electrodes.
2. Experimental details 2.1. Deposition of the Ti-DLC films The Ti-DLC films were deposited on Si (100) and Ti substrates at 300 ºC by a hybrid PVD/DC-PECVD process where a DC bias was applied to the substrate holder to generate glow discharge plasma for DLC deposition and a HiPIMS power source was applied to the Ti target (99.99 %) for doping with Ti. A schematic diagram of the deposition system is shown in 4
Fig. 1. All of the substrates were ultrasonically cleaned in acetone and alcohol for 15 min each, and before the deposition step all were additionally cleaned by Ar+ ion bombardment for 10 min. The base pressure of the vacuum chamber was lower than 5 × 10-5 torr. During the deposition process, a Ti interlayer was initially deposited onto the substrates by the HiPIMS power source in an Ar atmosphere. Pure DLC film was deposited in an Ar and C2H2 gas mixture at 5×10-2 torr in a single DC-PECVD process with a DC bias voltage of - 500 V. The Ti-DLC film was deposited in an Ar and C2H2 gas mixture at 1×10-2 torr in a hybrid PVD/PECVD process, where a DC bias voltage of - 500 V was applied to the substrate holder while the Ti target power was fixed at 1 kW by HiPIMS. The Ti content in the DLC films was altered by changing the Ar/C2H2 gas flow ratio from 90/6 to 90/14 (sccm). Details of the deposition condition of the films are summarized in Table 1.
2.2. Characterizations of the Ti-DLC films Scanning electron microscopy (SEM, Hitachi, S-4800) was used to analyzer the morphology of the Ti-DLC films. The microstructure and crystallinity of the Ti-DLC films were investigated by transmission electron microscopy (HR-TEM, TALOS F200X, USA). High-resolution dispersive Raman spectroscopy (Horiba Jobin Yvon, France) was used to characterize the hybridization variations of the carbon atoms in the Ti-DLC films. The chemical composition and bonding status of the Ti-DLC films were analyzed by X-ray photoelectron spectroscopy (XPS, K-ALPHA+XPS System, Thermo Fischer Scientific) with monochromatic Al Kα (1486.6 eV) as the excitation source at a spot size of 400 mm (in diameter) with a lowenergy electron and low-energy ion gun for charge compensation. The contamination layer on 5
the surface of each sample was etched by an Ar+ ion beam with energy of 4 keV for 180 s. All spectra obtained were calibrated through compensation with the spectra of adventitious carbon with a (C1s) core level peak at 284.6 eV as a reference. The electrical resistivity of the Ti-DLC films was calculated by the van der Pauw method using a Hall effect measurement system (HMS 3000, Ecopia). The ohmic contacts consisted of indium metal on the four edges of each side of the square-shaped films. The adhesion strength of the DLC films was tested by a scratch tester (JLST022, J&L Tech Co., Ltd.) using a Rockwell C indenter. During the scratch measurements, the load was gradually increased from 1 N to 20 N with a scratch length of 10 mm. The typical three-dimensional (3D) surface morphology and root-mean square (RMS) surface roughness of the Ti-DLC films were determined using an atomic force microscope (AFM, MFP-3D, Asylum Research).
2.3. Evaluation of electrochemical properties of Ti-DLC films The electrochemical properties of the Ti-DLC films on Ti substrates (Ti-DLC/Ti) with different Ti contents were evaluated by measuring cyclic voltammograms (CV) at a scan rate of 20 mV/s using a potentiostat (ZIVE SP2, WonATech Co., Ltd.) with a Pt counter electrode and a saturated KCl-Ag/AgCl (SSE) reference electrode. A 0.5 M Na2SO4 solution was used as the electrolyte to measure the potential window at which the oxygen and hydrogen evolution occurred, and a solution of 50 mM K3Fe(CN)6/K4Fe(CN)6 mixed into the 0.5 M Na2SO4 solution was used to evaluate the electrochemical activity.
3. Results and discussion 6
3.1. Composition and surface morphology Fig. 2 shows the Ti content in the DLC films as a function of the Ar partial ratio. The Ti content in the Ti-DLC films increased from 2.2 to 5.8 at. % as the Ar partial ratio increased from 0.87 to 0.94. In the DC-PECVD process used to deposit the DLC films, C2H2 gas was ionized by glow discharge plasma in an Ar + C2H2 gas mixture and deposited onto the substrates to form films, while in the hybrid PVD/DC-PECVD process, some of the Ar ions were attracted to the cathodic Ti target to generate sputtered plasma. As the Ar partial ratio increased, the plasma density of the sputtering process was enhanced, and more Ti ions were sputtered out and doped into the DLC films, leading to an increase of the Ti content in the films. The Ti doping effect on the morphology of the DLC films was investigated by SEM and AFM. Figs. 3 (a) and (c) show cross-sectional SEM images of pure DLC film and Ti-DLC film (2.8 at. %) samples, respectively, where both films exhibit a dense, uniform and glass-like structure. Figs. 3 (b) and (d) correspondingly show the 3D AFM morphologies and RMS roughness levels of the DLC and Ti-DLC films. It is clearly indicated that after Ti doping, the RMS roughness exhibited an obvious increase from 5.027 nm (pure DLC film) to 9.181 nm (Ti-DLC film).
3.2. Hybridization, bonding status, and phase structure Analyzing the Raman spectra is a very effective means of investigating the detailed bonding structure and domain sizes of carbon-based films. Fig. 4 (a) shows the Raman spectra of the pure DLC and Ti-DLC films with various Ti doping ratios. The Raman spectra were deconvoluted into three peaks with Gaussian fit. The D peak, which is associated with the breathing modes of sp2 atoms in full six-membered carbon rings, was observed at about 1360 7
cm−1 while the G peak, attributed to the bond stretching of all pairs of sp2 atoms in both rings and chains, was observed at approximately 1580 cm−1. The additional peak centered at around 1200 cm−1 indicates formation of trans-polyacetylene(TPA) [11-15]. The extracted Raman parameters of the pure DLC and Ti-DLC films such as the ID/IG (intensity of the D peak/intensity of the G peak) ratio, position and FWHM of the G peak were given in Table 2. The ID/IG ratio of the Ti-DLC films increased from 0.78 to 0.98 as the Ti content increased from 0 to 5.8 at. %. Such an increase in the ID/IG ratio of DLC films indicates an increase of the sp2-bonded clusters and graphitic domains in the amorphous carbon films. In addition, the shifting of G peak position to higher wave numbers and the decrease in the FWHM of G peak also indicated the size increase of sp2-rich cluster [10, 16]. It has been reported that the sp2 bonding ratio was increased as the content of the metal used, such as V or Cu, increased in the DLC films [17, 18], which is consistent with the Raman results of our study. In order to identify the atomic bonding evolution as a function of the Ti doping ratio, the Ti-DLC films were characterized by XPS. Fig. 5 (a) shows the XPS survey spectra of pure DLC film and Ti-DLC films with Ti contents of 2.8 and 5.8 at. %, respectively. It can be seen that the peaks corresponding to C1s and O1s appear in all samples. The small peaks of O1s can be attributed to impurities which were introduced into the films during deposition step or to surface contamination which arose during the exposure of the films to the ambient air before the analysis. Ti2p and Ti2s peaks were observed in Ti-DLC films, while no obvious Ti-related peaks were detected in the pure DLC film. In order to gain insight into the bonding state of the pure DLC and Ti-DLC films, the C1s and Ti2p peaks from XPS spectra were investigated further by Voigt-function fitting after subtraction of the inelastic background. As shown in 8
Figs. 5 (b) and (c), the C1s spectra of Ti-DLC films with Ti contents of 2.8 at. % and 5.8 at. % were deconvoluted into four sub-peaks at 282.5, 284.5, 285.3 and 288.4 eV, which were assigned to the C-Ti, sp2 C-C, sp3 C-C and C=O bonds, respectively [10, 19-22]. The C-Ti bond peak intensity increased as the Ti content was increased in the DLC films. In Figs. 5 (d) and (e), the Ti2p spectra of Ti-DLC films with Ti contents of 2.8 at. % and 5.8 at. % were deconvoluted into six sub-peaks corresponding to TiC, Ti2+ (TiO), Ti3+ (Ti2O3) in Ti2p1/2 and Ti2p3/2, which were located at 461 eV (TiC2p1/2), 455 eV (TiC2p3/2), 461.7 eV (Ti2+2p1/2), 456.0 eV (Ti2+2p3/2), 463.1 eV (Ti3+2p1/2) and 457.4 eV (Ti3+2p3/2), respectively [23-26]. The intensity of the TiC2p1/2 and TiC2p3/2 peaks increased significantly as the Ti content in the DLC films was increased. Given these results, it can be inferred that the Ti atoms exist mainly in the forms of Ti-C and Ti-O bonds in the DLC films. The ratio of sp2 and sp3 (sp2/sp3) in the films was reflected by the peak area ratio of sp2 and sp3 [27]. As shown in Fig. 5 (f), the sp2/sp3 ratio increased significantly from 1.1 to 2.1 as the Ti content was increased from 0 to 5.8 at. %, which is consistent with the Raman results. From both the Raman and XPS analyses, it can be concluded that the incorporation of Ti causes graphitization in DLC films. It was also considered that the Ti atoms within the DLC matrix act as catalysts during the formation of the sp2 sites; the embedded Ti carbide nanoparticles are surrounded by a sp2-rich boundary which is in contact with the amorphous carbon matrix [28, 29]. The transformation characteristics of the microstructure and phase structure of the Ti-DLC film with various Ti contents were analyzed further by high-resolution TEM (HR-TEM). Fig. 6 shows cross-sectional HR-TEM images of pure DLC and Ti-DLC films with Ti contents of 2.8, 4.0 and 5.8 at. %, respectively, with their corresponding fast Fourier transform (FFT) 9
images inserted. A typical amorphous phase structure was observed in the pure DLC film and in the Ti-DLC film (2.8 at. %), as confirmed by the HR-TEM and FFT images in Figs. 6 (a) and (b), respectively. On the other hand, a small amount of nanocrystalline TiC phase was observed in the Ti-DLC film (4.0 at. %), as shown in Fig. 6 (c). After the Ti doping ratio rising to a level of 5.8 at. % in Fig. 6 (d), the size and amount of TiC nanocrystals were increased in comparison with those of the Ti-DLC film (4.0 at. %). The TiC nanocrystal surrounded by an amorphous matrix were observed in the DLC films, with the (002) plane was identified from the crystalline d value (d value=0.215 nm, JCPDS#02-0943) and from FFT images. It can be inferred that the Ti atoms exist as amorphous-phase Ti carbides when doping with a small amount of Ti, while an increase of the Ti doping ratio to more than 4.0 at. % leads to the formation of the nanocrystalline TiC phase in the DLC films.
3.3. Electrical and electrochemical properties Electrical resistivity and electrochemical tests were conducted to investigate the performance capabilities of the Ti-DLC films as an electrode material. Fig. 7 (a) shows the electrical resistivity levels of the Ti-DLC films with different Ti contents. The electrical resistivity of the pure DLC film was relatively high (~ 3.6 × 102 Ω·cm). The electrical resistivity of the Ti-DLC films decreased sharply to ~ 1.6 × 10-3 Ω·cm after a small amount Ti doping of 2.8 at. %, while a slight increase of the electrical resistivity to ~ 1.9 × 10-3 Ω·cm in the Ti-DLC films was observed with a further increase of the Ti content to 5.8 at. %. A schematic illustration of the Ti doping mechanism is shown in Fig. 7 (a). The electronic transport model was active during thermally activated conduction along the linkages or chains 10
of sp2 carbon atoms with a variable range and with variable orientation hopping during the process, following percolation theory [30, 31]. According to this electronic transport model, the significant decrease of the electrical resistivity of the Ti-DLC films was considered to have stemmed from the formation of conduction pathways with an increase in the sp2 bonding ratio and with more Ti bonds in the amorphous state (at a low Ti doping content). Specifically, graphitization of the DLC films after Ti doping led to a decrease in the electrical resistivity, and the incorporated Ti atoms existing in the amorphous-phase Ti carbide also acted as good electron donors in the DLC films to reduce the resistivity. On the other hand, the slight increase of the electrical resistivity of the Ti-DLC films with a greater Ti contents exceeding 2.8 at. % is attributable to the formation of TiC nanocrystals, which cannot act as effective dopants and which have relatively high electrical resistivity in the range of 0.3 - 0.8 Ω·cm [32]. The significant reduction of the electrical resistivity of the Ti-DLC films by doping with a small amount of Ti made it possible to utilize this film as an electrode material for applications in the electrochemical field, which typically require a large current density. Fig. 7 (b) shows the CV curves of pure DLC/Ti and Ti-DLC/Ti electrodes in a 0.5 M Na2SO4 solution at the scan rate of 20 mV/s as a function of the Ti content. It was observed that the over-potentials of pure DLC/Ti and Ti-DLC/Ti electrodes for hydrogen evolution were similar and that the background current was not significantly changed, whereas the oxygen evolution potential shifted slightly toward the negative direction as the Ti content was increased. The electrochemical activity, catalytic ability and reversibility were evaluated by an analysis of the redox couples of Fe(CN)63-/4-. Fig. 7 (c) shows the CV curves of pure DLC/Ti and TiDLC/Ti electrodes in 50 mM of ferri/ferro cyanide in a 0.5 M Na2SO4 solution at a scan rate of 11
20 mV/s as a function of the Ti content. The Fe(CN)63- oxidation and Fe(CN)64- reduction peak potential separation (△Ep) of the pure DLC/Ti electrode was about 1089 mV, and the ratio of the anodic and cathodic peak currents (Ip,a/Ip,c, where Ip,a is the anodic peak current and Ip,c is the cathodic peak current) was 1.18. The values of △Ep and Ip,a/Ip,c of the Ti-DLC/Ti electrodes were in the ranges of 265 to 573 mV and 1.02 to 1.05, respectively. For reversible n-electron couples (n: the number of electrons exchanged) controlled by diffusive mass transfer in cyclic voltammetry, the ideal values of △Ep and Ip,a/Ip,c are 59/n mV (at 25 ºC in this study, n=1) and 1, respectively [6, 33]. The △Ep and Ip,a/Ip,c values of the Ti-DLC/Ti electrode were closer to the theoretical values of a reversible system, which suggested that there was a more reversible electrode reaction of Fe(CN)63-/Fe(CN)64- on the surfaces of the Ti-DLC/Ti electrodes in comparison with that of the pure DLC/Ti electrode. The Ip,a and electrochemical potential window values of the films are shown in Fig. 7 (d). It can be seen that the electrochemical potential window of the Ti-DLC/Ti electrodes decreased slightly from ~ 3.4 V to ~ 3 V as the Ti content of DLC films was increased from 0 to 5.8 at. %, which can be attributed to the reduction of the sp3 content in the films. The Ip,a value of the TiDLC/Ti electrodes increased gradually as the Ti content was increased to 2.8 at. %, while a further increase of the Ti content to 5.8 at. % led to a slight decrease in the Ip,a value. Higher Ip,a values of the Ti-DLC/Ti electrodes, as compared to those of a pure DLC/Ti electrode, indicate that doping with Ti led to improved electrochemical activity and catalytic ability levels of the Ti-DLC/Ti electrodes [6]. It was considered that there were three main contributions to the improved electrochemical properties of the DLC/Ti electrodes after being doped with Ti: (1) the electrical 12
resistivity of the DLC films was significantly reduced by doping with Ti, which led to a higher response current density of the electrodes; (2) the increased surface roughness, specifically the surface area, of the Ti-DLC films, also contributed to the increased response current density; and (3) the incorporation of Ti in the amorphous-phase bonding state activated the surfaces of the DLC/Ti electrodes, and the active C-Ti functional group enhanced the electrochemical reaction. In summary, with a small amount of Ti doping (2.8 at. %), the electrical conductivity, electrochemical activity and catalytic ability of DLC film electrodes were dramatically increased without greatly sacrificing the electrochemical potential window.
3.4. Adhesion strength The peeling off of film is one of the main causes of the failure of film electrodes in electrochemical wastewater treatment applications. Therefore, the adhesion strength is an important criterion to consider when fabricating DLC electrodes. To estimate the Ti doping effect on the adhesion strength of DLC films on Ti substrates, scratch tests were conducted. Fig. 8 shows the typical friction-load curves of the pure DLC and Ti-DLC films with optical micrographs of the scratch tracks as measured by a scratch tester. The Lc1 (critical load at which a fine crack occurs) values of the samples were difficult to determine, whereas the Lc2 (critical load at which peeling off starts, defined as the adhesion strength) values were clearly identified. It was easily observed that the adhesion strength of the DLC films was significantly improved by Ti doping and that a higher Ti doping ratio led to enhanced adhesion strength of Ti-DLC films.
13
4. Conclusion In this study, Ti-DLC films with Ti contents ranging from 0 to 5.8 at. % were synthesized on Ti substrates by a hybrid PVD/PECVD process. The effects of the Ti doping content on the microstructure, adhesion strength, and electrical and electrochemical properties of the DLC films were systematically investigated. The main results are summarized below. (1) As the Ti content was increased, the surface roughness and sp2/sp3 ratio of the Ti-DLC film increased. (2) Ti atoms existed as amorphous-phase Ti carbides at a low Ti doping ratio (< 2.8 at. %), while a further increase of the Ti doping ratio (> 4.0 at. %) led to the formation of the nanocrystalline TiC phase in the DLC films. (3) The electrical resistivity of the DLC films was significantly reduced to ~ 10-3 Ω·cm by Ti doping due to graphitization, and the Ti atoms existing in the amorphous-phase Ti carbide acted as good electron donors in the DLC films. (4) The electrochemical activity and reversibility of the DLC films were significantly improved by Ti doping, and the best performance was obtained at a Ti content of 2.8 at. %. (5) Ti atoms existed in the form of amorphous-phase Ti carbides in the DLC films enhanced the electrical and electrochemical properties of the films. (6) The adhesion strength of the DLC films was also significantly improved by Ti doping.
Acknowledgments This work was supported by the Global Frontier R&D Program (2013M3A6B1078874) of the Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning.
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plasma evaporation, Surf. Coat. Tech. 202 (2008) 5350-5355. [26] A.A. El Mel, B. Angleraud, E. Gautron, A. Granier, P.Y. Tessier, XPS study of the surface composition modification of nc-TiC/C nanocomposite films under in situ argon ion bombardment, Thin Solid Films 519 (2011) 3982-3985. [27] P. Merel, M. Tabbal, M. Chaker, S. Moisa, J. Margot, Direct evaluation of the sp(3) content in diamond-like-carbon films by XPS, Appl. Surf. Sci. 136 (1998) 105-110. [28] C. Corbella, E. Bertran, M.C. Polo, E. Pascual, J.L. Andujar, Structural effects of nanocomposite films of amorphous carbon and metal deposited by pulsed-DC reactive magnetron sputtering, Diam. Relat. Mater. 16 (2007) 1828-1834. [29] K. Bewilogua, R. Wittorf, H. Thomsen, M. Weber, DLC based coatings prepared by reactive d.c. magnetron sputtering, Thin Solid Films 447 (2004) 142-147. [30] A. Grill, Electrical and optical properties of diamond-like carbon, Thin Solid Films, 355 (1999) 189-193. [31] R. Sanjines, M.D. Abad, C. Vaju, R. Smajda, M. Mionic, A. Magrez, Electrical properties and applications of carbon based nanocomposite materials: An overview, Surf. Coat. Tech. 206 (2011) 727-733. [32] W.A. James F Shackelford, Materials Science and Engineering Handbook, CRC Press LLC, USA, 2001. [33] L.R.F. Allen J. Bard, Electrochemical Methods: Fundamentals and Applications, JOHN WILEY & SONS, New York, 2001.
18
Figure captions: Fig. 1. Schematic diagram of the hybrid PVD/PECVD system.
19
Fig. 2. Ti content of Ti-DLC films as a function of the Ar partial ratio.
20
Fig. 3. Cross-sectional SEM images, 3D AFM morphologies, and RMS roughness levels of the pure DLC and Ti-DLC films.
21
Fig. 4. Raman spectra of the pure DLC and Ti-DLC films.
22
Fig. 5. (a) XPS survey spectra of pure DLC and Ti-DLC films, the deconvolved C1s XPS spectra of Ti-DLC films with Ti contents of (b) 2.8 at. % and (c) 5.8 at. %, the deconvolved Ti2p XPS spectra of Ti-DLC films with Ti contents of (d) 2.8 at. % and (e) 5.8 at. %, and (f) the sp2/sp3 ratio of the Ti-DLC films as a function of the Ti content.
23
Fig. 6. HR-TEM images with corresponding FFT images of (a) pure DLC film and Ti-DLC films with Ti contents of (b) 2.8 at. %, (c) 4.0 at. % and (d) 5.8 at. %.
24
Fig. 7. (a) Resistivity of the Ti-DLC films as a function of the Ti content, (b) CV curves of a pure DLC/Ti electrode and Ti-DLC/Ti electrodes in a 0.5 M Na2SO4 solution, (c) CV curves of a 50 mM Fe(CN)63-/4- redox couple in a 0.5M Na2SO4 solution on a pure DLC/Ti electrode and on Ti-DLC/Ti electrodes, and (d) potential window and peak current density of pure DLC/Ti and Ti-DLC electrodes as a function of the Ti content.
25
Fig. 8. Scratch curves and their corresponding optical scratch track images of (a) pure DLC film, and Ti-DLC films with Ti contents of (b) 2.8 at. % and (c) 5.8 at. %.
26
Table captions: Table 1. Deposition conditions of the DLC Films. Table 1. Parameters
Ti interlayer
Pure DLC
Ti-DLC
Power source
HiPIMS
DC (Bias)
HiPIMS (Ti target), DC (Bias)
HiPIMS power (kW)
1
0
1
Ar : C2H2 gas flow (sccm)
95 : 0
10 : 85
90 : 6 - 14
Working pressure (torr)
7×10
5×10
1×10
DC bias voltage (V)
0
- 500
- 500
Deposition time (min)
15
300
30
-3
-2
-2
Table 2. The G peak position, FWHM of the G peak, and ID/IG ratio of the pure DLC and TiDLC films. Table 2. DLC sample
Position (G) (cm-1)
FWHM (G) (cm-1)
ID/IG ratio
Pure DLC
1571.60
133.10
0.78
Ti-DLC (2.2 at.%)
1571.99
123.49
0.88
Ti-DLC (2.8 at.%)
1572.14
120.11
0.91
Ti-DLC (4.0 at.%)
1572.32
119.98
0.95
Ti-DLC (5.8 at.%)
1575.02
114.44
0.98
27
S0169-4332(17)33102-1 https://doi.org/10.1016/j.apsusc.2017.10.151 APSUSC 37494
To appear in:
APSUSC
Received date: Revised date: Accepted date:
8-8-2017 12-10-2017 22-10-2017
Please cite this article as: Yeong Ju Jo, Teng Fei Zhang, Myoung Jun Son, Kwang Ho Kim, Synthesis and Electrochemical Properties of Tidoped DLC Films by a Hybrid PVD/PECVD Process, Applied Surface Science https://doi.org/10.1016/j.apsusc.2017.10.151 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and Electrochemical Properties of Ti-doped DLC Films by a Hybrid PVD/PECVD Process
Yeong Ju Jo1,2,†, Teng Fei Zhang2,3,†, Myoung Jun Son1,2 and Kwang Ho Kim1, 2, 3,*
1
School of Materials Science and Engineering, Pusan National University,
Busan 609-735, South Korea 2
Global Frontier R&D Center for Hybrid Interface Materials, Pusan National University,
Busan 609-735, South Korea 3
National Core Research Center for Hybrid Materials Solution, Pusan National University, Busan 609-735,
South Korea †
These two authors contributed equally to this work
*Corresponding author: E-mail: [email protected] Graphical abstract
1
Highlights
Ti-DLC films with various Ti content were synthesized a hybrid PVD/PECVD process.
Ti doping led to increased surface roughness and sp2/sp3 ratio of the DLC films.
Ti atoms existed amorphous-phase Ti carbide at low Ti doping ratio
Electrical and electrochemical properties of the films were improved by Ti doping.
The adhesion strength of the DLC films was also improved by Ti doping.
Abstract: Low electrical conductivity and poor adhesion to metallic substrates are the main drawbacks of diamond-like carbon (DLC) films when used in electrode applications. In this study, Ti-doped DLC films with various Ti contents were synthesized on metal Ti substrates by a hybrid PVD/PECVD process, where PECVD was used for deposition of DLC films and PVD was used for Ti doping. The effects of the Ti doping ratio on the microstructure, adhesion strength, and electrical and electrochemical properties of the DLC films were systematically investigated. An increase in the Ti content led to increased surface roughness and a higher sp2/sp3 ratio of the Ti-DLC films. Ti atoms existed as amorphous-phase Ti carbide when the Ti doping ratio was less than 2.8 at. %, while the nanocrystalline TiC phase was formed in DLC films when the Ti doping ratio was exceeded 4.0 at. %. The adhesion strength, electrical resistivity, electrochemical activity and reversibility of the DLC films were greatly improved by Ti doping. The influence of Ti doping ratio on the electrical and electrochemical properties of the DLC films were also investigated and the best performance was obtained at a Ti content 2
of 2.8 at. %. Keywords: Ti-doped DLC; electrical resistivity; electrochemical property; TEM; hybrid PVD/PECVD
1. Introduction In recent years, the electrochemistry of diamond-like carbon (DLC) films has drawn a remarkable amount of attention due to the unique properties of these films, such as high chemical inertness and a relatively wide electrochemical potential window, which make them promising candidates as new electrode materials for environmental and electroanalytical applications, especially in relation to waste-water treatment [1-3]. As compared to traditional boron-doped diamond (BDD) film electrodes, one great merit of DLC films is that it is readily synthesizable on various substrate materials at low temperatures (even at room temperature), which is beneficial for large-scale and low-cost industrial production processes [4]. However, there remain drawbacks to resolve before DLC films can be used as electrode materials, such as their high electrical resistivity, high internal stress, and poor adhesion on metallic substrates [5-7]. Doping various elements such as N and B into DLC films is an effective means of resolving these limitations, as these techniques can significantly improve the electrical conductivity and adhesion strength of DLC films. Thus far, a considerable amount of research has focused on N-doped and B-doped DLC film electrodes, showing that greatly enhanced electrode performance can be achieved via the doping of N and B atoms [8, 9], however, the electrochemical properties of metal-doped DLC films are rarely reported. On the other hand, it is very important to choose a proper substrate on which the carbon 3
films are coated for electrochemical electrodes in the real industrial applications. Ti is a desirable candidate of substrate because it is inexpensive, highly conductive and much more mechanically strong, as compared to the conductive Si wafer, which is not ideal for industrial applications due to its fragility [10]. Therefore, Ti plates were chosen to be used as the substrates for DLC film deposition in this study. Meanwhile, Ti element was also selected as the dopant in DLC films to improve the electrical conductivity and poor adhesion to Ti substrates. In this study, Ti-DLC films with various Ti contents were synthesized on Ti substrates. The effects of the Ti doping content on the microstructure, adhesion strength, and electrical and electrochemical properties of the DLC films were systematically investigated. A hybrid PVD/PECVD technique was utilized to deposit the Ti-DLC films, where PECVD was used for deposition of DLC films and PVD was used for Ti doping. This hybrid process facilitates both the advantages of the PECVD technique, such as a low deposition temperature and uniform and large-scale deposition, and those of the PVD technique, in this case various options of doping sources from solid targets, making it promising technique for the industrial mass production of doped DLC film electrodes.
2. Experimental details 2.1. Deposition of the Ti-DLC films The Ti-DLC films were deposited on Si (100) and Ti substrates at 300 ºC by a hybrid PVD/DC-PECVD process where a DC bias was applied to the substrate holder to generate glow discharge plasma for DLC deposition and a HiPIMS power source was applied to the Ti target (99.99 %) for doping with Ti. A schematic diagram of the deposition system is shown in 4
Fig. 1. All of the substrates were ultrasonically cleaned in acetone and alcohol for 15 min each, and before the deposition step all were additionally cleaned by Ar+ ion bombardment for 10 min. The base pressure of the vacuum chamber was lower than 5 × 10-5 torr. During the deposition process, a Ti interlayer was initially deposited onto the substrates by the HiPIMS power source in an Ar atmosphere. Pure DLC film was deposited in an Ar and C2H2 gas mixture at 5×10-2 torr in a single DC-PECVD process with a DC bias voltage of - 500 V. The Ti-DLC film was deposited in an Ar and C2H2 gas mixture at 1×10-2 torr in a hybrid PVD/PECVD process, where a DC bias voltage of - 500 V was applied to the substrate holder while the Ti target power was fixed at 1 kW by HiPIMS. The Ti content in the DLC films was altered by changing the Ar/C2H2 gas flow ratio from 90/6 to 90/14 (sccm). Details of the deposition condition of the films are summarized in Table 1.
2.2. Characterizations of the Ti-DLC films Scanning electron microscopy (SEM, Hitachi, S-4800) was used to analyzer the morphology of the Ti-DLC films. The microstructure and crystallinity of the Ti-DLC films were investigated by transmission electron microscopy (HR-TEM, TALOS F200X, USA). High-resolution dispersive Raman spectroscopy (Horiba Jobin Yvon, France) was used to characterize the hybridization variations of the carbon atoms in the Ti-DLC films. The chemical composition and bonding status of the Ti-DLC films were analyzed by X-ray photoelectron spectroscopy (XPS, K-ALPHA+XPS System, Thermo Fischer Scientific) with monochromatic Al Kα (1486.6 eV) as the excitation source at a spot size of 400 mm (in diameter) with a lowenergy electron and low-energy ion gun for charge compensation. The contamination layer on 5
the surface of each sample was etched by an Ar+ ion beam with energy of 4 keV for 180 s. All spectra obtained were calibrated through compensation with the spectra of adventitious carbon with a (C1s) core level peak at 284.6 eV as a reference. The electrical resistivity of the Ti-DLC films was calculated by the van der Pauw method using a Hall effect measurement system (HMS 3000, Ecopia). The ohmic contacts consisted of indium metal on the four edges of each side of the square-shaped films. The adhesion strength of the DLC films was tested by a scratch tester (JLST022, J&L Tech Co., Ltd.) using a Rockwell C indenter. During the scratch measurements, the load was gradually increased from 1 N to 20 N with a scratch length of 10 mm. The typical three-dimensional (3D) surface morphology and root-mean square (RMS) surface roughness of the Ti-DLC films were determined using an atomic force microscope (AFM, MFP-3D, Asylum Research).
2.3. Evaluation of electrochemical properties of Ti-DLC films The electrochemical properties of the Ti-DLC films on Ti substrates (Ti-DLC/Ti) with different Ti contents were evaluated by measuring cyclic voltammograms (CV) at a scan rate of 20 mV/s using a potentiostat (ZIVE SP2, WonATech Co., Ltd.) with a Pt counter electrode and a saturated KCl-Ag/AgCl (SSE) reference electrode. A 0.5 M Na2SO4 solution was used as the electrolyte to measure the potential window at which the oxygen and hydrogen evolution occurred, and a solution of 50 mM K3Fe(CN)6/K4Fe(CN)6 mixed into the 0.5 M Na2SO4 solution was used to evaluate the electrochemical activity.
3. Results and discussion 6
3.1. Composition and surface morphology Fig. 2 shows the Ti content in the DLC films as a function of the Ar partial ratio. The Ti content in the Ti-DLC films increased from 2.2 to 5.8 at. % as the Ar partial ratio increased from 0.87 to 0.94. In the DC-PECVD process used to deposit the DLC films, C2H2 gas was ionized by glow discharge plasma in an Ar + C2H2 gas mixture and deposited onto the substrates to form films, while in the hybrid PVD/DC-PECVD process, some of the Ar ions were attracted to the cathodic Ti target to generate sputtered plasma. As the Ar partial ratio increased, the plasma density of the sputtering process was enhanced, and more Ti ions were sputtered out and doped into the DLC films, leading to an increase of the Ti content in the films. The Ti doping effect on the morphology of the DLC films was investigated by SEM and AFM. Figs. 3 (a) and (c) show cross-sectional SEM images of pure DLC film and Ti-DLC film (2.8 at. %) samples, respectively, where both films exhibit a dense, uniform and glass-like structure. Figs. 3 (b) and (d) correspondingly show the 3D AFM morphologies and RMS roughness levels of the DLC and Ti-DLC films. It is clearly indicated that after Ti doping, the RMS roughness exhibited an obvious increase from 5.027 nm (pure DLC film) to 9.181 nm (Ti-DLC film).
3.2. Hybridization, bonding status, and phase structure Analyzing the Raman spectra is a very effective means of investigating the detailed bonding structure and domain sizes of carbon-based films. Fig. 4 (a) shows the Raman spectra of the pure DLC and Ti-DLC films with various Ti doping ratios. The Raman spectra were deconvoluted into three peaks with Gaussian fit. The D peak, which is associated with the breathing modes of sp2 atoms in full six-membered carbon rings, was observed at about 1360 7
cm−1 while the G peak, attributed to the bond stretching of all pairs of sp2 atoms in both rings and chains, was observed at approximately 1580 cm−1. The additional peak centered at around 1200 cm−1 indicates formation of trans-polyacetylene(TPA) [11-15]. The extracted Raman parameters of the pure DLC and Ti-DLC films such as the ID/IG (intensity of the D peak/intensity of the G peak) ratio, position and FWHM of the G peak were given in Table 2. The ID/IG ratio of the Ti-DLC films increased from 0.78 to 0.98 as the Ti content increased from 0 to 5.8 at. %. Such an increase in the ID/IG ratio of DLC films indicates an increase of the sp2-bonded clusters and graphitic domains in the amorphous carbon films. In addition, the shifting of G peak position to higher wave numbers and the decrease in the FWHM of G peak also indicated the size increase of sp2-rich cluster [10, 16]. It has been reported that the sp2 bonding ratio was increased as the content of the metal used, such as V or Cu, increased in the DLC films [17, 18], which is consistent with the Raman results of our study. In order to identify the atomic bonding evolution as a function of the Ti doping ratio, the Ti-DLC films were characterized by XPS. Fig. 5 (a) shows the XPS survey spectra of pure DLC film and Ti-DLC films with Ti contents of 2.8 and 5.8 at. %, respectively. It can be seen that the peaks corresponding to C1s and O1s appear in all samples. The small peaks of O1s can be attributed to impurities which were introduced into the films during deposition step or to surface contamination which arose during the exposure of the films to the ambient air before the analysis. Ti2p and Ti2s peaks were observed in Ti-DLC films, while no obvious Ti-related peaks were detected in the pure DLC film. In order to gain insight into the bonding state of the pure DLC and Ti-DLC films, the C1s and Ti2p peaks from XPS spectra were investigated further by Voigt-function fitting after subtraction of the inelastic background. As shown in 8
Figs. 5 (b) and (c), the C1s spectra of Ti-DLC films with Ti contents of 2.8 at. % and 5.8 at. % were deconvoluted into four sub-peaks at 282.5, 284.5, 285.3 and 288.4 eV, which were assigned to the C-Ti, sp2 C-C, sp3 C-C and C=O bonds, respectively [10, 19-22]. The C-Ti bond peak intensity increased as the Ti content was increased in the DLC films. In Figs. 5 (d) and (e), the Ti2p spectra of Ti-DLC films with Ti contents of 2.8 at. % and 5.8 at. % were deconvoluted into six sub-peaks corresponding to TiC, Ti2+ (TiO), Ti3+ (Ti2O3) in Ti2p1/2 and Ti2p3/2, which were located at 461 eV (TiC2p1/2), 455 eV (TiC2p3/2), 461.7 eV (Ti2+2p1/2), 456.0 eV (Ti2+2p3/2), 463.1 eV (Ti3+2p1/2) and 457.4 eV (Ti3+2p3/2), respectively [23-26]. The intensity of the TiC2p1/2 and TiC2p3/2 peaks increased significantly as the Ti content in the DLC films was increased. Given these results, it can be inferred that the Ti atoms exist mainly in the forms of Ti-C and Ti-O bonds in the DLC films. The ratio of sp2 and sp3 (sp2/sp3) in the films was reflected by the peak area ratio of sp2 and sp3 [27]. As shown in Fig. 5 (f), the sp2/sp3 ratio increased significantly from 1.1 to 2.1 as the Ti content was increased from 0 to 5.8 at. %, which is consistent with the Raman results. From both the Raman and XPS analyses, it can be concluded that the incorporation of Ti causes graphitization in DLC films. It was also considered that the Ti atoms within the DLC matrix act as catalysts during the formation of the sp2 sites; the embedded Ti carbide nanoparticles are surrounded by a sp2-rich boundary which is in contact with the amorphous carbon matrix [28, 29]. The transformation characteristics of the microstructure and phase structure of the Ti-DLC film with various Ti contents were analyzed further by high-resolution TEM (HR-TEM). Fig. 6 shows cross-sectional HR-TEM images of pure DLC and Ti-DLC films with Ti contents of 2.8, 4.0 and 5.8 at. %, respectively, with their corresponding fast Fourier transform (FFT) 9
images inserted. A typical amorphous phase structure was observed in the pure DLC film and in the Ti-DLC film (2.8 at. %), as confirmed by the HR-TEM and FFT images in Figs. 6 (a) and (b), respectively. On the other hand, a small amount of nanocrystalline TiC phase was observed in the Ti-DLC film (4.0 at. %), as shown in Fig. 6 (c). After the Ti doping ratio rising to a level of 5.8 at. % in Fig. 6 (d), the size and amount of TiC nanocrystals were increased in comparison with those of the Ti-DLC film (4.0 at. %). The TiC nanocrystal surrounded by an amorphous matrix were observed in the DLC films, with the (002) plane was identified from the crystalline d value (d value=0.215 nm, JCPDS#02-0943) and from FFT images. It can be inferred that the Ti atoms exist as amorphous-phase Ti carbides when doping with a small amount of Ti, while an increase of the Ti doping ratio to more than 4.0 at. % leads to the formation of the nanocrystalline TiC phase in the DLC films.
3.3. Electrical and electrochemical properties Electrical resistivity and electrochemical tests were conducted to investigate the performance capabilities of the Ti-DLC films as an electrode material. Fig. 7 (a) shows the electrical resistivity levels of the Ti-DLC films with different Ti contents. The electrical resistivity of the pure DLC film was relatively high (~ 3.6 × 102 Ω·cm). The electrical resistivity of the Ti-DLC films decreased sharply to ~ 1.6 × 10-3 Ω·cm after a small amount Ti doping of 2.8 at. %, while a slight increase of the electrical resistivity to ~ 1.9 × 10-3 Ω·cm in the Ti-DLC films was observed with a further increase of the Ti content to 5.8 at. %. A schematic illustration of the Ti doping mechanism is shown in Fig. 7 (a). The electronic transport model was active during thermally activated conduction along the linkages or chains 10
of sp2 carbon atoms with a variable range and with variable orientation hopping during the process, following percolation theory [30, 31]. According to this electronic transport model, the significant decrease of the electrical resistivity of the Ti-DLC films was considered to have stemmed from the formation of conduction pathways with an increase in the sp2 bonding ratio and with more Ti bonds in the amorphous state (at a low Ti doping content). Specifically, graphitization of the DLC films after Ti doping led to a decrease in the electrical resistivity, and the incorporated Ti atoms existing in the amorphous-phase Ti carbide also acted as good electron donors in the DLC films to reduce the resistivity. On the other hand, the slight increase of the electrical resistivity of the Ti-DLC films with a greater Ti contents exceeding 2.8 at. % is attributable to the formation of TiC nanocrystals, which cannot act as effective dopants and which have relatively high electrical resistivity in the range of 0.3 - 0.8 Ω·cm [32]. The significant reduction of the electrical resistivity of the Ti-DLC films by doping with a small amount of Ti made it possible to utilize this film as an electrode material for applications in the electrochemical field, which typically require a large current density. Fig. 7 (b) shows the CV curves of pure DLC/Ti and Ti-DLC/Ti electrodes in a 0.5 M Na2SO4 solution at the scan rate of 20 mV/s as a function of the Ti content. It was observed that the over-potentials of pure DLC/Ti and Ti-DLC/Ti electrodes for hydrogen evolution were similar and that the background current was not significantly changed, whereas the oxygen evolution potential shifted slightly toward the negative direction as the Ti content was increased. The electrochemical activity, catalytic ability and reversibility were evaluated by an analysis of the redox couples of Fe(CN)63-/4-. Fig. 7 (c) shows the CV curves of pure DLC/Ti and TiDLC/Ti electrodes in 50 mM of ferri/ferro cyanide in a 0.5 M Na2SO4 solution at a scan rate of 11
20 mV/s as a function of the Ti content. The Fe(CN)63- oxidation and Fe(CN)64- reduction peak potential separation (△Ep) of the pure DLC/Ti electrode was about 1089 mV, and the ratio of the anodic and cathodic peak currents (Ip,a/Ip,c, where Ip,a is the anodic peak current and Ip,c is the cathodic peak current) was 1.18. The values of △Ep and Ip,a/Ip,c of the Ti-DLC/Ti electrodes were in the ranges of 265 to 573 mV and 1.02 to 1.05, respectively. For reversible n-electron couples (n: the number of electrons exchanged) controlled by diffusive mass transfer in cyclic voltammetry, the ideal values of △Ep and Ip,a/Ip,c are 59/n mV (at 25 ºC in this study, n=1) and 1, respectively [6, 33]. The △Ep and Ip,a/Ip,c values of the Ti-DLC/Ti electrode were closer to the theoretical values of a reversible system, which suggested that there was a more reversible electrode reaction of Fe(CN)63-/Fe(CN)64- on the surfaces of the Ti-DLC/Ti electrodes in comparison with that of the pure DLC/Ti electrode. The Ip,a and electrochemical potential window values of the films are shown in Fig. 7 (d). It can be seen that the electrochemical potential window of the Ti-DLC/Ti electrodes decreased slightly from ~ 3.4 V to ~ 3 V as the Ti content of DLC films was increased from 0 to 5.8 at. %, which can be attributed to the reduction of the sp3 content in the films. The Ip,a value of the TiDLC/Ti electrodes increased gradually as the Ti content was increased to 2.8 at. %, while a further increase of the Ti content to 5.8 at. % led to a slight decrease in the Ip,a value. Higher Ip,a values of the Ti-DLC/Ti electrodes, as compared to those of a pure DLC/Ti electrode, indicate that doping with Ti led to improved electrochemical activity and catalytic ability levels of the Ti-DLC/Ti electrodes [6]. It was considered that there were three main contributions to the improved electrochemical properties of the DLC/Ti electrodes after being doped with Ti: (1) the electrical 12
resistivity of the DLC films was significantly reduced by doping with Ti, which led to a higher response current density of the electrodes; (2) the increased surface roughness, specifically the surface area, of the Ti-DLC films, also contributed to the increased response current density; and (3) the incorporation of Ti in the amorphous-phase bonding state activated the surfaces of the DLC/Ti electrodes, and the active C-Ti functional group enhanced the electrochemical reaction. In summary, with a small amount of Ti doping (2.8 at. %), the electrical conductivity, electrochemical activity and catalytic ability of DLC film electrodes were dramatically increased without greatly sacrificing the electrochemical potential window.
3.4. Adhesion strength The peeling off of film is one of the main causes of the failure of film electrodes in electrochemical wastewater treatment applications. Therefore, the adhesion strength is an important criterion to consider when fabricating DLC electrodes. To estimate the Ti doping effect on the adhesion strength of DLC films on Ti substrates, scratch tests were conducted. Fig. 8 shows the typical friction-load curves of the pure DLC and Ti-DLC films with optical micrographs of the scratch tracks as measured by a scratch tester. The Lc1 (critical load at which a fine crack occurs) values of the samples were difficult to determine, whereas the Lc2 (critical load at which peeling off starts, defined as the adhesion strength) values were clearly identified. It was easily observed that the adhesion strength of the DLC films was significantly improved by Ti doping and that a higher Ti doping ratio led to enhanced adhesion strength of Ti-DLC films.
13
4. Conclusion In this study, Ti-DLC films with Ti contents ranging from 0 to 5.8 at. % were synthesized on Ti substrates by a hybrid PVD/PECVD process. The effects of the Ti doping content on the microstructure, adhesion strength, and electrical and electrochemical properties of the DLC films were systematically investigated. The main results are summarized below. (1) As the Ti content was increased, the surface roughness and sp2/sp3 ratio of the Ti-DLC film increased. (2) Ti atoms existed as amorphous-phase Ti carbides at a low Ti doping ratio (< 2.8 at. %), while a further increase of the Ti doping ratio (> 4.0 at. %) led to the formation of the nanocrystalline TiC phase in the DLC films. (3) The electrical resistivity of the DLC films was significantly reduced to ~ 10-3 Ω·cm by Ti doping due to graphitization, and the Ti atoms existing in the amorphous-phase Ti carbide acted as good electron donors in the DLC films. (4) The electrochemical activity and reversibility of the DLC films were significantly improved by Ti doping, and the best performance was obtained at a Ti content of 2.8 at. %. (5) Ti atoms existed in the form of amorphous-phase Ti carbides in the DLC films enhanced the electrical and electrochemical properties of the films. (6) The adhesion strength of the DLC films was also significantly improved by Ti doping.
Acknowledgments This work was supported by the Global Frontier R&D Program (2013M3A6B1078874) of the Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning.
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Figure captions: Fig. 1. Schematic diagram of the hybrid PVD/PECVD system.
19
Fig. 2. Ti content of Ti-DLC films as a function of the Ar partial ratio.
20
Fig. 3. Cross-sectional SEM images, 3D AFM morphologies, and RMS roughness levels of the pure DLC and Ti-DLC films.
21
Fig. 4. Raman spectra of the pure DLC and Ti-DLC films.
22
Fig. 5. (a) XPS survey spectra of pure DLC and Ti-DLC films, the deconvolved C1s XPS spectra of Ti-DLC films with Ti contents of (b) 2.8 at. % and (c) 5.8 at. %, the deconvolved Ti2p XPS spectra of Ti-DLC films with Ti contents of (d) 2.8 at. % and (e) 5.8 at. %, and (f) the sp2/sp3 ratio of the Ti-DLC films as a function of the Ti content.
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Fig. 6. HR-TEM images with corresponding FFT images of (a) pure DLC film and Ti-DLC films with Ti contents of (b) 2.8 at. %, (c) 4.0 at. % and (d) 5.8 at. %.
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Fig. 7. (a) Resistivity of the Ti-DLC films as a function of the Ti content, (b) CV curves of a pure DLC/Ti electrode and Ti-DLC/Ti electrodes in a 0.5 M Na2SO4 solution, (c) CV curves of a 50 mM Fe(CN)63-/4- redox couple in a 0.5M Na2SO4 solution on a pure DLC/Ti electrode and on Ti-DLC/Ti electrodes, and (d) potential window and peak current density of pure DLC/Ti and Ti-DLC electrodes as a function of the Ti content.
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Fig. 8. Scratch curves and their corresponding optical scratch track images of (a) pure DLC film, and Ti-DLC films with Ti contents of (b) 2.8 at. % and (c) 5.8 at. %.
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Table captions: Table 1. Deposition conditions of the DLC Films. Table 1. Parameters
Ti interlayer
Pure DLC
Ti-DLC
Power source
HiPIMS
DC (Bias)
HiPIMS (Ti target), DC (Bias)
HiPIMS power (kW)
1
0
1
Ar : C2H2 gas flow (sccm)
95 : 0
10 : 85
90 : 6 - 14
Working pressure (torr)
7×10
5×10
1×10
DC bias voltage (V)
0
- 500
- 500
Deposition time (min)
15
300
30
-3
-2
-2
Table 2. The G peak position, FWHM of the G peak, and ID/IG ratio of the pure DLC and TiDLC films. Table 2. DLC sample
Position (G) (cm-1)
FWHM (G) (cm-1)
ID/IG ratio
Pure DLC
1571.60
133.10
0.78
Ti-DLC (2.2 at.%)
1571.99
123.49
0.88
Ti-DLC (2.8 at.%)
1572.14
120.11
0.91
Ti-DLC (4.0 at.%)
1572.32
119.98
0.95
Ti-DLC (5.8 at.%)
1575.02
114.44
0.98
27