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Gradient layers of boron-doped diamond on titanium substrates
Diamond & Related Materials 16 (2007) 899 – 904 www.elsevier.com/locate/diamond
Gradient layers of boron-doped diamond on titanium substrates I. Gerger, R. Haubner ⁎ University of Technology Vienna, Institute of Chemical Technologies and Analytics, Getreidemarkt 9/164-CT, A-1060 Vienna, Austria Available online 27 January 2007
Abstract For the deposition of well-adhesive, low-doped diamond layers on titanium substrates a gradient layer is designed. At first a highly borondoped diamond layer is deposited, which shows good adhesion to the titanium substrate, followed by a low-boron-doped diamond layer on the surface. The boron-doped diamond layers were deposited on titanium stretch metal substrates by the hot-filament CVD method. It is shown that with increasing boron content during diamond deposition above 6000 ppm B/C the intermediate Ti(C,B) layers becomes very thin and so at high-boron concentrations no problem with layer adhesion occurs. These Ti(C,B)-layers formed during diamond deposition were investigated by standard metallographic preparations. To form a diamond gradient layer on the highly boron-doped diamond the boron content was reduced and a lowdoped diamond layer was deposited. Electrochemical cyclic voltammetric measurements show that the lower boron contents at the diamond surface provide better electrochemical properties. These layers show extraordinary electrochemical properties in respect of the gained hydrogen and oxygen overvoltage. © 2007 Elsevier B.V. All rights reserved. Keywords: Diamond growth and charcterization; Boron doping; Electrochemistry; Gradient layer
1. Introduction In recent years there has been increasing interest in the electrochemical properties of boron-doped diamond (BDD) coated substrates [1–4]. For the most part BDD has been deposited on silicon substrates, however, the use of other metal discs or meshes as substrate materials is highly desirable for electrochemical applications because of the enhanced electrical conductivity and the increased substrate area. Titanium especially turns out to be a quite promising material for this application as it also shows other interesting properties like corrosion resistance due to a passivation by forming a titanium oxide layer. In previous papers it has been shown that diamond can be deposited on titanium substrates [5,6]. However, the diamond deposition on titanium is accompanied by various problems. On the one hand titanium is an active carbide and hydride forming element and therefore reacts with the used reaction
⁎ Corresponding author. E-mail address: [email protected] (R. Haubner). 0925-9635/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2007.01.023
gases under diamond deposition conditions by forming intermediate layers [7,8]. The formed TiC-layer is of a rough and porous nature and tends to split off easily, which causes severe adhesion problems of the diamond layers on these substrates. On the other hand the diamond nucleation on titanium is delayed due to the former described competing reaction of carbide formation. Only after the saturation of the surface when the carbide layer has reached a certain thickness that the carbon atoms form clusters and diamond can nucleate [7,9]. A maximum for nucleating diamond crystals is found at a boron concentration of 6000 ppm B/C and increases with higher temperature [10]. After diamond nucleation the individual crystals grow to form clusters and finally diamond layers [11]. The knowledge about the nature and thickness of the intermediate layers formed on the substrate during diamond deposition helped to optimise the layer adhesion [6]. In this paper boron-doped diamond gradient layers were designed. Before the real deposition of 16 h and low boron concentrations a pre-treatment was carried out at quite high boron concentrations. The influence of several deposition
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Table 1 Deposition parameters of diamond gradient layers by HF-CVD Filament temperature (Tfil) [°C] Substrate temperature (Tsub) [°C] Filament to substrate distance [mm] Total gas flow rate [sccm] Methane concentration CH4/H2 [%] B/C ratio [ppm] pre-treatment B/C ratio [ppm] deposition Pre-treatment [h] Deposition time [h] Deposition pressure [mbar]
2200 850 20 400 0.5 6000 500 1; 3; 5 16 20
parameters on the morphology and electrochemical properties of these layers are discussed. 2. Experimental details Boron-doped diamond gradient layers were synthesised by the hot-filament chemical vapor deposition technique (HFCVD) on titanium stretch metal substrates. The deposition experiments were carried out in a CVD reactor, which allows double side diamond deposition, as described in [5]. Typical deposition parameters are shown in Table 1. The amounts of dopants specified refer to their concentrations in the gas phase. As a boron source triethylboron (B(C2H5)3, vapor pressure at − 10 °C: 9.74 mbar) was used, whereby a supporting H2 gas flow was saturated with B(C2H5)3 and added to the reaction gas mixture (pressure in the evaporator: 1466 mbar; condenser temperature: − 10 °C). As substrates CP (commercially pure) titanium stretch metal (30 × 20 mm) were used, which were placed between two tantalum filaments. The pre-treatment of the titanium stretch metal included the ultrasonic seeding in a suspension of 0.25 μm diamond powder in acetone for 20 min. The diamond growth rates were calculated from the weight differences of the substrates before and after deposition. Diamond layer morphologies were determined by scanning electron microscopy (SEM) and Raman Spectroscopy was used to measure the film quality and phase purity. Cyclic voltammetric analyses were carried out in an airproof one-compartment glass cell using a three electrodes set-up. The
diamond sample has been used as a working electrode, a saturated mercury sulfate Hg/Hg2SO4 (SMSE — + 640 mV vs. standard hydrogen electrode (SHE)) as a reference and platinum as a counter electrode. In all the experiments the used equipment consists of an Agilent function generator (Agilent 33120A) and an Agilent Data Acquisition/Switch Unit (Agilent 34970A), which are connected to a Jaissle potentiostat–galvanostat IMP88PC. For electrical contacting a stainless steel conductor was attached to the diamond coated titanium meshes. The geometric area of the electrode was estimated to be 2 cm2. For the determination of the electrochemical potential windows different electrolytes (0.1 N H2SO4 and 6 N NaOH) were used. Cyclo-voltammograms were taken before and after an anodic treatment at a constant potential of +1.8 V vs. SMSE for 30 min to get stable conditions. For decomposition experiments phenol was used as a test component and the electrochemical decomposition of a 20 mmol/L phenol-solution was carried out at different potentialls (+ 1.85 V and +2.5 V vs. GMSE). As matrix solutions (100 ml) H2SO4-acid solution (pH 1.6) and NaOH-alkaline solution (pH 12) were used. All measurements were made at room temperature (23 ± 2 °C) and the solutions were stirred during the electrochemical experiments. The solutions were analysed by HPLC (high performance liquid chromatography) (Hewlett Packard Agilent 1100 with UV/vis — (UV adsorption: 210 nm) and MS detector) before and after the decomposition at the different potentials. The mobile phase was a methanol/water solvent gradient with ammonium acetate as buffer. 3. Results and discussion 3.1. The concept of boron-doped diamond gradient layers By cyclic voltammetric measurements it was shown [6,10] that the boron doping level has a great influence on the electrochemical properties of the diamond layers. Best results in regard to the size of their electrochemical working potential window were achieved at quite low boron contents. However, these low-doped layers show enormous problems in regard to
Fig. 1. Intermediate layers of the Ti-substrate formed during diamond deposition (a) and TiC-layer thickness in dependency with the B-concentration during deposition (b) [6].
I. Gerger, R. Haubner / Diamond & Related Materials 16 (2007) 899–904
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Fig. 2. SEM pictures of a gradient layer diamond film morphology from different surfaces of a Ti stretch metal (Tfil: 2200 °C, Tsub: 850 °C, H2 400 sccm, 0.5% CH4, 6000 ppm B/C, 15 mm, 20 mbar, 16 h).
layer adhesion as part of the layer or even the whole layer splits off the substrate. This problem is caused by the formation of intermediate layers during diamond deposition on titanium substrates. In a previous paper [6] the influence of various deposition parameters on these intermediate layers on the one hand and the impact of those formed layers on the diamond deposition on the other hand were discussed. The formed intermediate layers are clearly revealed in Fig. 1a. In the core a significant coarsening of the α-titanium can be observed, which is (4) martensitic titanium formed during the cooling process. Adjacent (3) the fine grained microstructure of Ti-hydride needles is found. Beneath the diamond layer a more or less thick (2) TiC/TiB/Ti (C,B) layer has been formed depending on the boron content. In Fig. 1a a pronounced intermediate layer of about 70 μm is found due to the deposition without boron doping. Boron shows positive influence on the diamond deposition also in regard to layer adhesion. With increasing boron content a decrease in the thickness of the TiC-intermediate layer is observed (Fig. 1b). It is assumed that a higher boron concentration in the gas phase impedes the further diffusion of C-atoms into the titanium by forming TixBy or Ti(C,B) compounds. At the surface (1) the deposited diamond layer can be seen (Fig. 1a). To get well-adhesive and continuous layers, which also show excellent electrochemical properties, so-called gradient layers were developed. By doing a pre-treatment step prior to the real deposition at low boron concentrations for 16 h this bad adhesion problem can be avoided. As already discussed and shown the TiC formation can be diminished by high boron
concentrations. That is why a pre-treatment step with a quite high boron concentration (6000 ppm B/C) is carried out. The boron concentration in the layer proceeds from high values at the interface to low concentrations at the surface. Different durations (1, 3 and 5 h) of this pre-treatment step were investigated. 3.2. Boron-doped diamond gradient layer deposition on titanium stretch metal In this research Ti stretch metals were coated on both sides, whereas the differentiation between inner and outer (filament facing) surface is made. The top panel in Fig. 2 shows a
Fig. 3. Raman spectrum of a boron-doped diamond gradient layer on titanium stretch metal (Tfil: 2200 °C, Tsub: 850 °C, H2 400 sccm, 0.5% CH4, 500 ppm B/C, 15 mm, 20 mbar, 16 h, pre-treatment: 1 h at 6000 ppm B/C).
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Fig. 4. Morphology and weight change of boron-doped diamond layers depending on the duration of pre-treatment at 6000 ppm B/C (Tfil: 2200 °C, Tsub: 850 °C, H2 400 sccm, 0.5% CH4, 500 ppm B/C, 15 mm, 20 mbar, 16 h).
representative segment of the Ti stretch metal onto which the boron-doped diamond gradient layer was deposited. Various magnifications of the stretch metal demonstrate that also for different substrate areas a high quality diamond coating exists. There is a continuous and homogenous layer without any cracks. Concerning the morphology of the deposited diamond layers well faceted (111)-faces are found. In Raman spectra the typical 1332 cm− 1 peak for diamond is found (Fig. 3). According to the boron addition the peak is relatively small. Independent of the duration well-adhesive and continuous diamond layers were found, whereas concerning the diamond morphology best results were achieved at shorter times (Fig. 4a). Layers with 1 h pre-treatment show well formed (111)-facets, whereas with longer pre-treatment duration the layers have increased defects and Ballas-diamond depositions occur.
With longer the pre-treatment duration a higher weight increase is found (Fig. 4b). 3.3. Electrochemical properties of BDD gradient layer titanium electrodes As already mentioned boron-doped diamond layers show outstanding electrochemical properties especially in regard to their wide working potential window. By cyclic voltammetric measurements those gradient layers show electrochemical potential windows between 4.1 V and 4.4 V in 0.1 N H2SO4 and 3.5 V and 3.6 V in 6 N NaOH (Fig. 5). After an anodic treatment at a constant potential of +1.8 V vs. SMSE the potential windows have increased about 0.4 V due to a change of surface termination. In the first scan of cyclic voltammetric measurements an oxidation peak at 0.65 V vs.
Fig. 5. Electrochemical potential window of a BDD — electrode on titanium stretch metal (Tfil: 2200 °C, Tsub: 850 °C, 400 sccm H2, 0.5% CH4, 500 ppm B/C, 20 mbar, 15 mm, 16 h) with different pre-treatment durations at 6000 ppm B/C in different electrolytes.
I. Gerger, R. Haubner / Diamond & Related Materials 16 (2007) 899–904
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Fig. 6. Cyclo-voltammetric measurements of a boron-doped diamond gradient layer on titanium stretch metal in 0.1 N H2SO4 (Tfil: 2200 °C, Tsub: 850 °C, 400 sccm H2, 0.5% CH4, 500 ppm B/C, 20 mbar, 15 mm, 16 h, pre-treatment: 1 h at 6000 ppm B/C).
SMSE can be observed (Fig. 6), which does not occur in the further scans. This peak occurs as irreversible oxidation peak as no reduction peak is seen in the backward scan. Martin et al. [12] also observed such an irreversible oxidation peak at 1.4 V vs. SHE in 0.5 M H2SO4 of a H-terminated diamond surface electrode, which he also ascribes to a surface oxidation. The as grown CVD diamond layers, which have been cooled in a hydrogen atmosphere show a hydrogen saturated surface, which is probably substituted by hydroxyl groups during the cyclic voltammetric measurements. A typical cyclo-voltammogram of a boron-doped diamond gradient layer is shown in Fig. 6, which is at the same time the largest found electrochemical working potential window (4.4 V) in 0.1 H2SO4.
acid solution (pH 1.6). It could be shown that different intermediate and final products are found depending on the applied potential. Fig. 7 shows the HPLC analyses of a 20 mmol/L phenol-solution before and after the decomposition at + 1.85 V vs. SMSE. Before the decomposition a sharp phenol peak at 9.7 min. can be observed, after 5 h electrochemical treatment at a constant potential of + 1.85 V only a small phenol peak is present and further peaks can be seen. At this potential organic products like p-benzochinone and hydrochinone are formed. A 93% phenol decomposition is achieved. The electrodes were also stable after long running experiments and there was no change in morphology of the diamond layers. 4. Conclusion
3.4. Application of BDD gradient layer titanium electrodes for industrial wastewater treatment This property of large electrochemical working potential windows enables the decomposition of organic substances – even with quite high positive potentials – in industrial wastewaters. As a test substance phenol was decomposed in
It could be shown that the deposition of low-doped boron diamond layers on titanium stretch metal via gradient layers is possible. These gradient layers with a pre-treatment step of high boron concentrations lead to well-adhesive, homogenous and continuous layers, which at the same time exhibit excellent electrochemical properties. Electrochemical windows up to
Fig. 7. HPLC analyses before and after the decomposition of phenol in a sulfuric acid solution (pH 1.6) at 1.85 V vs. SMSE for 5 h with a BDD titanium electrode.
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4.4 V in acid solutions and 3.6 V in basic solutions were observed. The main reason for the good adhesion of the gradient layer is the reduced TiC formation on the substrate at high-boron concentrations. It was also shown that the application of these electrodes in industrial wastewater is possible. Electrochemical oxidation of phenol (93% decomposition) was successfully done at different potentials. These kinds of electrodes offer an interesting field of electrochemical application for the future. References [1] R. Tenne, K. Patel, K. Hashimoto, A. Fujishima, Journal of Electroanalytical Chemistry 347 (1–2) (1993) 409. [2] A. Perret, W. Haenni, N. Skinner, X.M. Xang, D. Gandini, C. Comninellis, B. Corres, G. Foti, Diamond and Related Materials 8 (1999) 820.
[3] G.M. Swain, R. Ramesham, Analytical Chemistry 65 (1993) 345. [4] F. Bouamrane, A. Tadjeddine, J.E. Butler, R. Tenne, C. Levy-Clement, Journal of Electroanalytical Chemistry 405 (1996) 95. [5] I. Gerger, R. Haubner, Powdermetallurgy World Congress and Exhibition, EPMA Shrewsbury vol. 2 (2004) 417. [6] I.Gerger, R. Haubner: 15th IFHTSE — International Federation for Heat Treatment and Surface Engineering Congress 2006, Wein; 25.-29.09.2006 pp. 633-638. [7] P.O. Joffreau, R. Haubner, B. Lux, Journal of Refractory Metals & Hard Materials 7 (4) (1988) 186. [8] R. Haubner, A. Lindlbauer, B. Lux, Refractory Metals and Hard Materials 14 (1996) 119. [9] B. Lux, R. Haubner (Eds.), Proc. of NATO ASI, 11 Ciocco, Italien, 22.7–3.8, Plenum Press, 1990. [10] I. Gerger, R. Haubner, H. Kronberger, G. Fafilek, Diamond and Related Materials 13 (4–8) (2004) 1062. [11] R. Bichler, R. Haubner, B. Lux, High Temperatures, High Pressures 21 (1989) 576. [12] H.B. Martin, J.C. Angus, U. Landau, Journal of the Electrochemical Society 146 (1999) 2959.
Gradient layers of boron-doped diamond on titanium substrates I. Gerger, R. Haubner ⁎ University of Technology Vienna, Institute of Chemical Technologies and Analytics, Getreidemarkt 9/164-CT, A-1060 Vienna, Austria Available online 27 January 2007
Abstract For the deposition of well-adhesive, low-doped diamond layers on titanium substrates a gradient layer is designed. At first a highly borondoped diamond layer is deposited, which shows good adhesion to the titanium substrate, followed by a low-boron-doped diamond layer on the surface. The boron-doped diamond layers were deposited on titanium stretch metal substrates by the hot-filament CVD method. It is shown that with increasing boron content during diamond deposition above 6000 ppm B/C the intermediate Ti(C,B) layers becomes very thin and so at high-boron concentrations no problem with layer adhesion occurs. These Ti(C,B)-layers formed during diamond deposition were investigated by standard metallographic preparations. To form a diamond gradient layer on the highly boron-doped diamond the boron content was reduced and a lowdoped diamond layer was deposited. Electrochemical cyclic voltammetric measurements show that the lower boron contents at the diamond surface provide better electrochemical properties. These layers show extraordinary electrochemical properties in respect of the gained hydrogen and oxygen overvoltage. © 2007 Elsevier B.V. All rights reserved. Keywords: Diamond growth and charcterization; Boron doping; Electrochemistry; Gradient layer
1. Introduction In recent years there has been increasing interest in the electrochemical properties of boron-doped diamond (BDD) coated substrates [1–4]. For the most part BDD has been deposited on silicon substrates, however, the use of other metal discs or meshes as substrate materials is highly desirable for electrochemical applications because of the enhanced electrical conductivity and the increased substrate area. Titanium especially turns out to be a quite promising material for this application as it also shows other interesting properties like corrosion resistance due to a passivation by forming a titanium oxide layer. In previous papers it has been shown that diamond can be deposited on titanium substrates [5,6]. However, the diamond deposition on titanium is accompanied by various problems. On the one hand titanium is an active carbide and hydride forming element and therefore reacts with the used reaction
⁎ Corresponding author. E-mail address: [email protected] (R. Haubner). 0925-9635/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2007.01.023
gases under diamond deposition conditions by forming intermediate layers [7,8]. The formed TiC-layer is of a rough and porous nature and tends to split off easily, which causes severe adhesion problems of the diamond layers on these substrates. On the other hand the diamond nucleation on titanium is delayed due to the former described competing reaction of carbide formation. Only after the saturation of the surface when the carbide layer has reached a certain thickness that the carbon atoms form clusters and diamond can nucleate [7,9]. A maximum for nucleating diamond crystals is found at a boron concentration of 6000 ppm B/C and increases with higher temperature [10]. After diamond nucleation the individual crystals grow to form clusters and finally diamond layers [11]. The knowledge about the nature and thickness of the intermediate layers formed on the substrate during diamond deposition helped to optimise the layer adhesion [6]. In this paper boron-doped diamond gradient layers were designed. Before the real deposition of 16 h and low boron concentrations a pre-treatment was carried out at quite high boron concentrations. The influence of several deposition
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Table 1 Deposition parameters of diamond gradient layers by HF-CVD Filament temperature (Tfil) [°C] Substrate temperature (Tsub) [°C] Filament to substrate distance [mm] Total gas flow rate [sccm] Methane concentration CH4/H2 [%] B/C ratio [ppm] pre-treatment B/C ratio [ppm] deposition Pre-treatment [h] Deposition time [h] Deposition pressure [mbar]
2200 850 20 400 0.5 6000 500 1; 3; 5 16 20
parameters on the morphology and electrochemical properties of these layers are discussed. 2. Experimental details Boron-doped diamond gradient layers were synthesised by the hot-filament chemical vapor deposition technique (HFCVD) on titanium stretch metal substrates. The deposition experiments were carried out in a CVD reactor, which allows double side diamond deposition, as described in [5]. Typical deposition parameters are shown in Table 1. The amounts of dopants specified refer to their concentrations in the gas phase. As a boron source triethylboron (B(C2H5)3, vapor pressure at − 10 °C: 9.74 mbar) was used, whereby a supporting H2 gas flow was saturated with B(C2H5)3 and added to the reaction gas mixture (pressure in the evaporator: 1466 mbar; condenser temperature: − 10 °C). As substrates CP (commercially pure) titanium stretch metal (30 × 20 mm) were used, which were placed between two tantalum filaments. The pre-treatment of the titanium stretch metal included the ultrasonic seeding in a suspension of 0.25 μm diamond powder in acetone for 20 min. The diamond growth rates were calculated from the weight differences of the substrates before and after deposition. Diamond layer morphologies were determined by scanning electron microscopy (SEM) and Raman Spectroscopy was used to measure the film quality and phase purity. Cyclic voltammetric analyses were carried out in an airproof one-compartment glass cell using a three electrodes set-up. The
diamond sample has been used as a working electrode, a saturated mercury sulfate Hg/Hg2SO4 (SMSE — + 640 mV vs. standard hydrogen electrode (SHE)) as a reference and platinum as a counter electrode. In all the experiments the used equipment consists of an Agilent function generator (Agilent 33120A) and an Agilent Data Acquisition/Switch Unit (Agilent 34970A), which are connected to a Jaissle potentiostat–galvanostat IMP88PC. For electrical contacting a stainless steel conductor was attached to the diamond coated titanium meshes. The geometric area of the electrode was estimated to be 2 cm2. For the determination of the electrochemical potential windows different electrolytes (0.1 N H2SO4 and 6 N NaOH) were used. Cyclo-voltammograms were taken before and after an anodic treatment at a constant potential of +1.8 V vs. SMSE for 30 min to get stable conditions. For decomposition experiments phenol was used as a test component and the electrochemical decomposition of a 20 mmol/L phenol-solution was carried out at different potentialls (+ 1.85 V and +2.5 V vs. GMSE). As matrix solutions (100 ml) H2SO4-acid solution (pH 1.6) and NaOH-alkaline solution (pH 12) were used. All measurements were made at room temperature (23 ± 2 °C) and the solutions were stirred during the electrochemical experiments. The solutions were analysed by HPLC (high performance liquid chromatography) (Hewlett Packard Agilent 1100 with UV/vis — (UV adsorption: 210 nm) and MS detector) before and after the decomposition at the different potentials. The mobile phase was a methanol/water solvent gradient with ammonium acetate as buffer. 3. Results and discussion 3.1. The concept of boron-doped diamond gradient layers By cyclic voltammetric measurements it was shown [6,10] that the boron doping level has a great influence on the electrochemical properties of the diamond layers. Best results in regard to the size of their electrochemical working potential window were achieved at quite low boron contents. However, these low-doped layers show enormous problems in regard to
Fig. 1. Intermediate layers of the Ti-substrate formed during diamond deposition (a) and TiC-layer thickness in dependency with the B-concentration during deposition (b) [6].
I. Gerger, R. Haubner / Diamond & Related Materials 16 (2007) 899–904
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Fig. 2. SEM pictures of a gradient layer diamond film morphology from different surfaces of a Ti stretch metal (Tfil: 2200 °C, Tsub: 850 °C, H2 400 sccm, 0.5% CH4, 6000 ppm B/C, 15 mm, 20 mbar, 16 h).
layer adhesion as part of the layer or even the whole layer splits off the substrate. This problem is caused by the formation of intermediate layers during diamond deposition on titanium substrates. In a previous paper [6] the influence of various deposition parameters on these intermediate layers on the one hand and the impact of those formed layers on the diamond deposition on the other hand were discussed. The formed intermediate layers are clearly revealed in Fig. 1a. In the core a significant coarsening of the α-titanium can be observed, which is (4) martensitic titanium formed during the cooling process. Adjacent (3) the fine grained microstructure of Ti-hydride needles is found. Beneath the diamond layer a more or less thick (2) TiC/TiB/Ti (C,B) layer has been formed depending on the boron content. In Fig. 1a a pronounced intermediate layer of about 70 μm is found due to the deposition without boron doping. Boron shows positive influence on the diamond deposition also in regard to layer adhesion. With increasing boron content a decrease in the thickness of the TiC-intermediate layer is observed (Fig. 1b). It is assumed that a higher boron concentration in the gas phase impedes the further diffusion of C-atoms into the titanium by forming TixBy or Ti(C,B) compounds. At the surface (1) the deposited diamond layer can be seen (Fig. 1a). To get well-adhesive and continuous layers, which also show excellent electrochemical properties, so-called gradient layers were developed. By doing a pre-treatment step prior to the real deposition at low boron concentrations for 16 h this bad adhesion problem can be avoided. As already discussed and shown the TiC formation can be diminished by high boron
concentrations. That is why a pre-treatment step with a quite high boron concentration (6000 ppm B/C) is carried out. The boron concentration in the layer proceeds from high values at the interface to low concentrations at the surface. Different durations (1, 3 and 5 h) of this pre-treatment step were investigated. 3.2. Boron-doped diamond gradient layer deposition on titanium stretch metal In this research Ti stretch metals were coated on both sides, whereas the differentiation between inner and outer (filament facing) surface is made. The top panel in Fig. 2 shows a
Fig. 3. Raman spectrum of a boron-doped diamond gradient layer on titanium stretch metal (Tfil: 2200 °C, Tsub: 850 °C, H2 400 sccm, 0.5% CH4, 500 ppm B/C, 15 mm, 20 mbar, 16 h, pre-treatment: 1 h at 6000 ppm B/C).
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I. Gerger, R. Haubner / Diamond & Related Materials 16 (2007) 899–904
Fig. 4. Morphology and weight change of boron-doped diamond layers depending on the duration of pre-treatment at 6000 ppm B/C (Tfil: 2200 °C, Tsub: 850 °C, H2 400 sccm, 0.5% CH4, 500 ppm B/C, 15 mm, 20 mbar, 16 h).
representative segment of the Ti stretch metal onto which the boron-doped diamond gradient layer was deposited. Various magnifications of the stretch metal demonstrate that also for different substrate areas a high quality diamond coating exists. There is a continuous and homogenous layer without any cracks. Concerning the morphology of the deposited diamond layers well faceted (111)-faces are found. In Raman spectra the typical 1332 cm− 1 peak for diamond is found (Fig. 3). According to the boron addition the peak is relatively small. Independent of the duration well-adhesive and continuous diamond layers were found, whereas concerning the diamond morphology best results were achieved at shorter times (Fig. 4a). Layers with 1 h pre-treatment show well formed (111)-facets, whereas with longer pre-treatment duration the layers have increased defects and Ballas-diamond depositions occur.
With longer the pre-treatment duration a higher weight increase is found (Fig. 4b). 3.3. Electrochemical properties of BDD gradient layer titanium electrodes As already mentioned boron-doped diamond layers show outstanding electrochemical properties especially in regard to their wide working potential window. By cyclic voltammetric measurements those gradient layers show electrochemical potential windows between 4.1 V and 4.4 V in 0.1 N H2SO4 and 3.5 V and 3.6 V in 6 N NaOH (Fig. 5). After an anodic treatment at a constant potential of +1.8 V vs. SMSE the potential windows have increased about 0.4 V due to a change of surface termination. In the first scan of cyclic voltammetric measurements an oxidation peak at 0.65 V vs.
Fig. 5. Electrochemical potential window of a BDD — electrode on titanium stretch metal (Tfil: 2200 °C, Tsub: 850 °C, 400 sccm H2, 0.5% CH4, 500 ppm B/C, 20 mbar, 15 mm, 16 h) with different pre-treatment durations at 6000 ppm B/C in different electrolytes.
I. Gerger, R. Haubner / Diamond & Related Materials 16 (2007) 899–904
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Fig. 6. Cyclo-voltammetric measurements of a boron-doped diamond gradient layer on titanium stretch metal in 0.1 N H2SO4 (Tfil: 2200 °C, Tsub: 850 °C, 400 sccm H2, 0.5% CH4, 500 ppm B/C, 20 mbar, 15 mm, 16 h, pre-treatment: 1 h at 6000 ppm B/C).
SMSE can be observed (Fig. 6), which does not occur in the further scans. This peak occurs as irreversible oxidation peak as no reduction peak is seen in the backward scan. Martin et al. [12] also observed such an irreversible oxidation peak at 1.4 V vs. SHE in 0.5 M H2SO4 of a H-terminated diamond surface electrode, which he also ascribes to a surface oxidation. The as grown CVD diamond layers, which have been cooled in a hydrogen atmosphere show a hydrogen saturated surface, which is probably substituted by hydroxyl groups during the cyclic voltammetric measurements. A typical cyclo-voltammogram of a boron-doped diamond gradient layer is shown in Fig. 6, which is at the same time the largest found electrochemical working potential window (4.4 V) in 0.1 H2SO4.
acid solution (pH 1.6). It could be shown that different intermediate and final products are found depending on the applied potential. Fig. 7 shows the HPLC analyses of a 20 mmol/L phenol-solution before and after the decomposition at + 1.85 V vs. SMSE. Before the decomposition a sharp phenol peak at 9.7 min. can be observed, after 5 h electrochemical treatment at a constant potential of + 1.85 V only a small phenol peak is present and further peaks can be seen. At this potential organic products like p-benzochinone and hydrochinone are formed. A 93% phenol decomposition is achieved. The electrodes were also stable after long running experiments and there was no change in morphology of the diamond layers. 4. Conclusion
3.4. Application of BDD gradient layer titanium electrodes for industrial wastewater treatment This property of large electrochemical working potential windows enables the decomposition of organic substances – even with quite high positive potentials – in industrial wastewaters. As a test substance phenol was decomposed in
It could be shown that the deposition of low-doped boron diamond layers on titanium stretch metal via gradient layers is possible. These gradient layers with a pre-treatment step of high boron concentrations lead to well-adhesive, homogenous and continuous layers, which at the same time exhibit excellent electrochemical properties. Electrochemical windows up to
Fig. 7. HPLC analyses before and after the decomposition of phenol in a sulfuric acid solution (pH 1.6) at 1.85 V vs. SMSE for 5 h with a BDD titanium electrode.
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I. Gerger, R. Haubner / Diamond & Related Materials 16 (2007) 899–904
4.4 V in acid solutions and 3.6 V in basic solutions were observed. The main reason for the good adhesion of the gradient layer is the reduced TiC formation on the substrate at high-boron concentrations. It was also shown that the application of these electrodes in industrial wastewater is possible. Electrochemical oxidation of phenol (93% decomposition) was successfully done at different potentials. These kinds of electrodes offer an interesting field of electrochemical application for the future. References [1] R. Tenne, K. Patel, K. Hashimoto, A. Fujishima, Journal of Electroanalytical Chemistry 347 (1–2) (1993) 409. [2] A. Perret, W. Haenni, N. Skinner, X.M. Xang, D. Gandini, C. Comninellis, B. Corres, G. Foti, Diamond and Related Materials 8 (1999) 820.
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