Boron doped diamond thin films on large area Ti6Al4V substrates for electrochemical application

Abstract

Boron doped diamond thin films were grown on titanium alloy substrates (Ti6Al4V) with 36 × 35 × 1.3 mm at 873-933 K at 6.5 × 10³ Pa during 8 h by hot filament CVD assisted technique. The boron source was obtained from a H2 line forced to pass through a bubbler containing B2O3 dissolved in methanol (B\C = 6000 ppm). The films were grown on both sides of perforated and non-perforated substrates. Emphasis for diamond growing on perforated substrates have been done in order to increase the active surface area and hereafter to promote an easier electrolyte flow for wastewater treatment. The electrode performance was determined by cyclic voltammetry measurements in KCl, KNO3, Na2SO4, HCl, HNO3 and H2SO4 solutions and the reversibility behavior of the Fe(CN)6(3-/4-) at the Ti6Al4V/Diamond electrode were studied. Also, Scaning Electron Microcopy and Raman Scattering Spectroscopy were used for morphology and diamond quality evaluation, respectively.

boron doped-diamond; Ti6Al4V; electrochemistry

Boron doped diamond thin films on large area Ti6Al4V substrates for electrochemical application

Alessandra V. Diniz^I,II; Neidenêi G. Ferreira^I; Evaldo J. Corat^I; Vladimir J. Trava-Airoldi^I

^I

^IIInstituto Tecnológico de Aeronáutica, ITA, São José dos Campos - SP, Brazil

^{Address to correspondence}

ABSTRACT

Boron doped diamond thin films were grown on titanium alloy substrates (Ti6Al4V) with 36 × 35 × 1.3 mm at 873-933 K at 6.5 × 10³ Pa during 8 h by hot filament CVD assisted technique. The boron source was obtained from a H₂ line forced to pass through a bubbler containing B₂O₃ dissolved in methanol (B\C = 6000 ppm). The films were grown on both sides of perforated and non-perforated substrates. Emphasis for diamond growing on perforated substrates have been done in order to increase the active surface area and hereafter to promote an easier electrolyte flow for wastewater treatment. The electrode performance was determined by cyclic voltammetry measurements in KCl, KNO₃, Na₂SO₄, HCl, HNO₃ and H₂SO₄ solutions and the reversibility behavior of the Fe(CN)₆^3-/4- at the Ti6Al4V/Diamond electrode were studied. Also, Scaning Electron Microcopy and Raman Scattering Spectroscopy were used for morphology and diamond quality evaluation, respectively.

Keywords: boron doped-diamond, Ti6Al4V, electrochemistry

1. Introduction

The evolution of semiconductor diamond research has shown promising results with doping process during CVD growth. The standard methods used for CVD boron doped diamond were initially thermal diffusion and ion implantation^1,2. These processes were made ex situ, after the film growth, and the no contamination of the growth reactor is the main advantage. However, alternative techniques have shown highly doped films, using in situ process from a solid source of boron inside of the reactor³ and by introducing of B₂O₃ in a methanol-acetone mixture⁴. It was observed better results evidenced by more homogeneity in films bulk, also showing films with a linear relation between the doping levels and boron concentration in the precursor source.

The diamond electrodes, which correspond to a typical p-type semiconductor, offer high sensitivity, good precision and high stability when compared with vitreous carbon, for example. Its more remarkable property is a wide working potential window concerning to hydrogen and oxygen evolution in aqueous solutions. Consequently, this kind of electrodes can withstand corrosive environments and favor alternative reactions⁵.

The main used substrate was the Si once its presents good characteristics for many applications. More recently, titanium alloy substrate became a good alternative for diamond growth since they have higher mechanical strength and electrical conductivity⁶. However some problems related to diamond film delamination became a goal to be overcome. It usually occurs due to the generation of high residual stresses in both films and substrate. Thermal stresses are often high in diamond films as a result of high deposition temperatures and the low thermal expansion coefficient of diamond compared to Ti6Al4V⁷. Some solutions for this problem have been found, as the low deposition temperature and stress relieves by slow temperature dropping, and have been considered in this work. Also, an electrochemical investigation for characterization of boron-doped diamond electrodes on Ti6Al4V is one of the purposes of this work. We have carried out electrochemical experiments providing voltammetric behavior studies for diamond electrodes in different neutral and acid solutions, and in 1 mM of ferrocyanide at scan rates from 5 up to 500 mV/s. The morphology and films quality were analyzed by SEM and Raman spectroscopy.

2. Experimental Procedure

Depositions were performed using a tubular stainless steel HFCVD reactor 100 mm diameter with one H₂ line that passes through a bubbler containing B₂O₃ dissolved in methanol (B\C = 6000 ppm in solution, flux from the bubbler = 20 sccm), as has been described elsewhere⁸.

Ti6Al4V 35 × 35 × 1,3 mm perforated and non-perforated samples has been used. The laser made perforation increased geometric area by approximately 95%.

All samples were ground using 600 and 1000 SiC papers successively and polished using 9T and 5T Al₂O₃ powder, and then ultrasonically prepared in hexane bath with 0,25 mm diamond powder for 1 h.

The films were grown in both sides of the substrates with the aid of a tripod. The temperature and pressure were 900 ± 25 K and 6.5 × 10³ Pa respectively. The total gas flow was kept constant at 100 sccm during 8 h with gas mixture of 1.0% vol. of methane in hydrogen, plus the B₂O₃ in methanol. The reactor has a rotative mechanism for the substrate that provides better uniformity during film growth. After the growth period the Ti6Al4V/diamond were cooled down to room temperature continually during 4 h.

The non-perforated electrodes were assembled over a brass base and electrical contact was made with a silver-based paste to the rear side of the substrate. The group was isolated with silicone wax inside a Teflon box. The electrodes did not have surface pre-treatment before the electrochemical measurements.

Electrochemical activity of the diamond/Ti6Al4V and diamond/silicon electrodes was analyzed in 0.1 M of neutral (KCl, Na₂SO₄ and NaNO₃) and acidic (HCl, H₂SO₄ and HNO₃) solutions. The anodic and cathodic charge was evaluated from cyclic voltammety in 1mM of ferrocyanide/0.1 M KCl at scan rates from 5 up to 500 mV/s. The experiments were carried out at room temperature. The solution was exposed to air during electrochemical measurements. A conventional three-electrodes (work electrode, counter electrode and reference electrode), single-compartment electrochemical cell was used using a Microquímica potentiostat model MQPG-01. The solution was not stirred during the voltametry experiments. A reference electrode of Ag/AgCl was used in this study. A platinum foil was also used during all the electrochemical measurements as counter electrode.

The characterizations of the films morphology and diamond quality were made by a LEO 440 scanning electron microscopy system and by a Renishaw Microscope 2000 micro-Raman scattering system, respectively.

3. Results and Discussions

Scanning Electron Microscopy and Raman Spectroscopy

First of all, surface and quality analyses have been carried out. SEM analysis for boron-doped diamond films on Ti6Al4V is depicted in Fig. 1. It is observed a polycrystalline film with predominant (111) and (100) orientation also determinated by X-Ray analysis. The films grew on the surface (Fig. 1a) and on the border of the samples, as is shown in Fig. 1b. The grains are very twinned, with average size around 1mm for 1.3 mm film thickness, which was measured by SEM cross section. The thickness, the average grain size and the low growth rate are smaller in comparison with standard diamond electrodes on silicon⁹. This probably occurred due to the low deposition temperature¹⁰ at 600 °C. This temperature seems to be best for diamond growth on Ti6Al4V considering the best benefits in terms of diamond quality and thermal stress component value for keeping an adherent film^11,12.

A TiC interlayer is formed between the diamond films and the Ti6Al4V as observed by X-Ray analysis. It is already know that at low temperature the titanium carbide interlayer has low growth rate and small thickness⁷. This characteristic favors the adhesion once the debonding occurs at the interface region between the TiC layer and the diamond film. Since the TiC formation is lower, the diamond formation is favored by available methane concentration in the gas phase, near the surface, and high nucleation rate is observed.

The Raman spectrum, as depicted in Fig. 2, shows a narrow sp³ peak at 1340 cm^-1 and a wide band centered at ~1550 cm^-1. The displacement and the splitting of the diamond characteristic peak (1332 cm^-1 for natural diamond) is attribute to the high residual stress in the film, which Terranova et al.¹³ also observed for diamond films on Ti6Al4V. This stress probably occurred due to the high mechanical resistance of the titanium alloys which could not allow a partial relaxation of the high thermal stress and the different compositions of the interfacial layers¹⁴. The positive shifts have to be interpreted as films in compressive stress¹⁵.

Fig. 2a

The wide band centered at 1550 cm^-1 is attributed to sp² carbons bond in the film¹⁶. This result was expected once the methane concentration around 2% is relatively higher than other values normally used¹⁷. As shown in

¹⁵³

Also, the Raman spectrum doesn't show a broad band centered at 1200 cm^-1, that result of the breakdown of the k = 0 selection rule, probably because of the films are only slightly doped^8,18,19.

Electrochemical Behavior

The electrochemical behavior has been studied in order to observe the potential window and other parameters concerning different active area. The electrochemical responses from non-perforated and perforated diamonds electrodes in 0,1 M KCl solution are shown in

Fig. 3

The large potential window can be observed, as expected, for both electrodes. The anodic current density observed from the curve of perforated diamond is around twice higher than non-perforated diamond. This result is probably due to the difference between the real area and the geometric area, for both electrodes, which promote the double layer capacitance increase²⁰.

In order to verify if the diamond/Ti6Al4V electrodes have the same electrochemical properties of the standard diamond\silicon electrodes, the electrochemical activity at 2 and 2 V of the two diamond/Ti6Al4V electrodes in acid and neutral solutions were compared with diamond/silicon doped, as shown in

Fig. 45^20,21_maxmin⁺et al.²¹

The main objective of the next experiments were to verify the electrode response agreement, using some criteria of reversibility²², for the redox reactions in K₄Fe(CN)₆ solution. The results obtained from cyclic voltammetry of Fe(CN)₆^3-/4- in 0,1 M KCl solution with perforated boron-doped diamond/Ti6Al4V electrode and boron-doped diamond/Si are shown in

Figs. 6-8Fig. 6Fig. 7Fig. 8₆^{3-/4- 23}

However one of the conditions to consider the reaction quasi-reversible is that

_p²⁴

4. Conclusion

The thin films of boron-doped diamond were grown on perforated and non-perforated Ti6Al4V samples with good adhesion. SEM and Raman Scattering Spectroscopy characterized their quality and morphology indicating films with small average grain size, highly stressed, slightly doped and with some quantity of sp² carbon.

The electrodes electrochemical behavior showed similarity with the standard electrodes deposited on silicon. Although in the perforated samples the double layer capacitance was increased. This behavior turns them suitable for electrochemical capacitors applications. The electrochemical properties, as large potential window, electrochemical activity in different neutral and acid solutions and cyclic voltammetry behavior of the Fe(CN)₆^3-/4- at the electrodes, were the expected ones. This is very important to assure good performance in future applications.

Acknowledgements

The authors are very grateful to FAPESP for financial support and to Mr. Jognes Panasiewicz Junior from Instituto Nacional de Pesquisas Espaciais for SEM analyses and Mrs. Maria Auxiliadora S. Oliveira from Instituto Tecnológico de Aeronáutica (ITA) for potentiostat equipment.

References

23. Ferreira, N.G.; Silva, L.L.G.; Corat, E.J. in press.

e-mail: [e-mail]

Received: January 02, 2002

Revised: September 30, 2002

• 1. Tsai, W.; Delfino, M.; Ching, L.Y.; Reynolds, G.; Hodul, D.; Cooper, C.B New Diamond Science and Technology, MRS Int. Conf. Proc. P. 937, 1991.
• 2. Prawer, S. Diamond and Related Material, v. 4, p. 862, 1995.
• 3. Martin, H.B.; Argotia, A.; Landau, U.; Anderson A.B.; Angus, J.C. J. Electrochem. Soc, v. 143, n. 6, p. 133, 1996.
• 4. Okano, K.; Akiba, Y.; Kurosu, T.; Lida, M.; Nakamura, T. J. Cristal Growth, v. 99, p. 1192, 1990.
• 5. Tenne, R.; Lévy-Clément, C. Israel Journal of Chemistry, v. 38, p. 57-73, 1998.
• 6. Fisher, V.; Gandini, D.; Laufer, S.; Blank, E.; Comninellis, C. Electrochimica Acta, v. 44, p. 521-524, 1998.
• 7. Peng, X.L.; Clyne, T.W. Thin Solid Films, v. 293, p. 264-269, 1997.
• 8. Silva, L.L.G.; Corat, E.J.; Barros, R.C.M.; Trava-Airoldi ,V.J.; Leite, N.F.; Iha, K. Materials Research, v. 2, n. 2, p. 99-103, 1999.
• 9. Ferreira, N. G.; Abramof, E.; Corat, E.J.; Leite, N.F.; Trava-Airoldi, V.J. Diamond and Related Materials, v. 10, p. 750-754, 2001.
• 10. Fan, W.D; Jagannadham, K.; Narayan, J. Surface and Coatings Tecnology, v. 91, p. 32-36, 1997.
• 11. Rats, D.; Vandenbulcke, L.; Serin, V.; Sevely, J. Thin Solid Films, v. 270, p. 177-183, 1995.
• 12. Vandenbulcke, L.; Rats, D.; Herbin, R. Materials Letters, v. 27, p. 77-80, 1996.
• 13. Scardi, P.; Leoni, M.; Cappuccio, G.; Sessa, V.; Terranova, M.L. Diamond and Related Materials, v. 6, p. 807, 1997.
• 14. Perry, S.S.; Ager, J.W.; Somorjai, G.A. J. Appl. Phys, v. 74, p. 7542-7550, 1993.
• 15. Knight, D.S; White, W.B. J. Mater. Res, v. 4, n. 2, p. 385-393, 1989.
• 16. Nemanich, R.J.; Glass, J.T.; Lucovsky, G.; Shroder, R.E. J. Vac. Sci. Technol A, v. 6, p. 1783-1787, 1988.
• 17. Gheeraert, E.; Deneuville, A.; Bonnot, A.M. Diamond and Related Materials, p. 525-528, 1992.
• 18. Wang, W.L.; Pólo, M.C.; Sanchez, G.; Esteve, J. J. Appl. Phys, v. 80, p. 1846-1850, 1996.
• 19. Liao, X.Z.; Zhang, R.J.; Lee, C.S.; Lee, S.T.; Lam, Y.W. Diamond and Related Materials, v. 6, p. 521-525, 1997.
• 20. Ferreira, N.G.; Silva, L.L.G.; Corat, E.J. Diamond Related Materials, v. 11, p. 657-661, 2002.
• 21. Lévy-Clément, C.; Zenia, F.; Ndao, N.A.; Deneuville, A. New Diamond and Frontier Carbon Technology, v. 9, n. 3, p.189-206, 1999.
• 22. Greef, R.; Peat, R.; Peter, L.M.; Pletcher, D.; Robinson, J. Instrumental Methods in Electrochemistry, John Wiley & Sons, New York, cap. 3, 1985.
• 24. Ramesham, R.; Rose, M.F. Diamond Related Materials, v. 6, p. 17-27, 1997.

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Publication Dates

• Publication in this collection
  25 Mar 2003
• Date of issue
  Jan 2003

History

• Reviewed
  30 Sept 2002
• Received
  02 Jan 2002