Sol-gel derived pure titania is compared with vanadium and / or aluminium modified titania deposited by spin coating on pure titanium substrates annealed at 300C and 650C. Reflection high-energy electron diffraction indicated the presence of anatase from the surface layers of samples annealed at 300C with a transition to rutile with increasing annealing temperature and addition of vanadium. Cross-sectional transmission electron microscopy indicated a gradation of Ti-O phases through the layer with the dominant presence of rutile. Aluminium was found to inhibit grain growth while vanadium promoted crystallisation

Allison, F.

Brown, Paul D.

Grant, D.M.

Harrison, P.G.

Marlafeka, S.

Nottingham eTheses

Marlafeka, S. and Allison, F. and Grant, D.M. and Harrison, P.G. and Brown, Paul D. (2001) Microstructural characterisation of biocompatible sol-gel derived vanadium doped TiO2 on Ti substrates. In: Electron microscopy and analysis 2001: proceedings of the Institute of Physics Electron Microscopy and Analysis Group Conference, University of Dundee, 5-7 September 2001. Institute of Physics conference series (168). IOP Publishing Ltd, Bristol, pp. 381-384. ISBN 9780750308120 Access from the University of Nottingham repository: http://eprints.nottingham.ac.uk/1466/1/Inst._Phys._Conf._Ser._168_%282001%29_pp_381-384.pdfCopyright and reuse: The Nottingham ePrints service makes this work by researchers of the University of Nottingham available open access under the following conditions.This article is made available under the University of Nottingham End User licence and may be reused according to the conditions of the licence.  For more details see: http://eprints.nottingham.ac.uk/end_user_agreement.pdfA note on versions: The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher’s version. Please see the repository url above for details on accessing the published version and note that access may require a subscription.For more information, please contact eprints@nottingham.ac.uk 1   Microstructural characterisation of biocompatible sol-gel derived vanadium-aluminium doped TiO2 on Ti substrates   S. Marlafeka1, F. Allison2, D.M. Grant1, P.G. Harrison2 and P.D. Brown1  1School of Mechanical, Materials, Manufacturing Engineering and Management, University of Nottingham, University Park, Nottingham NG7 2RD, UK. 2School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, UK.  ABSTRACT: Sol-gel derived pure titania is compared with vanadium and / or aluminium modified titania deposited by spin coating on pure titanium substrates annealed at 300qC and 650qC. Reflection high-energy electron diffraction indicated the presence of anatase from the surface layers of samples annealed at 300qC with a transition to rutile with increasing annealing temperature and addition of vanadium. Cross-sectional transmission electron microscopy indicated a gradation of Ti-O phases through the layer with the dominant presence of rutile. Aluminium was found to inhibit grain growth while vanadium promoted crystallisation.   1. INTRODUCTION  Titanium and titanium alloys naturally exhibit a thin but adherent titanium oxide passivating surface that provides good corrosion and wear resistance (Pouilleau et al. 1997). These properties are particularly enhanced for titanium alloys such as TiAlV when used as a biomedical implant material as compared with conventional stainless steel and cobalt based alloys (Long and Rack 1998). There is interest in engineering thicker TiO2 coatings that may improve the material biocompatibility depending on the oxide chemical composition, thickness, topography, roughness, microstructure and polycrystallinity (Casaletto et al. 2001). The effect of the interaction of Ti with the implant environment, allowing the release of metal ions that might otherwise contaminate living tissue, may be reduced in the presence of a passivating TiO2 layer and even more so as the engineered coating is increased in thickness (Woodman et al. 1984, Lausmaa 1996). By way of example, coatings of nanocrystalline titania and alumina are particularly favoured by osteoblasts (Webster et al. 1999, Webster et al. 2000). Hence, it is of great interest to control the titania stability (Gouma et al. 1999, Byun et al. 1997, Hengerer et al. 2000), since anatase which is one of the unstable phases of TiO2 can have a nanocrystalline form at low temperature. Sol-gel, one of the most promising, easy and cheap preparation techniques for high quality thin films, is widely applied in synthetic implants (Haddow et al. 1996, Xu et al. 1998). As part of a larger research program, there is evidence to suggest that additions of vanadium to TiO2 promote the formation of the more stable rutile phase, while additions of aluminium act to lower the extent of polycrystallinity. The microstructural development of vanadium and aluminium doped TiO2 is investigated using a combination of reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM).   2. EXPERIMENTAL  Preparation of the TiO2 sol-gel modifications by evaporation of aqueous colloidal sols was followed by deposition of titania thin film coatings (up to 200nm thickness) onto 10mm diameter pure titanium discs using a custom-built spin coater controlled by an EMC TOP-5200 syringe pump. Published under licence in IOP Conference Series: by IOP Publishing Ltd. Inst. Phys. Conf. Ser. 168 (2001) pp 381-384 Electron Microscopy & Analysis Conference, EMAG 2001, Dundee  2 Figure 1. RHEED patterns from unmodified and vanadium and / or aluminium modified TiO2/Ti samples; (1a-1e) 80kV (1f) 200kV (acquired with a different camera length).   The following samples were examined: unmodified TiO2, TiO2-12wt.%Al, TiO2-6wt.%Al-4wt.%V and TiO2-16wt.%V annealed at 300qC; in addition to unmodified titania and TiO2-16wt.%V samples annealed at 650qC. Heat treatment took place in air for 18 hours using a Vecstar 91e tube furnace. Cross-sectional samples were prepared by sequential mechanical polishing and argon ion milling for examination using a JEOL 2000FX II TEM operated at 200kV. The associated titania electron diffraction patterns were indexed with reference to a standard titanium specimen used for instrument calibration. Samples were also studied using RHEED within either the 2000FX TEM or a Philips 410 TEM operated at 80kV, with specimens being held vertically on a RHEED stage positioned immediately below the projector lens. Accelerated electrons undergo glancing angle diffraction at the layer surface. The samples were tilted so that the shadow edge was positioned close to the primary beam to access near surface diffraction information.   3. RESULTS AND DISCUSSION   Figures 1a to 1f show RHEED patterns for a range of annealed unmodified and V-Al modified TiO2 coatings. The crystallographic phase of all the layers annealed at 300qC was indentified as being anatase. However, unmodified TiO2 annealed at 650qC revealed a transition to a mixed phase of anatase and rutile, while TiO2±16wt.%V annealed at 650qC transformed to rutile. RHEED patterns from the TiO2 and TiO2-6wt.%Al-4wt.%V annealed at 300qC also exhibited slight blurring of the diffraction rings implying that the size of the grains is relatively small. Conversely, the 12wt.%Al and the 16wt.%V TiO2 modifications, also annealed at 300qC displayed more well defined polycrystalline rings indicating a distribution of comparatively larger particles. Following heat treatment at 650qC the unmodified TiO2 and the TiO2-16wt.%V specimens additionally showed tentative evidence of preferred orientation. The sequence of more well defined polycrystallinity and increased spottiness of the diffraction rings signifies that a high vanadium modification is responsible for lowering the rutile phase transition point with a corresponding increase in TiO2 grain size.   More detailed TEM investigation was carried out on the TiO2-12wt.%Al and TiO2-16wt.%V samples annealed at 300 and 650qC respectively (Figures 2 and 3). The associated diffraction patterns acquired throughout both layers indicated the presence of titanium and a gradation of several other Ti-O based phases. Rutile was the most dominant phase identified in both samples.  It is of interest to note that the 12wt.%Al layer exhibited a transition from very small columnar grain polycrystallinity at the surface through to an amorphous (D-TiO2) layer beneath, in turn leading through to the polycrystalline titanium substrate, i.e. nucleation occurs at the surface and crystallisation of the amorphous sol-gel deposit proceeds towards the substrate. Conversely, for the 16wt.%V-TiO2, the coating exhibits regions of amorphous material at the top surface with a larger grained polycrystalline phase in contact with the Ti substrate. The implication in this case is that nucleation has occurred at the TiO2/Ti interface and crystallisation has proceeded towards the sol-gel a b c TiO2 @ 300qC (A) TiO2 @ 650qC (A & R) TiO2-12%Al @ 300qC (A) d e f TiO2-6%Al-4%V @ 300qC (A) TiO2-16%V @ 300qC (A) TiO2-16%V @ 650qC (R)  3 Figure 2. Dark field image of TiO2-12wt.%Al annealed at 300qC showing amorphous to crystalline transition.  Figure 3. Dark field image of TiO2-16wt.%V annealed at 650qC showing columnar mixed phase grains.   deposit surface. It is noted that there is evidence from other annealed systems such as amorphous Si for either surface or interface nucleated grain growth (Shimiju and Ishihara 1995, Saki et al. 1991). TEM images from the TiO2-6wt.%Al-4wt.%V and TiO2-16wt.%V samples annealed at 300qC are shown in Figures 4 and 5, respectively. For the case of the mixed Al-V modification, a very complicated microstructure has developed similar to the case of the 12wt.%Al modification, with a predominant columnar rutile phase on amorphous TiO2 on a polycrystalline Ti substrate. The 16wt.%V sample annealed at 300qC shows complete polycrystallinity through the entire anatase / rutile phase layer but with a significantly smaller grain size as compared with the equivalent sample annealed at 650qC (Figure 3). The increased annealing temperature results in approximately a doubling of the oxide layer. RHEED data is very sensitive to the top few atomic layers with a large ~ 1mm2 sampling surface area. The implication from this part of the study is a transition from anatase to rutile with increasing annealing temperature and vanadium addition. However, cross-sectional TEM provides more depth information indicating a greater presence of rutile within all the layers examined. Graded Ti-O layers were found for the case of Al and Al-V modifications, indicating that the presence of aluminium acts to lower the rate of crystallisation, while the presence of vanadium clearly accentuates crystallisation. TiO2-12%Al@300qC  a. Ti+D-TiO2+rutile  b. Ti+D-TiO2+rutile  c. Ti+rutile                d. Ti+D-TiO2+rutile  e. Ti+TiO2-(hex) a b c d e 100nm Layer Ti 200nm a b c d Layer TiO2-16%V@650qC  a. D-TiO2+rutile+TiO-(hex), [whole layer] b. Ti+D-TiO2 +rutile+anatase              c. Ti+D-TiO2+rutile      d. Ti+D-TiO2+rutile Ti  4         4. CONCLUSIONS  RHEED investigation of sol-gel derived vanadium-aluminium modified titania on titanium substrates indicated the presence of an anatase microstructure from the top few atomic layers of samples annealed at 300qC. At higher annealing temperatures (650qC) a transition to a mixed anatase / rutile phase was observed for unmodified TiO2, while the addition of vanadium promotes a transformation to the stable rutile phase. However, when observed in cross-section by TEM, the majority of the layers revealed a dominant rutile phase throughout the layer. The addition of aluminium was found to inhibit the extent of crystallisation.   5. ACKNOWLEDGMENTS  This work was supported under EPSRC contract GR/M74603 and GR/L55209.  REFERENCES Byun C, Jang JW, Kim IT, Hong KS and Lee B-W 1997 Materials Research Bulletin 32, 4 Casaletto MP, Ingo GM, Kaciulis S, Mattogno G, Pandolfi L and Scavia G 2001 Applied Surface Science 172, 167 Gouma PI, Dutta PK and Mills MJ 1999 NanoStructured Materials 11, 8 Haddow DB, Kothari S, James PF, Short RD, Hatton PV and Noort R van 1996 Biomaterials 17, 501 Hengerer R, Bolliger B, Erbudak M and Grätzel M 2000 Surface Science 460, 162 Lausmaa J 1996 J. Electron Spectroscopy and Related Phenomena 81, 343 Long M and Rack HJ 1998 Biomaterials 19, 1621 Pouilleau J, Devilliers D, Garrido F, Durand-Vidal S and Mahé E 1997 Materials Science Engineering B47, 235 Saki A, Ono H, Ishida K, Niino T and Tatsumi 1991 Jpn J. Applied Physics 30, L941 Shimiju T and Ishihara S 1995 J. Electrochemistry Society 142, 298 Webster TJ, Siegel RW and Bizios R 1999 Biomaterials 20, 1221 Webster TJ, Ergun C, Doremus RH, Siegel RW and Bizios R 2000 Biomaterials 21, 1803 Woodman JL, Jacobs JJ, Galante JO and Urban RM 1984 J. of Orthopaedic Research: Official Publication of the Orthopaedic Research Society 1, 4 Xu W-X, Zhu S and Fu X-C 1998 Applied Surface Science 136, 194 100nm 100nm Figure 4. Bright field TEM image of TiO2-6wt.%Al/4wt.%V annealed at 300qC showing an amorphous to crystalline transition Figure 5. Dark field TEM image of TiO2-16wt.%V annealed at 300qC showing small mixed anatase-rutile grains. Layer Ti Layer Ti 

Microstructural characterisation of biocompatible sol-gel derived vanadium doped TiO2 on Ti substrates

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Microstructural characterisation of biocompatible sol-gel derived vanadium doped TiO2 on Ti substrates

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