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Anatase–rutile transformation of TiO2 sol–gel coatings deposited on different substrates
Author's Accepted Manuscript
Anatase-rutile transformation of TiO2 sol-gel coatings deposited on different substrates Sebastian Miszczak, Bożena Pietrzyk
www.elsevier.com/locate/ceramint
PII: DOI: Reference:
S0272-8842(15)00285-0 http://dx.doi.org/10.1016/j.ceramint.2015.02.066 CERI9988
To appear in:
Ceramics International
Received date: Accepted date:
11 February 2015 11 February 2015
Cite this article as: Sebastian Miszczak, Bożena Pietrzyk, Anatase-rutile transformation of TiO2 sol-gel coatings deposited on different substrates, Ceramics International, http: //dx.doi.org/10.1016/j.ceramint.2015.02.066 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 galley proof before it is published in its final citable 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.
Anatase-rutile transformation of TiO2 sol-gel coatings deposited on different substrates Sebastian Miszczak*, BoĪena Pietrzyk Institute of Materials Science and Engineering Technical University of Lodz, Stefanowskiego 1/15, 90-924 Lodz, Poland *Corresponding author: [email protected] Tel. +48 426312283; Fax: +48 426366790 Abstract Titanium dioxide is widely used in a lot of applications. The properties of TiO2 strongly depend on its phase composition. The transformation temperature between phases is influenced by a lot of factors. One of them is a type of substrate under the TiO2 film. In presented work, thin films of TiO2 were deposited by the sol-gel method on silicon, stainless steel (304L) and Co-Cr-Mo alloy (Vitallium). The process of anatase–rutile phase transformation was investigated by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) studies of deposited coatings. The results were compared with anatase– rutile transformations temperature of TiO2 powders obtained by analogous sol-gel process. The temperature of anatase–rutile phase transformation changed in the range of 700–1000oC and strongly depends on a kind of substrate. It was found that anatase-rutile transformation of TiO2 coating proceeded at a higher temperature than rutilization of titania powders.
Keywords: titania, TiO2, anatase, rutile, crystallization, sol-gel
1. Introduction Titanium dioxide is a material attracting a great deal of attention due to its series of useful properties, such as photocatalytic [1], self-cleaning [2], bioactive [3,4 ] electronic and optical [5] ones. They result in perpetually growing spectrum of medical [3,6,7], electrotechnical [8] and surface engineering [1,2,8,9] applications, where titania can be used as coating. The titanium dioxide in nature can occur in one of the following crystalline types: brookite, anatase and rutile, or as an amorphous material [9, 10]. Among all titanium dioxide polymorphic types, only rutile is thermodynamically stable, whereas the other ones are metastable phases that can be transformed into rutile. The amorphous phase and crystalline forms of titania possess different features, thus their type and the participation has a direct impact on the material properties [10,11]. Rutile, for instance, is often used due to its optical properties, whereas anatase possesses better photocatalytic properties [12]. The temperature of anatase-rutile phase transition depends on numerous factors, such as: production method, grain size, presence of impurities, atmosphere type, etc. [9-11,13]. As far as the coatings are concerned, one of the factors is also the kind of substrate, on which titania films have been
1
deposited [14]. TiO2 coatings can be produced with the use of various methods, such as chemical vapor deposition, magnetron sputtering, ion beam evaporation and sol-gel method, however, effective production of coatings with determined functionality depends largely on the ability to control their structure and properties, which is the subject of intense researches [14-18]. This paper presents studies on polymorphic transformations of TiO2 coatings produced through sol-gel method on different substrates. The obtained results have been compared to the temperature of phase transformations of powders produced at analogous parameters during the sol-gel process.
2. Material and methods In this work both the TiO2 coatings and the powders produced with the use of the sol-gel method have been studied. The sol was obtained from titanium(IV) butoxide (Ti[O(CH2)3CH3)4] by dissolving it in anhydrous ethanol. The acetic acid aqueous solution was catalyst of hydrolysis. The sol was prepared preserving the molar ratio [Ti]:[H2O]:[CH3COOH] = 1:10:1. The coatings were deposited by means of dip-coating method. The substrates were emerged from sol at withdrawal speed of 30 mm/min. Further processing consisted of 15-minute drying at the room temperature and 15-minute annealing at temperature of 300÷1000oC in a muffle furnace. This process was repeated several times in order to obtain a coating of 1µm thickness. After the last deposition cycle coated substrates were annealed for 60 minutes. The substrates were: austenitic steel 304L, cobalt-based alloy (Vitallium) and monocrystalline silicon wafers (100). Chemical composition of metal substrates was specified by an X-ray spectrometer Siemens SRS-303 and carbon-sulphur analyzer LECO CS-200 (Table 1). The metal substrates (discs with diameter of 14-16 mm) were ground (abrasive paper with gradation 200÷1500), polished and ultrasonically cleaned in ethanol before deposition of coatings. Silicon substrate delivered in thin polished slices was cut into rectangular pieces and subjected only to the ultrasonic cleaning in ethanol. The titania powders were obtained from sol by drying it at the room temperature for 48 hours and then annealing at the temperature of 300÷800oC for 60 minutes. The phase transitions of titanium dioxide coatings and powders was studied by means of phase composition analysis through X-ray diffraction (XRD) method with SIEMENS D-500 diffractometer. Co KĮ emission lines and a graphite monochromator were used in the analysis. The tests were conducted by the stepwise method (of 0,05 step degree) with counting time of 3 seconds, X-ray source voltage of 30 kV and electron beam current of 40mA. An identification of phase composition was carried out with the help of X-RAYAN computer software, supported by the ICDD database. The microstructure of coatings was observed by means of scanning electron microscope (SEM) Jeol JSM-6610.
3. Results & discussion X-ray diffraction patterns of the TiO2 powders annealed at temperature range 300-800oC are shown in Fig. 1. The TiO2 powder obtained by means of the sol-gel method and annealed at the temperature of 300oC possesses an amorphous structure – there is no diffraction signal on the diffraction pattern. After
2
thermal annealing at the temperature of 400oC, some maxima corresponding to the anatase structure have been observed (ICDD card no. 21-1272). Further temperature increase to 600oC intensifies these signals and indicates an intensive growth of anatase phase in the powder. On the diffraction pattern of the powder annealed at 700oC, together with the anatase signals observed earlier, rutile signals appeared (ICDD card no. 21-1276). It proves the beginning of anatase–rutile transformation, what corresponds to the other reports [10]. The diffraction pattern of the powder annealed at 800oC revealed disappearance of anatase diffraction peak and increase of rutile signals intensity. Brookite was not observed in the annealed powders. Diffraction patterns of the titanium dioxide coatings produced on a steel substrate (304L) are shown in Fig. 2. The coatings annealed at 500oC exhibit anatase structure, while those annealed at 800oC – rutile structure. It appears that as far as TiO2 coatings produced on 304 steel are concerned, the temperature range of anatase–rutile transformation is similar to the same transition in TiO2 powders. Diffraction pattern of coating annealed at 800oC also contains signals of Fe2O3 (ICDD card no. 33-0664) resulted from substrate metals oxidation. Diffraction patterns of coatings produced on the Co-Cr-Mo alloy (Vitallium) substrate are shown in Fig. 3. The TiO2 coating annealed at 500oC shows the structure of anatase, as in the case of powder and coating produced on a stainless steel substrate. This polymorph is stable up to the temperature of 800oC and signals of crystallizing rutile are not visible. Only when the coating is annealed at 850oC, the mixed structure anatase–rutile is created. Diffraction pattern obtained at 900oC indicates the end of the anatase rutile transformation. The signals associated with Cr2O3 (ICDD card no. 38-1479) formed during the substrate oxidation, become also visible in this spectrum. In the analyzed spectra for 2Θ angles within the 50-60º range, signals from the substrate can be seen. Their intensity interrelations are subject to change. Intensity relation changeability comes from texture effect connected with the coarse-graininess of the Vitallium alloy substrate. Figure 4 presents phase composition of TiO2 coatings produced on the silicon wafer. The coating annealed at 500oC shows the anatase structure, similarly as in the case of the powder and coatings produced on the other substrates at the same temperature. Diffraction patterns of the coatings annealed at 600÷900oC revealed no significant changes: the anatase structure remained stable and no rutile signals were observed. Only in the diffraction pattern of the coating annealed at 1000oC a rutile diffraction signal becomes visible together with anatase signals. It points to the creation of mixed coating structure consisting of anatase and rutile, proving that the initiation of the anatase–rutile polymorphic transformation is considerably delayed. Microstructures of the anatase and rutile coatings are shown in Fig. 5 on the example of coatings produced on Si as well as on Co-Cr-Mo alloy substrates. The anatase coatings are homogenous and smooth regardless of the type of substrate. Crystallites are not visible at applied magnification. The microscopic images changes with the beginning of rutilation: the grain size explicitly increases. The largest crystallites are observed for the rutile coatings on Co-Cr-Mo alloy annealed at 900oC. The grain
3
size of mixed anatase/rutile structure is much smaller despite the annealing at temperature of 1000°C. It means, that the grain size results primarily of anatase-rutile transition progress. The presented results clearly show that the nature of the substrate has a significant impact on the course of anatase–rutile transformation in TiO2 coatings. This effect may be due to the possibility of doping layer by diffusion of substrate atoms into the coating during the annealing process. The influence of foreign atoms (ions) additives on the crystallization of titanium dioxide has been the subject of extensive research [10]. The impact of impurities on the course of anatase–rutile transformation depends on their effect on the formation of structural defects, in particular oxygen vacancies. Growing number of such defects causes the increase of ions mobility in TiO2 lattice, required for the phase transformation [10,19]. The literature suggests that the presence of Si causes a delay in anatase–rutile transformation while Fe, Cr and Co impurities should promote this process in TiO2 powders [10]. Such changes do not coincide with a tendency observed in the research results regarding the TiO2 coatings. The transformation temperature of the coatings produced on the silicon substrate is much higher than on the remaining substrates. This phenomenon remains in accordance with the Si additive effect. However, the desired change in anatase–rutile transformation of titania coatings produced on metal substrates is an acceleration whereas a significant delay is observed for the coatings deposited on Co-CrMo alloy. What is more, on the steel substrates the acceleration effect is also not clearly visible. That is why the transformation delay has to be associated also with other factors. It seems that one of the essential factors might be stresses occurring in the coating during the annealing and phase transition processes because of the interaction with substrate. During the forming process of coatings with the use of the sol-gel method, tensile stresses occur. They are the result of capillary forces present during drying, gelation and sintering of the coatings [20-22]. Since the anatase– rutile transformation is connected with volume decrease of 8% [10], the tensile stresses will work against the coating contraction. The phase transformation in coatings will then require more energy than in the case of the powders. In consequence, one should think that the anatase–rutile transformation in the titania coatings will be delayed as a rule (i.e. will take place in a higher temperature) in comparison with the same transformation in the powders. Other factors, such as additives presence, may only modify this rule. The presence of tensile stresses in TiO2 coatings manufacturing on metal substrates is caused not only by structural stress, but also by thermal stress existing between the coating and substrate at a temperature of anatase–rutile transformation. Thermal expansion coefficient Į for the anatase, austenitic steel and Vitallium alloy are respectively: 10,2•10-6K-1, 16,9•10-6K-1 and 17,1•10-6K-1 [23-25]. Lower thermal expansion coefficient of the coating material to the substrate will cause tensile stresses during the heating step of annealing process. Therefore the state of stress in the TiO2 coatings on metal substrate will favor delay of anatase–rutile transformation.
4. Conclusions The results presented in the paper show that both the powder and coating obtained by sol-gel method crystallize as a result of thermal processing at temperatures higher than 300oC. After annealing at
4
500oC all tested coatings and powders had the anatase structure. The anatase–rutile transformation needs further increase of the annealing temperature. The temperature of anatase–rutile transformation of TiO2 powder begins over 600oC and ends below 800oC. It was found that for TiO2 coating anatase–rutile transformation temperature is strongly dependent on the type of substrate.For coatings deposited on the steel substrate it occurs in similar temperature range like in powders, but for coatings deposited on the Co-Cr-Mo alloy and on silicon it is subject to significant delay. Therefore the crystalline structure and properties of the TiO2 coatings produced on different substrates may vary, despite the same deposition and thermal processing conditions. It was found that the grain size depends mainly on anatase-rutile transition progress rather than on heating temperature. Intensive growth of grains starts from the beginning of rutilization. The delay of anatase–rutile transformation in coatings may be the result of the summary effect of the tensile stresses occurring in the film after sol-gel process and influence of additives diffusing from the substrate during annealing of the coating. As a result, anatase-rutile transformation of TiO2 coating conducts at a higher temperature than the anatase-rutile transformation of titania powders.
References [1] Allen NS, Edge M; Verran J, Stratton J, Maltby J, Bygott C. Photocatalytic titania based surfaces: Environmental benefits. Polym Degrad Stabil, 2008; 93(9):1632-46 [2] Piispanen M, Hupa L. Comparison of self-cleaning properties of three titania coatings on float glass. Appl Surf Sci, 2011; 258(3):1126-31 [3] Guo YP, Tang HX, Zhou Y, Jia DC, Ning CQ, Guo YJ. Effects of mesoporous structure and UV irradiation on in vitro bioactivity of titania coatings. Appl Surf Sci, 2010; 256(16): 4945-52 [4] Heidenau F, Mittelmeier W, Detsch R, Haenle M, Stenzel F, Ziegler G, et al. A novel antibacterial titania coating: Metal ion toxicity and in vitro surface colonization. J Mater Sci-Mater M, 2005; 16(10):883 - 88 [5] Mo SD, Ching WY. Electronic and optical properties of three phases of titanium dioxide: Rutile, anatase, and brookite. Phys Rev B, 1995; 51(18):13023–32 [6] Wen CE, Xu W, Hu WY, Hodgson PD. Hydroxyapatite/titania sol–gel coatings on titanium– zirconium alloy for biomedical applications. Acta Biomater, 2007; 3(3):403-10 [7] Jouanny I, Labdi S, Aubert P, Buscema C, Maciejak O, Berger MH, et al. Structural and mechanical properties of titanium oxide thin films for biomedical application. Thin Solid Films, 2010; 518(12):3212-17 [8] Nowotny J, Bak T, Nowotny MK, Sheppard LR. Titanium dioxide for solar-hydrogen I. Functional properties. Int J Hydrogen Energ, 2007; 32(14):2609-29 [9] Chen X, Mao SS. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications. Chem Rev, 2007; 107(7):2891–959 [10] Hanaor DAH, Sorrell CC. Review of the anatase to rutile phase transformation. J Mater Sci, 2011; 46(4):855-74 [11] Diebold U. The surface science of titanium dioxide. Surf Sci Rep, 2003; 48(5-8):53-229 [12] Liu L, Zhao H, Andino JM, Li Y. Photocatalytic CO2 reduction with H2O on TiO2
nanocrystals: Comparison of anatase, rutile, and brookite polymorphs and exploration of surface chemistry. ACS Catal. 2012; 2, 1817–1828. [13] Ogden A, Corno JA, Hong JI, Fedorov A, Gole JL. Maintaining particle size in the transformation of anatase to rutile titania nanostructures. J Phys Chem Solids, 2008; 69:2898–906 [14] Nikolic´ LM, Radonjic´L, Srdic VV. Effect of substrate type on nanostructured titania sol–gel coatings for sensors applications. Ceram Int, 2005; 31:261–6.
5
[15] Szymanowski H, Sobczyk-Guzenda A, Rylski A, Jakubowski W, Gazicki-Lipman M, Herberth U, Olcaytug F: Photo-induced properties of thin TiO2 films deposited using the radio frequency plasma enhanced chemical vapor deposition method. Thin Solid Films 515, 2007, 5275–5281 [16] Ali HM, Abou-Mesalam MM, El-Shorbagy MM. Structure and optical properties of chemically synthesized titanium oxide deposited by evaporation technique. J Phys Chem Solids, 2010; 71(1):5155 [17] Onifade AA, Kelly PJ. The influence of deposition parameters on the structure and properties of magnetron-sputtered titania coatings. Thin Solid Films, 2006;494(1-2):8-12 [18] Huang JH, Wong MS. Structures and properties of titania thin films annealed under different atmosphere. Thin Solid Films, 2011; 520(5):1379-84 [19] Radecka M, Rekas M. Charge and mass transport in ceramic TiO2. J Eur Ceram Soc, 2002; 22:200112 [20] Brinker CJ, Scherer GW. Sol-Gel Science. Academic Press, San Diego, 1990 [21] Watchman J, Haber R. Ceramic films and coatings. Noyes Publications, 1993 [22] Scherer GW. Sintering of Sol-Gel Films. J Sol-Gel Sci Technol, 1997; 8(1-3):353-63 [23] Brandes EA, Brook GB. Smithells Metals Reference Book. Seventh Edition, ButterworthHeinemann, 1992 [24] Bauccio M. ASM Engineered Materials Reference Book. Second Edition, ASM International, 1994. [25] Lide DR. CRC Handbook of Chemistry and Physics. Ed. 80th Edition, CRC Press, 1999.
Table 1. Chemical composition of metal substrates
Co-Cr-Mo (VITALIUM)
Co
Cr
Mo
Si
Mn
Fe
Ni
C
63.09
28.3
6.36
0.77
0.67
0.36
0.13
0.32
Stainless steel (304L)
Fe
Cr
71.27 18.04
Ni
Mn
Mo
Si
V
C
8.24
1.61
0.41
0.31
0.1
0.022
6
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5a
Figure 5b
Figure 5c
Figure 5d
Fig. 1 XRD patterns of the TiO2 powders annealed at 300÷800oC. Fig. 2 XRD patterns of the sol-gel TiO2 coatings deposited on 304L steel and annealed at 500oC and 800oC. Fig. 3 XRD patterns of the TiO2 coatings deposited on Co-Cr-Mo alloy and annealed at o
500 C÷900oC. Fig. 4 XRD patterns of the TiO2 coatings deposited on a silicon substrate and annealed at o
500 C÷1000oC. Fig. 5 SEM images of the TiO2 coatings produced on: a) Si, annealed at 700oC – anatase; b) Si, annealed at 1000oC – anatase + rutile; c) Co-Cr-Mo alloy, annealed at 500oC – anatase; d) Co-Cr-Mo alloy annealed at 900oC – rutile.
Anatase-rutile transformation of TiO2 sol-gel coatings deposited on different substrates Sebastian Miszczak, Bożena Pietrzyk
www.elsevier.com/locate/ceramint
PII: DOI: Reference:
S0272-8842(15)00285-0 http://dx.doi.org/10.1016/j.ceramint.2015.02.066 CERI9988
To appear in:
Ceramics International
Received date: Accepted date:
11 February 2015 11 February 2015
Cite this article as: Sebastian Miszczak, Bożena Pietrzyk, Anatase-rutile transformation of TiO2 sol-gel coatings deposited on different substrates, Ceramics International, http: //dx.doi.org/10.1016/j.ceramint.2015.02.066 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 galley proof before it is published in its final citable 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.
Anatase-rutile transformation of TiO2 sol-gel coatings deposited on different substrates Sebastian Miszczak*, BoĪena Pietrzyk Institute of Materials Science and Engineering Technical University of Lodz, Stefanowskiego 1/15, 90-924 Lodz, Poland *Corresponding author: [email protected] Tel. +48 426312283; Fax: +48 426366790 Abstract Titanium dioxide is widely used in a lot of applications. The properties of TiO2 strongly depend on its phase composition. The transformation temperature between phases is influenced by a lot of factors. One of them is a type of substrate under the TiO2 film. In presented work, thin films of TiO2 were deposited by the sol-gel method on silicon, stainless steel (304L) and Co-Cr-Mo alloy (Vitallium). The process of anatase–rutile phase transformation was investigated by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) studies of deposited coatings. The results were compared with anatase– rutile transformations temperature of TiO2 powders obtained by analogous sol-gel process. The temperature of anatase–rutile phase transformation changed in the range of 700–1000oC and strongly depends on a kind of substrate. It was found that anatase-rutile transformation of TiO2 coating proceeded at a higher temperature than rutilization of titania powders.
Keywords: titania, TiO2, anatase, rutile, crystallization, sol-gel
1. Introduction Titanium dioxide is a material attracting a great deal of attention due to its series of useful properties, such as photocatalytic [1], self-cleaning [2], bioactive [3,4 ] electronic and optical [5] ones. They result in perpetually growing spectrum of medical [3,6,7], electrotechnical [8] and surface engineering [1,2,8,9] applications, where titania can be used as coating. The titanium dioxide in nature can occur in one of the following crystalline types: brookite, anatase and rutile, or as an amorphous material [9, 10]. Among all titanium dioxide polymorphic types, only rutile is thermodynamically stable, whereas the other ones are metastable phases that can be transformed into rutile. The amorphous phase and crystalline forms of titania possess different features, thus their type and the participation has a direct impact on the material properties [10,11]. Rutile, for instance, is often used due to its optical properties, whereas anatase possesses better photocatalytic properties [12]. The temperature of anatase-rutile phase transition depends on numerous factors, such as: production method, grain size, presence of impurities, atmosphere type, etc. [9-11,13]. As far as the coatings are concerned, one of the factors is also the kind of substrate, on which titania films have been
1
deposited [14]. TiO2 coatings can be produced with the use of various methods, such as chemical vapor deposition, magnetron sputtering, ion beam evaporation and sol-gel method, however, effective production of coatings with determined functionality depends largely on the ability to control their structure and properties, which is the subject of intense researches [14-18]. This paper presents studies on polymorphic transformations of TiO2 coatings produced through sol-gel method on different substrates. The obtained results have been compared to the temperature of phase transformations of powders produced at analogous parameters during the sol-gel process.
2. Material and methods In this work both the TiO2 coatings and the powders produced with the use of the sol-gel method have been studied. The sol was obtained from titanium(IV) butoxide (Ti[O(CH2)3CH3)4] by dissolving it in anhydrous ethanol. The acetic acid aqueous solution was catalyst of hydrolysis. The sol was prepared preserving the molar ratio [Ti]:[H2O]:[CH3COOH] = 1:10:1. The coatings were deposited by means of dip-coating method. The substrates were emerged from sol at withdrawal speed of 30 mm/min. Further processing consisted of 15-minute drying at the room temperature and 15-minute annealing at temperature of 300÷1000oC in a muffle furnace. This process was repeated several times in order to obtain a coating of 1µm thickness. After the last deposition cycle coated substrates were annealed for 60 minutes. The substrates were: austenitic steel 304L, cobalt-based alloy (Vitallium) and monocrystalline silicon wafers (100). Chemical composition of metal substrates was specified by an X-ray spectrometer Siemens SRS-303 and carbon-sulphur analyzer LECO CS-200 (Table 1). The metal substrates (discs with diameter of 14-16 mm) were ground (abrasive paper with gradation 200÷1500), polished and ultrasonically cleaned in ethanol before deposition of coatings. Silicon substrate delivered in thin polished slices was cut into rectangular pieces and subjected only to the ultrasonic cleaning in ethanol. The titania powders were obtained from sol by drying it at the room temperature for 48 hours and then annealing at the temperature of 300÷800oC for 60 minutes. The phase transitions of titanium dioxide coatings and powders was studied by means of phase composition analysis through X-ray diffraction (XRD) method with SIEMENS D-500 diffractometer. Co KĮ emission lines and a graphite monochromator were used in the analysis. The tests were conducted by the stepwise method (of 0,05 step degree) with counting time of 3 seconds, X-ray source voltage of 30 kV and electron beam current of 40mA. An identification of phase composition was carried out with the help of X-RAYAN computer software, supported by the ICDD database. The microstructure of coatings was observed by means of scanning electron microscope (SEM) Jeol JSM-6610.
3. Results & discussion X-ray diffraction patterns of the TiO2 powders annealed at temperature range 300-800oC are shown in Fig. 1. The TiO2 powder obtained by means of the sol-gel method and annealed at the temperature of 300oC possesses an amorphous structure – there is no diffraction signal on the diffraction pattern. After
2
thermal annealing at the temperature of 400oC, some maxima corresponding to the anatase structure have been observed (ICDD card no. 21-1272). Further temperature increase to 600oC intensifies these signals and indicates an intensive growth of anatase phase in the powder. On the diffraction pattern of the powder annealed at 700oC, together with the anatase signals observed earlier, rutile signals appeared (ICDD card no. 21-1276). It proves the beginning of anatase–rutile transformation, what corresponds to the other reports [10]. The diffraction pattern of the powder annealed at 800oC revealed disappearance of anatase diffraction peak and increase of rutile signals intensity. Brookite was not observed in the annealed powders. Diffraction patterns of the titanium dioxide coatings produced on a steel substrate (304L) are shown in Fig. 2. The coatings annealed at 500oC exhibit anatase structure, while those annealed at 800oC – rutile structure. It appears that as far as TiO2 coatings produced on 304 steel are concerned, the temperature range of anatase–rutile transformation is similar to the same transition in TiO2 powders. Diffraction pattern of coating annealed at 800oC also contains signals of Fe2O3 (ICDD card no. 33-0664) resulted from substrate metals oxidation. Diffraction patterns of coatings produced on the Co-Cr-Mo alloy (Vitallium) substrate are shown in Fig. 3. The TiO2 coating annealed at 500oC shows the structure of anatase, as in the case of powder and coating produced on a stainless steel substrate. This polymorph is stable up to the temperature of 800oC and signals of crystallizing rutile are not visible. Only when the coating is annealed at 850oC, the mixed structure anatase–rutile is created. Diffraction pattern obtained at 900oC indicates the end of the anatase rutile transformation. The signals associated with Cr2O3 (ICDD card no. 38-1479) formed during the substrate oxidation, become also visible in this spectrum. In the analyzed spectra for 2Θ angles within the 50-60º range, signals from the substrate can be seen. Their intensity interrelations are subject to change. Intensity relation changeability comes from texture effect connected with the coarse-graininess of the Vitallium alloy substrate. Figure 4 presents phase composition of TiO2 coatings produced on the silicon wafer. The coating annealed at 500oC shows the anatase structure, similarly as in the case of the powder and coatings produced on the other substrates at the same temperature. Diffraction patterns of the coatings annealed at 600÷900oC revealed no significant changes: the anatase structure remained stable and no rutile signals were observed. Only in the diffraction pattern of the coating annealed at 1000oC a rutile diffraction signal becomes visible together with anatase signals. It points to the creation of mixed coating structure consisting of anatase and rutile, proving that the initiation of the anatase–rutile polymorphic transformation is considerably delayed. Microstructures of the anatase and rutile coatings are shown in Fig. 5 on the example of coatings produced on Si as well as on Co-Cr-Mo alloy substrates. The anatase coatings are homogenous and smooth regardless of the type of substrate. Crystallites are not visible at applied magnification. The microscopic images changes with the beginning of rutilation: the grain size explicitly increases. The largest crystallites are observed for the rutile coatings on Co-Cr-Mo alloy annealed at 900oC. The grain
3
size of mixed anatase/rutile structure is much smaller despite the annealing at temperature of 1000°C. It means, that the grain size results primarily of anatase-rutile transition progress. The presented results clearly show that the nature of the substrate has a significant impact on the course of anatase–rutile transformation in TiO2 coatings. This effect may be due to the possibility of doping layer by diffusion of substrate atoms into the coating during the annealing process. The influence of foreign atoms (ions) additives on the crystallization of titanium dioxide has been the subject of extensive research [10]. The impact of impurities on the course of anatase–rutile transformation depends on their effect on the formation of structural defects, in particular oxygen vacancies. Growing number of such defects causes the increase of ions mobility in TiO2 lattice, required for the phase transformation [10,19]. The literature suggests that the presence of Si causes a delay in anatase–rutile transformation while Fe, Cr and Co impurities should promote this process in TiO2 powders [10]. Such changes do not coincide with a tendency observed in the research results regarding the TiO2 coatings. The transformation temperature of the coatings produced on the silicon substrate is much higher than on the remaining substrates. This phenomenon remains in accordance with the Si additive effect. However, the desired change in anatase–rutile transformation of titania coatings produced on metal substrates is an acceleration whereas a significant delay is observed for the coatings deposited on Co-CrMo alloy. What is more, on the steel substrates the acceleration effect is also not clearly visible. That is why the transformation delay has to be associated also with other factors. It seems that one of the essential factors might be stresses occurring in the coating during the annealing and phase transition processes because of the interaction with substrate. During the forming process of coatings with the use of the sol-gel method, tensile stresses occur. They are the result of capillary forces present during drying, gelation and sintering of the coatings [20-22]. Since the anatase– rutile transformation is connected with volume decrease of 8% [10], the tensile stresses will work against the coating contraction. The phase transformation in coatings will then require more energy than in the case of the powders. In consequence, one should think that the anatase–rutile transformation in the titania coatings will be delayed as a rule (i.e. will take place in a higher temperature) in comparison with the same transformation in the powders. Other factors, such as additives presence, may only modify this rule. The presence of tensile stresses in TiO2 coatings manufacturing on metal substrates is caused not only by structural stress, but also by thermal stress existing between the coating and substrate at a temperature of anatase–rutile transformation. Thermal expansion coefficient Į for the anatase, austenitic steel and Vitallium alloy are respectively: 10,2•10-6K-1, 16,9•10-6K-1 and 17,1•10-6K-1 [23-25]. Lower thermal expansion coefficient of the coating material to the substrate will cause tensile stresses during the heating step of annealing process. Therefore the state of stress in the TiO2 coatings on metal substrate will favor delay of anatase–rutile transformation.
4. Conclusions The results presented in the paper show that both the powder and coating obtained by sol-gel method crystallize as a result of thermal processing at temperatures higher than 300oC. After annealing at
4
500oC all tested coatings and powders had the anatase structure. The anatase–rutile transformation needs further increase of the annealing temperature. The temperature of anatase–rutile transformation of TiO2 powder begins over 600oC and ends below 800oC. It was found that for TiO2 coating anatase–rutile transformation temperature is strongly dependent on the type of substrate.For coatings deposited on the steel substrate it occurs in similar temperature range like in powders, but for coatings deposited on the Co-Cr-Mo alloy and on silicon it is subject to significant delay. Therefore the crystalline structure and properties of the TiO2 coatings produced on different substrates may vary, despite the same deposition and thermal processing conditions. It was found that the grain size depends mainly on anatase-rutile transition progress rather than on heating temperature. Intensive growth of grains starts from the beginning of rutilization. The delay of anatase–rutile transformation in coatings may be the result of the summary effect of the tensile stresses occurring in the film after sol-gel process and influence of additives diffusing from the substrate during annealing of the coating. As a result, anatase-rutile transformation of TiO2 coating conducts at a higher temperature than the anatase-rutile transformation of titania powders.
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[15] Szymanowski H, Sobczyk-Guzenda A, Rylski A, Jakubowski W, Gazicki-Lipman M, Herberth U, Olcaytug F: Photo-induced properties of thin TiO2 films deposited using the radio frequency plasma enhanced chemical vapor deposition method. Thin Solid Films 515, 2007, 5275–5281 [16] Ali HM, Abou-Mesalam MM, El-Shorbagy MM. Structure and optical properties of chemically synthesized titanium oxide deposited by evaporation technique. J Phys Chem Solids, 2010; 71(1):5155 [17] Onifade AA, Kelly PJ. The influence of deposition parameters on the structure and properties of magnetron-sputtered titania coatings. Thin Solid Films, 2006;494(1-2):8-12 [18] Huang JH, Wong MS. Structures and properties of titania thin films annealed under different atmosphere. Thin Solid Films, 2011; 520(5):1379-84 [19] Radecka M, Rekas M. Charge and mass transport in ceramic TiO2. J Eur Ceram Soc, 2002; 22:200112 [20] Brinker CJ, Scherer GW. Sol-Gel Science. Academic Press, San Diego, 1990 [21] Watchman J, Haber R. Ceramic films and coatings. Noyes Publications, 1993 [22] Scherer GW. Sintering of Sol-Gel Films. J Sol-Gel Sci Technol, 1997; 8(1-3):353-63 [23] Brandes EA, Brook GB. Smithells Metals Reference Book. Seventh Edition, ButterworthHeinemann, 1992 [24] Bauccio M. ASM Engineered Materials Reference Book. Second Edition, ASM International, 1994. [25] Lide DR. CRC Handbook of Chemistry and Physics. Ed. 80th Edition, CRC Press, 1999.
Table 1. Chemical composition of metal substrates
Co-Cr-Mo (VITALIUM)
Co
Cr
Mo
Si
Mn
Fe
Ni
C
63.09
28.3
6.36
0.77
0.67
0.36
0.13
0.32
Stainless steel (304L)
Fe
Cr
71.27 18.04
Ni
Mn
Mo
Si
V
C
8.24
1.61
0.41
0.31
0.1
0.022
6
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5a
Figure 5b
Figure 5c
Figure 5d
Fig. 1 XRD patterns of the TiO2 powders annealed at 300÷800oC. Fig. 2 XRD patterns of the sol-gel TiO2 coatings deposited on 304L steel and annealed at 500oC and 800oC. Fig. 3 XRD patterns of the TiO2 coatings deposited on Co-Cr-Mo alloy and annealed at o
500 C÷900oC. Fig. 4 XRD patterns of the TiO2 coatings deposited on a silicon substrate and annealed at o
500 C÷1000oC. Fig. 5 SEM images of the TiO2 coatings produced on: a) Si, annealed at 700oC – anatase; b) Si, annealed at 1000oC – anatase + rutile; c) Co-Cr-Mo alloy, annealed at 500oC – anatase; d) Co-Cr-Mo alloy annealed at 900oC – rutile.