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Growth and structure of TiO2 thin films deposited inside borosilicate tubes by spray pyrolysis
Surface & Coatings Technology 200 (2006) 4111 – 4116 www.elsevier.com/locate/surfcoat
Growth and structure of TiO2 thin films deposited inside borosilicate tubes by spray pyrolysis M. Miki-Yoshidaa,b,*, W. Antu´nez-Floresa, K. Gomez-Fierroa, L. Villa-Pandoa, R. Silveyra-Moralesa, P. Sa´nchez-Santiago, R. Martı´nez-Sa´ncheza, M. Jose´-Yacama´nb,c a
Centro de Investigacio´n en Materiales Avanzados, Miguel de Cervantes 120, Chihuahua, Chih., C.P. 31109, Me´xico b Texas Materials Institute, University of Texas, Austin, TX 78712, USA c Department of Chemical Engineering, University of Texas, Austin, TX 78712, USA Received 6 October 2004; accepted in revised form 6 March 2005 Available online 12 May 2005
Abstract Titanium dioxide thin films were deposited inside borosilicate glass tubes by a simple, reproducible spray pyrolysis technique. The tubes had 22 mm of internal diameter and 120 cm of length. Films were transparent, uniform on almost 80% of the tube length, non-light scattering, and well adhered to the substrate. Transmission electron microscopy analysis shows that titanium oxide films are polycrystalline, and their structure corresponds to a mixed phase of anatase and rutile. The microstructure of the films was built by irregular closed-packed grains of around 100 – 500 nm wide. Cross sections of the films were analyzed by scanning electron microscopy and Z-contrast techniques. This analysis showed that the grains were composed by very small crystallites of about 3 – 5 nm. D 2005 Elsevier B.V. All rights reserved. Keywords: Scanning electron microscopy (SEM); Transmission electron microscopy (TEM); Transmission high energy electron defraction; Electron energy loss spectroscopy; Titanium oxide; Borosilicate tube coating
1. Introduction TiO2 has become a photocatalyst in environmental decontamination for a diversity of organics [1 –3], viruses, bacteria, fungi, algae, and cancer cells [4,5], which can be degraded and mineralized to CO2, H2O, and harmless inorganic anions. The photoinduced processes originate when photons of higher energy than the band gap are absorbed; consequently an electron can be promoted to the conduction band, leaving a hole in the valence band. This excited electron and the highly oxidizing hole can be used directly to drive a chemical reaction, called photocatalysis. TiO2 is mainly used in the form of powders. Nevertheless, the use of photocatalytic thin films has advantages; the
* Corresponding author. Centro de Investigacio´n en Materiales Avanzados, Miguel de Cervantes 120, Chihuahua, Chih., C.P. 31109, Me´xico. E-mail address: [email protected] (M. Miki-Yoshida). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.03.025
active material is immobilized on the substrate, avoiding the separation process required when powders are used. In addition, the thickness, as well as the amount, of the active material can be optimised in order to use the minimum required to obtain the best activity. TiO2-covered tube could be used to decompose organic compounds or to sterilize microbial cells on air or water flows, in solar panel reactors or indoor panel irradiated with long-wave UV radiation. This feature renders the photocatalyst-covered tube applicable to environmental protection, especially in medical facilities, production, or experimental environments where biological contamination must be prevented. Moreover, the method described in this work has potential application in photo-thermal solar energy collectors to deposit transparent selective materials. Titanium dioxide films can be prepared by many deposition techniques: sol – gel [6], atmospheric pressure metal organic chemical vapour deposition [7], reactive cathodic vacuum arc deposition [8], electron beam evapo-
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90% degradation of butane in 2 h. Details of these tests will be presented elsewhere.
2. Experimental Titanium dioxide thin films were obtained inside borosilicate glass tubes by a new, very simple SP technique. The overall dimensions of the tubes were an internal diameter of 22 mm and a length of 120 cm. The schematic principle of the spraying system is illustrated in Fig. 1. The borosilicate glass tube (a) has been coupled to a medical nebulizer (b), which was used as an atomizer. A three-zone cylindrical furnace M-HTF 55667C heated this tube (c), with a precise temperature control (T 1 K). The starting solution was a 0.1 mol dm 3 dilution of titanyl acetylacetonate (Merck > 98%) in absolute ethanol. The process started with the aerosol generation of precursor solution in the nebulizer. This aerosol was subsequently conveyed by the carrier gas and injected directly into the heated tube inside the cylindrical furnace. The carrier gas was micro-filtered air, the pressure was kept at 310 kPa, and the flux was controlled with a mass flow control between 142 and 250 cm3 s 1. A three-step deposition method was applied. In the first step, samples were obtained by heating only zone 1 (exit) of the tube furnace at 623 K. The other two zones were left under the natural processes of heat transfer; the temperature of each section was recorded. After the first deposition step, other films were deposited by heating zones 1 and 2 (exit and centre) of the cylindrical furnace at 623 K. Finally, some films were obtained by an additional third step; in this case, the furnace temperature was fixed at 623 K in the three zones. In order to elucidate the effect of furnace temperature, some samples were
Fig. 1. Schematic diagram of the spraying system.
ration [9], reactive magnetron sputtering [10], and spray pyrolysis (SP) [3,11,12]. Among them, the SP technique has a relatively low cost; it is simple to manipulate, applicable to large-scale area, and, as demonstrated in this work, susceptible to use in thin film deposition on tube walls. This work reports on the preparation of uniform, transparent, non-light scattering thin films of TiO2 inside borosilicate glass tubes of 22 mm internal diameter and 120 cm length. We have used a simple, reproducible SP technique similar to that previously reported to deposit TiO2 and ZnO films inside fused silica tubing [3]; the main difference is that, in this work, the cylindrical furnace had three heating zones of 30 cm each, and it was fixed in relation with the tube. The microstructure of the films was analysed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). In addition, the thickness of the films was determined by UV – VIS reflectance spectrophotometry. Preliminary results about the photocatalytic activity of the films showed more than
Table 1 Principal preparation conditions of the TiO2 films (see text) Step (1)
T [K]
Step (2)
T [K]
Step (3)
T [K]
SampleY
A
B
C
D
E
F
G
H
I
J
Tube’s exit Exit zone 1 Centre zone 2 Entry zone 3 Tube’s entry t [s] Zones 1 and 2 Zone 3 t [s] Zone 1, 2, and 3 t [s] f AIR [cm3 s 1] f S [ml s 1] d [nm] rr v d [nm s 1]
416 598 529 494 295 600
471 623 544 505 297 600
488 648 562 515 303 600
499 673 600 553 306 600
514 698 625 573 307 600
528 723 648 593 309 600
– 623 551 514 – 480
– 623 548 510 – 480
– 623 541 505 – 480 623 551 288
142 0.033 141 0.23 0.23
142 0.036 161 0.30 0.27
142 0.034 185 0.28 0.31
150 0.033 262 0.42 0.44
150 0.030 232 0.55 0.39
150 0.028 234 0.54 0.39
217 0.051 153 0.54 0.32
250 0.054 120 0.20 0.25
250 0.056 200 0.21 0.26
– 623 537 497 – 480 623 539 288 623 192 250 0.055 361 0.22 0.38
Furnace temperature measured at the middle of each zone (exit zone 1, centre zone 2, and entry zone 3); total spraying time t; air flux f AIR; solution flux f S; average film thickness d; relative standard deviation of thickness r r; average deposition rate v d. For samples A – F, the temperatures at the tube’s exit and entry are included.
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were mounted on a 1000-mesh copper grid. Some samples were analyzed by electron energy loss spectroscopy (EELS) to evaluate the atomic ratio Ti/O using an Enfina system.
3. Results and discussion 3.1. Thickness profile and deposition rate
Fig. 2. Thickness profile for samples obtained at different temperatures by applying only step (1) of the preparation method (films A – F). The error bars indicate the standard deviation of the thickness of each sample.
deposited, considering only the first step and varying the furnace temperature in the range of 598– 723 K. Table 1 presents the principal preparation parameters used in this work. All the samples were prepared with intermittent spraying to improve film thickness uniformity and overall quality. During the rest period, a ventilation flow was maintained. The spraying time was between 60 and 120 s, and the rest time was 300 s. After deposition, all the films were heated in air at 725 K for 2 h to decompose all organic residues deposited in the surface and to stabilize the films’ microstructure; after that, the samples were left to cool down inside the furnace naturally. UV –VIS reflectance spectra were used for thickness determination. These spectra were obtained in an F-20 UV optical fibber reflectance spectrophotometer in contact probe mode. The tubes were cut in many sections to establish the film thickness profile, axially and azimuthally. The same pieces were used in the analysis by SEM and for the preparation of TEM samples. SEM analysis served to determine potential contaminants and to establish the surface morphology and cross-sectional microstructure of the films. This examination was realized in a JSM-5800LV scanning electron microscope, coupled with a DX-Prime energy-dispersive X-ray spectrometer (EDS). TEM analysis has been used primordially to evaluate the crystalline structure because X-ray diffraction analysis was not possible due to the curvature of the samples. TEM micrographs and selected area electron diffraction patterns (SAED) were obtained in a 200 kV high-resolution transmission electron microscope (JEM-2010F), equipped with a field emission gun and coupled with an Inca EDS system. Scanning transmission electron microscopy (STEM) images were also acquired with a high-angle annular dark field (HAADF) detector; the uniformity and homogeneity of the samples were analysed by this method. For TEM studies, the films were peeled off the borosilicate glass substrate by immersion in diluted HF (¨ 5 vol.%); immediately after, they were floated and rinsed in deionized water; finally, they
Thickness profile (or distribution) was obtained by measuring several circumferential points in different sections along the length of the tube. Fig. 2 shows the average thickness at each section as a function of the axial coordinate (z) of samples obtained at different temperatures, applying only step 1 (films A – F). The distribution has some interesting features. Near the entry of the tube (i.e., around the second section; ¨ 30 cm from the entry), the maximum of the profile increases very sharply as the furnace temperature increases and shifts towards the entry of the tube. It is shown that the film grew faster in this portion of the tube, despite the higher temperature at the exit section. In the other region, from around the middle to near the exit of the tube, the thickness profile was almost the same for all the samples (i.e., the average thickness in each section was independent of the furnace temperature). The data obtained from these films were used to analyze the deposition kinetics along the length of the tube (see below). The average thickness of each tube and its relative standard deviation (standard deviation divided by the average thickness) are tabulated in Table 1. Fig. 3 shows the thickness profile as a function of the air flux (: 142; r: 217; >: 250 cm3 s 1) for films prepared applying only step 1 (films B, G, and H); the shift of the profile’s maxima towards the exit of the tube as the air flux increases can be observed. The thickness profiles for the films obtained using one (film H; g), two (film I; +), and three (film J; ?) steps are presented in Fig. 4. It is worthwhile to indicate that a uniform thickness along the
Fig. 3. Thickness profile along the length of the tube for films obtained at different air fluxes (, sample B: 142; r, sample G: 217; >, sample H: 250 cm3 s 1). Only step (1) of the preparation method was employed. The error bars indicate the standard deviation of the thickness of each sample.
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Fig. 4. Thickness profile for films obtained at a fixed air flux (250 cm3 s 1) by applying the different steps of preparation: one step (g, sample H), two steps (+, sample I), and three steps (?, sample J). The error bars indicate the standard deviation of the thickness of each sample.
tube is obtained using one step or two steps (i.e., by heating only the exit or the exit and central zones of the tube). In these cases, at a fixed section, the circumferential variation of the thickness can be as low as 5– 10% of the section average, and the average thickness of one section varies about 10– 15% around the average thickness of the tube. For non-optimized deposition conditions, the variation of the thickness can be very large (see Fig. 2 and Table 1). At this regard, we are working to improve the uniformity of the film, axially and azimuthally. We are implementing a rotating device in order to rotate the tube around its axis, and a tube displacement system with a fixed furnace and nozzle to distribute more uniformly the precursor along the length of the tube. The average deposition rate of each section was determined as a function of the section’s temperature, in order to analyze the deposition kinetics along the length of
Fig. 6. Arrhenius plot of deposition rate versus reciprocal Ts in the sections of the tubes. Films were obtained at different temperatures (samples A – F), using deposition step (1).
the tube. The deposition rate of each section was obtained, considering the average thickness and total deposition time. In addition to the three temperatures measured inside the furnace, we have measured two more points, near the tube’s entry and exit. Linear interpolation was used to determine the temperature along the length of the tube. Fig. 5 illustrates the location of the different sections (S1 – S6). As an example, the thickness profile (d), furnace temperature (T) of sample E, and temperature of each section (Ts) used in the analysis of the deposition kinetics are also presented. Fig. 6 shows the Arrhenius plot of deposition rate versus reciprocal temperature (Tsi ) of each section of the tube, where Tsi is the temperature of section i. It can be observed two different regimes for the deposition rate. In sections 2, 3, and 4, the growth rate increases with the temperature of the section (i.e., they are activated regions, in which the deposition rate is limited by the surface reaction kinetics, by processes occurring at or near the tube surface) [13]. In the second regime, for sections 1, 5, and 6, the growth rate is almost independent of the tube temperature; in this case, the process is mass transport-controlled. In section 1, probably as a consequence of the low furnace temperature and the
Table 2 Activation energy of each section determined from the slope of the Arrhenius plot
Fig. 5. Schema showing the six different sections (Si ) used for the Arrhenius plot of Fig. 6. As an example, the thickness profile (d), furnace temperature distribution (T), and interpolated temperatures (Ts) of each section in the sample are also shown.
Section
z [cm]
E a [kJ mol 1]
1 2 3 4 5 6
16.5 33.0 49.5 66.0 82.5 99.0
0 38 15 12 0.1 0.1
It was found that the activation energy diminished continuously from sections 2 to 6.
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Table 3 Interplanar distances obtained from the analysis of SAED patterns of several samples
Fig. 7. Typical EELS spectra of TiO2 films. A good matching between this spectrum and that reported in the EELS atlas of Bryant [13] can be observed.
(hkl)
d ICDD [nm]
Phase
d exp [nm]
(101) (004) (200) (105) (213) (116) (107) (202) (303) (410) (411)
0.35200 0.2378 0.18920 0.17000 0.14930 0.13640 0.12800 0.12441 0.11725 0.11143 0.10425
A A A A A A A R A R R
0.348 0.238 0.189 0.167 0.150 0.137 0.127 0.122 0.116 0.109 0.104
We found diffraction rings or spots corresponding to anatase (A; PDF 211272) and rutile (R; PDF 21-1276).
short residence time, the precursor salt did not heat up sufficiently to evaporate or decompose to form the gaseous reactants; therefore, the concentration of reactants should be very low. In sections 5 and 6, on the contrary, most of the reactants have already decomposed and reacted. For sections 2– 5, the activation energy, determined from the slope of the Arrhenius plot, diminished continuously (see Table 2) from 38 to ¨ 0 kJ mol 1, then it stayed constant in sections 5 and 6. The change in the rate-limiting mechanism from kinetics (sections 2– 4) to mass transport (sections 5 and 6) is probably due to the depletion of reactants [14], principally as a consequence of the precursor decomposition, and to the homogeneous reaction with the oxygen present as the flow of sprayed solution traverses the tube. 3.2. Composition EDS testing indicates that carbon was the main contaminant of the as-deposited films, probably due to organic residues from the decomposition of the precursor salts. Ti/O atomic ratio determined by EDS was in the range between 0.5 and 0.6. Furthermore, EELS analysis indicates that the atomic ratio Ti/O was around 0.5. Fig. 7 shows a typical
Fig. 8. Characteristic TEM micrographs of undoped titanium dioxide film. (a) Bright field image showing the close-packed grains in the microstructure of the film. Inset shows SAED pattern of the sample. (b) STEM (Z-contrast) image showing the small crystallites (3 – 5 nm) inside the grains.
Fig. 9. Secondary electron SEM micrographs showing surface morphology and cross-sectional image of TiO2 film.
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EELS spectrum of TiO2 film; a very good matching between the EELS spectrum and the one reported in the literature can be observed [15]. 3.3. Microstructure Fig. 8a shows a distinctive bright field TEM micrograph of sample A. Irregular closed-packed grains, around 100 – 500 nm wide, form the film microstructure. The inset shows a typical SAED pattern of the films. From the analysis of the several SAED patterns, we concluded that titanium oxide films were polycrystalline, and that their structure corresponded to a mixed phase of anatase [16] and rutile [17]. Table 3 shows the interplanar distance obtained from these patterns. A typical STEM Z-contrast image of the films is presented in Fig. 8b. It is shown that the films were homogeneous and composed by very small crystallites of around 3 – 5 nm. A more detailed analysis of the microstructure of the films in relation to the parameters of synthesis is in course. Surface morphology and cross-sectional microstructure of the films were also analyzed by SEM. Fig. 9 shows a typical secondary electron SEM micrograph of sample H. The microstructure consists of densely packed grains with a smooth surface, which can be interpreted as a transition structure between a porous arrangement of tapered crystallites and dense columnar grains. 4. Conclusions The feasibility of depositing homogeneous, transparent, non-light-scattering films of TiO2 inside a borosilicate glass tube by a simple, reproducible spray pyrolysis technique has been demonstrated. No important contamination was detected by EDS and EELS analyses. The stoichiometry was close to TiO2, probably with some O deficiency. SAED patterns showed predominantly two phases: anatase and rutile.
Acknowledgments The authors would like to thank to F. Paraguay, M. Roma´n, P. Castillo, D. Lardizabal, S. Miranda, and M. Moreno for their valuable assistance. This work was partially supported by a grant from CONACYT (project no. 34325-U).
References [1] Y. Paz, A. Heller, J. Mater. Res. 12 (1997) 2759, (and references therein). [2] Jean-Marie Herrmann, Catal. Today 53 (1) (1999) 115. [3] M. Miki-Yoshida, V. Collins-Martinez, P. Amezaga-Madrid, A. Aguilar-Elguezabal, Thin Solid Films 419 (2002) 60. [4] K. Sunada, Y. Kikuchi, K. Hashimoto, A. Fujishima, Environ. Sci. Technol. 32 (5) (1998) 726. [5] W.A. Jacoby, P. Ching Maness, E.J. Wolfrum, D.M. Blake, J.A. Fennell, Environ. Sci. Technol. 32 (17) (1998) 2650. [6] J.A. Navı´o, G. Colo´n, M. Macı´as, C. Real, M.I. Litter, Appl. Catal., A Gen. 177 (1999) 111. [7] V. Gauthier, S. Bourgeois, P. Sibillot, M. Maglione, M. Sacilotti, Thin Solid Films 340 (1999) 175. [8] H. Takikawa, T. Matsui, T. Sakakibara, A. Bendavid, P.J. Martin, Thin Solid Films 348 (1999) 145. [9] Y. Leprince-Wang, K. Yu-Zhang, V. Nguyen Van, D. Souche, J. Rivory, Thin Solid Films 307 (1997) 38. [10] M. Strfmme, A. Gutarra, G.A. Niklasson, C.G. Granqvist, J. Appl. Phys. 79 (7) (1996) 3749. [11] C. Natarajan, N. Fukunaga, G. Nogami, Thin Solid Films 322 (1998) 6. [12] W.A. Badawy, J. Mater. Sci. 32 (1997) 4979. [13] W.A. Bryant, J. Mater. Sci. 12 (1977) 1285. [14] K.L. Choy, Prog. Mater. Sci. 48 (2003) 57. [15] C.C. Ahn, O.L. Krivanek, Publication of Center for Solid State Science, Arizona State University and Gatan, Inc., 1983. [16] Joint Committee on Powder Diffraction Standards, Powder Diffraction File, International Center for Diffraction Data, Swarthmore, PA, 1996, (card 21-1272). [17] Joint Committee on Powder Diffraction Standards, Powder Diffraction File, International Center for Diffraction Data, Swarthmore, PA, 1996, (card 21-1276).
Growth and structure of TiO2 thin films deposited inside borosilicate tubes by spray pyrolysis M. Miki-Yoshidaa,b,*, W. Antu´nez-Floresa, K. Gomez-Fierroa, L. Villa-Pandoa, R. Silveyra-Moralesa, P. Sa´nchez-Santiago, R. Martı´nez-Sa´ncheza, M. Jose´-Yacama´nb,c a
Centro de Investigacio´n en Materiales Avanzados, Miguel de Cervantes 120, Chihuahua, Chih., C.P. 31109, Me´xico b Texas Materials Institute, University of Texas, Austin, TX 78712, USA c Department of Chemical Engineering, University of Texas, Austin, TX 78712, USA Received 6 October 2004; accepted in revised form 6 March 2005 Available online 12 May 2005
Abstract Titanium dioxide thin films were deposited inside borosilicate glass tubes by a simple, reproducible spray pyrolysis technique. The tubes had 22 mm of internal diameter and 120 cm of length. Films were transparent, uniform on almost 80% of the tube length, non-light scattering, and well adhered to the substrate. Transmission electron microscopy analysis shows that titanium oxide films are polycrystalline, and their structure corresponds to a mixed phase of anatase and rutile. The microstructure of the films was built by irregular closed-packed grains of around 100 – 500 nm wide. Cross sections of the films were analyzed by scanning electron microscopy and Z-contrast techniques. This analysis showed that the grains were composed by very small crystallites of about 3 – 5 nm. D 2005 Elsevier B.V. All rights reserved. Keywords: Scanning electron microscopy (SEM); Transmission electron microscopy (TEM); Transmission high energy electron defraction; Electron energy loss spectroscopy; Titanium oxide; Borosilicate tube coating
1. Introduction TiO2 has become a photocatalyst in environmental decontamination for a diversity of organics [1 –3], viruses, bacteria, fungi, algae, and cancer cells [4,5], which can be degraded and mineralized to CO2, H2O, and harmless inorganic anions. The photoinduced processes originate when photons of higher energy than the band gap are absorbed; consequently an electron can be promoted to the conduction band, leaving a hole in the valence band. This excited electron and the highly oxidizing hole can be used directly to drive a chemical reaction, called photocatalysis. TiO2 is mainly used in the form of powders. Nevertheless, the use of photocatalytic thin films has advantages; the
* Corresponding author. Centro de Investigacio´n en Materiales Avanzados, Miguel de Cervantes 120, Chihuahua, Chih., C.P. 31109, Me´xico. E-mail address: [email protected] (M. Miki-Yoshida). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.03.025
active material is immobilized on the substrate, avoiding the separation process required when powders are used. In addition, the thickness, as well as the amount, of the active material can be optimised in order to use the minimum required to obtain the best activity. TiO2-covered tube could be used to decompose organic compounds or to sterilize microbial cells on air or water flows, in solar panel reactors or indoor panel irradiated with long-wave UV radiation. This feature renders the photocatalyst-covered tube applicable to environmental protection, especially in medical facilities, production, or experimental environments where biological contamination must be prevented. Moreover, the method described in this work has potential application in photo-thermal solar energy collectors to deposit transparent selective materials. Titanium dioxide films can be prepared by many deposition techniques: sol – gel [6], atmospheric pressure metal organic chemical vapour deposition [7], reactive cathodic vacuum arc deposition [8], electron beam evapo-
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90% degradation of butane in 2 h. Details of these tests will be presented elsewhere.
2. Experimental Titanium dioxide thin films were obtained inside borosilicate glass tubes by a new, very simple SP technique. The overall dimensions of the tubes were an internal diameter of 22 mm and a length of 120 cm. The schematic principle of the spraying system is illustrated in Fig. 1. The borosilicate glass tube (a) has been coupled to a medical nebulizer (b), which was used as an atomizer. A three-zone cylindrical furnace M-HTF 55667C heated this tube (c), with a precise temperature control (T 1 K). The starting solution was a 0.1 mol dm 3 dilution of titanyl acetylacetonate (Merck > 98%) in absolute ethanol. The process started with the aerosol generation of precursor solution in the nebulizer. This aerosol was subsequently conveyed by the carrier gas and injected directly into the heated tube inside the cylindrical furnace. The carrier gas was micro-filtered air, the pressure was kept at 310 kPa, and the flux was controlled with a mass flow control between 142 and 250 cm3 s 1. A three-step deposition method was applied. In the first step, samples were obtained by heating only zone 1 (exit) of the tube furnace at 623 K. The other two zones were left under the natural processes of heat transfer; the temperature of each section was recorded. After the first deposition step, other films were deposited by heating zones 1 and 2 (exit and centre) of the cylindrical furnace at 623 K. Finally, some films were obtained by an additional third step; in this case, the furnace temperature was fixed at 623 K in the three zones. In order to elucidate the effect of furnace temperature, some samples were
Fig. 1. Schematic diagram of the spraying system.
ration [9], reactive magnetron sputtering [10], and spray pyrolysis (SP) [3,11,12]. Among them, the SP technique has a relatively low cost; it is simple to manipulate, applicable to large-scale area, and, as demonstrated in this work, susceptible to use in thin film deposition on tube walls. This work reports on the preparation of uniform, transparent, non-light scattering thin films of TiO2 inside borosilicate glass tubes of 22 mm internal diameter and 120 cm length. We have used a simple, reproducible SP technique similar to that previously reported to deposit TiO2 and ZnO films inside fused silica tubing [3]; the main difference is that, in this work, the cylindrical furnace had three heating zones of 30 cm each, and it was fixed in relation with the tube. The microstructure of the films was analysed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). In addition, the thickness of the films was determined by UV – VIS reflectance spectrophotometry. Preliminary results about the photocatalytic activity of the films showed more than
Table 1 Principal preparation conditions of the TiO2 films (see text) Step (1)
T [K]
Step (2)
T [K]
Step (3)
T [K]
SampleY
A
B
C
D
E
F
G
H
I
J
Tube’s exit Exit zone 1 Centre zone 2 Entry zone 3 Tube’s entry t [s] Zones 1 and 2 Zone 3 t [s] Zone 1, 2, and 3 t [s] f AIR [cm3 s 1] f S [ml s 1] d [nm] rr v d [nm s 1]
416 598 529 494 295 600
471 623 544 505 297 600
488 648 562 515 303 600
499 673 600 553 306 600
514 698 625 573 307 600
528 723 648 593 309 600
– 623 551 514 – 480
– 623 548 510 – 480
– 623 541 505 – 480 623 551 288
142 0.033 141 0.23 0.23
142 0.036 161 0.30 0.27
142 0.034 185 0.28 0.31
150 0.033 262 0.42 0.44
150 0.030 232 0.55 0.39
150 0.028 234 0.54 0.39
217 0.051 153 0.54 0.32
250 0.054 120 0.20 0.25
250 0.056 200 0.21 0.26
– 623 537 497 – 480 623 539 288 623 192 250 0.055 361 0.22 0.38
Furnace temperature measured at the middle of each zone (exit zone 1, centre zone 2, and entry zone 3); total spraying time t; air flux f AIR; solution flux f S; average film thickness d; relative standard deviation of thickness r r; average deposition rate v d. For samples A – F, the temperatures at the tube’s exit and entry are included.
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were mounted on a 1000-mesh copper grid. Some samples were analyzed by electron energy loss spectroscopy (EELS) to evaluate the atomic ratio Ti/O using an Enfina system.
3. Results and discussion 3.1. Thickness profile and deposition rate
Fig. 2. Thickness profile for samples obtained at different temperatures by applying only step (1) of the preparation method (films A – F). The error bars indicate the standard deviation of the thickness of each sample.
deposited, considering only the first step and varying the furnace temperature in the range of 598– 723 K. Table 1 presents the principal preparation parameters used in this work. All the samples were prepared with intermittent spraying to improve film thickness uniformity and overall quality. During the rest period, a ventilation flow was maintained. The spraying time was between 60 and 120 s, and the rest time was 300 s. After deposition, all the films were heated in air at 725 K for 2 h to decompose all organic residues deposited in the surface and to stabilize the films’ microstructure; after that, the samples were left to cool down inside the furnace naturally. UV –VIS reflectance spectra were used for thickness determination. These spectra were obtained in an F-20 UV optical fibber reflectance spectrophotometer in contact probe mode. The tubes were cut in many sections to establish the film thickness profile, axially and azimuthally. The same pieces were used in the analysis by SEM and for the preparation of TEM samples. SEM analysis served to determine potential contaminants and to establish the surface morphology and cross-sectional microstructure of the films. This examination was realized in a JSM-5800LV scanning electron microscope, coupled with a DX-Prime energy-dispersive X-ray spectrometer (EDS). TEM analysis has been used primordially to evaluate the crystalline structure because X-ray diffraction analysis was not possible due to the curvature of the samples. TEM micrographs and selected area electron diffraction patterns (SAED) were obtained in a 200 kV high-resolution transmission electron microscope (JEM-2010F), equipped with a field emission gun and coupled with an Inca EDS system. Scanning transmission electron microscopy (STEM) images were also acquired with a high-angle annular dark field (HAADF) detector; the uniformity and homogeneity of the samples were analysed by this method. For TEM studies, the films were peeled off the borosilicate glass substrate by immersion in diluted HF (¨ 5 vol.%); immediately after, they were floated and rinsed in deionized water; finally, they
Thickness profile (or distribution) was obtained by measuring several circumferential points in different sections along the length of the tube. Fig. 2 shows the average thickness at each section as a function of the axial coordinate (z) of samples obtained at different temperatures, applying only step 1 (films A – F). The distribution has some interesting features. Near the entry of the tube (i.e., around the second section; ¨ 30 cm from the entry), the maximum of the profile increases very sharply as the furnace temperature increases and shifts towards the entry of the tube. It is shown that the film grew faster in this portion of the tube, despite the higher temperature at the exit section. In the other region, from around the middle to near the exit of the tube, the thickness profile was almost the same for all the samples (i.e., the average thickness in each section was independent of the furnace temperature). The data obtained from these films were used to analyze the deposition kinetics along the length of the tube (see below). The average thickness of each tube and its relative standard deviation (standard deviation divided by the average thickness) are tabulated in Table 1. Fig. 3 shows the thickness profile as a function of the air flux (: 142; r: 217; >: 250 cm3 s 1) for films prepared applying only step 1 (films B, G, and H); the shift of the profile’s maxima towards the exit of the tube as the air flux increases can be observed. The thickness profiles for the films obtained using one (film H; g), two (film I; +), and three (film J; ?) steps are presented in Fig. 4. It is worthwhile to indicate that a uniform thickness along the
Fig. 3. Thickness profile along the length of the tube for films obtained at different air fluxes (, sample B: 142; r, sample G: 217; >, sample H: 250 cm3 s 1). Only step (1) of the preparation method was employed. The error bars indicate the standard deviation of the thickness of each sample.
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Fig. 4. Thickness profile for films obtained at a fixed air flux (250 cm3 s 1) by applying the different steps of preparation: one step (g, sample H), two steps (+, sample I), and three steps (?, sample J). The error bars indicate the standard deviation of the thickness of each sample.
tube is obtained using one step or two steps (i.e., by heating only the exit or the exit and central zones of the tube). In these cases, at a fixed section, the circumferential variation of the thickness can be as low as 5– 10% of the section average, and the average thickness of one section varies about 10– 15% around the average thickness of the tube. For non-optimized deposition conditions, the variation of the thickness can be very large (see Fig. 2 and Table 1). At this regard, we are working to improve the uniformity of the film, axially and azimuthally. We are implementing a rotating device in order to rotate the tube around its axis, and a tube displacement system with a fixed furnace and nozzle to distribute more uniformly the precursor along the length of the tube. The average deposition rate of each section was determined as a function of the section’s temperature, in order to analyze the deposition kinetics along the length of
Fig. 6. Arrhenius plot of deposition rate versus reciprocal Ts in the sections of the tubes. Films were obtained at different temperatures (samples A – F), using deposition step (1).
the tube. The deposition rate of each section was obtained, considering the average thickness and total deposition time. In addition to the three temperatures measured inside the furnace, we have measured two more points, near the tube’s entry and exit. Linear interpolation was used to determine the temperature along the length of the tube. Fig. 5 illustrates the location of the different sections (S1 – S6). As an example, the thickness profile (d), furnace temperature (T) of sample E, and temperature of each section (Ts) used in the analysis of the deposition kinetics are also presented. Fig. 6 shows the Arrhenius plot of deposition rate versus reciprocal temperature (Tsi ) of each section of the tube, where Tsi is the temperature of section i. It can be observed two different regimes for the deposition rate. In sections 2, 3, and 4, the growth rate increases with the temperature of the section (i.e., they are activated regions, in which the deposition rate is limited by the surface reaction kinetics, by processes occurring at or near the tube surface) [13]. In the second regime, for sections 1, 5, and 6, the growth rate is almost independent of the tube temperature; in this case, the process is mass transport-controlled. In section 1, probably as a consequence of the low furnace temperature and the
Table 2 Activation energy of each section determined from the slope of the Arrhenius plot
Fig. 5. Schema showing the six different sections (Si ) used for the Arrhenius plot of Fig. 6. As an example, the thickness profile (d), furnace temperature distribution (T), and interpolated temperatures (Ts) of each section in the sample are also shown.
Section
z [cm]
E a [kJ mol 1]
1 2 3 4 5 6
16.5 33.0 49.5 66.0 82.5 99.0
0 38 15 12 0.1 0.1
It was found that the activation energy diminished continuously from sections 2 to 6.
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Table 3 Interplanar distances obtained from the analysis of SAED patterns of several samples
Fig. 7. Typical EELS spectra of TiO2 films. A good matching between this spectrum and that reported in the EELS atlas of Bryant [13] can be observed.
(hkl)
d ICDD [nm]
Phase
d exp [nm]
(101) (004) (200) (105) (213) (116) (107) (202) (303) (410) (411)
0.35200 0.2378 0.18920 0.17000 0.14930 0.13640 0.12800 0.12441 0.11725 0.11143 0.10425
A A A A A A A R A R R
0.348 0.238 0.189 0.167 0.150 0.137 0.127 0.122 0.116 0.109 0.104
We found diffraction rings or spots corresponding to anatase (A; PDF 211272) and rutile (R; PDF 21-1276).
short residence time, the precursor salt did not heat up sufficiently to evaporate or decompose to form the gaseous reactants; therefore, the concentration of reactants should be very low. In sections 5 and 6, on the contrary, most of the reactants have already decomposed and reacted. For sections 2– 5, the activation energy, determined from the slope of the Arrhenius plot, diminished continuously (see Table 2) from 38 to ¨ 0 kJ mol 1, then it stayed constant in sections 5 and 6. The change in the rate-limiting mechanism from kinetics (sections 2– 4) to mass transport (sections 5 and 6) is probably due to the depletion of reactants [14], principally as a consequence of the precursor decomposition, and to the homogeneous reaction with the oxygen present as the flow of sprayed solution traverses the tube. 3.2. Composition EDS testing indicates that carbon was the main contaminant of the as-deposited films, probably due to organic residues from the decomposition of the precursor salts. Ti/O atomic ratio determined by EDS was in the range between 0.5 and 0.6. Furthermore, EELS analysis indicates that the atomic ratio Ti/O was around 0.5. Fig. 7 shows a typical
Fig. 8. Characteristic TEM micrographs of undoped titanium dioxide film. (a) Bright field image showing the close-packed grains in the microstructure of the film. Inset shows SAED pattern of the sample. (b) STEM (Z-contrast) image showing the small crystallites (3 – 5 nm) inside the grains.
Fig. 9. Secondary electron SEM micrographs showing surface morphology and cross-sectional image of TiO2 film.
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EELS spectrum of TiO2 film; a very good matching between the EELS spectrum and the one reported in the literature can be observed [15]. 3.3. Microstructure Fig. 8a shows a distinctive bright field TEM micrograph of sample A. Irregular closed-packed grains, around 100 – 500 nm wide, form the film microstructure. The inset shows a typical SAED pattern of the films. From the analysis of the several SAED patterns, we concluded that titanium oxide films were polycrystalline, and that their structure corresponded to a mixed phase of anatase [16] and rutile [17]. Table 3 shows the interplanar distance obtained from these patterns. A typical STEM Z-contrast image of the films is presented in Fig. 8b. It is shown that the films were homogeneous and composed by very small crystallites of around 3 – 5 nm. A more detailed analysis of the microstructure of the films in relation to the parameters of synthesis is in course. Surface morphology and cross-sectional microstructure of the films were also analyzed by SEM. Fig. 9 shows a typical secondary electron SEM micrograph of sample H. The microstructure consists of densely packed grains with a smooth surface, which can be interpreted as a transition structure between a porous arrangement of tapered crystallites and dense columnar grains. 4. Conclusions The feasibility of depositing homogeneous, transparent, non-light-scattering films of TiO2 inside a borosilicate glass tube by a simple, reproducible spray pyrolysis technique has been demonstrated. No important contamination was detected by EDS and EELS analyses. The stoichiometry was close to TiO2, probably with some O deficiency. SAED patterns showed predominantly two phases: anatase and rutile.
Acknowledgments The authors would like to thank to F. Paraguay, M. Roma´n, P. Castillo, D. Lardizabal, S. Miranda, and M. Moreno for their valuable assistance. This work was partially supported by a grant from CONACYT (project no. 34325-U).
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