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Morphological study of TiO₂ thin films doped with cobalt by Metal Organic Chemical Vapor Deposition
Results in Physics 16 (2020) 102891
Contents lists available at ScienceDirect
Results in Physics journal homepage: www.elsevier.com/locate/rinp
Morphological study of TiO₂ thin films doped with cobalt by Metal Organic Chemical Vapor Deposition
T
⁎
Néstor Méndez-Lozanoa, ,1, Miguel Apátiga-Castrob, Alejandro Manzano-Ramírezc, Eric M. Rivera-Muñozb, Rodrigo Velázquez-Castillod, Carlos Alberto-Gonzáleza, Marco Zamora-Antuñanoa a
Universidad del Valle de México, Campus Querétaro, Blvd. Juriquilla No. 1000 A, Del. Santa Rosa Jáuregui, Querétaro, Qro C.P. 76230, Mexico Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México, A.P. 1-1010, Querétaro, Qro C. P. 76000, Mexico c Centro de Investigación y de Estudios Avanzados, Unidad Querétaro, Libramiento Norponiente # 2000. Fracc. Real de Juriquilla, Querétaro, Qro 76230, Mexico d División de Investigación y Posgrado, Universidad Autónoma de Querétaro, Cerro de las Campanas S/N, Querétaro, Qro C.P 76010, Mexico b
A R T I C LE I N FO
A B S T R A C T
Keywords: Thin films Anatase films Chemical vapor deposition Nanostructure
In this work, we present the synthesis of thin films of TiO2 doped with cobalt using the MOCVD technique, allowing the growth of coatings with controlled morphology and a faster deposit speed, as compared to other techniques. In addition, the synthesis parameters necessary for the reproducibility of the films were obtained. Anatase thin films doped with cobalt were prepared by the Metal Organic Chemical Vapor Deposition technique working in pulsed injection mode using a mixture of titanium (IV) isopropoxide and acetyl acetonate of cobalt at different concentrations as the precursor. The films were deposited on silicon (1 0 0) substrates at temperature of 650 °C. The crystalline structure of the deposited films was characterized by X-ray diffraction (XRD) and Raman spectroscopy. The characteristic XRD peaks and vibrational modes of TiO2 corresponding to the anatase phase were observed in all cases, without any secondary or mixed phase. On the other hand, the scanning electron microscopy (SEM) observations show that the film surfaces are formed by a higher number of porous, which are characteristic of the anatase films and the corresponding Energy Dispersion Spectroscopy (EDS) shows the elemental composition.
Introduction Solid materials grown in thin film form fall into two general classifications, depending on application: passive as for the majority of optical and mechanical coatings, and active, as for electro-optical applications. The latter include transparent conductive and photo-active coatings. For several years, the thin films of titania have attracted attention due to their electro-optical and photochemical applications [1], for example UV-light shielding, solar energy conversion and detoxification of pollutants, among others. Titania coatings generate specific response or initiate reactions when they are excited by external stimuli. For example, incident light of energy greater than the 3-eV band edge of titania generates holes and electrons. These carriers produce the oxidation and reduction reactions on the surface of titania layers responsible for the photocatalytic activity [2]. For these applications, it is generally accepted that anatase phase has demonstrated a higher activity than rutile and brookite, therefore it is of great importance to
fabricate titania thin films with a well-controlled nanostructure. On the other hand, it is possible to improve the photocatalytic properties of titania films by doping it with noble metals [3] as well as with transition metals [4,5]. Recently, the use of transition metals for the doping of anatase films like Co for example, has gained much importance due to its ferromagnetic properties and also for the improvement of the photocatalytic response in the visible region. In recent works cobaltdoped TiO2 thin films by MOCVD have been obtained to study the effect of temperature on grain size [6]. Many deposition techniques have been used to obtain Co:TiO2 thin films, such as sputtering, laser ablation, ion implantation, sol-gel and MOCVD. Among the wide range of film growth techniques, the Pulsed Injection MOCVD technique is the most feasible and widely applicable to tailor stable and high-quality films on a nanometer-scale range [7–10]. MOCVD technique is a suitable method for stoichiometric and microstructural deposition of thin films [11]. Due to the titania anatase structure provides greater photocatalytic activity than the rutile phase, the growth conditions were
⁎
Corresponding author. E-mail address: [email protected] (N. Méndez-Lozano). 1 ORCID: 0000-0001-5622-9283. https://doi.org/10.1016/j.rinp.2019.102891 Received 22 March 2019; Received in revised form 16 December 2019; Accepted 17 December 2019 Available online 19 December 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
Results in Physics 16 (2020) 102891
N. Méndez-Lozano, et al.
Characterization of Co:TiO2 films
adjusted to predominantly favor the formation of the anatase structure. Our recent works on nanostructured titania thin films and nanoparticles suggest that the preparation of highly ordered and crystalline anatase coatings requires a perfect tuning between the chemicals and the experimental conditions [12,13]. In this paper cobalt doped titanium dioxide (Co:TiO2) thin films were deposited on crystalline silicon substrates by Metal Organic Chemical Vapor Deposition (MOCVD) technique, using a liquid metalorganic precursor. The crystallinity, vibrational modes and surface morphology of the Co:TiO2 films were studied by XRD, Raman Spectroscopy and SEM respectively. This paper focuses on the investigation of the effect of cobalt as a doping agent on the structural and morphological quality of Co:TiO2 thin films deposited on silicon substrates. Highlighting the advantages of the MOCVD technique such as: possibility of epitaxial growth and control of most of the growth parameters.
X-ray powder diffraction was used to identify the crystalline phases contained in all samples. Wide angle X-ray experiments were carried out in a Rigaku Mini Flex diffractometer using the Cu k α radiation (λ = 1.5406 å), an accelerating voltage of 40 kV and a current of 30 mA. Diffractograms were recorded with a Solid-State D/teX-ULTRA Detector from 5 through 80 ° on a 2θ scale with a rate of 10 ° per minute. Spectrum analysis software, MDI Jade V 5.0.37, was used. To analyze the characteristic vibrational modes of hydroxyapatite a Raman spectrometer Bruker model Senterra was used. The operation condition was a voltage of 100 mV, a resolution of 3 to 5 cm−1 and a 20X objective. Morphological, topological and microstructural analyses of all samples were carried out in a JEOL JSM5900-LV Scanning Electron Microscope. The analysis was performed using 20 kV electron acceleration voltage and the images were formed by secondary electrons. All samples were placed on a stainless-steel cylinder and fixed with doublesided tape and covered with a gold thin film done by sputtering to avoid the electrostatic charge accumulation. In order to obtain the elemental composition of thin films, a Scanning Electron Microscope SU8230 Hitachi equipment was used with a voltage of 20 kV and a current of 0.10 nA.
Methods A vertical MOCVD reactor was used to grow the Co:TiO2 films from metal organic liquid precursors in pulsed injection mode. The reactor consists of a quartz tube and stainless-steel bodies interconnected by Orings, a detailed description has been published elsewhere [9,10]. A pulsed injection mechanism that consist of an injector, similar to those used regularly for fuel injection in internal combustion engines, delivers the exact amount of liquid precursor into the evaporation zone at 280 °C. In this way, the liquid precursor is instantaneously evaporated and then transported by a carrier gas (Ar) towards the reaction chamber, where the chemical reaction and film growth on the hot substrate occurs at 650 °C. Liquid solutions of 10 ml of titanium isopropoxide (Sigma Aldrich) mixed with different quantities of powder of cobalt acetyl acetonate (Sigma Aldrich) were used as precursor, concentrations from 2, 4, 6 and 8 at.% vol were used. The final mixture was a homogeneous liquid solution. The total amount of liquid precursor was 19 ml, which was injected at two pulses per second at a dose of 7.6 × 10−3 mL per pulse. Most of the experimental conditions such as, injector frequency (dose), substrate temperature, carrier gas flow and total pressure were measured and controlled by a computer driven system. The reaction chamber is a quartz tube surrounded by a resistive furnace that can heat the substrate up to 1000° C. Substrates as long as to 2 in. in diameter can be used easily. The pressure in the system was kept at 600 Pa with a carrier gas (Ar) flowing at 0.1 l/min. Schematics of the MOCVD apparatus for Co-doped TiO2 thin films synthesis is showed in Fig. 1.
Results and discussion Phase composition: XRD Fig. 2 shows XRD patterns of all the films deposited under the same experimental conditions but with different concentrations of cobalt. At the bottom, it is included the pattern of the TiO2 films, deposited without Co concentration in the precursor, which can be taken as reference. In all cases the Bragg reflections attributed to the crystalline anatase phase are clearly observed (JCPDS file No. 21-1272) and no peaks corresponding to cobalt or cobalt oxide phases were observed on the cobalt doped samples, suggesting that a very low amount of Co is homogeneously distributed in anatase, at a concentration below the detection limits of the XRD equipment [14,15]. In addition, a decrease in the intensity of the peaks as the cobalt concentration is increased was also observed indicating that the cobalt dopants might occupy the substitutional sites in the anatase lattice [14,16].
Fig. 2. Comparison on the diffractograms obtained for different concentrations of Co in thin films of TiO₂: (a) 2% Co: TiO₂, (b) 4% Co: TiO₂, (c) 6% Co: TiO₂, (d) 8% Co: TiO₂, (e) 10% Co: TiO₂. The Indices correspond to the anatase phase of TiO₂.
Fig. 1. Schematic of the MOCVD apparatus. 2
Results in Physics 16 (2020) 102891
N. Méndez-Lozano, et al.
Table 1 Crystallite size for all samples. Sample
% of Co
Crystallite size (nm)
TiO₂ Co:TiO₂ Co:TiO₂ Co:TiO₂ Co:TiO₂ Co:TiO₂
0 2 4 6 8 10
41 44 47 42 45 42
Fig. 5. Micrographs for 6% Co: TiO₂.
preferential growth in the direction (1 0 1) independently of the concentration of cobalt dopants, the presence of cobalt in the structure of the anatase suggests this preferential growth. Other works have reported the obtaining of TiO₂ thin films with preferential direction of growth (2 2 0) without the presence of cobalt [12]. It is also possible to consider that small amounts of cobalt oxide segregated in the samples may be invisible in the diffraction patterns. Analyzing the full width at half maximum of the (1 0 1) peak, the estimated grain size using the Scherrer equation [18] is shown in Table 1. It can be seen that there is no significant change in crystallite size by increasing the amount of cobalt impurities, which suggests an incorporation of the impurities within the structure crystalline TiO₂. Other works report the increase in crystallite size as the concentration of dopants increases [19,20]. The difference with other works is the way to deposit the precursor inside the reactor through discrete pulses.
Fig. 3. Raman spectra obtained for each of the samples of Co: TiO₂.
The diffraction peaks observed in 2 theta value of 25.5, 38 and 48 corresponding to the planes (1 0 1), (0 0 4) and (2 0 0) and are attributed to the anatase phase of the tetragonal structure. This confirms the presence of the anatase phase, which is well known as the most suitable for photocatalysis processes. The most intense peak corresponding to the plane (1 0 1), which is energetically the most stable and predominant in the crystal structure of the samples [17]. It was possible to observe that the nanostructures present a
Vibrational modes: Raman spectroscopy Fig. 3 shows the Raman spectra for all the samples in the frequency range from 100 to 800 cm−1 employing a helium-neon laser with an exciting light of 783 nm. In the spectra, it is clear the presence of the anatase phase due to all
Fig. 4. Micrographs for each of the films made of TiO₂: (a) TiO₂, (b) 2% Co: TiO₂, (c) 4% Co: TiO₂, (d) 6% Co: TiO₂, (e) 8% Co: TiO₂, (f) 10% Co: TiO₂. 3
Results in Physics 16 (2020) 102891
N. Méndez-Lozano, et al.
Fig. 6. EDS spectra confirm the elements in all samples.
the bands at 142, 398 and 637 cm−1 attributed to the anatase phase. The strong peak located at 520 cm−1 is due to the vibrational mode of silicon (1 0 0) substrate. For all concentration, the same bands are observed, the absence of vibrational modes of cobalt confirms that impurities are occupying substitutional sites in the crystalline structure of TiO₂ [16], in agreement with the results obtained by XRD. These results show a strong interaction between the titanium dioxide film and the silicon substrate. In agreement with other works, no signals related to cobalt particles are identified due to the relatively low concentration, besides a weak Raman scattering was observed [21]. Fig. 3 shows that the intensity of the bands decreased as the concentration of cobalt increases, but the Raman signal remains in the same position. This indicates that the interaction between cobalt and TiO2 affects the resonance phenomena. These results show the correct deposition of cobalt on TiO2 without phase transition [22,23]. Vibrational modes corresponding to cobalt were not observed regardless of the concentration of dopants which suggests that a small amount of them are substituted in the lattice while another amount is segregated. Other works also reported the presence of anatase phase in titanium dioxide coatings regardless of the concentration of dopants [20].
Morphology and microstructure: SEM The micrographs in Fig. 4 show a view to 10,000 amplifications of all the samples. In the first micrograph (a) a thin film of titanium dioxide without the incorporation of some dopant is observed, which is characteristic of the polycrystalline thin TiO2 films grown on silicon substrates in accordance with reported in other works [12]. When the doping agent is added in the titanium dioxide films the morphology changes radically as shown in the micrographs (b, c, d, e, f), a polycrystalline growth with agglomerates of small crystals is observed. The growth of the film on the substrate is not uniform in comparison with other works [16,24] that used the same method of synthesis but the difference is in the deposit of the precursor on the reactor, the precursor is injected by discrete pulses which suggests the formation of these agglomerates [25]. Fig. 5 shows the micrograph for a 6% concentration of cobalt, this film presents the most uniform growth, in agreement with other research reports as the concentration of dopant increases the number of pores decreases. In this study it can be observed that the amount of pores in the morphology of the film decreases but there is a large amount of agglomerates, it can be suggested that 6% of cobalt is the limit quantity to achieve a continuous growth of the film since a This concentration shows the greater uniformity of the film. 4
Results in Physics 16 (2020) 102891
N. Méndez-Lozano, et al.
Elemental composition: EDS [2]
Fig. 6 shows the EDS spectra for each sample. The elemental composition can be observed in each spectrum. Besides, in all samples the presence of titanium, oxygen and cobalt is observed. The aluminum signal is due to the sample holder.
[4]
Conclusions
[5]
[3]
[6]
Polycrystalline Co:TiO2 films were deposited onto Si substrates by the MOCVD technique in pulsed injection mode using liquid precursors at different Co concentrations. Our XRD, SEM, and Raman spectroscopy studies show the typical polycrystalline thin TiO2 anatase structure, formed by grains from 41 to 47 nm in diameter. A decrease in the intensity of the XRD peaks as the cobalt concentration is increased was also observed indicating that the cobalt dopants might occupy the substitutional sites in the anatase lattice. This result was confirmed by the absence of vibrational modes of cobalt in the Raman analysis. The SEM studies showed the influence of the doping agent (cobalt) on the surface morphology of the TiO2, also the best concentration of dopants in the structure was determined to obtain homogeneous coatings without a large amount of agglomerated crystals on the surface. The EDS results confirmed the success of the synthesis process. Finally, the experimental conditions ensure a high reproducibility of the synthesis.
[7] [8]
[9]
[10]
[11] [12]
[13]
Financing
[14]
This work was financed with CONACYT resources as part of a grant.
[15]
Declaration of Competing Interest [16]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[17] [18]
Acknowledgments
[19]
The authors wish to thank Dr. B. Millan, G. Hernndez and C. Peza for the XRD, SEM, Raman and UV characterization, respectively. In addition, the authors acknowledge the Universidad Autónoma de Querétaro for the facilities to achieve the UV-Vis measurements at Nanotechnology Lab.
[20]
[21]
[22]
Appendix A. Supplementary data [23]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.rinp.2019.102891.
[24]
References [25] [1] Ganesh V, Alizadeh M, Shuhaimi A, Adreen A, Pandikumar A, Jayakumar M, et al. Correlation between indium content in monolithic InGaN/GaN multi quantum well
5
structures on photoelectrochemical activity for water splitting. J Alloy Compd 2017;706(2017):629–36. Sakatani Y, Grosso D, Nicole L, Boissiere C, et al. Effect of sintering on transparent TiO2 18NR-T type thin films as the working electrode for transparent solar cells”. J Mater Chem 2006;16:77–82. Zaho G, Kozuka H, Yoko T. Sol-gel preparation and photoelectrochemical properties of TiO2 films containing Au and Ag metal particles. Thin Solid Films 1996;277:147. Mardare D, Tasca M, Delibas M, Rusu GI. On the structural properties and optical transmittance of TiO2 r.f. sputtered thin films. Appl Surf Sci 2000;156:200. Mardare D, Rusu GI. Structural and electrical properties of TiO2 RF sputtered thin films. Mater Sci Eng B 2000;75:68. Saripudin A, Saragih H, Khairurrijal K, Arifin P. Effect of growth temperature on cobalt-doped TiO2 thin films deposited on Si (100) substrate by MOCVD technique. Advanced materials research. Trans Tech Publications Ltd; 2014. p. 192–6. Krumdieck S, Raj R. Growth rate and morphology for ceramic films by pulsedMOCVD”. Surf Coat Technol 2001;141:7–14. Kang B-C, Lee S-B, Boo J-H. Growth of TiO2 thin films on Si(100) substrates using single molecular precursors by metal organic chemical vapor deposition. Surf Coat Technol 2000;131:88–92. Apátiga LM, Castaño VM. Magnetic behavior of cobalt oxide films prepared by pulsed liquid injection chemical vapor deposition from a metal-organic precursor. Thin Solid Films 2006;496:576–9. Abrutis A, Hubert-Pfalzgraf LG, Pasko SV, Bartasyte A, Weiss F, Janickis V. Hafnium oxoneopentoxide as a new MOCVD precursor for hafnium oxide films. J Cryst Growth 2004;267:529–37. Nam SH, Cho SJ, Boo JH. Growth behavior of titanium dioxide thin films at different precursor temperatures. Nanoscale Res Lett 2012;7(1):89. Apátiga LM, Rubio E, Rivera E, Castaño VM. Surface morphology of nanostructured anatase thin films prepared by pulsed liquid injection MOCVD. Surf Coat Technol 2006;201:4136–8. Apátiga LM, Rivera E, Castaño VM. Nucleation and growth of titania nanoparticles prepared by pulsed injection metal organic chemical vapor deposition from a single molecular precursor. J Am Ceram Soc 2007;90:932–5. Xu J, Shi S, Li X, Zhang Y, Wang X, Chen J, et al. Structural, optical, and ferromagnetic properties of Co-doped TiO2 films annealed in vacuum. Appl Phys 2010;107:053910. Matsumoto Y, Murakami M, Shino T, Hasegawa T, Fukumura T, Kawasaki M, et al. Room-temperature ferromagnetism in transparent transition metal-doped titanium dioxide. Science 2001;291:854. Subramanian M, Vijayalakshmi S, Venkataraj S, Jayavel R. Effect of cobalt doping on the structural and optical properties of TiO2 films prepared by sol–gel process. Thin Solid Films 2008;516:3776–82. Thamaphat K, Limsuwan P, Ngotawornchai B. Phase Characterization of TiO2 Powder by XRD and TEM. Kasetsart J (Nat Sci) 2008;42:357–61. Azaroff LA. Elements of X-ray crystallography. New York: McGraw-Hill; 1968. p. 552. Iwasaki Mitsunobu, Hara Masayoshi, Kawada Hiromi, Tada Hiroaki, Ito Seishiro. Cobalt ion-doped TiO2 photocatalyst response to visible light. J Colloid Interface Sci 2000;224:202–4. Whang CM, Kim JG, Kim EY, Kim YH, Lee WI. Effect of Co, Ga, and Nd additions on the photocatalytic properties of TiO2 nanopowders. Glass Phys Chem 2005;31(3):390–5. Lim SP, Pandikumar A, Huang NM, Lim HN. Enhanced photovoltaic performance of silver@ titania plasmonic photoanode in dye-sensitized solar cells. RSC Adv 2014;4(72):38111–8. Pandikumar A, Sivaranjani K, Gopinath CS, Ramaraj R. Aminosilicate sol–gel stabilized N-doped TiO 2–Au nanocomposite materials and their potential environmental remediation applications. RSC Adv 2013;3(32):13390–8. Lim SP, Pandikumar A, Huang NM, Lim HN, Gu G, Ma TL. Promotional effect of silver nanoparticles on the performance of N-doped TiO 2 photoanode-based dyesensitized solar cells. RSC Adv 2014;4(89):48236–44. Bouaine Abdelhamid, Schmerber G, Ihiawakrim D, Derory A. Structural, optical and magnetic properties of Co-doped SnO2 powders synthesized by the coprecipitation technique. Mater Sci Eng B 2012;177:1618–22. Kern W, Ban WS. Chemical vapor deposition of inorganic thin films, thin film processes. New York: Academic Press; 1897. p. 257–331.
Contents lists available at ScienceDirect
Results in Physics journal homepage: www.elsevier.com/locate/rinp
Morphological study of TiO₂ thin films doped with cobalt by Metal Organic Chemical Vapor Deposition
T
⁎
Néstor Méndez-Lozanoa, ,1, Miguel Apátiga-Castrob, Alejandro Manzano-Ramírezc, Eric M. Rivera-Muñozb, Rodrigo Velázquez-Castillod, Carlos Alberto-Gonzáleza, Marco Zamora-Antuñanoa a
Universidad del Valle de México, Campus Querétaro, Blvd. Juriquilla No. 1000 A, Del. Santa Rosa Jáuregui, Querétaro, Qro C.P. 76230, Mexico Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México, A.P. 1-1010, Querétaro, Qro C. P. 76000, Mexico c Centro de Investigación y de Estudios Avanzados, Unidad Querétaro, Libramiento Norponiente # 2000. Fracc. Real de Juriquilla, Querétaro, Qro 76230, Mexico d División de Investigación y Posgrado, Universidad Autónoma de Querétaro, Cerro de las Campanas S/N, Querétaro, Qro C.P 76010, Mexico b
A R T I C LE I N FO
A B S T R A C T
Keywords: Thin films Anatase films Chemical vapor deposition Nanostructure
In this work, we present the synthesis of thin films of TiO2 doped with cobalt using the MOCVD technique, allowing the growth of coatings with controlled morphology and a faster deposit speed, as compared to other techniques. In addition, the synthesis parameters necessary for the reproducibility of the films were obtained. Anatase thin films doped with cobalt were prepared by the Metal Organic Chemical Vapor Deposition technique working in pulsed injection mode using a mixture of titanium (IV) isopropoxide and acetyl acetonate of cobalt at different concentrations as the precursor. The films were deposited on silicon (1 0 0) substrates at temperature of 650 °C. The crystalline structure of the deposited films was characterized by X-ray diffraction (XRD) and Raman spectroscopy. The characteristic XRD peaks and vibrational modes of TiO2 corresponding to the anatase phase were observed in all cases, without any secondary or mixed phase. On the other hand, the scanning electron microscopy (SEM) observations show that the film surfaces are formed by a higher number of porous, which are characteristic of the anatase films and the corresponding Energy Dispersion Spectroscopy (EDS) shows the elemental composition.
Introduction Solid materials grown in thin film form fall into two general classifications, depending on application: passive as for the majority of optical and mechanical coatings, and active, as for electro-optical applications. The latter include transparent conductive and photo-active coatings. For several years, the thin films of titania have attracted attention due to their electro-optical and photochemical applications [1], for example UV-light shielding, solar energy conversion and detoxification of pollutants, among others. Titania coatings generate specific response or initiate reactions when they are excited by external stimuli. For example, incident light of energy greater than the 3-eV band edge of titania generates holes and electrons. These carriers produce the oxidation and reduction reactions on the surface of titania layers responsible for the photocatalytic activity [2]. For these applications, it is generally accepted that anatase phase has demonstrated a higher activity than rutile and brookite, therefore it is of great importance to
fabricate titania thin films with a well-controlled nanostructure. On the other hand, it is possible to improve the photocatalytic properties of titania films by doping it with noble metals [3] as well as with transition metals [4,5]. Recently, the use of transition metals for the doping of anatase films like Co for example, has gained much importance due to its ferromagnetic properties and also for the improvement of the photocatalytic response in the visible region. In recent works cobaltdoped TiO2 thin films by MOCVD have been obtained to study the effect of temperature on grain size [6]. Many deposition techniques have been used to obtain Co:TiO2 thin films, such as sputtering, laser ablation, ion implantation, sol-gel and MOCVD. Among the wide range of film growth techniques, the Pulsed Injection MOCVD technique is the most feasible and widely applicable to tailor stable and high-quality films on a nanometer-scale range [7–10]. MOCVD technique is a suitable method for stoichiometric and microstructural deposition of thin films [11]. Due to the titania anatase structure provides greater photocatalytic activity than the rutile phase, the growth conditions were
⁎
Corresponding author. E-mail address: [email protected] (N. Méndez-Lozano). 1 ORCID: 0000-0001-5622-9283. https://doi.org/10.1016/j.rinp.2019.102891 Received 22 March 2019; Received in revised form 16 December 2019; Accepted 17 December 2019 Available online 19 December 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
Results in Physics 16 (2020) 102891
N. Méndez-Lozano, et al.
Characterization of Co:TiO2 films
adjusted to predominantly favor the formation of the anatase structure. Our recent works on nanostructured titania thin films and nanoparticles suggest that the preparation of highly ordered and crystalline anatase coatings requires a perfect tuning between the chemicals and the experimental conditions [12,13]. In this paper cobalt doped titanium dioxide (Co:TiO2) thin films were deposited on crystalline silicon substrates by Metal Organic Chemical Vapor Deposition (MOCVD) technique, using a liquid metalorganic precursor. The crystallinity, vibrational modes and surface morphology of the Co:TiO2 films were studied by XRD, Raman Spectroscopy and SEM respectively. This paper focuses on the investigation of the effect of cobalt as a doping agent on the structural and morphological quality of Co:TiO2 thin films deposited on silicon substrates. Highlighting the advantages of the MOCVD technique such as: possibility of epitaxial growth and control of most of the growth parameters.
X-ray powder diffraction was used to identify the crystalline phases contained in all samples. Wide angle X-ray experiments were carried out in a Rigaku Mini Flex diffractometer using the Cu k α radiation (λ = 1.5406 å), an accelerating voltage of 40 kV and a current of 30 mA. Diffractograms were recorded with a Solid-State D/teX-ULTRA Detector from 5 through 80 ° on a 2θ scale with a rate of 10 ° per minute. Spectrum analysis software, MDI Jade V 5.0.37, was used. To analyze the characteristic vibrational modes of hydroxyapatite a Raman spectrometer Bruker model Senterra was used. The operation condition was a voltage of 100 mV, a resolution of 3 to 5 cm−1 and a 20X objective. Morphological, topological and microstructural analyses of all samples were carried out in a JEOL JSM5900-LV Scanning Electron Microscope. The analysis was performed using 20 kV electron acceleration voltage and the images were formed by secondary electrons. All samples were placed on a stainless-steel cylinder and fixed with doublesided tape and covered with a gold thin film done by sputtering to avoid the electrostatic charge accumulation. In order to obtain the elemental composition of thin films, a Scanning Electron Microscope SU8230 Hitachi equipment was used with a voltage of 20 kV and a current of 0.10 nA.
Methods A vertical MOCVD reactor was used to grow the Co:TiO2 films from metal organic liquid precursors in pulsed injection mode. The reactor consists of a quartz tube and stainless-steel bodies interconnected by Orings, a detailed description has been published elsewhere [9,10]. A pulsed injection mechanism that consist of an injector, similar to those used regularly for fuel injection in internal combustion engines, delivers the exact amount of liquid precursor into the evaporation zone at 280 °C. In this way, the liquid precursor is instantaneously evaporated and then transported by a carrier gas (Ar) towards the reaction chamber, where the chemical reaction and film growth on the hot substrate occurs at 650 °C. Liquid solutions of 10 ml of titanium isopropoxide (Sigma Aldrich) mixed with different quantities of powder of cobalt acetyl acetonate (Sigma Aldrich) were used as precursor, concentrations from 2, 4, 6 and 8 at.% vol were used. The final mixture was a homogeneous liquid solution. The total amount of liquid precursor was 19 ml, which was injected at two pulses per second at a dose of 7.6 × 10−3 mL per pulse. Most of the experimental conditions such as, injector frequency (dose), substrate temperature, carrier gas flow and total pressure were measured and controlled by a computer driven system. The reaction chamber is a quartz tube surrounded by a resistive furnace that can heat the substrate up to 1000° C. Substrates as long as to 2 in. in diameter can be used easily. The pressure in the system was kept at 600 Pa with a carrier gas (Ar) flowing at 0.1 l/min. Schematics of the MOCVD apparatus for Co-doped TiO2 thin films synthesis is showed in Fig. 1.
Results and discussion Phase composition: XRD Fig. 2 shows XRD patterns of all the films deposited under the same experimental conditions but with different concentrations of cobalt. At the bottom, it is included the pattern of the TiO2 films, deposited without Co concentration in the precursor, which can be taken as reference. In all cases the Bragg reflections attributed to the crystalline anatase phase are clearly observed (JCPDS file No. 21-1272) and no peaks corresponding to cobalt or cobalt oxide phases were observed on the cobalt doped samples, suggesting that a very low amount of Co is homogeneously distributed in anatase, at a concentration below the detection limits of the XRD equipment [14,15]. In addition, a decrease in the intensity of the peaks as the cobalt concentration is increased was also observed indicating that the cobalt dopants might occupy the substitutional sites in the anatase lattice [14,16].
Fig. 2. Comparison on the diffractograms obtained for different concentrations of Co in thin films of TiO₂: (a) 2% Co: TiO₂, (b) 4% Co: TiO₂, (c) 6% Co: TiO₂, (d) 8% Co: TiO₂, (e) 10% Co: TiO₂. The Indices correspond to the anatase phase of TiO₂.
Fig. 1. Schematic of the MOCVD apparatus. 2
Results in Physics 16 (2020) 102891
N. Méndez-Lozano, et al.
Table 1 Crystallite size for all samples. Sample
% of Co
Crystallite size (nm)
TiO₂ Co:TiO₂ Co:TiO₂ Co:TiO₂ Co:TiO₂ Co:TiO₂
0 2 4 6 8 10
41 44 47 42 45 42
Fig. 5. Micrographs for 6% Co: TiO₂.
preferential growth in the direction (1 0 1) independently of the concentration of cobalt dopants, the presence of cobalt in the structure of the anatase suggests this preferential growth. Other works have reported the obtaining of TiO₂ thin films with preferential direction of growth (2 2 0) without the presence of cobalt [12]. It is also possible to consider that small amounts of cobalt oxide segregated in the samples may be invisible in the diffraction patterns. Analyzing the full width at half maximum of the (1 0 1) peak, the estimated grain size using the Scherrer equation [18] is shown in Table 1. It can be seen that there is no significant change in crystallite size by increasing the amount of cobalt impurities, which suggests an incorporation of the impurities within the structure crystalline TiO₂. Other works report the increase in crystallite size as the concentration of dopants increases [19,20]. The difference with other works is the way to deposit the precursor inside the reactor through discrete pulses.
Fig. 3. Raman spectra obtained for each of the samples of Co: TiO₂.
The diffraction peaks observed in 2 theta value of 25.5, 38 and 48 corresponding to the planes (1 0 1), (0 0 4) and (2 0 0) and are attributed to the anatase phase of the tetragonal structure. This confirms the presence of the anatase phase, which is well known as the most suitable for photocatalysis processes. The most intense peak corresponding to the plane (1 0 1), which is energetically the most stable and predominant in the crystal structure of the samples [17]. It was possible to observe that the nanostructures present a
Vibrational modes: Raman spectroscopy Fig. 3 shows the Raman spectra for all the samples in the frequency range from 100 to 800 cm−1 employing a helium-neon laser with an exciting light of 783 nm. In the spectra, it is clear the presence of the anatase phase due to all
Fig. 4. Micrographs for each of the films made of TiO₂: (a) TiO₂, (b) 2% Co: TiO₂, (c) 4% Co: TiO₂, (d) 6% Co: TiO₂, (e) 8% Co: TiO₂, (f) 10% Co: TiO₂. 3
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Fig. 6. EDS spectra confirm the elements in all samples.
the bands at 142, 398 and 637 cm−1 attributed to the anatase phase. The strong peak located at 520 cm−1 is due to the vibrational mode of silicon (1 0 0) substrate. For all concentration, the same bands are observed, the absence of vibrational modes of cobalt confirms that impurities are occupying substitutional sites in the crystalline structure of TiO₂ [16], in agreement with the results obtained by XRD. These results show a strong interaction between the titanium dioxide film and the silicon substrate. In agreement with other works, no signals related to cobalt particles are identified due to the relatively low concentration, besides a weak Raman scattering was observed [21]. Fig. 3 shows that the intensity of the bands decreased as the concentration of cobalt increases, but the Raman signal remains in the same position. This indicates that the interaction between cobalt and TiO2 affects the resonance phenomena. These results show the correct deposition of cobalt on TiO2 without phase transition [22,23]. Vibrational modes corresponding to cobalt were not observed regardless of the concentration of dopants which suggests that a small amount of them are substituted in the lattice while another amount is segregated. Other works also reported the presence of anatase phase in titanium dioxide coatings regardless of the concentration of dopants [20].
Morphology and microstructure: SEM The micrographs in Fig. 4 show a view to 10,000 amplifications of all the samples. In the first micrograph (a) a thin film of titanium dioxide without the incorporation of some dopant is observed, which is characteristic of the polycrystalline thin TiO2 films grown on silicon substrates in accordance with reported in other works [12]. When the doping agent is added in the titanium dioxide films the morphology changes radically as shown in the micrographs (b, c, d, e, f), a polycrystalline growth with agglomerates of small crystals is observed. The growth of the film on the substrate is not uniform in comparison with other works [16,24] that used the same method of synthesis but the difference is in the deposit of the precursor on the reactor, the precursor is injected by discrete pulses which suggests the formation of these agglomerates [25]. Fig. 5 shows the micrograph for a 6% concentration of cobalt, this film presents the most uniform growth, in agreement with other research reports as the concentration of dopant increases the number of pores decreases. In this study it can be observed that the amount of pores in the morphology of the film decreases but there is a large amount of agglomerates, it can be suggested that 6% of cobalt is the limit quantity to achieve a continuous growth of the film since a This concentration shows the greater uniformity of the film. 4
Results in Physics 16 (2020) 102891
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Elemental composition: EDS [2]
Fig. 6 shows the EDS spectra for each sample. The elemental composition can be observed in each spectrum. Besides, in all samples the presence of titanium, oxygen and cobalt is observed. The aluminum signal is due to the sample holder.
[4]
Conclusions
[5]
[3]
[6]
Polycrystalline Co:TiO2 films were deposited onto Si substrates by the MOCVD technique in pulsed injection mode using liquid precursors at different Co concentrations. Our XRD, SEM, and Raman spectroscopy studies show the typical polycrystalline thin TiO2 anatase structure, formed by grains from 41 to 47 nm in diameter. A decrease in the intensity of the XRD peaks as the cobalt concentration is increased was also observed indicating that the cobalt dopants might occupy the substitutional sites in the anatase lattice. This result was confirmed by the absence of vibrational modes of cobalt in the Raman analysis. The SEM studies showed the influence of the doping agent (cobalt) on the surface morphology of the TiO2, also the best concentration of dopants in the structure was determined to obtain homogeneous coatings without a large amount of agglomerated crystals on the surface. The EDS results confirmed the success of the synthesis process. Finally, the experimental conditions ensure a high reproducibility of the synthesis.
[7] [8]
[9]
[10]
[11] [12]
[13]
Financing
[14]
This work was financed with CONACYT resources as part of a grant.
[15]
Declaration of Competing Interest [16]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[17] [18]
Acknowledgments
[19]
The authors wish to thank Dr. B. Millan, G. Hernndez and C. Peza for the XRD, SEM, Raman and UV characterization, respectively. In addition, the authors acknowledge the Universidad Autónoma de Querétaro for the facilities to achieve the UV-Vis measurements at Nanotechnology Lab.
[20]
[21]
[22]
Appendix A. Supplementary data [23]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.rinp.2019.102891.
[24]
References [25] [1] Ganesh V, Alizadeh M, Shuhaimi A, Adreen A, Pandikumar A, Jayakumar M, et al. Correlation between indium content in monolithic InGaN/GaN multi quantum well
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structures on photoelectrochemical activity for water splitting. J Alloy Compd 2017;706(2017):629–36. Sakatani Y, Grosso D, Nicole L, Boissiere C, et al. Effect of sintering on transparent TiO2 18NR-T type thin films as the working electrode for transparent solar cells”. J Mater Chem 2006;16:77–82. Zaho G, Kozuka H, Yoko T. Sol-gel preparation and photoelectrochemical properties of TiO2 films containing Au and Ag metal particles. Thin Solid Films 1996;277:147. Mardare D, Tasca M, Delibas M, Rusu GI. On the structural properties and optical transmittance of TiO2 r.f. sputtered thin films. Appl Surf Sci 2000;156:200. Mardare D, Rusu GI. Structural and electrical properties of TiO2 RF sputtered thin films. Mater Sci Eng B 2000;75:68. Saripudin A, Saragih H, Khairurrijal K, Arifin P. Effect of growth temperature on cobalt-doped TiO2 thin films deposited on Si (100) substrate by MOCVD technique. Advanced materials research. Trans Tech Publications Ltd; 2014. p. 192–6. Krumdieck S, Raj R. Growth rate and morphology for ceramic films by pulsedMOCVD”. Surf Coat Technol 2001;141:7–14. Kang B-C, Lee S-B, Boo J-H. Growth of TiO2 thin films on Si(100) substrates using single molecular precursors by metal organic chemical vapor deposition. Surf Coat Technol 2000;131:88–92. Apátiga LM, Castaño VM. Magnetic behavior of cobalt oxide films prepared by pulsed liquid injection chemical vapor deposition from a metal-organic precursor. Thin Solid Films 2006;496:576–9. Abrutis A, Hubert-Pfalzgraf LG, Pasko SV, Bartasyte A, Weiss F, Janickis V. Hafnium oxoneopentoxide as a new MOCVD precursor for hafnium oxide films. J Cryst Growth 2004;267:529–37. Nam SH, Cho SJ, Boo JH. Growth behavior of titanium dioxide thin films at different precursor temperatures. Nanoscale Res Lett 2012;7(1):89. Apátiga LM, Rubio E, Rivera E, Castaño VM. Surface morphology of nanostructured anatase thin films prepared by pulsed liquid injection MOCVD. Surf Coat Technol 2006;201:4136–8. Apátiga LM, Rivera E, Castaño VM. Nucleation and growth of titania nanoparticles prepared by pulsed injection metal organic chemical vapor deposition from a single molecular precursor. J Am Ceram Soc 2007;90:932–5. Xu J, Shi S, Li X, Zhang Y, Wang X, Chen J, et al. Structural, optical, and ferromagnetic properties of Co-doped TiO2 films annealed in vacuum. Appl Phys 2010;107:053910. Matsumoto Y, Murakami M, Shino T, Hasegawa T, Fukumura T, Kawasaki M, et al. Room-temperature ferromagnetism in transparent transition metal-doped titanium dioxide. Science 2001;291:854. Subramanian M, Vijayalakshmi S, Venkataraj S, Jayavel R. Effect of cobalt doping on the structural and optical properties of TiO2 films prepared by sol–gel process. Thin Solid Films 2008;516:3776–82. Thamaphat K, Limsuwan P, Ngotawornchai B. Phase Characterization of TiO2 Powder by XRD and TEM. Kasetsart J (Nat Sci) 2008;42:357–61. Azaroff LA. Elements of X-ray crystallography. New York: McGraw-Hill; 1968. p. 552. Iwasaki Mitsunobu, Hara Masayoshi, Kawada Hiromi, Tada Hiroaki, Ito Seishiro. Cobalt ion-doped TiO2 photocatalyst response to visible light. J Colloid Interface Sci 2000;224:202–4. Whang CM, Kim JG, Kim EY, Kim YH, Lee WI. Effect of Co, Ga, and Nd additions on the photocatalytic properties of TiO2 nanopowders. Glass Phys Chem 2005;31(3):390–5. Lim SP, Pandikumar A, Huang NM, Lim HN. Enhanced photovoltaic performance of silver@ titania plasmonic photoanode in dye-sensitized solar cells. RSC Adv 2014;4(72):38111–8. Pandikumar A, Sivaranjani K, Gopinath CS, Ramaraj R. Aminosilicate sol–gel stabilized N-doped TiO 2–Au nanocomposite materials and their potential environmental remediation applications. RSC Adv 2013;3(32):13390–8. Lim SP, Pandikumar A, Huang NM, Lim HN, Gu G, Ma TL. Promotional effect of silver nanoparticles on the performance of N-doped TiO 2 photoanode-based dyesensitized solar cells. RSC Adv 2014;4(89):48236–44. Bouaine Abdelhamid, Schmerber G, Ihiawakrim D, Derory A. Structural, optical and magnetic properties of Co-doped SnO2 powders synthesized by the coprecipitation technique. Mater Sci Eng B 2012;177:1618–22. Kern W, Ban WS. Chemical vapor deposition of inorganic thin films, thin film processes. New York: Academic Press; 1897. p. 257–331.