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Vol. 8. Issue 2.
Pages 2350-2358 (April 2019)
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Vol. 8. Issue 2.
Pages 2350-2358 (April 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.04.018
Open Access
Comparison on the TiO2 crystalline phases deposited via dip and spin coating using green sol–gel route
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Nur Dalilah Joharia,b, Zulkifli Mohd Roslia,
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zmr@utem.edu.my

Corresponding author.
, Jariah Mohamad Juoib, Shuhadah A. Yazida,b
a Fakulti Teknologi Kejuruteraan Mekanikal dan Pembuatan, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia
b Fakulti Kejuruteraan Pembuatan, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia
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Tables (2)
Table 1. Comparison of TiO2 crystalline phases obtained via dip and spin coating (A, anatase; B, brookite; R, rutile).
Table 2. Comparison of the single brookite film via spin coating obtained in the present work with previously reported in the literature (D, crystallite size; T, thickness).
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Abstract

TiO2 exist in three polymorphs known as anatase, rutile and brookite. Amongst these polymorphs, brookite are less reported and mostly produced as a by-product. In this work, two deposition methods which are dip and spin coating are selected to compare the effect of deposition methods on TiO2 crystalline phase formation, particularly on identifying the brookite presence. The sol used was made free from solvent as an attempt for a green sol–gel route. The heat treatment temperature is varied at 200°C, 300°C, 400°C and 500°C for 3h. The produced thin films are then characterized by X-ray diffraction (XRD), Raman spectroscopy (RS) and transmission electron microscopy (TEM). Crystallite size was calculated using Scherrer's equation. The cross sectional morphology of the thin films was examined with the scanning electron microscope (SEM). Results show that the deposition methods influence the phase formation and crystallinity. TiO2 thin films produced via dip coating composed of anatase and rutile despite the temperatures variation. In contrast, spin coating produced single brookite (111) with Raman spectra of 319cm−1 and 320cm−1 at 200°C and 300°C. The brookite crystallite size is 47.9nm at 200°C and 58.4nm at 300°C. TEM results had confirmed the brookite presence with the lattice fringes of 0.28nm. However, at 400°C and 500°C, XRD pattern reveals no formation of TiO2 phases. Therefore, in this green sol–gel route, it is found that spin coating deposition at low temperature is preferable for brookite formation whereas dip coating is more suitable for anatase and rutile.

Keywords:
Brookite
Crystallite size
TiO2 thin film
Spin coating
Dip coating
Full Text
1Introduction

Commonly, TiO2 nanoparticles exist in three different crystalline phases which are anatase, brookite, and rutile. The rutile and anatase phases are well recognized and their synthesis and application have been well documented [1–4]. Unlike rutile and anatase, brookite is quite rare and least known for its application. This is probably due the difficulties in obtaining pure brookite under lab scale and in most cases exist as a by-product [5]. Nevertheless, studies have also shown that brookite phase has notably better in photocatalytic performance as compared to those of rutile and anatase [6–14]. For example, Mukarami et al. shows that the brookite powder produced 1100ppm in 500min for photocatalytic decomposition of acetaldehyde and exhibit higher photocatalytic activity compared to the other TiO2 powders [9]. In addition, Kandiel et al. had reported that the brookite nanoparticles produced ∼17% photonic efficiency which is higher than anatase in methanol photooxidation reaction [7]. Tran et al. had also compared the photocatalytic activity of brookite, anatase and rutile in the pharmaceutical ibuprofen (IBP), the recalcitrant phenol (Ph), and the more reactive cinnamic acid (CA) [6]. The results show that the mineralization shows of brookite (51%)>anatase (50%)>rutile (7%) for photocatalytic degradation in IBP, brookite (15%)>anatase (13%)>rutile (3%) for photocatalytic degradation in Ph and brookite (52%)>anatase (36%)>rutile (38%) for photocatalytic degradation in CA, respectively [6]. Meanwhile, for the brookite film, Alotaibi et al. shows that the brookite film has four times (1.17×10−4 molecules per incident photon) superior photocatalytic activity in the destruction of stearic acid compared to an anatase film [15]. Komariah et al. also reported that the brookite film shows 92% photocatalytic degradation of methylene blue after 240min under visible light irradiation [16]. Therefore, based on the high performance of brookite during photocatalytic activity (particularly in visible light environment), it is of great interest to acquire comprehensive understanding on the influence of deposition method with its formation during processing.

Many techniques available to deposit the TiO2 thin films which are hydrothermal treatment, pulsed laser deposition [17], chemical vapour deposition [15], vapor phase [18], spray pyrolysis [19] and sol–gel process [16]. In sol–gel process, the precursor, solvent, catalyst and water plays a remarkable role in the synthesis of TiO2 thin film [20]. Hafizah et al. stated that the precursor such as titanium (IV) isopropoxide (TTIP), titanium tetrachloride (TiCl4), titanium tetrabutoxide (TBT) and titanium alkoxides serves to develop the TiO2 polymorphs and its crystallinity [21]. Based on different precursors, TTIP is widely used in sol–gel process due to generating stable solution at low hydrolysis ratio [20]. In another hand, catalyst serves to growth and influence the type of phase's formation as well as its crystallite size. Meanwhile, solvent attends to slow the rate of the hydrolysis and condensation in the sol–gel process that consequently influence the films crystallization, films microstructures, crystallite sizes as well as their photo electrochemical properties [22]. For example, Mahyar and Amani-Ghadim reported that ethanol favourable to produced mixture of anatase and rutile with crystallite size of 14.5nm and lowest specific surface area, 9.07m2g−1[23]. Thus in common, solvent is widely used for almost all sol formulation during TiO2 thin film deposition. However, it should be highlighted that the long term exposure to solvents can be toxic and harmful to the environmental and human body. This is because, solvents consist of different chemical groups which lead to difference properties especially in the physiological and toxicological aspects [24]. For instance, used of benzene can caused cancer in humans [25]. Moreover, Uzma et al. [26] and Rama et al. [27] also stated that the solvents exposure lead to damage on respiratory, haematological and thyroid functioning. For this reason, researchers had started to initiate work on a green chemistry approach for TiO2 synthesis. Recently, a systematically work carried out as an attempt to produce sol–gel TiO2 thin films with the desired grain size and preferable phases without the use of solvent had been reported by Yazid et al. [20]. Also, Spada et al. had managed to produce anatase and rutile TiO2 nanoparticles without the use of any solvents [28]. Yet, there are no published report on the brookite formation without the use of solvent. Therefore, this work is pioneering an effort on TiO2 deposition for brookite formation via a green sol–gel route.

Generally, sol–gel is one of the techniques to deposit TiO2 thin film. Sol–gel method is the simplest synthesis technique for preparing TiO2 nanoparticles and economical processes for film fabrication [29,30]. Furthermore, sol–gel technique allows either thick or thin films to be easily formed on many types of substrate via various deposition methods such as dip coating and spin coating method. The difference in the deposition methods is reflected in the way of the coating being deposited which also influence the way of the crystallization taking place. For instance, during a dip coating, the glass substrate is normally withdrawn vertically from the sol at a constant speed. When the glass substrate was aligned upwards, a gravitational force will be exerted on the gel layer that tends to “pulls” the colloidal particles downwards away from the substrate along with its dispersion onto the substrate [31,32]. While, in a spin coating method, a small volume of sol is place onto the centre of the substrate, where an attached vacuum pump is spin at a controllable speed. The centrifugal force generated will then act to disperse the coating solution. Later, the evaporation of the solution will take place and become the main process for the coating formation. In common, spin coating is a deposition technique used to gain a uniform thin layer film on a flat surface such as glass substrate [32].

In reviewing related works on TiO2 deposition, few reported works are discussing on the effect of deposition method on the produced TiO2 phases. Many works are just simply mentioning the deposition method and phases produced without focusing on explaining the effect of the deposition method on the results obtained. In focus to brookite formation, several deposition methods such as dip coating and spin coating had produce this crystalline phase as a minor product or mixture phases. For example, using the dip coating method, the produced TiO2 film deposited on the glass substrate contained brookite as a by-product to anatase [33]. While, TiO2 films with the 70% weight fraction of brookite with the remaining fractions were anatase and amorphous TiO2 produce by Djoued et al. [34]. Djoued et al. claimed that the amount of polyethylene glycol (PEG) in the sol formulation influenced the nucleation of the brookite and anatase phases. In addition, Ohara and his co-workers observed an overlapping of crystalline peaks of brookite (120) and anatase (101) in TiO2 film deposited on soda lime glass [35]. He suggests that the Na ion from the substrate diffused to the film surface and promotes the brookite phase. Despite less report on a single brookite formation, Novotna et al. had observed formation of single brookite film on soda lime glass substrate with crystallite size of 50±10nm [36]. He claimed that the diffusion Na+ lead to the formation of brookite. While, using the spin coating method, more work had been able to synthesize single brookite film. For instance, Arier and Tepehan managed to synthesize brookite thin film (with crystallite size of ∼12nm) on Corning 2947 glass substrate using titanium butoxide as a precursor and ethanol as a solvent [37]. Besides, Komaraiah et al. had reported producing brookite film on glass substrate with crystallite sizes of 54–67nm [16]. Singh et al. had also produced brookite film with crystallite size of 28–48nm via spin coating using ethyl alcohol as a solvent [38]. While, Katsumata et al. reported brookite formation in film deposited on polyimide substrate. Here, the brookite formation is due to the concentration of Na ion from sodium oleate during hydrothermal treatment [39].

It is reported that the choice of deposition method (e.g. dip coating and spin coating) may affect the crystalline structure of titania films [40]. As an example, Kment et al. reported that the deposition methods had influenced the surface, structure, and density of the TiO2 thin film deposited on sodium-lime glass [41]. It was found that brookite phase with crystallite size of 46±2nm is produced via dip coating method while spin coating produced brookite and anatase with the crystallite size of 50±2nm. [41]. Besides, Wang et al. had carried out study on different deposition method to understand its effect on the TiO2 thin film phase formation, grain size, morphology and thickness towards greater photocatalytic performance [29]. In their research, dip coating, spin coating, and combination of dip and spin coating method (dip/spin coating) are utilizing to fabricate TiO2 thin films on homemade porous α-Al2O3 disks with the use of solvents. Based on Wang findings, he proposed that sol–gel with a dip/spin coating is suitable to produce TiO2 films with smaller and more uniform grains of anatase. In summary, the deposition methods had a significant effect on the crystallite size of the TiO2 produced. Therefore, in this work, further analyze on the influence of two deposition methods (dip coating and spin coating) on the deposited sol–gel TiO2 thin film phase formation and crystallite sizes (with emphasis on brookite formation) were reported. The sol formulation utilized for the TiO2 thin films deposition is prepared without the use of any solvents as an approach for a green sol–gel route synthesis.

2Experimental2.1Preparation of TiO2 sol

The TiO2 sol was prepared by dissolving 0.2M of titanium (IV) isopropoxide precursor (TTIP, 97%, Sigma Aldrich) into 64ml of deionized water (DI) water under constant stirring. A 0.4ml of hydrochloric acid (37% of HCl) was then dropped slowly using pipette into the solution drop by drop under constant stirring conditions. The entire solution was continuously stirred using a magnetic stirrer at room temperature. After 3h the solution was then kept ageing for 48h at room temperature before it is ready to be used.

2.2Deposition method of TiO2 thin films

Glass slides with dimensions 25.4mm×10.0mm×1.0mm were used as substrate. Prior to deposition, acetone, ethanol and distilled water were used to clean the glass slides in an ultrasonic bath for 10min. The glass slides were then dried in an oven at 110°C for 2h. Two deposition method which are dip and spin coating were used to deposit the TiO2 thin films.

For the dip coating, the glass slides were immersed into the TiO2 sol using a precision single dip coater apparatus of TEFINI Model DP1000. The dipping withdrawal speed was set at 30mm/min with 5s dwelling time. The dipped glass slides were dried for 30min at ambient temperature then dried in an oven at 110°C for 30min. The dipping was repeated for ten times in order to produce homogenous coating as determined by Musa et al. [42].

For the spin coating, 90μl of TiO2 sol was dropped using Eppendorf Pipette onto the surface of glass substrate which was attached on a vacuum pump with a motor spinning at a fixed speed of 1500rpm for 30s [16]. This process was repeated two times. Then, the deposited TiO2 thin films were allowed to dry in an oven at 110°C for 1h.

For the final treatment, both TiO2 thin films deposited via dip and spin coating process were heat-treated at 200°C, 300°C, 400°C and 500°C for 3h.

2.3Characterization of TiO2 thin films

Crystalline structure of the deposited TiO2 thin films were characterized using X-ray diffractometer (XRD) PANalytical X’PERT PRO MPD Model PW 3060/60 with a Cu Kα radiation (λ=1.54046Å) operating at 40kV and 30mA. The diffraction angle of 2θ was varied within the range of 10–80°. For comparison purposes, the crystallite size obtained at only dominant XRD peaks of 2θ=25° (anatase), 27° (rutile) and 31° (brookite) were calculated using Scherrer's formula (1):

where D is the mean size of the crystalline (nm), K as a constant, 0.94, λ is the wavelength of X-rays, θ is the diffraction angle and β is the full width at half maximum (FWHM). Further analysis on crystalline structures was affirmed by Raman Spectrometer (UniRAM-3500) with 532nm laser wavelength. The morphology and lattice information of the TiO2 thin films were observed using a transmission electron microscope (TEM) with a HT-7700 model. The TEM analysis was carried out by using field emission gun at 120kV and magnification power up to 600k. The cross sectional morphology of the TiO2 thin films was observed by scanning electron microscope (SEM) with a JEOL model JSM-6010PLUS/LV. The average thickness of TiO2 thin film was calculated based on measurement at 8 different locations.

3Results and discussion

Fig. 1 shows the XRD patterns of the deposited TiO2 thin films produced via dip coating and spin coating at various heat treatment temperatures. For the dip coating (Fig. 1(a)), the deposited TiO2 thin films XRD patterns exhibit a mixture of anatase (JCPDS No. 21-1272) and rutile (JCPDS No. 75-1753). The anatase (101) and rutile (110) are identified only at an angle of 25° and 27°, respectively for temperature at 300°C. At 400°C and 500°C, three anatase peaks of (101), (200) and (105) are identified at an angle of 25°, 48° and 54° along with rutile (110) at an angle of 27°. The relative intensity of anatase peak increases with increased heat treatment temperature. This indicated increased degree of crystallization as suggest by Bakri et al. [43]. Here, the crystallization process take place through nucleation and growth processes when heat treatment temperature is applied [44]. In Bakri et al. work, the intensity of anatase (101) peak at 300°C is 1173.5 and had increased to 5930.1 at 900°C [43]. Fig. 1(b) shows the XRD patterns of TiO2 thin films deposited via spin coating. The XRD patterns of TiO2 thin film produce via spin coating displayed a single brookite (111) peak (JCPDS No. 84-1750) at an angle of 31° for heat treatment temperature of 200°C and 300°C. This is in contrast with XRD pattern of TiO2 thin film deposited via dip coating where a mixture of anatase and rutile were obtained. When the heat treatment temperature is increased at 400°C and 500°C, the XRD pattern reveal amorphous characteristic that indicates no formation of TiO2 crystalline phase. The absence of brookite phase formation at high temperature (400°C and 500°C) could be due to its thermodynamically metastable character where it is preferably existing at low temperature [45]. This is also aligned with the finding reported by Allen et al. where in their work, 39.7% brookite crystalline phase at 110°C had reduced to 15.1% when the temperature is increased at 600°C [46]. In addition, brookite is not identified at temperature 700°C and above [46]. The similar observation is also obtained by Li et al. where the amount of brookite decreased when the temperature was increased from 200°C to 400°C with the crystallite size <10nm and until it did not identify above 400°C [47]. The absence of the TiO2 crystalline phase could be due to the no solvent use in the sol formulation. Solvent serves to slow the rate of the hydrolysis and condensation in the sol–gel process. Herman et al. reported that the desorption temperature for water is lower (∼250K) compared to methanol (610K) [48]. Thus, in this work where no solvent is used, it is expected the higher rate of hydration is observed particularly at high temperature thus crystallization is hindered.

Fig. 1.

X-ray diffraction (XRD) patterns of the deposited TiO2 thin films produced via (a) dip coating and (b) spin coating at various temperatures.

(0.15MB).

Based on Fig. 1, it is clear that deposition method of dip and spin coating influenced the TiO2 phase formation. Results shows that dip coating produced a mixture of anatase and rutile (Fig. 1(a)) while spin coating produced a single brookite (Fig. 1(b)). The findings of the current study are consistent with those of Yazid et al. who reported that deposition of TiO2 thin film via dip coating produced a mixture of anatase and rutile [20]. These findings also similar to Bakri et al. which had produced TiO2 thin film with a mixture of anatase (101) and rutile (110) via dip coating [43]. In the other hand, Komaraiah et al. had also observed a single brookite with orientation of B (110), B (111) and B (023) via spin coating [16]. Furthermore, Arier and Tepehan produced single brookite (211) via spin coating [37]. Table 1 shows the comparison of TiO2 crystalline phases obtained via dip and spin coating. Based on the comparison in Table 1, it is evidence that brookite formation is preferable via spin coating. Further review on single brookite formation via spin coating method is shown in Table 2. It can be seen most single brookite formation via the spin coating method involved the use of solvent such as ethanol, ethyl alcohol and sodium oleate in its sol formulation. The difference types of solvent had influenced the crystallite size, orientation, thickness coating and surface morphology. In contrast to our work, brookite had been produced without any solvent used. It should be noted that the brookite crystallite size produced around 47.9–58.4nm which is almost similar to the Komaraiah et al. (54–67nm) [16]. It is also of our interest to report that the heat treatment temperature is much lower 200°C and 300°C compared to Komaraiah et al. [16]. In this work, when water is added into the TTIP solution, it is creating fast hydrolysis and condensation process where the rate of hydrolysis is not been slow as there is no solvent used in the formulation [49]. Therefore, it is possible to expect that the nucleation start at crystalline growth at lower temperature leading to formation of metastable form of brookite.

Table 1.

Comparison of TiO2 crystalline phases obtained via dip and spin coating (A, anatase; B, brookite; R, rutile).

Deposition method  Phase obtained  Orientation  Angle, 2θ  Remarks  Reference 
Dip coating  Anatase with the tiny amount of brookite  A (101), A(004), A (200), A(106), A (215)    • Crystallite size ∼10nm  Kwon et al. [33] 
Dip coating  Mixture of anatase, rutile with minor brookite    A=25°, 48°, 54°, 56°, 59°, 63°, 68°, 73°R=27°, 35°, 41°, 44°B=31°  • Crystallite size of anatase ∼21nm• Crystallite size of rutile ∼34nm  Yazid et al. [20] 
Dip coating  Mixture of anatase and rutile  A (101),R (110)  A=24.2°,R=27.4°  • Crystallite size of anatase in range of 20.94–26.17nm  Bakri et al. [43] 
Spin coating  Single brookite  B (110),B (111),B (023)  B=31.39°  • Crystallite size 54–67nm  Komaraiah et al. [16] 
Spin coating  Single brookite  B (211)    • Crystallite size 4–11nm  Arier and Tepehan [37] 
Spin coating  Single brookite  B (120),B (121)  B=31.06°  Crystallite size 28–48nm  Singh et al. [38] 
Table 2.

Comparison of the single brookite film via spin coating obtained in the present work with previously reported in the literature (D, crystallite size; T, thickness).

Reference  Process type  Solvent used  Heat treatment  Orientation  D (nm)  Angle, 2θ  JCPDS No.  T (nm)  Other characteristic 
Komaraiah et al. [16]  • Sol–gel method• 1500rpm• 30Ethanol  • 400°C• 500°C  B (110),B (111),B (023)  54–67  31.39°  84–1750  220  Uniform coatingCrack freeNano spherical structure 
Arier and Tepehan [37]  • Sol–gel method• 1000rpm  Ethanol  • 400°C  B (211)  4–11    75–1582    HomogeneousSpherical microstructures 
Singh et al. [38]  • Sol–gel method• Spin coating  Ethyl alcohol    B (120),B (121)  28–48  31.06°    100–300  Band gap of 3.55–3.35eV 
Katsumata et al. [39]  • Hydrothermal treatment• 2000rpm• 30Sodium oleate  • 350°C  B (001),B (210)      29–1360    Brookite nanoparticles was squarish without sharp angles and the particle size was 20–40nm 
This work  • Sol–gel• 1500rpm• 30Without solvent  • 200°C• 300°C  B (111),B (023)  47–58  31°  84–1750  428–618  Crack coating 

Fig. 2 shows the crystallite sizes of the deposited TiO2 thin films via dip and spin coating with regard to temperatures. In this work, the dominant XRD peaks at an angle of 25°, 27° and 31° which is denoted to anatase (101), rutile (110) and brookite (111) were the only peaks considered for phases comparison. Thus, for dip coating, the crystallite size of anatase was found to be 9.6±6.0nm while rutile was 14.5±0.1nm at the temperature of 300°C. At 400°C, the crystallite size of anatase was found to be 10.8±1.1nm and rutile to be 24.7±7.0nm. The crystallite size of anatase and rutile increased up to 28.8±3.6nm and 24.8±0.1nm, respectively as the temperature increased to 500°C. The same observation on the increase of the crystallite size was also found for the spin coating. The crystallite size is 47.9±19.9nm and 58.4±12.0nm as temperature increases at 200°C and 300°C, respectively. However, for the spin coating, only brookite peak was identified. The effect of temperature increased (from 250°C to 900°C) on promoting crystallite growth (from 6.8 to 22.7nm) was reported by Chen et al. [50].

, brookite; □, rutile).

'> Crystallite sizes of the deposited TiO2 thin films via (a) dip and (b) spin coating with regard to temperatures (■, anatase; , brookite; □, rutile).
Fig. 2.

Crystallite sizes of the deposited TiO2 thin films via (a) dip and (b) spin coating with regard to temperatures (■, anatase;

, brookite; □, rutile).

(0.09MB).

Fig. 3 shows the Raman spectra for TiO2 thin films deposited via dip and spin coating at various temperatures. The Raman spectra for the deposited TiO2 thin films using dip coating show mixture of brookite (319cm−1), rutile (442cm−1) and anatase (511cm−1) throughout the temperature variation (Fig. 3(a)). However, with the spin coating, only TiO2 thin films deposited at 200°C shows mixture of brookite (319cm−1), rutile (447cm−1), and anatase (512cm−1) as in Fig. 3(b). At 300°C, 400°C and 500°C, the Raman spectra shows peaks of only brookite (320cm−1) and anatase (514cm−1) without the presence of rutile. It seems that the Raman result is slightly different with the obtained XRD results. With the Raman, all titania phases of brookite, rutile, and anatase are detected regardless of the deposition method and the varied temperatures. Whereas with the XRD, anatase and rutile were detected for the TiO2 thin films deposited using dip coating at 300°C, 400°C and 500°C and only single brookite is detected when deposited using spin coating at 200°C and 300°C. Therefore, TEM analysis is conducted to further characterize the microstructure and type of phases produced for the TiO2 thin films.

Fig. 3.

Raman spectra for TiO2 thin films deposited via (a) dip and (b) spin coating at various temperatures (A, anatase; B, brookite; R, rutile).

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Fig. 4 displays the TEM microstructures of the TiO2 thin films deposited via dip coating. It is obvious that the deposited TiO2 thin film had a spherical shape with a lattice fringes of 0.35nm and 0.33nm which can be assigned to the d-spacing of the of anatase (101) and rutile (110), respectively [51,52]. Meanwhile, Fig. 5 shows the TEM microstructures of the TiO2 thin films deposited via spin coating. As shown in the figures, the microstructures had a spherical shape with lattice fringes of 0.28nm which belong to the (111) plane of brookite [51,52]. Thus, it can be confirmed that anatase and rutile are present in the TiO2 thin film produced via dip coating and only brookite is present in the TiO2 thin films produced via spin coating.

Fig. 4.

TEM microstructures of the TiO2 thin films deposited via dip coating.

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Fig. 5.

TEM microstructures of the TiO2 thin films deposited via spin coating at (a) 200°C and (b) 300°C.

(0.31MB).

Fig. 6 shows the SEM cross sectional morphologies of the TiO2 thin films deposited via dip coating at 300°C (Fig. 6(a)) and spin coating at 200°C (Fig. 6(b)) and 300°C (Fig. 6(c)). The TiO2 thin films deposited via dip coating produced 706.3±167.5nm of thickness with the presence of anatase and rutile. Zhang et al. had also observed anatase and rutile in TiO2 thin films produced via dip coating method despite that the thickness is only of 125nm [53]. While for the spin coating, the thickness of the TiO2 thin films is 438.2±167.5nm at 200°C and 608.6±195.2nm at 300°C with the presence of single brookite. Komaraiah et al. had also produced brookite in TiO2 thin films via spin coating with the thickness of 220nm [16]. It can be observed that difference in the film thickness produced by different deposition methods is insignificant in the types of phases produced. Therefore, it can be confirmed that the deposition method is more significant in influencing the type of TiO2 phases formation rather than the film's thickness.

Fig. 6.

SEM cross sectional morphologies of the TiO2 thin films deposited via (a) dip coating at 300°C and spin coating at (b) 200°C and (c) 300°C.

(0.08MB).
4Conclusions

This work shows that the choice of deposition method can significantly influenced the phases formation and crystallinity of the deposited TiO2 thin films. Results of XRD and Raman show that the dip coating produced only a mixture of anatase and rutile at 200°C, 300°C, 400°C and 500°C. While with the spin coating, the films produced only single brookite at 200°C and 300°C. Further TEM analysis on the films deposited via spin coating confirmed the presence of only brookite crystallite with lattice fringes of 0.28nm. Meanwhile, for the dip coating, only anatase and rutile were presence with a lattice fringe of 0.35nm and 0.33nm, respectively. The crystallite size of the brookite is found to be 47.9±19.9nm at 200°C and 58.4±12.0nm at 300°C using Scherrer calculation. Thus this work proved that a single brookite TiO2 films was successfully produced via spin coating using green sol–gel route. Further works on evaluating the photocatalytic performance of the deposited brookite thin films produced via spin coating is currently in progress.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The authors thank the Ministry of Higher Education Malaysia and Universiti Teknikal Malaysia Melaka (UTeM) through Grant FRGS/1/2016/TK05/FKP-AMC/F00319 and all Sustainable Material for Green Technology (SM4GT) group members under Advanced Manufacturing Centre for the support given throughout this research.

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