High Temperature Stable Anatase Phase Titanium Dioxide Films Synthesized by Mist Chemical Vapor Deposition

nanomaterials

Article High Temperature Stable Anatase Phase Titanium Dioxide Films Synthesized by Mist Chemical Vapor Deposition

Qiang Zhang 1 and Chaoyang Li 1,2,*

1 School of Systems Engineering, Kochi University of Technology, Kami, Kochi 782-8502, Japan; [email protected] 2 Center for Nanotechnology, Kochi University of Technology, Kami, Kochi 782-8502, Japan * Correspondence: [email protected]; Tel.: +81-887-57-2106

Received: 31 March 2020; Accepted: 6 May 2020; Published: 9 May 2020

Abstract: Pure anatase-phase titanium dioxide films stable up to high temperatures were successfully fabricated by the mist chemical vapor deposition method. A post-annealing treatment of the synthesized films was carried out in oxygen atmosphere in the temperature range from 600 to 1100 ◦C and no anatase to rutile transformation was observed up to 1000 ◦C. Based on the grazing incidence X-ray diffraction data, the average crystallite size of the titanium dioxide films increased gradually with increasing annealing temperature. The structural analysis revealed that the high thermal stability of the anatase phase can be attributed to the small crystallite size and a sheet-like grain structure. An incomplete anatase to rutile transformation was observed after annealing at 1100 ◦C.

Keywords: anatase; titanium dioxide; thin ﬁlm; thermal stability; mist chemical vapor deposition

1. Introduction

Titanium dioxide (TiO2) is one of the most promising semiconductor materials due to its unique properties such as a wide bandgap, abundance in nature, and high physical and chemical stability [1,2]. TiO2 has been extensively investigated for potential application in photocatalysts, photovoltaic devices and gas sensors [3]. Generally, TiO2 has three crystalline phases: anatase, rutile, and brookite. Compared with other phases of TiO2, anatase phase TiO2 has better photocatalytic and photovoltaic activity because of its larger bandgap and higher surface energy. However, anatase is the metastable phase and can easily transform to rutile phase, the most stable phase of TiO2, after high temperature heating. Reported research has shown that the anatase to rutile transformation occurs at temperatures between 450 and 850 ◦C, depending on fabrication method and precursors [4–8]. Various methods such as metal dopant and morphology control have been investigated to resist the anatase-rutile transformation [9,10]. However, until now, there was little report on successfully fabricating pure anatase TiO2 ﬁlm stabilized at high temperature of over 900 ◦C. In order to apply anatase phase TiO2 for various environmental applications such as gas sensors, anti-microbial sanitary wares, and self-cleaning ceramic tiles, high temperature stable anatase TiO2 without phase transformation is essential [11,12]. Therefore, the development of a high temperature stable (above 1000 ◦C) anatase phase TiO2 is desirable. The anatase to rutile transformation has been investigated by a number of groups [9,10,13–22]. According to the literature [9,10,13–21], the anatase to rutile transformation could be inﬂuenced by several factors including particle size, particle shape, and the presence of {112} facets. Related researches predicted that anatase became more stable than rutile when the particle size was smaller than the critical size (6.9~22.7 nm) [13,14]. Furthermore, it has been reported that rutile could easily nucleate at

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{112} twin interfaces during the anatase to rutile transformation [15–19]. As a result, the presence of {112} facets in anatase phase TiO2 will decrease the thermal stability of anatase phase TiO2. According to our previous research, mist chemical vapor deposition (mist CVD) method was found to have specific advantages in terms of precise growth controllability, large area deposition and simplicity to synthesize TiO2 thin films [23,24]. In this research, the mist CVD method was used to synthesize TiO2 thin film, and the thermal stability of obtained TiO2 films was investigated.

2. Materials and Methods

2.1. Fabrication of TiO2 Thin Films

TiO2 thin films with a thickness of 300 nm were deposited on quartz glass substrate (MITORIKA Co., Ltd., Mito, Japan) by mist CVD. The deposition condition of TiO2 films is shown in Table1. A solution of precursor was prepared by dissolving titanium tetraisopropoxide (TTIP, purity > 95.0%, Wako Pure Chemical Industries, Ltd., Osaka, Japan) in ethanol (purity > 99.5%, Wako Pure Chemical Industries, Ltd., Osaka, Japan). The concentration of TTIP was 0.10 mol/L. The solution was ultrasonically atomized by ultrasonic transducers (2.4 MHz). Subsequently, the mist droplets of precursor were transferred to the reaction chamber using compressed air as a carrier gas and dilution gas, with flow rates controlled at 2.5 L/min and 4.5 L/min respectively. The substrate was set in the reaction chamber, which was heated and kept at 400 ◦C.

Table 1. Deposition condition of TiO2 ﬁlms.

Solute TTIP Solvent Ethanol Concentration (mol/L) 0.10 Deposition temperature (◦C) 400 Carrier gas, ﬂow rate (L/min) Compressed air, 2.5 Dilution gas, ﬂow rate (L/min) Compressed air, 4.5

2.2. Annealing Treatment

According to previous publications and TiO2 phase diagram, both the annealing ambient and pressure effect the temperature and speed of anatase to rutile phase transformation. However, the effect of annealing ambient on the phase transformation can be ignored because the variation of transformation temperature was less than 50 ◦C. Moreover, the effect of annealing ambient on the phase transformation was dependent on the structural properties of anatase TiO2. In gas sensor applications, the working conditions of gas sensors were oxidizing conditions (O2 or air) at room pressure (1 Bar). Therefore, in order to investigate the thermal stability, the as-deposited TiO2 films were calcined at a temperature in the range of 600–1100 ◦C in a pure oxygen ambient (1 Bar) for 1 h with a rapid thermal annealing furnace (RTA, ULVAC-RIKO, MILA-3000, Yokohama, Japan). The conditions of annealing treatment are shown in Table2.

Table 2. Conditions of annealing treatment.

Ambient Pure Oxygen Pressure (bar) 1 Annealing time (h) 1 Annealing temperature (◦C) 600, 800, 1000, 1100 Speed of warming up (◦C/min) 15 Speed of cooling down (◦C/min) 10

2.3. Characterization

The structural properties of TiO2 ﬁlms were investigated by grazing incidence X-ray diﬀraction (GIXRD, ATX-G, Rigaku, Tokyo, Japan) with Cu Kα X-ray source (1.54184 Å) at a 0.35◦ incidence Nanomaterials 2020, 10, x FOR PEER REVIEW 3 of 8

2.3. Characterization

The structural properties of TiO2 films were investigated by grazing incidence X-ray diffraction (GIXRD, ATX-G, Rigaku, Tokyo, Japan) with Cu Kα X-ray source (1.54184 Å) at a 0.35° incidence angle, Raman spectroscopy (LabRAM HR-800, Horiba Jobin Yvon, Longjumeau, France) with a 532.8 nm excitation laser, and X-ray photoelectron spectroscopy (XPS, AXIS-HS, Shimadzu/KRATOS, Kyoto, Japan) with Mg Kα X-ray source (1253.6 eV). The morphological properties of TiO2 films were Nanomaterialsevaluated 2020by ,a10 ,field 911 emission scanning electron microscope (FE-SEM, SU-8020, Hitachi, Tokyo,3 of 8 Japan). All measurements were carried out at room temperature. angle, Raman spectroscopy (LabRAM HR-800, Horiba Jobin Yvon, Longjumeau, France) with a 3. Results 532.8 nm excitation laser, and X-ray photoelectron spectroscopy (XPS, AXIS-HS, Shimadzu/KRATOS, Kyoto,The Japan) GIXRD with patterns Mg Kα X-rayof as-deposited source (1253.6 TiO eV).2 films The and morphological TiO2 films after properties annealing of TiO are2 films shown were in evaluatedFigure 1a. by Comparing a field emission as-deposited scanning TiO electron2 films microscope with TiO2 (FE-SEM,films calcined SU-8020, at 600, Hitachi, 800, and Tokyo, 1000 Japan). °C, it Allwas measurements found that all were of the carried diffraction out at peaks room corresponded temperature. with the diffractions from (101), (200), (211), (201), (204), and (215) crystal planes of the anatase phase TiO2. This suggested that pure anatase phase 3.TiO Results2 thin films were obtained by mist CVD. No anatase to rutile transformation was observed after TiO2 films calcined at 600, 800, and 1000 °C. However, the TiO2 films calcined at 1100 °C showed a The GIXRD patterns of as-deposited TiO2 films and TiO2 films after annealing are shown in different GIXRD pattern. More peaks were observed at 2 theta of 27.57°, 36.24°, 41.44°, 54.52°, and Figure1a. Comparing as-deposited TiO 2 films with TiO2 films calcined at 600, 800, and 1000 ◦C, it was found56.73° that except all of the the anatase diffraction phase peaks peaks. corresponded Those peaks with thecorresponded diffractions with from (101),rutile (200),phase (211), TiO2 (201),. This suggested that incomplete anatase to rutile transformation occurred under 1100 °C annealing. (204), and (215) crystal planes of the anatase phase TiO2. This suggested that pure anatase phase TiO2 Compared with other diffraction peaks for all samples, the (101) peak located at 2 theta of 25.45° thin films were obtained by mist CVD. No anatase to rutile transformation was observed after TiO2 was dominant with much higher intensity. The intensities of (101) peaks are shown in Figure 1b. films calcined at 600, 800, and 1000 ◦C. However, the TiO2 films calcined at 1100 ◦C showed a different Compared with as-deposited sample, the TiO2 films calcined at 600 °C showed higher (101) peak GIXRD pattern. More peaks were observed at 2 theta of 27.57◦, 36.24◦, 41.44◦, 54.52◦, and 56.73◦ except intensity. As the annealing temperature was increased from 600 to 1000 °C, (101) peak intensity the anatase phase peaks. Those peaks corresponded with rutile phase TiO2. This suggested that gradually increased. When the annealing temperature was increased from 1000 °C to 1100 °C, the incomplete anatase to rutile transformation occurred under 1100 ◦C annealing. (101) peak intensity decreased due to the anatase to rutile transformation.

(a) (b)

FigureFigure 1. 1.GIXRD GIXRD results results of of as-deposited as-deposited TiO TiO2 2films films and and TiO TiO2 2films films after after annealing. annealing. (( ((aa)) GIXRD GIXRD patterns patterns ofof TiO TiO2 2films; films; ( b(b)) (101) (101) peak peak intensities intensities of of TiO TiO2 2films). films).

ComparedIt is well-known with other that di theﬀraction results peaks of XRD, for all including samples, peak the (101) intensity peak locatedand the at full 2 theta width of 25.45at half◦ wasmaximum dominant (FWHM) with muchvalue, highercan be intensity.used to calculate The intensities the grain ofsize (101) and peaks the thickness are shown of defective in Figure layer1b. Compared[25–27]. According with as-deposited to the Scherrer sample, equation the TiO (Equation2 ﬁlms calcined (1)), the at(101) 600 orientation◦C showed crystallite higher (101) size peak(L) of intensity.anatase TiO As2 can the be annealing calculated temperature using the FWHM was increased value [25,26]. from 600 to 1000 ◦C, (101) peak intensity gradually increased. When the annealing temperatureK wasλ increased from 1000 ◦C to 1100 ◦C, the (101) peak intensity decreased due to the anatase toL= rutile transformation. (1) cos It is well-known that the results of XRD, including peak intensity and the full width at half maximumwhere K is (FWHM) a constant value, related can to becrystallite used to shape calculate (taken the as grain 0.89 sizefor anatase), and the λ thickness the X-ray of wavelength defective layerin nanometer [25–27]. According (taken as to 0.154184 the Scherrer nm equationin this research), (Equation β (1)), the theFWHM (101) orientationvalue of diffraction crystallite sizepeak (L) in ofradians, anatase and TiO θ2 canthe diffraction be calculated angle. using the FWHM value [25,26].

Kλ L = (1) β cos θ where K is a constant related to crystallite shape (taken as 0.89 for anatase), λ the X-ray wavelength in nanometer (taken as 0.154184 nm in this research), β the FWHM value of diﬀraction peak in radians, and θ the diﬀraction angle. Nanomaterials 2020, 10, 911 4 of 8

Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 8 Figure2 shows the average crystallite size along the (101) orientation calculated from GIXRD Figure 2 shows the average crystallite size along the (101) orientation calculated from GIXRD data. The calculated crystallite size of as-deposited TiO2 ﬁlms was around 16.7 nm. After annealing, data. The calculated crystallite size of as-deposited TiO2 films was around 16.7 nm. After annealing, the crystallite size of TiO2 ﬁlms slightly increased. When the annealing temperature increased from the crystallite size of TiO2 films slightly increased. When the annealing temperature increased from 600 ◦C to 1100 ◦C, the crystallite size of anatase TiO2 increased from 17.7 nm to 21.4 nm gradually. The600 increase°C to 1100 in crystallite°C, the crystallite size could size be of attributedanatase TiO to2several increased factors, fromincluding 17.7 nm to the 21.4 anatase nm gradually. to rutile transformation.The increase in crystallite Compared size with could the increasebe attributed of anatase to several crystallite factors, size including caused bythe anataseanatase toto rutilerutile transformation. Compared with the increase of anatase crystallite size caused by anatase to rutile transformation reported in other publications [20,28], the crystallite size increase of TiO2 synthesized bytransformation mist CVD was reported very gradual in other and publications slight. Therefore, [20,28], we the conclude crystallite that size the increase increase of inTiO crystallite2 synthesized size wasby mist not causedCVD was by very the phase gradual transformation. and slight. Therefore, we conclude that the increase in crystallite size was not caused by the phase transformation.

18 Crystallitesize (nm)

as-deposited 600 800 1000 1100 Annealing temperature (°C)

Figure 2. The (101) orientation crystallite sizes of as-deposited TiO films and TiO films after annealing. Figure 2. The (101) orientation crystallite sizes of as-deposited2 TiO2 films2 and TiO2 films after annealing. Raman spectra of as-deposited TiO2 films and TiO2 films after annealing are shown in Figure3a. Three peaks were observed in the spectra of as-deposited TiO films and TiO films calcined at 600, Raman spectra of as-deposited TiO2 films and TiO2 films after2 annealing are2 shown in Figure 3a. 1 1 800, and 1000 ◦C. The peaks at around 395 cm− and 638 cm− corresponded with the B mode and Eg Three peaks were observed in the spectra of as-deposited TiO2 films and TiO2 films 1gcalcined at 600, mode of anatase phase TiO respectively [29,30]. The peaks at around 515 cm 1 corresponded with a 800, and 1000 °C. The peaks2 at around 395 cm−1 and 638 cm−1 corresponded with− the B1g mode and Eg doublet of the A and B modes of the anatase phase TiO [29,30]. All of the peaks corresponded mode of anatase1g phase TiO1g 2 respectively [29,30]. The peaks2 at around 515 cm−1 corresponded with a with the anatase phase TiO , which indicated the as-deposited TiO films were pure anatase phase and doublet of the A1g and B1g2 modes of the anatase phase TiO2 [29,30].2 All of the peaks corresponded the anatase to rutile transformation did not occur after calcination at 600, 800, and 1000 C. For TiO with the anatase phase TiO2, which indicated the as-deposited TiO2 films were pure anatase◦ phase2 filmsand the calcined anatase at to 1100 rutile◦C, transformation as shown in the did inserted not occur image, after somecalcination spots at with 600, a 800, diameter and 1000 of around °C. For 20 µm were observed using the Raman system optical microscope. After checking these spots by TiO2 films calcined at 1100 °C, as shown in the inserted image, some spots with a diameter of around Raman20 μm spectroscopy,were observed two using Raman the Raman peaks weresystem observed optical andmicroscope. corresponded After checking with rutile these phase spots TiO by2. Other areas were also measured and identified as anatase phase TiO . This suggested that incomplete Raman spectroscopy, two Raman peaks were observed and corresponded2 with rutile phase TiO2. anatase-rutile transformation occurred during 1100 C annealing. The results of Raman measurement Other areas were also measured and identified as anatase◦ phase TiO2. This suggested that incomplete wereanatase-rutile in agreement transformation with those ofoccurred the GIXRD during measurement. 1100 °C annealing. Figure3 Theb shows results the of relationship Raman measurement between thewere intensities in agreement of anatase with those Raman of the peaks GIXRD and measur annealingement. temperatures. Figure 3b shows Compared the relationship with as-deposited between sample,the intensities the TiO 2offilms anatase showed Raman higher peaks Raman and peakannealing intensities temperatures. after annealing Compared at different with temperatures. as-deposited As the annealing temperature was increased from 600 to 1100 C, the intensities of all Raman peaks sample, the TiO2 films showed higher Raman peak intensities◦ after annealing at different increasedtemperatures. gradually. As the annealing temperature was increased from 600 to 1100 °C, the intensities of all Raman peaks increased gradually. Since the presence of dopants may strongly affect the thermal stability of anatase phase TiO2, the purity of as-deposited sample was investigated by XPS. Figure 4 shows the XPS survey spectrum of as-deposited TiO2 films. As shown in the XPS survey spectrum, several distinct peaks were observed and corresponded with various electron orbitals of titanium and oxygen [21,31–34]. The XPS peak at around 521.3 eV corresponded with the satellite peak of O 1s, which was attributed to Mg Kα 3. The Si 2p peak was observed at around 101 eV and attributed to the substrate (quartz glass). A C 1s peak at around 284.5 eV was observed due to environmental contamination. No peaks corresponding with other elements were observed, indicating that there were no obvious dopants in

Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 8

Nanomaterials 2020, 10, 911 5 of 8 the as-deposited sample. Based on the results of XPS, the effects of impurities on the thermal stability of anatase phase TiO2 can be safely discounted.

Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 8

the as-deposited sample. Based on the results of XPS, the effects of impurities on the thermal stability of anatase phase TiO2 can be safely discounted.

(a) (b)

Figure 3. Raman results of as-deposited TiO2 films and TiO2 films after annealing. ((a) Raman spectra Figure 3. Raman results of as-deposited TiO2 films and TiO2 films after annealing. ((a) Raman spectra 2 2 ofof TiO TiO films;2 films; (b ()b Raman) Raman peak peak intensities intensities of of TiO TiO films).2 films).

Since the presence of dopants may strongly affect the thermal stability of anatase phase TiO2, the purity of as-deposited sample was investigated by XPS. Figure4 shows the XPS survey spectrum of as-deposited TiO2 films. As shown in the XPS survey spectrum, several distinct peaks were observed and corresponded with various electron orbitals of titanium and oxygen [21,31–34]. The XPS peak at around 521.3 eV corresponded with the satellite peak of O 1s, which was attributed to Mg Kα 3. The Si 2p peak was observed at around 101 eV and attributed to the substrate (quartz glass). A C 1s peak at around 284.5 eV was(a) observed due to environmental contamination.(b No) peaks corresponding with other elements were observed, indicating that there were no obvious dopants in the as-deposited Figure 3. Raman results of as-deposited TiO2 films and TiO2 films after annealing. ((a) Raman spectra sample. Based on the results of XPS, the effects of impurities on the thermal stability of anatase phase of TiO2 films; (b) Raman peak intensities of TiO2 films). TiO2 can be safely discounted.

Figure 4. XPS survey spectrum of as-deposited TiO2 films.

The top view FE-SEM images of as-deposited TiO2 films and TiO2 films after annealing are shown in Figure 5. It was confirmed that the as-deposited TiO2 film exhibited a uniform surface. As shown in Figure 5a, the intertwined TiO2 nanosheets were observed in as-deposited TiO2 film. Compared with as-deposited sample, the morphology of TiO2 films calcined at 600 °C and 800 °C including the length and thickness of sheet-like grains showed little change. As the annealing temperature was increased from 800 °C to 1000 °C, the thickness of TiO2 nanosheets increased slightly. However, the length of TiO2 nanosheets showed little variation. As shown in Figure 5f, the TiO2 films calcined at 1100 °C showed different morphology as other samples. Some spots with a diameter of around 20 μmFigure were 4. observed,XPS survey which spectrum was of similar as-deposited to the TiO image2 films. obtained by the optical Figure 4. XPS survey spectrum of as-deposited TiO2 films. microscope in Raman system. The detail of spots and other areas were checked and shown in Figure The top view FE-SEM images of as-deposited TiO films and TiO films after annealing are shown 5e and Figure 5g respectively. According to Raman results,2 these spots2 were identified as rutile phase The top view FE-SEM images of as-deposited TiO2 films and TiO2 films after annealing are in Figure5. It was confirmed that the as-deposited TiO 2 film exhibited a uniform surface. As shown TiO2, and other areas were identified as anatase phase. However, the morphology of anatase phase inshown Figure in5 Figurea, the intertwined 5. It was confirmed TiO nanosheets that the as-deposited were observed TiO in2 film as-deposited exhibited TiOa uniformfilm. Comparedsurface. As area was different to other anatase2 phase samples. The grain size of anatase phase2 area was much withshown as-deposited in Figure sample,5a, the intertwined the morphology TiO2 of nanosheets TiO films calcinedwere observed at 600 Cinand as-deposited 800 C including TiO2 film. the bigger than other samples. This suggested that the incomplete2 anatase-rutile◦ transformation◦ occurred lengthCompared and thicknesswith as-deposited of sheet-like sample, grains the showed morphology little change.of TiO2 films As the calcined annealing at 600 temperature °C and 800 was °C including the length and thickness of sheet-like grains showed little change. As the annealing temperature was increased from 800 °C to 1000 °C, the thickness of TiO2 nanosheets increased slightly. However, the length of TiO2 nanosheets showed little variation. As shown in Figure 5f, the TiO2 films calcined at 1100 °C showed different morphology as other samples. Some spots with a diameter of around 20 μm were observed, which was similar to the image obtained by the optical microscope in Raman system. The detail of spots and other areas were checked and shown in Figure 5e and Figure 5g respectively. According to Raman results, these spots were identified as rutile phase TiO2, and other areas were identified as anatase phase. However, the morphology of anatase phase area was different to other anatase phase samples. The grain size of anatase phase area was much bigger than other samples. This suggested that the incomplete anatase-rutile transformation occurred

Nanomaterials 2020, 10, 911 6 of 8 Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 8 duringincreased 1100 °C from annealing, 800 ◦C to 1000which◦C, corresponds the thickness well of TiO to 2 thenanosheets results increasedof the GIXRD slightly. and However, Raman the measurement.length of TiO 2 nanosheets showed little variation. As shown in Figure5f, the TiO 2 films calcined at 1100According◦C showed to related different research morphology [6,22], asif the other anatase samples. phase Some TiO spots2 is not with thermodynamically a diameter of around stable 20 µm at thewere critical observed, temperature, which was the similar anatase to theto imagerutile obtainedtransformation by the opticalwill oc microscopecur immediately. in Raman At system.the beginningThe detail of phase of spots transformation, and other areas the were reaction checked rate and is almost shown a in constant. Figure5e,g Accordingly, respectively. the According rutile phaseto RamanTiO2 can results, be detected these spots after were several identified minutes as rutile annealing, phase TiOwhich2, and has other been areas proved were by identified some as publicationsanatase phase. [6,20]. However,In this research, the morphology the anatase of phase anatase TiO phase2 kept areaits sheet-like was different structure to other after anatase one hour phase annealingsamples. at The1000 grain °C, and size ofrutile anatase phase phase TiO area2 was was not much found bigger by GIXRD than other and samples. Raman Thismeasurement. suggested that Therefore,the incomplete we can conclude anatase-rutile that transformationthe anatase phase occurred TiO2 films during synthesized 1100 ◦C annealing, in this research which correspondsshowed highwell thermal to the stability results ofup the to 1000 GIXRD °C. and Raman measurement.

(a) (b) (c) (d)

200 nm 200 nm 200 nm 200 nm

(e) (f) (g)

200 nm 20 μm 200 nm

FigureFigure 5. FE-SEM 5. FE-SEM images images of as-deposited of as-deposited TiO TiO2 films2 films (a) (anda) and TiO TiO2 films2 films calcined calcined at at different different temperatures temperatures((b) 600 ◦ C;((b ()c )600 800 °C;◦C; (c ()d 800) 1000 °C;◦ (C;d) (1000e–g) 1100°C; (e◦–C).g) 1100 °C).

AccordingAccording to the to relatedGIXRD research results, [the6,22 as-deposited], if the anatase TiO phase2 film TiO showed2 is not a thermodynamically small crystallite size stable of at aroundthe critical16.7 nm. temperature, As mentioned the in anatase the introduction, to rutile transformation anatase phase will is occurthermodynamically immediately. At more the beginningstable thanof rutile phase phase transformation, for such crystallite the reaction size. rate Moreover is almost, the a constant.rutile transformation Accordingly, would the rutile be phaseunlikely TiO to2 can occurbe if detected the crystallite after several size does minutes not increase, annealing, which which maybe has been a reason proved for by the some high publications thermal stability [6,20]. Inof this TiO2research, films. As the mentioned anatase phase in the TiO introduction,2 kept its sheet-like the presence structure of {112} after facets one hourin anatase annealing phase at TiO 10002 will◦C, and decreaserutile the phase thermal TiO2 wasstability not foundof anatase by GIXRD phase andTiO Raman2. Therefore, measurement. a diffraction Therefore, peak that we corresponded can conclude that withthe {112} anatase facets phase was TiO not2 films observed synthesized in the inGIXRD this research pattern showed of as-deposited high thermal sample, stability which up to also 1000 ◦C. improvedAccording the thermal to stability the GIXRD of TiO results,2 film. the as-deposited TiO2 film showed a small crystallite size of aroundAnother 16.7 reason nm. Asfor mentionedhigh temperature in the introduction, stabilized pure anatase anatase phase TiO is2 film thermodynamically could be the fabrication more stable method.than As rutile annealing phase for temperature such crystallite was increased size. Moreover, from 600 the to rutile 1000 transformation°C, the morphology would of beTiO unlikely2 films to includingoccur ifthe the length crystallite and sizethickness does notof sheet-like increase, whichgrains maybedid not a reasonshow much for the change. high thermal These unique stability of sheet-likeTiO2 films. grains As with mentioned fewer ininterfaces the introduction, had a gr theeat presence influence of {112}on the facets anatase in anatase to rutile phase phase TiO2 will transformation.decrease the thermalFewer grain stability interfaces of anatase can phase suppress TiO2. Therefore,the phase a ditransformation.ffraction peak thatThe corresponded packing characteristicswith {112} facetsof the wassheet-like not observed TiO2 grains in the suppressed GIXRD pattern the interface of as-deposited nucleation sample, of the which rutile alsophase improved and significantlythe thermal limited stability the ofphase TiO 2transformationfilm. at a relatively high temperature. Moreover, the weak connectionsAnother among reason sheet-like for high temperaturegrains worked stabilized as purebarriers, anatase which TiO2 filmblocked could beanatase-rutile the fabrication transformation.method. As annealing temperature was increased from 600 to 1000 ◦C, the morphology of TiO2 films includingAs the temperature the length andincreased thickness to over of sheet-like 1000 °C, grainsthe interface did not and show weak much connections change. Thesecould uniquenot suppresssheet-like the anatase grains with to rutile fewer transformation. interfaces had a greatThe sheet-like influence ongrains the anatase were merged to rutile into phase a large transformation. grain withFewer many grainvoids. interfaces can suppress the phase transformation. The packing characteristics of the sheet-like TiO2 grains suppressed the interface nucleation of the rutile phase and significantly limited 4. Conclusionsthe phase transformation at a relatively high temperature. Moreover, the weak connections among sheet-like grains worked as barriers, which blocked anatase-rutile transformation. Pure anatase phase TiO2 films with high thermal stability were successfully synthesized by mist CVD method. Sheet-like grains were observed in obtained TiO2 film. As calcined temperature was increased from 600 to 1000 °C, the TiO2 films did not show any obvious difference in terms of morphological and structural properties. The TiO2 films were in the pure anatase phase until the

Nanomaterials 2020, 10, 911 7 of 8

As the temperature increased to over 1000 ◦C, the interface and weak connections could not suppress the anatase to rutile transformation. The sheet-like grains were merged into a large grain with many voids.

4. Conclusions

Pure anatase phase TiO2 films with high thermal stability were successfully synthesized by mist CVD method. Sheet-like grains were observed in obtained TiO2 film. As calcined temperature was increased from 600 to 1000 ◦C, the TiO2 films did not show any obvious difference in terms of morphological and structural properties. The TiO2 films were in the pure anatase phase until the annealing temperature reached to 1000 ◦C. Anatase to rutile transformation of TiO2 film occurred at a temperature of 1100 ◦C. It was shown that the sheet-like grains, small crystallite size, and the absence of {112} facets might contribute to the high temperature stability of TiO2 film. Anatase TiO2 films with high temperature stability therefore demonstrate great potential for application in high temperature gas sensors.

Author Contributions: Conceptualization, Q.Z.; methodology, Q.Z.; investigation, Q.Z.; data curation, Q.Z.; writing—original draft preparation, Q.Z.; writing—review & editing, C.L.; visualization, C.L.; supervision, C.L.; project administration, C.L.; funding acquisition, C.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan, grant number [17K06394]. Acknowledgments: The authors gratefully acknowledge the financial support by Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Conflicts of Interest: The authors declare no conflict of interest.

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