1 Introduction

Superhydrophilic titanium dioxide (TiO2) thin films have been applied in various fields, such as for biomedical applications [1], antireflection and antifogging [2, 3], antimicrobial activities [4], and self-cleaning [2, 5], owing to TiO2 thin films possessing hydrophilic, chemical, and outstanding optical properties. Anatase TiO2, with an energy band of about 3.2 eV, which causes the superhydrophilicity of TiO2 thin films, vanishes rapidly without ultraviolet (UV) light irradiation, meaning that the current applications of TiO2 thin films are limited mainly to outdoor use [6, 7].

When the TiO2 thin films are treated with a given chemical composition to increase the surface roughness, this can render the thin films more hydrophobic or hydrophilic. Therefore, porous TiO2 thin films can enhance the surface roughness. Porous TiO2 thin films have been fabricated by various processes such as the sol–gel method [5, 8–10], sol–gel and evaporation-induced self-assembly [11], sputtering techniques [12], direct deposition from aqueous solutions [13], spray pyrolysis deposition technique [14], and template hydrothermal synthesis [15]. Among these processes, the sol–gel process is an important method for fabricating the porous TiO2 thin films, because it possesses a number of advantages, such as the atomic-scale mixing of constituents, easy operation, and cost factor [16].

Transparent, nanosized, crystalline TiO2 thin films with high photocatalytic activity, fabricated on soda-lime glass substrates, using the TiO2 sol solution prepared by the sol–gel process, have been reported by Yu et al. [9]. They demonstrated that the TiO2 gel films were converted to anatase TiO2 thin films with the highest photocatalytic activity when the TiO2 gel films had been heat-treated at 500 °C. Zhao et al. [5] pointed out a new method for enhancing the photocatalytic activity of TiO2 thin films, when the thin films were directly coated on the soda-lime glass, prepared by the sol–gel process, and the films were treated in an acidic solution. In addition, Wang et al. [11] pointed out that the mesostructured TiO2 thin films were fabricated by the sol–gel and evaporation-induced self-assembly processes, using the triblock copolymer Pluronic F127 (EO106PO70EO106) as the templating agent and (Ti(OC4H9)4, Ti(OBu)4) as the titanium source. They also reveal that the as-synthesized mesoporous TiO2 thin films have excellent thermal stability and possess pores with a diameter larger than 7 nm, with a narrow pore size distribution, and thick inorganic walls composed of nanocrystalline anatase.

On the other hand, the effect of ultrasonic treatment on the surface of highly hydrophilic TiO2 was reported by Sakai et al. [17]. They reveal that the contact angle of surface hydrophilicity decreased to approximately 11° when the pure water was added after ultrasonic treatment. Watanabe et al. [18] studied the single-crystal structure of rutile and polycrystalline anatase TiO2 dependence on the photoinduced hydrophilic conversion and pointed out that the wettability conversion rate of the surface with the bridging oxygen site is low, compared to that without the bridging site. Moreover, the relationship between the natural superhydrophilic and structurally porous TiO2 thin films has also been reported by various researchers [19–22]. However, the effect of heat treatment on the crystalline structure and superhydrophilic properties of TiO2 porous thin films without ultraviolet (UV) light irradiation has not been discussed in detail.

In the present study, the porous TiO2 thin films were synthesized using acetylacetone (AcAc) and diethanolamine (DEA) as complexing agents, prepared by a sol–gel spin-coating process. The aim of the present work was to: (1) study the effect of heat treatment on the crystalline structure of TiO2 powders and thin films; (2) observe the morphologies of TiO2 thin films before and after heat treatment; and (3) evaluate the effect of heat treatment on the superhydrophilic properties of TiO2 thin films.

2 Experimental procedure

2.1 Materials

In the present study, tetrabutyl titanate (Ti(OBu)4) (Ti(OC4H9)4, purity >98.0 %), absolute ethanol (C2H5OH, purity >99.7 %), acetylacetone (AcAc) (C5H8O2, purity >99.0 %), diethanolamine (DEA) (C4H11NO2, purity >98.0 %), and nitric acid (HNO3, purity ≥65–68 %) were used as the initial materials. All initial materials were supplied by Sinopharm Chemical Reagent Co. Ltd., China. The soda-lime silicate glasses with a dimension of 2.5 cm × 2.5 cm × 0.1 cm were used as the substrates.

2.2 Preparation of TiO2 thin films

The TiO2 thin films were prepared using sol–gel spin-coating on soda-lime silicate glass substrates, according to the chart shown in Fig. 1 [23]. The “A” solution was made by Ti(OBu)4, absolute ethanol, AcAc, and DEA in volumes of 10, 27, 2, and 2 ml, respectively. The “B” solution was made by mixing absolute ethanol, deionized water, and HNO3 in volumes of 53, 10, and 1 ml, respectively. The sol–gel of TiO2 was prepared by sequentially premixing the “B” solution into the “A” solution and stirring at room temperature for 10 min to form sol 1. Then, sol 1 was stirred at room temperature for 90 min to form an insoluble gel. The gel was then aged at room temperature for 24 h.

Fig. 1
figure 1

Flowchart for preparing TiO2 thin films

Full size image

Before spin-coating, the soda-lime silicate glass substrates with a dimension of 7.5 cm × 2.5 cm × 0.1 cm were degreased, cleaned thoroughly, and dried in an oven at 80 °C for 30 min. First, the gel was spin-coated on the substrate once with a screw rotation rate of 750 rpm for 15 s and subsequently at 5000 rpm for 30 s. Then, the films were dried at 105 °C for 30 min. In order to heat-treat the dried sample, it was transferred to another electric furnace at heating rates of 5 °C/min to 200, 300, 350, 400, 450, 500, and 550 °C, respectively, in air for 60 min. Then, porous TiO2 thin films were obtained. Moreover, the dried samples were also heat-treated at 450 °C for 10, 30, and 120 min, respectively. In order to compare the developed phase, the titanium oxide gels were also directly dried, and calcined between 300 and 550 °C for 1 h. Then, the TiO2 powders were obtained.

2.3 Sample characterization

The crystalline phase of TiO2 powders and TiO2 thin films was identified by X-ray diffraction (XRD, D8 Advance, Germany), with a CuKα target and an Ni filter, operated at 30 kV, 200 mA, and a scanning rate (2θ) of 1°/min. The morphologies of the surface and cross section of the TiO2 thin films were observed by field emission scanning electron microscopy (FESEM, Zeiss Ultra Plus, Germany). The Fourier transform infrared spectra of TiO2 sols spread on KBr pellets were recorded on a Fu Liye transform infrared spectrometer (FTIR, Nicolet6700, America), with 0.019 cm−1 resolution in the frequency range from 4000 to 400 cm−1. The water contact angles (WCAs) of TiO2 thin films without UV irradiation were measured at ambient temperature, by an automatic contact angle measuring device (Dataphysics OCA35, Germany). Water droplets were generated automatically, with a volume of 5 μl. Samples were kept in a dark box for 48 h before the measurement of WCAs. Typically, the average value of measurements at five different positions of the TiO2 thin films’ surface was adopted as the value of the WCAs.

3 Results and discussion

3.1 FTIR analysis of TiO2 sols

Figure 2a shows the FTIR spectrum of TiO2 sols prepared by using Ti(OBu)4, absolute ethanol, AcAc, and DEA as the initial materials, where the OH flexion bands face out and the low-energy band of 435.8 cm−1 appear due to a stretching vibration of Ti–O band in titania [24]. The weak absorption peak is seen at 659.5 cm−1 because of the hydrolysis and polycondensation reaction of –(Ti–O)–n [25]. The Ti–O bond bands are found at 555.0 and 773.3 cm−1 of the low-energy interval, and a well-defined band typical of hydroxyl groups (≡Ti–OH) is found at 929.5 cm−1 [24]. The weak absorption peak to be found at 1064.5 cm−1 is attributed to the stretching vibration of C–O band [26]. The absorption peak at 1272.8 cm−1 is attributed to the asymmetric vibration of C–O [27, 28]. The absorption peaks at 1378.7 and 1432.9 cm−1 are due to the symmetric band of α-CH3 [29] and symmetric stretching vibration of CH3 [27, 29]. Two absorption peaks at 1521.6 and 1589.2 cm−1 are due to the acetylacetonate groups bound to titanium [25]. The C–H stretching vibration of –CH2– and –CH3– is found at 3262.3 and 3399.4 cm−1, respectively, which is caused by the OH stretching band [24, 25].

Fig. 2
figure 2

FTIR spectrum of TiO2 sols prepared by using: a Ti(OC4H9)4, absolute ethanol, AcAc, DEA and b Ti(OC4H9)4, absolute ethanol, AcAc, DEA, HNO3, and deionized water

Full size image

The FTIR spectrum of TiO2 sols prepared by using Ti(OC4H9)4, absolute ethanol, AcAc, DEA, HNO3, and deionized water as initial materials is shown in Fig. 2b. It is seen that the result of Fig. 2b is similar to that of Fig. 2a, except the peaks at 2867.6 and 3262.3 cm−1 disappear in Fig. 2b.

3.2 Crystalline structure of the TiO2 powder and TiO2 thin films

Figure 3 shows the XRD patterns of TiO2 precursor films after being calcined at various temperatures for 1 h. The XRD patterns of the TiO2 precursor powders, after calcination at 300 and 350 °C, are shown in Fig. 3a, b, respectively, which reveals that the powders still maintained the amorphous state. Figure 3c, d shows the XRD patterns of the TiO2 precursor after being calcined at 400 and 450 °C for 1 h, respectively, which indicates that the powders only have a single phase of anatase TiO2, due to the reflections of (101), (004), (200), (105), (211), (204), (116), (220), and (215) of anatase TiO2 (JCPDS Cards: 89-4921). However, Fig. 3c also reveals that the broad and weak intensities of the peak are a result of the fine crystals in the nanometer-to-submicron-meter range and/or the poor crystallinity of anatase TiO2 [30, 31]. Figure 3e, f shows the XRD patterns of the TiO2 precursor powders after being calcined at 500 and 550 °C for 1 h, respectively, which reveals that the patterns still maintained the single phase of anatase TiO2. It is also found that the intensity of the reflection peaks was greater than the corresponding peaks, as shown in Fig. 3d. This result was attributed to the crystallinity, and the size of the anatase TiO2 crystalline increased as the calcination temperature rose from 450 to 550 °C.

Fig. 3
figure 3

XRD patterns of TiO2 powder samples calcined at various temperatures for 1 h: a 300 °C; b 350 °C; c 400 °C; d 450 °C; e 500 °C; and f 550 °C (A: anatase TiO2)

Full size image

High-purity and spherical nanocrystalline anatase TiO2 is synthesized by a modified sol–gel process, using anhydrous TiCl4 mixed with ethanol at 0 °C. Li et al. [32] reported that a yellowish sol was first obtained and then transformed into the phase of anatase TiO2 when baked at 87 °C for 3 days. They also pointed out that while heating the powders after baking between 87 and 600 °C for 2 h, the powders still maintained the phase of anatase TiO2. Katoch et al. [33] demonstrated that nanocrystalline anatase TiO2 is dispersed uniformly in the stable sol using the processes of hydrolysis and condensation of titanium isopropoxide in ethanol, with HNO3. The results of the present study were in agreement with previous results [26, 27].

Yeh et al. [34] reported the use of titanium tetrachloride (TiCl4) as the initial material for preparing TiO2 nanocrystallines using a coprecipitation process and pointed out that when the precursor powders had been calcined at temperatures between 200 and 400 °C for 2 h, the calcined powders only contained single phase of anatase TiO2. After being calcined at 500 °C for 2 h, the XRD pattern shows that anatase TiO2 and rutile TiO2 coexist. Yeh et al. [35] also synthesized mesoporous TiO2 nanocrystalline powder by a simple coprecipitation process, without adding surfactant, and demonstrated that the XRD pattern shows that anatase TiO2 and type II NH4Cl coexist when the TiO2 precursor powders had been calcined at 300 °C for 2 h. In addition, the XRD pattern shows that the calcined powders only contained a single phase of anatase TiO2 when the precursor powders have been calcined at 400 °C for 2 h. The results of Yeh et al. [34, 35] are only in partial agreement with the present study. The difference between the present study and previous works was due to the different initial materials and synthesis process.

The anatase TiO2 is thermodynamically less stable than the rutile TiO2. Therefore, the phase transformation from the anatase to rutile TiO2 occurs at an elevated temperature. Liao et al. [36] pointed out that the Gibb’s free energy (∆G) transformation from anatase (A) to rutile (R) TiO2 is expressed as:

$$\begin{aligned}\Delta G_{A \to R} & = 2374.3 + 0.8569\,T - 1.524\,T \ln T - 1.89 \times 10^{ - 3} T \\ & \quad + 1.43 \times 10^{ - 7} T^{ - 1} + \,1.1 \times 10^{ - 7} T^{3} + 305.525\sqrt T \\ \end{aligned}$$
(1)

At 500 °C, the value of ∆G A→R is 3.9 × 103 kJ/mol [23]. Because the value of ∆G A→R at 500 °C is positive, the phase from anatase to rutile TiO2 can be considered as a movement from a stable state to a metastable state. This result means that the rutile TiO2 was not visible in the present study. Moreover, the energy of 3.9 × 103 kJ/mol can be considered as the driving force for the rutile nucleation that occurred [37].

The XRD patterns of TiO2 thin films prepared by spin-coating after being calcined in air at various temperatures for 1 h are shown in Fig. 4. Figure 4a–c shows the XRD patterns of the thin films after being calcined at temperatures between 300 and 400 °C, which reveals that the thin films are in the amorphous state. This result is almost fully in agreement with the results of Fig. 3a–c. Figure 4d–f shows the XRD patterns of thin films after being calcined at a temperature between 450 and 550 °C and reveals that only (101) the peak of the anatase TiO2 appeared. However, the intensity of the (101) peak increased slightly with an increase in the calcined temperature, but was still weak and broad due to poor crystallinity and small crystals in the submicron-meter -to-nanometer scale [30, 31].

Fig. 4
figure 4

XRD patterns of porous films after being calcined at different temperatures for 1 h: a 300 °C; b 350 °C; c 400 °C; d 450 °C; e 500 °C; and f 550 °C (A: anatase TiO2)

Full size image

The use of titanium isopropoxide (TTiP), absolute ethanol, and acetone as initial materials for fabricating TiO2 thin films prepared by sol–gel spin-coating has been reported [23, 38, 39]. These studies have pointed out that the TiO2 thin films only contained single phase of anatase TiO2 when the as-coated thin films had been calcined at 500 °C for 1 h. These results are in agreement with the present study. When the TiO2 thin films with a thickness of <0.5 μm had been calcined at 520 °C for 1 h, the XRD patterns show that the phase formation is either brookite TiO2 or sodium titanate (Na2O·xTiO2), due to the reaction of Na2O migration from the soda-lime glass substrate to TiO2 thin films [8, 40]. However, the XRD patterns show the obvious diffraction peaks of anatase TiO2 with some preferred orientation of anatase (101) when the thickness of TiO2 thin films is >0.5 μm, after being calcined at 520 °C for 1 h [8]. This result was partially in agreement with the present study.

3.3 Microstructure and morphology of the TiO2 thin films before and after heat treatment

The cross-sectional FESEM microstructures of TiO2 thin films before and after heat treatment at various temperatures for 1 h are shown in Fig. 5. Figure 5a shows the cross-sectional microstructure of TiO2 thin films before heat treatment, which reveals that the thickness of thin films is about 381.8 ± 49.6 nm, but the thickness distribution was not uniform. Moreover, a pore exists in the interior of thin films, which was speculated that the air dissolved in film while spin-coating. Figure 5b–c shows the TiO2 thin films after heat treatment at 300–450 °C for 1 h. Figure 5a shows that the thickness of thin films decreases from 381.8 ± 49.6 to 204.0 ± 17.5 nm from before to after heat treatment at 450 °C. This phenomenon is attributed to the volatility amount of organic solvent and H2O increases with increasing heat treatment temperature, which leads to enhance the films densification and decrease the thickness of thin films.

Fig. 5
figure 5

Cross-sectional FESEM microstructures of TiO2 thin films before and heat treatment at various temperatures for 1 h: a before; b 300 °C; c 400 °C; and d 450 °C

Full size image

The FESEM microstructures of the TiO2 thin films before and after heat treatment in air at various temperatures for 1 h are shown in Fig. 6. Figure 6a shows the obvious macropores thin films before heat treatment with an average size around 3.32 μm. This result is due to the phase separation that is occurred in the course of condensation. Figure 6b shows the microstructure of TiO2 pore thin films after heat treatment at 300 °C for 1 h, which reveals the thin films with an average pore size of about 3.66 μm. The pore size of TiO2 thin films before heat treatment was smaller than after heat treatment, and this is due to the removal of solvents leading to pore size increase. This result also shows that the pore size of TiO2 thin films is affected by the heat treatment temperature. Moreover, Fig. 6c, d shows the TiO2 pore thin films with an average size approximately 3.67 and 3.64 μm after heat treatment at 400 and 450 °C for 1 h, respectively. The results are similar to Fig. 6b due to solvent of ethanol evaporated completely.

Fig. 6
figure 6

FESEM microstructures of TiO2 thin films before and heat treatment at different temperatures for 1 h: a before; b 300 °C; c 400 °C; and d 450 °C

Full size image

Figure 7 shows the relationship between relative amount of pores and pore size for TiO2 thin films before and after heat treatment at various temperatures for 1 h. It is found that the pore size of the thin films before heat treatment decreased with increasing heat treatment temperature. This result is due to the Ti4+ oligomers that are rapidly formed at a certain pH condition increasing the comparability to low degree and phase separation [25, 41]. In the polycondensation process, phase separation occurred by the reduction in miscibility between the polar solvent and polycondensed inorganic species [25], and As the polarity of solvents decreased, the polarity gap between solvent and gel phase reduced, leading to the reduction in the rate of phase separation and the pore size becoming smaller with increase in the heat treatment temperature.

Fig. 7
figure 7

Effect of heat treatment temperature on the relative pore size distribution of TiO2 pore thin films: a before heat treatment; b 300 °C; c 400 °C; and d 450 °C

Full size image

In the present study, AcAc and DEA were used as complexing agents simultaneously. Huang et al. [25] have pointed out that the isolated or interconnected pores were formed because phase separation occurred in the sol–gel process in the presence of AcAc and DEA. Figure 6 shows the isolated or interconnected macropores with an average size greater than 3.0 μm in TiO2 surface because of the presence of AcAc and DEA agents [25]. These two complexing agents have been used as an effective addition for controlling the rate of hydrolysis and precipitation in the sol–gel process for metal alkoxides through complexing reaction [25]. The AcAc displays a stronger chelate-forming ability with the Ti(OBu)4 precursor, which leads to decreases in the rate of hydrolysis and precipitation of the Ti(OBu)4 precursor [42]. DEA possessing the hydrogen bonding is used as a highly polar solvent, and it was hydrolyzed to produce ammonia in the presence of HNO3 [43]. While mixing DEA with strong acid, the pH changes, accelerating the polycondensation stage of TiO2 oligomers. The polarity of the gel phase slightly decreases when the dehydration and debutanation polycondensation occurred, leading to the consumption of hydroxyl groups, while the polarity of the solvent is still high [43]. The pore size becomes smaller with the same amount of acid and base reported by Huang et al. [25]. In the present study, the gels were completely miscible before adding HNO3 to organic compounds. Adding HNO3 to the weak alkaline DEA accelerates the polycondensation reaction of TiO2 oligomers and eventually induces the sol–gel transition. The schematic diagram of the phase separation process of adding HNO3 is shown in Fig. 8. In the polycondensation process, the reduction in miscibility between the polycondensed inorganic species and polar solvent decreases leads to a phase separation [41, 44]. Malfatti et al. [45] suggested that directly using propylene glycol (PPG) triggers demixing. The difference between the mechanisms of phase separation was due to the different initial materials.

Fig. 8
figure 8

Schematic diagram of the phase separation process after adding HNO3

Full size image

The various sol–gel systems on the basis of polymerization-induced phase separation have been successfully used for designing and fabricating the well-defined macroporous monoliths [41, 46]. Chen et al. [47] used the sol–gel process for preparing the ZnO/TiO2 thin films and pointed out that the polymerization-induced phase separation leads to the formation of porous structure.

On the other hand, the time of the onset of phase separation relative to the sol–gel transition was adjusted, and then the size of the macropores can be controlled due to the domain formation when the phase separation involves a coarsening process. The pore size control was readily accomplished through only a change in the starting composition [41]. The sol–gel method that accompanied phase separation is one of the promising techniques for fabricating monolithic materials with a bimodal pore structure [41, 48].

In order to control the bicontinuous morphology of macroporous in the present study, the following three key points must be attained. The first key point is the use of Ti(OBu)4 as TiO2 precursor leading to the reactivity of decreasing polycondensation [41]. Moreover, the complexing agent AcAc is used to reduce the reactivity of Ti(OBu)4 polycondensation for controlling the rate of hydrolysis and precipitation of metal alkoxides [25, 42, 49]. The second key point is the control of the pH of synthetic system [25]. HNO3 is used as the strong acid catalyst, which also reduces the reactivity of Ti(OBu)4 to H2O [41]. The third key point was the use of DEA in the present study which can promote the control over the rate of condensation and effectively cause the phase separation [41]. Moreover, in order to understand the mechanism of pore formation, Huang et al. [25] also pointed out that due to the composition difference between the phases and domains continues to enhance. Therefore, the phase size also started to increase that was strongly governed by the surface energy [48].

On the other hand, since the thin films of present study were obtained by a sol–gel spin-coating process, an evaporation-induced gelation causes rapid solidification of the developing domains in the coated layer. This rapid freezing transient structure created the two-dimensionally phase-separated structure on a substrate [43]. When the gel was spin-coated on the substrate, the micropores formed after evaporation drying [48]. In addition, when the domains of solvent phase were dispelled from the substrate, the unoccupied regions formed, whereas the porous skelecton formed due to the domains of gel phase spread on the substrate surface. This result leads to the pore segregation in the surface of thin films [43, 50].

The macropores were distributed sharply, and the pore size decreased with increasing molar ratio of H2O to Ti(OBu)4 in the starting composition [41]. In the system with a larger amount of H2O, the hydrolysis reaction of titanium alkoxide was more strongly promoted to enhance the number of hydroxyl groups on the surface of the TiO2 oligomers. Therefore, the compatibility of the two-size distribution of pores increased in the thin films. The increase in amount of H2O makes the phase separation tendency become weaker, and a finer bicontinuous structure is obtained as a result of the later onset of phase separation, the small pores being left behind after evaporation drying, as reported by Konishi et al. [41].

However, Bu et al. [42] noted a pore size in the range of 200–500 nm for the sol at a molar ratio of H2O/Ti(OBu)4 = 1 with triethanolamine (TEA). This result was not in agreement with the present study, owing to the molar ratio of H2O/Ti(OBu)4 in the present study being 19.2, which was greater than that reported by Bu et al. [42]. Moreover, the starting composition of the present study used the DEA and AcAc as the complexing agents. On the other hand, the phase separation occurred much earlier than the gelation, leading to bicontinuous structure fragments changing into spherical particles to reduce the interfacial energy when the molar ratio of H2O/Ti(OBu)4 was smaller than 20.75, as reported by Konishi et al. [41]. This result was not in agreement with the present study because of the different initial materials and the composition of the sol.

3.4 Hydrophilic properties of porous TiO2 thin films

The surface wettability of the porous TiO2 thin films after heat treatment at different temperatures for 1 h, and heat treatment at the same temperature for various holding times, was investigated by measuring the water contact angles (WCAs) on the surface of the thin films. The optical images of water spreading on the surface of porous TiO2 thin films after heat treatment at different temperatures for 1 h are shown in Fig. 9. After heat treatment at 300 °C, the contact angle (CA) of water spreading on the surface of porous TiO2 thin films was 97.4°, as shown in Fig. 9a. The figure reveals that the surface of porous TiO2 thin films has a hydrophobic property. The optical image of water spreading on the surface of porous TiO2 thin films after heat treatment at 350 °C for 1 h is shown in Fig. 9b, which reveals that the CA was 64.7°; thus, the films are both hydrophobic and hydrophilic. After heat treatment at 400 °C for 1 h, the optical image of water spreading on the porous TiO2 thin films shows that the CA was 9.2°, as shown in Fig. 9c, which reveals the hydrophilic property. Figure 9d shows the image of water spreading on the surface of porous TiO2 thin films after heat treatment at 450 °C for 1 h and demonstrates that the CA was 2.0°. Therefore, the surface of porous TiO2 thin films has a superhydrophilic property. The results of Fig. 9 also show that the wetting ability of porous TiO2 thin films gradually transits from hydrophobic to superhydrophilic with a rise in the calcined temperature, from 300 to 450 °C. In the report of Wenzel [51], it is well established that for a given chemical composition to increase the roughness of a surface can render it more hydrophobic or more hydrophilic. The introduction of porosity to the surface makes hydrophobic amorphous TiO2 films, and hydrophilic crystalline TiO2 films exhibit better hydrophobicity and hydrophilicity, respectively.

Fig. 9
figure 9

Optical images of water spreading on TiO2 porous films after heat treatment at different temperatures for 1 h: a 300 °C; b 350 °C; c 400 °C; and d 450 °C

Full size image

The water contact angles of the porous TiO2 thin films, before and after heat treatment, at temperatures between 200 and 450 °C for 1 h, are listed in Table 1. It can be seen that the contact angle is more than 90° or less than 5° when the calcined temperatures were lower, 300 °C, or higher, 450 °C, respectively. As shown in Fig. 6, the microstructure and morphology of TiO2 pore thin films were different before and after heat treatment in air at 300 °C for 1 h. However, the water contact angles of the porous TiO2 thin films maintained substantially, and no other changes can be detected. In addition, the microstructure and morphology of films have influenced by heat treatment process while annealing over 300 °C for 1 h; however, the water contact angles of the porous TiO2 thin films decreased, indicating that the natural hydrophilic property of TiO2 porous thin films depended on the microstructure and morphology, but the surface of TiO2 thin films with amorphous and crystalline anatase TiO2 led to the above-mentioned results. Wenzel [44] also pointed out that the wettability of the rough surface is greater than that of the smooth surface. However, the results of Figs. 6 and 7 show that after heat treatment, the porous TiO2 thin films have different microstructures and pore sizes. Therefore, the difference of water contact angles of the porous TiO2 thin films after heat treatment at temperature lower than 300 °C and higher than 450 °C can be attributed to the surface with an amorphous and crystalline state. Moreover, Wang et al. [52] noted that oxygen adsorption of the TiO2 thin film surface becomes greater, leading to the surface wettability quickly reconverting from hydrophilicity to hydrophobicity.

Table 1 WCAs of the TiO2 films on glass substrates before annealing at various temperatures for 1 h
Full size table

Figure 10 shows the dependence of the contact angle on the heat treatment temperature and wetting times for a water droplet added to the surface. It was seen that the porous TiO2 thin films show the hydrophobic property when the thin films have been heat-treated at 300 °C for 1 h, due to the surface with the amorphous state. In addition, Fig. 10 also shows that the water contact angle decreased rapidly with the wetting time for a water droplet added to the surface with a rise in the heat treatment temperature. However, the water contact angle decreased and was saturated at 9.2° and 2.0°, when the porous TiO2 thin films had been calcined at 400 and 450 °C for 1 h, respectively. This result occurred because the various calcined temperatures created different surfaces of porous TiO2 thin films.

Fig. 10
figure 10

Dependence of contact angle on heat treatment temperature and wetting times for a water droplet added to the surface: a 300 °C; b 400 °C; and c 450 °C

Full size image

Wang et al. [52] pointed out that the water contact angle rapidly decreased with the illumination time of ultraviolet irradiation because the (110) and (100) reflections indicate that defects can be relatively easily produced on the (110) and (100) reflections, and the surface wettability conversion of TiO2 greatly depends on the amount of light-induced defect sites that are relevant to the surface structure of TiO2 single crystal. However, the porous TiO2 thin films of the present study were observed without ultraviolet light irradiation. The behavior with decreased water contact was similar to the reported results of Wang et al. [52]. Therefore, it can be speculated that the effect of heat treatment temperatures on the surface structure of porous TiO2 thin films, calcined at higher temperature, is similar to the larger amount of ultraviolet light-induced defect sites.

Figure 11 shows the relationship between the contact angle, calcined time, and wetting time for a water droplet added to the surface, for porous TiO2 thin films have been heat-treated at 450 °C for various times. It can be seen that the water contact angle decreased with the wetting time for a water droplet added to the surface time for the same heat treatment time of porous TiO2 thin films. On the other hand, the water contact angle decreased rapidly and was saturated at 34.4°, 5.0°, and 2.0°, when heat-treated at 450 °C for 10, 30, and 60 min, respectively. This result, ascribed to the heat treatment time increasing from 10 to 60 min, enhanced the crystallinity of anatase TiO2, which created the defect sides of (100) and (110) reflections on the surface of porous TiO2 thin films. This result increases the amount of the ultraviolet light irradiated. The surface wettability conversion greatly depends on the amount of light-induced defect sites that are relevant to the surface structure of the TiO2 crystal [52].

Fig. 11
figure 11

Relationship between contact angle, calcined time, and wetting time added to surface of porous TiO2 thin films after heat treatment at 450 °C for various times: a 10 min; b 30 min; and c 60 min

Full size image

As mentioned above, the superhydrophilic properties of TiO2 porous thin films without UV light radiation can be obtained for thin films that are first fabricated by a sol–gel spin-coating process and, subsequently, for TiO2 thin films after suitable heat treatment.

4 Conclusion

TiO2 porous thin films were fabricated successfully using Ti(OBu)4 and absolute ethanol as the initial materials, with AcAc and DEA as the complexing agents, prepared by a sol–gel spin-coating process. The effect of heat treatment on the crystalline structure and hydrophilic properties of TiO2 porous thin films was investigated by XRD, FTIR, FESEM, and WCA measurements. The conclusions of the present work are summarized as follows:

  1. 1.

    XRD results show that the calcined powders still maintained the amorphous state when the precursor powders had been calcined at 300 and 350 °C for 1 h. In addition, the calcined powders only contained a single phase of anatase TiO2 when the precursor powders had been calcined at 400–550 °C for 1 h.

  2. 2.

    The TiO2 thin films revealed the amorphous state after being heat-treated at temperatures between 300 and 400 °C, whereas the thin films contained a single phase of anatase TiO2 after heat treatment at temperatures between 450 and 550 °C.

  3. 3.

    The size of TiO2 pore thin films, before and after heat treatment, can be divided into two: 7.28 and 7.96 μm; 4.16 and 4.64 μm, respectively. This result shows that the pore size of TiO2 thin films was affected by the heat treatment.

  4. 4.

    The TiO2 thin films, before and after heat treatment, at temperatures lower than 200 °C for 1 h, where the WCA was >96.8°, revealed the hydrophobic property. On the other hand, with heat treatment at temperatures greater than 400 °C for 1 h, the TiO2 thin films exhibited the hydrophilic property with a WCA smaller than 9.2°.

  5. 5.

    When the TiO2 porous thin films had been heat-treated at 450 °C for 30 and 60 min, the thin films possessed WCAs smaller than 5° and showed the superhydrophilic property.