1 Introduction

As one of the most promising semiconductors, titanium dioxide (TiO2) has been extensively utilized in organic pollutant degradation [1, 2], solar energy conversion [3,4,5,6,7], and photoelectrochemical water splitting [8,9,10,11]. Although TiO2 shows good stability, non-toxicity, low cost, and high photocatalytic activity, its wide bandgap (3.2 eV for anatase) limits its applications in visible light [12, 13]. Owing to their recycling ability, TiO2 thin films deposited on various substrates have gained immense attention over the past 2 decades [14, 15].

Liquid phase deposition (LPD) is a novel soft wet chemical technique for preparing various metal oxide films. Unlike other film-preparation techniques, LPD does not require vacuum, high temperature, expensive equipment, and special substrates [16, 17]. However, TiO2 thin films prepared by the LPD method are highly susceptible to cracking [18]. TiO2 thin films undergo cracking during drying because of the presence of capillary stress.

Gong et al. [19] reported that the cracking of TiO2 films can be mitigated by doping them with tungsten. At the tungsten content of 5% (at. %) the cracking of the film could be eliminated completely. Zhang et al. [20] reported that the deposition of WO3 on TiO2 films result in the formation of flower-like WO3 crystals, and the cracks in LPD TiO2 films can be completely eliminated by coating the surface of TiO2 with WO3. The morphology of TiO2 films can be tailored by adjusting the experimental conditions. Cheng et al. [21] reported that the substrate type affects the structural properties of TiO2 thin films. The surfaces of TiO2 films deposited on Si substrates are rougher than those of the films deposited on glass. Lei et al. [22] reported that TiO2 monolayer films show an acicular rod-like surface morphology, whereas SnO2−TiO2 bilayer composite films exhibit a network-like surface morphology with aggregated tiny SnO2 nanosheets. Huang et al. [23] reported that the deposition rate and surface roughness of TiO2 films can be controlled by adjusting the H3BO3 concentration. The results showed that high boric acid concentrations neutralized HF in the reactive solution, thereby generating films with high surface roughness and large particle sizes.

In this study, we investigated the relationship between the deposition processing parameters (precursor concentration, solution pH, and precursor molar ratio) and the microstructure of TiO2 films prepared by LPD. The cracking of the films could be mitigated by controlling their morphology by varying the deposition conditions.

2 Materials and experiment

2.1 Chemicals

All the chemicals used in this study including ammonium fluorotitanate (95% purity), boric acid (99.8% purity), hydrofluoric acid, hydrochloric acid, acetone, and absolute ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd (China) with analytical grade and were used without any further purification. Deionized (DI) water was used to prepare the reaction solution.

2.2 Preparation of TiO2 thin films

The LPD method was used for preparing the thin films. Prior to the preparation of the TiO2 thin films, the glass substrates (2 cm × 2 cm) were washed sequentially with acetone, absolute ethanol, and DI water for 10 min. Dilute HF (1:4 in volume) was used to etch the glass substrates for 1–3 min. The substrates were then rinsed with DI water and dried in air. The pH of the mixture of (NH4)2TiF6 and H3BO3 was adjusted by adding diluted HCl. The pre-treated substrates were then horizontally immersed (with the surface upside down) in the prepared solution in a beaker. The beaker was then heated at 80 °C for 3 h. The substrates were then taken out, rinsed with DI water, and dried in air at 60 °C.

2.3 Characterization

The phase compositions of the products were analyzed by X-ray diffraction (XRD, Germany Bruker Science, Co. Ltd Bruker D8 ADVANCE) with monochromatized Cu–K radiation (λ = 0.1541874 nm). The morphologies of the products were examined using scanning electron microscopy [SEM, JEOL JSM-7500F (Japan)] at an acceleration voltage of 10 kV. The transmittance of the products was measured using ultraviolet–visible spectroscopy (UV–Vis, UV-8000S Yuanxi Instrument Co. Ltd, China).

3 Results and discussion

3.1 Phase structure analysis of the TiO2 films

The XRD patterns of the samples with different raw material molar ratios are shown in Fig. 1. All the samples showed three sharp diffraction peaks corresponding to the (101), (004) and (200) [24] planes of anatase TiO2 (JCPDS No. 21-1272). This indicates that the prepared samples were anatase titanium dioxide and showed good crystallinity. The samples showed no impurity peaks. No significant difference was observed in the shapes of patterns (a), (b), and (c) and their peak positions. This indicates that the (NH4)2TiF6: H3BO3 molar ratio did not affect the crystallinity of the obtained films.

Fig. 1
figure 1

XRD patterns of the thin films with different (NH4)2TiF6: H3BO3 molar ratios. a 1:1.5; b 1:2, and c 1:3

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3.2 Effect of the precursor concentration on the TiO2 films

The morphologies of the TiO2 thin films obtained using 0.2 and 0.1 M (NH4)2TiF6 at pH = 2.0 and (NH4)2TiF6:H3BO3 = 1:1.5 are shown in Fig. 2a and b, respectively. The surfaces of both the films showed a large number of cracks. These cracks resulted in the warping and peeling off of the films from the substrate (Fig. 2a2), which severely limited the practical applications of the films. Anatase TiO2 films prepared by LPD generally show cracks [25, 26]. The optical microscopy observations revealed that the films showed cracking only after complete drying and not after cooling and washing with DI water. This indicates that the films cracked because of the mismatch in their coefficients of thermal expansion with that of the substrate [18]. According to the Laplace equation [27],

$$ \Delta P \, = \, - 2\gamma \;{ \cos }\;\theta /r, $$
(1)

where γ is the surface tension and r is the pore radius. When the pore size was less than 10 nm, the capillary stress, which caused the shrinkage of the films was significant. The grain size of films prepared from liquid solutions can be controlled by varying their growth and nucleation velocities. These velocities can be controlled by adjusting the ion concentration of the solution. The ion concentration can be simply adjusted by varying the concentration of the deposited solution. Therefore, crack-free TiO2 films are obtained at low solution concentrations.

Fig. 2
figure 2

Surface morphologies of TiO2 thin films prepared using different (NH4)2TiF6 concentrations: a 0.20 M; b 0.10 M; c 0.05 M; d 0.03 M and the 100,000 × magnification micrographs of the TiO2 thin films with the (NH4)2TiF6 concentrations of e 0.20 M, f 0.10 M

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Figure 2c shows the SEM image of the film prepared using 0.05 M (NH4)2TiF6. The surface of the film was dense and uniform. The film was composed of sub-micron-sized spherical clusters along with nano-sized grains (~ 80 nm). Figure 2d shows the SEM image of the film prepared using 0.03 M (NH4)2TiF6. The film showed flower-like morphology. There were some blanks among these flowers, and the surface density was low. The grains were elongated, but the size did not increase. This phenomenon occurred as follows [28]:

$$ \left[ {{\text{TiF}}_{ 6} } \right]^{ 2- } + n{\text{H}}_{ 2} {\text{O}} \leftrightarrow \left[ {{\text{TiF}}_{ 6- n} \left( {\text{OH}} \right)_{n} } \right]^{ 2- } + n{\text{H}}^{ + } + n{\text{F}}^{ - } , $$
(2)
$$ \left[ {{\text{TiF}}_{ 6- n} \left( {\text{OH}} \right)_{n} } \right]^{ 2- } + \left( { 6- n} \right){\text{H}}_{ 2} {\text{O}} \leftrightarrow \left[ {{\text{Ti}}\left( {\text{OH}} \right)_{ 6} } \right]^{ 2- } + \left( { 6- n} \right){\text{HF,}} $$
(3)
$$ {\text{H}}_{ 3} {\text{BO}}_{ 3} + 4 {\text{HF}} \leftrightarrow {\text{BF}}_{ 4}^{ - } + {\text{H}}_{ 3} {\text{O}}^{ + } + 2 {\text{H}}_{ 2} {\text{O,}} $$
(4)
$$ \left[ {{\text{Ti}}\left( {\text{OH}} \right)_{ 6} } \right]^{ 2- } + 2 {\text{H}}^{ + } \leftrightarrow {\text{TiO}}_{ 2} + 4 {\text{H}}_{ 2} {\text{O}} . $$
(5)

These reactions proceeded smoothly towards the right when the concentration of [TiF6]2− was high (0.1 M and 0.2 M). During the nucleation process, large quantities of raw materials were supplied and the solutions reacted quickly because of their high ionic concentrations. In this case, the nucleation velocity was much higher than the growth velocity, therefore, a large amount of TiO2 grains were accumulated. However, the size of the grains did not increase, and the pore size remained small. The ΔP value increased according to Eq. (1), resulting in the generation of cracks. The thin film prepared using 0.2 M (NH4)2TiF6 showed severe cracking as compared to the one prepared using 0.1 M (NH4)2TiF6. This is because the higher (NH4)2TiF6 concentration resulted in smaller grains and pores. Fine clusters with the sizes of about 60–80 nm composed of a large number of fine grains with the sizes of less than 10 nm were observed (Fig. 2e, f). On the contrary, in the case of the low ion concentration (0.03 M), the nucleation velocity was low and the difference between the growth velocities of different crystal planes increased because of the lack of growth units. Therefore, single grains grew into short rods with the size of ~ 100 nm. These short rods shared one end and aggregated to form flower-like particles because of the lack of nucleation sites.

The thin films prepared with different ion concentrations showed significantly different optical transmittance owing to their different microstructures. The films prepared using the high concentration solutions (0.10 and 0.20 M) were transparent to visible light. However, the films prepared using the low concentration solutions (0.03 and 0.05 M) were white and opaque to visible light. Figure 3 shows the transmittance of the films prepared at different ionic concentrations. The transmittances of the films (in the visible light range of the spectrum) with the solution concentrations of 0.03 and 0.05 M were less than 30% (Fig. 3c, d), while those of the films with the solution concentrations of 0.10 and 0.20 M were higher than 70% (Fig. 3a, b). It can be observed that the cracks on the 0.10 and 0.20 M films showed no significant effect on their transmittance (> 70%). The low transmittance of the 0.03 and 0.05 M films can be attributed to light scattering. This is consistent with the microstructure observations of the films (Fig. 2). The grain size of the 0.10 and 0.20 M films was less than 10 nm, which is much smaller than the visible light wavelength. Therefore, these films were transparent to visible light. However, the grain size of the 0.03 and 0.05 M films was ~ 100 nm, which is of the same order of magnitude as the wavelength of visible light. The films acted as inhomogeneous media for visible light. When photons hit the sub-micrometer TiO2 grains, serious scattering occurred, which resulted in a low transmittance. Moreover, it can be observed that there were interference fringes on the 0.20 M film. This indicates that the thickness and optical constants of this film remained constant. When the film was irradiated with photons, light interfered on the surface and interference fringes were generated.

Fig. 3
figure 3

UV–Vis transmittance spectra of the films with different (NH4)2TiF6 concentrations

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The 0.1 and 0.2 M TiO2 thin films showed absorption edges at ~ 306 and ~ 310 nm. On the other hand, the 0.03 and 0.05 M films showed absorption edges at ~ 300 nm. The absorption edge of the films prepared at high concentrations showed a red shift. This can be explained by the surface effect of nano-materials. The high-concentration films showed crystalline grains with sizes less than 10 nm. The high surface energy induced lattice distortions in the films, which caused variations in the bond length of the films.

3.3 Effect of solution pH on the TiO2 films

The reaction ion concentration of the deposition solution was affected not only by the concentration of the raw materials, but also by the pH of the solution and the molar ratio of the raw materials. Therefore, the effect of the solution pH on the size and morphology of the TiO2 films was investigated. Figure 4 shows the SEM images of the samples prepared with 0.03 M (NH4)2TiF6 and 0.045 M H3BO3 at different pH values. A flower-like TiO2 thin film was obtained at the solution pH of 1.5 (Fig. 4a). The grain size of this film was about 200 nm. The grains accumulated at particular sites and formed flower-like clusters. Furthermore, the flower-like clusters grew sparsely on the surface of the substrate and a discontinuous film was observed. At the solution pH of 2.0, the grain size of the obtained film decreased to about 100 nm, and the micromorphology of the film transformed to sphere-like clusters composed of ~ 100-nm grains. With a further increase in the solution pH, the grain size of the films decreased further and the density of the films increased. As can be observed from Fig. 4c the film prepared with the solution pH of 2.3 was uniform and dense and the grain size was only about 80 nm. The morphology of the film with the solution pH of 2.5 is shown in Fig. 4d. No significant difference was observed in the morphologies of films shown in Fig. 4d and c. The grains agglomerated into spheres with a size of ~ 150 nm and the film was dense and compact. The grain size was less than 80 nm. The density of the TiO2 film on the surface of the substrate increased with an increase in the pH from 2 to 2.3. The morphology of the film changed gradually from sparse flower-like to dense and compact clusters with an increase in the solution pH.

Fig. 4
figure 4

Surface morphologies of the TiO2 thin films prepared at different pH values. a 1.5; b 2.0; c 2.3; d 2.5

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This phenomenon can also be explained by reactions (2), (3), (4), and (5). According to reaction (2), when pH value was low (1.5 and 2), there were less OH− ions in the solution but more H+ ions. As a result, the process of TiO2 formation was disadvantageous. When the pH value was relatively high (2.3 and 2.5), there were a large number of OH− ions in the solution and reaction (2) was more likely to proceed to the right. Therefore, the film was continuous and consisted of a large amount of TiO2. Moreover, during the growth of the thin film, the amount of OH− increased and the grains tended to nucleate. The film was very compact and continuous (Fig. 4c, d).

The UV–Vis transmittance spectra of the samples are shown in Fig. 5. The absorption edges of all the as-prepared TiO2 thin films appeared at ~ 300 nm. The effect of the solution pH on the transmittance of the films was not significant (Fig. 5). However, the transmittance of the film with the lowest pH (1.5) was the highest. This can be attributed to the very low TiO2 content and the discontinuous growth of this film. This film also showed the leakage of light during the testing process. In some areas, photons irradiated the glass substrate. At the same time, the transmittance of the glass substrate (not the TiO2 thin film) was investigated. Hence, its transmittance was relatively high.

Fig. 5
figure 5

UV–Vis transmittance spectra of the films with different pH values

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3.4 Effect of the raw material molar ratio on the TiO2 films

The grain size and morphology of TiO2 films can be tailored by controlling the raw material molar ratio. Figure 6 shows the morphologies of the films prepared with different (NH4)2TiF6:H3BO3 ratios (1:1.5, 1:2, and 1:3). The 1:1.5 film (Fig. 6a) was composed of micro clusters of ~ 100-nm gains. The substrate was not completely covered with the clusters. In other words, this film was not fully dense (Fig. 6a). The grain size of the 1:2 thin film (~ 80 nm) was smaller than that of 1:1.5 film (Fig. 6b). The fine grains grew and agglomerated into microspheres. However, the compactness of the obtained film did not improve. The 1:3 film was dense and homogeneous (Fig. 6c). The film was compact and consisted of fine grains. Therefore, the grain size of the films decreased with an increase in the amount of H3BO3. The morphology of the films changed from micro-sized flowers to spheres and then to compact nano-sized grains with an increase in the ratio of H3BO3.

Fig. 6
figure 6

Surface morphologies of the as-prepared TiO2 thin films with different (NH4)2TiF6:H3BO3 molar ratios. a 1:1.5; b 1:2; c 1:3

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According to reactions (2), (3), (4), and (5), H3BO3 acted as a F− scavenger [28]. As can be observed from Fig. 6, the films became denser and more uniform with an increase in the H3BO3 concentration. This is because at high H3BO3 concentrations, the solution consisted of a few F− ions. As a result, reaction (4) proceeded towards the right. Reactions (2), (3), and (5) proceeded towards the right when the F− content was low. A large number of crystal nuclei were formed rapidly on the substrate at high H3BO3 concentrations. The process of nucleus formation consumed most of the [Ti(OH)6]2− ions, which caused the reaction to quickly reach the equilibrium state before the growth of the nuclei. As a result, the obtained film was uniform and compact.

Figure 7 shows the transmittance spectra of the thin films. All the samples showed adsorption edges at 300 nm. No significant difference was observed between the curve shapes and transmittances of the samples. As can be observed from Fig. 6, the scattering phenomenon was caused by the submicron cluster of nanograins. The size of the clusters exactly matched with the wavelength detected. Therefore, the transmittances of thin films were relatively low. All the three curves showed slight interference fringes, indicating the uniformity of the thin films.

Fig. 7
figure 7

UV–Vis transmittance spectra of the films with different (NH4)2TiF6:H3BO3 molar ratios

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4 Conclusion

In summary, TiO2 films with controllable grain size and morphology were prepared using the LPD technique. The grain size and morphology of the TiO2 films were controlled by adjusting the deposition parameters including the raw material concentration, solution pH, and raw material molar ratio. Here, the ion concentration, which was controlled by varying the deposition parameters, affected the nucleation and growth velocities of the TiO2 films, which in turn affected the grain size of the films. The capillary stress among the grains was controlled accordingly. Therefore, crack-free LPD-derived TiO2 films were obtained by simply adjusting the deposition parameters. At very low nucleation velocities (lower concentration, 0.03 M; or lower pH, 2.0; or higher ratio, 1:1), the grains tended to grow and the morphology of the film transformed to flower-like. At very high nucleation velocities (higher concentration, 0.1 M and 0.2 M), the grain size decreased and the film became dense. However, numerous micro-scale cracks were observed on the film because of its large capillary stress.