Filename: b315777f

Back | Show only these items: | Comparison of photochemical properties of brookite and anatase TiO2 films

1
Comparison of photochemical properties of brookite and anatase TiO₂ films

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

The photocatalytic oxidation activity and photoinduced hydrophilicity of brookite-rich TiO₂ film were compared to those of anatase TiO₂ film under ultraviolet (UV) light irradiation.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

The photocatalytic oxidation activities were evaluated by the initial photodegradation rates of methylene blue and cis-9-octadecenoic acid.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met1

The rates for the methylene blue bleaching were nearly identical (−2.2 × 10⁻³ Abs min⁻¹) for the brookite-rich and anatase films, and the rate for cis-9-octadecenoic acid decomposition on the brookite-rich film was slightly less than that on the anatase one (−0.08 ± 0.01 μg cm⁻² min⁻¹ and −0.10 ± 0.01 μg cm⁻² min⁻¹, respectively).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

However, the rate constants for photoinduced hydrophilicity, which were evaluated by the changes in water contact angles (θ) under rather weak UV light irradiation (5 μW cm⁻²), were better for the brookite surface than the anatase one.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

Moreover, the brookite-rich film became more hydrophilic (θ = 10°) than the anatase one (θ = 18°) after long time exposure to weak UV light irradiation, and showed a good ability for the photoinduced hydrophilicity.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

Introduction

In recent years, numerous researchers have investigated the photocatalytic activities of TiO₂ due to the strong oxidation power of its photogenerated holes under ultraviolet (UV) light irradiation,¹ as well as its chemical inertness and nontoxicity.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

In addition to the strong oxidation power, we reported that UV light irradiation causes the TiO₂ surface to become highly hydrophilic.²

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

These useful properties have been applied to commercial products, such as anti-fogging mirrors, self-cleaning ceramic tiles, etc. At normal pressure, TiO₂ has three different crystal structures, rutile, anatase and brookite.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

As photocatalysts, both rutile and anatase phases have been widely investigated, and it is well known that the photocatalytic activities of the anatase phase are mostly superior to those of the rutile one.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

In contrast, there are a limited number of reports on brookite phase TiO₂^3–14 and most of these papers are simply on its preparation.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

There are a few reports on its photocatalytic and electrochemical properties,^10,12–14 but none on photoinduced hydrophilicity of brookite films.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot1

Here, the photocatalytic oxidation activity and the photoinduced hydrophilic property of a brookite-rich film are compared with those of an anatase film.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

Experimental

TiO₂ films with a thickness around 270–280 nm were prepared on SiO₂-coated glass plates by a spin-coating method of TiO₂ sols (1500 rpm for 10 s), followed by a calcination at 500 °C for 30 min.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

Two types of films were prepared: film A was from NTB-01 (brookite type TiO₂ sol: SHOWA DENKO K. K., Japan) and film B was from STS-01 (anatase sol: Ishihara Sangyo Kaisha, Ltd., Japan).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

For comparison, corresponding powders A and B were prepared by desiccating the NTB-01 and STS-01 sols at 500 °C for 30 min, respectively.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

The crystal phases of the films were identified by glancing incidence X-ray diffraction (RINT-2100, Rigaku Denki Ltd., Japan) at a constant incident angle of 0.8° using Cu-Kα radiation operated at 40 kV–30 mA.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

Conventional θ–2θ scanning was used in the powder XRD measurements.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

The optical absorption spectra were recorded on a UV-VIS-NIR scanning spectrophotometer (UV-3100PC, Shimadzu Co., Japan).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

The microstructures of these films were observed by atomic force microscopy (AFM, SPA300, Seiko Instruments Inc., Japan) and scanning electron microscopy (SEM, S-4200, Hitachi Co., Japan).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp7

The morphologies of the powders were observed using a transmission electron microscope (TEM, JEM-2010, JEOL, Japan).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp8

Degradation of Methylene Blue (MB) was measured to evaluate the photocatalytic oxidation activity.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met1

MB is a brightly coloured, blue cationic thiazine dye with λ_max between 550 and 600 nm, and is not photoblenched by UV light irradiation in the absence of TiO₂ under the present experimental conditions.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

MB was applied on the TiO₂ films by dipping the films in 1 mmol l⁻¹ methylene blue solution for 30 min.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp9

The change of the absorbance peak in the visible region was measured by the spectrophotometer.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met1

It is known that reduction of MB leads to colorless leucoMB, which can be reoxidized to MB by oxygen.¹⁵

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

To exclude the contribution of this bleaching reduction process, the irradiated TiO₂ samples were stored in the dark until the absorption remained constant before the measurement.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met1

The photocatalytic oxidation activity of the TiO₂ films was also measured by the weight change of cis-9-octadecenoic acid under UV light irradiation.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met1

Approximately 10 mg of cis-9-octadecenoic acid was first applied onto the 6.25 cm² samples and their weight changes were measured by an electronic balance (AG285, Mettler Toledo, Switzerland, 10 μg precision).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp10

The weight was determined averaging 5 different measurements from the same sample.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met1

Details have been given elsewhere.¹⁶

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

Because cis-9-octadecenoic acid on a glass substrate was stable when irradiating with UV light, we concluded that the weight changes were not due to the evaporation of cis-9-octadecenoic acid but from the photooxidization by the TiO₂ films.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met1

The photoinduced hydrophilic property was evaluated by measuring the water contact angle (θ) of the TiO₂ film's surfaces by the sessile drop method with a commercial water contact meter (CA-X, Kyowa Interface Science, Japan).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp11

The droplet size used for the measurements was 0.7 μL.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp12

The value of the contact angle was determined averaging at least 5 different measurements from the same sample.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met1

All the above UV light irradiation experiments were done using a cylindrical black-light lamp as a light source.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp13

The UV intensity was measured by a UV radiometer (UVR-2, TOPCON, Japan).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp14

Results and discussion

Fig. 1 shows the XRD patterns of Films A, B and Powders A, B. Although Film B and Powder B showed only anatase peaks, Film A and Powder A displayed characteristic peaks for brookite (JCPDS #29-1360) in addition to weak anatase ones.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

In order to estimate the ratio (R) of brookite for Powder A, the following equation was used.¹⁷

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

Note that the equation was obtained from the literature by excluding the contribution of the rutile peak.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

(k_anatase = 0.886, k_brookite = 2.721)

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

Here Ianatase101 and Ibrookite121 represent the integrated intensities of the anatase (101) peak and the brookite (121) peak, respectively.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

The anatase (101) peak and the brookite (120) and (111) peaks were separated using a numerical deconvolution method.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

The R value was estimated to be about 0.75 from Fig. 1 pattern (c), indicating that Powder A consists of mostly brookite phase with some anatase one.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

The estimated value agrees well with the previously reported value for the NTB-01 sol,¹³ suggesting that crystal phases did not change by the thermal treatment at 500 °C for 30 min.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

Because Film A was prepared under the same thermal conditions with Powder A, it is reasonably expected that the film also consists of mostly brookite phase with some anatase one.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp1

Hereafter, Film A, Film B, Powder A and Powder B are referred to brookite-rich film, anatase film, brookite-rich powder and anatase powder, respectively.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

The SEM and AFM measurements showed that the surface roughnesses of the brookite-rich and anatase films were nearly identical.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

The average roughnesses (Ra), which indicate surface roughness estimated from AFM measurements, were 5.2 nm and 4.3 nm for the brookite-rich and anatase films, respectively, with the same grain size around 10–20 nm.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

These films were transparent between 400–800 nm.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

The UV-VIS spectra showed that the main absorption edges for the brookite-rich and anatase films were located around 380 nm, which corresponds to the photoelectrochemically determined band gap energy values E = 3.26 eV for brookite and E = 3.20 eV for anatase (not shown here).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

Fig. 2 is the TEM image of the brookite-rich powders, showing the characteristic crystal shape, a plate-like aspect with a square plane.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

Electron diffraction patterns identified the indices of the square surface as brookite (010) and (210).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

Now, let us evaluate the photocatalytic oxidation activities of the brookite-rich and anatase films.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa2

Fig. 3 shows the change in the absorbance of the MB peak in visible region under 1 mW cm⁻² UV light irradiation.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

The continuous UV light irradiation caused the adsorbed MB absorption to decrease.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs10

Here we estimated the degradation rate (k) for MB from the initial slope to be k_brookite = k_anatase = −2.2 × 10⁻³ Abs min⁻¹ under the assumption of a pseudo-zero-order kinetics.¹⁶

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

Fig. 4 shows the weight change of the applied cis-9-octadecenoic acid adsorbed on the TiO₂ films.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs11

The weight immediately started decreasing upon UV light irradiation accompanied by CO₂ production.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs12

Continuous UV light irradiation decomposed almost all the reactants.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs13

The initial decomposition rates (k′) for the brookite-rich film and the anatase one were estimated to be k′_brookite = −0.08 ± 0.01 μg cm⁻² min⁻¹ and k′_anatase = −0.10 ± 0.01 μg cm⁻² min⁻¹, respectively.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

Irradiating with UV light generates electrons and holes in the bulk TiO₂ that diffuse to the surface and then react with organic materials adsorbed on the surface directly or via various active oxygen species, such as ˙OH and ˙HO₂.^18–21

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

These organic compounds eventually decompose into CO₂ and H₂O, due to the strong oxidation power of the generated radicals and holes.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac11

Since these two films have almost identical surface roughness, the difference in the photocatalytic decomposition activities is attributed to that in the amounts of the active species produced by UV light irradiation.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

The ratio of the total absorbed photon numbers for the present brookite-rich to anatase films was calculated to be 1.26.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs14

Therefore, the present results suggest that the photocatalytic oxidation activity of the anatase film is roughly the same or rather greater than that of the brookite-rich film.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

Ohtani et al. reported that the brookite-type TiO₂ showed almost the same activity for photocatalytic oxidation reaction (photocatalytic mineralization of acetic acid) as that of anatase-type TiO₂,¹⁴ which coincides with our present results.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

Next, let us examine the photoinduced hydrophilicity of these films.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa3

Fig. 5 shows the changes in the water contact angle for the brookite-rich and anatase films under the irradiation with sufficient UV light (1 mW cm⁻²).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs15

Before irradiation, the contact angles of these two TiO₂ films were around 30°.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs16

Although a small difference was initially observed between the two films in Fig. 5, the hydrophilicizing behaviors were very similar after 30 min and lead to a decent hydrophilic conversion when θ = approximately 5°.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

In contrast, Fig. 6 demonstrates the changes when irradiating with a rather weak UV light (5 μW cm⁻²).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs17

Under this condition, not enough photons were produced to fully create hydrophilic surfaces and both films were partially hydrophilic, reflecting the sensitivity for the hydrophilicizing reaction.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

We previously reported that plots of the reciprocal of the water contact angles versus irradiation time have a linear relationship and its slope can be regarded as the hydrophilicizing rate constant.^22,23

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

From the inset of Fig. 6, it is estimated that the rate constants of the brookite-rich and anatase films for hydrophilic conversion were 7.2 × 10⁻⁵ degree⁻¹ min⁻¹ and 2.7 × 10⁻⁵ degree⁻¹ min⁻¹, respectively.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs18

In addition, the contact angle of the brookite-rich film became to 10°, although that of the anatase one was only ∼18° after long time exposure to the weak UV light irradiation.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs19

The contact angle of the heterogeneous surface is determined by well-known Cassie equation.²⁴

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

In the case of brookite-rich film, the contact angle is given bycosθ = f_a cosθ_a + f_bcosθ_bwhere f_a, f_b are the fractions of anatase and brookite, and θ_a, θ_b, θ are the contact angles of anatase, brookite and the mixed surface, respectively.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

Since the brookite-rich surface showed lower contact angle than that of the pure anatase film under UV light irradiation, it is reasonable to consider that the brookite phase exhibits a better ability for the photoinduced hydrophilicity than the anatase one.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

It is important to note that the brookite-rich film showing the better hydrophilic conversion property is not superior in oxidation ability to the anatase one.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3

We previously reported that SrTiO₃ film having a good oxidation property did not become highly hydrophilic under UV light irradiation.²⁵

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac13

These results indicate that photoinduced hydrophilicity is not simply induced by the oxidation of adsorbed organic impurities on the surface.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con4

In fact, nearly zero degrees of water contact angle on the UV light irradiated TiO₂ surface increases to about 10 degrees when the TiO₂ is sonicated in ultra-pure water.²⁶

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac14

Such a deterioration of the wettability is also observed when the highly hydrophilic TiO₂ is stored in high vacuum chamber.²⁷

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac15

We proposed that the highly hydrophilic state is due to the formation of unstable OH group on TiO₂ surface²³ and the surface with bridging site oxygen may give a better change for the formation of unstable OH species.²⁸

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con5

The (010) face of brookite, which is considered to be one of the main faces of the present brookite particles, has such a two-fold oxygen atoms with rather high surface concentration (7.1 × 10¹⁴ site cm⁻²) as shown in Fig. 7.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs20

This might be one of the reasons for a good hydrophilic property of the present brookite-rich film.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

The photoinduced hydrophilicity of TiO₂ (mostly anatase) has been already used commercially in various materials.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac16

However, highly hydrophilicity can be obtained only under rather strong UV light irradiation, above 100 μW cm⁻², and thus its application field is almost limited in outdoor use.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac17

The present results show that the brookite-rich TiO₂ film is more sensitive in hydrophilicity than anatase one especially under weak UV light irradiation, indicating that the application field potentially expanded by using the brookite film.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6

Conclusion

This study compared the photocatalytic oxidation and photoinduced hydrophilicity of brookite-rich TiO₂ to the anatase one.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

It is noteworthy that the activity for the photoinduced hydrophilic reaction of the brookite was more sensitive than that of anatase, while the photocatalytic oxidation activities of these two phases were nearly identical or rather lower.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

These results showed that the photoinduced hydrophilic conversion was not simply explained by the degradation of organic impurities, and supported the existence of alternative mechanism for it.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con4

Moreover, the current work reveals a high potential for brookite type TiO₂ as a coating material.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

It suggests that the hydrophilicity may be improved by using the brookite type TiO₂.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6