Fabrication and characteristics of visible light active TiO2 films by ...

Fabrication and characteristics of visible light active TiO2 films by reduction treatment in carbon powder Y. Lu*1, L. Hao2, K. Matsuzaka2, F. Pan3 and H. Yoshida4 TiO2 film balls were fabricated by mechanical coating technique and the following high temperature oxidation, reduction reaction by carbon powder. The films were characterised by Xray diffraction, SEM and electron probe microanalyser. X-ray diffraction revealed that rutile TiO2 films were formed during the oxidation and reserved in the following reduction reaction. Scanning electron microscopy results showed that the films had a composite microstructure of TiO2 and Ti films. After the reduction reaction, the films showed enhanced photocatalytic activity under ultraviolet (UV) or visible light irradiation. Absorption spectra analysis proved that the films’ absorption spectra extended towards visible light. The balance of crystallinity and specific surface area is considered to be the main influencing factor of photocatalytic activity under UV light irradiation. The red shift of TiO2 films’ absorption spectra due to the introduction of oxygen vacancy in the reduction reaction is believed to be the crucial reason of improving the visible light photocatalytic activity significantly. Keywords: Titanium dioxide, Composite films, Mechanical coating technique, Reduction reaction, Photocatalytic activity

Introduction TiO2 as a potential photocatalyst has received increasing attention for its applications in degradation of pollutants, environmental cleaning and hydrogen generation.1,2 To lower its recycling cost and increase the degradation efficiency of pollutants, recent investigations on TiO2 photocatalyst are oriented towards photocatalyst immobilisation in the form of thin films.3,4 Until now, a variety of techniques including plasma electrolytic oxidation,5 electron beam irradiation,6 sol–gel method,7 chemical vapour deposition8 and sputtering method9 have been used to prepare TiO2 films. TiO2 can degrade organic pollutants or split water into hydrogen and oxygen only when it is activated by UV light due to its large band gap energies (3?2 eV for anatase or 3?0 eV for rutile).10 However, UV light and visible light occupy ,5 and 50% of the solar radiation energy respectively. It means only a small part of solar radiation can be utilised. It limits the practical applications of TiO2 photocatalyst. To increase utilisation efficiency of visible light, it is necessary to extend the light adsorption spectrum

1

Graduate School and Faculty of Engineering, Chiba University 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan Graduate School, Chiba University 1-33, Yayoi-cho, Inage-ku, Chiba 2638522, Japan 3 College of Materials Science and Engineering, Chongqing University, No. 174 Shazhengjie, Shapingba, Chongqing 400044, China 4 Industrial Technology Research Institute of Chiba, 889 Kasori-cho, Wakaba-ku, Chiba 264-0017, Japan 2

*Corresponding author, email [email protected]

ß 2013 W. S. Maney & Son Ltd. Received 13 September 2012; accepted 7 October 2012 DOI 10.1179/106678512X13518574234731

of TiO2 towards the range of visible light. In other words, we have to prepare visible light response TiO2 photocatalyst. So far, many efforts have been made including coupling with semiconductor particles,10–12 doping with metal or non-metal ions,13–15 dye photosensitisation16,17 and introduction of oxygen vacancy into TiO2 by reduction reaction.18,19 However, the investigation to improve photocatalytic activity of TiO2 films under visible light irradiation by introduction of oxygen vacancy is scarce. In our early works,20,21 we have proposed and carried out a novel fabrication technique of TiO2 photocatalyst films called mechanical coating technique (MCT). The procedure of MCT is as follows. First, we prepared Ti or other metal films on Al2O3 balls with the diameter of 1 mm by ball milling. The evolution of metal films during the process also has been studied.22,23 Second, the Al2O3 balls coated with Ti films were oxidised at elevated temperature and TiO2 films could be formed. The other option is to form TiO2 films through coating Ti or other metal films that had been formed on Al2O3 balls with TiO2 powder by ball milling. By the above processes, single TiO2 films or TiO2/metal composite films can be fabricated. The investigations indicated that these films fabricated by MCT show relatively high photocatalytic activity under the UV irradiation. Compared with other preparation technology for films, MCT is a more simple and economic technology to prepare photocatalyst films. However, their photocatalytic activity under visible light irradiation has not been investigated so far. In this work, we fabricated Ti films by MCT and TiO2 films by subsequent high temperature oxidation.

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Fabrication and characteristics of visible light active TiO 2 films by reduction treatment in carbon powder

Afterwards, the TiO2 films were reduced in carbon powder at elevated temperatures to introduce oxygen vacancy into TiO2 structure. These films were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalyser (EPMA). Their photocatalytic activities under ultraviolet–visible (UV-vis) light irradiation were evaluated. The possible influencing mechanisms of photocatalytic activity were discussed.

Experimental Fabrication of TiO2 films Ti powder and Al2O3 balls were used as the coating material and the substrates respectively with the relevant parameters listed in Table 1. The source materials were charged into a pot made of Al2O3 with a volume of 250 mL. After that, ball milling was carried out by a planetary ball mill (Pulverisette 6, Fritsch). Its rotation speed was fixed at 480 rev min21, and the milling time was 10 h. The samples were labelled with M10–Ti. During the fabrication, the milling operation was carried out 10 min followed by 2 min intermission to avoid excess heating of the pot. After the ball milling, Ti films were formed on the surface of Al2O3 balls. Subsequently, the Ti film coated Al2O3 balls were oxidised in air at elevated temperatures from 973 to 1273 K holding for 10 h and then cooled in the furnace to room temperature. The samples were named M10–tK–10h, where t refers to oxidation temperature.

Reduction reaction of TiO2 films in carbon powder M10–1073K–10h samples (3 g) were charged into a crucible made of alumina, while abundant carbon powder was put into the crucible as shown in Fig. 1. The crucible was sealed at one end and open at the other end. After the loading, the M10–1073K–10h samples were totally surrounded by carbon powder. Then, the crucible was put into another cylindrical Al2O3 crucible with a larger diameter. It should be pointed out that the open end of the former contacted with the sealed end of the latter. Subsequently, the space between the two crucibles was charged into carbon powder. Finally, the open end of the larger crucible was enclosed. This can avoid the penetration of oxygen and obtain a reduction atmosphere. The reduction temperatures were set at 873, 973 and 1073 K. The holding times were 0?5, 1, 3, 5 and 10 h. The reduction treatments were performed by a furnace in air. The treated samples were tagged with C– xK–yh, where x and y refer to the reduction temperature and holding time respectively.

Characterisation The chemical contents and crystal forms of the samples were examined by XRD (JDX-3530, JEOL). Cu Ka irradiation in the condition of 30 kV and 20 mA was adopted. Diffraction data were recorded in the 2h

1 Schematic diagram of reduction reaction of TiO2 films by carbon powder

angular range of 23–50u with a step width of 0?02u s21. The morphology and microstructure of the samples were observed by SEM (JSM-6510, JEOL). Element distribution was analysed by EPMA (JXA-8900, JEOL). Ultraviolet–visible absorption spectra were measured by a scan UV-vis spectrophotometer (MSV-370; JASCO).

Evaluation of photocatalytic activity Before evaluation of photocatalytic activity, all the samples were cleaned by ultrasonic (frequency, 28 kHz) to remove adsorbent matters on the samples’ surface. The evaluation procedure of photocatalytic activity under UV light irradiation can be found in our early work.20 In the evaluation under visible light irradiation, all the conditions are the same as that under UV light irradiation except the irradiation light source. A 20 W fluorescent lamp (FL20SS?ECW/18, Panasonic) with an illuminance of 5000 lx was used as the light source. The wavelength of the light emitted by the fluorescent lamp was confirmed to be from 400 to 700 nm (within the wavelength range of visible light). Therefore, the filter for UV light was not used. A colorimeter (miniphoto 10, Sanshin) with UV radiation wavelength of 660 nm, which is near the peak of absorption spectrum of methylene blue (MB) solution (664 nm), was used to measure the absorbance of MB solution. The measurement was carried out from 1 to 24 h. The gradient k (nmol L21 h21) of the MB solution concentration–irradiation time curve was calculated by the least squares method with the data from 1 to 12 h. They were used as the degradation rate constants.

Results and discussion Crystal form analysis of films Figure 2 shows the XRD patterns of the samples. From Fig. 2a, the peaks of Ti can be seen, while those of Al2O3 cannot be found, which means Ti films on the surfaces of Al2O3 balls were formed after the ball milling for 10 h. After the rutile TiO2 films were heated at 1073 K for 10 h, they were formed by oxidising Ti films. After the reduction treatment in carbon atmosphere at elevated

Table 1 Source materials for one coating procedure

206

Source materials

Weight/g

Purity/%

Average diameter/mm

Manufacturer

Al2O3 balls Ti powder

60 40

98.5 99.1

1 0.03

Nikkato Osaka Titanium Technologies


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2 X-ray diffraction patterns of samples fabricated in different stages of whole process

temperatures from 873 to 1073 K for 0?5 h, rutile TiO2 was reserved and no other matter was formed. From Fig. 2b, the peaks of rutile TiO2 hardly changed with the increase in holding time to 10 h at 1073 K in the reduction reaction and no other matter was formed. It indicates that no phase transformation happened in the reduction reaction. However, in our early work,24 part oxygen in TiO2 was reduced by the same reduction reaction and non-stoichiometric TiO22x was formed. In this work, mass evolution of the sample could not be

detected by thermogravimetric/differential thermal analysis, which might be due to the relatively low reduction temperature. Although the Ti and TiO2 films were formed on Al2O3 balls, the diffraction peaks of Al2O3 were not seen. That is because the penetration of X-ray was less than the thickness of Ti films and/or TiO2 films.

Morphology and microstructure of films Scanning electron micrographs of the samples are shown in Fig. 3. It can be clearly seen that Ti films were formed

a backscattered electron image (BEI) of surface of M10–Ti; b SEI image of surface of M10–Ti; c BEI image of cross-section of M10–Ti; d BEI image of cross-section of M10–1073K–10h 3 Images (SEM) of samples


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a M10–1073K–10h; b C–873K–0?5h; c C–973K–0?5h; d C–1073K–0?5h; e C–1073K–5h; f C–1073K–10h 4 Images (SEM) of TiO2 films

after the milling operation for 10 h, and the average thickness was ,50 mm (Fig. 3a and c). From Fig. 3b, Ti flakes were connected with each other and formed Ti films. After the surface layer of Ti films was heated at 1073 K for 10 h, it was oxidised to rutile TiO2 with a thickness between 10 and 20 mm (Fig. 3d). These results are in good agreement with the XRD pattern analysis (Fig. 2). Morphologies of the TiO2 films are also given in Fig. 4. It can be seen that TiO2 column crystals were formed after being oxidised at 1073 K for 10 h (Fig. 4a). The crystals grew up along the direction of column with the increase in reduction temperature and holding time. When above 973 K, these crystals merged with each other.

Element distribution analysis of films Element distribution on the cross-sections of the samples after the ball milling and the oxidation are given by EPMA analysis as shown in Fig. 5. It can be clearly seen that the oxygen content in Ti films was rather low (Fig. 5a–c). After the oxygen content in the surface layer was heated at 1073 K for 10 h, it was higher than the

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inner layer, which indicated that the surface layer was oxidised and rutile TiO2 films were formed as described as Fig. 2. In the inner layer of Ti films, the oxygen content was also high in some areas. It means that part inner layer was also oxidised. From the above results, it can be known that a composite microstructure of TiO2 and Ti was formed after the high temperature oxidation. Electron probe microanalyser analysis was also performed for the samples after the following reduction reaction in carbon powder as shown in Fig. 6. As seen in the figures, the thickness of TiO2 films increased as the reduction temperature and/or the holding time increased. It means more oxygen penetrated into the inner layer of the films. However, the carbon content in the films was rather tiny and did not change observably, which indicates that carbon did not penetrate into the TiO2 films.

Ultraviolet–visible absorption spectra and band gap energy of films Figure 7 shows the absorption spectra of all the samples. From Fig. 7a, the absorbance intensity of the TiO2 films in the range of visible light increased after the reduction

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a–c M10–Ti; d–f M10–1073K–10h 5 Electron probe microanalyser analysis of samples

reaction compared with M10–1073K–10h. In addition, the absorbance intensity also increased with the increase in the reduction temperature except 973 K (Fig. 7a). When the reduction temperature was 1073 K, the intensity increased with the increase in holding time except for 5 h (Fig. 7b). Band gap energies were determined from a plot (ahn)1/2 versus (hn) by the following relation, which shows indirect relationship of the absorption coefficient a and band gap Eg15 ðahnÞ1=2 ?hn{Eg

(1)

where h is the Plank constant, and n is the frequency. Then, the calculated band gap energies and wavelength as well as those of some commercial TiO2 with different crystal types are shown in Table 2. Compared with the commercial TiO2 photocatalysts with the same crystal type, the band gap energies of the samples after the reduction reaction decreased in varying degrees from 0?19 to 0?35 eV. Meanwhile, the wavelength shifted 29–56 nm in the direction of visible light. Besides, compared with that before the reduction reaction, the absorption spectra of the films after reduction reaction also showed red shift from 5 to 27 nm. It means reduction reaction by carbon powder made the absorption spectra of the TiO2 films shifted in the direction of visible light wavelength.

Photocatalytic activity The concentration evolution of MB solution as function of irradiation time is shown in Fig. 8. Even if irradiated

under UV or visible light, MB degradation fit zero order kinetics, expressed by the following equation: ½C0 {½C~kt

(2)

where C0 is the initial concentration of the MB solution, C is the concentration of the MB solution at irradiation time t and k is the degradation rate constant related to the reaction. The result is in good agreement with the work of Rana et al.25 Under the visible light irradiation, the MB solution without TiO2 films degraded spontaneously. However, the concentration of MB solution with the sample C–973K–0?5h was decreased at the highest rate as shown in Fig. 8a and c. In the case of UV light irradiation, MB solution with the C–973K–0?5h sample also showed the greatest concentration decrease rate (Fig. 8b and d). Taking account of the spontaneous degradation of MB solution, the degradation rate constant k recalculated from the concentration–irradiation time curves is represented in Fig. 9. Under the visible light irradiation, TiO2 films fabricated by MCT and the oxidation (M10–tK–10h) showed relatively low photocatalytic activity. In addition, their visible light photocatalytic activity was not improved with the increase in oxidation temperature. However, the photocatalytic activity under visible light irradiation was greatly improved after the reduction reaction by carbon powder. The activity increased as the reduction temperature rose to 973 K, above which it decreased again. C–973K–0?5h sample showed the highest activity, which was 5?5 times of that before the reduction reaction. Under the UV irradiation, the photocatalytic activity of the TiO2 films


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a C–873K–0?5h; b C–973K–0?5h; c C–1073K–0?5h; d C–1073K–3h; e C–1073K–5h; f C–1073K–10h 6 Electron probe microanalyser analysis of samples

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7 Absorption spectra of TiO2 films fabricated in present work

fabricated by MCT and oxidation decreased with the increase in oxidation temperature and then increased at 1273 K. The photocatalytic activity under UV irradiation was also improved by the reduction reaction. It first increased and then decreased as the reduction temperature was increased to 1073 K. The C–973K–0?5h sample also showed the highest photocatalytic activity under UV light irradiation. A simple model for TiO2 photocatalyst is as follows.26,27 When a photo is absorbed by a TiO2 particle, a hole–electron pair is produced according by equation (3). The hole–electron pairs may migrate to the surface of the photocatalyst particle. The holes react with surface hydroxyl groups (OH2) and adsorbed H2O to form hydroxyl radicals (?OH). The basis of photocatalytic reaction is related to the production of very active ?OH obtained primarily according to equations (4) and (5) TiO2 zhv?hz ze{

(3)

Table 2 Band gap energies and wavelength of samples and commercial TiO2* Samples

Band gap energy Eg/eV

Wavelength l/nm

M10–1073K–10h C–873K–0.5h C–973K–0.5h C–1073K–0.5h C–1073K–3h C–1073K–5h C–1073K–10h Wako (rutile, 300 nm) Ishiraha (anatase, 7 nm) Kishida (anatase, 450 nm)

2.77 2.71 2.74 2.61 2.71 2.66 2.64 2.96 3.21 3.16

448 458 453 475 458 466 470 419 386 392

*Wako, Ishihara and Kishida are the manufacturers.

OH{ zhz ?OH

(4)

H2 Ozhz ?OHzHz

(5)

As seen in Fig. 4, during the reduction reaction, column crystals of TiO2 grew along the direction perpendicular to the surface of TiO2 films. Therefore, the specific surface area of TiO2 films increased in the initial stage. As reduction temperature rose to 1073 K, the crystals connected with each other, which resulted in the decrease in the specific surface area. The increase in specific surface area can improve the photocatalytic activity of TiO2.28,29 From Fig. 2, it can be known that the phase transformation from rutile to anatase did not happen and rutile was reserved in the reduction reaction. From Fig. 4, the crystallinity of rutile increased as the reduction temperature and holding time increased. The crystallinity improvement of TiO2 is considered to be helpful in enhancing photocatalytic activity. Therefore, the balance of crystallinity and specific surface area is very important to improve photocatalytic activity under UV light irradiation. The analysis is suitable for the samples fabricated by MCT and the oxidation. The above results are in good agreement with the work of Jitputti et al.30 From the EPMA analysis in Fig. 6, the carbon content in the films during the reduction reaction was rather tiny and did not change remarkably, which means that carbon did not penetrate into the films. Therefore, it can be inferred that the improvement on photocatalytic activity under visible light irradiation is not from carbon doping. In our published study on thermoelectric materials,24 oxygen vacancy was successfully introduced into TiO2 and non-stoichiometric TiO22x by reduction treatment in carbon powder. In this work, the band gap energy of the TiO2 films (M10–1073K–10h) was 2?77 eV


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a, c visible light irradiation; b, d UV light irradiation 8 Methylene blue solution concentration as function of irradiation time

rather than 3?0 eV of rutile TiO2. It suggests that oxygen vacancy was introduced into the TiO2 films by the reduction reaction. Pure TiO2 as a semiconductor photocatalyst is characterised by low quantum efficiency because of high band gap energy (y3?2 eV) and high recombination rate of the hole–electron pairs.31 To increase the charge separation efficiency, the hole–electron pair recombination rate needs to be reduced. One way is to introduce defects into TiO2 lattice by doping with transition metal ions, e.g. Fe3z cations.27 However, oxygen defects were introduced into TiO2 by reduction reaction in this work. The oxygen defects may serve as trapping sites for holes and/or electrons. Therefore, the introduction of oxygen defects may be a crucial factor to improve the photocatalytic activity under the irradiation of UV light. However, some researchers hold opposite opinions. The study of Takeda et al.32 showed that structural defects associated with the

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oxygen vacancies can work as recombination centres of photogenerated holes and electrons and therefore decreasing the photocatalytic activity of TiO2. TiO2 with oxygen vacancy has been prepared by the reduction reactions in hydrogen and carbon atmosphere.18,19 Their studies revealed that the energy state of oxygen vacancy is located between the valence band and the conduction band of TiO2. Therefore, TiO2 with oxygen vacancy could be activated by the photons with lower energies. In other words, the band gap energy of TiO2 was reduced by the introduction of oxygen defects, and hence, the absorption spectra of the TiO2 have shifted in the range of visible light wavelength. The analysis is consistent with the results in Fig. 7 and Table 2. Therefore, the introduction of oxygen vacancy into TiO2 lattice was considered to be the main reason why the photocatalytic activity under visible light irradiation was improved.

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9 Degradation rate constants k of samples under UV-vis light irradiation

Conclusions TiO2 films were successfully fabricated by MCT and the following high temperature oxidation. After further reduction treatment in carbon powder, the films showed improved photocatalytic activity even if under UV or visible light irradiation. Compared with that before the reduction treatment, the photocatalytic activity after the reduction treatment under UV-vis light irradiation was increased by ,1?2 and 4?5 times. For the improvement on photocatalytic activity under UV light irradiation, the balance of crystallinity and specific surface area is considered to the main reason. As for the case under visible light irradiation, the red shift of TiO2’s absorption spectra due to the introduction of oxygen vacancy into TiO2 lattice explained the improvement on photocatalytic activity.

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