总气压与Ar/O2流量比对直流对向靶磁控溅射TiO2薄膜光催化性能的影响

陈 芃 谭 欣 于 涛

(1天津大学环境科学与工程学院，天津300072;2国家海洋局天津海水淡化与综合利用研究所,天津300192; 3天津大学化学工程学院，天津300072)

1 Introduction

In 1972,Fujishima and Honda1discovered the phenomenon of photocatalytic splitting of water on a TiO2electrode under ultraviolet(UV)irradiation.Due to its non-toxicity,high activity,and chemical stability,many researchers have focused on the environmental applications of TiO2photocatalyst,including the production of self-clean materials,2decomposition of water,3,4dye-sensitized solar cells,5,6mineralization of toxic organic compounds7-9and so on.

However,the filtration of TiO2powders has limited its practical or industrial applications.Thus,TiO2thin films have attracted a great deal of attention due to their excellent properties such as easy recycling and stable coating on various kinds of substrates.10Bahnemann and co-workers11,12have reported TiO2thin films on different substrates such as polycarbonate substrates and soda-lime glass for degradation of organic compounds.Kiwi and co-workers13focused on the stable parylene/ TiO2films for the photoinduced discoloration of methyl orange and the acceptable kinetics during the dye discoloration.However,most of the researches focused on the photocatalytic TiO2thin films prepared by wet process such as a sol-gel method, only a limited number of reports were concerned on the synthesis of TiO2films by physical deposition.Here we proposed a new device named direct current facing-target magnetron sputtering(DCFTMS)system for thin film deposition.Compared to the traditional film preparation methods including sol-gel method14and chemical vapor deposition,15TiO2thin films prepared by magnetron sputtering showed a higher uniformity and improved the combination between the films and the substrate.16Besides,parameters such as sputtering pressure,17power,18target-substrate distance,19and substrate temperature20can be controlled precisely during the sputtering process.Therefore,TiO2thin films with well-controlled stoichiometry,high purity,and strong adhesion can be fabricated by regulating sputtering parameters and post-annealing treatments.

As a physical depositon method,magnetron sputtering has been developed very fast in recent years.In conventional magnetron sputtering system there is only one target paralleled to the substrate,which easily cause high energetic ion bombardment on the film surface during the sputtering process.21In DCFTMS system the substrate is vertical to a pair of facing targets with the aim of prevention of high energetic ions bombardment in the new system.This new technology has shown many advantages including high sputtering efficiency and stable substrate temperature.So far,there is seldom study focusing on the crystal structures and photocatalytic activities of TiO2thin films prepared by DCFTMS.In the previous work22we have investigated the influence of annealing temperature on the properties of TiO2thin films deposited by DC facing-target magnetron sputtering method.In this paper we focus on effects of different sputtering pressures and Ar/O2flow ratios on the photocatalytic activities of the prepared TiO2thin films.A pair of Ti plates were used as the sputtering targets in the TiO2thin film fabrication.Argon gas was selected as the sputtering gas for its inert properties.The characterizations of TiO2thin films including X-ray diffraction(XRD),field emission scanning electron microscopy(FESEM),atomic force microscopy(AFM),and UV-visible(UV-Vis)spectrophotometer were also investigated.

2 Experimental

2.1 Preparation of TiO2thin films

TiO2thin films were prepared by DCFTMS with the vacuum pressure at approximately 6.0×10-4Pa.Schematic diagram of the reactive DCFTMS system is shown in Fig.1.The facing targets were a pair of Ti discs(purity of 99.99%)with 100 mm in diameters.Fluorine-doped SnO2(FTO)glasses(NSG,Japan) served as the substrate with an area of 20 cm×20 cm.Argon gas(purity of 99.999%)and oxygen gas(purity of 99.999%) were selected as the working gas and reactive gas,respectively. The sputtering deposition began when the plasma color changed to blue.In order to clean the target surface completely,the pair of Ti discs was pre-sputtered for 5 min.All of the TiO2thin films were sputter-deposited at room temperature using a power of 350 W.The total working pressure was controlled at 1.0,1.5,2.0 Pa by an ion gauge.The depositions were carried out with the Ar/O2flow ratios of 3:1,2:1,and 1:1. The substrate-target distance was kept at 70 mm for the deposition of all samples.After deposition,the prepared films were sintered at 550°C in the furnace with a speed of 4°C·min-1and then kept at 550°C for 2 h.

2.2 Characterization

The thicknesses of TiO2thin films were measured by a surface roughness tester(Vecco Dektak 6M,America).The crystal structure and phase composition of samples were characterized by XRD(Rigaku Rint-2500,Japan)with a 2θ method using Cu Kαradiation.The scanning rate was 2(°)·min-1.The surface morphology of the films was observed by FESEM(Hitachi S-4800,Japan).For a detailed study of the film surface,AFM (Digital MMAFM-2,America)was used with contact mode. The images with an area of 4 μm×4 μm were obtained.

Fig.1 Schematic diagram of the reactive DCFTMS system

2.3 Photocatalytic activity measurement

The photocatalytic activities of the prepared TiO2thin films were evaluated by measuring the decomposition of gaseous iso-propanol(IPA)under UV light irradiation.In order to remove the impurity from film surfaces,samples were cleaned using nitrogen before measurement.The prepared TiO2thin films were placed at the center of the rector and then the IPA gas was injected with an initial concentration of 100×10-6.At the beginning of the reaction,the reactor was kept in the dark for 1 h to obtain the absorption balance of IPA.Then the reactor was irradiated using an UV light for 2 h at room temperature.The reaction products were calculated using a gas chromatograph(GC-2014,Shimadzu)equipped with a methanizer and a flame-ionized detector.

2.4 Photoinduced superhydrophilicity measurement

The photoinduced hydrophilicity of samples was evaluated by a sessile drop method.The TiO2thin films were kept in a dry box under darkness for 24 h and then determined at every 15 min interval for 2 h under the UV irradiation.All water droplets were blown with nitrogen gas before the film was irradiated.The water contact angles were measured at five different positions using a goniometer,with the average value selected to be the contact angle.

3 Results and discussion

3.1 Structure and morphology

The average thickness of all the prepared films was about 200 nm,measured using a surface roughness tester.Conditions of the fabrication of the TiO2thin films were investigated by optimizing the sputtering pressures and Ar/O2flow ratios.The XRD patterns of a series of TiO2samples deposited on FTO glasses are shown in Fig.2.It should be noted that the diffraction patterns of FTO substrates were much sharper than the TiO2patterns due to the thickness of the TiO2films.On the other hand,the XRD images clearly showed typical diffraction patterns of around 25°and 27°,which indicate anatase(101) phase and rutile(110)phase,respectively.

All films deposited at Ar/O2flow ratio of 1:1 showed the best crystal structure under the same total pressure.As an important parameter,a threshold of Ar/O2flow ratio existed during the sputtering process.The number density of Ti atom on the substrate is directly related to the oxygen flow rate and its partial pressure in the reaction chamber.23

Due to lack of oxygen atoms,it was not good for the formation of TiO2at low O2flow ratio.However,the sputtering process might stop if keeping increasing the O2flow ratio.In the DCFTMS system Ti targets would be oxidized by superfluous O atoms which were unconducive to the formation of TiO2.Besides,the excessive O2flow ratio caused the decreasing of the utilization of initial Ti targets.Therefore there was a threshold of the Ar/O2ratio existing in DCFTMS process.In our experiment,the threshold ofAr/O2flow ratio was 1:1.

Fig.2 XRD patterns of the TiO2thin films deposited on FTO substrates with differentAr/O2flow ratios of 3:1,2:1,1:1The sputtering pressures(ptot)were(a)1.0 Pa,(b)1.5 Pa,and (c)2.0 Pa,respectively.

On the other hand,the phase of deposited films changed from the mixture of anatase and rutile to pure anatase as the sputtering pressure increased.This might be related to the impinging particles with high energy under the high total pressure condition.In order to obtain crystalline TiO2thin films,the energy of the deposited particles should be higher than the crystalline nucleation energy of anatase or rutile.As the sputtering pressure increased from 1.0 to 2.0 Pa,the energy of impinging particles increased,conducing to the formation of anatase phase.The XRD patterns are consistent with the above analysis.As shown in Fig.2,the TiO2thin films deposited at 2.0 Pa with Ar/O2flow ratio of 1:1 showed the highest crystallinity with anatase phase only.

Fig.3 FESEM images of TiO2thin films deposited on FTO substrates with differentAr/O2flow ratios under different total pressures

In addition,the surface morphology of the TiO2thin films is an important factor in photocatalytic reaction.The FESEM images of TiO2thin films deposited on FTO substrates with different Ar/O2flow ratios under different total pressures are shown in Fig.3.The surfaces of the prepared films were composed of polygonal particles with about 100 nm in diameter.Moreover, the particles are not closely contacted,with some‘little crack’between the particles.This phenomenon could be explained by the affection of FTO layers during the deposition.It can be seen from FESEM images that the films deposited with higher Ar/O2flow ratios show higher surface roughness.Therefore, the reactants can be easily adsorbed on the film surface and the products can be quickly transferred out of the film.

Fig.4 shows AFM images of TiO2thin films deposited with Ar/O2flow ratio of 1:1 under different total pressures.The prepared films became rougher and higher uniformity as the sputtering pressure increases during the deposition process.The effect of increasing pressure on the surface morphology of TiO2films may attribute to the high surface mobility caused by the increasing kinetic energy of sputtered atoms.

3.2 Photocatalytic activity

The photocatalytic performance of TiO2thin films was evaluated by the decomposition of IPA under UV irradiation.It could be seen from the FESEM images that the film was compact and no pore existed on the film surface.So the decrease in IPA was independent of the absorption during the photocatalytic reaction.Furthermore,the IPA concentration remained the same during 2 h in the absence of TiO2thin film,indicating the absence of IPAself-degradation under UV irradiation.

Under UV irradiation,the electron and hole,which have high activity and ability for oxidizing many organic compounds,were generated and then transformed to the surface of TiO2films.24-26IPA was oxidized to acetone(CH3COCH3)and further to carbon dioxide(CO2).

Fig.4 AFM images of TiO2thin films deposited on FTO substrates withAr/O2flow ratio of 1:1 under different total pressuresptot/Pa:(a)1.0,(b)1.5,(c)2.0

Fig.5 and Fig.6 record the concentrations of acetone and CO2as a function of UV irradiation time respectively in presence of different samples.Acetone was the main product while both acetone and CO2were evolved from the photocatalytic degradation of IPA in our experiment.It was determined that under the same sputtering pressure conditions,the TiO2thin film deposited with Ar/O2flow ratio of 1:1 showed that the amounts of the generated acetone and CO2were the largest. XRD patterns of the film samples showed that the decreasing Ar/O2flow ratios resulted in the formation of anatase phase.It has been reported that the photocatalytic activities of TiO2thin film composed by the mixed phase of rutile and anatase increase with the increasing content of anatase.27,28Simultaneously,if the Ar/O2flow ratios were kept constant,then TiO2thin film deposited at 2.0 Pa would show that the amounts of the generated acetone and CO2were the largest.As evident from Fig.6,TiO2film deposited at 2.0 Pa with the Ar/O2flow ratio of 1:1 showed the highest photocatalytic activity.The concentration of acetone reached the maximum value of 93.2×10-6after 120 min irradiation.

For all the prepared samples,the concentration of the generated CO2changed in a low range during the whole reaction process.However,among all the prepared TiO2thin films,the sample deposited at 2.0 Pa with Ar/O2flow ratio of 1:1 still showed the highest CO2yield of around 11.1×10-6after 120 min irradiation.

Fig.5 CO2concentration during the photocatalytic degradation of IPAin the presence of TiO2thin films deposited on FTO substrates with differentAr/O2flow ratios of 3:1,2:1,1:1 under different pressuresptot/Pa:(a)1.0,(b)1.5,(c)2.0

The superior photocatalytic activity of TiO2film deposited at 2.0 Pa with Ar/O2flow ratio of 1:1 can be attributed to the high degree of crystallization and typical crystal structure.The higher sputtering pressures are good for the formation of anatase phase but inhibit the growth of rutile phase.Besides,both of XRD and FESEM images show that the crystalline perfection of TiO2thin film increases with the increasing sputtering pressure.Compared to other samples,TiO2thin films deposited at 2.0 Pa exhibitd a higher crystallinity and photocatalytic activity.Therefore,it can be referred that crystalline perfection played an important role in the photocatalytic process of TiO2thin films.

3.3 Chemical stability

This paper investigates the chemical stability of the prepared TiO2thin films with the highest photocatalytic activity,which is on the basis of the possible application for TiO2films.The repetitive experiment,namely the decomposition of IPA in the presence of TiO2thin film deposited at 2.0 Pa with Ar/O2flow ratio of 1:1 was done for 5 cycles in our study.In order to remove the impurities from the film surface,the TiO2thin films were heated at 150°C in muffle furnace for 2 h before the photocatalytic reaction.According to the above experiments,acetone was the main product in the decomposition of IPA.Therefore,the concentration of the generated acetone was used to measure the chemical stability of TiO2thin film.Fig.7 shows the change of acetone concentration during the degradation of IPA.The photocatalytic activity evidently decreased both in the second and third cycles,after which the value remained stable.The decreased photocatalytic activity of TiO2thin film may be due to the adsorbed species of reactant.However,the concentration of acetone could reach up to 83.2×10-6after 2 h irradiation in the fifth cycle,which was still higher than that of the other samples with different sputtering pressures and Ar/O2flow ratios.This phenomenon proved that the TiO2thin film deposited at 2.0 Pa with Ar/O2flow ratio of 1:1 kept high stability during the degradation of IPA.

Fig.6 Acetone concentration during the photocatalytic degradation of IPAin the presence of the TiO2thin films deposited on FTO substrates with differentAr/O2flow ratios of 3:1,2:1,1:1 under different pressuresptot/Pa:(a)1.0,(b)1.5,(c)2.0

Fig.7 Evaluation of stability of TiO2thin film deposited at 2.0 Pa withAr/O2flow ratio of 1:1 from the repetitive photocatalytic degradation of IPA

3.4 Photoinduced superhydrophilicity

Photoinduced superhydrophilicity of the sample was determined by changes in contact angles for water under UV irradiation.The mechanism of photoinduced superhydrophilicity can be explained as follows:as UV-induced surface structure changed,the amount of OH groups on the TiO2film surface increased.29

Fig.8 UV irradiation time dependence on water contact angle of TiO2thin film deposited at 2.0 Pa withAr/O2flow ratio of 1:1Inset images illustrating water contact angles of samples:(a)at the beginning, (b)after 15 min,(c)after 90 min of UV irradiation

Fig.8 shows the relationship between irradiation time and contact angles for water on the sample.The initial water contact angle was 71°,which meant that the prepared film was initially hydrophobic.After 15 min of UV illumination,the water contact angle decreased from 71°to 28°.It could be seen that the water contact angle decreased with increasing irradiation time under UV light.The water contact angle finally decreased to 3°,indicating that the sample became superhydrophicity.According to the mechanism of photoinduced superhydrophilicity,the amount of water molecules adsorbed on the surface hydroxyls increased with UV irradiation,resulting in the high hydrophilicity of the prepared TiO2thin film.

4 Conclusions

In our study,TiO2thin films were successfully deposited on FTO substrates by DC facing-target magnetron sputtering.The deposition parameters,including the sputtering pressures and Ar/O2flow ratios were optimized.Our work can be summarized as follows.

(1)The decreasing Ar/O2flow ratios are beneficial for the formation of the mixed phase of anatase and rutile.The higher sputtering pressures result in the increase of crystalline perfection of TiO2thin film and inhibit the growth of rutile.The TiO2thin films deposited at 2.0 Pa with Ar/O2flow ratio of 1:1 showed the highest degree of crystallinity and only the crystal structure of anatase phase.

(2)A threshold of Ar/O2flow ratios,which proved to be 1:1 in our experiment,existed for the TiO2thin film formation. Sputtering pressure of 2.0 Pa was considered to be the most suitable ptot.The TiO2thin film deposited at 2.0 Pa with Ar/O2flow ratio of 1:1 showed significantly higher photocatalytic activity than other samples and confirmed with high chemical stability.The concentration of the generated acetone and CO2could reach up to 93.2×10-6and 11.1×10-6after 120 min UV irradiation,respectively.

(3)The TiO2thin films deposited at 2.0 Pa with Ar/O2flow ratio of 1:1 showed photoinduced superhydrophilicity after 90 min UV irradiation.

In summary,the DC facing-target magnetron sputtering is one of the useful and promising methods for the preparation of photocatalysts with the uniformed structure,high density coatings of large surface areas with a strong adhesion.The control for parameters in deposition process is important for fabricating TiO2thin films with high effective photocatalytic properties.

Acknowledgements:The authors thank Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology for the help on the TiO2thin film preparation.And the authors are grateful to Photocatalytic Materials Center at National Institute for Materials Science(Japan)for fruitful scientific discussions.

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