Photo-assisted Deposited Titanium Dioxide Film and the Enhancement of Its Photocatalytic Water Splitting Activity①

XU Chao-Sheng LV Pei-Wen

a(Fuzhou University, College of Chemical Engineering, Fuzhou 350116, China)

b (Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

ABSTRACT Photo-assisted deposited method is often employed in the metal-organic chemical vapor deposition whose ion source is organic compounds. It has been proved to increase the deposition rate and improve the crystallinity of the films. We demonstrate a photo-assisted sputtering deposited method which is used to prepare high quality TiO2 films. The crystallinity of the films is improved by the photo assistance without changing the morphology. And the structural and optical properties remain the same. The photo-assisted deposited TiO2 film shows a H2 evolution rate of 1.62 μmol·cm-2·h-1 that is about twice more than that of the pristine TiO2 film. It is found the Mott-Schottky effect responds for the photocatalytic activity. Photo-assisted deposited films show an enhanced photocatalytic activity due to the reduction of interface recombination and the high efficiency in the transferring of photo-generated carriers.

Keywords: TiO2, photo-assisted, Mott-Schottky, photocatalyst;

1 INTRODUCTION

As a well-established technology to grow high quality films, magnetron sputtering has now been applied in many industries including solar cells, sensor, thin film transistor(TFT), optical lens, circuits, light-emitting diode (LED),etc[1-4]. Particularly, sputtering has the advantage of low temperature deposition which can be used to deposit lots of inorganic element and compounds on various substrate such as glass, ceramics, metals, textiles and flexible polymer substrate[5,6].

The development of magnetron sputtering technique experiences several stages. From the classical planar magnetron sputtering, inductively coupled plasma source magnetron sputtering, microwave amplified magnetron sputtering and high power impulse magnetron sputtering,both of them focus on the modification of the flux of ions sputtering from the target[7-10]. Photo-assisted growth methods have been studied since 1980s[11]. Most photoassisted growth methods are metal-organic chemical vapor deposition (MOCVD), sol-gel in which the react species are organic compounds[12]. The photo is induced to activate or decompose the react species[13]. Fujita et al. reduced the growth temperature of ZnSe from 500 to 300 ℃ by photo assisted MOCVD to reduce the defects in the ZnSe epilayers[14]. Li et al. improved the quality ZnO films due to the ethyl radical releasing from the metal atom, thus leading to incorporation into the lattice[15]. However, it is lack of knowledge about the affection of photo on the film’s growth for magnetron sputtering.

In this work, we induce a photo-assisted magnetron sputtering method to improve the quality of as-grown titanium dioxide film. The influence of incident light on the photocatalytic activity has been studied. And the mechanism of photo-assisted growth has been discussed.

2 EXPERIMENTAL

TiO2thin films are deposited on stainless steel substrates(2cm × 2cm) by sputtered titanium target. During the sputtering process, the working gas is a mixture of Ar(99.999% pure) and O2(99.99% pure), and the base vacuum is 3 × 10-5Pa. For all the samples, the sputtering power, total working pressure, Ar flow rate and O2flow rate are set to 140 W, 2.0 Pa, 14 sccm (standard cubic centimeter per minute)and 6 sccm, respectively. The incident light is provided by a 300 W Xenon lamp (PLSSXE300C). The as-prepared samples are labelled as FL, VL, IR and NL for deposition under light with full spectrum, light filtered with UV-cutoff filter (λ≥S420 nm) and light filtered with UV-vis cutoff filter (λ≥600 nm) and without light respectively.

Crystallinity of the films is determined by Rikagu Miniflex 600 X-ray diffraction. Electron microscope analysis is carried on a JEOLJSM-6700F Scanning electronic microscopy(SEM). The transmittance spectra of the films are obtained via the Shimadzu UV-2500 UV/Vis Spectrometer. The photoelectrochemical measurements of the as-prepared samples are also performed in 4% formaldehyde aqueous solution with 0.1 M K2SO4to increase the conductivity of the solution. All electrochemical measurements are conducted on a CHI 660D electrochemical workstation, using a three-electrode test cell. Transient absorption Spectra are obtained by an ultrafast HELIOS Femtosecond Transient Absorption Spectrometer. XPS measurements are performed in a Thermo Fisher K-Alpha+ system with an AlKαsource.

The simulated sunlight is provided by a xenon source lamp.The photocatalytic H2evolution test is conducted in an online photocatalytic hydrogen production system. A xenon arc lamp (Perfect Light PLS-SXE300) is used as a light source and positioned 20 cm away from the film. The sample fixed on bracket made of PTFE is dipped in 100 mL formaldehyde aqueous solution (4%) using a magnetic stirrer.And the hydrogen is analyzed by an online gas chromatograph (GC-2014, Shimadzu) which is equipped with a TCD detector and a packed column (carbon molecular sieve)using Argon (99.999%) as the carrier gas.

3 RESULTS

3. 1 Structures of the films

Fig. 1 shows the XRD pattern of as-grown films. There are only three diffraction peaks listed in the XRD pattern. Except two peaks attributed to the substrate, all films exhibit anatase(101) preferred orientation which is in accordance with our previous study[16]. While inducing light during film growth,the peak intensity of anatase (101) increases. It indicates that the crystallinity of the films is improved with photo assistance.

Fig. 1. XRD pattern of the as-grown films

The surface morphology is analyzed by AFM. As shown in Fig. 2, the films show a typical island growth pattern. The surface morphology of the films doesn’t show a remarkable change for depositing with or without photo-assistance. To analyze the microstructure of the films, SEM and TEM tests are carried out and shown in Figs. 3 and 4. SEM images of the films show a columnar structure pattern. Corresponding to island growth pattern observed in AFM, the particle coalescence phenomenon is clearly observed. It is known that the crystal structure of TiO2belongs to tetragonal system with space groupI41/amd, while the steel substrate is of cubic system in space groupFm3m. The films show a typical Volmer-Weber growth mode due to the low substrate temperature and lattice mismatch. TEM image in Fig. 4 clearly shows lattice fringes and the observed lattice fringes of all samples haved-spacing of 0.35 nm, which is in accordance withd(101) of anatase TiO2. TEM images suggest an anatase structure of the films, which is consistent with the XRD results. The morphology and structure of the films remain unchanged, indicating that the film growth is independent to the incident light.

Fig. 2. AFM images of (a) VL, (b) IR, (c) FL and (d) NL

Fig. 3. SEM images of (a) NL and (b) FL

3. 2 Optic properties

Fig. 5 shows UV-vis absorption spectra of the TiO2films prepared on the glass substrate. All films exhibit the same absorption edge and spectral interference, indicating the same film thickness and refractive index[17,18]. Fig. 6 shows the reflectance spectra of TiO2prepared on the steel substrates.The steel substrate possesses high reflectance rate. After being coated with TiO2films, the reflectance decreases significantly. Consistent with the transmittance spectra, all films show the same spectral interference. The similar spectral interference indicates the film is uniform and homogeneous with the consistent film thickness which is ideal for comparative study on the photocatalytic performance.

3. 3 Photocatalytic activity

Fig. 7 shows the photocatalytic H2evolution rate as the function of the wavelength of the incident photon.Interestingly, the photocatalytic performance is changedwhile the morphology and optical properties of the films remain the same. The photocatalytic performance of the film is improved for deposition under infrared light and further enhanced for deposition under visible light. However, the performance reduces while depositing under full spectrum lighting. Electrochemical measurement also shows the different behaviors of the films. The photocurrent response is carried out to analyze the efficient charge separation during photocatalytic activity. As shown in Fig. 8, all films exhibit stable photocurrent response. The photocurrent density increases sharply under each irradiation and recovers rapidly in the dark. The reproducibility of the photocurrent indicates the stability of photocatalytic activity of the films. Compared with NL, the photocurrent density of the photo-assisted deposited films increases for sample VL and drops dramatically for sample FL. Consistent with the photocatalytic H2evolution activity, the results reveal the notable difference in the photocatalytic performance of the films which are dependent to the incident light during photo-assisted deposition.

Fig. 5. UV-vis transmittance spectra of the TiO2 films on the glass substrate

Fig. 6. Reflectance spectra of TiO2 prepared on the steel substrates

Fig. 7. Photocatalytic H2 evolution rate of the as-preparedfilms

Fig. 8. Photocurrent response of the as-prepared films under zero bias (Ag/AgCl as the reference electrode)

4 DISCUSSION

We have previously demonstrated that the defects in TiO2thin films could be tuned by the incident photons during sputtering. We now focus on the effects of defects on the photocatalytic activity of TiO2. It is known that defects in TiO2such as Ti3+are beneficial for the utilization of visual light[19,20]. And the hydrogen evolution rate of TiO2nanoparticles grown under 254 nm UV lamp is 3.7 fold compared to that of the normal TiO2[21]. However, we observe a decrease of photocatalytic activity of sample FL.To verify the different behaviors during photocatalytic activity, the electrochemical impedance spectroscopy (EIS) is employed to analyze the carrier transferring mechanism.Fig. 9 shows the Nyquist plots of the impedance data. VL possesses small capacitive arcs, showing that it has higher efficiency in transferring the carriers. The single capacitive arcs are modeled by the simple equivalent circuit (Fig. 9),consisting of a resistorRsin series with a resistorRctand capacitorCPEin parallel, whereRctandCPErepresent the charge transfer resistance and the double layer capacitance,respectively. As shown in Table 1, theRctof FL is higher than that of VL, suggesting the low efficiency in the transferring of photo-generated carriers.

Fig. 9. Impendence spectra of the as-prepared films

Table 1. Fitted Parameter of Element in the Equivalent Circuit Model of Steel/TiO2/Solution

To clarify the impact of the defects induced by UV light on the kinetics of the carriers in TiO2, transient absorption spectra are employed to study the carrier behaviors under excitation. In Fig. 10, the absorption peaks of trapped electrons and holes are clearly shown in the spectra[22,23]. The similar spectrum and lifetime of the trapped state in FL and VL indicate the same type of trapped state in the two samples.FL has much higher absorption peaks than VL, which means the concentration of the trapped states of FL is higher than that of VL. It is known that the trapped states would significantly reduce the carrier mobility and lifetime. The different behaviors of the carrier under excitation in the films which are consistent with the EIS spectra clearly show the influence of defects on the transferring efficiency of the carriers. To study the trapped states in the films, XPS survey are conducted. Fig. 11 shows the Ti 2pspectra of FL and VL.Compared with VL, the Ti 2pspectrum in FL is slightly broadened and exhibits a tail in the region of lower binding energy. It is known that the Ti3+has a lower binding energy than Ti4+, which would result in the shift of peak. The O 1s XPS peak of VL in Fig. 12 is deconvoluted into two peaks at 531 and 529.6 eV attributed to -OH absorbed on the surface and Ti4+-O bond of TiO2, respectively. Additional peak located at 528.7 eV attributed to Ti3+-O bond of TiO2appears in the FL sample[24]. Accordingly, the change of XPS spectra of FL and VL reveals the existence of trapped state which is related to Ti3+defect state.

Fig. 10. a) Transient absorption spectra of VL and FL after excitation by 300 nm laser pulses,b) Time profiles of transient absorption signals recorded at 500 nm for VL and FL

Fig. 11. XPS spectrum of Ti 2p

Fig. 12. XPS spectrum of O 1s

To reveal the photocatalytic mechanism, the interface contacts of steel/TiO2and TiO2/solution are analyzed. Our previous study demonstrates the ohmic contact between Al and TiO2[16]. As shown in Fig. 13, the I-V characteristics of Al/TiO2/steel also show an ohmic contact between steel and TiO2due to the lower Fermi energy level (EF) of TiO2compared to the work function of steel. The band bending effect would accumulate electron in the steel and block the hole migration. Thus, the hydrogen should generate at the surface of steel. A movie clip filmed during photocatalytic experiment clearly shows the generation of hydrogen bubble at the edge of steel rather than on the TiO2film. As to the interface between TiO2and aqueous solution,a liquid junction is formed between TiO2and the solution redox couple (the potential of the formaldehyde oxidation reaction)[25,26]. Equilibrium could be achieved through the transfer of electrons from TiO2to aqueous solution. The energy band bends owing to electron depletion in TiO2. And a Schottky barrier formed in TiO2is expected. Accordingly,the I-V characteristics of TiO2/solution (Fig. 13) show a rectifying behavior.

Fig. 13. I-V characteristics of (a) Al/TiO2/steel and (b) steel/TiO2/solution (Ag/AgCl as the reference electrode)

According to the catalytic performance and I-V characteristics, a possible photocatalytic mechanism for hydrogen evolution is presented in Scheme 1. An ohmic contact is formed at the interface of steel and TiO2film, and Schottky barrier is induced at the interface of TiO2and solution. Photo-generated electrons are driven by the built-in electric field into steel while the holes are transferred to oxidize the formaldehyde. The enhancement of photocatalytic performance is due to high efficiency in the transferring of photo-generated carriers in the photo-assisted deposited samples. Unlike the nanoparticles with a huge specific surface area, the contact interface between thin film and solution is limited. The defects located on the surface of TiO2films would significantly hinder the transfer of photo-induced carriers from TiO2to solution. The ineffective electron transferring would reduce its photocatalytic performance.

Scheme 1. Energy diagram of TiO2/steel in the formaldehyde solution

5 CONCLUSION

TiO2thin films are deposited on the steel with photo assistance. The columnar grain structure and the nanoparticle morphology show a typical Volmer-Weber growth mode.The films exhibit (101) anatase preferred orientation and a sharp absorption edge. All films demonstrate photocatalytic H2evolution activity. The Mott-Schottky effect is supposed to promote the separation of photo-generated carrier.Photo-assisted deposited films show an enhanced photocatalytic activity due to the reduction of interface recombination and the high efficiency in the transferring of photo-generated carriers.