Structure and Photocatalysis of TiO2/ZnO Double-Layer FilmPrepared by Pulsed Laser Deposition

Lei Zhao, Maosheng Xia, Yuhua Liu, Biju Zheng, Qing Jiang and Jianshe Lian+

Key Lab of Automobile Materials, Ministry of Education, College of Materials Science and Engineering,Jilin University, Nanling Campus, Changchun, 130025, P. R. China

Nanocrystalline double-layer film of TiO2/ZnO was prepared by pulsed laser deposition on quartz substrate and their microstructure,optical property and photocatalysis were investigated. The first/bottom ZnO layer grew on its [001] direction of the wurtzite structure, while thesecond/top TiO2 layer consisted of nanorods grown on the [001] direction of its anatase structure. The TiO2/ZnO film showed improved photo-absorption in visible light in the UV­visible absorption spectrum; and showed weaken UV emission and enhanced deep level emission in thephotoluminescence spectrum. By the degradation of methyl orange and methylene blue, the TiO2/ZnO film exhibited evident enhanced photo-degradation efficiency in a wide wavelength range. That can be explained by the enhanced wavelength response in visible range, more effectiveuse of irradiation light and the retarded recombination of electron­hole pairs in the double layer films. [doi:10.2320/matertrans.M2011345]

(Received November 8, 2011; Accepted December 14, 2011; Published February 25, 2012)

Keywords: thin films, electron microscopy, microstructure, optical properties

1. Introduction

In the past few decades semiconductor photocatalysis dueto its great potential applications in energy and environmentareas has been becoming more attractive.1­4) Among varioussemiconductors employed, titanium dioxide (TiO2) is wellknown to be an excellent photocatalyst because of its hightransparency in visible light, good photocatalytic behaviorand favorable chemical stability.5,6) Zinc oxide, as a directwide-band-gap semiconductor with a band gap of 3.37 eVand a large exciton binding energy of 60meV at roomtemperature, is discovered to be important semiconductorphotocatalysis in recent years.7­10) And there have beenseveral examples of ZnO displaying more impressive photo-catalytic activity than widely studied titanium dioxide.11) Ingeneral, the photocatalytic activity is based on the generationof electron/hole pairs and limited by the recombination ofelectron/hole pairs. Many methods, such as doping,12) metalmodification13) and coupling of composite semiconduc-tors,3,4,14) have been investigated to suppress the recombina-tion and increase the life time of separated electron and hole,hence to improve the photocatalyst properties. Among themthe coupling of different semiconductor seems useful toachieve a more efficient electron/hole pair separation andprevent the recombination of electron/hole pairs effi-ciently.15,16) In this direction, different TiO2/ZnO compositematerials were investigated in consideration of that bothcomponents have good photocatalytic activity. For example,ZnO/TiO2 composite was fabricated by site-specific deposi-tion and the structure of the composite was analyzed.17) ZnO/TiO2 films were deposited on quartz substrate through E-beam evaporation and their photolumincence properties werediscussed.18) ZnO was also used as a buffer layer betweenthe Si (111) substrate and the deposited rutile TiO2 film bymagnetron sputtering.19) Nano-sized TiO2/ZnO compositefilm with different atomic ratio of Ti/(Ti + Zn) was preparedvia sol­gel process and the photodegradation activity ofmethyl orange was detected and analyzed.3) Although these

researches focused on the fabrication of TiO2/ZnO compositematerials and their photocatalytic activity, there is still a needto study the structure and growth relation of the TiO2/ZnOcomposite and photocatalytic activity as well as the relevancybetween photoluminescence and photcatalytic activity of thecomposite.

In the present work, TiO2/ZnO double layer film wasprepared on the quartz glass substrate by a simple pulsedlaser deposition.20) The structure and composition of the as-deposited film was characterized with XRD, SEM, EDS andHRTEM. The optical properties were investigated with UV­Vis and photoluminescence (PL) spectra and their photo-catalytic efficiency were evaluated by the photodegradationof methyl orange and methylene blue solution.

2. Experiment

TiO2/ZnO film was prepared on quartz substrate by pulselaser deposition and the pure TiO2 and ZnO films were alsoprepared for comparison. Pulsed Nd:YAG laser with awavelength of 1064 nm was used. The distance between thetarget and the quartz substrate was kept at 2.5 cm. Thechamber was evacuated first to a base pressure (below5 © 10¹4 Pa) using a turbo molecular pump and then the gaspressure was kept at 10 Pa by feeding oxygen gases (99.99%purity) into the chamber. Table 1 shows the detail preparingcondition of pure ZnO and TiO2 films. The TiO2/ZnO doublelayer film was obtained through the deposition of first ZnOfilm then TiO2 film following the procedure of Table 1.

Table 1 Preparing condition of ZnO and TiO2 film.

LayerTargetmaterial

Laserfrequency

(Hz)

Substratetemperature

(°C)

Depositiontime(min)

ZnOfilm

Metallic Zn(99.9%)

10 200 30

TiO2

filmMetallic Ti(99.9%)

20 600 60+Corresponding author, E-mail: lianjs@jlu.edu.cn

Materials Transactions, Vol. 53, No. 3 (2012) pp. 463 to 468©2012 The Japan Institute of Metals

The crystal structure of the films was analyzed by XRD(Rigaku D/max) with a Cu target and a monochromator at50 kV and 300mA. Field-emission Scanning Electron Mi-croscopy (FESEM, JEOL-JSM-6700F) was used to observethe surface morphology of the films. The chemicalstoichiometry of the film was analyzed by an EnergyDispersive X-ray Spectrometer (EDX, Oxford). Field-emis-sion Transmission Electron Microscopy (FETEM, JEOL-JEM-2100F) was used to characterize the structure of TiO2/ZnO film and interface of them through the TEM andHRTEM images and selected area electron diffraction(SAED) patterns at working voltage of 200 kV. The opticalproperties of the TiO2 thin films were characterized by UV­vis spectrophotometer and photoluminescence with an Arion laser as a light source using an excitation wavelength of325 nm.

The visible-light photocatalytic efficiency was estimatedby the photo-degradations of aqueous methyl orange andmethylene blue, respectively. The visual light was a 500Whigh-pressure mercury lamp (100mm long), which wassurrounded by a circulating water jacket to cool the lamp.The films were put in the reaction vessel containing 30mL ofaqueous methyl orange (or methylene blue) of 20mg/L,which were stirred through piping air into the beaker at a fluxof 50mL/min. The reaction vessel was exposed to the high-pressure mercury lamp perpendicularly. The distance be-tween the lamp and the vessel was kept at 20 cm. Followingthe exposure to visible-light, the decolorizations of methylorange at its maximum absorption wavelength (449 nm) ormethylene blue at its maximum absorption wavelength(665 nm) was analyzed by using UV­Visible (UV­Vis)spectrophotometer and recorded as a function of time. TheUV­Vis spectrophotometer was also used to measure theabsorbance spectrum of the films. All operations wereconducted at ambient temperature.

3. Results and Discussions

3.1 Structure characterization of TiO2/ZnO filmsFigure 1 shows the XRD patterns of ZnO, TiO2 and TiO2/

ZnO films deposited on quartz substrate. From the pattern ofthe ZnO film only two peaks of (002) and (103) plane weredetected. It shows typical hexagonal structure and strongc-axis orientation. The as-deposited TiO2 film shows typicalanatase structure with two peaks of (101) and (004) planes. Itis interesting that the TiO2 layer on TiO2/ZnO film showsanatase phase with high c-axis (004) orientation vertical tothe bottom ZnO film. The degree of crystal-axis orientationcan be estimated using the Lotgering orientation factor f:21)

f ¼ Phkl � P0hkl

1� P0hkl

ð1Þ

where Phkl is peak intensity ratio of the oriented (hkl)direction to the sum of all diffraction directions for orientedsample and P0

hkl is the corresponding value for the standardsample (from JSPDS card). The orientation factor f = 1means the complete orientation growth of the (hkl) direction;on the other end, complete random state is shown by f = 0.22)

The values of f(004) for as-deposited TiO2 film and the topTiO2 layer on the double-layer film are calculated to be 0.07

and 0.92, respectively. That is, the TiO2 film shows nearlyrandom growth, while TiO2 layer of the double-layer filmshows nearly complete (004) c-axis orientation growth.Meanwhile, only the (002) peak of the bottom ZnO can stillbe detected. That indicates that the bottom ZnO layer grewdirectionally in the process of TiO2 film deposition. Similarstudy has been reported before: the (001) oriented BaTiO3

thin films by using [0001] texture ZnO template weresuccessfully fabricated.23)

The crystal structure of anatase TiO2 is tetragonal crystallattice with the standard constants: a = 0.37854 nm andc = 0.95143 nm. ZnO has wurtzite structure with thestandard lattice constant of a = 0.32498 nm and c =0.52066 nm. The experimental lattice constant c of TiO2 filmdeposited on ZnO film can be calculated by Bragg equation:2dsin ª = n­ through the diffraction peak angle of (004)plane. The calculated result is c = 0.9561 nm, which is muchbigger than the standard value. As the matching relationshipbetween TiO2 and ZnO is ð004ÞTiO2

== ð0002ÞZnO, the valueof the lattice mismatch ¤ can be calculated by the equationbelow:24)

¤ ¼ af � asas

ð2Þ

where af and as denoted the lattice parameter of the film(TiO2, af = 0.37854 nm) and the substrate (ZnO, is consid-ered as the substrate for TiO2, as = 0.32498 nm), respec-tively. Therefore, the calculated lattice mismatch value is¤ = 16.5%. As af > as, this bigger lattice mismatch maycause a compressive internal stress on TiO2 lattice inhorizontal direction and hence a lattice shrinkage of TiO2

in this direction. To balance this compressive internal stress,a lattice expansion in vertical direction or c-axis direction(0.9561 nm/0.95143 nm = 1.005) is reasonably expected.

Figure 2 shows the FESEM images of TiO2, ZnO andTiO2/ZnO thin films. It is seen from Figs. 2(a) and 2(b) thatthe TiO2 film shows wafer grains with average grain size of24 nm and ZnO film shows column grains with average grainsize of 22 nm. Figure 2(c) shows the 45° perspective viewimage of TiO2/ZnO film and the EDX spectra of point A andB. It can be confirmed that the bottom layer of the film(point A) is ZnO film and the top layer is TiO2 (point B). Inorder to observe the morphology and structure of the film

200

500

1000

1500

2000

2500

Z(103)

A(105)A(101) Z(002)

A(101) A(004)

ZnO

TiO2

Inte

nsity

, I

/ ar

b.un

its.

2Theta, 2θ

TiO2/ZnO

A(004)

/ °7060504030

Fig. 1 XRD patterns of TiO2, ZnO and TiO2/ZnO thin films on quartzglass substrate.

L. Zhao et al.464

clearly, a high resolution FESEM image was shown inFig. 2(d). The thicknesses of the ZnO layer and TiO2 layerwere measured to be about 100 and 450 nm, respectively. TheZnO film deposited on quartz substrate shows column grainswith average diameter of about 40 nm, while the TiO2

deposited on ZnO film are nanorods of rectangular orpolygon column with average diameter about 100 nm andlength of about 450 nm, whose growth direction is [001]direction [see Figs. 2(d) and 2(e)].

Further structural characterization of the TiO2/ZnO filmwas revealed by using TEM. From the TEM image of a pieceof the double layer film shown in Fig. 3(a) it can be seen thatthe film was very dense and uniform. The short bottom (left)part is ZnO and the long right part is TiO2. The SAED[Fig. 3(b)] diffracted on Fig. 3(a) reveals the coexistence ofboth TiO2 and ZnO. The TEM observation of one selectedTiO2 nanorod is shown in Fig. 3(c). It is seen that the TiO2

nanorod presents obviously rectangular column. The attached

Fig. 2 FESEM images of thin film grown on quartz glass (a) TiO2 film; (b) ZnO film; (c) 45° perspective view and corresponding EDXspectra of TiO2/ZnO film; (d) high resolution of 45° perspective view of TiO2/ZnO film; (e) top view of TiO2/ZnO film.

Structure and Photocatalysis of TiO2/ZnO Double-Layer Film Prepared by Pulsed Laser Deposition 465

SAED shown in Fig. 3(d) indicates that the TiO2 nanorod is asingle crystalline throughout its length. Figure 3(e) shows theHRTEM image of the selected area of Fig. 3(c). In order toreveal it clearly, Fig. 3(f ) shows the enlarged filtered imageand the atomic arrangement simulation image of the zoneframed in Fig. 3(e). By the filtered image the growthorientation of the nanorod is determined to be along [001]direction, which is the growth axis parallel to the substratesurface according to Fig. 2(d). Both the filtered image andthe atomic simulation image (simulation state: indices ofzone axis: [010]; JEM-2100F: 200KV, Cs = 1.0mm; thick-

ness: 11 nm; defocus: 92 nm; no aperture) show that the TiO2

nanorods grown on the ZnO film have high crystal quality.

3.2 Optical and photocatalytic propertiesFigure 4 shows UV­vis absorption spectra of TiO2, ZnO

and TiO2/ZnO films. Compared with that of the TiO2 film,the absorption spectrum of TiO2/ZnO film obviously shiftsto visible light and displays an evident enhancement ofphoto-absorptions in visible region. The optical absorptioncoefficient ¡ near the absorption edge obeys the followingrelation for high photon energies: (¡h¯) = A(h¯ ¹ Eg)n,25)

Fig. 3 The TEM and HRTEM images: (a) TEM image of the TiO2/ZnO double-layer film; (b) the SAED of (a); (c) TEM image of theTiO2 nanorods; (d) the SAED of (c); (e) the HRTEM image of the selected area of (c); (f ) enlarged filtered image and the atomicarrangement simulation image of the zone framed in (e).

L. Zhao et al.466

where A is a constant, ¡ (cm¹1) is the absorption coefficientand h¯ (eV) is the energy of excitation, n is 0.5 and 2.0 for adirect transition semiconductor and indirect transition semi-conductor, respectively. Assuming the absorption coefficient¡ corresponding to the band gap energy, in the fundamentalabsorption region, better linearity was observed from the(¡h¯)2(or 0.5) vs. (h¯) plot, which was used to determine theband gap energy (Eg).26) The band gaps of TiO2, ZnO andTiO2/ZnO film were estimated to be 3.33, 3.26 and 3.03 eV,respectively. The significant red shift has also been reportedby other studies.27,28) It may be owing to significantdifferences in the surface state, which promote the separateefficiency of photogenerated charges and extend the range ofexcited spectrum.

Figure 5 shows the room temperature photoluminescence(PL) spectra of TiO2, ZnO and TiO2/ZnO thin films on quartzglass substrate. The TiO2 film shows no UV emission andweak visible emission, while the ZnO film shows strong UVemission (around 376 nm) with a narrow full width at halfmaximum (FWHM) of 18 nm (or 0.15 eV) and a weak deeplevel emission around 558 nm (2.22 eV). The strong UVemission from ZnO film is attributed to the high quality ZnOfilm preferred grown on c-axis orientation.29) Compared to

the spectra of single TiO2 film and ZnO film, the PL spectrumof TiO2/ZnO film shows a lower UV emission and a slightlyhigher deep level emission. Because TiO2 shows no UVemission, the UV emission should come from the bottomZnO film and was weakened by the top TiO2 layer. The deeplevel emission may result from the cooperation of TiO2 andZnO films.30) That is, some excitated UV photons from ZnOmay have been absorbed by the coated TiO2 nanorods, andthe TiO2 nanorods were excited by both the laser light andUV emission from the bottom ZnO. These two excitationchannels enhance the intensity of deep level emission of theTiO2/ZnO thin film to be higher than that of the as-depositedTiO2 film.

Table 2 shows the degradation efficiency of the TiO2 ZnOand TiO2/ZnO films for methyl orange and methylene bluefor 10 h. The TiO2/ZnO film shows the highest degradationefficiency of methyl orange: 32.9%, which is 29% higherthan that of TiO2 film and 73% higher than that of ZnO film.All the films show low-rise degradation efficiency at highwavelength range of 665 nm. Comparing with the naturaldegradation efficiency of 5.9% of methylene blue withoutcatalysts, ZnO film shows no evident increase of degradationefficiency (8.7%), and TiO2 film shows a limited increase of12.3%. TiO2/ZnO double layer film still exhibits the highestdegradation efficiency of 18.6%, which is 50% higher thanthat of TiO2 film. So the TiO2/ZnO double layer film exhibitsimproved photocatalytic activity in a relative wide visiblelight region and can be considered as a potential photo-catalytic material for pollutant degradation in water environ-ment, as the fabrication processing and cost are similar tothose of TiO2 and ZnO films. The improved photocatalyticproperty can be explained by the following three reasons:

(1) The wavelength response range of TiO2/ZnO filmestimated by the optical band-gap is <409 nm (3.03 eV),which is larger than those of TiO2 film and ZnO film(<372 nm (3.33 eV) and <380 nm (3.26 eV), respectively), asshown by Fig. 4. The double layer film also shows obviouslyenhanced absorption ability in wide visible-light region. So,more visible-light can be utilized in photocatalytic process.

(2) Although the ZnO and TiO2 have similar band gap, thecomposite of them may modify the electrons and holesmigration process. Both the electrons transition between theconduction band of TiO2 and conduction band of ZnO andthe corresponding holes transition between the ZnO valenceband and TiO2 valence band increase the channels for

300 400 500 600 700 8000.0

0.5

1.0

1.5

2.0

2.5

3.0

methylene blue

665

TiO2

ZnO TiO2/ZnO

Abs

orpt

ion,

A /

arb.

units

.

Wavelength, λ / nm

Visible region

449

methyl orange

Fig. 4 Optical absorption spectra of the TiO2, ZnO and TiO2/ZnO thinfilms.

350 400 450 500 550 600 650 7000

100

200

300

400

500

600

700

Wavelength, λ / nm

Inte

nsity

I /

arb.

units

.

TiO2

ZnO TiO2/ZnO

Inte

nsity

, I /

arb.

units

.

Wavelength, λ / nm

enlarge image

376

500 520 540 560 580 600 62010

20

30

40

50

593

558

518

Fig. 5 PL spectra obtained from TiO2, ZnO and TiO2/ZnO films.

Table 2 Degradation of methyl orange and methylene blue.

SampleA*1 at449 nm(a.u.)

A at665 nm(a.u.)

D*2 ofmethylorange

(10 h) (%)

D ofmethylene

blue(10 h) (%)

No sample 6.8 5.9

TiO2 film 0.054 0.006 25.5 12.3

ZnO film 0.097 0.033 19 8.7

TiO2/ZnOfilm

0.38 0.101 32.9 18.6

*1A is the absorption of the film.*2D (Degradation efficiency) = A0 ¹ Ai/A0 (Ai is the absorption of theindicator after degraded, A0 is the absorption of the original indicator).

Structure and Photocatalysis of TiO2/ZnO Double-Layer Film Prepared by Pulsed Laser Deposition 467

electrons and holes transition and hence can greatly reducethe recombination probability of electron­hole pairs.14)

(3) When the double layer film irradiated by light not onlythe TiO2 nanorods involved in the photocatalytic process, butalso the ZnO in the bottom joined in the photocatalyticreaction since the top TiO2 has high transmittance (>90%) inwave length region >370 nm. Since the PL process proceedsin the same time as the light irradiation, the top TiO2

nanorods were excited by both the exposure light and UVemission from the bottom ZnO film just as shown in Fig. 6.More effective use of irradiation light can obviously improvethe photodegradation efficiency.

4. Conclusions

Nanocrystalline double layer thin films of TiO2/ZnO wereprepared by pulsed laser deposition on quartz glass substrate.The microstructure and the optical property of the film wereinvestigated. It was found that the TiO2 film on the ZnO filmgrow with nanorod mode and the growth matching relation-ship between TiO2 and ZnO is ð004ÞTiO2

== ð0002ÞZnO. TiO2/ZnO film showed enhanced absorption in visible light. Thetop TiO2 nanorods were excited by both the laser light andUVemission from the bottom ZnO, so its deep level emissionwas extended. The double layer TiO2/ZnO film showedimproved photocatalytic activity in the degradations ofmethyl orange, which is attributed to its wide wavelengthresponse range, more effective use of irradiation light and theretarded recombination of electron­hole pairs of the twophase film.

Acknowledgment

This work was supported by National Nature ScienceFoundation (Grant No. 50871046), the Foundation of Na-tional Key Basic Research and Development Program(No. 2010CB631001), the Program for Changjiang Scholars.This work also was supported by Innovative Research Team

in University and 2009 Natural Science Basic Research OpenFoundation of the Key Lab of Automobile Materials (09-421060242467), Ministry of Education, Jilin University.

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substrate

Exposure light

TiO2

ZnO

UV emission

Fig. 6 Scheme of the photocatalytic process for TiO2/ZnO film.

L. Zhao et al.468