Surface & Coatings Technology 205 (2011) 2919–2923

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Surface & Coatings Technology

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Enhanced visible-light absorption from Ag2O nanoparticles in nitrogen-doped TiO2

thin films

Feng Fang a,b,⁎, Qi Li b, Jian Ku Shang b

a Jiangsu Key Lab for Advanced Metallic Materials, Southeast University, Nanjing 211189, Chinab Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA

⁎ Corresponding author. Jiangsu Key Lab for AdvanceUniversity, Nanjing 211189, PR China. Tel.: +86 25 520

E-mail address: fangfeng@seu.edu.cn (F. Fang).

0257-8972/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.surfcoat.2010.10.068

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 July 2010Accepted in revised form 29 October 2010Available online 4 November 2010

Keywords:Silver nanoparticlesNitrogen-doped TiO2

Thin filmVisible-light absorption

Ion-beam-assisted deposition was used to prepare TiON thin film with ultrafine Ag2O semiconductornanoparticles. These Ag2O nanoparticles were about 3 nm. The obtained thin films showed an obvious “red-shift” in the optical absorption spectrum. The absorption in the visible-light range is resulting from theabsorption by metallic-like nanoparticles as silver changed its chemical state from Ag+ to Ag in nitrogen-doped titanium oxide matrix under visible-light illumination.

d Metallic Materials, Southeast90630; fax: +86 25 52090634.

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Titanium dioxide (TiO2) thin film has been extensively investigat-ed as a photocatalyst since the discovery of its photosensitizationeffect by Honda and Fujishima in 1972 [1]. Its strong photo-oxidizingpotential, high chemical stability, nontoxicity, etc., havemotivated thestudy for environmental applications ranging from deodorization tothe purification of air and water [1–8]. However, pure TiO2 has verylow photocatalytic efficiency outdoors, where only 5% of the solarenergy in the ultraviolet range is capable of activating the photo-catalytic reaction, due to its wide energy band gap (3.2–3.8ev). To usethe solar energy effectively, it is essential to extend the absorptionspectrum of TiO2 into the visible-light region, where a much higherproportion (45%) of the solar energy may be used. To address thisrequirement, early studies had focused on alloying TiO2 withtransition metals [2] and doping TiO2 with carbon [3], sulfur [4],fluorine [5], or nitrogen [6–10]. Research results showed that nitrogendoping TiO2 had an excellent effect on extending the absorptionspectrum into the visible-light region.

The assembly of metal or semiconductor nanoparticles intodielectric thin films has been investigated extensively [11–14]because of their unique optical and electrical properties comparedwith their counterparts in bulk forms. Noble-metal nanoparticles,such as Au and Pd, are known to exhibit characteristic opticalabsorption in the UV–visible region caused by the surface plasmonresonance (SPR) originating from collective oscillations of free

electrons [15–21]. For semiconductor nanoparticles, their opticalspectra may display a quantum size effect. As semiconductornanoparticles decrease in size, their excitation energy generallyincreases, resulting in a shift of their absorption band to a shorterwavelength region (“blue-shift”). Ag2O is a noble-metal oxide ofwidespread technological interests, whose reported band gap rangesfrom 0.49 to 3.1 eV [22–24]. As the Ag2O nanoparticle size goes downto a couple of nanometers, a “red-shift” may occur in the opticalabsorption spectrum because of the quantum size effect.

The present study focused on the improvement of the utility ofsolar energy of TiO2 thin film. With the help of ion-beam-assisteddeposition (IBAD) technique [25], we prepared nitrogen-doped TiO2

thin film with ultrafine Ag2O semiconductor nanoparticles andattempted to determine if Ag2O nanoparticles might have effectedon the “red-shift” in the optical absorption spectrum of TiO2 (TiON)thin film.

Ag2O/TiON and Ag2O/TiO2 thin films were deposited by electronbeam evaporation of TiO2/Ag2O pellet with/without simultaneouslynitrogen ions bombarding the growing film, respectively. Forcomparison purpose, TiON and TiO2 thin films were also preparedby electron beam evaporation of pure TiO2 pellet with/withoutnitrogen ion bombardment, respectively, under the same experimen-tal conditions.

2. Experimental procedure

The ion-beam-assisted deposition (IBAD) system used in thisstudy consisted of a 3-cm-diameter Kaufman-type ion source, a 3-kWelectron-beam evaporator, and a substrate holder with a heater madeof tantalum, as shown schematically in Fig. 1. A quartz-crystal monitor

Fig. 1. Schematic diagram of the ion-beam-assisted deposition (IBAD) system.

200

300

400

A

ount

s

A-Anatase

2920 F. Fang et al. / Surface & Coatings Technology 205 (2011) 2919–2923

was used to control the film thickness. The film thickness wasconfirmed after deposition by Gaertner Ellipsometer (Model L116c,Wavelength: 633 nm). Micro-slides glasses were used as substrate inthis study. They were ultrasonically cleaned in acetone and methanolbaths successively and blow dried under nitrogen purge. Prior todeposition, the substrate was cleaned by 800 eV Ar ions for 5 min toremove surface contamination. Nitrogen-doped TiO2 was depositedby electron-beam evaporation of TiO2, anatase, 99.9% (Alfa Aescar,USA). Simultaneously, the growing TiO2 film was bombarded bynitrogen ions. During deposition, the nitrogen partial pressure wascontrolled at a stable value at 1.0×10−4 Torr. The deposition wascarried out at a substrate temperature of 400 °C, which was chosenbased on the previous report that TiON film tended to have anatasestructure around this temperature[19,21]. The deposition conditionsare listed in Table 1. After the deposition, the thin film was heated toabout 150 °C and was bombarded by ions (nitrogen 50% andoxygen 50%) for 3 min and then holding at 80 °C for 20 min in oxygenatmosphere.

The structures of the TiON film were characterized by X-raydiffraction (XRD) measurements on a Philips Exert diffractometer,using Cu Ka radiation (45 kV, 40 mA). Transmission electron micros-copy (TEM) cross-sectional images and electron diffraction (ED)patterns of the films were taken by a JEOL 2100 Cryo transmissionelectron microscope (TEM) at 200 kV. X-ray photoelectron spectros-copy (XPS) spectra were obtained on a Physical Electronics PHI 5400X-ray photoelectron spectrometer with a Mg Ka anode (15 kV,300 W) at a take-off angle of 45°. Binding energies have beencorrected by using adventitious carbon (C 1s at 284.6 eV) as reference[26,27]. The films' surface morphology was examined by atomic forcemicroscope (AFM) (Digital Instruments Dimension 3100). The optical

Table 1Ion-beam-assisted deposition conditions.

Substrate temperature (°C) 400Ion acceleration voltage (V) 150–550Ion-beam voltage (V) 500Ion incident angle to the substrate 45°Base pressure (Torr) 4.0×10−7

Working pressure (Torr) 1.0×10−4

Film thickness (nm) about 200

properties were determined by measuring optical transmittance byusing a Cary 5G UV–Vis–NIR spectrophotometer at room temperature,in the wavelength range 325–800 nm.

3. Results and discussion

Fig. 2 is the XRD pattern of Ag2O/TiON thin film obtained by IBADtechnique. The pattern shows that an anatase phase exists in the film.A very weak peak belonging to Ag2O is observed in this pattern, whichsuggests that Ag dopant exists as Ag2O in the obtained film at a verysmall quantity.

Fig. 3 shows the curve of XPS high-resolution scan over Ti(2p) andAg(3d) peaks on Ag2O/TiON thin film. There are twomain peaks in theTi(2p) binding energy region [Fig. 3a]. The binding energy of Ti 2p3/2 is459.0 eV and Ti 2p1/2 is 464.6 eV. The slitting between Ti(2p3/2) andTi(2p1/2) is 5.6 eV, which indicates that a normal state of Ti4+ in theanantase TiO2 film [28]. The binding energy of Ag(3d5/2) is 367.80 eVand Ag(3d3/2) is 361.80 eV [Fig. 3b], which indicates clearly that Agdopant mainly exists as Ag2O in the obtained film. The same resultsobtained from Ag2O/TiO2 thin film.

With the help of XPS, the relative element composition ratio ofthin films was determined by multiplex high-resolution scans overthe N 1s, O 1s, Ag 3d, and Ti 2p spectral regions. Experimental resultsindicate that the average N/Ti atomic ratio was about 0.40 (the samefor TiON thin film), and Ag/Ti atomic ratio was about 0.03 (the samefor Ag2O/TiO2 thin film).

Fig. 4 shows the AFM images of the Ag2O/TiON and Ag2O/TiO2 thinfilm. The obtained Ag2O/TiON thin film has a dense surface and thegrain size in the film is 20 nm [Fig. 4a], while the grain size of theAg2O/TiO2 thin film is 70 nm [Fig. 4b].

Fig. 5a and b is the Z-contrast images of Ag2O/TiON and Ag2O/TiO2

thin films by using the high-angle annular dark field (HAADF)scanning transmission electron microscopy (STEM) technique. Bothof them show similar structures with bright nanoparticles embeddedin the dark matrix. Fig. 5c shows the energy dispersive X-ray analysis(EDS) spectra from the bright particles in Ag2O/TiON thin film. Theresult shows that the bright particles were mainly contained silverelement. Combining XPS analyses result, experimental results showthat Ag exists as Ag2O [bright particles in Fig. 5a] in a highly dispersedstate in TiON film matrix [dark matrix in Fig. 5a]. A similar result wasobtained for Ag2O/TiO2 thin film, except that its matrix is TiO2.

Although Ag2O/TiON and Ag2O/TiO2 thin films have similarstructures, an obvious difference on their Z-contrast images exists.The average size of the individual Ag2O nanoparticles in the TiONmatrix is 3 nm, while the individual Ag2O nanoparticles in the TiO2

matrix are about one order of magnitude larger, 20 nm. Because of

10 20 30 40 50 60 700

100 Ag2O AAA

A

C

2θ

Fig. 2. X-ray diffraction pattern of Ag2O/TiON thin film obtained by IBAD technique.

Fig. 3. XPS high-resolution scan over Ti(2p) and Ag(3d) peaks on Ag2O/TiON thin film. (a) Ti(2p) peak; (b) Ag(3d) peak.

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their similar Ag2O concentrations, the number of Ag2O nanoparticlesin Ag2O/TiON is far greater than that in Ag2O/TiO2 thin films, and thedistance between Ag2O nanoparticles in Ag2O/TiON is much shorterthan that in Ag2O/TiO2 thin films. Since Ag2O/TiO2 thin film wasdeposited at the same condition as Ag2O/TiON thin film except for the

Fig. 4. Atomic force microscopic (AFM) images of Ag2O/TiON

Fig. 5. HAADF STEM image of Ag2O/TiON and Ag2O/TiO2 thin films, respectively. (a) Ag2O/TiOTiON thin film.

nitrogen ion bombardment, our work demonstrates that ion beambombardment is beneficial in minimizing the embedded Ag2Onanoparticle size to no larger than a few nanometers.

Fig. 6 shows the light absorption spectra of Ag2O/TiON and Ag2O/TiO2 thin films collected by using a Varian Cary 5G spectrophotometer

and Ag2O/TiO2 thin film. (a) Ag2O/TiON; (b) Ag2O/TiO2.

N thin film; (b) Ag2O/TiO2 thin film; (c) EDS spectra from the bright particles in Ag2O/

Fig. 6. Optical absorbance (in terms of arbitrary units) of TiO2, TiON, Ag2O/TiON, andAg2O/TiO2 thin films.

Fig. 7. In situ XPS high-resolution scan over Ag 3d peaks on Ag2O/TiON thin film in darkand after 2-h visible-light illumination.

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in transmission mode. For comparison, the light absorption spectra ofTiON and TiO2 thin films are listed. The small fluctuations of theabsorption spectra came from interferences occurring at the interfaceof the thin film and the glass substrate. As expected, TiO2 thin filmabsorbs only UV light (wavelength λb400 nm) [1], whose energyexceeds the relatively wide band gap of 3.2 eV in the TiO2 anatasecrystalline phase. Ag2O/TiO2 thin film shows a very similar lightabsorbance behavior as TiO2 thin film. This similarity indicates thatthe embedded Ag2O nanoparticle does not affect the light absorbanceof the TiO2 matrix in a significant manner. On the other hand, TiONthin film shows a noticeable light absorption ability in the visible-lightregion (400 nmbλb500 nm) due to the substitutional doping of Nand the subsequent band gap narrowing, as Asahi et al. reported [6].The TiON thin film still shows light absorption ability in the visible-light region (λN500 nm) due to some defects produced during thenitrogen ion bombardment process. Most strikingly, Ag2O/TiON thinfilm shows a much higher absorption in the visible-light range thanTiON thin film. Its light absorbance spectrum extends to over 700 nm,covering most of the visible-light region. From the Tauc plot [29], theband gap of the TiO2 thin film is determined to be 3.27 eV and that ofthe Ag2O/TiO2 thin film is 3.20 eV. TiON thin film has a smaller bandgap at 3.00 eV, which is in accordance with its visible-lightabsorbance ability as reported previously [6]. For Ag2O/TiON thinfilm, its Tauc curve is mostly nonlinear and does not show a clearlinear Tauc region, which suggests that a mechanism other than bandgap narrowing may have caused the red-shift in optical absorptionspectrum and the much better absorption in the visible-light range.

To understand such an enhanced visible-light absorption observedin Ag2O/TiON thin film, an in situ XPS study was conducted toexamine potential changes in the valence states of the chemicalspecies. The sample was first placed in the dark for 8 h right beforeXPS high-resolution scans over the Ag 3d peak in the dark. Then, XPSscans were performed with a visible-light illumination on the samplesimultaneously after 2 h. The result of the comparison was showed inFig. 7.

Differences on the Ag 3d peak shape and position were observedbetween scans in the dark and under visible-light illumination. Forcomparison purpose, the same in situ XPS study was also conductedon the Ag2O/TiO2 sample. In the dark, the binding energy of Ag 3d5/2 is367.80 eV, indicating that Ag dopant exists as Ag2O. Under visible-light illumination, however, Ag peaks are broadened and the bindingenergy of Ag 3d5/2 is shifted to 368.20 eV. The broad Ag 3d5/2 is bestfitted as a combination of Ag2O 3d5/2 peak at 367.80 eV and Ag 3d5/2peak at 368.20 eV. It demonstrates that a part of Ag2O nanoparticlesin Ag2O/TiON thin film has been reduced to metallic Ag nanoparticles

under visible-light illumination. The change in the valence state of Agis consistent with the notion that metal ion dopants in TiO2 can act aselectron traps to alter the electron-hole pair recombination rate [30–32]. However, Ag2O/TiO2 sample shows no change in Ag 3d peakshape and position under the same visible-light illuminationcondition. The binding energy of Ag 3d5/2 in Ag2O/TiO2 sample is367.80 eV, indicating that Ag dopant exists as Ag2O in the dark orunder visible light illumination.

We can understand the optical shift in Ag2O/TiON thin film fromthe chemical state transition of Ag2O nanoparticles. Under visible-light illumination, TiON thin film can produce electron-hole pairs. Theelectrons can move from TiON into Ag2O nanoparticles due to theirconduction band offset. Because of their ultrafine size (about 3 nm), arelatively large volume of Ag2O nanoparticles in Ag2O/TiON wouldquickly be populated with electrons, which then reduce these Ag2Osemiconductor nanoparticles to metallic Ag nanoparticles as seen inthe in situ XPS study. These metal nanoparticles can create the SPRoriginating from collective oscillation of free electrons, whichenhances the visible-light absorption of Ag2O/TiON thin film. In thisregard, it is not just the Ag2O nanoparticles that determine Ag2O/TiONthin film properties but also the interaction between these nanopar-ticles and TiON matrix as described above. On the other hand, TiO2

thin film absorbs only UV light and the diameter of Ag2O nanoparticlesin Ag2O/TiO2 is about one magnitude larger than that of Ag2Onanoparticles in Ag2O/TiON. Thus, no significant interaction should beexpected.

4. Conclusions

Ion-beam-assisted deposition was used to prepare ultrafine Ag2Osemiconductor nanoparticles into TiON thin film. The obtained Ag2O/TiON thin film has a dense surface and the grain size in the film isabout 20 nm, and the average size of Ag2O nanoparticles is about3 nm, while the grain size of the Ag2O/TiO2 thin film is about 70 nmand the average size of Ag2O nanoparticles is about 20 nm. Thechemical state transition of silver in Ag2O/TiON thin film results in anobvious optical red-shift in its absorption spectra. In the Ag2O/TiO2

films, no significant chemical state transition occurs and no clearoptical shift was observed from the Ag2O nanoparticles.

Acknowledgments

The authors would like to thank C. H. Lei in the Frederick SeitzMaterials Research Laboratory, University of Illinois at Urbana-Champaign, for the help on STEM. This work was supported by the

2923F. Fang et al. / Surface & Coatings Technology 205 (2011) 2919–2923

Center of Advanced Materials for the Purification of Water withSystems, National Science Foundation (grant no. CTS-0120978) andwas funded by the China Scholarship Council (grant no. 2006A45003)and Jiangsu Province University—Industry Cooperation Project (grantno. BY2009154). XPS and XRD measurements were carried out at theFrederick Seitz Materials Research Laboratory, University of Illinois atUrbana-Champaign, which is partially supported by the U.S. Depart-ment of Energy under grant no. DEFG02-91-ER45439.

References

[1] A. Fujishima, K. Honda, Nature 37–38 (1972) 238.[2] V. Subramanian, E. Wolf, P.V. Kamat, J. Phys. Chem. B 11439–11446 (2001) 105.[3] S. Khan, M. Al-Shahry, W. Ingler, Science 2243–2245 (2002) 297.[4] T. Umebayashi, T. Yamaki, S. Tanaka, et al., Chem. Lett. 772–773 (2003) 32.[5] J.C. Yu, J.G. Yu, W.K. Ho, et al., Chem. Mater. 3808–3816 (2002) 14.[6] R. Asahi, T. Morikawa, T. Ohwaki, et al., Science 269–71 (2001) 293.[7] S. Sakthivel, H. Kisch, Chem. Phys. Chem. 487 (2003) 4.[8] C. Burda, Y. Lou, X. Chen, et al., Nano Lett. 1049 (2003) 3.[9] P. Romero-Gomez, A. Palmero, F. Yubero, et al., Scr. Mater. 574–577 (2008) 60.

[10] H. Huang, L. Jiang, W.K. Zhang, et al., Sol. Energy Mat. Sol. Cells. 355–359 (2010)94.

[11] M.-J. Ko, M. Birnboim, J. Plawsky, Adv. Mater. 909–913 (1997) 9.[12] B. Palpant, B. Prevél, J. Lermé, et al., Phys. Rev. B 1963–1970 (1998) 57.[13] H. Amekura, N. Kishimoto, J. Appl. Phys. 2585–2589 (2003) 94.[14] J. Okumu, C. Dahmen, A.N. Sprafke, et al., J. Appl. Phys. 094305 (2005) 97.[15] R.G. Freeman, K.C. Grabar, K.J. Allison, et al., Science 1629–1632 (1995) 267.[16] H. Xu, E.J. Bjerneld, M. Kall, et al., Rev. Lett. 4357–4360 (1999) 83.[17] R.F. Aroca, P. Goulet, D. Santos, et al., Anal. Chem. 378–382 (2005) 77.[18] Q. Li, W. Liang, J.K. Shang, Appl. Phys. Lett. 0631091–3 (2007) 90.[19] JiningGao , XianglingRen , DongChen , et al., Scr. Mater. 687-690 (2007) 57.[20] J.W. Yoon, T. Sasaki, N. Koshizaki, et al., Scr. Mater. 1865–1868 (2001) 44.[21] A.P. Alivisatos, J. Phys. Chem. 13226–13239 (1996) 100.[22] A.J. Varkey, A.F. Fort, Sol. Energy Mater. Sol. Cells 253–259 (1993) 29.[23] D.A. Kudryashov, S.N. Grushevskaya, A.V. Vvedenskii, Prot. Met. 591–599 (2007)

43.[24] ZhangXi-Yao , PanXin-Yu , ZhangQi-Feng , et al., Acta Phys. Chim. Sin. 203-207

(2003) 19.[25] P.G. Wu, C.H. Ma, J.K. Shang, Appl. Phys. A 1411–1417 (2005) 81.[26] O. Akhavan, J. Colloid Interface Sci. 117–124 (2009) 336.[27] WenyanLi. , Sudipta Seal, EdwardMegan , et al., J. Appl. Phys. 93 (2003) 9553.[28] O. Akhavan, E. Ghaderi, Surf. Coat. Technol. 3676–3683 (2010) 204.[29] J. Tauc, R. Grigorovici, A. Vancu, Phys. Status Solidi. 627–637 (1966) 15.[30] O. Akhavan, E. Ghaderi, J. Phys. Chem. C 113 (2009) 20214.[31] W. Choi, A. Termin, M.R. Hoffmann, J. Phys. Chem. 13669–13679 (1994) 98.[32] S.I. Shah, W. Li, C.-P. Huang, O. Jung, et al., Proc. Natl Acad. Sci. USA 6482–6486

(2002) 99.