Characteristics and Optical Properties of Ni Nanograins Reduced on TiO
2Film
Hong-Hsin HUANG, Hung-Peng CHANG1, Fang-Hsing WANG1, Yuan-Shing LIU,Moo-Chin WANG2, and Ding-Fwu LII
Department of Electrical Engineering, Cheng Shiu University, 840 Cheng Ching Rd., Niaosong, Kaohsiung 83347, Taiwan
1Department of Electrical Engineering and Graduate Institute of Optoelectronic Engineering, National Chung Hsing University,
250 Kuo Kuang Rd., Taichung 402, Taiwan
2Faculty of Fragrance and Cosmetics, Kaohsiung Medical University, 100 Shi-Chuan 1st Rd., Kaohsiung 80708, Taiwan
(Received July 12, 2007; accepted October 4, 2007; published online January 22, 2008)
NiO/TiO2films with various NiO film thicknesses ranging from 10 to 320 nm were deposited on silicon and glass substrates
by e-beam evaporation at 200C, and then annealed in H
2atmosphere at 500C for 1 h in order to reduce the NiO film to Ni
grains on the TiO2film. The structures of titanium oxide, NiO, and Ni/TiO2were determined by X-ray diffraction (XRD)
analysis, and the morphology of the Ni/TiO2films was observed by scanning probe microscopy. The ultraviolet–visible (UV–
vis) transmittance and vis absorption of the Ni/TiO2films were measured by UV–vis spectrophotometry. The results showed
that an amorphous titanium oxide was obtained as deposited at 200C and that the structure change to the anatase phase after
500C annealing. As deposited, crystalline NiO films with XRD patterns similar to that of powder were obtained; however,
diffraction peaks of (111) and (200) Ni appeared after annealing in H2atmosphere. Ni nanograins, coarsened grains, and films
were obtained on the TiO2films when the NiO film with a thickness from 10 to 320 nm was reduced in H2atmosphere at
500C. The transmittance of the Ni/TiO
2films decreased with an increase in Ni particle size. The vis absorption measurement
showed that the peak shifted toward a shorter wavelength with a decrease in Ni particle size. [DOI:10.1143/JJAP.47.764]
KEYWORDS: Ni nanoparticle, visible-light absorption shift, transmittance, e-beam evaporation, morphology
1. Introduction
Titanium dioxide (TiO2) films have beneficial physical
and chemical properties and is one of the most important materials in optical, electronic, and chemical fields. It is used as an anti- and high-reflection coating, and as capacitor, solar cell, photocatalytic device,1,2) and hydrophilic
materi-als. New trends in reducing the size of microelectronic devices have enforced the use of high-permittivity oxides to fabricate metal–oxide–semiconductor field-effect transistor (MOSFET) structures with thicker oxides.3)Titanium oxide, zirconium oxide, and hafnium oxides are of immediate interest owing to their high-permittivity factor and compat-ibility with silicon technology.4–6)TiO2is generally
consid-ered to be the best semiconductor photocatalyst available for photocatalysis (PC) at present. Of possible greater signifi-cance are the recent findings of Fujishima et al.7,8)that some
semiconductors also exhibit a photoinduced superhydrophil-ic effect (PSH), i.e., they are rendered more wettable by water after exposure to ultra-band gap irradiation and that this process is reversible, albeit slow, in the dark.
Metal-support interactions can affect both catalyst activity and stability.9)The techniques of metal-doped TiO
2
photo-catalyst preparation, thermal treatment, and doping limita-tion, as well as doped titania properties and photoactivity were reviewed.10) Anpo et al.11) reported that visible light absorption is found in Cr–TiO2films by ion implanting and
the order of the effectiveness in the redshift was found to be V > Cr > Mn > Fe > Ni ions. Such a shift allows the metal ion-implanted titanium oxide to use solar light more efficiently and effectively, at up to 20 – 30%. In the other case, the Cr-ion doped catalyst showed no shift in absorption band but a new absorption shoulder at aproximately 420 nm. The modified method of placing metal on TiO should affect the absorption of visible light.
A Ni/TiO catalyst was prepared by Yan et al.12)for the
partial oxidation of the methane reaction (POM) and it showed high activity and long-term stability in the CO-reforming reaction. Yan et al. reported that CH4 is
effi-ciently converted to CO and H via an oxidation mechanism when Ni/TiO is reduced, and pulse reaction studies provide evidence that the oxidation state of nickel controls the methane activation mechanism and product distribution. Titania-supported nickel catalysts are more active in CO hydrogenations than silica- or alumina-supported cata-lysts.13)Two groups of catalyst have been used for methane activation to generate syngas: Ni-based catalysts14) and noble-based catalysts.15) The high-cost and limited avail-ability of noble metals highlights the need to develop Ni-based catalysts, which can exhibit stability for extended periods.
Titanium oxide thin films have been fabricated by e-beam evaporation,16) reactive and rf sputtering,17,18) thermal
chemical vapor deposition (CVD),19) and plasma-enhanced
CVD20) techniques. E-beam evaporation is a powerful
technique used to prepare thin films at higher deposition rate and with controlled stoichiometry by adjusting the deposition parameters such as substrate temperature, oxygen pressure, and evaporation rate.21,22)In this study, NiO/TiO
films with various NiO film thicknesses were deposited by e-beam evaporation on glass and silicon substrates and then annealed at 500C in H
2 atmosphere for 1 h in order to
obtained Ni nanograins with various sizes on the crystalline TiO film. The structure, morphology, ultraviolet–visible (UV–vis) transmittance, and visible light absorption were studied.
2. Experimental Procedure
The target materials of TiO2 powder (ADMAT MIDAS,
99.99%) and NiO powder (Alfa Aesar, 99%) were formed and sintered at 1200C for 3 h. The substrates were cleaned
with isopropyl alcohol (EPA) and deionized water (DI E-mail address: [email protected]
Japanese Journal of Applied Physics Vol. 47, No. 1, 2008, pp. 764–767
#2008 The Japan Society of Applied Physics
water), and then dried under a flow of nitrogen gas. The substrate was fastened in a curved holder with a working distance of 20 cm and spun at a spin speed of 40 rpm. The substrates were heated using lamps to 200C. A
diffusion-pumped vacuum system with a base pressure of 5 10 6
Torr was used for deposition. The deposition rate was controlled by e-beam power and monitored by a thickness control system (ULVAC CRTM-6000). TiO2thin films with
a thickness of 500 nm were deposited on silicon and glass substrates by e-beam evaporation at 200C, and NiO thin
films with thicknesses ranging from 10 to 320 nm were deposited on the TiO2 film under the same deposition
parameters. Then, the NiO films were annealed at 500C in
H2 atmosphere for 1 h under a heating rate of 10C/min.
The structures of NiO, TiO2, and Ni/TiO2 films were
identified by X-ray diffraction (XRD) analysis with Cu K radiation and a Ni filter, operated at 30 kV, 30 mA, a scanning rate of 4/min, and a 2 in the range of 20 – 80 (Simens D5000). The morphology of the Ni/TiO2films was
observed and analyzed by scanning probe microscopy (SPM; Seiko HV300). The optical transmittance and absorption levels of the Ni nanoparticle/TiO2 thin film and glass
substrate were measured by UV–vis spectrophotometry (HP Agilent 8453) at 190 –1100 nm and (Hitachi U3010) at 200 – 800 nm.
3. Results and Discussion
A titanium oxide film was deposited on glass and silicon substrates by e-beam evaporation at 200C, and its structure
was determined by XRD analysis, as shown in Fig. 1. The analysis revealed that the amorphous titanium oxide was obtained on both the silicon and glass substrates. For thin-film deposition, amorphous titanium oxide is usually obtained at low deposition temperatures. The mobility of the absorbed species is relatively low owing to the low substrate temperature, thus preventing these species from migrating to more energetic sites where nucleation can occur.23)Amor et al.24)reported that the amorphous titanium
oxide film prepared by radio frequency magnetron sputtering was obtained at a lower temperature than 350C although
high-energy ion bombardment was used in a sputtering process, and the unheated films are amorphous regardless of the sputtering parameters.25)The color of the titanium oxide films prepared at room temperature was transparent, but a slight gray color was observed when deposited at 200C.
Although the color of the thin film had changed, the structure of the titanium oxide was still amorphous, as shown in Fig. 1.
The NiO film was deposited on TiO2/silicon or TiO2/
glass by e-beam evaporation and the structure was analyzed by XRD analysis, as shown in Fig. 2. In Fig. 2, the (111), (200), (220), and (311) reflections of the NiO thin film were obtained and the intensity ratio is close to the data of JCPDs card, which reveals that a polycrystalline NiO film with a random orientation was obtained. There was no preferred orientation, which is usually found in thin-film deposition processes.
NiO films were deposited on TiO2/silicon or TiO2/glass
by e-beam evaporation at various thicknesses and then annealed at 500C in H
2atmosphere for 1 h. The structure of
the Ni/TiO2 films determined by XRD analysis is shown in
Fig. 3. Although titanium oxide exists in three different phases, namely, anatase, rutile, and brookite, only amor-phous, anatase, and rutile films have been found for TiO2up
to now.26)The key factors influencing the microstructure of
TiO2films are deposition temperature and annealing
temper-ature.24,27)At a low temperature, TiO
2films with the anatase
phase are more stable and change to the rutile phase at about 800C.24)In Fig. 3, it was found that an anatase TiO
2 film
was obtained, meanwhile, the diffraction peaks maintained nearly constant after being annealed at 500C. The structures of the crystalline TiO2 films are the anatase phase
with reflections of (101), (112), (200), and (204), as shown in Fig. 3. Battiston et al.28)reported that thermal treatment at
500C in air for 3 h is not sufficient for inducing phase
transformation to anatase and a transformation is observed only after 6 h of annealing in air at 500C. Such reluctance
to crystallization is more marked in samples grown at low temperatures. In this study, the TiO2 film was prepared by
e-beam evaporation followed by annealing at 500C for 1 h, Fig. 1. XRD pattern of titanium oxide film deposited on silicon substrate
at 200C.
Fig. 2. XRD pattern of nickel oxide film deposited on titanium oxide at 200C.
Jpn. J. Appl. Phys., Vol. 47, No. 1 (2008) H.-H. HUANGet al.
which is a shorter annealing time than that used by Battiston et al.28) However, sharp diffraction peaks of the anatase
phase were found in Fig. 3. For application as photocatalyst, the anatase phase has the highest photocatalytic activity29) and is a good choice in that it provides an anticorrosion effect under UV illumination. Either the anatase or the rutile phase is usually observed in films prepared by PVD or CVD. In Fig. 3, we can also see that the intensity of the Ni diffraction peaks, i.e., (111), (200), and (220), increases as the thickness of the NiO film increases, which indicates that the particle size of Ni increased with the thickness of the NiO film increasing. Before the thickness of NiO film was higher than 80 nm, the Ni grains joined and began to form a thin film after annealing in H2 atmosphere for 1 h. These
samples with metallic color were observed and a very low resistivity was easily measured.
After annealing, Ni nanograins were reduced from the NiO film on the TiO2 film. The surface morphologies of the
Ni/TiO2 films on the silicon substrates were observed by
SPM, as shown in Fig. 4, and the influence of particle size was studied. In Fig. 4(a), isolated Ni nanograins formed, whose average size was less than 100 nm. It was found that the thicker NiO film, the larger the Ni particle. Furthermore, isolated grains coarsened and contacted each other resulting in island grain formation, and then a Ni film finally formed. In Fig. 4(c), a distinct morphology was found because of nickel film formation. From Fig. 4, it was demonstrated that the Ni nanograins on the TiO2 film could be prepared by
NiO film reduction and the average particle size of Ni could be decided by NiO film thickness.
Titanium oxide films are transparent in the visible region and its transparency exhibits a sharp decrease in the UV region.30)The transmittance of the Ni–TiO
2 films is shown
in Fig. 5, which shows that the transparent limitations are
Fig. 3. XRD patterns of Ni/TiO2 films reduced from NiO/TiO2 films
with NiO film thicknesses of (a) 10, (b) 40, (c) 80, and (d) 160 nm.
Fig. 4. SPM analysis of Ni/TiO2films reduced from NiO/TiO2films with
NiO film thicknesses of (a) 10, (b) 40, and (c) 160 nm.
400
600
800
1000
Wavelength (nm)
0
20
40
60
80
100
Transmittance (%)
(a)
(b)
(c)
(d)
(e)
Fig. 5. UV–vis transmittance of Ni/TiO2films reduced from NiO/TiO2
films with NiO film thicknesses of (a) 10, (b) 20, (c) 40, (d) 80, and (e) 160 nm.
Jpn. J. Appl. Phys., Vol. 47, No. 1 (2008) H.-H. HUANGet al.
nearly constant because the TiO2films deposited in the same
run were annealed at the same temperature of 500C. Martin
et al.31) reported similar results when TiO
2 films were
annealed at a temperature lower than 700C, but the
limitation shifted to a higher wavelength after annealing at 900 and 1100C. Hou and Choy32) reported that the
transparent limitation shifts to longer wavelength because of the rutile/anatase ratio of the films after various-temperature annealing. It can be observed that the shape of the spectra depends not only on the changes in nickel layer thickness, but also on changes in particle size. The transmittance decreased with an increase in Ni particle size, and a very low transmittance was found when the Ni film formed. In Fig. 5, a similar peak shape was found, but these peaks shift to a shorter wavelength with an increase in Ni nanoparticle size. At the same time, the transmittance decreased with an increase in Ni nanoparticle size, because the transparent area was occupied by Ni grains. Yoon et al.33)reported that the
visible-light absorption wavelength moves the shorter wave-length forward because the size of nanometallic particles decreased. According to our previous research, the highest transparent ratio of TiO2films on glass is higher than 90%;
however, the lower transparent ratio was found because of Ni nanograins.
The vis absorption of Ni/TiO2 films is showed in Fig. 6,
which reveals the effect of Ni nanoparticle/TiO2film on the
visible light absorption clearly. We found that the absorption peak shifts to longer wavelengths with Ni grain size increases.
4. Conclusions
Ni nanograins reduced on TiO2 film can be prepared by
depositing NiO/TiO2 films using e-beam evaporation
fol-lowed by annealing in H2 atmosphere at 500C for 1 h. The
Ni particle size increases with an increase in NiO film thickness, and grains coarsened and finally joined to form a thin film when the NiO film thickness is higher than 80 nm.
For transmittance measurement, the peak shifts to a shorter wavelength and this trend was enhanced with a decrease in Ni particle size. For visible-light absorption measurement, the absorption peak shifted toward a longer wavelength with an increase in Ni particle size.
Acknowledgements
The financial support from the Nation Science Council, Taiwan, R.O.C. through a grant (No. NSC93-2216-E-2310-006) is greatly appreciated. The authors also sincerely thank Dr. S. J. Huang and Dr. C. H. Tai for their kind assistance in the transmittance measurements.
1) M. Takahashi, K. Mita, and H. Toyuki: J. Mater. Sci. 24 (1989) 243. 2) K. Ishibashi, Y. Nosaka, K. Hashimoto, and A. Fujishima: J. Phys.
Chem. B 102 (1998) 2117.
3) C. Hobbs, R. Hegde, and B. Maiti: Symp. VLSI Technology Dig. Tech. Pap., 1999, p. 133J.
4) P. S. Peercy: Nature 406 (2000) 1023.
5) A. I. Kingon, J.-P. Maria, and S. K. Streiffer: Nature 406 (2000) 1032. 6) H. Sim, S. Jeon, and H. Hwang:Jpn. J. Appl. Phys. 40 (2001) 6803. 7) A. Fujishima, R. Wang, K. Hashimoto, T. Watanabe, M. Chikuni, E. Kojima, A. Kitamura, and M. Shimohigoshi: Adv. Mater. 10 (1998) 135.
8) A. Fujishima, T. N. Rao, and D. Tryk: J. Photochem. Photobiol. C 1 (2000) 1.
9) M. C. J. Bradford and M. A. Vannice: Appl. Catal. A 142 (1996) 73. 10) M. I. Litter: Appl. Catal. B 23 (1999) 89.
11) M. Anpo, M. Takeuchi, K. Ikeue, and S. Dohshi: Curr. Opin. Solid State Mater. Sci. 6 (2002) 381.
12) Q. G. Yan, W. Z. Weng, H. L. Wan, H. Toghiani, R. K. Toghiani, and C. U. Pittman, Jr.: Appl. Catal. A 239 (2003) 43.
13) M. A. Vannice and R. L. Garten: J. Catal. 56 (1979) 236.
14) D. Dissanayake, M. P. Rosynek, K. C. C. Kharas, and J. H. Lunsford: J. Catal. 132 (1991) 117.
15) C. T. Au and H. Y. Wang: J. Catal. 167 (1997) 337.
16) L.-J. Meng, M. Andritschky, and M. P. dos Santos: Thin Solid Films 223(1993) 242.
17) M. Nakamura, T. Aoki, and Y. Hatanaka: Vacuum 59 (2000) 506. 18) M. Ivanda, S. Music, S. Popovic, and M. Gotic: J. Mol. Struct. 480 –
481(1999) 645.
19) D. H. Lee, Y. S. Cho, T. S. Kim, J. K. Lee, and H. J. Jung: Appl. Phys. Lett. 66 (1995) 815.
20) Y. H. Lee, K. K. Chan, and M. J. Brady: J. Vac. Sci. Technol. A 13 (1995) 596.
21) K. Kawasaki, J. F. Despres, S. Kamei, M. Ishikawa, and O. Odawara: J. Mater. Chem. 7 (1997) 2117.
22) F. Zhang, X. Liu, Y. Mao, N. Huang, Y. Chen, Z. Zheng, Z. Zhou, A. Chen, and Z. Jiang: Surf. Coat. Technol. 103 –104 (1998) 146. 23) X. Hou and K.-L. Choy: Surf. Coat. Tech. 180 –181 (2004) 15. 24) S. B. Amor, L. Guedri, G. Baud, M. Jacquet, and M. Ghedira: Mater.
Chem. Phys. 77 (2002) 903.
25) S. B. Amor, G. Baud, M. Jacquet, and N. Pichon: Surf. Coat. Technol. 102(1998) 63.
26) P. Lo¨bl, M. Huppertz, and D. Mergel: Thin Solid Films 251 (1994) 72. 27) Y.-Q. Hou, D.-M. Zhuang, G. Zhang, M. Zhao, and M.-S. Wu: Appl.
Surf. Sci. 218 (2003) 98.
28) G. A. Battistona, U. R. Gerbasia, A. Gregoria, M. Porchiaa, S. Cattarinb, and G. A. Rizzic: Thin Solid Films 371 (2000) 126. 29) B. Ohtani, Y. Ogawa, and S. I. Nishimoto: J. Phys. Chem. B 101
(1997) 3746.
30) D. Mardare, M. Tasca, M. Delibas, and G. I. Rusu: Appl. Surf. Sci. 156 (2000) 200.
31) N. Martin, C. Rousselot, D. Rondot, F. Palmino, and R. Mercier: Thin Solid Films 300 (1997) 113.
32) X. Hou and K.-L. Choy: Surf. Coat. Technol. 180 –181 (2004) 15. 33) J. W. Yoon, T. Sasaki, and N. Koshizaki: Electrochem. Solid-State
Lett. 5 (2002) A256. Fig. 6. Vis absorption levels of Ni/TiO2films reduced from NiO/TiO2
films as function of NiO film thickness.
Jpn. J. Appl. Phys., Vol. 47, No. 1 (2008) H.-H. HUANGet al.
