NANO LETTERS
Zirconia−Titania Nanofilm with Composition Gradient
2002 Vol. 2, No. 6 669-672
Jianguo Huang,† Izumi Ichinose,† Toyoki Kunitake,*,† and Aiko Nakao‡ Topochemical Design Laboratory, Spatio-Temporal Function Materials Research Group, Frontier Research System, and Surface Characterization DiVision, Characterization Center, The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198, Japan Received April 3, 2002; Revised Manuscript Received April 25, 2002
ABSTRACT Zirconia−titania hybrid films with composition gradient along film normal were successfully synthesized with nanometer precision via the surface sol−gel process and low-temperature oxygen plasma treatment. The polymer component was removed by plasma etching from (zirconia/ PAA)(titania/PAA) nanocomposite films to give wormlike random mesopores with diameters of around 2 nm, as confirmed by TEM. The nanogradients of zirconia and titania components were preserved for PAA-hybrid films based on XPS measurement, but it was not the case without PAA.
Metal oxide thin films with compositional gradients normal to the growth surface will lead to a large variety of applications in functional devices and surface coating because of their unique mechanical, electrical, and optical properties that are not observed in uniform films.1-3 Compositionally graded BaxSr1-xTiO3 (BST) films with thicknesses of 1.2 µm were synthesized on platinum substrates by successive deposition of films, followed by annealing in flowing oxygen at 1000 °C.4 Similar BST graded films were fabricated on silicon using unbalanced magnetron sputter deposition.5 A modified sol-gel technique was developed to prepare 0.50.6 µm thick gradient (Pb,Ca)TiO3 (PCT) thin films on metal oxide electrodes and platinum-coated silicon substrates, and the compositional gradient was improved by annealing at 550-650 °C.6,7 Pulsed laser deposition was another technique for growing compositionally varied multilayer structures, and 0.3 µm thick Pb(Zr,Ti)O3 (PZT) graded films were deposited on platinum-coated silicon at 600 °C.8 Such compositionally graded films have never been achieved at the nanometer precision, despite their intrinsic importance in electronic devices. We have shown that tightly complexed layers of metal oxides and hydroxyl polymers, as illustrated by titania/poly(acrylic acid) (PAA) nanocomposite films, can be easily formed by the layer-by-layer (LbL) sol-gel process from metal alkoxide and polymer solutions.9 In the present research, a zirconia-titania nanoporous film with the composition gradient normal to the substrate was prepared at room temperature by combining the surface sol* To whom correspondence should be addressed. Tel: +81-48-467-9601. Fax: +81-48-464-6391. E-mail:
[email protected]. † Topochemical Design Laboratory. ‡ Surface Characterization Division. 10.1021/nl0255653 CCC: $22.00 Published on Web 05/10/2002
© 2002 American Chemical Society
gel technique9-11 and low-temperature oxygen plasma treatment.12-14 Titania and zirconia were chosen for our purpose because of their wide applications in photovoltaics and catalysis. The preparative procedure is illustrated schematically in Figure 1. Zirconia/PAA layers are first deposited with molecular precision onto the surface of given solid substrates, and titania/PAA layers are deposited subsequently. It is expected that low-temperature oxygen plasma treatment produces a compositional gradient with high titania in the upper part of the film and high zirconia at the lower part. The zirconia layer was deposited by immersing a solid substrate in 20 mM Zr(OnBu)4 in 1:1 (v/v) toluene/ethanol for 1 min, followed by rinsing in ethanol for 1 min and hydrolysis in pure water for 1 min, and finally dried by flushing with nitrogen gas.9 Deposition of a titania layer was similarly conducted, except that the concentration of Ti(OnBu)4 was 100 mM and the immersion time was 3 min. Adsorption of the PAA layer was achieved by 10 min immersion of the solid substrate in 1 mg/mL PAA in ethanol, followed by rinsing with ethanol for 1 min and drying with nitrogen gas.9 Oxygen plasma treatment was carried out on a PE-2000 Plasma Etcher (South Bay Technology, RF 13.56 MHz). The as-prepared sample was placed directly on the lower RF electrode, and the reactor was evacuated to 75 mTorr. Subsequently, oxygen gas was introduced into the reactor to a pressure of 176 mTorr, and the plasma was ignited with an RF power of 10 W. The treatment time was varied depending on the sample. Figure 2 shows QCM (quartz crystal microbalance) frequency shifts during the successive deposition of 7-cycle
Figure 1. Schematic diagram illustrating the formation of zirconia-titania gradient nanofilm.
Figure 2. QCM frequency shifts (-∆F) with film deposition of (ZrO2/PAA)7(TiO2/PAA)7 [Zr(OnBu)4, 20 mM in 1:1 v/v toluene/ ethanol, 25 °C (b); Ti(OnBu)4, 100 mM in 1:1 v/v toluene/ethanol, 25 °C (1); poly(acrylic acid), 1 mg/mL in ethanol, 25 °C (O)] and frequency shift after oxygen plasma treatment [(3) RF power 10 W, 10 min].
zirconia/PAA layers and 7-cycle titania/PAA layers (denoted as (ZrO2/PAA)7(TiO2/PAA)7 hereafter) onto the surface of a 2-mercaptoethanol modified, gold-coated QCM resonator. Regular film growth proceeds, as seen from the almost linear frequency decrements during film fabrication. In the current setup, 1 Hz frequency decrement corresponds to a thickness increment of 0.273/F Å, where F is the density of the adsorbed film.9,15,16 Using the bulk densities of metal oxide gel and PAA,9 the thickness of the individual zirconia/PAA layer and titania/PAA layer is calculated to be 2.1 ( 0.5 and 0.9 ( 0.3 nm, respectively. The thickness of the whole film is thus estimated to be around 20 nm. When the film assembly on the QCM electrode was subjected to oxygen plasma treatment, its frequency was found to increase, indicating mass lost from the resonator surface. As shown in Figure 2, the QCM frequency increased by 354 Hz upon plasma treatment of the (ZrO2/PAA)7(TiO2/PAA)7 film. This shift agrees with the frequency decrement of 341 Hz due to PAA adsorption in the as-prepared film. For a titania film prepared by means of stepwise adsorption of titanium alkoxide alone, such a frequency increase (mass loss) was 670
not observed upon plasma treatment. It is inferred that plasma etching removed the mass corresponding to the adsorbed PAA. The removal of PAA was also confirmed by disappearance of the CdO stretching vibration of a free carboxyl group (1720 cm-1) and of vibrational bands of titanium carboxylate (1560, 1450, and 1410 cm-1) in FTIR spectra and by disappearance of the C(1s) peak corresponding to COOH (288 eV, relative to standard C(1s) at 285.0 eV) in XPS spectra of the as-prepared film. The plasma etching is mainly caused by breaking of C-C and C-H bonds due to active species in the plasma (O+, O-, O2+, O2-, O, O3, ionized ozone, metastably excited O2, free electrons, etc.).12 The gas product from polymer decomposition, such as CO2, CO, H2O, and hydrocarbons of low molecular weights, is released from the sample and leaves a thin film of metal oxide alone. Figure 3a shows angle-resolved XPS measurements of the Ti/Zr atomic ratio in the film before and after oxygen plasma treatment. A VG-ESCALAB 250 spectrometer was employed using Al KR X-rays (anode operated at 15 kV, 20 mA) with the analysis chamber pressure less than 8-10 Pa. The majority of the detected signal originates from a depth of 3λsinR from the surface, where λ is the photoelectron mean free path in the sample and R is the takeoff angle (the angle between the detector lens axis and the film surface).17 Thus, the band intensities depend on the depth profile of individual elements in the film,18 and quantitative information on the depth composition not exceeding about 10 nm is obtainable.17,18 For the as-prepared (ZrO2/PAA)7(TiO2/PAA)7 film, the Ti/ Zr ratio was 26 at a takeoff angle of 5°, and the ratio decreased to 3.3 at a takeoff angle of 90°. These data indicate that the upper part of the film is mainly composed of titania, and the lower layer contains more zirconia, in agreement with the deposition procedure. This characteristic is lessened for the plasma-treated film, as also shown in Figure 3a. The Ti/Zr ratio decreased remarkably at each takeoff angle compared with those of the as-prepared one. The ratio is 2.5 at a takeoff angle of 5° (as opposed to 26 in the asNano Lett., Vol. 2, No. 6, 2002
Figure 3. Angle-resolved XPS measurements of the Ti/Zr atomic ratio for as-prepared (b) and oxygen-plasma-treated (O) nanocomposite films: (a) (ZrO2/PAA)7(TiO2/PAA)7; (b) (ZrO2/PAA//TiO2/PAA)5; (c) (ZrO2)7(TiO2)7. The inset in (a) shows a magnified curve of the plasma-treated film.
prepared film) and decreases to 1.1, 1.0, 0.94, and 0.95 at takeoff angles of 45°, 60°, 75°, and 90°, respectively. This result is clearly derived from diffusion of the zirconia component to the upper layer and the titania component to the lower layer upon oxygen plasma treatment. The molar ratio of TiO2 and ZrO2 is estimated to be 0.56 in the film on the basis of the QCM frequency decrement during film fabrication (Figure 2). This ratio is smaller than the XPS ratio of 0.95 at 90° takeoff angle. XPS measurement is highly depth-sensitive and the elemental composition in the upper layer is still emphasized. In any case, the existence of the compositional gradient along the film normal cannot be doubted. A UV-vis absorption spectrum of a plasma-treated (ZrO2/PAA)7(TiO2/PAA)7 film deposited on a quartz plate did not show the characteristic band near 250 nm that is Nano Lett., Vol. 2, No. 6, 2002
known for nanosized titania layers. In contrast, this peak was clearly observed for a pure titania ultrathin film that resulted from plasma treatment of a (TiO2/PAA)n film. These spectroscopic features reveal that there do not exist extended titania-alone layers in the former nanofilm. The composition gradient was not found in the films that were formed by alternate deposition of individual zirconia/ PAA and titania/PAA layers. Figure 3b shows the Ti/Zr ratio as a function of takeoff angle for a film of five alternate cycles of zirconia/PAA and titania/PAA film (denoted as (ZrO2/PAA//TiO2/PAA)5) before and after plasma treatment. The ratios are 0.3-0.4 at all the takeoff angles, and this value agrees well with the result of QCM frequency measurement during the film deposition process. The frequency decrement due to titania and zirconia adsorption was 222 and 1036 Hz, 671
respectively, and the molar ratio of TiO2 and ZrO2 is thus estimated to be 0.33. It is clear that a nongradient homogeneous zirconia-titania binary nanofilm is formed in this case. We have also examined the depth profile of a titania/ zirconia binary film that was prepared without PAA. Its composition gradient after oxygen plasma treatment is not pronounced. Figure 3c displays angle-dependent XPS data of a metal oxide gel film composed of seven lower layers of zirconia and seven upper layers of titania (denoted as (ZrO2)7(TiO2)7, PAA not included). Compared with the corresponding PAA-containing films (Figure 3a), the angle dependence, as well as the Ti/Zr ratio at each takeoff angle, is much smaller for both as-prepared and plasma-treated samples. Metal oxide gel films prepared without PAA show rough surfaces, as observed by scanning electron microscopy (SEM). Therefore, the zirconia and titania layers are partly mixed without complete separation in the prepared sample, as schematically illustrated in Figure 3c. The Ti/Zr curves of the as-prepared and plasma-treated films are close, suggesting that plasma treatment did not alter the inner film structure much, and a dense film composed of zirconia and titania layers resulted. Our previous study showed that metal oxides and PAA produce tightly bound alternate layers, giving flat and uniform film surfaces. 9 In the current work, effective separation of the zirconia and titania layers is achieved only in the presence of PAA layers. The PAA layers are required to avoid mixing of zirconia and titania layers in the as-prepared oxides/PAA film (Figure 3a) and must play a key role in the formation of composition gradients. We have found that removal of PAA from a titania/PAA nanocomposite film by oxygen plasma treatment results in a mesoporous titania film. In contrast, a titania film prepared without PAA does not show such mesoporosity. Transmission electron microscopy (TEM) was employed to probe the inner morphology of the (ZrO2/PAA)7(TiO2/PAA)7 film before and after plasma treatment. The film was fabricated on quartz plates and then scraped off in ethanol, and a dispersion of the scraped flakes was placed on a carboncoated copper grid and left to dry in air. As shown in Figure 4, the as-prepared film is uniform and dense, without any porosity; whereas wormhole-like, interconnected channels with diameters of around 2 nm are observed in the plasmatreated film. Thus, removal of PAA by plasma treatment leads to the formation of nanopores. In conclusion, we have developed an effective, mild method for making compositionally graded metal oxide films with nanometer thickness. This approach should be ap-
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Figure 4. TEM micrographs of (ZrO2/PAA)7(TiO2/PAA)7 nanocomposite film before (a) and after (b) oxygen plasma treatment. The film edge is selected here to make the comparison more apparent. TEM images were acquired on a JEOL JEM-2000 at an acceleration voltage of 100 kV.
plicable to various metal oxides. The synthesis can be integrated with silicon technology and can be of importance in nanotechnology. References (1) Schubring, N. W.; Mantese, J. V.; Micheli, A. L.; Catalan, A. B.; Lopez, R. J. Phys. ReV. Lett. 1992, 68, 1778. (2) Mantese, J. V.; Schubring, N. W.; Micheli, A. L.; Catalan, A. B.; Mohammed, M. S.; Naik, R.; Auner, G. A. Appl. Phys. Lett. 1997, 71, 2047. (3) Bull, S. J. J. Vac. Sci. Technol. A 2001, 19, 1404. (4) Mohammed, M. S.; Auner, G. W.; Naik, R.; Mantese, J. V.; Schubring, N. W.; Micheli, A. L.; Catalan, A. B. J. Appl. Phys. 1998, 84, 3322. (5) Jin, F.; Auner, G. W.; Naik, R.; Schubring, N. W.; Mantese, J. V.; Catalan, A. B.; Micheli, A. L. Appl. Phys. Lett. 1998, 72, 2838. (6) Bao, D.; Mizutani, N.; Yao, X.; Zhang, L. Appl. Phys. Lett. 2000, 77, 1041. (7) Bao, D.; Mizutani, N.; Zhang, L.; Yao, X. J. Appl. Phys. 2001, 89, 801. (8) Brazier, M.; McElfresh, M.; Mansour, S. Appl. Phys. Lett. 1998, 72, 1121. (9) Ichinose, I.; Kawakami, T.; Kunitake, T. AdV. Mater. 1998, 10, 535. (10) Kleinfeld, E. R.; Ferguson, G. R. Mater. Res. Soc. Symp. Proc. 1994, 351, 419. (11) Ichinose, I.; Senzu, H.; Kunitake, T. Chem. Mater. 1997, 9, 1296. (12) Kalachev, A. A.; Mathauer, K.; Ho¨hne, U.; Mo¨hwald, H.; Wegner, G. Thin Solid Films 1993, 228, 307. (13) Spatz, J. P.; Mo¨ssmer, S.; Hartmann, C.; Mo¨ller, M.; Herzog, T.; Krieger, M.; Boyen, H.-G.; Ziemann, P.; Kabius, B. Langmuir 2000, 16, 407. (14) Jang, H. K.; Lee, S. K.; Lee, C. E.; Noh, S. J.; Lee, W. I. Appl. Phys. Lett. 2000, 76, 882. (15) Sauerbrey, G. Z. Phys. 1959, 155, 206. (16) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (17) Solletti, J. M.; Botreau, M.; Sommer, F.; Brunat, W. L.; Kasas, S.; Duc, T. M.; Celio, M. R. Langmuir 1996, 12, 5379. (18) Kurihara, K.; Kawahara, T.; Sasaki, D. Y.; Ohto, K.; Kunitake, T. Langmuir 1995, 11, 1408.
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Nano Lett., Vol. 2, No. 6, 2002
