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Role of surface OH groups in surface chemical properties of metal oxide films
Materials Science and Engineering B 119 (2005) 265–267
Role of surface OH groups in surface chemical properties of metal oxide films Satoshi Takeda ∗ , Makoto Fukawa Research Center, Asahi Glass Co., Ltd., 1150 Hazawa-cho, Kanagawa-ku, Yokohama 221-8755, Japan Received 22 April 2004; received in revised form 15 September 2004; accepted 9 December 2004
Abstract We investigated the effects of surface OH groups on surface chemical properties, such as wetting and surface reactivity of various metal oxide films. Linear relationship was observed between hydrophobicity caused by adsorption of organic substances from the atmosphere and surface OH group density. It was also found that surface reactivity with a fluoroalkyl isocyanate silane was strongly dependent upon the surface OH group density of the film. These results indicate that surface OH groups are a major factor governing the surface chemical properties of the films. It was also revealed that the surface OH group density depended on the film materials. This dependence can be explained in terms of electronegativity of the metal element directly bound to oxygen atoms. Utilizing these findings, the surface chemical properties of SiO2 films were successfully controlled. © 2005 Elsevier B.V. All rights reserved. Keywords: Surface OH group; Metal oxide; TOF-SIMS; Wetting; Surface reactivity
1. Introduction Considerable effort has been made to control surface properties of materials, because it is extremely important to obtain high-quality devices. Recent progresses in fabrication techniques of micro- and nanometer-scale devices make it more important because surface-to-volume ratio becomes relatively larger due to downsizing. Therefore, more precise surface characterization is required to control the surface properties of materials. It is known that surface OH groups are present on the surface of metal oxide materials. In previous paper [1,2], we reported that the surface OH group plays an extremely important role in surface chemical properties of metal oxide films and various glasses, since it can work as effective adsorption or reactive sites. In this paper, we review our recent works on characterization of surface OH groups, and discuss the role of surface OH groups in surface chemical properties of metal ∗
Corresponding author. E-mail address: [email protected] (S. Takeda).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.12.078
oxide materials. Here, we investigated the relationship between the surface OH group density and wetting or surface reactivity by time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy and contact angle measurements. Also, we demonstrate that metal doping is an effective way to alter the surface chemical properties of SiO2 films due to the change in the surface OH group density induced by the doping [3].
2. Experimental Metal oxide films, SiO2 , SnO2 , TiO2 , CrOx and ZrO2 with a thickness of 40 nm, were deposited onto glass by rf magnetron sputtering. Subsequently, the films were stored in same desiccators whose atmosphere was controlled at 50 ◦ C, 95% RH. The adsorption of organic substances from the atmosphere to the film surface was evaluated by contact angle and XPS measurements. The contact angle of water was measured as a function of elapsed time. Its accuracy was within ±1◦ . XPS measurements were performed to quantify the carbon
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S. Takeda, M. Fukawa / Materials Science and Engineering B 119 (2005) 265–267
adsorbed on the films, which were stored in the desiccators for 18 days. Same experiments were performed for SiO2 thin films, doped with aluminum (Al), titanium (Ti) or zirconium (Zr). The concentration of doped element in the film was adjusted at ∼3 at.%. The surface OH group density was evaluated by TOFSIMS. A low dose and pulsed Cs+ or Ga+ primary ion beam was employed. In the evaluation of surface reactivity of the film, a polyfluoroalkyl isocyanate silane [C8 F17 C2 H4 Si(NCO)3 ] was used as a reactive agent [4]. The sample surfaces were cleaned using UV/O3 for 10 min to remove contaminants prior to the treatment. The formation ability of the surface OH group on the films was evaluated from XPS O 1s spectra analysis. XPS measurements were carried out with a monochromatized Al K␣ source. The detection angle of the X-ray photoelectrons was 70◦ to the sample surface. The binding energy was referenced to the C 1s line at 284.6 eV.
3. Results and discussion
Fig. 2. The relationship between the secondary ion intensity ratio of 17 OH− /16 O− and (a) the contact angle of water droplets at 18 days elapsed, or (b) F/(F + M) (M = Si, Sn, Ti, Cr and Zr) atomic ratio obtained from XPS.
3.1. Wetting and surface reactivity Fig. 1 shows the contact angle of water droplet for the films as a function of elapsed time. The contact angles are nearly 0◦ for all the films immediately after UV/O3 cleaning. Generally, the contact angle of water droplets is close to 0◦ for oxide materials without contaminants, because the surface energy is essentially very large compared with that of water. Therefore, the surfaces are considered to be with no contaminants immediately after UV/O3 cleaning. The contact angles of the films gradually increase with time and reach at an asymptotic value (θs) in approximately 10 days elapsed. The value remains constant for the following 5 days. The hydrophobicities (θs) are different among the films.
Fig. 1. Contact angle of water droplets for metal oxide films SiO2 (a), SnO2 (b), TiO2 (c), CrOx(d) and ZrO2 (e) as a function of elapsed time.
Here, it is known that the contact angle is affected by surface roughness as well as by surface energy [5]. However, no significant change in the surface roughness was observed, indicating that the difference in the θs is not due to the surface roughness but due to the surface chemical properties. XPS analysis revealed that carbon concentration on the film surface linearly increase with increasing θs. These results indicate that the increase in the contact angle results from the adsorption of organic substances in the atmosphere, and the difference in the θs is caused by the difference in the amount of these adsorbed organic substances. As shown in Fig. 2, 17 OH− /16 O− ratio, which represents the surface OH group density of the film steadily increase with increasing the θs, indicating that the hydrophobicity (θs), resulting from the adsorption of organic substances from the atmosphere depends on the surface OH group density. This suggests that the wetting, one of the most important surface chemical properties, can be controlled by the surface OH group density. In Fig. 2, the surface fluorine concentration on the films measured by XPS as a result of reaction with a polyfluoroalkyl isocyanate silane [C8 F17 C2 H4 Si(NCO)3 ] is plotted versus 17 OH− /16 O− ratio. It is found that F/(F + M) (M = Si, Sn, Ti, Cr and Zr) atomic ratio is larger for the film having higher surface OH group density, indicating that the surface reactivity with the agent also depends on the surface OH group density. These results mean that the surface OH groups can work as effective adsorptive and reactive sites, and that the surface OH groups are considered to be a major factor governing the surface chemical properties of the films.
S. Takeda, M. Fukawa / Materials Science and Engineering B 119 (2005) 265–267
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Fig. 4. Contact angle of water droplets for undoped and metal doped SiO2 films as a function of elapsed time.
Fig. 3. The relationship between the secondary ion intensity ratio 17 OH− /16 O− and (a) O 1s binding energy obtained from the XPS spectra, or (b) χM–O which represents the electronegativity difference between oxygen and the metal element directly bound to the oxygen atoms of the metal oxide films.
3.2. Formation mechanism of surface OH group As shown in Fig. 2, the surface OH group density is different among the film material. To elucidate the origin of this difference, XPS O 1s analyses were carried out. Fig. 3 shows the relationship between the O 1s peak position and the 17 OH− /16 O− ratio. It is found that the O 1s peak is observed at lower binding energy side for the films having a higher surface OH density. This means that negative charge density on the oxygen atoms of the films is different among the film materials, and that the formation ability of surface OH groups is dependent on the negative charge density on the oxygen atoms of the films. Generally, the binding energy of the electrons in a certain atom is highly influenced by the electronegativity of the atoms directly bound to that particular atom [6]. Therefore, this difference is considered to be related to the electronegativity of the metal element directly bound to the oxygen atom. In Fig. 3, the 17 OH− /16 O− ratio is plotted as a function of the electronegativity difference between the oxygen and metal element directly bound to the oxygen atom (χM–O ). It is clearly found that the surface OH group density increases with χM–O . This indicates that the formation ability of surface OH groups is dependent on χM–O . 3.3. Control of surface OH group density by metal doping Next, we applied above findings to modify the surface chemical properties of metal oxide materials. Here, the effects of metal doping on the surface chemical properties of sputtered SiO2 films were investigated. As shown in Fig. 4, the hydrophobicity resulting from the adsorption of organic
substances from the atmosphere is significantly altered by metal doping. TOF-SIMS analysis revealed that this alteration is due to the change in the surface OH group density of the films. That is, the surface chemical properties are effectively altered by metal doping. Furthermore, XPS and FT-IR analyses revealed that the origin of the change in the surface OH group density is ascribed to the change in the number of non-bridging oxygen (Si–O− ) in the SiO2 film, because the doped metal strongly affects negative charge density on the oxygen atoms of the film, resulting in change in SiO2 network structure [3].
4. Conclusions In this paper, we reported the effects of surface OH group density on surface properties, such as wetting and surface reactivity of metal oxide films. It was found that the wetting and surface reactivity depended on the surface OH group density. Also, it was revealed that the surface OH group density was dependent on the film material. This dependence can be explained with respect to the electronegativity of metal element directly bound to the oxygen atoms. Furthermore, we successfully demonstrated that the surface chemical properties of sputtered SiO2 films can be altered by the surface OH group density because of the change in the surface OH group density induced by the doping. We expect that these findings offer a novel method to control and design the surface properties of metal oxide materials. References [1] S. Takeda, M. Fukawa, Y. Hayashi, K. Matsumoto, Thin Solid Films 339 (1999) 220–224. [2] S. Takeda, K. Yamamoto, Y. Hayasaka, K. Matsumoto, J. Non-Cryst. Solids 249 (1999) 41–46. [3] S. Takeda, M. Fukawa, Thin Solid Films 444 (2003) 153–157. [4] T. Yoneda, T. Morimoto, Thin Solid Films 351 (1999) 279– 283. [5] R.W. Wenzel, Ind. Eng. Chem. 28 (1936) 988–994. [6] W.J. Stec, W.E. Morgan, R.G. Albridge, J.R. van Wazer, Inorg. Chem. 11 (1972) 219–225.
Role of surface OH groups in surface chemical properties of metal oxide films Satoshi Takeda ∗ , Makoto Fukawa Research Center, Asahi Glass Co., Ltd., 1150 Hazawa-cho, Kanagawa-ku, Yokohama 221-8755, Japan Received 22 April 2004; received in revised form 15 September 2004; accepted 9 December 2004
Abstract We investigated the effects of surface OH groups on surface chemical properties, such as wetting and surface reactivity of various metal oxide films. Linear relationship was observed between hydrophobicity caused by adsorption of organic substances from the atmosphere and surface OH group density. It was also found that surface reactivity with a fluoroalkyl isocyanate silane was strongly dependent upon the surface OH group density of the film. These results indicate that surface OH groups are a major factor governing the surface chemical properties of the films. It was also revealed that the surface OH group density depended on the film materials. This dependence can be explained in terms of electronegativity of the metal element directly bound to oxygen atoms. Utilizing these findings, the surface chemical properties of SiO2 films were successfully controlled. © 2005 Elsevier B.V. All rights reserved. Keywords: Surface OH group; Metal oxide; TOF-SIMS; Wetting; Surface reactivity
1. Introduction Considerable effort has been made to control surface properties of materials, because it is extremely important to obtain high-quality devices. Recent progresses in fabrication techniques of micro- and nanometer-scale devices make it more important because surface-to-volume ratio becomes relatively larger due to downsizing. Therefore, more precise surface characterization is required to control the surface properties of materials. It is known that surface OH groups are present on the surface of metal oxide materials. In previous paper [1,2], we reported that the surface OH group plays an extremely important role in surface chemical properties of metal oxide films and various glasses, since it can work as effective adsorption or reactive sites. In this paper, we review our recent works on characterization of surface OH groups, and discuss the role of surface OH groups in surface chemical properties of metal ∗
Corresponding author. E-mail address: [email protected] (S. Takeda).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.12.078
oxide materials. Here, we investigated the relationship between the surface OH group density and wetting or surface reactivity by time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy and contact angle measurements. Also, we demonstrate that metal doping is an effective way to alter the surface chemical properties of SiO2 films due to the change in the surface OH group density induced by the doping [3].
2. Experimental Metal oxide films, SiO2 , SnO2 , TiO2 , CrOx and ZrO2 with a thickness of 40 nm, were deposited onto glass by rf magnetron sputtering. Subsequently, the films were stored in same desiccators whose atmosphere was controlled at 50 ◦ C, 95% RH. The adsorption of organic substances from the atmosphere to the film surface was evaluated by contact angle and XPS measurements. The contact angle of water was measured as a function of elapsed time. Its accuracy was within ±1◦ . XPS measurements were performed to quantify the carbon
266
S. Takeda, M. Fukawa / Materials Science and Engineering B 119 (2005) 265–267
adsorbed on the films, which were stored in the desiccators for 18 days. Same experiments were performed for SiO2 thin films, doped with aluminum (Al), titanium (Ti) or zirconium (Zr). The concentration of doped element in the film was adjusted at ∼3 at.%. The surface OH group density was evaluated by TOFSIMS. A low dose and pulsed Cs+ or Ga+ primary ion beam was employed. In the evaluation of surface reactivity of the film, a polyfluoroalkyl isocyanate silane [C8 F17 C2 H4 Si(NCO)3 ] was used as a reactive agent [4]. The sample surfaces were cleaned using UV/O3 for 10 min to remove contaminants prior to the treatment. The formation ability of the surface OH group on the films was evaluated from XPS O 1s spectra analysis. XPS measurements were carried out with a monochromatized Al K␣ source. The detection angle of the X-ray photoelectrons was 70◦ to the sample surface. The binding energy was referenced to the C 1s line at 284.6 eV.
3. Results and discussion
Fig. 2. The relationship between the secondary ion intensity ratio of 17 OH− /16 O− and (a) the contact angle of water droplets at 18 days elapsed, or (b) F/(F + M) (M = Si, Sn, Ti, Cr and Zr) atomic ratio obtained from XPS.
3.1. Wetting and surface reactivity Fig. 1 shows the contact angle of water droplet for the films as a function of elapsed time. The contact angles are nearly 0◦ for all the films immediately after UV/O3 cleaning. Generally, the contact angle of water droplets is close to 0◦ for oxide materials without contaminants, because the surface energy is essentially very large compared with that of water. Therefore, the surfaces are considered to be with no contaminants immediately after UV/O3 cleaning. The contact angles of the films gradually increase with time and reach at an asymptotic value (θs) in approximately 10 days elapsed. The value remains constant for the following 5 days. The hydrophobicities (θs) are different among the films.
Fig. 1. Contact angle of water droplets for metal oxide films SiO2 (a), SnO2 (b), TiO2 (c), CrOx(d) and ZrO2 (e) as a function of elapsed time.
Here, it is known that the contact angle is affected by surface roughness as well as by surface energy [5]. However, no significant change in the surface roughness was observed, indicating that the difference in the θs is not due to the surface roughness but due to the surface chemical properties. XPS analysis revealed that carbon concentration on the film surface linearly increase with increasing θs. These results indicate that the increase in the contact angle results from the adsorption of organic substances in the atmosphere, and the difference in the θs is caused by the difference in the amount of these adsorbed organic substances. As shown in Fig. 2, 17 OH− /16 O− ratio, which represents the surface OH group density of the film steadily increase with increasing the θs, indicating that the hydrophobicity (θs), resulting from the adsorption of organic substances from the atmosphere depends on the surface OH group density. This suggests that the wetting, one of the most important surface chemical properties, can be controlled by the surface OH group density. In Fig. 2, the surface fluorine concentration on the films measured by XPS as a result of reaction with a polyfluoroalkyl isocyanate silane [C8 F17 C2 H4 Si(NCO)3 ] is plotted versus 17 OH− /16 O− ratio. It is found that F/(F + M) (M = Si, Sn, Ti, Cr and Zr) atomic ratio is larger for the film having higher surface OH group density, indicating that the surface reactivity with the agent also depends on the surface OH group density. These results mean that the surface OH groups can work as effective adsorptive and reactive sites, and that the surface OH groups are considered to be a major factor governing the surface chemical properties of the films.
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Fig. 4. Contact angle of water droplets for undoped and metal doped SiO2 films as a function of elapsed time.
Fig. 3. The relationship between the secondary ion intensity ratio 17 OH− /16 O− and (a) O 1s binding energy obtained from the XPS spectra, or (b) χM–O which represents the electronegativity difference between oxygen and the metal element directly bound to the oxygen atoms of the metal oxide films.
3.2. Formation mechanism of surface OH group As shown in Fig. 2, the surface OH group density is different among the film material. To elucidate the origin of this difference, XPS O 1s analyses were carried out. Fig. 3 shows the relationship between the O 1s peak position and the 17 OH− /16 O− ratio. It is found that the O 1s peak is observed at lower binding energy side for the films having a higher surface OH density. This means that negative charge density on the oxygen atoms of the films is different among the film materials, and that the formation ability of surface OH groups is dependent on the negative charge density on the oxygen atoms of the films. Generally, the binding energy of the electrons in a certain atom is highly influenced by the electronegativity of the atoms directly bound to that particular atom [6]. Therefore, this difference is considered to be related to the electronegativity of the metal element directly bound to the oxygen atom. In Fig. 3, the 17 OH− /16 O− ratio is plotted as a function of the electronegativity difference between the oxygen and metal element directly bound to the oxygen atom (χM–O ). It is clearly found that the surface OH group density increases with χM–O . This indicates that the formation ability of surface OH groups is dependent on χM–O . 3.3. Control of surface OH group density by metal doping Next, we applied above findings to modify the surface chemical properties of metal oxide materials. Here, the effects of metal doping on the surface chemical properties of sputtered SiO2 films were investigated. As shown in Fig. 4, the hydrophobicity resulting from the adsorption of organic
substances from the atmosphere is significantly altered by metal doping. TOF-SIMS analysis revealed that this alteration is due to the change in the surface OH group density of the films. That is, the surface chemical properties are effectively altered by metal doping. Furthermore, XPS and FT-IR analyses revealed that the origin of the change in the surface OH group density is ascribed to the change in the number of non-bridging oxygen (Si–O− ) in the SiO2 film, because the doped metal strongly affects negative charge density on the oxygen atoms of the film, resulting in change in SiO2 network structure [3].
4. Conclusions In this paper, we reported the effects of surface OH group density on surface properties, such as wetting and surface reactivity of metal oxide films. It was found that the wetting and surface reactivity depended on the surface OH group density. Also, it was revealed that the surface OH group density was dependent on the film material. This dependence can be explained with respect to the electronegativity of metal element directly bound to the oxygen atoms. Furthermore, we successfully demonstrated that the surface chemical properties of sputtered SiO2 films can be altered by the surface OH group density because of the change in the surface OH group density induced by the doping. We expect that these findings offer a novel method to control and design the surface properties of metal oxide materials. References [1] S. Takeda, M. Fukawa, Y. Hayashi, K. Matsumoto, Thin Solid Films 339 (1999) 220–224. [2] S. Takeda, K. Yamamoto, Y. Hayasaka, K. Matsumoto, J. Non-Cryst. Solids 249 (1999) 41–46. [3] S. Takeda, M. Fukawa, Thin Solid Films 444 (2003) 153–157. [4] T. Yoneda, T. Morimoto, Thin Solid Films 351 (1999) 279– 283. [5] R.W. Wenzel, Ind. Eng. Chem. 28 (1936) 988–994. [6] W.J. Stec, W.E. Morgan, R.G. Albridge, J.R. van Wazer, Inorg. Chem. 11 (1972) 219–225.