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Hydrogen behavior in gasochromic tungsten oxide films investigated by elastic recoil detection analysis
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NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 301–307 www.elsevier.com/locate/nimb
Hydrogen behavior in gasochromic tungsten oxide films investigated by elastic recoil detection analysis Aichi Inouye a,*, Shunya Yamamoto b, Shinji Nagata a, Masahito Yoshikawa b, Tatsuo Shikama a b
a Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Quantum Beam Science Directorate, Japan Atomic Energy Agency, 1233 Watanuki, Takasaki 370-1292, Japan
Received 10 October 2007; received in revised form 15 November 2007 Available online 22 November 2007
Abstract Changes in the composition and crystalline structure of gasochromic tungsten oxide films resulting from the incorporation of hydrogen were investigated; the oxide films were prepared by reactive RF magnetron sputtering on SiO2 and glassy carbon substrates simultaneously. X-ray diffraction analysis of the deposited films at 600 °C showed a uniaxial oriented structure in the (0 1 0) plane of monoclinic WO3 for both substrates. The elastic recoil detection analysis (ERDA) and Rutherford backscattering spectroscopy (RBS) for the films on glassy carbon revealed that the hydrogen impurity was uniformly distributed up to a concentration of 0.24 H/W. The Pd-coated films on SiO2 turned blue when they were exposed to a mixture of Ar and 5% H2 gases. When the sample became colored, the hydrogen concentration in the film increased to 0.47 H/W and the crystalline structure of the film changed from monoclinic to tetragonal. These results indicated that the gasochromic coloration of the tungsten oxide films coincided with incorporation of hydrogen atoms into the crystalline lattice, corresponding to the formation of hydrogen tungsten bronze (HxWO3). Ó 2007 Elsevier B.V. All rights reserved. PACS: 81.15.Cd Keywords: Tungsten oxide; Hydrogen; Gasochromism; ERDA
1. Introduction Electrochromic materials such as WO3, MoO3 and V2O5 can be used in optical devices because the materials can be reversibly colored by an external voltage. Tungsten oxides exhibit cathodic coloration when incorporated with cations such as hydrogen, lithium and sodium ions [1]. When the surface of tungsten oxide is covered with a catalyst such as palladium, platinum and nickel, the optical absorption of the oxide drastically changes when exposed to hydrogen gas; this phenomenon is called gasochromism. The optical changes occur reversibly between the colored and bleached states in the presence of hydrogen and oxygen gases. The *
Corresponding author. Tel.: +81 22 215 2063; fax: +81 22 215 2061. E-mail address: [email protected] (A. Inouye).
0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.11.016
gasochromic films are expected to be used in optical hydrogen-gas-leak detectors that use fiber optics because they are considered to be operational at room temperature in addition to being highly sensitive; moreover, they do not produce noise in response to electronic effects [2]. However the mechanism of gasochromic coloration is not clearly understood. One of the widely accepted gasochromic coloration mechanism is that illustrated by the double-injection model, which is similar to the mechanism of electrochromic coloration [3]. In this model, dissociated hydrogen atoms and the electrons in the catalyst dissolve in the tungsten oxide film, and the film is colored due to the formation of tungsten bronze (HxWO3). The change in the valency of tungsten from 6+ to 5+ gives rise to the blue color [4]. In another model, the dissociated hydrogen atoms diffuse along a pore and grain boundary of tungsten oxide and
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react with oxygen on the surface of tungsten oxide, thereby resulting in the formation of water and oxygen vacancies [5]. The oxygen vacancies diffuse to the interior of tungsten oxide, making it oxygen deficient; this gives rise to the blue color. Therefore, it is necessary to understand the behavior of hydrogen and oxygen in tungsten oxide during the coloration in the presence of hydrogen gas. Moreover, it is also important to determine whether the tungsten oxide structure transforms to the HxWO3 or the WO3x structure. However, there are few reports on the crystalline structure of gasochromic materials because most of the studies have focused on amorphous tungsten oxide films. To determine the hydrogen and oxygen concentrations of tungsten oxide films in the colored and bleached states, ion-beam analyses such as the elastic recoil detection analysis (ERDA), Rutherford backscattering spectroscopy (RBS) and nuclear reactions (NR) are suitable for studying not only the surface but also the interior of samples [6–11]. In electrochromic tungsten oxide films, the nuclear reaction measurements using 15N ions showed a variation in the hydrogen content as a function of the coloration state [8]. In gasochromic tungsten oxide films, an unchanged hydrogen concentration was observed before and after coloration by nuclear reaction analysis using 11 B ions [9]. The increase in the hydrogen concentration with gasochromic coloration was also observed by nuclear reaction analysis using 15N ions [10]. However, the change in the composition of the samples during the analysis due to irradiation defects resulting from the heavy ions is a matter of concern. Therefore, ERDA performed by using 4 He ions is suitable for determining the hydrogen concentration in gasochromic tungsten oxide films; this analysis method suffers little from the irradiation effects of the analysis beam. In our previous studies, various hydrogen concentrations were observed between the bleached and colored states in amorphous tungsten oxide films by ERDA performed using 4He ions [11]. In order to reveal the gasochromic mechanism to a greater extent, it is necessary to investigate the hydrogen behavior in crystalline tungsten oxides whose crystalline structure can be observed. The incorporation of hydrogen changes the structure of tungsten oxide to HxWO3 electrochemically [12–14]. However, the crystalline structure of colored tungsten oxide films was hardly observed in previous reports because only amorphous tungsten oxide films were considered in many studies. It has been known that polycrystalline WO3 produced by heat treatment in air can hardly be colored by using hydrogen gas. By employing X-ray diffraction (XRD), the crystalline structure of a WO3 nanowire has been observed to change to the HxWO3 structure during gasochromic coloration [15]. In this study, we prepared oriented tungsten oxide films by employing the reactive RF magnetron sputtering method. Before and after gasochromic coloration, the hydrogen and oxygen concentrations and crystalline structure of the films were investigated by performing ERDA using 4He ions and XRD, respectively. Further, the hydro-
gen behavior in the tungsten oxides during gasochromic coloration is discussed. 2. Experimental Tungsten oxide films were prepared by employing the reactive RF magnetron sputtering method with a W target (purity: 3 N) in a vacuum chamber that was evacuated up to a base pressure of 5 104 Pa by using a turbomolecular pump. The tungsten oxides were deposited in a defined argon (purity: 6 N) and oxygen (purity: 4 N) mixture controlled by mass flow meters and an absolute pressure gauge (Baratron 626, MKS) under the pumping condition. The distance between the tungsten target and the substrates was fixed at approximately 100 mm. The films were deposited on SiO2 substrates, glassy carbon and a silicon (0 0 1) wafer by varying the temperature from 30 to 600 °C. The partial pressure of argon and oxygen were 135 and 20 MPa, respectively, and the sputtering power was 40 W. These deposition conditions indicate a sputtering rate of 15 nm/min. For the incorporation of hydrogen into the tungsten oxide films by exposure to hydrogen gas, a 15-nm Pd catalyst was coated on the tungsten oxide films by sputtering a Pd target (purity: 3 N) in argon at a temperature of 200 °C. In order to characterize the crystalline structure of the samples, h–2h scans of X-ray diffraction were performed by high-resolution diffractometers (X’Pert-MRD, PANalytical and Geigerflex, Rigaku). Cu Ka radiations were used with a 200-lm-thick Ni filter to minimize the Kb components. Continuous scans were performed with the following parameters; an integration time of 1 s per step, 2h resolution of 0.01° per step and a scan range of 10°–60°. For the observation of the surface morphology of the samples, a scanning electron microscope (SEM, JSM-6700F, JEOL) was employed. Ion-beam analyses were performed for the determination of the oxygen and hydrogen atomic concentration ratios (O/W and H/W) in tungsten oxide films by using a 3.0-MV single-ended accelerator at JAEA and a 1.7-MV tandem accelerator at the Institute for Materials Research, Tohoku University. For measuring the oxygen concentration, the 2.0 MeV He ions were irradiated at an incident angle of 90° with respect to the surface of the samples on glassy carbons. The backscattered He ions were detected at an angle of 165° with respect to the incident direction by using a surface barrier detector. For the estimation of the hydrogen concentration, the samples were irradiated with He ions at an incident angle of 20° with respect to the sample surface. The recoiled hydrogen atoms were detected at an angle of 30° with respect to the incident direction by using a surface barrier detector with 12-lmthick aluminum foil; this thickness was selected to avoid detecting backscattered He ions. The irradiation was performed with a flux of approximately 8 1012 ions/cm2/s and a fluence of up to 1.6 1015 ions/cm2. The absolute hydrogen concentration was estimated by considering the
A. Inouye et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 301–307
2.8 MeV, 4 He2+
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Optical window Monochrometer with CCD camera Fig. 1. Experimental apparatus for simultaneous examination of coloration properties by using hydrogen gas, and the hydrogen and oxygen concentrations in tungsten oxide films coated with a 15-nm Pd layer on SiO2 substrates.
yields of the recoiled hydrogen atoms and by using titanium hydrate as the standard sample with a know hydrogen concentration (TiH1.95). To simultaneously examine the gasochromic coloration properties and the ion-beam analysis, the light transmittance by the Pd/WO3/SiO2 samples was measured in the ion-beam analysis chamber, as shown in Fig. 1. The sample was fixed on a holder with a slit to measure the light transmittance by using a white-light-emitting diode (LED) in the wavelength range from 400 to 1000 nm. The light transmitted by the samples was focused by using double optical lenses and transported to a CCD camera by employing a monochrometer. The gasochromic coloration caused by exposure to argon and 5% hydrogen (Ar–5%H2) gas was measured as a function of time during exposure, pumping and purging. 3. Results and discussions Fig. 2 shows XRD patterns of the tungsten oxide films deposited on SiO2 substrates at substrates temperatures of (a) 600, (b) 500 and (c) 400 °C. The XRD pattern for WO3 powder is also shown in Fig. 2(d). For a substrate temperature below 400 °C, the XRD pattern shows a broad peak, indicating an amorphous structure. Above 500 °C, a diffraction peak appears at 2h = 23.5° and the intensity of the peak increases with the substrate temperature. For deposition at 600 °C, a few diffraction peaks appeared such as the side peaks at 23.1° and 24.1°. The three peaks at 23.1, 23.5 and 24.1° can be related to three strong peaks in the XRD pattern of the WO3 powder, as shown in
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2θ [degree] Fig. 2. XRD patterns of tungsten oxide films deposited by reactive RF sputtering method on SiO2 substrates at temperatures of (a) 600, (b) 500 and (c) 400 °C; (d) the pattern of WO3 powder.
Fig. 2(d). The three peaks obtained in the XRD patterns of the deposited films might be attributed to the monoclinic or orthorhombic phase of c- or b-WO3, respectively. Monoclinic and orthorhombic WO3 are nearly identical to each other; the reported lattice parameters for the mono˚ , b = 7.540 A ˚ , c = 7.692 A ˚ and clinic phase are a = 7.306 A b = 90.88°, while those for the orthorhombic phase are ˚ , b = 7.570 A ˚ and c = 7.754 A ˚ [16]. The oriena = 7.341 A tation and structure of monoclinic WO3 on amorphous SiO2 substrates have been obtained by employing sputtering methods [15,17]. Monoclinic WO3 has also been epitaxially grow on SrTiO3 (1 0 0) planes, MgO (1 0 0) planes and Al2O3 r-planes by sputtering, thermal evaporation and chemical vapor deposition methods [18–20]. For simplicity, we will continue to refer to the crystalline structure of the deposited films as the monoclinic phase, and the strong peak at 23.5° can be attributed to the (0 2 0) planes. In the case of deposition at a substrate temperature of 600 °C, the intensity of the peaks at 23.1° and 24.1° originating from (0 0 2) and (2 0 0), respectively, is considerably lower than the peak from the (0 2 0) planes. These results reveal that monoclinic WO3 can be strongly oriented on the SiO2 substrate above a temperature of 500 °C by employing reactive sputtering methods. To investigate the orientation of the tungsten oxide films deposited at 600 °C, pole figure measurement was performed at 2h = 41.9°, which is assigned to the h2 2 2i axis of monoclinic WO3. A diffraction ring was obtained at w = 55° from the (0 2 0) phase indicating that the oriented tungsten oxide films were uniaxially oriented along the h0 1 0i axis with the h0 0 1i and h1 0 0i axes being randomly directed. For the observation of a microstructure, the cleavage surface morphology at a cross section of the uniaxially oriented tungsten oxide films the on the silicon (0 0 1) wafer
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was observed by the SEM, as shown in Fig. 3. We can recognize the columnar structure with a width of 100 nm on the substrate. It is considered that the crystalline structure of the columnar particles is oriented along the (0 1 0) plane of monoclinic WO3 with the h0 0 1i and h1 0 0i axes in the plane being randomly directed. Fig. 4 shows a typical RBS spectrum for the oriented tungsten oxide film deposited on glassy carbon at the substrate temperature of 600 °C. The peaks originating from the tungsten, oxygen and carbon atoms of the sample are
Fig. 3. SEM image of a cross section of the oriented tungsten oxide films deposited on silicon (0 0 1) wafer at a substrate temperature of 600 °C.
found in the backscattering energy ranges of 1620– 1840 keV, 530–700 keV and below 330 keV, respectively. With respect to the peaks originating from tungsten and oxygen, the peak shape can be explained by the relation between the scattering cross sections and the energy loss of the incident ions, indicating that the oxygen/tungsten atomic ratio (O/W) is homogeneous throughout the WO3 film. The O/W ratio in the film was estimated to be 3.0 ± 0.05 by counting the total backscattered yields from the tungsten and oxygen atoms. The oxygen concentration in the tungsten oxide films was unchanged when the substrate temperature was varied from 30 to 600 °C. It was clarified that the WO3 films could be obtained at any substrate temperature by deposition at an oxygen partial pressure of 20 MPa in the sputtering gas. By measuring the width of the tungsten peak in the RBS spectrum and considering the WO3 composition, the film thickness is estimated to be approximately 380 nm. A film thickness of 390 nm was obtained by using a surface profilometer for the same sample. By comparing the estimated film thickness with the measured one, the atomic density of the film can be calculated to be 7.4 1022 atoms/cm3 (7.3 g/cm3); this is approximately equal to the density of polycrystalline monoclinic WO3, which has been reported to be 7.55 1022 atoms/cm3 (7.3 g/cm3) (JCPDS card No. 431035). Fig. 5 shows the ERDA spectrum for the tungsten oxide film deposited on glassy carbon at 600 °C. This shape of the ERDA spectrum indicates that the hydrogen atoms are uniformly distributed in throughout the film. The hydrogen concentration is estimated to be approximately 0.24 H/W (5.7 at.%) by counting the total yields in this spectrum and comparing it with the yields of the standard TiH1.95 sample. The hydrogen present in the film is assumed to
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Energy [keV] Fig. 5. Typical ERDA spectrum (obtained by using 2.8 MeV He ions) for the oriented tungsten oxide film deposited on glassy carbon at a substrate temperature of 600 °C.
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have originated from water impurities in the sputtering gas or sputtering chamber and is considered to be incorporated in WO3 after the deposition procedure due to the release of hydrogen below the temperature of 400 °C [11]. The incorporated hydrogen is considered to form water molecules in tungsten oxide films deposited by thermal evaporation; this assumption is supported by the high value of the estimated O/W ratio (above 3) [6]. There is a difference between the reported O/W ratio and the estimated one in this study; the film composition is found to almost stoichiometric WO3 from the RBS spectrum. If it is considered that all the hydrogen atoms in the films form H2O, the composition of the tungsten oxide films should be nonstoichiometric WO3x such as WO2.88. This oxygen deficiency is considered to make the tungsten oxides bluer as compared to the color of WO3. Hydrogen tungsten bronze (HxWO3) also had a blue or yellow color when the hydrogen concentration was changed [1]. The light green color of the as-prepared WO3 films obtained in this study indicates that hydrogen in the films does not form bronzes, hydrates, or water molecules; however, it might be trapped in vacancies or at grain boundaries. To examine the gasochromic properties of the tungsten oxide films, the light transmittance of the oriented tungsten oxide film deposited on the SiO2 substrates coated with a 15-nm-thick Pd layer was measured in the as-prepared state, after being colored by exposure to Ar–5%H2 gas, and after being bleached by exposure to air on subsequent coloration. Fig. 6 shows the transmittance spectra for the Pd-coated samples in the (a) as-prepared, (b) colored and (c) bleached states in addition to that of (d) the sample without a Pd layer. The transmittance of the as-prepared
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sample is approximately 30% for wavelengths above 450 nm. After the sample is exposed to the Ar–5%H2 gas for 20 min, the transmittance decreases to 1% at wavelengths from 650 to 1000 nm. When exposed to air after the coloration, the sample is bleached up to a transmittance of 20% in all the wavelength regions. It was also confirmed that this bleaching cannot be caused in Ar, N2 and vacuum, indicating that we can handle the colored samples by maintaining the coloration levels in vacuum for the determination of atomic concentrations with the ion-beam analysis. The ERDA spectra for the as-prepared and colored samples on SiO2 substrates are shown in Fig. 7(a) and (b), respectively. In the case of the as-prepared sample, the ERDA spectrum shows that the hydrogen atoms are uniformly distributed throughout the film, with a concentration of approximately 0.24 H/W (5.8 at.%). As the film is colored by the hydrogen gas, hydrogen profiling shows that the hydrogen concentration of 0.23 H/W (5.4 at.%) increases throughout the film. It was also confirmed that the hydrogen concentration in the bleached states was similar to that in as-prepared films. It is elucidated that the hydrogen atoms are incorporated into the tungsten oxide films and diffuse throughout the film during gasochromic coloration. The release of the incorporated hydrogen by exposure to oxygen or air is also revealed. With respect to the oxygen concentration in the colored samples, the RBS measurements for the films deposited on glassy carbon showed that the oxygen concentration in the films was unchanged during the repeated changes between colored and bleached states. The determination of the hydrogen and oxygen concentrations by the ERDA and RBS measurements reveals that the gasochromic coloration of tungsten oxide films by hydrogen gas coincides with the incorporation of hydrogen; this occurrence is similar to
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Wavelength [nm] Fig. 6. Light transmittance spectra for 15-nm Pd-coated tungsten oxide films on SiO2 substrates (a) for the as-prepared state, (b) for the colored state after 20 min of exposure to hydrogen gas, (c) for the bleached state after 20 min of exposure to the air after the coloration and (d) light transmittance spectrum for the sample without the Pd layer.
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Energy [keV] Fig. 7. ERDA spectra for 15-nm Pd-coated tungsten oxide films on SiO2 substrates: (a) for the as-prepared state and (b) for the colored state after 20 min of exposure to hydrogen gas.
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that in the mechanism corresponding to the double-injection model [3]. In another model for gasochromism, hydrogen atoms of H3O+ ions diffuse into the tungsten oxide films in the initial stage of gasochromic coloration and finally the H2O molecules are released from the films [5]. In this study, if all the incorporated hydrogen atoms are released with the formation of H2O molecules, 0.11 O/W of the oxygen concentration should decrease. As the error in the O/W measurements is estimated to be less than 0.05 from the yields detected from the tungsten and oxygen elements in the RBS spectra, a difference of 0.11 O/W should be able to be detectable. It is confirmed that the behavior of oxygen atoms in the tungsten oxides is not related to the gasochromic coloration. These results are also observed in the amorphous tungsten oxides from the ERDA and NR measurements [10,11]. The XRD patterns of the oriented tungsten oxide films on the SiO2 substrates coated with the Pd catalyst before and after coloration are shown in Fig. 8(a) and (b), respectively. In the pattern of the as-prepared sample, the strong peak at 23.5° and the weak peaks at 23.1° and 24.1° indicate that the monoclinic WO3 structure is oriented along the h0 1 0i axis. When the sample is colored by exposure to hydrogen gas, the pattern changes as shown in Fig. 8(b). This pattern of the colored sample can be related to that of the tetragonal tungsten bronze (HxWO3) shown in JCPDS card 42-1261. Based on the ERDA and RBS analyses, this result corresponds to an increase in the hydrogen concentration and a constant oxygen concentration during the gasochromic coloration. These results reveal that the transformation of the crystalline structure between the monoclinic WO3 and the tetragonal HxWO3 structures is caused due to the gasochromic coloration by hydrogen gas. With regard to the transformation of the crystalline
structure from that of WO3 to that of HxWO3, it suggested that the crystal lattice of the tungsten oxide film extended in the h0 0 1i direction and shrunk along the h1 0 0i and h0 1 0i directions in a certain manner, which was caused by the incorporation of hydrogen into the crystal lattice. 4. Conclusion To investigate the hydrogen behavior in tungsten oxide films during the coloration caused by the incorporation of hydrogen, the composition and crystalline structure of the oriented tungsten oxide films were examined in bleached and colored states. XRD analysis revealed that the as-prepared films have a uniaxially oriented structure along the h0 1 0i axis of monoclinic WO3 on the SiO2 substrates at a temperature above 500 °C. The results of the ERDA and RBS for films on glassy carbon showed that the hydrogen concentration in the oriented tungsten oxide films was 0.24 H/W and it was uniformly distributed throughout the entire film. When the oriented tungsten oxide films on the SiO2 substrates coated with a 15-nm Pd layer were colored by exposure to Ar–5%H2, the light transmittance of the sample decreased to below 1% in the visible region. In the colored states, the hydrogen concentration was higher than that in the as-prepared state by 0.23 H/W, but the oxygen concentration remained constant. The crystalline structure was transformed to the tetragonal hydrogen tungsten bronze (HxWO3) structure by the incorporation of hydrogen. These results indicate that the gasochromic coloration was caused by the incorporation of hydrogen into the WO3 crystalline lattice and the transformation of the lattice between monoclinic and tetragonal structures. This result supports the double-injection model for gasochromic coloration mechanism of the tungsten oxide films.
(0 0 2)
This work was supported by a Grant-in-Aid for Scientific Research (C) from the Japan Society for the Promotion of Science (No. 19560717). The authors wish to thank Dr. Katsuyoshi Takano of the Quantum Beam Science Directorate at JAEA for several useful discussions. References
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Acknowledgments
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2θ [degree] Fig. 8. XRD patterns of 15-nm Pd-coated tungsten oxide films on SiO2 substrates: (a) for the as-prepared state and (b) for the colored state after 20 min of exposure to hydrogen gas.
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NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 301–307 www.elsevier.com/locate/nimb
Hydrogen behavior in gasochromic tungsten oxide films investigated by elastic recoil detection analysis Aichi Inouye a,*, Shunya Yamamoto b, Shinji Nagata a, Masahito Yoshikawa b, Tatsuo Shikama a b
a Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Quantum Beam Science Directorate, Japan Atomic Energy Agency, 1233 Watanuki, Takasaki 370-1292, Japan
Received 10 October 2007; received in revised form 15 November 2007 Available online 22 November 2007
Abstract Changes in the composition and crystalline structure of gasochromic tungsten oxide films resulting from the incorporation of hydrogen were investigated; the oxide films were prepared by reactive RF magnetron sputtering on SiO2 and glassy carbon substrates simultaneously. X-ray diffraction analysis of the deposited films at 600 °C showed a uniaxial oriented structure in the (0 1 0) plane of monoclinic WO3 for both substrates. The elastic recoil detection analysis (ERDA) and Rutherford backscattering spectroscopy (RBS) for the films on glassy carbon revealed that the hydrogen impurity was uniformly distributed up to a concentration of 0.24 H/W. The Pd-coated films on SiO2 turned blue when they were exposed to a mixture of Ar and 5% H2 gases. When the sample became colored, the hydrogen concentration in the film increased to 0.47 H/W and the crystalline structure of the film changed from monoclinic to tetragonal. These results indicated that the gasochromic coloration of the tungsten oxide films coincided with incorporation of hydrogen atoms into the crystalline lattice, corresponding to the formation of hydrogen tungsten bronze (HxWO3). Ó 2007 Elsevier B.V. All rights reserved. PACS: 81.15.Cd Keywords: Tungsten oxide; Hydrogen; Gasochromism; ERDA
1. Introduction Electrochromic materials such as WO3, MoO3 and V2O5 can be used in optical devices because the materials can be reversibly colored by an external voltage. Tungsten oxides exhibit cathodic coloration when incorporated with cations such as hydrogen, lithium and sodium ions [1]. When the surface of tungsten oxide is covered with a catalyst such as palladium, platinum and nickel, the optical absorption of the oxide drastically changes when exposed to hydrogen gas; this phenomenon is called gasochromism. The optical changes occur reversibly between the colored and bleached states in the presence of hydrogen and oxygen gases. The *
Corresponding author. Tel.: +81 22 215 2063; fax: +81 22 215 2061. E-mail address: [email protected] (A. Inouye).
0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.11.016
gasochromic films are expected to be used in optical hydrogen-gas-leak detectors that use fiber optics because they are considered to be operational at room temperature in addition to being highly sensitive; moreover, they do not produce noise in response to electronic effects [2]. However the mechanism of gasochromic coloration is not clearly understood. One of the widely accepted gasochromic coloration mechanism is that illustrated by the double-injection model, which is similar to the mechanism of electrochromic coloration [3]. In this model, dissociated hydrogen atoms and the electrons in the catalyst dissolve in the tungsten oxide film, and the film is colored due to the formation of tungsten bronze (HxWO3). The change in the valency of tungsten from 6+ to 5+ gives rise to the blue color [4]. In another model, the dissociated hydrogen atoms diffuse along a pore and grain boundary of tungsten oxide and
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react with oxygen on the surface of tungsten oxide, thereby resulting in the formation of water and oxygen vacancies [5]. The oxygen vacancies diffuse to the interior of tungsten oxide, making it oxygen deficient; this gives rise to the blue color. Therefore, it is necessary to understand the behavior of hydrogen and oxygen in tungsten oxide during the coloration in the presence of hydrogen gas. Moreover, it is also important to determine whether the tungsten oxide structure transforms to the HxWO3 or the WO3x structure. However, there are few reports on the crystalline structure of gasochromic materials because most of the studies have focused on amorphous tungsten oxide films. To determine the hydrogen and oxygen concentrations of tungsten oxide films in the colored and bleached states, ion-beam analyses such as the elastic recoil detection analysis (ERDA), Rutherford backscattering spectroscopy (RBS) and nuclear reactions (NR) are suitable for studying not only the surface but also the interior of samples [6–11]. In electrochromic tungsten oxide films, the nuclear reaction measurements using 15N ions showed a variation in the hydrogen content as a function of the coloration state [8]. In gasochromic tungsten oxide films, an unchanged hydrogen concentration was observed before and after coloration by nuclear reaction analysis using 11 B ions [9]. The increase in the hydrogen concentration with gasochromic coloration was also observed by nuclear reaction analysis using 15N ions [10]. However, the change in the composition of the samples during the analysis due to irradiation defects resulting from the heavy ions is a matter of concern. Therefore, ERDA performed by using 4 He ions is suitable for determining the hydrogen concentration in gasochromic tungsten oxide films; this analysis method suffers little from the irradiation effects of the analysis beam. In our previous studies, various hydrogen concentrations were observed between the bleached and colored states in amorphous tungsten oxide films by ERDA performed using 4He ions [11]. In order to reveal the gasochromic mechanism to a greater extent, it is necessary to investigate the hydrogen behavior in crystalline tungsten oxides whose crystalline structure can be observed. The incorporation of hydrogen changes the structure of tungsten oxide to HxWO3 electrochemically [12–14]. However, the crystalline structure of colored tungsten oxide films was hardly observed in previous reports because only amorphous tungsten oxide films were considered in many studies. It has been known that polycrystalline WO3 produced by heat treatment in air can hardly be colored by using hydrogen gas. By employing X-ray diffraction (XRD), the crystalline structure of a WO3 nanowire has been observed to change to the HxWO3 structure during gasochromic coloration [15]. In this study, we prepared oriented tungsten oxide films by employing the reactive RF magnetron sputtering method. Before and after gasochromic coloration, the hydrogen and oxygen concentrations and crystalline structure of the films were investigated by performing ERDA using 4He ions and XRD, respectively. Further, the hydro-
gen behavior in the tungsten oxides during gasochromic coloration is discussed. 2. Experimental Tungsten oxide films were prepared by employing the reactive RF magnetron sputtering method with a W target (purity: 3 N) in a vacuum chamber that was evacuated up to a base pressure of 5 104 Pa by using a turbomolecular pump. The tungsten oxides were deposited in a defined argon (purity: 6 N) and oxygen (purity: 4 N) mixture controlled by mass flow meters and an absolute pressure gauge (Baratron 626, MKS) under the pumping condition. The distance between the tungsten target and the substrates was fixed at approximately 100 mm. The films were deposited on SiO2 substrates, glassy carbon and a silicon (0 0 1) wafer by varying the temperature from 30 to 600 °C. The partial pressure of argon and oxygen were 135 and 20 MPa, respectively, and the sputtering power was 40 W. These deposition conditions indicate a sputtering rate of 15 nm/min. For the incorporation of hydrogen into the tungsten oxide films by exposure to hydrogen gas, a 15-nm Pd catalyst was coated on the tungsten oxide films by sputtering a Pd target (purity: 3 N) in argon at a temperature of 200 °C. In order to characterize the crystalline structure of the samples, h–2h scans of X-ray diffraction were performed by high-resolution diffractometers (X’Pert-MRD, PANalytical and Geigerflex, Rigaku). Cu Ka radiations were used with a 200-lm-thick Ni filter to minimize the Kb components. Continuous scans were performed with the following parameters; an integration time of 1 s per step, 2h resolution of 0.01° per step and a scan range of 10°–60°. For the observation of the surface morphology of the samples, a scanning electron microscope (SEM, JSM-6700F, JEOL) was employed. Ion-beam analyses were performed for the determination of the oxygen and hydrogen atomic concentration ratios (O/W and H/W) in tungsten oxide films by using a 3.0-MV single-ended accelerator at JAEA and a 1.7-MV tandem accelerator at the Institute for Materials Research, Tohoku University. For measuring the oxygen concentration, the 2.0 MeV He ions were irradiated at an incident angle of 90° with respect to the surface of the samples on glassy carbons. The backscattered He ions were detected at an angle of 165° with respect to the incident direction by using a surface barrier detector. For the estimation of the hydrogen concentration, the samples were irradiated with He ions at an incident angle of 20° with respect to the sample surface. The recoiled hydrogen atoms were detected at an angle of 30° with respect to the incident direction by using a surface barrier detector with 12-lmthick aluminum foil; this thickness was selected to avoid detecting backscattered He ions. The irradiation was performed with a flux of approximately 8 1012 ions/cm2/s and a fluence of up to 1.6 1015 ions/cm2. The absolute hydrogen concentration was estimated by considering the
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2.8 MeV, 4 He2+
Sample
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Optical window Monochrometer with CCD camera Fig. 1. Experimental apparatus for simultaneous examination of coloration properties by using hydrogen gas, and the hydrogen and oxygen concentrations in tungsten oxide films coated with a 15-nm Pd layer on SiO2 substrates.
yields of the recoiled hydrogen atoms and by using titanium hydrate as the standard sample with a know hydrogen concentration (TiH1.95). To simultaneously examine the gasochromic coloration properties and the ion-beam analysis, the light transmittance by the Pd/WO3/SiO2 samples was measured in the ion-beam analysis chamber, as shown in Fig. 1. The sample was fixed on a holder with a slit to measure the light transmittance by using a white-light-emitting diode (LED) in the wavelength range from 400 to 1000 nm. The light transmitted by the samples was focused by using double optical lenses and transported to a CCD camera by employing a monochrometer. The gasochromic coloration caused by exposure to argon and 5% hydrogen (Ar–5%H2) gas was measured as a function of time during exposure, pumping and purging. 3. Results and discussions Fig. 2 shows XRD patterns of the tungsten oxide films deposited on SiO2 substrates at substrates temperatures of (a) 600, (b) 500 and (c) 400 °C. The XRD pattern for WO3 powder is also shown in Fig. 2(d). For a substrate temperature below 400 °C, the XRD pattern shows a broad peak, indicating an amorphous structure. Above 500 °C, a diffraction peak appears at 2h = 23.5° and the intensity of the peak increases with the substrate temperature. For deposition at 600 °C, a few diffraction peaks appeared such as the side peaks at 23.1° and 24.1°. The three peaks at 23.1, 23.5 and 24.1° can be related to three strong peaks in the XRD pattern of the WO3 powder, as shown in
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2θ [degree] Fig. 2. XRD patterns of tungsten oxide films deposited by reactive RF sputtering method on SiO2 substrates at temperatures of (a) 600, (b) 500 and (c) 400 °C; (d) the pattern of WO3 powder.
Fig. 2(d). The three peaks obtained in the XRD patterns of the deposited films might be attributed to the monoclinic or orthorhombic phase of c- or b-WO3, respectively. Monoclinic and orthorhombic WO3 are nearly identical to each other; the reported lattice parameters for the mono˚ , b = 7.540 A ˚ , c = 7.692 A ˚ and clinic phase are a = 7.306 A b = 90.88°, while those for the orthorhombic phase are ˚ , b = 7.570 A ˚ and c = 7.754 A ˚ [16]. The oriena = 7.341 A tation and structure of monoclinic WO3 on amorphous SiO2 substrates have been obtained by employing sputtering methods [15,17]. Monoclinic WO3 has also been epitaxially grow on SrTiO3 (1 0 0) planes, MgO (1 0 0) planes and Al2O3 r-planes by sputtering, thermal evaporation and chemical vapor deposition methods [18–20]. For simplicity, we will continue to refer to the crystalline structure of the deposited films as the monoclinic phase, and the strong peak at 23.5° can be attributed to the (0 2 0) planes. In the case of deposition at a substrate temperature of 600 °C, the intensity of the peaks at 23.1° and 24.1° originating from (0 0 2) and (2 0 0), respectively, is considerably lower than the peak from the (0 2 0) planes. These results reveal that monoclinic WO3 can be strongly oriented on the SiO2 substrate above a temperature of 500 °C by employing reactive sputtering methods. To investigate the orientation of the tungsten oxide films deposited at 600 °C, pole figure measurement was performed at 2h = 41.9°, which is assigned to the h2 2 2i axis of monoclinic WO3. A diffraction ring was obtained at w = 55° from the (0 2 0) phase indicating that the oriented tungsten oxide films were uniaxially oriented along the h0 1 0i axis with the h0 0 1i and h1 0 0i axes being randomly directed. For the observation of a microstructure, the cleavage surface morphology at a cross section of the uniaxially oriented tungsten oxide films the on the silicon (0 0 1) wafer
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was observed by the SEM, as shown in Fig. 3. We can recognize the columnar structure with a width of 100 nm on the substrate. It is considered that the crystalline structure of the columnar particles is oriented along the (0 1 0) plane of monoclinic WO3 with the h0 0 1i and h1 0 0i axes in the plane being randomly directed. Fig. 4 shows a typical RBS spectrum for the oriented tungsten oxide film deposited on glassy carbon at the substrate temperature of 600 °C. The peaks originating from the tungsten, oxygen and carbon atoms of the sample are
Fig. 3. SEM image of a cross section of the oriented tungsten oxide films deposited on silicon (0 0 1) wafer at a substrate temperature of 600 °C.
found in the backscattering energy ranges of 1620– 1840 keV, 530–700 keV and below 330 keV, respectively. With respect to the peaks originating from tungsten and oxygen, the peak shape can be explained by the relation between the scattering cross sections and the energy loss of the incident ions, indicating that the oxygen/tungsten atomic ratio (O/W) is homogeneous throughout the WO3 film. The O/W ratio in the film was estimated to be 3.0 ± 0.05 by counting the total backscattered yields from the tungsten and oxygen atoms. The oxygen concentration in the tungsten oxide films was unchanged when the substrate temperature was varied from 30 to 600 °C. It was clarified that the WO3 films could be obtained at any substrate temperature by deposition at an oxygen partial pressure of 20 MPa in the sputtering gas. By measuring the width of the tungsten peak in the RBS spectrum and considering the WO3 composition, the film thickness is estimated to be approximately 380 nm. A film thickness of 390 nm was obtained by using a surface profilometer for the same sample. By comparing the estimated film thickness with the measured one, the atomic density of the film can be calculated to be 7.4 1022 atoms/cm3 (7.3 g/cm3); this is approximately equal to the density of polycrystalline monoclinic WO3, which has been reported to be 7.55 1022 atoms/cm3 (7.3 g/cm3) (JCPDS card No. 431035). Fig. 5 shows the ERDA spectrum for the tungsten oxide film deposited on glassy carbon at 600 °C. This shape of the ERDA spectrum indicates that the hydrogen atoms are uniformly distributed in throughout the film. The hydrogen concentration is estimated to be approximately 0.24 H/W (5.7 at.%) by counting the total yields in this spectrum and comparing it with the yields of the standard TiH1.95 sample. The hydrogen present in the film is assumed to
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Energy [keV] Fig. 5. Typical ERDA spectrum (obtained by using 2.8 MeV He ions) for the oriented tungsten oxide film deposited on glassy carbon at a substrate temperature of 600 °C.
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have originated from water impurities in the sputtering gas or sputtering chamber and is considered to be incorporated in WO3 after the deposition procedure due to the release of hydrogen below the temperature of 400 °C [11]. The incorporated hydrogen is considered to form water molecules in tungsten oxide films deposited by thermal evaporation; this assumption is supported by the high value of the estimated O/W ratio (above 3) [6]. There is a difference between the reported O/W ratio and the estimated one in this study; the film composition is found to almost stoichiometric WO3 from the RBS spectrum. If it is considered that all the hydrogen atoms in the films form H2O, the composition of the tungsten oxide films should be nonstoichiometric WO3x such as WO2.88. This oxygen deficiency is considered to make the tungsten oxides bluer as compared to the color of WO3. Hydrogen tungsten bronze (HxWO3) also had a blue or yellow color when the hydrogen concentration was changed [1]. The light green color of the as-prepared WO3 films obtained in this study indicates that hydrogen in the films does not form bronzes, hydrates, or water molecules; however, it might be trapped in vacancies or at grain boundaries. To examine the gasochromic properties of the tungsten oxide films, the light transmittance of the oriented tungsten oxide film deposited on the SiO2 substrates coated with a 15-nm-thick Pd layer was measured in the as-prepared state, after being colored by exposure to Ar–5%H2 gas, and after being bleached by exposure to air on subsequent coloration. Fig. 6 shows the transmittance spectra for the Pd-coated samples in the (a) as-prepared, (b) colored and (c) bleached states in addition to that of (d) the sample without a Pd layer. The transmittance of the as-prepared
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sample is approximately 30% for wavelengths above 450 nm. After the sample is exposed to the Ar–5%H2 gas for 20 min, the transmittance decreases to 1% at wavelengths from 650 to 1000 nm. When exposed to air after the coloration, the sample is bleached up to a transmittance of 20% in all the wavelength regions. It was also confirmed that this bleaching cannot be caused in Ar, N2 and vacuum, indicating that we can handle the colored samples by maintaining the coloration levels in vacuum for the determination of atomic concentrations with the ion-beam analysis. The ERDA spectra for the as-prepared and colored samples on SiO2 substrates are shown in Fig. 7(a) and (b), respectively. In the case of the as-prepared sample, the ERDA spectrum shows that the hydrogen atoms are uniformly distributed throughout the film, with a concentration of approximately 0.24 H/W (5.8 at.%). As the film is colored by the hydrogen gas, hydrogen profiling shows that the hydrogen concentration of 0.23 H/W (5.4 at.%) increases throughout the film. It was also confirmed that the hydrogen concentration in the bleached states was similar to that in as-prepared films. It is elucidated that the hydrogen atoms are incorporated into the tungsten oxide films and diffuse throughout the film during gasochromic coloration. The release of the incorporated hydrogen by exposure to oxygen or air is also revealed. With respect to the oxygen concentration in the colored samples, the RBS measurements for the films deposited on glassy carbon showed that the oxygen concentration in the films was unchanged during the repeated changes between colored and bleached states. The determination of the hydrogen and oxygen concentrations by the ERDA and RBS measurements reveals that the gasochromic coloration of tungsten oxide films by hydrogen gas coincides with the incorporation of hydrogen; this occurrence is similar to
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Wavelength [nm] Fig. 6. Light transmittance spectra for 15-nm Pd-coated tungsten oxide films on SiO2 substrates (a) for the as-prepared state, (b) for the colored state after 20 min of exposure to hydrogen gas, (c) for the bleached state after 20 min of exposure to the air after the coloration and (d) light transmittance spectrum for the sample without the Pd layer.
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Energy [keV] Fig. 7. ERDA spectra for 15-nm Pd-coated tungsten oxide films on SiO2 substrates: (a) for the as-prepared state and (b) for the colored state after 20 min of exposure to hydrogen gas.
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that in the mechanism corresponding to the double-injection model [3]. In another model for gasochromism, hydrogen atoms of H3O+ ions diffuse into the tungsten oxide films in the initial stage of gasochromic coloration and finally the H2O molecules are released from the films [5]. In this study, if all the incorporated hydrogen atoms are released with the formation of H2O molecules, 0.11 O/W of the oxygen concentration should decrease. As the error in the O/W measurements is estimated to be less than 0.05 from the yields detected from the tungsten and oxygen elements in the RBS spectra, a difference of 0.11 O/W should be able to be detectable. It is confirmed that the behavior of oxygen atoms in the tungsten oxides is not related to the gasochromic coloration. These results are also observed in the amorphous tungsten oxides from the ERDA and NR measurements [10,11]. The XRD patterns of the oriented tungsten oxide films on the SiO2 substrates coated with the Pd catalyst before and after coloration are shown in Fig. 8(a) and (b), respectively. In the pattern of the as-prepared sample, the strong peak at 23.5° and the weak peaks at 23.1° and 24.1° indicate that the monoclinic WO3 structure is oriented along the h0 1 0i axis. When the sample is colored by exposure to hydrogen gas, the pattern changes as shown in Fig. 8(b). This pattern of the colored sample can be related to that of the tetragonal tungsten bronze (HxWO3) shown in JCPDS card 42-1261. Based on the ERDA and RBS analyses, this result corresponds to an increase in the hydrogen concentration and a constant oxygen concentration during the gasochromic coloration. These results reveal that the transformation of the crystalline structure between the monoclinic WO3 and the tetragonal HxWO3 structures is caused due to the gasochromic coloration by hydrogen gas. With regard to the transformation of the crystalline
structure from that of WO3 to that of HxWO3, it suggested that the crystal lattice of the tungsten oxide film extended in the h0 0 1i direction and shrunk along the h1 0 0i and h0 1 0i directions in a certain manner, which was caused by the incorporation of hydrogen into the crystal lattice. 4. Conclusion To investigate the hydrogen behavior in tungsten oxide films during the coloration caused by the incorporation of hydrogen, the composition and crystalline structure of the oriented tungsten oxide films were examined in bleached and colored states. XRD analysis revealed that the as-prepared films have a uniaxially oriented structure along the h0 1 0i axis of monoclinic WO3 on the SiO2 substrates at a temperature above 500 °C. The results of the ERDA and RBS for films on glassy carbon showed that the hydrogen concentration in the oriented tungsten oxide films was 0.24 H/W and it was uniformly distributed throughout the entire film. When the oriented tungsten oxide films on the SiO2 substrates coated with a 15-nm Pd layer were colored by exposure to Ar–5%H2, the light transmittance of the sample decreased to below 1% in the visible region. In the colored states, the hydrogen concentration was higher than that in the as-prepared state by 0.23 H/W, but the oxygen concentration remained constant. The crystalline structure was transformed to the tetragonal hydrogen tungsten bronze (HxWO3) structure by the incorporation of hydrogen. These results indicate that the gasochromic coloration was caused by the incorporation of hydrogen into the WO3 crystalline lattice and the transformation of the lattice between monoclinic and tetragonal structures. This result supports the double-injection model for gasochromic coloration mechanism of the tungsten oxide films.
(0 0 2)
This work was supported by a Grant-in-Aid for Scientific Research (C) from the Japan Society for the Promotion of Science (No. 19560717). The authors wish to thank Dr. Katsuyoshi Takano of the Quantum Beam Science Directorate at JAEA for several useful discussions. References
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Acknowledgments
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2θ [degree] Fig. 8. XRD patterns of 15-nm Pd-coated tungsten oxide films on SiO2 substrates: (a) for the as-prepared state and (b) for the colored state after 20 min of exposure to hydrogen gas.
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