WO3 and the role of hydrophilicity of tungsten oxide films

Sensors and Actuators B 188 (2013) 127–136

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Hydrogen sensing by wet-gasochromic coloring of PdCl2 (aq)/WO3 and the role of hydrophilicity of tungsten oxide films M. Allaf Behbahani, M. Ranjbar ∗ , P. Kameli, H. Salamati Department of Physics, Isfahan University of Technology, Isfahan 84156-83111, Iran

a r t i c l e

i n f o

Article history: Received 13 December 2012 Received in revised form 23 April 2013 Accepted 30 June 2013 Available online xxx Keywords: Tungsten oxide films Gasochromic Pulsed laser deposition (PLD) Aqueous PdCl2 Hydrophilicity FTIR

a b s t r a c t In this study, WO3 /glass thin films were prepared by pulsed laser deposition (PLD) at 100 mTorr oxygen pressure and substrate temperatures of 25, 100, 200, 300 and 400 ◦ C for gasochromic investigations. In the presence of H2 , a wet-gasochromic switching with an edge-to-center coloring nature was observed when aqueous PdCl2 was used as hydrogen catalyst. A correlation between substrate temperature, hydrophilicity, palladium growth and gasochromic coloring of tungsten oxide films was observed and analyzed by Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscope (FE-SEM) and water contact angle measurement. Results showed that the samples made at 200 ◦ C have the best gasochromic properties; coloring rate and deepness. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Recently, the gasochromic coloration of WO3 films coated with noble metal (Pt, Pd) catalysts have became more interesting due to their applications in smart windows and optical gas sensor devices. They are transparent in the visible spectral range but change their optical properties reversibly from a transparent to a dark blue, when exposed to H2 gas [1–4]. In gasochromic process, H2 molecules undergoes catalytic dissociation into H+ ions and electrons (spillover process) then these ion–electron pairs transfer into the WO3 layer and cause change in the optical transmittance based on small polaron transitions mechanism [4]. This process is often reversible, i.e. whenever the colored film is exposed to O2 gas, the small polarons states are recovered and finally the layer becomes bleached. Many techniques had been used to fabricate tungsten oxide films, including sputtering [5–7], evaporation [8–10], sol–gel [11–14], pulsed laser deposition (PLD) [11,15–18], etc. PLD technique is a successful method to fabricate compound films such as metal oxides. It can be accompanied with oxygen to retain the stoichiometry of films [16,19]. As mentioned, it is essential to use thin Pt or Pd layers as hydrogen catalyst over the surface of prepared WO3 films in gasochromic systems. Therefore the characteristic of coated catalyst would

∗ Corresponding author. Tel.: +98 3113912375; fax: +98 3113912376. E-mail address: [email protected] (M. Ranjbar). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.06.097

control the dissociation rate, hence the coloring-bleaching rate [20]. Pd is a well-known catalyst for hydrogen and there are different ways for its deposition, such as sputtering [21–23], evaporation [24], electroless [17] and hydrogen reduction of PdCl2 [11]. Electroless deposition of palladium has a well-established mechanism. it is based on the reduction of a meta-stable metallic salt complex on an activated substrate in which metal ions undergo normally a reducing mechanism from the liquid to a metallic phase over the whole substrate or some selective positions [25]. In the reduction method, PdCl2 is drop-dried over the surface and steam of hydrogen containing gas convert the PdCl2 (solid)/WO3 into Pd/WO3 , through a reducing reaction. Hydrogen gas acts as a common reducing agent with no residual chemical impact on the system hence the produced catalyst layer has high purity. Recently we have observed an interesting wet-gasochromic coloring properties for PdCl2 (aq)/WO3 . There, substrate films were prepared by spin-coating sol–gel method and the effect of its annealing temperature on gasochromic coloring was studied [26]. In this work, we examined this idea for pulsed laser deposited WO3 thin films and the effect of substrate temperature was traced. The main motivation is the enhanced films’ porosity, as an effective factor in gas-surface interaction process, obtained at PLD method [11]. Due to aqueous nature of PdCl2 , the hydrophilicity of layers is expected to play important role in feature of palladium growth as well as the gasochromic behavior. We observed that the WO3 film properties including structural, optical, surface morphology and also hydrophilicity changes with substrate temperature.

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(002) (112) (202) (141)

(221)

(020) (200) (111)

Intensity (a.u.)

Tungsten oxide films were fabricated by PLD method from tungsten oxide pressed powder onto circular glass substrates of 11 mm diameter which were cleaned ultrasonically by methanol and DI water. The employed deposition system was a stainless steel chamber with a base pressure of 1 × 10−5 Torr. In order to deposit stoichiometric tungsten oxide films, the PLD process was carried out in 100 mTorr oxygen (purity 99.9%) pressure. The substrates could be held at different temperatures ranging from room temperature up to 1000 ◦ C by means of an electric heater directly placed on the back of substrate. The substrate temperatures were RT (S25), 100 ◦ C (S100), 200 ◦ C (S200), 300 ◦ C (S300), 400 ◦ C (S400). To ablate the tungsten oxide targets, 5000 pulses of a KrF laser ( = 248 nm,  = 10 ns, a laser energy of 200 mJ and R.R = 10 Hz) was delivered on the surface of rotating target at a 45◦ angle. The substrate to target distance was 7 cm. The main aqueous PdCl2 (aq-PdCl2 ) catalyst solution were prepared by solving a 0.02 g PdCl2 powder (5 N), 99.9 cm3 DI water and 0.1 cm3 HCl. Then drops of 0.07 cm3 of 0.2 g/l PdCl2 solution were put on the surface of layers. The crystalline structure of the films deposited at different substrate temperatures were characterized using X-ray diffractometer (Philips EXPERT MPD) with Cu-k␣ ( = 0.154 nm) radiation. The morphology of the films was investigated using FE-SEM (Hitachi model S4460) and AFM (Bruker, model Nanos 1.1). The chemical bonds of the samples were determined by FTIR spectroscopy in the mid-infrared range of 600–4000 cm−1 using Bruker spectrometer (model Tensor 27). The contact angle measurements were performed in atmospheric air at room temperature using a commercial contact angle meter (Data physics OCA 15plus) with ±1◦ accuracy. A droplet was injected on the surface using a 2 ␮l micro-injector. Optical transmission spectra of the tungsten oxide films were investigated in the visible and near IR region, using a UV–vis spectrophotometer (model Perkin-Elmer lambda 25). To record the coloring/bleaching response of the samples, we used a sealed gas chamber. A steam of 10% H2 /Ar (2 l/min) or O2 (2 l/min) gas were alternatively delivered over the PdCl2 (aq)/WO3 sample for gasochromic experiments. The gas flow was controlled by means of mass flow controllers (MFC). A laser beam ( = 780 nm) was delivered to 1 mm distance from the samples’ edge and transmission beams were measured by a phototransistor detector on the other side.

(001)

2. Experimental

(220)

128

S400 S300 S200 S100 S25

10

30

50

70

2θ (degree) Fig. 1. XRD patterns of WO3 /glass films deposited at different substrate temperature.

refractive index of films with substrate and the interference of multiple reflections originated from film and substrate surface [28]. The sharp reduction in transmittance in the region of 300–400 nm in the transmission spectra of the films corresponds to the fundamental absorption edge of WO3 films. From the transmission spectra, it is evident that the most transparent films were obtained at room

3. Results and discussion 3.1. Characterization 3.1.1. XRD measurements X-ray diffraction (XRD) measurements were carried out to study the temperature dependence of the crystalline structure of the laser ablated WO3 thin films. Fig. 1 shows XRD patterns of deposited films at different substrate temperatures. The films made up to temperature of 300 ◦ C, were amorphous, but crystallized films were obtained at 400 ◦ C. The crystallization of WO3 thin films is usually reported to be in the range of 300–400 ◦ C [16,27]. The sample S400 show an almost well defined diffraction peaks corresponding to lattice reflection planes, characteristics of orthorhombic structural of WO3 , as reported by the JC-PDS 20-1324. The average grain sizes of the sample S400 were calculated to be about 22 nm from Scherrer’s formula and using a shape factor of 0.9. 3.1.2. Optical properties The optical transmission spectra of the films, deposited at different temperature, for the range of 300–1100 nm wavelength, are shown in Fig. 2(a). The oscillations in the transmission spectra are the product of optical interference arising due to difference of

Fig. 2. (a) Optical transmission spectra and (b) plots of (˛h)1/2 vs. h (tauc plots) of WO3 thin films deposited at different substrate temperatures.

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Table 1 FTIR vibrational band frequencies (in cm−1 ) and related band assignments for WO3 films deposited at different temperatures. Band assignment

Deposition temperature and vibrating bands

␯(W O W) ␯(O W O) ␯(W O W) ␯(W O) ␦(W OH OH2 ) ␦(H2 O) ␯(OH)

RT

100 ◦ C

200 ◦ C

300 ◦ C

400 ◦ C

– 756 889 1066 1426 1620 2928 3180 3420

634 770 889 1066 1426 1626 2930 3236 3503

634 765 891 1060 1426 1628 2960 3210 3455

632 767 902 1060 1426 1630 2974 3210 3451

617 802 962 1060 1456 1659 3020 3288 3534

temperature and increasing the substrate temperature reduces the transparency of the films in the visible region. Moreover, it is also found that the absorption edge shifted to larger wavelength regions by increasing the substrate temperature, this is an indication of a shift in the optical band gap toward lower energy. From the optical transmission spectra of the films below the absorption edge, one can determine the band gap energy using the following relation [29]: ˛h = B(h − Eg )

2

(1)

where h is the photon energy, Eg the optical energy gap and ˛ is the optical absorption coefficient and B is a constant. Using the linear extrapolation of the (˛h)1/2 vs. photon energy, the optical band gap energy is obtained (Fig. 2(b)). The band gap values are found to be 3.35, 3.3, 3.2, 3.18 and 2.98 eV for the samples S25, S100, S200, S300 and S400, respectively. Dominant reduction of optical band gap energy at 400 ◦ C, resulting from the crystallization of the WO3 films at this temperature. 3.1.3. Chemical bonds variations In order to study the effect of substrate temperature on the chemical bonds of the WO3 thin films, the FTIR spectroscopy was used. FTIR spectra of WO3 thin films deposited at different substrate temperatures after 25 days of storage in atmospheric conditions allowing interaction with air, are shown in Fig. 3 (a and b). Table 1 presents the vibration band related to each absorption peak. The bands in the range 600–700 cm−1 and 800–900 cm−1 are attributed to the W O W bridging mode of the WO6 corner-sharing species [30,31]. The bands located at ∼770 cm−1 and ∼1060 cm−1 are due to the O W O inter bridging stretching and W O modes in WO3 , respectively [31–34]. According to Fig. 3(a), one can see that the W O W band at ∼630 cm−1 is rather weak in films deposited at low substrate temperatures but its intensity increases with temperature and being intense for sample S400 in agreement with its crystalline structure [34]. FTIR is very sensitive to the presence of the OH groups and direct evidence of presence water in films can be deduced from the region 1300–4000 cm−1 . As Fig. 3(b) shows, all samples except S400, exhibit two intense peaks at ∼1426 and ∼1626 cm−1 in addition to a broad band consisting of three major peaks at 2700–3700 cm−1 . The intense peak at ∼1426 cm−1 is due to the ␦(W OH OH2 ) mode of hydroxyl groups flanked by tungsten on one side and hydrogen bonded with water molecules, on the other side [5,31,33]. Two last bands correspond to ␦(H2 O) and ␯(OH) vibration modes of molecular water, respectively [5,9]. Three components in ␯(OH) arising from different modes of molecular water involving hydroxylation (3400–3500 cm−1 ), hydration (2950–3050 cm−1 ) and surface adsorbed water (3200–3250 cm−1 ) [8]. As Fig. 3(b) shows, intensity of ␦(W OH OH2 ) and ␦(H2 O) bands as well as splitting the

Fig. 3. FTIR absorption spectra in range of (a) 600–1300 cm−1 and (b) 1300–4000 cm−1 of 25 days aged WO3 /glass films deposited at different substrate temperatures, and (c) absorption spectra of sample S100 in range of 1300–4000 cm−1 , at different aging stages: immediately after deposition, after 2, 9 and 25 days of storage in atmosphere condition. *Denotes CO2 absorption peaks.

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δ (W-OH-OH2) Surface H-OH mode

Relative area (%)

70

Hydration mode Hydroxylation mode

50

30

10 0

100

200

300

400

Substrate temperature (°C) Fig. 4. The relative area of ␦(W OH OH2 ) and different components of ␯(OH) mode in WO3 films deposited at different substrate temperatures after 25 days aging.

high-frequency ␯(OH) band into components, differs for different substrate temperatures. The great difference takes place about sample S400, in which ␦(W OH OH2 ) band was disappeared and splitting of ␯(OH) is not clearly observable. In order to trace the origin of water in the films and better interpretation of these differences, the FTIR spectra of a typical sample (S100), were recorded at different aging stages including just after deposition, after 2, 9 and 25 days (Fig. 3(c)). As Fig. 3(c) shows, at early aging stages only the ␦(H2 O) and broad ␯(OH) vibration modes are present indicating structural water could partially originate from deposition process. During aging period, ␦(W OH OH2 ) band at 1426 cm−1 appears and gradually becomes intense over the aging time. At the same time, the broad ␯(OH) peak deforms and the intensities of its corresponding component change. Decomposition of ␯(OH) band into the three components (Fig. 3(c)) shows that the dominant feature of the structural H2 O for as-deposited film is the hydroxylation mode and only a small amount of the hydration mode of that is exist, moreover, there is some amount of surface water, too. Aging causes the contribution of hydration and surface water modes to increase. The bending vibration of hydration mode is certainly responsible of the appearance of the ␦(W OH OH2 ) band at 1426 cm−1 [5,32]. In the earlier studies by Daniel [5] and Rougier [16], the splitting of the broad band and the presence of the peak at 1425 cm−1 , were also observed and has been attributed to adsorbed water from air moisture followed by gradual interaction with tungsten oxide matrix by hydroxylation and/or hydrolysis process during aging period. These observations suggest that the observed differences in Fig. 3(b) for films deposited at different temperature are resulted from differences in the sensitivity of that to air moisture. A high sensitivity of a film to ambient air most likely arises from its porosity [16]. Therefore, the degree of films porosity is perceivable based on FTIR spectra. To realize more precisely the role of substrate temperature on porosity, the relative area of ␦(W OH OH2 ) as well as three modes of ␯(OH) were plotted versus substrate temperature in Fig. 4. As the figure shows, at Ts = 200 ◦ C the relative peak area of both ␦(W OH OH2 ) and hydration mode of ␯(OH) has maximum values. So sample S200 in comparison with other samples, has been more affected by aging because it probably has the highest sensitivity to moisture and the highest porosity, whereas the sample S400, for example, is not so. These findings are confirmed by the FE-SEM micrographs (next section). Fig. 4 shows that the relative area of hydroxylation mode and surface H-OH is also varied by substrate temperature which will be discussed for hydrophilicity properties of samples.

3.1.4. Morphology The effect of substrate temperature on the surfaces morphology of WO3 thin films was investigated by FE-SEM and atomic force microscope (AFM) (Fig. 5). As seen from FE-SEM micrographs, all sample types demonstrate a porous surface consisting of many small discrete islands separated by different surface cracks. By increasing temperature up to 200 ◦ C the islands size and cracks dimensions become smaller while the number of cracks increases which result in an enhanced surface porosity. At higher substrate temperatures, Ts > 200 ◦ C, grains are fused partially together and hence the surface become slightly denser. The variation of surface porosity with substrate temperature is in agreement with the FTIR results. Porosity is an effective factor for gas-surface interaction of metal oxide semiconductor thin films because the gas sensor action is usually a surface phenomenon and is expected to be accomplished better in highly porous films. Since PLD involves complicated physical phenomena, the effect of substrate temperature on surface morphology is not very clear for us and should be further studied. The root mean square (RMS) surface roughness of the films derived from AFM data are plotted as a function of substrate temperature in Fig. 5(f). The high surface roughness at low temperatures (25 and 100 ◦ C) is partially due to the presence of large surface cracks. By increasing substrate temperature up to 300 ◦ C, the surface roughness decreases and films with smooth surface obtained. By further increasing substrate temperature to 400 ◦ C, the surface roughness of the films increases slightly and the film surface becomes uniformly high-density granular. The temperature dependence of surface roughness and morphology of WO3 thin films can be explained on the basis of the difference in the mobility of ablated species on the substrate surface as follows. When the substrate temperature is low, adatom migration on substrate and self-surface diffusion are very limited and consequently nuclei are randomly oriented which leads to high surface roughness [35]. With increasing the substrate temperature, the surface diffusion is activated [35,36]. As a result of noticeable self-surface diffusivity of adatoms, smooth surface develops. Aided by the high substrate temperature, the grains in the films are fused compactly together to form polycrystalline film. The polycrystalline films have a highdensity granular structure, which can be seen in the AFM image of the sample S400 (Fig. 5(e)).

3.1.5. Hydrophilicity The effect of substrate temperature on hydrophilicity of WO3 thin films was investigated by water contact angle measurement. The contact angle of water drop as a function of deposition temperature is shown in Fig. 6. As figure shows, the contact angle for sample S25 is ∼35◦ and by increasing the substrate temperature, the contact angle decreases to ∼27◦ for sample S100 then increases up to 56◦ for S400. The behavior of hydrophilicity variation with substrate temperature is similar to surface water and opposite of hydroxylation modes (Fig. 4). The higher the hydrophilicity, the more absorption of surface water. Opposite behavior of hydrophilicity with the structural water mode were also observed elsewhere for the WO3 [13] and SiO2 thin films [37]. These findings suggest the amount of hydroxylation mode determines the hydrophilicity of WO3 thin films. In examination of wet-gasochromic switching of PdCl2 (aq)/WO3 , droplet of PdCl2 would spread differently on the surface of samples with different hydrophilicity. Since in our aqueous catalyst based route, the palladium nanoparticles are produced through a reduction reaction within the droplets, the surface hydrophilicity of tungsten oxide films is expected to play important role in palladium growth and gasochromic response.

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Fig. 5. FE-SEM and typical AFM micrographs of WO3 /glass films deposited at different substrate temperatures of (a) room temperature, (b) 100, (c) 200, (d) 300 and (e) 400 ◦ C, (f) RMS surface roughness versus substrate temperature.

4. Gasochromic properties By exposing PdCl2 (aq)/WO3 to hydrogen containing gas, we observed an interesting wet-gasochromic coloring effect. Fig. 7 shows photograph images of the various coloring stages for samples made at different substrate temperatures. As can be seen, coloring started from the boundary edge of catalyst drop as a tiny blue ring and over the time its area gradually extends into the center until the beneath surface of drop is uniformly colored. This evolution suggests that the hydrogen initially enters from side to the edge of the droplet where triple points are consisting of substrate, liquid and gas. By hydrogen exposing, palladium nanoparticles probably with PdHx composition are produced over the surface as well as in the triple points (see gray stains in Fig. 7). The surface diffusion of the nanoparticles from the surface to the triple points is

also possible and thus the hydrogen transferring into the tungsten oxide film and consequent coloring begins. The formation of palladium in the period of hydrogen exposure is accompanied by gasochromic coloring of tungsten oxide layer. At initial hydrogen exposure, however, the reduction of palladium is dominant. It is predicted that hydrogen as reducing agent could help accumulation a layer of palladium nanoparticles that will be left after drop evaporation. It is schematically shown in Fig. 7(b). At temperatures below 200 ◦ C, a uniform color state is obtained and the coloring uniformity is reduced with increasing deposition temperature which is more pronounced for 400 ◦ C. Additionally, at 200 ◦ C we have the deepest blue color. Due to the interaction of hydrogen with PdCl2 drop, palladium nanoparticles gradually begin to reduce within or on the surface of the drop. The produced palladium layers feature, after allowing solution to be dried, was found

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Fig. 6. Contact angle of water for the WO3 /glass thin films as a function of substrate temperature.

to depend on the hydrophilicity of tungsten oxide surface. FESEM micrographs of dried samples after two complete coloring cycles corresponding to different temperatures were compared in Fig. 8. For all cases, two different kinds of palladium agglomerates exist; particles attached to the surface forming a continuous layer of palladium and particles accumulated on the surface as fractal-like structures (denoted A-Pd). The initial granular surfaces of tungsten oxide substrates were completely covered by the continuous layer hence are not longer observable. The second type particles originate from hydrogen-induced nucleation inside the solution which are inherently under random Brownian motions. The fractal-like assembly resulting from the Brownian motion of pre-nucleated Pd nanoparticles which is known as diffusion limited aggregation (DLA) phenomenon [38]. The individual agglomerates after sorting by size (shown in the corner of Fig. 8) make obvious the growth and branching steps of a large fractallike structure. The fractals are not very gasochromically important because they are not firmly attached to the substrate and did not participate in covering the tungsten oxide pores. However, there is a relation between the hydrophilicity and the number of fractals. Sample S100 with the highest hydrophilicity involves the lowest number of fractals. The number of fractals increases with decreasing hydrophilicity. Especially in the case of S400, two-dimensional branching of fractals transform into the threedimensional flower-like constructions. We believe in this method thickness of palladium layer increases with hydrophilicity because the more hydrophilic surface undergoes more palladium deposition at substrate-solution interface. H2 –Pd interaction induces irreversible stress in palladium layer and crack formation at the surface [17,39,40]. The presence of long cracks in sample S100 may be a result of its higher thickness. Growth of palladium on the pores’ wall promotes the diffusion and dissociation of hydrogen molecules and consequently the gasochromic effect. This result shows that the adhesion of palladium particles on the hydrophilic surfaces is stronger than the adhesion between palladium particles themselves. In other word, the tendency of palladium nanoparticles to attach each other is more than their tendency to attach to the film of tungsten oxide at low hydrophilicity. In order to investigate the effect of substrate temperature on gasochromic response time of the PdCl2 (aq)/WO3 films, time evolution of optical density difference (OD) at  = 780 nm was measured in the presence of 10%H2 /Ar or O2 gases. Optical density difference is equal to −ln[T (t)/T0 ] where T and T0 are transmittance at time t and 0, respectively. The responses for two first coloring/bleaching cycles are shown in Fig. 9. This data were taken from

Fig. 7. (a) Photograph images of coloration process of PdCl2 (aq)/WO3 /glass samples deposited at different temperature at different stages of hydrogen gas exposing. (b) Schematic view of the gasochromic coloration of PdCl2 (aq)/WO3 /glass samples: by hydrogen exposure, palladium nanoparticles are produced over the surface as well as in the triple points and then coloring started from the boundary edge of catalyst drop as a tiny blue ring and over the time its area gradually extends into the center. After the drying process, a thin layer of palladium is formed. (For interpretation of the references to color in the artwork, the reader is referred to the web version of the article.)

the edge of the drop i.e. very close to triple points. In the first cycles of all cases, shortly after exposure of hydrogen gas optical density is almost constant. In this period, the reduction reaction has not yet completed and therefore hydrogen ions are not injecting sufficient into the WO3 layer hence coloring begins with delay. The time delay at the first cycle of sample S400 is longer than those of other cases. Since the main reduction reaction takes place during the first cycle, this effect is attributed to the ineffective reduction of palladium at the interface of low hydrophilic surface which is also confirmed by FE-SEM micrographs. By further hydrogen exposure, optical density gradually increases over the time and reaches saturation after a while. By replacing hydrogen steam with oxygen, optical density decreases and almost returned to its initial values. The coloring/bleaching processes were repeated again with

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Fig. 8. FE-SEM micrograph of palladium growth after two coloring cycles followed by drying. The corresponding WO3 /glass films were deposited at (a) 25, (b) 100, (c) 200, (d) 300 and (e) 400 ◦ C. Two different kinds of palladium morphology can be seen including a continuous Pd layer and fractal-like aggregates (A-Pd). The individual agglomerates after sorting by size (the corner of part e) make obvious the growth and branching steps of a large fractal-like structure.

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2.5

10

(a)

8

2

6 4 1

2

2.5

10

(b) Axis Title

2

8 6

Max coloring rate (ms-1)

Saturation value of ΔOD

1.5

1.5 4 1

2 0

0.5 0

100

200

300

400

500

Substrate temperature (˚C) Fig. 10. Saturation values of OD and maximum of coloring velocity determined by time derivation of curves in Fig. 9, as a function of substrate temperature at (a) first cycle and (b) second cycle.

Fig. 9. Two first cycles of time evolutions of OD ( = 780 nm) at the triple points and in the presence of 10%H2 /Ar or O2 gases obtained for PdCl2 (aq)/WO3 samples with different substrate temperatures.

alternative exposure of hydrogen and oxygen gases. The initial delay is not observed in the second cycles because the significant portion of PdCl2 could be converted in the first cycle. The maximum values of OD and coloring velocity in the first and second cycles for different substrate temperatures are compared in Fig. 10. Here, the maximum coloring rates were obtained from maximum time-derivative of OD curves. One can see in comparison with

the other samples, the saturated optical density of samples S200 and S400 is considerably higher. But the sample S400 undergoes very slow and non-uniform gasochromic coloring which is probably due to the crystalline structure as well as non-uniform growth of catalyst on the surface of that. At the second cycles the saturated optical densities remain almost unchanged while the coloring rates increase. Moreover one can see in the first cycle, sample S100 shows the highest velocity while in the second, sample S200 has the highest coloring velocity. According to hydrophilic properties, this can be attributed to the faster accumulation of palladium layer in the first cycle over the sample S100 which showed the most hydrophilicity. In the second cycle, one can assume that palladium layer was completely deposited and tungsten oxide layer properties play dominant role in coloring process. Indeed the first cycles could be considered as a preparation stage. Bleach and colored optical properties of samples after a long coloring experiment followed by a drying stage are compared in Fig. 11. ODs as a function of wavelength were calculated as ln(Tb ()/Tc ()) where Tb () and Tc () are the bleached and colored transmutation at wavelength , respectively. As can be seen, optical density of coloring is higher in near IR region, which is a result of formation of small polarons responsible for gasochromic coloring of tungsten oxide bronze [41]. Due to fluctuations in the bleach state spectra, OD function is not of sufficient accuracy. However, the total optical densities obtaining from OD integral can be better compared. The total optical densities are shown in part (d) for different deposition temperature indicating a maximum for samples S200. Regarding to the maximum coloring velocity and optical densities, it can be concluded that the sample S200 has the best gasochromic performance. As indicated by FTIR, this sample is the most sensitive to moisture due to its high porosity.

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100

(a) (a) Transmitance (%)

80 60 40 20

100 0

310(b) (b)410

510

610

80

Transmitance (%)

710

810

910

1010 1110

Ts= 25 °C

Axis Title

S25

Ts= 100 °C S100

Ts= 200 °C S200

60

135

at 200 ◦ C. It was attributed to the high porous nature of this layer, which was confirmed by FE-SEM images. We found the hydrophilicity of layers depends on the substrate temperature so that the most hydrophilicity was found at a temperature of 100 ◦ C. The surface of deposited films was covered with few drops of PdCl2 solution as aqueous palladium source for catalytic hydrogen dissociation in gasochromic process. By exposure hydrogen, PdCl2 (aq)/WO3 was converted to Pd/WO3 through a wet reduction reaction. At the same time, a wet-gasochromic switching began to spread from the edge and extended to the center of circular substrates. FE-SEM micrographs of Pd/WO3 samples after drying show two types of morphology; a continuous covering and distinct fractal-like structures of palladium. Number of fractals and branching generally increases with decreasing hydrophilicity. It was explained by assuming palladium ions and particles, like to water molecules, to be absorbed better by a hydrophilic surface. We found that a deposited sample at 200 ◦ C is the best in terms of rate and depth of coloring. Based on FESEM and FTIR observations this sample has higher surface porosity.

Ts= S30300 0 °C S40400 0 °C Ts=

40

Acknowledgment The authors would like to thank the Iranian National Science Foundation (INSF) for their financial support.

20 3 0

310(c) (c)410

510

2.5

610

710

810

910

1010 1110

ΔOD

2 1.5 1 0.5 0 310

410

510

610

710

810

910

1010 1110

300

400

Wavelength (nm)

(d)

1400

Integral of ΔOD

References

Wavelength (nm)

1200

1000

800

600 0

100

200

Substrate temperature (°C) Fig. 11. Optical transmission spectra of Pd/WO3 thin films deposited at different substrate temperatures, (a) bleached states, (b) colored states, (c) corresponding optical density difference (OD) and (d) integral of OD curves as a function of substrate temperature.

5. Conclusion WO3 films were prepared by PLD method at different substrate temperatures of 25, 100, 200, 300 and 400 ◦ C on circular glass substrates. XRD showed an amorphous structure for temperatures less than 300 ◦ C and orthorhombic crystalline structure for 400 ◦ C. FTIR showed the highest sensitivity to moisture for sample deposited

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Biographies M. Allaf Behbahani is a M.S. student in Department of Physics, Isfahan University of Technology, Iran. She received her Bachelor of Physics from Persian Gulf University of Bushehr, Iran (2010). She is working on gasochromic effects of tungsten oxide thin films for gas sensor applications. M. Ranjbar is an Assistant Professor at Department of Physics, Isfahan University of Technology, Iran. He received his Ph.D. from Sharif University of Technology in 2009. Main interests include the deposition and characterization of metal oxide thin films, smart materials, nanoparticles in liquid via laser ablation. P. Kameli is an Associate Professor at Department of Physics, Isfahan University of Technology, Iran. He received his Ph.D. from Isfahan University of Technology in 2003. Main interests are magnetic properties of ferrite and manganate nanoparticles. H. Salamati is a Professor in the Department of Physics, Isfahan University of Technology, Iran. He received his PhD in Solid State Physics, from Oklahoma State University, USA in 1980. Main interests are magnetic properties of ferrite, manganate, superconductivity and fuel cells.