Electroless plating of palladium on WO3 films for gasochromic applications

ARTICLE IN PRESS Solar Energy Materials & Solar Cells 94 (2010) 201–206

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Electroless plating of palladium on WO3 films for gasochromic applications M. Ranjbar a,n, N. Tahmasebi Garavand a, S.M. Mahdavi a,b,nn, A. Iraji zad a,b a b

Department of Physics, Sharif University of Technology, P.O. Box; 11155-9196, Tehran, Iran The Institute for Nanoscience and Nanotechnology (INST), Sharif University of Technology, P.O. Box 11155-8639, Tehran, Iran

a r t i c l e in fo

abstract

Article history: Received 16 May 2009 Accepted 4 September 2009 Available online 6 October 2009

Nowadays, gasochromic Pd/WO3 coatings as optically switchable materials during H2 absorption have become more interesting due to their novel applications in smart windows and gas sensor devices. In this study, WO3 films were fabricated by Pulsed Laser Deposition (PLD) on glass substrates. Using an electroless method, Pd nanoparticles were deposited on the surface of the produced films. The effect of Pd growth time on coloring/bleaching rate was studied for different submerging times (1, 2, 5, 10, 15, 20 and 40 min). The best coloring/bleaching velocity was attained with a growth time of 2 min. The effect of growing time and growth feature of Pd particles was further studied using Scanning Electron Microscope (SEM) and X-ray Photoelectron Spectroscopy (XPS). It was revealed that spherical or agglomerate metallic particles have nucleated upon many surface cracks. The size of particles depends on the growing time and also on the size of cracks. It was observed that hydrogen interaction with a Pd/WO3 layer provides additional surface cracks leading to gasochromic improvement. Furthermore the effect of substrate on Pd growth and gasochromic response was studied. It was observed that the size and distribution of particles as well as gasochromic response depend strongly on WO3 substrate characteristics. & 2009 Elsevier B.V. All rights reserved.

Keywords: Gasochromic Tungsten oxide Palladium Pulsed laser deposition Electroless

1. Introduction Recently Pd-covered tungsten oxide films have attracted a great deal of interest because of their high potential in various solar energy systems such as smart windows [1–5] and also optical H2 sensors [6,7]. They are transparent in the visible spectral range but change their optical absorption reversibly from a transparent to a dark blue state when exposed to a H2-containing gas. In the gasochromic process, adsorbed H2 molecules on the Pd/WO3 surface are dissociated to H + ions and electrons by a catalytic reaction on the Pd metal surface (spill-over mechanism). These ion–electron pairs diffuse through the grain boundaries then transfer into the lattice sites and as a result small polarons are subsequently produced. The small polarons transitions are responsible for the optical absorption of colored tungsten oxide films [8]. This process is often reversible, i.e. whenever the colored film is flushed with O2 gas, the small polarons are recovered and the initial transparency is attained. The velocity of the coloring/bleaching process is directly related to the formation/recovery rates of small polarons at the surface of gasochromic film. Therefore one expects that the characteristic of

n

Corresponding author. Tel.: +98 21 661 645 68; fax: + 98 21 660 227 11. Corresponding author. Tel.: +98 21 661 645 17; fax: + 98 21 660 227 11. E-mail addresses: [email protected] (M. Ranjbar), [email protected] (S.M. Mahdavi). nn

0927-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2009.09.002

Pd catalyst at the surface plays an important role in the dissociation of H2 (O2) molecules for fast coloring (bleaching). To date, different techniques such as sputtering [5,9], evaporation [10] and electroless plating [7] have been utilized for catalyst deposition at gasochromic WO3 layers. In this study the PLD technique for WO3 together with electroless plating for Pd was employed. PLD technique is a successful fabrication method of compound films such as metal oxides [11,12]. It can also be accompanied with O2 partial pressure to retain the stoichiometry of films. Since no crucible is involved in PLD, higher purity end products are achievable. Electroless method has also attracted great interest due to simplicity of operation, cost effectiveness, high throughput, lack of elaborated equipment and feasibility of fabricate metal nanoparticles. In this approach metal ions normally undergo a reducing mechanism, which is used by three different ways; they are autocatalytic, substrate catalyzed and have galvanic displacement. In the first one, the reduced novel metal serves as the catalyst for further reduction of the metal by the external reducing agent. In the second one, the substrate surface catalyzes the reduction of metal salt by the reducing agent. However, in galvanic displacement the surface serves as the reducing agent and electron source for reduction of the metal salt. In this process deposition can continue as long as ions can penetrate and electrons can transfer through the film [13]. Since in galvanic displacement metal nanoparticles can start to nucleate on the surface, surface characteristics such as morphology and chemistry are expected to have an important role in the Pd

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Tungsten oxide films were fabricated by PLD method from tungsten oxide pressed powder (5N) onto glass substrates. The employed deposition system was a stainless steel chamber with a base pressure of 1 10 5 Torr. In the case of stoichiometric tungsten oxide films deposition, the PLD process was carried out in vacuum or in 100 mTorr oxygen (purity of 99.999%) environment. The substrates could be held at different temperatures ranging from room temperature up to 300 1C by means of an ohmic heater directly placed on the back. To ablate the tungsten oxide targets, 3600 pulses of an Nd:YAG Q-switched laser (l = 1064 nm, energy density around 4 J/cm2 and R.R =5 Hz) was delivered over the target surface at a 451 incident angle. The substrate to target distance was 30 mm and for some samples it was increased to 55 mm. To avoid texturing of target surface, it was rotated with 12 rpm during the ablation process. Prior to the deposition, the glass substrates were cleaned ultrasonically and then degreased in acetone. Electroless plating of Pd on tungsten oxide substrates was accomplished via submerging of substrates into a 0.2 g/lit PdCl2 solution. This solution was prepared by ultrasonically solving a 0.02 g PdCl2 powder (5N), 99.9 cc DI water and 0.1 cc HCl. Next to the electroless deposition, samples were removed from the plating bath and washed with DI water. The SEM observations were achieved by means of a Philips XL30 Electron Microscope. The surface morphology of the as-deposited films was observed by an atomic force microscope (AFM model Veeco CP Research) in the contact force mode at room temperature in air. The metallic composition of grown Pd was confirmed by X-ray Photoelectron Spectroscopy (XPS). A concentric hemispherical analyzer (CHA) (Specs model EA10 plus) was used, and to excite the X-ray photoelectrons, an Al Ka line at 1486.6 eV was used. Energy scale was calibrated against the carbon binding energy (284.8 eV). For recording of coloring/bleaching response of the samples we used a transparent sealed gas chamber with 40 cc volume. A He–Ne laser beam (l = 632.8 nm) was delivered into the chamber and transmission of the light was measured by a photodiode on the outer side. Coloring/bleaching of samples proceeds under a steady stream of 10%H2/Ar or pure O2.

3. Results and discussion

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a fast coloring/bleaching response. A mean-thickness dependence of colouration rate has also been observed for Pt catalytic layers sputter deposited on WO3 in reference [15]. They reported a maximum coloration rate for 3.5 nm Pt thickness. In the electroless plating method, particle size can be controlled by different parameters such as solution concentration, temperature and growing time. In a fixed bath and at room temperature we studied the effect of deposition time on growth of Pd as well as gasochromic response. The WO3 substrates were prepared in PO2 = 0.1 Torr, Tsubstrate = room temperature and 55 mm substrate to target distance. Here we first look at the changes in optical density (defined as DOD=–ln[T(t)/T0]; T and T0 are transmittance at time t and 0, respectively) as a function of gas exposure time. In Fig. 1 variation of DOD is shown for different submerging times (1, 2, 5, 15, 20 and 40 min). For all cases, with the onset of 10%H2/Ar gas flushing the change in optical density first undergoes a rapid drop to small negative values and then gradually increases upon coloring process. We attribute the first small decrease to the immediate production of the PdHx complex by hydrogen absorption on Pd particles. In fact, optical variations at the beginning of H2 treatment are more complex because two different mechanisms including bleaching due to forming Pd–H and the coloring of WO3 itself exist together. From Fig. 1(a), we found that the time width of this effect almost increases with submerging time. When O2 is gas introduced into the test box, DOD lowers and approximately reaches its initial value. Fig. 1(a) shows the coloring and bleaching rates as well as DOD dependence on the submerging time. For example, the sample that was submerged for 2 min exhibits the highest DOD after about 820 s. Furthermore, the coloring and bleaching for submerging times longer than 10 min become noticeably slower as it is obvious from the increase in their saturation time. It is possible to extract the maximum of coloring or bleaching velocity curve by plotting the derivative of DOD function in order to

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growth, and consequently in gasochromic kinetics. Therefore it is of interest from both fundamental and technological perspectives to study the different aspects of Pd growth on tungsten oxide substrates by electroless deposition. Extensive literature can be found on electroless deposition of Pd on different metallic or semiconductor substrates [13,14]. In case of metal oxide substrates such as WO3, more investigations are needed. We attempted to investigate the electroless growth of Pd and its influence on gasochromic behavior. To correlate the growth feature with different deposition time and surface morphology, SEM observations were performed. Furthermore, surface morphological modification by hydrogen exposure is introduced here as a benefit for further Pd growth as well as for an excellent gasochromic response.

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As we described above, dissociation of adsorbed H2 molecules on Pd/WO3 surface is crucial for injection of spill-over hydrogen and hence for coloring of the WO3 layer. Therefore, the characteristics of Pd layer are expected to be important to achieve

Fig. 1. (a) Variation of optical density (DOD) as a function of 10%H2/Ar or O2 flushing for Pd/WO3 films with different submerging times. (b) Maximum of coloring and bleaching velocity determined by time derivation of curves in part (a). Large arrows show the onset of hydrogen or oxygen exposure.

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compare the switching behaviors more precisely. In Fig. 1(b) coloring/bleaching maximum velocities are shown as a function of submerging time. Obviously, it reveals an optimum at t= 2 min that both coloring and bleaching velocities are at their maximum level. To justify better, the Backscatter Electron (BSE mode) SEM images corresponding to t = 1, 2 and 40 min are illustrated in Fig. 2. BSE mode permits having a contrast between elements with different atomic numbers. From Fig. 2(a), by 1 min submersion, Pd particles appear as bright dots over the surface (shown by white arrows) with size ranging from less than 100 nm to more than 1 mm. By 2 min submerging time, a greater number of particles can be seen, and after a long time of 40 min, it seems that the average size was enhanced but the number of particles was not considerably increased. Therefore, for long-term submerging, the particle growth mechanism dominants the new nucleation on unoccupied sites. The metallic nature of these particles was also confirmed using XPS core-level spectrum of Pd3d transition peaks. In Fig. 3, two peaks at 335.2 and 340.5 eV correspond to Pd3d5/2 and Pd3d3/2, respectively, that are characteristics of metallic palladium [16]. It should be mentioned that due to low surface concentration of Pd many XPS scans were needed to obtain this spectrum. The bleaching of a colored film is a revised process by which oxygen molecules incorporate into the Pd metal and subsequently polaron sites became inactive. For short submerging time, the number of Pd particles, and hence the amount of catalyst, might be too little to indicate a good catalytic activity, for both hydrogen and oxygen, and consequently a fast gasochromic switching. Since small clusters are more chemically active than larger ones [17], for long submerging time the dissociation rate of hydrogen will be partly limited due to the increase in Pd particles size. Also the covering of some WO3 pores by palladium in electroless bath, which inhibit hydrogen diffusion into the layer, may be a reason. So only for a critical submerging time Pd particles are synthesized with a mean optimum size. Regarding SEM images, the majority of grown particles are found being close to the surface crack’s edges. This phenomenon indicates that the nucleation of Pd ions initiates first beside the cracks and then the subsequent ions are attracted from any direction of solution allowing the growth of single nanoparticles or agglomerates. A single spherical Pd particle grown on the corner of a ‘‘boomerang-like’’ crack over the sample of t =2 min is shown in Fig. 2(d). It’s spherical shape proposes a Volmer–Weber (3D island growth) mechanism [18] for electroless deposition through galvanic displacement on WO3 layers. Our SEM observation from various places of different samples showed that the dimensions of many agglomerated or single Pd particles increase with crack size as almost micron-size particles could be found just beside the large cracks. For example, in Fig. 2(c) such different features are identified as labels A, B and C for relatively small, mean and large cracks, respectively. As shown by arrow C, an agglomeration of particles can occur for a high expanded crack probably due to the high reduction potential value at that point. In addition, the majority reduction of Pd ions proceeds on special sites in such a way so that various cracks remain unoccupied, resulting in a spread distribution of particles over the substrate surface. Pd growth from PdCl2 solution upon pore surfaces was reported in the literature [19,20]. Zhongliang et. al. [20] have observed that Pd nanoparticles start to agglomerate over the pores’ wall area of aluminum surface and consequently a bridge has been fabricated on the pores. Considering the galvanic displacement as dominant reduction mechanism for a metal oxide substrate such as WO3, Pd ions are preferentially attracted by those local sites that provide a relatively high redox reactivity. At the boundary of cracks the enhanced local electric field owing to accumulation of excess charges. Consequently, diffusion of Pd ions toward the nucleation sites is determined by local electric

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Fig. 2. SEM (BSE mode) images of Pd/WO3 films for (a) 1 min, (b) 2 min and (c) 40 min submerging times (d) SEM (SE mode) image of a spherical Pd particle on a ‘‘boomerang-like’’ crack.

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field they experience. This is why the amount of deposited Pd particles increases with crack size and why a great number of cracks are bare of Pd metal.

These stresses lead to irreversible changes in microstructural and morphological properties at Pd/WO3 system. Therefore different effects such as surface cracks, porosity and even cleavage of layer from the substrate may be observed. As an evidence, typical SEM images (Fig. 4) corresponding to a WO3 layer with a relatively more compact morphology that passed the electroless stage for a 20 min submerging time, before and after 1500 s hydrogen exposure, indicate an increased porosity with more fine splits between the grains. To investigate the effect of such a hydrogen exposing process on the gasochromic switching response, we compared the sample of 40 min (from Fig. 1) with the one that was submerged first for 20 min, subsequently exposed to H2 for 1500 s, then submerged again for a further 20 min (totally 40 min). We named them one-step and two-step sample, respectively. Fig. 5 shows that the coloring/bleaching rates as well as maximum DOD of the two-step sample are obviously greater than those of the one-step sample. We believe that the morphology variation of the gas-exposed sample provides more nucleation sites for Pd metal not only on the surface but also inside the new pores area. The hydrogen-induced surface stress of Pd/WO3 is extensively reported in the literature [5,21]. Although such irreversible effects may be regarded as a main reason for this improvement of gasochromic response, it must be noted that hydrogen can inherently reduce the PdO phases to Pd and in the case of electroless deposition of PdCl2, it can also reduce those PdCl2 components that have possibly remained on the surface. Such effects are usually observed as a little longer switching time in the first cycles of gasochromic switching. Data of Fig. 6 are related to the stabilized stages, so such effects could not have important roles.

3.2. Surface modifications by hydrogen exposure

3.3. The effect of substrate morphology

It is known that H2 absorption on Pd surface produces PdHx complexes due to a gas–solid solving process. As H atoms incorporate into the Pd lattice its volume increases with x and simultaneously some surface stresses are created at the layer.

As we discussed above, electroless growth of Pd particles depends on the WO3 surface properties. Using the PLD technique, different surface morphologies can be produced by modulation of various deposition conditions [7,22]. In the following we study the

Fig. 4. SEM images of a dense Pd-covered WO3 film (a) before and (b) after 1500 s hydrogen exposure.

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Fig. 6. SEM images of Pd/WO3. The substrate samples were fabricated under different conditions as PO2 = 0.1 Torr and Tsubstrate =300 1C (a) or Pvacuum = 1 1 5 Torr and Tsubstrate =RT (b), (c) sample (b) annealed in air at 400 1C for 1 h.

effect of substrate morphology on Pd growth from a PdCl2 solution bath using three different WO3 samples. One was prepared at PO2 = 0.1 Torr and Tsubstrate = 300 1C and the other one at Pvacuum = 1 1 5 Torr and Tsubstrate = RT. The substrate to target distance was constant at 55 mm. Chemical compositions of these two samples using XPS (not shown here) revealed WO3 and WOx (x o 3), respectively. The third sample was achieved by annealing of the latter one in air at 400 1C for 1 hour. Annealing allows the sample to become stoichiometric through compensating oxygen vacancies that are usually produced by ablation in vacuum environment. At this temperature the amorphous structure turns to a crystalline phase. Using AFM data, RMS surface roughness of these 3 sample was measured as 4.2, 0.7 and 0.6 nm, respectively.

For investigation of palladium growth, these substrates were all immersed in the 0.2 g/lit PdCl2 solution for 210 s. The fixed time and concentration allows studying the effect of surface morphology on Pd growth. SEM images after electroless plating process are shown in Fig. 6(a–c). For deposition in an oxygen environment, spherical Pd particles (shown by arrow) with an average diameter less than 200 nm are observed on unevenly distributed sites. The distribution of particle is not very uniform, which may be due to non-uniform redox reactivity. The deposit in vacuum condition has not many particles compared to the deposit in oxygen environment but instead the size distribution is relatively narrow. However, after submerging the annealed sample into PdCl2 bath, spherical Pd nanoparticles with a narrow size distribution are uniformly overlaid upon the surface. The size of particles is approximately 200 nm as is seen in the inset of Fig. 6(c). The narrow size distribution of the two later samples can be attributed to their low RMS surface roughness. A nearly uniform surface provides a uniform distribution of redox relativity on the surface, hence a homogenous growth at different sites. However, the number of grown particles seems to be correlated with the presence of oxygen at the surface, according to XPS data, as deposit in oxygen environment and annealed one includes more number of particles. These effects propose that surface characteristics such as morphology, surface roughness and surface composition play important roles in electroless growth of Pd over the tungsten oxide films. The gasochromic behavior of these three samples was examined and it was revealed that only the deposited sample at oxygen environment exhibited a coloring response. The coloring/ bleaching switching kinetics of this sample is shown in Fig. 7. Two other samples did not exhibit a proper response despite the possibility of Pd growth. This is due to the known fact that the amorphous structures and near stoichiometric samples exhibit an appropriate gasochromic effect. These observations indicate that to use the electroless deposition as a cheap approach in fabrication process of gasochromic layers, a precise control is needed at deposition steps of WO3 layer from both structural and chemical viewpoints.

4. Conclusion In summary, gasochromic WO3 films were fabricated by pulsed laser deposition under different experimental conditions. Via a simple electroless plating method, Pd metal particles as a good catalyst for hydrogen were deposited through submerging of WO3 substrates into a 0.2 g/lit PdCl2 solution. It was revealed that

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there is an optimum submerging time by which the gasochromic response of Pd/WO3 layers to hydrogen/oxygen become maximum due to the size-dependence catalytic activity of Pd. SEM shows that Pd particles nucleate with spherical or agglomerate forms just on the surface cracks and their sizes increase with cracks’ dimension. Hydrogen exposure provides extra surface cracks, as new nucleation sites for Pd, and a porosity enhancement by which gasochromic response becomes fast. The general feature of Pd growth depends strongly on surface morphology and chemistry; smooth surfaces lead to a uniform distribution while the number of particles increases by oxygen surface concentration.

Acknowledgment The authors wish to thank the Research Council of Sharif University of Technology and also wish to thank Mr. Rafiey at the ESCA/AES laboratory and also Mrs. Z. Mohammadzadeh for her kind help and Mr. M.R. Nematollahi. References [1] C.G. Granqvist, in: Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995. [2] V. Wittwer, M. Datz, J. Ell, A. Georg, W. Graf, G. Walze, Gasochromic windows, Sol. Energy Mater. Sol. Cells 84 (2004) 305–314. [3] A. Georg, A. Georg, W. Graf, V. Wittwer, Switchable windows with tungsten oxide, Vacuum 82 (2008) 730–735. [4] M. Ranjbar, S.M. Mahdavi, A. Irajizad, Pulsed laser deposition of W–V–O composite films: preparation, characterization and gasochromic studies, Sol. Energy Mater. Sol. Cells 92 (2008) 878–883. [5] C. Salinga, H. Weis, M. Wuttig, Gasochromic switching of tungsten oxide films: a correlation between film properties and coloration kinetics, Thin Solid Films 414 (2002) 275–282. [6] W.C. Hsu, C.C. Chan, C.H. Peng, C.C. Chang, Hydrogen sensing characteristics of an electrodeposited WO3 thin film gasochromic sensor activated by Pt catalyst, Thin Solid Films 516 (2007) 407–411.

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