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Vacuum 82 (2008) 956–961 www.elsevier.com/locate/vacuum
Nanostructural palladium films for sensor applications M. Koz"owskia,b,, R. Diduszkoa, K. Olszewskaa, H. Wronkaa, E. Czerwosza a
Tele and Radio Research Institute, 03-450 Warsaw, Ratuszowa 11, Poland b Institute of Physics PAS, 02-668 Warsaw, Al. Lotnikow 32/46, Poland
Abstract The composite films composed of palladium (Pd) nanocrystals and Pd–fullerenes nanocrystals obtained by physical vapor deposition (PVD) method were studied. These types of films can be applied as active material for sensor application. The films were prepared by PVD technique. They were deposited on different substrates and contained Pd in various concentrations. The structure and composition of the Pd films were studied by X-ray diffraction, scanning probe microanalysis, transmission and scanning electron microscopy. It was found that these films were composed of Pd nanocrystals placed in Pd-C60 matrix. Changes of some electric properties of the film upon an influence of a toxic liquid have been characterized by DC electrical measurements. They were performed for different volatile organic compounds (VOCs). At 22 1C the electrical resistance was studied for VOC such as benzene and toluene. r 2008 Elsevier Ltd. All rights reserved. Keywords: Nanocrystals; PVD; Thermal treatment; Standard roughness
1. Introduction Palladium (Pd) is an ideal material for sensors active toward the hydrogen containing compounds because it selectively absorbs molecules with hydrogen and forms a chemical species known as a Pd hydride. Films with Pd change their resistance after adsorption of hydrogen because electrical resistance of hydride is greater than the Pd’s resistance [1,2]. For example hydrogen penetrates Pd metal to form Pd hydride, resulting in a 5% increase in resistance for 2% hydrogen concentrations [3,4]. Nanostructured Pd films electrochemically deposited from the hexagonal lyotropic liquid crystalline phase of a nonionic surfactant, octaethyleneglycol monohexadecyl ether, on to micromachined Si hotplate structures show such a feature. These Pd films have high surface areas (28 m2 g1) and are effective and stable catalysts for the detection of methane in air on heating to 500 1C [5]. Especially, in a thin film based on nanosized Pd particles formation of the hydride causes a formation of new electrical connections with their neighbors. The increased Corresponding author at: Tele and Radio Research Institute, Dluga 44/50, 00-241 Warsaw, Poland. E-mail address:
[email protected] (M. Koz"owski).
0042-207X/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.01.035
number of conducting pathways results in an overall net decrease in resistance. Creation of such conduction paths in the composite films composed of Pd nanocrystals was reported by us earlier [6]. In this paper we report on the electrical changes in prepared Pd composite films observed after introducing different volatile organic compounds (VOCs). At the ambient atmosphere the electrical resistance changes were studied for VOCs such as benzene and toluene.
2. Experimental Pd nanocrystalline films were prepared by the physical vacuum deposition (PVD) method. The evaporation from two separated sources of fullerene C60 (99.95 of C60) and Pd acetate was performed. The films were deposited on various substrates (glass, Si, metal foils). The distance from sources to the substrate was the same for all processes. The sources’ temperatures were changed and the composition and structure depended on these parameters. The concentration of Pd in the studied films is presented in Table 1. The structure, morphology, topography and composition of obtained films were studied by X-ray diffraction
ARTICLE IN PRESS M. Koz!owski et al. / Vacuum 82 (2008) 956–961 Table 1 Palladium content in the particular samples Pd (wt%)
66 73 44
15 30 39
500 450 400 intensity (a.u.)
Sample no.
957
350 300 250 200 150 100 50 0
10
20
30 40 2 theta
10
20
30 2 theta
40
50
60
10
20
30 2 theta
40
50
60
50
60
500 450 400
Fig. 1. Experimental set-up for resistivity measurements.
intensity (a.u.)
350 300 250 200 150
3. Results In Figs. 2a–c XRD patterns for studied films are shown. In most of these diffraction patterns a reflex at 2y ¼ 111 was found. This reflex could be connected to structure
100 50 0
800 700 600 intensity (a.u.)
(XRD), transmission electron microscopy (TEM), atomic force microscopy (AFM). XRD were performed with the Siemens D-500 powder diffractometer. XRD patterns for films deposited on glass and/or Mylar foil substrate were obtained for CuKa line for the lamp working with U ¼ 40 kV, I ¼ 30 mA. The measurements were done with step 0.051 and counting time for each step 10 s. The geometry of the experiment was of the Bragg–Brentano type. TEM was applied to study structure of deposited film. TEM investigations were performed with the JEOL2000EX electron transmission microscope operating at 200 keV incident beam energy. Film topography was investigated by the scanning tunnelling microscope EXPLORER 2000 in AFM contact mode. All measurements were performed at ambient atmosphere. The changes of film resistivity were studied in the experimental set-up presented in Fig. 1. The change of signal resistivity was measured as a function of time after introducing a drop of a liquid onto the sample surface and it was registered by computerized system. The change of resistivity was calculated as a function of measured voltage changes on the sample at known current intensity.
500 400 300 200 100 0
Fig. 2. X-ray diffraction patterns for studied films deposited on glass (a, b) and on molybdenum (c).
composed of C60 and Pd. It is also observed that film with the lowest Pd content exhibits the lowest ordering, while the films with Pd content of 39 wt% of Pd show an ordering that cannot be attributed to any known Pd crystalline–carbon structure. The reflex observed for all
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Fig. 3. TEM images for films of sample with content 15% Pd (a), 30% Pd (b) and 39% Pd (c).
Fig. 4. Electron diffraction from selected area (EDSA) patterns for films of sample with content 15% Pd (a), 30% Pd (b) and 39% Pd (c).
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Fig. 5. Distribution of Pd nanocrystals size for sample with 30 wt% of Pd.
films is connected with interplanar distances d ¼ 0.8 nm, which could be assigned to (h k l) ¼ (0 0 2) for hexagonal fullerite structure or to (1 1 1) of cubic fullerite structure. The absence of other reflexes connected to the mentioned above structures could be related to the preferential orientation of the fullerite type crystals in respect to the substrate surface. We can conclude that studied film’s structure is based on the fullerite type structure. TEM images for these films are presented in Figs. 3a–c. These images show Pd nanocrystals with various sizes depending on the Pd content in a film. For a film with content of 15 wt% Pd almost no Pd grains were observed and most of the observed grains are C60 grains. In electron diffraction pattern a diffused ring attributed to small (with size lower than 1 nm) Pd nanocrystals are observed, but these nanocrystals were not observed with the resolution of our TEM measurements. In TEM images of samples with content higher than 15 wt% Pd the size of the discovered Pd nanocrystals is 1.5–3 nm. Electron diffraction from selected area (EDSA) patterns related to the structures are presented in Figs. 4a–c. For the smallest Pd nanocrystals only diffused diffraction rings characteristic for d(1 1 1), d(0 0 2) spacing of Pd fcc crystalline structure are seen (Fig. 4a). When the Pd content increases and the size of Pd nanocrystals grows, diffraction reflexes connected with fullerite structure appear together with diffraction rings attributed to Pd fcc structure. For samples with 30 wt% Pd content, the Pd nanocrystals with various sizes can be found. Presented in Fig. 5 the distribution of sizes of Pd nanocrystals was obtained from the analysis of a dark field TEM image for the sample. AFM images for one of the studied films are presented in Figs. 6a–c. For all samples the topography of the films surface is similar. The surface is highly developed and hillock-like objects are basic forms of this surface. The only difference, observed for samples with different Pd content,
Fig. 6. AFM images for films of sample with content 15% Pd (a), 30% Pd (b) and 39% Pd (c).
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Fig. 7. The standard roughness of studied films’ surfaces.
Fig. 8. Resistivity measurement results for selected sample for toluene (a) and benzene (b) treatment.
is the roughness of surface that is connected to various sizes of hillocks (Fig. 7). Resistivity measurements of samples were performed for ‘‘as-grown’’ and for after thermal treatment samples. In Fig. 8 the results of electric measurements obtained after introducing the VOCs on the sample surface are presented. The observed response signals for toluene were not enough strong. Thermal treatment for the sample with 39 wt% of Pd was done and this sample measurements for toluene were performed. The film sample was annealed in the temperature of 660 1C in a hydrocarbon atmosphere. In Fig. 9 the resistivity measurement result for toluene applied on the annealed film surface is presented. The change of resistivity for the film with toluene placed on film’s surface is much higher than for benzene.
Fig. 9. Resistivity measurement result for selected sample (after annealing process) for toluene treatment.
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Fig. 10. Structural changes of film with 39 wt% Pd before (upper row) and after (lower row) thermal treatment. (a) Electron diffraction pattern, (b) TEM structure of film and (c) SEM morphology of film.
The structural changes undergoing in the annealed film sample were studied with the TEM (with electron diffraction) and SEM methods in order to determine structural changes connected to the annealing process. In Figs. 10a and b images of film’s fragments (TEM) and surface (SEM) obtained for a sample before and after annealing process are presented. The agglomeration of Pd nanocrystals leading to the formation of bigger nanocrystals was a result of this process. 4. Conclusions Nanostructural Pd films are electrically sensitive to the presence of some VOCs on their surface. The resistivity of the film surface changes, what means that its sensitivity increases after the film annealing in the temperature of 660 1C. Additionally, thermal treatment in hydrocarbon
atmosphere improves current conductivity of the film that is connected to the agglomeration of Pd nanocrystals and formation of bigger nanocrystals. This effect could cause the formation of conductivity path after a VOC has been placed on the film surface.
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