versión impresa ISSN 0366-1644
Bol. Soc. Chil. Quím. v.47 n.2 Concepción jun. 2002
Bol. Soc. Chil. Quím., 47, 113-121 (2002)
PREPARATION OF PALLADIUM THIN FILMS AND THEIR
USEFULNESS AS MODIFIED ELECTRODES
aFacultad de Ciencias, Universidad de Valparaíso, Gran Bretaña 1111, Playa Ancha,
Valparaíso-CHILE. E-mail: email@example.com.
bInstituto de Química, Universidad Católica de Valparaíso, Av. Brasil 2950,
cFacultad de Química, Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860,
(Received: September 10, 2001 - Accepted: January 22, 2002)
Nanostructured palladium thin films have been photochemically obtained by direct UV irradiation (254 nm) of a Pd[C8H17COCHCOC4H 9]2 amorphous film deposited on Si(100) and on ITO glass by means of the spin-coating technique. The UV photolysis of thin films of palladium b-diketonate complex results in the loss of all ligands from the coordination sphere. The acetylacetone derivative was chosen for its solid photochemistry, because the presence of branched hydrocarbons substituents lowers the intermolecular interactions, and allows the formation of high quality films upon spin-coating. The product of this photolysis was analyzed by XRD, SEM, and microanalysis. Afterwards, the film photodeposited on ITO glass was electrochemically studied, applying successive triangular scannings in a potential range of 0.00-1.30 V vs SCE for 15 minutes, in an electrolytic solution consisting in H2SO4 0.50 M. The modified electrode showed a profile similar to that of a palladium foil, although it is possible to observe that the peak potential shifts in 0.04 V approximately towards less positive values for the modified system, which is indicative of a possible catalytic effect. Based on these data, the modified ITO/Pd system was tested as an electrodic substrate for aniline polymerization in 0.10 M H2SO4; there obtaining a polymeric deposit, at lower potentials than those required when palladium foil was used as electrode, thus indicating that the method described may constitute a very interesting tool for the preparation of modified electrodes which may be advantageously used in the preparation of polymeric materials.
KEYWORDS: Pd thin film, modified electrode, conducting polymers, polyaniline.
Se han obtenido, fotoquímicamente, películas delgadas nanoestructuradas de paladio, por irradiación UV directa (254nm) de una película amorfa de Pd[C8H17COCHCOC4H 9]2 depositada sobre Si(100) y sobre vidrio ITO, mediante la técnica de spin-coating. La fotólisis de la película de complejo b-dicetonato de paladio, por luz UV, conduce a la pérdida de los ligandos de la esfera de coordinación. Los derivados de las acetilacetonas fueron escogidos por su fotoquímica en estado sólido, ya que la presencia de sustituyentes ramificados hidrocarbonados disminuye las interacciones moleculares y, mediante spin-coating, conduce a la formación de películas de alta calidad. El producto de la fotólisis fue analizado por difracción de rayos-X (XRD), microscoscopía electrónica de barrido (SEM) y microánalisis. Posteriormente, la película depositada sobre vidrio ITO se estudió eletroquímicamente, aplicando ciclos triangulares sucesivos a potenciales entre 0,00 y 1,30 V vs ECS por 15 minutos, en una disolución electrolítica de H2SO4 0,50 M. El electrodo modificado presenta un perfil similar al de una lámina de paladio, sin embargo, es posible observar en el sistema modificado un desplazamiento de los potenciales de pico, de aproximadamente 0,04 V hacia valores menos positivos, lo cual es indicativo de un posible efecto catalítico. En base a ello, el sistema modificado ITO/Pd fue usado como sustrato electródico para la polimerización de anilina en H2SO4 0,10 M, obteniéndose un deposito polimérico, a potenciales menores que los requeridos sobre la lámina de paladio, indicando que el método descrito puede consituir una interesante herramienta para la preparación de electrodos modificados, que podrían ser ventajosamente usados en la preparación de materiales poliméricos.
PALABRAS CLAVES: Películas delgadas de Pd, electrodo modificado, polímeros conductores, polianilina.
Since the seventies, the development of microstructures on conductive substrates, with a specific activity, such as electrocatalysis, electrochromism, electroluminiscence, 1,2 or the avoidance of corrosion by electromigration in alloys, 3 has generated a great field for the study and the experimentation in the area of modified electrodes.
There are many physical and chemical methods to achieve these objectives, such as the conventional methods: evaporation, electrochemistry, polymerization,4 etc. However, the range of methods to obtain very thin metallic films of high quality has increased lately, adjusting to the modern technology demands. Among these methods, it is possible to mention the chemical vapour deposition (CVD), and the physical deposition in vapour phase (PVD). To decrease the temperature and to improve the quality of the films obtained by both techniques, the assistance of plasm has been introduced. More recent is the use of photochemistry in CVD, represented by the acronysm PCVD, as well as the photochemistry of organometallic complexes for the deposition in vapour phase (MOCVD). 5
In recent years, a new methodology for the obtention of thin metallic films has been developed. This consists in the direct irradiation of a coordination complex with ultraviolet light. The simplicity of the method allows for the deposition of very thin films of metallic materials or metallic oxides on substrates which are not affected by the UV light. The complex should fulfil two fundamental requirements. First, it must be photoreactive, and second, it should provide amorphous and uniform films, before and after the UV irradiation. 6 - 13
The application of this new method suggested in this paper for the obtention of thin Pd films is presented in detail. These materials of noble origin are frequently used in: electrochemistry, electric contacts (replacing gold), multilayer chip capacitors, electrode coating materials,14 catalysis,15 and membrane materials for either catalysis or gas separation.16 Moreover, taking into account the characteristics of these deposits, i.e., their extraordinary thinness, uniformity and great contact surface, it is possible to suggest its use it for the production of new electrodic substrates and their applications.
Materials and instrumentation
FT-IR spectra were obtained with 4 cm-1 resolution in a Perkin-Elmer 1605 FT-IR spectrophotometer. UV-Vis spectra were obtained in a Hewlett-Packard 8452-A diode array spectrophotometer. SEM analysis was performed on a JEOL 5410 scanning electron microscope with an EDAX microanalysis attachment, in the Electronic Microscopy Laboratory, Metallurgic Engineering Department, University of Santiago, Chile. Grazing incidence X-Ray diffraction patterns were obtained using a D5000 X-Ray diffractometer. The X-Ray source was Cu, 40kV/30 MA.
The photolysis in solid state and at room temperature was carried out under a UVLMS-38 lamp of 254 nm, 230/50/0,16 V/Hz/Amp with two 8 Watt tubes, from UVP-USA.
The substrates used were type-n silicon(100) obtained from WaferNet, San Diego, Ca., and conducting ITO glass. Prior to use, the wafers were rinsed successively with ether, methylene chloride, and ethanol.
Cyclic voltammograms were obtained with the aid of a potentiostat (LA-128) and a Kipp & Zonen X-Y recorder. The reagents used in the electrochemical study were H2SO4 p.a. and aniline p.a. from Merck and Milli-Q water
Figure 1 (a) FT-IR spectra of a film composed by 2.148, 4.296, 6.444, 8.592, 10.740, 12.888, 15.036 and 17.184 molecules per Å2 of Pd [C8H17COCHCOC4H 9]2 on a Si (100) surface; (b) Plot of the absorbance of Pd [C8H17COCHCOC4H 9]2 at 1556 cm-1vs coverage.
Figure 2 FT-IR spectral changes associated with the photolysis, during 48 h, of a Pd [C8H17COCHCOC4H 9]2 film deposited on Si(100).
The reagents used in the synthesis of palladium b-diketonate were from Aldrich Chemical Co., and were used without any previous purification, while the ester (ethyl-3,5,5-thrimethylhexanoate) employed to prepare the ligand b-diketone was synthesized starting from the 3,5,5-trimethylhexanal by oxidation with KMnO4/acetone17 and further esterification with ethanol/toluene/H2SO4.18
The procedure presented by Adams and Hauser19 was used for the obtention of the b-diketone, 2,8,10,10-tetramethyl-4,6-undecanedione. In the synthesis of the palladium complex, palladium (II) hydrated nitrate dissolved in water was used; it was vigorously stirred with the ligand in a 1:2 molar ratio during 24 hours. The product was filtered and rinsed with ethanol. An amorphous, light yellow coloured product was obtained, with an approximate yield of 50 %, and a melting point of 121 °C. The molar extinction coefficient (log e) shown in the UV-Vis (in CHCl3) corresponds to 4.30 at 238 nm, 4.11 at 256 nm, 3.94 at 334 nm, and 1.68 at 382 nm. The carbonyl group band was observed in the FTIR spectra at 1556 cm-1. Elem. Anal. for Pd[C8H17COCHCOC4H 9]2, calc.: C=61.57; H=9.30; found: C=61.55; H= 9.33.
Preparation of amorphous thin films
The thin films of precursor complex were prepared by the following procedure: a silicon chip was placed on a spin coater and it was rotated at 1500 rpm. A portion (0.1 mL) of the solution of palladium diketonate complex in chloroform was dispensed onto Si(100) or ITO glass, and it was allowed to spread and dry. Then the motor was stopped and a thin film of the complex remained on the chip. The quality of the films was examined through optical microscopy.
Calibration of the FT-IR absorption on Si surfaces
A Pd[C8H17COCHCOC4H 9]2 solution in CH2Cl2 (0.00577g in 5.00 mL) was prepared. With a microsyringe, 4.0 mL of this solution was placed on the silicon surface and it was allowed to dry. The area containing the dry complex was of 22.90 mm2 (2.29·1015 Å2 ) and corresponds to a coverage of 2.15 molecules /Å2. This process was repeated several times and the spectra were collected with each additional drop added (figure 1a). The plot of the absorbance at 1556 cm-1 vs coverage was linear, as shown in figure 1b. This plot was used to determine the concentration of the complex on the substrate.
Photolysis of the complex as a film on Si(100) and ITO glass
All photolysis experiments were carried out applying the same procedure. Here is a description of a typical experiment. A film of Pd[C8H17COCHCOC4H 9]2 was deposited on Si(100) or on ITO glass, by spin coating. This resulted in the formation of a smooth, uniform coating on the substrate. The FT-IR spectrum of the starting film was first obtained. The chip was then placed under a Hg lamp. After the FT-IR spectrum showed no evidence of the starting material, the chip was rinsed several times with acetone to remove any organic product remaining on the surface.
The procedure described above was performed on tin and indium oxide (ITO) conductive glass. The palladium deposit on the conductive substrate, obtained from the photochemical treatment, was introduced in an electrochemical cell as a work electrode, in an electrolytic medium, consisting of 0.50 M H2SO4. As a reference electrode a saturated calomel was used. As an auxiliary electrode, a platinum one having a large area was employed. The electrolytic solution was previously degassed by bubbling high purity argon bubbling. Room temperature was 20°C. Both, the ITO/Pd electrode and Pd foil were perturbed by successive triangular linear sweepings for their activation and comparison between them, which considered the potential intervals included between 0.00 V and 1.30 V. Scan rate in all cases was of 0.10 V·s-1 for a period of 15 min.
After the activation of the ITO/Pd systems, the Pd foil, through the procedure described above, was introduced in an electrolytic solution consisting of 0.50 M H2SO4 and recently distilled aniline, with the purpose of determining the existence of the ITO/Pd electrode catalytic activity, in relation to the Pd foil.
The experiment was performed applying a potential sweep from 0.04 to 0.90 V, at a scan rate of v =0.005 V·s-1.
RESULTS AND DISCUSSION
The complex palladiumbis[2,8,10,10-tetramethyl-4,6-undecanedionate] turned out to be an excellent precursory complex for the formation of amorphous and uniform thin films, in different substrates, such as Si(100), conductive ITO glass, and borosilicate glass, by means of the spin-coating technique. This would demonstrate that heavily branched coordination complexes and those having low melting points, as the one used in this case, constitute a potential precursory material for different deposition techniques, like CVD and PVD in their different forms.
Furthermore, and considering that this complex is a b-diketonate, it is even more interesting since it has been demonstrated that these compounds are photochemically reactive both in solution and in solid state.12, 21, 22
The evolution of the film photolysis of the precursory complex whose initial concentration was 17.55 molecules/Å2, deposited by means of the spin coating technique, was monitored through the decrease of the most intense band in the FT-IR spectrum (figure 2), which corresponds to the frequency at 1556 cm-1, that is assigned to the keto-enol system of the b-diketone ligand coordinated to the metallic Pd (II) ion. At the end of the photolysis, there are no detectable absorptions in the infrared spectrum. These results suggest that diketonate groups on the precursor are photodissociated on the surface forming volatile products which readily dissapear from the surface.
Since only the Si(100) has certain degree of transparency in the IR, this was chosen to carry out the monitoring of the reaction and to determine the necessary time for the total photo-extrusion of the ligand from the precursor complex film. For this same reason, Si(100) was used to construct the absorbance vs molecules/Å2 calibration curve. This one allows the determination of the concentration of the complex being irradiated and, at the same time, the useful concentration range that can be used in these reactions. It also allows to ascertain that the film was completely irradiated, that is to say, from the surface to the substrate.
This same calibration mentioned above may be used not only for Si(100) but also for other types of substrates, and it requires a similar photo-extrusion time. The only condition demanded is that the concentration used for Si(100) be the same for any other substrrate.
The microphotography obtained by scanning electron microscopy with a magnification of 5000, for the photolysis product under an oxygen atmosphere on Si(100), shows a very uniform film, formed by very small particles, i.e. 70 nm, covering the substrate completely (figure 3a).
A cross-section analysis of the same film (figure 3b) indicates the presence of very small amounts of palladium, carbon, oxygen, and silicon. The largest peak corresponding to silicon.
The characterization of the photoproducts was carried out by X-Ray diffraction. For this, it was necessary to use Si(100), because the products are amorphous and the signals are quite flat, which makes it necessary to anneal and crystalize the deposit in a Lindberg oven at temperatures where the conductive glass doest not stand. Nevertheless, to check the Pd presence, the difference of the substrate would not be important.
The peaks at 33.56 and 33.89 2-q degrees of the X-ray spectra, which correspond to the (002) and (001) orientations, respectively, demonstrate the presence of a small amount of PdO, while peaks at 40.11 and 46.66 2-q, corresponding to  and  orientations, respectively, pertaining to Pd, which is the largest composition of the film when these were photolyzed in the presence of oxygen (figure 4).
This agrees completely with the microanalysis done to the film, because there would be Pd, PdO, and C contamination, which would come from the photochemical decomposition of organic ligands or from the atmospheric pollution.
Figure 3 Scanning electron microphotograph and EDAX of Pd film produced by the irradiation of a thin film of Pd [C8H17COCHCOC4H 9]2 on Si(100) substrate.
Figure 4 . X-ray diffraction of a film produced by the irradiation, during 120 h, of five successive thin films of Pd [C8H17COCHCOC4H 9]2 on Si(100) substrate after annealing at 700 C for 3 h.
Figure 5 shows the response, under potentiodynamic perturbation, of the deposit prepared in the conditions summarized herein. It is noticeable that the voltammetric profile (figure 5b) is very similar to that obtained for a palladium foil electrode (figure 5 a), in the same electrolytic solution, which would indicate that the technique employed is adequate for the preparation of this type of electrodes, at least in what refers to its macroscopic response. However, it is necessary to point out, that the film prepared by this photochemical method shows a potential shift for the ITO/Pd system towards less positive values, which is clearly indicated by the respective reduction peak. This peak is observed at 0.367 V (figure 5b), while the Pd foil (figure 5a) shows the cathodic peak at 0.420 V which has been reported for palladium bulk electrode under identical conditions is of 0.430 V.23
The above mentioned results would be another proof that the Pd film is nanostructured and that is possesses catalytic properties, just as it was reported by Yassar and col.,24 who demonstrated that when they decrease in size from 300 to 6 nm in the particulate material of platinum, as well as palladium, a 0.04 V shift towards less positive values takes place in the redox potential.
Taking into account the previous results, this modified electrode was tested for the electro-formation of a polymer, selecting the aniline monomer, since its behaviour has been largely studied25 and because the polymerization may be carried out in the same electrolyte that was used for the ITO/Pd electrochemical behaviour.
This study was carried out for the Pd foil electrode as well as for the Pd/ITO and ITO glass system, of macroscopically identical areas. The experimental conditions described above, and the program for applied potentials were identical in the three cases.
Figure 5. Cyclic voltammograms in 0.50 M H2SO4 at 25 C; continous triangular potential, 0.00 to 1.30 V; scan rate 0.100 V·s-1: (a) Pd foil; (b) Pd/ITO system.
Figure 6 shows the variation of the anodic current density between 0.04 and 0.90 V, as a function of the potential, for the first cycle of the aniline oxidation towards the formation of polyaniline. The values of the current density were obtained dividing the current by the correction factor of the real areas of the respective electrodic systems. These factors were obtained dividing the anodic charge between 0.00 and 0.75 V (which is the region considered for the normal surface reduction of one PdO monolayer26) of the ITO/Pd and Pd foil electrodes obtained in figure 5, for the real area of one monolayer, (the real value of the area of the reduction process of one oxide monolayer is 0.424 mC·cm-2, value estimated by Woods27). The factor values obtained were 0.018 for ITO/Pd and 0.100 for Pd foil.
Figure 6. Anodic current density vs potential curves in 5·10-2 M aniline and 0.50 M H2SO4 solution at 25C; scan restricted to the 0.04-0.90 V potential range; potential scan rate: 0.005V·s-1 on : (.......) Pd foil; () ITO/Pd; (!) ITO glass.
The Pd foil and ITO/Pd electrodes basically present the same redox systems with the difference that the processes carried out on the particulate Pd take place at lower potentials than on the Pd foil (difference of approximately 0.04V). The actual current density generated in this voltammetric cycle is always higher when the nanostructured Pd is used as an electrode.
Consequently, it is demonstrated that the use of electrodic materials prepared by this method may be highly beneficial, from all aspects herein discussed, to which it is necessary to add the economic aspect, since, for the same geometric area, a much smaller amount of noble metal is required.
The results presented here reveal that the Pd deposits obtained by means of the spin coating technique, the photolysis and electrochemical reduction, permit the generation of very homogeneous thin deposits (nanostructured), having electrocatalytic properties.
This conclusion suggests that this methodology allows the preparation of noble metal electrodic substrates whose responses are similar to those of the respective bulk electrodes of the same material, using very small quantities of the metal.
We would like to thank FONDECYT-Chile (Project N 3990026) for financial support.
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