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versión impresa ISSN 0366-1644
Bol. Soc. Chil. Quím. v.47 n.4 Concepción dic. 2002
USE OF A SINGLE-SOURCE MIXED-METAL PRECURSOR FOR
THE PHOTODEPOSITION OF Pd/Ni OXIDES THIN FILMS.
1 Instituto de Química, Universidad Católica de Valparaíso, Valparaíso (Chile)
2 Facultad de Ciencias, Universidad de Valparaíso, Valparaíso (Chile)
3 Department of Chemistry, Simon Fraser University, Burnaby, B.C. (Canada)
(Received: April 25, 2002 - Accepted: August 9, 2002)
Irradiation of thin films of a polyketonate Pd/Ni heteronuclear complex, PdNi(DBA)2 [DBA = dibenzoylacetone)], produced uniform films containing Pd and Ni as shown by EDAX analysis. The X-ray powder diffraction spectrum showed peaks associated with elemental Pd and NiO. The morphology of the films was examined by SEM measurements showing a uniform granular surface with particle sizes of 100 nm or less. The XPS results show that the ratio of Pd/Ni (1:17) present in the film does reflect the stoichiometry in the starting complex. The large amount of carbon (52 %) detected on the surface of the film may be due to the presence of phenyl rings in the precursor complex.
KEY WORDS: thin films, palladium, nickel oxide, photodeposition.
La irradiación de películas delgadas de un complejo heteronuclear policetonato de Pd/Ni, PdNi(DBA)2 [DBA = dibenzoilacetona)], produjo películas uniformes conteniendo Pd y Ni como lo demostraron los análisis EDAX. El espectro de difracción de rayos-X mostró peaks asociados con Pd elemental y NiO. La morfología de los films fue examinada por SEM, mostrando una superficie granular con tamaño de partículas igual o menores a 100 nm. Los resultados de análisis por XPS demuestran que la relación Pd/Ni (1:17) refleja la estequiometría del complejo precursor. El alto porcentaje de carbono (52%) detectado en la superficie de las películas puede deberse a la presencia de grupos fenilos en el complejo precursor.
PALABRAS CLAVES: películas delgadas, paladio, óxido de níquel, fotodeposición.
Palladium and Pd-based alloys are extensively used in electronics, inorganic membranes for H2 gas separation, and as catalysts and gas sensors. Most of the hydrogen sensors use palladium to trap hydrogen1) . However, it has been found that palladium alloys are more suitable than palladium alone for use in hydrogen gas sensor devices, due to their greater mechanical strength and resistance to poisoning by other chemical species2) . Many of the published works on Pd-Ni alloy thin films have shown that the films give durable and quickly reversible detection of hydrogen at concentrations between 0.1 and 100% H2 3). For this reason, there has been much interest in the chemical vapor deposition (CVD) of Pd alloy films, especially on porous substrates which has wide applications in inorganic membranes H2 gas separation and membrane reactor4-6) . The typical approach in CVD of binary materials is based on individual molecular sources which carry the particular constituents of the final thin film. The composition of the film is then controlled by the precise adjustment and optimization of the process parameters. This includes the relation of the molar fractions of the precursor molecules in the gas phase, as well as the total pressure and the substrate temperature. Using this approach, Pd-Ni thin films have recently been prepared by metalorganic chemical vapor deposition (MOCVD) using metal-organic b -diketonate mixed precursors7) . The use of single source precursors for the preparation of binary metal alloys and oxides has drawn much attention in recent years due to their potential to simplify engineering procedures8) . Theoretically, a single source precursor should contain the same stoichiometric metal content as in the final binary metal film. However, in most cases, films have metal ratios different from the precursor, probably due to the relatively high temperatures required in CVD (and laser assisted depositions) which may cause decomposition of the precursor into individual metal fragments during the deposition process9-11) .
Previously, we have reported that photolysis of thin films of coordination complexes of transition metals, can lead to the formation of pure metal oxide films. By this technique, we have been able to deposit thin films of CuO, NiO and UO3, among others12,13) . In these studies, the entire process could be conducted at room temperature in air. We expected that, by using the appropiate precursors this method could also be applied to making binary metal oxides. This would immensely broaden the scope of potential materials that could be deposited by our technique.
Herein, we report the use of a Pd/Ni heteronuclear polyketonate complex, PdNi(DBA)2, 1, (DBA = dibenzoylacetone) as a single source precursor for the photodeposition of Pd/Ni oxide thin films.
The Si wafers were obtained from WaferNet, San Diego, Ca. The Si(100) surface was used in these studies and the wafers were p-type silicon with tolerances and specifications as per SEMI Standard M1.1STD.5 cut to the approximate dimensions of 1cm x 1cm. Prior to use the wafers were cleaned successively with ether, methylene chloride, ethanol, aqueous HF (50:1) for 30 seconds and finally with deionized water. They were dried in an oven at 110oC and stored in glass containers.
The FT-IR spectra were obtained with 4 cm-1 resolution in a Perkin-Elmer 1605 FT-IR spectrophotometer. UV spectra were obtained in a Hewlett-Packard 8452-A diode-array spectrophotometer. SEM analysis were performed on a JEOL 5410 scanning electron microscope equipped with EDX spectrometer, in the Electronic Microscopy Laboratory, Departamento de Ingeniería Metalúrgica, Universidad de Santiago de Chile. Grazing incidence X-ray diffraction patterns were obtained using a Siemens D5000 X-ray diffractometer, Department of Physics, Simon Fraser University. The X-ray source was a CuKa (1.54Å ) beam. The film thickness was determined using a Leitz Laborlux 12 ME S with an interference attachment. X-ray photoelectron spectra were obtained using a (PHI) Physical Electronics double pass CMA (cylindrical minor analyses) at 0.85 eV resolution at the Surface Physics Laboratory, Department of Physics, Simon Fraser University. Elemental analyses were performed on a Perkin-Elmer 2400 CHN elemental analyzer at the Microanalysis Laboratory at Simon Fraser University.
For photodeposition experiments the irradiation source was a 200 W high pressure Hg-Xe lamp in a Photon Technologies Inc. housing equipped with condenser lenses and filtered through a 10 cm water filter with quartz optics..
MATERIAL AND METHODS
Solvents were Merck (p.a. grade) and were distilled and dried before use. Benzoylacetone, sodium hydride, monoglyme, NiCl2 and Pd(II) nitrate were obtained from Aldrich and used as received.
The ligand 1,5-diphenyl-1,3,5-pentanetrione (H2DBA) was prepared by the method of Miles, Harris and Hauser14) . This method involves the condensation of a 1,3-diketone (benzoylacetone) with an appropiate ester (methyl benzoate) using NaH in refluxing monoglyme. The triketone was characterized by FT-IR and mass spectrometry.
Synthesis of Ni(HDBA)2(H2O)2 :
To a solution containing H2DBA (3.3 mmol, 880 mg) and KOH (150 mg) in methanol (10 ml), was added NiCl2 (1.66 mmol, 396 mg) in water (5 ml). Immediately after mixing, a yellow green solid was formed. The mixture was left stirring for 30 min and after vaccum filtration the solid was washed several times with water and cold methanol, and then dried in a vacuum dessicator. The crude product was purified by recrystallization from ethanol to give Ni(HDBA)2(H2O)2 as a green yellowish powder (401 mg, 68 %). IR: (NaCl, cm-1) 1681.4 (m, unchelated C=O), 1521.5 (s), 1455.0 (s), 1396.3 (s) , Analysis: Calcd., C 65.31, H 4.84; Found., C 65.46, H 4.79.
Synthesis of PdNi(DBA)2(H2O)4 , 1:
To a solution of Ni(HDBA)2(H2O)2 (0.2 mmol, 125 mg) in methanol (200 ml) under nitrogen atmosphere, was added Pd(II) nitrate (0.2 mmol, 47 mg), after which the mixture was left under nitrogen, with stirring at room temperature for 8 h. After solvent evaporation under reduced pressure, a black shiny solid was obtained (137 mg). Recrystallization from ethanol produced fine needles of 1 (106 mg, 82 % yield). IR (NaCl, cm-1) 1594.2 (s), 1566.1 (s), 1517.9 (s). Analysis. Calcd: C 63.20, H 3.79; Found: C 63.29, H 3.67.
Preparation of amorphous thin films.
Thin films of the precursor complex were prepared by the following procedure: A silicon chip was placed on a spin coater and rotated at a speed of 1500 RPM. A portion (0.5 ml) of a solution of complex 1 in CHCl3 was dipensed onto the silicon chip and allowed to spread. The motor was then stopped and a thin film of the complex remained on the chip. The quality of the films was examined by optical microscopy and in some cases by SEM.
Photolysis of complex 1 as a film on Si surfaces.
All photolysis experiments were done following the same procedure. Here is the description of a typical experiment. A film of PdNi(DBA)2 was deposited on p-type Si(100) by spin-coating from a CHCl3 solution. This resulted in the formation of a smooth, uniform coating on the chip. The FT-IR spectrum of the starting film was first obtained. The chip was then placed on a brass sample holder in an optical bench equipped with an illumination system as described above. The 200 W Hg-Xe lamp was placed 40 cm from the film and a small cooling fan was used to keep the temperature of the sample below 30 oC during irradiation. After the IR spectrum showed no evidence of the starting material, the chip was rinsed several times first with hexane and then with dry acetone to remove any organic products remaining on the surface, prior to analysis.
RESULTS AND DISCUSSION
The precursor complex 1, PdNi(DBA)2, was synthesized by reacting the mononuclear bis(1,5-diphenyl-1,3,5-pentanetrionato)niquel(II) complex with palladium(II) nitrate. Although both metal ions can form mononuclear complexes with the triketonate ligand, it has been reported that mononuclear bis(1,3,5-triketonate)palladium(II) complexes behave entirely differently from the analogous Ni(II) complexes, because they possess a trans configuration15) . Mononuclear Ni(II) complexes having a cis configuration are capable of binding a second metal ion, while clearly an inert trans isomer is not capable of forming a molecular binuclear complex.
Complex 1 can be spin-coated from chloroform onto a substrate such as borosilicate glass or a Si(100) chip, forming amorphous films showing no sign of crystallization on examination under an optical microscope up to 1000x magnification. In addition, 1 is highly photoreactive in solution upon irradiation with a mercury lamp (254 nm).
Irradiation of a thin film (~ 400 nm) of PdNi(DBA)2 under air atmosphere, led to the disappearance of the absorptions associated with the ligand, as shown by the FT-IR monitoring of the reaction. At the end of the photolysis there are no detectable absorptions in the infrared spectrum. These results suggest that the triketonate groups on the precursor are photodissociated on the surface forming volatile products which are readily desorbed from the surface. The morphology of the films was examined by SEM measurements. Figure 1 shows top view SEM micrographs of a film photodeposited on Si(100). This show the formation of a uniform film which consists of white grains with particle sizes around 100 nm or less, deposited on a smooth dark surface.
|Fig. 1. SEM Micrographs (top view) of a film produced by the irradiation for 3 h of a thin film (450 nm) of PdNi(DBA)2(H2O)4 on Si(100) substrate. a) 5K magnification; b) 10K magnification; c) 50K magnification.|
EDAX analysis showed that both Pd and Ni were present in the film, and metal mapping indicated that both elements were reasonably evenly distributed throughout the film. EDS mapping also indicated that the white grains on the surface of the film were most likely NiO phases, while the dark smooth surface consists mainly of elemental Pd. This was confirmed by XRD measurements which showed signals due to elemental Pd at 2q of 40.2, 46.8, 68 and 82º 16). Small intensities peaks due to NiO at 2q of 37.2, 43.3 and 63º could also be distinguished from the XRD pattern17) . X-ray photoelectron spectroscopy was carried out to examine the elemental composition of the film and the extent of C contamination (Table 1). The XPS results show that the ratio of Pd/Ni (1:17) present in the film surface does not reflect the stoichiometry in the starting compound. Generally, attempts to prepare mixed transition-metal films by CVD of single-source precursors have always resulted in fragmentation of the complexes so that the films contain metal ratios different from the precursors9,10,18) . Since temperature should not be a factor in our purely photochemical method, a possible explanation for the slightly higher Pd content found in the final film may have to do with the photoreactivity of the starting complex. Irradiation of this heteronuclear complex may preferentially generate in a primary photochemical process a volatile metal-containing intermediate, leaving one of the metals on the surface. In fact, we have found in our laboratory, that Pd-diketonates are far more reactive than Ni-diketonates when irradiated as solid films on glass substrates, the former producing elemental Pd films, while the latter forms NiO films. The large amount of carbon (52 %) detected on the surface of the film may be due to the presence of phenyl rings in the precursor, which often introduces carbon impurities in MOCVD of other metals8,19). However, Ar ion sputtering of the films for 3 min in the XPS experiments, indicates that the films contain carbon from the surface to the interface (Table 1).
This work has demonstrated that homogeneous Pd/Ni oxide films can be prepared by the photochemical deposition of Pd/Ni heteronuclear poliketonate complexes. To our knowledge, this is the first report on the use of single source precursors for the preparation of binary metal oxides films. This technique should in principle be extendable to heteronuclear complexes of various metal compositions so as to give a wide range of metal and oxide phases. Experiments under inert conditions are underway for the photodeposition of binary alloys films from single-source precursors.
We would like to thank FONDECYT, Chile (Project No. 1980317) and DGIP-UCV (Project No. 125.797) for financial support.
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