Clusters and thin films prepared by DC-sputtering: morphology and catalytic properties

PREPARATION OF CATALYSTSVI Sciemific Bases for the Preparation of HeterogeneousCatalysts G. Poncelet et al. (Editors) 9 1995 Elsevier ScienceB.V. All rights reserved.

Clusters and thin films prepared morphology and catalytic properties.

941

by

DC-sputtering:

D. Duprez and O. Enea Laboratoire de Catalyse en Chimie Organique. URA CNRS 350. 40, Avenue du Recteur Pineau, 86022 Poitiers Cedex, France.

The morphology of clusters and thin films of Pt, Pd, Au and Ni deposited on model substrates (Si, pyrolytic graphite, glass... ) by direct-current sputtering (DCS) was studied by scanning tunneling microscopy and X-ray reflectometry. Noble metals were also deposited by DCS on conventional supports (SiO 2, TiO 2, A120 3, CeO 2) and their properties in various catalytic reactions (benzene hydrogenation, cyclopentane hydrogenolysis, transient CO oxidation, photooxidation of alcohols) were investigated. 1. INTRODUCTION Physical vapor deposition methods (PVD) offer the possibility of preparing catalysts in which no foreign ions or molecules are introduced as is the case in the conventional "wet" impregnation methods. In evaporation methods however, the contact between metal and substrate produced by the deposition of metallic vapors is too weak to favor strong interactions and to enhance the catalytic activity and stability. By contrast, when a high-energy method like ion implantation is used, the metal is buried too deeply in the substrate and only a limited number of sites are available for the catalytic reactions. So far, directcurrent sputtering has been the only PVD method whereby reasonable amounts of active catalysts could be prepared [ 1]. In this study the morphology of clusters and of thin films of Pt, Pd, Au, Ni deposited on silicon, conducting glass or highly oriented pyrolitic graphite (HOPG) was studied by scanning tunneling microscopy (STM) and X-ray reflectometry. Noble metals (Pt, Pd, Pt-Rh .... ) were also deposited on powders like TiO 2, A120 3, SiO 2 or CeO 2, by using appropriate sputtering parameters (current intensity, voltage and time). The physico-chemical properties of these catalysts were examined by hydrogen chemisorption and TEM while their catalytic activity and selectivity were tested in various model reactions like the conversion of hydrocarbons (cyclopentane dehydrogenation and hydrogenolysis, benzene hydrogenation, transient CO oxidation for oxygen storage capacity, etc...), or the photooxidation of alcohols.

942

EXPERIMENTAL The sputtering device used in the present work (Fig. 1) ensured the uniform deposition of metallic d u s t e r s (Pt, Pd, Au, Ni .... ) on powdered or p l a n a r substrates [1]. The samples were placed in an A1 container used as an anode while the cathode was a 80 mm diameter foil of Pt, Pd, Au, Ni .... A vacuum of 10 -6 m b a r was created by means of a turbomolecular pump in the glass cylindrical vessel, flushed several times with pure Ar. The sputtering medium was Ar (high purity) introduced at a 1 mbar pressure and which flowed continuously during the whole deposition time. When the metallic target (Pt, Pd, Au, Ni .... ) used as a cathode was submitted to a DC potential of 500V, clusters of Pt, Pd, Au, Ni... were formed by an Ar ion bombardment (ionization energy = 13.5 eV); a plasma current of 20 to 25 mA can thus be m a i n t a i n e d constant during the whole deposition process. HOP Graphite samples (10 x 10 m m 2) were used to deposit small clusters and 10 x 20 mm 2 sheets of Si(100), glass or evaporated gold were used to prepare DC-sputtered films of Pt, Pd, Ni or Au [2][5]. In the case of powdered substrates like SiO2, TiO2, A1203 or CeO2, an uniform exposure was achieved by the mechanical vibration of the powder. Such vibration ensured a quasi-fluidization of the substrate and thus a rapid turnover of the powder surfaces exposed to the flux of metal clusters. Photocatalytic experiments were performed in a 80 ml Pyrex flask filled with 50 ml of PtfriO 2 suspension containing 0.5 M alcohol and illuminated at ~>350 nm with a 900 W Xenon l_~_mp. Gas aliquots were analyzed by gas chromatography with a 5m Porapak Q column.

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Figure 1. DC-sputtering set-up, a: cathode (Pt, Pd,..), b: screen, c: sample

943 Hydrocarbon catalytic reactions were carried out in a dynamic reactor under the following conditions : benzene hydrogenation at 120~ H2/benzene molar ration of 20 ; cyclopentane dehydrogenation and hydrogenolysis at 460~ H2/cyclopentane molar ratio of 20. The main products (cyclohexane in benzene hydrogenation, cyclopentene, cyclopentadiene and C1-C 4 hydrocarbons in cyclopentane conversion) were analyzed by GC. A pulse chromatograph apparatus was used for H 2 chemisorption and oxygen storage capacity [6, 7]. 3. T H I N FILMS D E P O S I T E D ON M O D E L S U B S T R A T E S

3.1. Morphology of films Thicknesses and densities of thin films of Pt, Pd, Ni,.Au.. sputtered for 15 to 120 minutes on glass or Si(100) were obtained from the reflected profiles recorded by X-ray reflectometry. In the case of Pt films, density ranged from 5.0 to 7.7 g cm "3 and was always much lower than that of bulk Pt (21.4 g cm-3). The presence of amorphous Pt oxides, containing only 20 to 30% platinum, was confirmed by X-ray diffraction measurements under a grazing incidence. The thickness of Pt films, calculated from the interference fringes of the reflected profiles (Fig. 2), increases from 20 nm to 45 nm when the sputtering time increases from 15 to 30 minutes. The deposition rate of Pt was = 1.5 nm min.-1 in the experimental conditions used in this work for DC-sputtering. In the case of Pd, the density of DC-sputtered films (7 to 8.7 g cm "3) was closer to that of bulk Pd (12 g cm "3) and the Pd content reached up to 70%. Cristalline PdO having the 0.266 nm interatomic distance was detected by X-ray diffraction measurements.

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Figure 2. X-ray reflection profiles of some DC-sputtered films, a: Ni/glass 60 rain., b: Au/Si sputtered for 3 rain. (dotted curve) or for 30 rain. (full curve).

944 Pd films sputtered for 30 to 60 minutes were only 13 to 27 nm thick respectively : the deposition rate of Pd (0.4 nm min -1) was found to be, under the same experimental conditions, about four times lower than for platinum. The density of DC-sputtered Ni films deposited at a rate of 0.6 nm min -1 was significantly lower (4.8 to 5.6 g cm "3) than the density of bulk nickel (8.9 g cm'3). DC-sputtered gold films had densities ranging between 9.86 g cm -3 and 12.65 g cm "3, i.e. significantly lower than the density of bulk gold (19.3 g cm-3). The examination of DC-sputtered films by STM shows surfaces with increasing roughness for longer deposition times (Fig. 3a,b), presumably due to a columnar type of growth. On the STM images of Pt films are seen large formations (700-1000 nm in size, 150 nm high) of small clusters between 7 and 20 nm large by 2 nm high. They are surrounded by holes 500 nm deep and 2000 3500 nm large. With this low density voided structure, DC-sputtered Pt films are not conductive enough (less than 10 -6 S) and show poor mechanical properties. A less rough topography is observed in the case of Pd films, probably due to a lower deposition rate, favoring the formation of both a crystalline PdO and a smoother film (Fig. 3c). The roughness of DC-sputtered Ni films imaged by STM is always greater than that of films prepared by other deposition methods, such as radio frequency. Depth concentration profiles determined by XPS-SIMS show that close to the silicon substrate there is more metallic Ni [4]. On the STM images recorded for gold films the increase of roughness with sputtering time is also observed. Large formations (2000 nm) are formed by coalescent, elongated gold particles of 10 nm.

Figure 3. STM images of Pt films at different sputtering times. X,Y = 2000 nm. a : 60 min., Az = 293 rim; b: 120 min., Az = 352 nm. STM conditions: It = 1 nA, U = 0.5 V.

945 3.2. T h e r m a l - r e d u c t i o n of DC-sputtered films under hydrogen The r o u g h n e s s of Pt a n d Pd reduced films ( l h at 300~ u n d e r H2 flow) decreases (Fig. 4) while Pt density values a n d m e t a l content increase from 5.5 g cm -3 a n d 25%Pt to 8.8 g cm "3 a n d 38% Pt after 30 m i n u t e s sputtering. Clusters, a p p r o x i m a t e l y 50 x 100 n m 2 in size a n d 6 to 10 n m h i g h are formed by the reduction a n d sintering of Pt or P d oxides. Moreover, the electrical conductivity of Pt a n d P d layers s p u t t e r e d on glass or silicon increases up to 5 S.

Figure 4. STM images of films reduced at 300~ u n d e r H 2. a: Pt, X,Y = 4000 nm, Az = 33 nm; b: Pt, X,Y = 500 nm, Az = 30 n m c: Pd, X,Y = 4000 nm, Az = 33 nm; d: Pd, X,Y = 500 nm, Az = 30 n m

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4 . C L U S T E R S D E P O S I T E D ON H O P G A N D ON P O W D E R S 4.l.Morphology of clusters The conventional planar diode DC sputtering device allows a satisfactory mass-transfer rate so that highly uniform deposits are obtained, as shown by TEM micrographs (Fig. 5). The size distribution of Pt clusters ranges between 2.0 and 3.5 nm as seen in Fig. 5 where Ti planes 0.35 nm apart can be also observed. The HOP Graphite surface bombardment with Pt, Pd or Au clusters having a significant kinetic energy upsets the cristal structure (Fig. 5b). The density charge waves observed around the clusters on the STM images could be due to the strong interactions created between metal and substrate (SMSI).

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10 nm Figure 5. Pt clusters a: TEM micrograph of Pt clusters sputtered 80 min. on TiO 2 powder (P25 Degussa) ; b: STM image of Pt clusters sputtered on HOPG.

4.2.Low-temperature catalytic a c t i v i t y Dispersion measurements (D%) deduced from hydrogen chemisorption (H/Pt s = H/Rh s = 1) and catalytic activity in benzene hydrogenation (molec. at "1. h "1) are given in Table 1. The performance of DCS catalysts (60min., 500V, 20mA) are compared in some cases with those of catalysts prepared by "wet" impregnation. DCS catalysts are very active in BH even though they are not prereduced at high temperature (300-5000C) : they are "ready to use" while conventional catalysts require a HT reduction pretreatment to develop a good hydrogenation activity. Bimetallic clusters of Rh/Pt prepared by using a

947 Rh(10%)-Pt alloy as a cathode p r e s e n t strong synergy effects : the activity .of s i l i c a - a n d alumina- supported Rh-Pt catalysts is 5 to 10 times higher t h a n t h a t of Pt catalysts. Moreover we can notice t h a t the b u l k composition of bimetallic catalysts is very close to t h a t of the alloy used as a cathode : there is no preferential sputtering of one of the metals. ~ Table 1 Benzene hydrogenation at 120~ The catalysts were reduced at 120~ their use in reaction (except Pt/TiO2"B "wet" 300, reduced at 300~ Catalyst

wt.%Pt

Pt/A120 3 Pt/AI20 3''wet'' Pt/SiO 2 Ptfrio 2 Pt/TiO 2-B"wet" PtfriO2-B"wet"300 Pt-Rh/AI20 3 Pt-Rh/SiO 2

0.56 0.98 0.32 0.30 0.67 0.67 0.56 0.32

wt%Rh

D%

rHS

0.040 0.035

45% 63% 39% 28% 24 % 24% 49% 56%

2~480 540 1 470 1 560 15 840 8 :~90 15 200

, before

T.O.N. (h- 1) 5 500 860 3 800 5 600 60 3 500 17 700 27 100

The photocatalytic conversion of alcohols was investigated on "sputtered" a n d on "wet" PtfriO 2 catalysts. The reaction involves several steps : (i) chemisorption and dissociation of alcohol molecules on the surface; (ii) creation of electron-hole pairs under illumination; ('di) hole consumption by O H - and alkoxide ions; (iv)reactions between the radicals formed at the surface; (v) photocatalytic decarboxylation of the corresponding acids into hydrocarbons; (vi) hydrogen formation by H + or water reduction on the surface of cathodically charged Pt deposits which act as microelectrodes. The rates of formation of the different products, given in Table 2, show t h a t the sputtered catalyst is a l w a y s more active t h a n the catalyst prepared by wet impregnation. Table 2 Rates of production of H 2, CO 2 and hydrocarbons (L h -1 gPt -1) on i l l u m i n a t e d suspensions of "sputtered" and "wet" photocatalysts. 0.1g Pt/TiO 2, 0.5M alcohol, pHinit.=5, ~>350 nm.

Alcohol

Ptfrio 2 H 2 CO 2

Methanol Ethanol n-Propanol

20.8 23.1 21.5

4.2 0.8 1.4

"sputtered" CH 4 C2H 6 1.6 -

4.1

H2 5.5 6.0 3.8

Pt/TiO 2 "wet" CO 2 CH 4 C2H 6 0.7 0.7 0.3

, 0.1 -

0.5

948

4.3. High-temperature catalytic activity The conversion of cyclopentane (dehydrogenation and hydrogenolysis) was investigated at 460~ (Table 3). No significant difference is found between impregnated and sputtered catalysts, which seems to show that the benefits of the sputtering method are cancelled at elevated temperature. An exception however is observed with Pt/TiO2-B which exhibit a very high selectivity in dehydrogenation. Table 3 Conversion of cyclopentane at 460~ SelectiviW % Catalyst

Pt/CeO 2 Pt/CeO 2''wet'' Pt/TiO2-B Ptfrio 2 PtfriO2"wet"

% Pt

0.32 0.92 0.30 o. 18 0.45

Activity molec.at Pt" l h ' l 4200 5100 3400 3000 3100

C 1- C4 7 14 1 5 4

n-C 5 16 13 1 18 20

CPE+ CPD* 77 73 98 77 76

*CPE : cyclopentene ; CPD : cyclopentadiene Oxygen storage capacity of Pt/CeO 2 was measured at 350-500~ by titration with CO of the active oxygen available at the preoxidized surface. OSC values are about four times higher on the sputtered catalyst than on the s~mple prepared by wet impregnation (at 450~ : 600 instead of 150~mol CO 2 gPt-1). This proves that the mobility and the availability of surface oxygen ions of ceria are better when the catalyst is prepared by soft methods without solvent or foreign ions (C1 in this case). In conclusion, DC-sputtering appears as a convenient method of preparing active and selective catalysts, especially designed for low-temperature processes. Extremely clean model catalysts can also be prepared by this technique.

REFERENCES 1. P. Albers, K. Seibold, A.J. McEvoy and J. Kiwi, J. Phys. Chem., 93 (1989) 1510 2. O. Enea, M. Rafai and A. Naudon, Ultramicroscopy, 42-44 (1992) 572 3. O. Enea and A. Naudon, in A. Davenport and J. G. Gorden II, X-ray Methods in Corrosion and Interfacial Electrochemistry, EC. Set., PV 92-1, Electrochem. Soc., New-York, 1992, p. 194. 4. O. Enea, M. Rafai, A. Naudon, M. Cahoreau, and A.J.McEvoy, ISE Abstracts 43(1992)403 5. O. Enea and M. Rafai, Ultramicroscopy, submitted. 6. D. Duprez, J. Chim. Phys., 80 (1983) 487 7. S.Kacimi, J. Barbier Jr, R.Taha and D.Duprez, Catal. Lett., 22 (1993) 343.