NANO LETTERS
AFM Study of Pt Clusters Electrochemically Deposited onto Boron-doped Diamond Films
2002 Vol. 2, No. 3 241-244
O. Enea,† B. Riedo,‡ and G. Dietler*,‡ UMR 6505 CNRS, Poitiers UniVersity, 86000 Poitiers, France, and Department of Physics, Lausanne UniVersity, 1015-Lausanne, Switzerland Received November 15, 2001; Revised Manuscript Received December 19, 2001
ABSTRACT Pt particles and clusters have been electrochemically deposited onto boron-doped diamond thin films supported by silicon wafers (SiBDD). The careful study of the morphology and dispersion of Pt deposits by AFM (air, tapping mode) proves the existence of a small number of preferential diamond sites on which the initially deposited Pt particles are growing fast. The behavior evidenced may result from the high electrical conductivity near some sites, presumably due to a higher local concentration of boron used in the BDD films as doping centers.
Introduction. Polycrystalline, boron-doped diamond (BDD) thin films are chemically, mechanically, and thermally very resistant and have a good electrical conductivity and the widest known electrochemical window. These new materials have been recently studied for a variety of electrochemical applications including metal recovery1 and the oxidation of the various organic residues1-5 contained in industrial wastewaters. The electrochemical behavior of boron-doped polycrystalline diamond electrodes depends on factors such as substrate pretreatment, growth conditions and doping concentrations.6 The activity of electrodes having a boron concentration range from 8 × 1017 to 9 × 1021 B cm-3 was found6 to be mainly controlled by their doping level and more precisely by the occurrence and behavior of their impurity band, which could supply either electrons or holes to the electrolyte through hopping between their electronic levels. The sp2 carbon impurities on the diamond surface as prepared by chemical vapor deposition can be eliminated by a mild anodic polarization.7 Such treatment also converts the hydrogen-terminated surface of BDD into an oxygenterminated surface and thus favors the attachment of IrO2 clusters8 or TiO2 thin films9 to the surface, on which these are deposited in order to improve the electrochemical properties of boron-doped diamond films. The incorporation of Pt and Pt/Ru particles into BDD thin films also led to promising materials for electrocatalytic applications.10 The metallic deposits of Pt anchored into the diamond matrix present good stability under long term * Corresponding author. † Poitiers University. ‡ Lausanne University. 10.1021/nl015666l CCC: $22.00 Published on Web 01/08/2002
© 2002 American Chemical Society
potential cycling, while this is not the case for Pt simply electrodeposited on the diamond surface.11 Despite of the number of characterization methods (microscopy, X-ray analysis, Raman and Auger spectroscopy, SIMS, cyclic voltammetry) devoted to study the various BDD films, it remains still unclear which are the most active diamond sites in the electrochemical processes. Therefore, the objective of the present work is to obtain more insight on their number and position through the study of the growth of electrochemically deposited Pt clusters, the formation of which can be followed by the I/E profiles at the macroscopic scale and by the local probe microscopy images at the microscopic scale. Experimental Section. Boron-doped diamond films were grown on conductive p-Si wafers (Siltronix, 10 mm in diameter, < 0.1 ohm.cm) via the hot filament chemical deposition technique (HF CVD). The gas mixture (1% methane in dihydrogen containing 1 ppm of trimethylboron) was supplied at a flow rate of 5 L min.-1 The temperature range of the filament was 2440-2560 °C and that of the substrate was 830 °C. A pinhole-free diamond film one micron thick was obtained at a growth rate of 240 nm h-1, under the form of a random textured, polycrystalline film. Several small sheets (10 × 25 mm) of BDD films deposited on silicon wafers have been treated under cycling at a 100 mV s-1 scan rate during 4 h in order to eliminate the impurities of sp2 carbon and to convert the hydrogen terminated surface to oxygen-termination. Next, these samples have been immersed into a solution of 5 g/L H2PtCl6 contained in a small electrolytic cell (30 mL) and various potential sequences (at fixed - 0.1 V/NHE potential or 3 to 5 cycles between - 0.05 and 1.4 V/NHE at a scan rate of
Figure 1. Cyclic voltammetric I/E curve of SiBDD/Pt in 0.1 N H2SO4 at 50 mV s-1 scan rate.
50 mV s-1) have been applied by using a three-electrode setup. The very first I/E profiles of SiBDD/Pt were recorded in a second cell filled out with 0.1 M H2SO4 by using a conventional equipment (PAR 432 potentiostat, X-Y Kipp Zonen recorder).
The morphology of the SiBDD/Pt deposits made on the anodically treated SiBDD has been examined in air by atomic force microscopy with a Nanoscope IIIa (Digital Instruments) instrument used in tapping mode. The best images were obtained with a scan frequency of 1 Hz. This low-frequency permitted to obtain minimal shift between the trace and the retrace signals. The adhesion force between tip and sample was estimated from the analysis of a force curve to be ∼30 nN12. Results and Discussion. The I/E profile recorded for a SiBDD/Pt sample at a scan rate of 50 mV s-1 in the potential range between 0 and 1.4 V (vs a normal hydrogen electrode, NHE) is given in Figure 1. This typical cyclic voltammogram displays the well-known characteristic peaks of the bulk Pt electrodes, which are due to the desorption of oxygen species and respectively to the adsorption/desorption of atomic hydrogen at Pt surface. As previously reported,11 long term potential cycling between 0 and 1.4 V/NHE leads to the decrease of the characteristic Pt peaks due to the loss of Pt,
Figure 2. AFM images (tapping mode in air) of SiBDD/Pt: (a) topography for an AFM window of 10 × 10 µm2 and (b) topography and amplitude for a window of 5 × 5 µm2. 242
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Figure 3. Histogram of the large Pt clusters electrochemically deposited (during five cycles between -0.05 and 1.4 V/NHE at a scan rate of 50 mV s-1) on SiBDD. The number of Pt clusters was plotted either toward their (a) height or their (b) size.
and thus these peaks are useless for an accurate evaluation of the electrochemically active area of Pt. Next, several dozens of spots on the first SiBDD/Pt sample have been examined by AFM used in the tapping mode, and a detailed analysis of the relatively large Pt clusters shown in the height and amplitude images (Figure 2a,b) was made. On the 10 × 10 µm2 image (Figure 2a) a number of 38 Pt clusters can be seen. Their total surface was estimated to be 6 µm2, and thus the ratio with respect to the surface examined (10 × 10 µm2) is 0.06. For a more precise estimation of clusters sizes and heights, only 5 × 5 µm2 images, as the one given in Figure 2b, have to be used. The histograms obtained for 68 Pt clusters (Figure 3a,b) display maxima for heights around 140 nm and an average size of 449 nm. When the heights of the 68 Pt clusters examined are plotted toward their sizes, a quite linear dependence is obtained as shown in Figure 4. On these bases, an ellipsoidal form can be reasonably attributed to the model average Pt cluster, the surface area of which can be therefore calculated to be 0.158 µm2. This led to a surface of 10.7 µm2 for the total area (141 µm2) of the AFM windows examined. The ratio between the total surface (10.7 µm2) of Pt particles and clusters toward the AFM window examined (141 µm2) is 0.075. Small Pt particles are hardly observed on the relatively large AFM windows needed for a systematic evaluation of sizes and heights of large Pt clusters, but can be only observed on narrow AFM windows, which, in contrast, are not so convenient for the evaluation of histograms. AFM images recorded at low scan rates and AFM windows of 2 × 2 µm2 or 1 × 1 µm2 are necessary for obtaining more details on the morphology of the diamond film and Pt clusters (Figure 5a,b). Diamond microcrystals, having facets mainly [111] oriented with some [100] orientation, the size of which varies Nano Lett., Vol. 2, No. 3, 2002
Figure 4. Dependence of the large Pt clusters heights with their sizes.
from 0.2 to 0.7 microns, are seen on the largest part of the examined area (Figure 5a). Round shaped Pt deposits are seen only in two areas, where a number of one to two dozen small Pt particles (35 to 55 nm in size, 15 to 20 nm high) have been deposited near three large Pt clusters 260-330 nm in size, 80-150 nm high (Figure 5b). The total surface area of the two large Pt clusters (0.109 µm2) seen in Figure 5b, added to that of the 47 small Pt particles (about 0.006 µm2 each) gives 0.391 µm2 for an AFM window of 1 µm2. If an AFM window of 2 × 2 µm2 is considered (Figure 5a), the total surface area of the three large Pt clusters added to 243
film has a thickness of around 1 µm micron and a roughness of 200 nm. In contrast, the seeds previously deposited on the silicon support are only 2.5 nm in diameter, and this hardly explains the preferential growth of Pt clusters around such seeds. More likely, the preferential electrodeposition of Pt is due to the boron doping which, at the microscopic scale, can be quite different from site to site. This may even lead to the formation of a Pt-boride complex, which was earlier assumed10 because the B signal on the SIMS profiles was 2 to 3 orders of magnitude larger at the surface than into the bulk. Conclusions. The morphological study of the electrodeposited Pt clusters and particles onto boron-doped diamond thin films has for the first time evidenced the presence of a limited number of BDD sites onto which the electrochemical processes preferentially occur. This may have importance for the use of BDD electrodes in a variety of applications such as wastewater treatment involving a large amount of consumed electricity. Improving the electrical conductivity of a higher number of diamond sites may lead to more efficient electrochemical processes occurring at a lower charge density and thus open up the possibility of saving electrical power. Boron-doped diamond films have therefore to be carefully examined by using local probe microscopy (AFM, STM) in order to reach more insight on the local properties of such materials at the nanometer scale. Acknowledgment. The samples of Si/BDD have been kindly provided by W. Haenni (CSEM, Neuchaˆtel, Switzerland). References
Figure 5. AFM images (tapping mode in air) of SiBDD/Pt at (a) 2 × 2 µm2 and (b) 1 × 1 µm2.
that of 92 small Pt particles amounts to 0.76 µm2 and thus to a ratio of 0.19 between the Pt area and the total area of SiBDD/Pt. No Pt clusters or particles are observed on many diamond microcrystals. This clearly shows that the electrodeposition of Pt on the SiBDD surface preferentially occurs at the specific diamond sites, presumably having higher conduction properties than those of the neighboring areas. The BDD
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(1) Perret, A.; Haenni, W.; Skinner, N.; Tang, X.-M.; Gandini, D.; Comninellis, C.; Correa, B.; Foti, G. Diamond Relat. Mater. 1999, 8, 820. (2) Fryda, M.; Dietz, A.; Herrmann, D.; Hampel, A.; Scha¨fer, L.; Klages, C.-P.; Perret, A.; Haenni, W.; Comninellis, C.; Gandini, D. Electrochem. Soc. Proc. 1999, 99-32, 473. (3) Gandini, D.; Mahe´, E.; Michaud, P.-A.; Haenni, W.; Perret, A.; Comninellis, Ch. J. Appl. Electrochem. 2000, 30, 1345. (4) Rodrigo, M. A.; Michaud, P.-A.; Duo, I.; Panizza, M.; Cerisola, G.; Comninellis, Ch. J. Electrochem. Soc. 2001, 148, D60. (5) Panizza, M.; Michaud, P.-A.; Cerisola, G.; Comninellis, Ch. Electrochem. Comm. 2001, 3, 336. (6) Zenia, F.; Ndao, N. A.; Deneuville, A.; Le´vy-Cle´ment, G. Electrochem. Soc. Proc. 1999, 99-32, 389. (7) Panizza, M.; Duo, I.; Michaud, P.-A.; Cerisola, G.; Comninellis, Ch. Electrochem. Solid-State Lett. 2000, 3, 429. (8) Duo, I.; Michaud, P.-A.; Haenni, W.; Perret, A.; Comninellis, Ch. Electrochem. Solid-State Lett. 2000, 3, 325. (9) Enea, O.; Correa, B.; Haenni, W.; Perret, A. Electrochem. Soc. Proc., 2002 in press. (10) Wang, J.; Swain, G.; Tachibana, T.; Kobashi, K. Electrochem. Soc. Proc. 1999, 99-32, 428. (11) Awada, M.; Strojek, J. W.; Swain, G. M. J. Electrochem. Soc. 1995, 142, L42. (12) Cappella, B.; Dietler, G. Surf. Sci. Rep. 1999, 34, 1-104.
NL015666L
Nano Lett., Vol. 2, No. 3, 2002
