- Email: [email protected]
Highly stable platinum nanoparticles on diamond
Electrochimica Acta 112 (2013) 493–499
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Highly stable platinum nanoparticles on diamond Fang Gao, Nianjun Yang ∗ , Christoph E. Nebel Fraunhofer-Institute for Applied Solid State Physics (IAF), Tullastraße 72, Freiburg 79108, Germany
a r t i c l e
i n f o
Article history: Received 1 July 2013 Received in revised form 21 August 2013 Accepted 2 September 2013 Available online 13 September 2013 Keywords: Pt nanoparticles Boron-doped diamond Thermal annealing Particle stability Electrochemical activity
a b s t r a c t Platinum nanoparticles electrodeposited on diamond substrate show poor stability. Their electrochemical activities vary with different substrates and deposition methods. In this study Pt nanoparticles were prepared using a two-step deposition method. The stability and electrochemical activities of Pt nanoparticles on diamond were investigated in detail. The deposition method includes a wet-chemical seeding process and an electrochemical overgrowth of the seeds. The wet-chemical seeding process can be applied as well for other kinds of metal particles on diamond. H-terminated diamond surface is more favorable for seeding than O-terminated surface. Rapid thermal annealing process was applied to enhance the stability of Pt particles on diamond. Electrochemical activation and further overgrowth of annealed Pt nanoparticles were applied to improve the hydrogen adsorption/desorption activities of Pt nanoparticles on diamond with cyclic voltammetry in 0.1 M sulfuric acid solution. As confirmed by ultrasound removal and atomic force microscope (AFM) removal experiments as well as electrochemical tests, highly stable and active Pt nanoparticles on diamond were achieved after thermal annealing and electrochemical activation/overgrowth processes. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Platinum and Pt–C composite catalyst have been widely studied for electrochemical energy conversion [1,2]. The carbon matrix provides high surface area while Pt catalyzes various reactions such as oxygen reduction (ORR) [3], hydrogen evolution (HER) [4] as well as the oxidation of methanol and of carbon monoxide [5,6]. However, due to the etching of sp2 carbon in electrolyte solutions, the life-time of conventional Pt–C electrodes are relatively short [7]. Diamond, on the other hand, is extremely resistive to both chemical and physical corrosions. Heavily boron-doped diamond shows metal-like properties and has been proved to have the widest potential window in aqueous solutions compared with other electrode materials [8]. Therefore, platinum nanoparticles deposited on conductive diamond electrodes are promising for providing a highly stable and active system for electrochemical catalytic reactions. However, two problems have been shown for this system in previous studies [7,9]. The first and the most prominent one is the low coverage of metal particles on diamond electrodes. This is due to the non-uniform electronic properties of diamond. The second one is the unstable binding between Pt particles and diamond surface. The first problem was solved recently by us after including a wetchemical seeding process before electrochemical deposition, due
to the fact that the seeding process is regardless of the electronic property of the diamond electrode [10]. However, the interaction between diamond and the Pt particles is still weak. The particles are easily to be removed by mechanical removal such as ultrasound cleaning or soft AFM (atomic force microscope) scratching. In this study, we report a novel way to improve the stability of Pt nanoparticles on diamond. Electrochemical activity Pt/diamond before and after stabilization of Pt particles was investigated as well. We firstly ascertained the effect of the oxygen (O-) and hydrogen (H-) surface termination of diamond on our wet-chemical seeding process. This method was then applied to seed several other metal particles besides Pt. In the second part, we focused on the characterization of the stability and electrochemical activity of Pt particles on diamond after applying rapid thermal annealing (RTA) process under N2 atmosphere. Scanning electron microscopy (SEM), contact mode AFM and ultrasound removal experiments confirmed improved stability of Pt particles on diamond. However, the formation of a passivation layer was detected on the nanoparticle surfaces, resulting in greatly reduced electrochemical activity of Pt nanoparticles for hydrogen adsorption/desorption. In the last part of this paper, we show a novel way to overcome this problem. It is a combination of electrochemical activation in 0.1 M H2 SO4 solution and a subsequent electrochemical overgrowth of a new Pt layer on the surface of Pt particles. 2. Experimental
∗ Corresponding author. Tel.: +49 7615159647; fax: +49 761515971647. E-mail address: [email protected] (N. Yang). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.09.005
Polycrystalline boron-doped diamond (BDD) films were grown with a microwave plasma enhanced chemical vapor deposition
494
F. Gao et al. / Electrochimica Acta 112 (2013) 493–499
reactor and using trimethyl boron (TMB) as the boron source. The boron concentration was measured to be in the range of 1021 cm−3 by secondary ion mass spectroscopy (SIMS). All chemicals were bought from Sigma–Aldrich without further purification. The BDD electrodes were electrochemically oxidized and hydrogenated as reported [11,12]. O-termination was obtained by anodic oxidation of BDD electrodes in 2 M H2 SO4 at +5 V for 1 min, while H-termination was obtained by cathodic treatment at −35 V for 1 min in the same solution. Scanning electron microscope images were recorded by a Hitachi 4500 microscope (Hitachi, Japan) at an acceleration voltage of 30 kV. High resolution X-ray photoelectron spectra (XPS) were obtained with the Al K␣ as excitation source and a detection angle of 15◦ . AFM measurements were made with a NanoWizard3 scanning probe microscopy system (JPK Instruments, Germany) and NCST tips (NanoWorld, Switzerland). Electrochemical experiments were conducted using a VMP-3 Biologic Multi-channel Potentiostat (Biological Inc., France). A three-electrode system was applied with Ag/AgCl reference and Pt counter electrodes. The working electrode was a BDD with or without metal particles. The geometric area of the BDD is 0.083 cm2 . Ultrasound removal of Pt particles from diamond surface was performed using an ultrasound probe (VCX 130, Vibra Cell) in a 3 ml beaker filled with de-ionized water. The distance between the probe and the sample was 1.5 cm, and the removal lasted for 1 min. The deposition of Pt nanoparticles on diamond were prepared as reported previously [10]. Briefly, 1.0 M NaBH4 dissolved in 0.1 M
Fig. 1. SEM images of Pt particles on (a) H-terminated and (b) O-terminated diamond surface. The samples were prepared using the procedure only with one-time seeding process. The inset is the XPS spectra of Pt 4f7/2 , 5/2 .
NaOH was first dropped onto the diamond electrode for impregnation. The diamond surface was H-terminated or O-terminated. The electrode was then washed with water and blew dry with N2 . After that, 1.0 mM H2 PtCl6 solution was dropped onto the sample to react with adsorbed NaBH4 to generate Pt nanoparticles. For the deposition of Ni, Au, and Cu nanoparticles, only H-terminated diamond was used. The deposition was performed in the same way as the deposition of Pt nanoparticles except that 1.0 mM H2 PtCl6 solution was replaced by 1.0 M NiSO4 , 2.4 mM HAuCl4 , or 10 mM CuSO4 solution.
Fig. 2. SEM images of wet chemically deposited (a) Au, (b) Cu, and (c) Ni particles on diamond.
F. Gao et al. / Electrochimica Acta 112 (2013) 493–499
After seeding, electrochemical overgrowth of Pt seeds was conducted in 1.0 mM H2 PtCl6 solution with a fixed charge of 3.6 mC under a constant potential of −0.2 V. After overgrowth, the particle coated diamond electrode was treated with RTA at 700 ◦ C for 5 min in N2 atmosphere. The annealed sample was then electrochemically activated between −0.5 V and 1.5 V at a scan rate of 200 mV/s for 100 cycles. After that, a further overgrowth was performed using a deposition voltage of −0.2 V and a fixed charge of 3.6 mC to renew the particle surface.
495
positively charged C H dipole surface [12]. On the other side, the key functional group of NaBH4 for its adsorption is the non-polarized BH4 − . Therefore in the case of H-termination the attractive force is generated and thus enhances the adsorption of NaBH4 . In contrast, a repulsive interaction prevents the adsorption of NaBH4 on O-termination. The chemical reaction between NaBH4 and H2 PtCl6 generating Pt nanoparticles is well-known and widely used [15–17]. NaBH4 + 3H2 O + H2 PtCl6 → Pt + H3 BO3 + 5HCl + NaCl + 2H2 ↑ (1)
3. Results and discussion 3.1. Universal wet-chemical seeding process Surface termination of diamond provides different dipole layers on the surface [12] and thus affects the adsorption of NaBH4 for wet-chemical seeding of metal particles. Fig. 1 shows SEM images of wet-chemically seeded BDD electrodes when different surface terminations were applied. On H-terminated diamond (Fig. 1a) a dense Pt nanoparticle layer is formed. The average diameter of the particles is below 10 nm and the density is ∼1011 cm−2 as previously reported [13]. The chemical composition of these seeds is confirmed by XPS. The inset of Fig. 1(a) shows the XPS spectra of Pt 4f7/2 , 5/2 . The Pt 4f7/2 peak centers at 71.0 eV, confirming Pt is in the metallic state [14]. On O-terminated diamond (Fig. 1b) nearly no Pt seeds can be detected. Therefore, H-terminated diamond surface is more favorable for wet-chemical seeding of Pt nanoparticles than O-terminated diamond. This is due to stronger adsorption of NaBH4 on the H-terminated surface. Taking the dipole layer on diamond into account, H-terminated surface generates a
However, the mechanism of using this method to generate Pt nanoparticles is seldom discussed. Based on classical nucleation theory (CNT) [18], the generation of nanoparticles requires first a supersaturation of the solvent followed by a burst of nucleation. In the case of our seeding method, because of the strong driving force of the reaction between NaBH4 and H2 PtCl6 (the standard reduction potential of NaBH4 is −1.24 V vs. NHE [19]), the adsorbed BH4 − cannot diffuse far from the electrode surface before it reacts with the Pt precursor. Therefore, a supersaturation is created within a distance close to the electrode. Two routes will then lead to the formation of diamond-supported Pt particles. In the first route, the nucleation happens in the vicinity of diamond electrode, and the nuclei have a high surface free energy which is given by [20]:
G = 4
2r 3 r − ∗ 3r
2
(2)
where is the interfacial energy of a nucleus with the surroundings, r is the size of the nucleus, and r* is the critical radius of nucleation.
Fig. 3. SEM images (a, b) and histogram (c, d) of Pt particles before (a, c) and after (b, d) rapid thermal annealing process at 700 ◦ C for 5 min under N2 atmosphere.
496
F. Gao et al. / Electrochimica Acta 112 (2013) 493–499
This energy must be lowered by either nuclei growth (increasing r), or by attaching to surfactant molecules, or in our case diamond surface (lowering ). In the second route, the non-uniform polycrystalline diamond surface provides local defects and impurities which lower the nucleation barrier, and then the nucleation starts directly at the substrate. In both cases, the prerequisite for the formation of Pt particles on diamond is that the supersaturation is sufficiently close to the substrate. Otherwise, in the first case, the nuclei will grow big enough to reach a balanced state when diffusing to the surface of diamond; in the second case, the supersaturated solution cannot meet the substrate where the nucleation is supposed to take place. In order to confirm the importance of the distance of the supersaturation with the diamond electrodes two control experiments were conducted. In the first one, we directly dropped firstly NaBH4 and then H2 PtCl6 onto a diamond electrode without removing excessive NaBH4 solution. In the second control experiment, we first mixed NaBH4 solution with H2 PtCl6 to generate nanoparticles and then drop the suspension of Pt particles onto a diamond electrode. Both of the two samples in the control experiment were finally rinsed with water and blow dry with nitrogen. In the first control experiment the supersaturation of Pt happened once Pt precursor met NaBH4 , the nucleation happened tens of microns (the thickness of the droplet) away from the electrodes; in the second control experiment, the nucleation finished in the suspension and there was no further nucleation after the suspension was dropped onto the electrodes. SEM images show that in neither of cases platinum-seeds adsorb to diamond electrodes. These results confirm that the key to a successful seeding is that only surface
adsorbed NaBH4 is involved before the generation of Pt nanoparticles, leading to a very close supersaturation to the surface. Similar supersaturation was also possible to be generated via high overpotential (>1 V) pulse deposition [21]. In this way the nucleation density is actually enhanced on a diamond electrode surface. As proved previously, the wet-chemical seeding process is actually controlled by NaBH4 adsorption [10]. Therefore, this seeding process is expected to be applicable on other metal salts if they can be reduced by NaBH4 to metal. We thus tested wet chemical seeding experiments on diamond surface by replacing 1.0 mM H2 PtCl6 with 1.0 M NiSO4 , 2.4 mM HAuCl4 , or 10 mM CuSO4 . Fig. 2 provides SEM images of Au (a), Cu (b), and Ni (c) nanoparticles seeded diamond surface. For these seeding experiments, only H-terminated surfaces were used. As seen in Fig. 2, homogenously distributed nanoparticles for three metals are detected on the diamond surface. The sizes and the densities of these particles vary with the types of metals, due to different reaction kinetics of adsorbed NaBH4 with metal ions. All these results indicate that wet chemical seeding method proposed here is a universal metal deposition method on diamond surface. 3.2. Enhanced stability of Pt nanoparticles via thermal annealing Pt nanoparticles deposited via electrochemical methods are mechanically unstable. They are easily removed by rigorous water stream or cleaning process in an ultrasound bath [10]. Since carbon dissolves in platinum [22], thermal annealing is helpful to form an epitaxial lattice structure between diamond and Pt beneath the particles. Rapid thermal annealing (RTA) of Pt nanoparticles coated
Fig. 4. (a) AFM images of Pt coated diamond sample before (a, b) and after (c, d) rapid thermal annealing; images (a, b, and c) are recorded with tapping mode, while image (d) is recorded with contact mode; in image (b), contact mode removal of Pt particles was done in the center of the area.
F. Gao et al. / Electrochimica Acta 112 (2013) 493–499
diamond was then conducted at 700 ◦ C under N2 atmosphere for 5 min. Fig. 3(a) shows the SEM image of Pt nanoparticles fabricated by electrochemical overgrowth of Pt seeds shown in Fig. 1(b). Fig. 3(b) shows the SEM image of these Pt nanoparticles after thermal annealing. The density decreases from 1.4 ± 0.3 × 1011 cm−3 before annealing (Fig. 3a) to 0.9 ± 0.3 × 1011 cm−3 after annealing (Fig. 3b). The growth of the nanoparticles was analyzed from the histogram of these particles (Fig. 3c and d). The mean diameter of the particles grows from 16.1 ± 2.8 nm (Fig. 3c) to 22.3 ± 4.1 nm (Fig. 3d), showing the coalesced particles. AFM removal experiments were further applied to test the stability of Pt nanoparticles on diamond before and after thermal annealing. For convenience, polished nanocrystalline diamond films were used. It has a root mean square (RMS) surface roughness of 1.38 nm, measured over an area of 5 × 5 m2 . Fig. 4(a) shows a tapping mode AFM image of Pt particles deposited on diamond before thermal annealing process. Pt particles distribute homogeneously on diamond surface, the same as those seen from SEM images. Removal experiments were then applied in an area of 600 × 600 nm2 with a vertical deflection of 0.25 V, which is equivalent to a loading force of 0.15 N. Even with this small loading force, Pt nanoparticles were completely removed. One typical AFM image of such an experiment is shown in Fig. 4(b) where a clear diamond surface without any particles is seen. Although after thermal annealing Pt particles still distribute well on diamond surface (as seen from one typical AFM image in Fig. 4c), the AFM removal experiments show different results from Fig. 4(b). One typical contact mode AFM image is shown in Fig. 4(d) where a vertical deflection of 1.5 V was applied, which is equivalent to a loading force of 0.88 N. This loading force is 5 times higher than that
Fig. 5. Cyclic voltammograms in 0.1 M H2 SO4 at a scan rate of 100 mV/s on the Pt nanoparticles coated diamond electrodes (a) as-deposited, (b) after rapid thermal annealing, (c) after rapid thermal annealing and electrochemical activation.
497
applied in Fig. 4(b), but few of Pt particles were removed. Therefore thermal annealing process does enhance the adsorption force of Pt nanoparticles on diamond, namely the stability of Pt particles on diamond. According to Winterbottom’s theory on the equilibrium shape of supported small particles [23], in order to obtain stable particles on a substrate, the energy configuration must satisfy: SP < SV + PV
(3)
SP , SV , and PV are the interfacial energies of substrate–particle, substrate–vacuum, and particle–vacuum, respectively. The energy gain for the formation of a Pt (1 1 1)-diamond (1 1 1) interface is given by first-principal calculation as 3.61 J/m2 [24], which means the Eq. (3) is satisfied. Although this energy (∼60 kJ/mol carbon atom) is much smaller than common covalent bond on diamond surface such as C H 410 kJ/mol, C C 335 kJ/mol [25], it still indicates that Pt particles can be energetically stable on diamond surface. Then the experimental observation of an weak adsorption of the electrodeposited particles without annealing [26] is most likely to be a “dirty” interface. It includes surface atoms/terminations, adsorbed water layer, as well as other impurities. After annealing, however, first carbon layer dissolves in Pt and a clean Pt–diamond interface is then formed. Because the first-principal calculation shows no bonding between Pt and C, the dangling bonds at carbon surface probably induce a surface dipole
Fig. 6. (a) SEM image of overgrown Pt nanoparticles after rapid thermal annealing and electrochemical activation; (b) cyclic voltammogram in 0.1 M H2 SO4 at a scan rate of 100 mV/s on overgrown Pt nanoparticles after rapid thermal annealing and electrochemical activation.
498
F. Gao et al. / Electrochimica Acta 112 (2013) 493–499
at the Pt side of the interface. This dipole interaction contributes to the formation energy of the interface. 3.3. Refreshed activity of Pt nanoparticles via electrochemical activation and overgrowth Although the annealing process provides a convenient and effective route in stabilizing Pt nanoparticles on diamond surface, the surface passivation of the metal particles after high temperature treatment seems to be very serious. Therefore, electrochemical polishing was conducted as mentioned in experimental session before performing experiments in Figs. 5 and 6. Fig. 5(a) and (b) provides the CV measurement on Pt nanoparticles coated diamond electrodes in 0.1 M H2 SO4 before and after annealing, respectively. Before annealing, two couples of peaks are clearly seen for hydrogen adsorption/desorption, indicating that Pt nanoparticles are electrochemically active. After annealing, these peaks disappear and only background current similar to bare diamond is recorded, indicating the loss of reactivity of Pt nanoparticles toward hydrogen adsorption/desorption. This is probably due to the formation of a passivation film since carbon dissolves in platinum at high temperature and precipitates at the metal surface during cooling down [22]. In order to obtain active Pt nanoparticles, electrochemical reactivation of such a surface was conducted by cycling in 0.1 M H2 SO4 solution. The idea is to repeatedly oxidize and reduce the surface of Pt nanoparticles and then the surface will be refreshed
[27]. As shown in Fig. 5(c), the hydrogen adsorption/desorption peaks show up again after such a treatment. Comparing with those for the sample before annealing, the current scale however decreases. This might be due to the incomplete removal of the passivation layer, which leads to a smaller electrochemically active Pt area. An electrochemical overgrowth was then performed on the electrochemically activated particles. Fig. 6(a) shows one SEM image of a surface after such a growth. A rough surface is detected with voids and nanostructures. Hydrogen adsorption/desorption was then recorded on such a surface in 0.1 M H2 SO4 solution using cyclic voltammogram. As shown in Fig. 6(b), the responses of hydrogen desorption/adsorption are fully recovered (Fig. 5a). Please note that the current scales in cyclic voltammograms of Figs. 5 and 6 are slightly lower than the zero axes. This is probably due to notoptimized process for electrochemically polishing Pt nanoparticles. To conclude, the process introduced here provides an easy and efficient way to re-generate fresh Pt surface after thermal treatment. The stability was tested as well using ultrasound removal. An ultrasound probe was used to mechanically remove particles from electrode. After each removal, the hydrogen adsorption/desorption peaks were measured and the peak areas were used to calculate the removal rate of the particles. Fig. 7 summarizes normalized peak area as a function of the ultrasound power applied. These results suggest an improved physical adsorption for Pt particles after our annealing-activation-overgrowing process. At a power of 130 W, about 35% Pt particles are removed from the sample without annealing, but less than 1% of the particles are removed from the sample after annealing. AFM removal experiments (not shown) were conducted as well. It is found that Pt particles are hard to be removed even by a high vertical deflection of 1.5 V. As a result, Pt particles deposited with this new technique have higher adsorption stability and are thus more promising for catalytic reactions with a long lift-time. 4. Conclusion In summary, we developed a novel technology to form stable and active Pt nanoparticles on diamond surfaces. This process consists of steps of wet chemical seeding, electrochemical overgrowth, rapid thermal annealing, electrochemical surface activation and further overgrowth. Due to the negative formation energy of a Pt–diamond interface, the adsorption stability of the particle is enhanced, as confirmed by contact mode AFM removal and ultrasound removal tests. Electrochemical activities of these nanoparticles can be restored by electrochemical activation and subsequent overgrowth. Moreover, this technology can be potentially applied for seeding different kinds of metal nanoparticles. Before applying these systems for industrial applications, further experiments are important with respect to the stability of these electrode systems for electrocatalytic reactions such as oxidation of fuel liquids, reduction of carbon dioxide, and catalytic reactions during organic synthesis. Acknowledgements We thank Ms. Georgia Lewes-Malandrakis for the growth and laser cutting of diamond samples, and Dr. René Hoffmann for the fruitful discussions. The financial support by the European Commission under the Seventh Framework Program (MATCON, Grant No. 238201) is gratefully acknowledged.
Fig. 7. Normalized peak area as a function of the power of the ultrasound probe for Pt–diamond electrode after annealing-activation-overgrowth (black squares) and for seeded diamond electrode directly fabricated by seeding-overgrowth (red dots). The peak area is calculated from the hydrogen desorption peak shown in cyclic voltammograms and normalized with respect to the starting peak area before ultrasound treatment (0 W power). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
References [1] X. Sun, R. Li, D. Villers, J.P. Dodelet, S. Desilets, Composite electrodes made of Pt nanoparticles deposited on carbon nanotubes grown on fuel cell backings, Chemical Physics Letters 379 (2003) 99–104.
F. Gao et al. / Electrochimica Acta 112 (2013) 493–499 [2] S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Ordered nanoporous arrays of carbon supporting high dispersions of platinum nanoparticles (vol 412, p 169, 2001), Nature 414 (2001) 470. [3] U.A. Paulus, T.J. Schmidt, H.A. Gasteiger, R.J. Behm, Oxygen reduction on a highsurface area Pt/vulcan carbon catalyst: a thin-film rotating ring-disk electrode study, Journal of Electroanalytical Chemistry 495 (2001) 134–145. [4] S.A. Grigoriev, P. Millet, V.N. Fateev, Evaluation of carbon-supported Pt and Pd nanoparticles for the hydrogen evolution reaction in PEM water electrolysers, Journal of Power Sources 177 (2008) 281–285. [5] Z.L. Liu, X.Y. Ling, X.D. Su, J.Y. Lee, Carbon-supported Pt and PtRu nanoparticles as catalysts for a direct methanol fuel cell, Journal of Physical Chemistry B 108 (2004) 8234–8240. [6] S. Alayoglu, A.U. Nilekar, M. Mavrikakis, B. Eichhorn, Ru–Pt core–shell nanoparticles for preferential oxidation of carbon monoxide in hydrogen, Nature Materials 7 (2008) 333–338. [7] J. Wang, G.M. Swain, Fabrication and evaluation of platinum/diamond composite electrodes for electrocatalysis – preliminary studies of the oxygen-reduction reaction, Journal of the Electrochemical Society 150 (2003) E24–E32. [8] A. Kraft, Doped diamond: a compact review on a new, versatile electrode material, International Journal of Electrochemical Science 2 (2007) 355–385. [9] J. Hu, X. Lu, J.S. Foord, Nanodiamond pretreatment for the modification of diamond electrodes by platinum nanoparticles, Electrochemistry Communications 12 (2010) 676–679. [10] F. Gao, N. Yang, W. Smirnov, H. Obloh, C.E. Nebel, Size-controllable and homogeneous platinum nanoparticles on diamond using wet chemically assisted electrodeposition, Electrochimica Acta 90 (2013) 445–451. [11] R. Hoffmann, H. Obloh, N. Tokuda, N. Yang, C.E. Nebel, Fractional surface termination of diamond by electrochemical oxidation, Langmuir 28 (2012) 47–50. [12] R. Hoffmann, A. Kriele, H. Obloh, J. Hees, M. Wolfer, W. Smirnov, N. Yang, C.E. Nebel, Electrochemical hydrogen termination of boron-doped diamond, Applied Physics Letters 97 (2010), 052103–052103. [13] F. Gao, N. Yang, H. Obloh, C.E. Nebel, Shape-controlled platinum nanocrystals on boron-doped diamond, Electrochemistry Communications 30 (2013) 55–58. [14] D. Briggs, in: C.D. Wanger, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg (Eds.), Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp., Physical Electronics Division, Eden Prairie, MN, USA, 1979, p. 190, $195, Surface and Interface Analysis, 3 (1981) v-v.
499
[15] P. Kim, J.B. Joo, W. Kim, J. Kim, I.K. Song, J. Yi, NaBH4 -assisted ethylene glycol reduction for preparation of carbon-supported Pt catalyst for methanol electrooxidation, Journal of Power Sources 160 (2006) 987–990. [16] L. La-Torre-Riveros, E. Abel-Tatis, A.E. Méndez-Torres, D.A. Tryk, M. Prelas, C.R. Cabrera, Synthesis of platinum and platinum–ruthenium-modified diamond nanoparticles, Journal of Nanoparticle Research 13 (2011) 2997–3009. [17] L. La-Torre-Riveros, R. Guzman-Blas, A.E. Mendez-Torres, M. Prelas, D.A. Tryk, C.R. Cabrera, Diamond nanoparticles as a support for Pt and PtRu catalysts for direct methanol fuel cells, ACS Applied Materials & Interfaces 4 (2012) 1134–1147. [18] V.K. LaMer, R.H. Dinegar, Theory, production and mechanism of formation of monodispersed hydrosols, Journal of the American Chemical Society 72 (1950) 4847–4854. [19] E. Gyenge, Electrooxidation of borohydride on platinum and gold electrodes: implications for direct borohydride fuel cells, Electrochimica Acta 49 (2004) 965–978. [20] V.K.L. Mer, Nucleation in phase transitions, Industrial & Engineering Chemistry 44 (1952) 1270–1277. [21] L. Hutton, M.E. Newton, P.R. Unwin, J.V. Macpherson, Amperometric oxygen sensor based on a platinum nanoparticle-modified polycrystalline boron doped diamond disk electrode, Analytical Chemistry 81 (2009) 1023–1032. [22] J.C. Hamilton, J.M. Blakely, Carbon layer formation on the Pt (1 1 1) surface as a function of temperature, Journal of Vacuum Science and Technology 15 (1978) 559–562. [23] W.L. Winterbottom, Equilibrium shape of a small particle in contact with a foreign substrate, Acta Metallurgica 15 (1967) 303–310. [24] M. Shafiei, A.R. Riahi, F.G. Sen, A.T. Alpas, Improvement of platinum adhesion to carbon surfaces using PVD coatings, Surface and Coatings Technology 205 (2010) 306–311. [25] N. Yang, J. Yu, H. Uetsuka, C.E. Nebel, Characterization of diamond surface terminations using electrochemical grafting with diazonium salts, Electrochemistry Communications 11 (2009) 2237–2240. [26] J.A. Bennett, Y. Show, S. Wang, G.M. Swain, Pulsed galvanostatic deposition of Pt particles on microcrystalline and nanocrystalline diamond thin-film electrodes, Journal of the Electrochemical Society 152 (2005) E184–E192. [27] K. Yamamoto, D.M. Kolb, R. Kotz, G. Lehmpfuhl, Hydrogen adsorption and oxide formation on platinum single-crystal electrodes, Journal of Electroanalytical Chemistry 96 (1979) 233–239.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Highly stable platinum nanoparticles on diamond Fang Gao, Nianjun Yang ∗ , Christoph E. Nebel Fraunhofer-Institute for Applied Solid State Physics (IAF), Tullastraße 72, Freiburg 79108, Germany
a r t i c l e
i n f o
Article history: Received 1 July 2013 Received in revised form 21 August 2013 Accepted 2 September 2013 Available online 13 September 2013 Keywords: Pt nanoparticles Boron-doped diamond Thermal annealing Particle stability Electrochemical activity
a b s t r a c t Platinum nanoparticles electrodeposited on diamond substrate show poor stability. Their electrochemical activities vary with different substrates and deposition methods. In this study Pt nanoparticles were prepared using a two-step deposition method. The stability and electrochemical activities of Pt nanoparticles on diamond were investigated in detail. The deposition method includes a wet-chemical seeding process and an electrochemical overgrowth of the seeds. The wet-chemical seeding process can be applied as well for other kinds of metal particles on diamond. H-terminated diamond surface is more favorable for seeding than O-terminated surface. Rapid thermal annealing process was applied to enhance the stability of Pt particles on diamond. Electrochemical activation and further overgrowth of annealed Pt nanoparticles were applied to improve the hydrogen adsorption/desorption activities of Pt nanoparticles on diamond with cyclic voltammetry in 0.1 M sulfuric acid solution. As confirmed by ultrasound removal and atomic force microscope (AFM) removal experiments as well as electrochemical tests, highly stable and active Pt nanoparticles on diamond were achieved after thermal annealing and electrochemical activation/overgrowth processes. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Platinum and Pt–C composite catalyst have been widely studied for electrochemical energy conversion [1,2]. The carbon matrix provides high surface area while Pt catalyzes various reactions such as oxygen reduction (ORR) [3], hydrogen evolution (HER) [4] as well as the oxidation of methanol and of carbon monoxide [5,6]. However, due to the etching of sp2 carbon in electrolyte solutions, the life-time of conventional Pt–C electrodes are relatively short [7]. Diamond, on the other hand, is extremely resistive to both chemical and physical corrosions. Heavily boron-doped diamond shows metal-like properties and has been proved to have the widest potential window in aqueous solutions compared with other electrode materials [8]. Therefore, platinum nanoparticles deposited on conductive diamond electrodes are promising for providing a highly stable and active system for electrochemical catalytic reactions. However, two problems have been shown for this system in previous studies [7,9]. The first and the most prominent one is the low coverage of metal particles on diamond electrodes. This is due to the non-uniform electronic properties of diamond. The second one is the unstable binding between Pt particles and diamond surface. The first problem was solved recently by us after including a wetchemical seeding process before electrochemical deposition, due
to the fact that the seeding process is regardless of the electronic property of the diamond electrode [10]. However, the interaction between diamond and the Pt particles is still weak. The particles are easily to be removed by mechanical removal such as ultrasound cleaning or soft AFM (atomic force microscope) scratching. In this study, we report a novel way to improve the stability of Pt nanoparticles on diamond. Electrochemical activity Pt/diamond before and after stabilization of Pt particles was investigated as well. We firstly ascertained the effect of the oxygen (O-) and hydrogen (H-) surface termination of diamond on our wet-chemical seeding process. This method was then applied to seed several other metal particles besides Pt. In the second part, we focused on the characterization of the stability and electrochemical activity of Pt particles on diamond after applying rapid thermal annealing (RTA) process under N2 atmosphere. Scanning electron microscopy (SEM), contact mode AFM and ultrasound removal experiments confirmed improved stability of Pt particles on diamond. However, the formation of a passivation layer was detected on the nanoparticle surfaces, resulting in greatly reduced electrochemical activity of Pt nanoparticles for hydrogen adsorption/desorption. In the last part of this paper, we show a novel way to overcome this problem. It is a combination of electrochemical activation in 0.1 M H2 SO4 solution and a subsequent electrochemical overgrowth of a new Pt layer on the surface of Pt particles. 2. Experimental
∗ Corresponding author. Tel.: +49 7615159647; fax: +49 761515971647. E-mail address: [email protected] (N. Yang). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.09.005
Polycrystalline boron-doped diamond (BDD) films were grown with a microwave plasma enhanced chemical vapor deposition
494
F. Gao et al. / Electrochimica Acta 112 (2013) 493–499
reactor and using trimethyl boron (TMB) as the boron source. The boron concentration was measured to be in the range of 1021 cm−3 by secondary ion mass spectroscopy (SIMS). All chemicals were bought from Sigma–Aldrich without further purification. The BDD electrodes were electrochemically oxidized and hydrogenated as reported [11,12]. O-termination was obtained by anodic oxidation of BDD electrodes in 2 M H2 SO4 at +5 V for 1 min, while H-termination was obtained by cathodic treatment at −35 V for 1 min in the same solution. Scanning electron microscope images were recorded by a Hitachi 4500 microscope (Hitachi, Japan) at an acceleration voltage of 30 kV. High resolution X-ray photoelectron spectra (XPS) were obtained with the Al K␣ as excitation source and a detection angle of 15◦ . AFM measurements were made with a NanoWizard3 scanning probe microscopy system (JPK Instruments, Germany) and NCST tips (NanoWorld, Switzerland). Electrochemical experiments were conducted using a VMP-3 Biologic Multi-channel Potentiostat (Biological Inc., France). A three-electrode system was applied with Ag/AgCl reference and Pt counter electrodes. The working electrode was a BDD with or without metal particles. The geometric area of the BDD is 0.083 cm2 . Ultrasound removal of Pt particles from diamond surface was performed using an ultrasound probe (VCX 130, Vibra Cell) in a 3 ml beaker filled with de-ionized water. The distance between the probe and the sample was 1.5 cm, and the removal lasted for 1 min. The deposition of Pt nanoparticles on diamond were prepared as reported previously [10]. Briefly, 1.0 M NaBH4 dissolved in 0.1 M
Fig. 1. SEM images of Pt particles on (a) H-terminated and (b) O-terminated diamond surface. The samples were prepared using the procedure only with one-time seeding process. The inset is the XPS spectra of Pt 4f7/2 , 5/2 .
NaOH was first dropped onto the diamond electrode for impregnation. The diamond surface was H-terminated or O-terminated. The electrode was then washed with water and blew dry with N2 . After that, 1.0 mM H2 PtCl6 solution was dropped onto the sample to react with adsorbed NaBH4 to generate Pt nanoparticles. For the deposition of Ni, Au, and Cu nanoparticles, only H-terminated diamond was used. The deposition was performed in the same way as the deposition of Pt nanoparticles except that 1.0 mM H2 PtCl6 solution was replaced by 1.0 M NiSO4 , 2.4 mM HAuCl4 , or 10 mM CuSO4 solution.
Fig. 2. SEM images of wet chemically deposited (a) Au, (b) Cu, and (c) Ni particles on diamond.
F. Gao et al. / Electrochimica Acta 112 (2013) 493–499
After seeding, electrochemical overgrowth of Pt seeds was conducted in 1.0 mM H2 PtCl6 solution with a fixed charge of 3.6 mC under a constant potential of −0.2 V. After overgrowth, the particle coated diamond electrode was treated with RTA at 700 ◦ C for 5 min in N2 atmosphere. The annealed sample was then electrochemically activated between −0.5 V and 1.5 V at a scan rate of 200 mV/s for 100 cycles. After that, a further overgrowth was performed using a deposition voltage of −0.2 V and a fixed charge of 3.6 mC to renew the particle surface.
495
positively charged C H dipole surface [12]. On the other side, the key functional group of NaBH4 for its adsorption is the non-polarized BH4 − . Therefore in the case of H-termination the attractive force is generated and thus enhances the adsorption of NaBH4 . In contrast, a repulsive interaction prevents the adsorption of NaBH4 on O-termination. The chemical reaction between NaBH4 and H2 PtCl6 generating Pt nanoparticles is well-known and widely used [15–17]. NaBH4 + 3H2 O + H2 PtCl6 → Pt + H3 BO3 + 5HCl + NaCl + 2H2 ↑ (1)
3. Results and discussion 3.1. Universal wet-chemical seeding process Surface termination of diamond provides different dipole layers on the surface [12] and thus affects the adsorption of NaBH4 for wet-chemical seeding of metal particles. Fig. 1 shows SEM images of wet-chemically seeded BDD electrodes when different surface terminations were applied. On H-terminated diamond (Fig. 1a) a dense Pt nanoparticle layer is formed. The average diameter of the particles is below 10 nm and the density is ∼1011 cm−2 as previously reported [13]. The chemical composition of these seeds is confirmed by XPS. The inset of Fig. 1(a) shows the XPS spectra of Pt 4f7/2 , 5/2 . The Pt 4f7/2 peak centers at 71.0 eV, confirming Pt is in the metallic state [14]. On O-terminated diamond (Fig. 1b) nearly no Pt seeds can be detected. Therefore, H-terminated diamond surface is more favorable for wet-chemical seeding of Pt nanoparticles than O-terminated diamond. This is due to stronger adsorption of NaBH4 on the H-terminated surface. Taking the dipole layer on diamond into account, H-terminated surface generates a
However, the mechanism of using this method to generate Pt nanoparticles is seldom discussed. Based on classical nucleation theory (CNT) [18], the generation of nanoparticles requires first a supersaturation of the solvent followed by a burst of nucleation. In the case of our seeding method, because of the strong driving force of the reaction between NaBH4 and H2 PtCl6 (the standard reduction potential of NaBH4 is −1.24 V vs. NHE [19]), the adsorbed BH4 − cannot diffuse far from the electrode surface before it reacts with the Pt precursor. Therefore, a supersaturation is created within a distance close to the electrode. Two routes will then lead to the formation of diamond-supported Pt particles. In the first route, the nucleation happens in the vicinity of diamond electrode, and the nuclei have a high surface free energy which is given by [20]:
G = 4
2r 3 r − ∗ 3r
2
(2)
where is the interfacial energy of a nucleus with the surroundings, r is the size of the nucleus, and r* is the critical radius of nucleation.
Fig. 3. SEM images (a, b) and histogram (c, d) of Pt particles before (a, c) and after (b, d) rapid thermal annealing process at 700 ◦ C for 5 min under N2 atmosphere.
496
F. Gao et al. / Electrochimica Acta 112 (2013) 493–499
This energy must be lowered by either nuclei growth (increasing r), or by attaching to surfactant molecules, or in our case diamond surface (lowering ). In the second route, the non-uniform polycrystalline diamond surface provides local defects and impurities which lower the nucleation barrier, and then the nucleation starts directly at the substrate. In both cases, the prerequisite for the formation of Pt particles on diamond is that the supersaturation is sufficiently close to the substrate. Otherwise, in the first case, the nuclei will grow big enough to reach a balanced state when diffusing to the surface of diamond; in the second case, the supersaturated solution cannot meet the substrate where the nucleation is supposed to take place. In order to confirm the importance of the distance of the supersaturation with the diamond electrodes two control experiments were conducted. In the first one, we directly dropped firstly NaBH4 and then H2 PtCl6 onto a diamond electrode without removing excessive NaBH4 solution. In the second control experiment, we first mixed NaBH4 solution with H2 PtCl6 to generate nanoparticles and then drop the suspension of Pt particles onto a diamond electrode. Both of the two samples in the control experiment were finally rinsed with water and blow dry with nitrogen. In the first control experiment the supersaturation of Pt happened once Pt precursor met NaBH4 , the nucleation happened tens of microns (the thickness of the droplet) away from the electrodes; in the second control experiment, the nucleation finished in the suspension and there was no further nucleation after the suspension was dropped onto the electrodes. SEM images show that in neither of cases platinum-seeds adsorb to diamond electrodes. These results confirm that the key to a successful seeding is that only surface
adsorbed NaBH4 is involved before the generation of Pt nanoparticles, leading to a very close supersaturation to the surface. Similar supersaturation was also possible to be generated via high overpotential (>1 V) pulse deposition [21]. In this way the nucleation density is actually enhanced on a diamond electrode surface. As proved previously, the wet-chemical seeding process is actually controlled by NaBH4 adsorption [10]. Therefore, this seeding process is expected to be applicable on other metal salts if they can be reduced by NaBH4 to metal. We thus tested wet chemical seeding experiments on diamond surface by replacing 1.0 mM H2 PtCl6 with 1.0 M NiSO4 , 2.4 mM HAuCl4 , or 10 mM CuSO4 . Fig. 2 provides SEM images of Au (a), Cu (b), and Ni (c) nanoparticles seeded diamond surface. For these seeding experiments, only H-terminated surfaces were used. As seen in Fig. 2, homogenously distributed nanoparticles for three metals are detected on the diamond surface. The sizes and the densities of these particles vary with the types of metals, due to different reaction kinetics of adsorbed NaBH4 with metal ions. All these results indicate that wet chemical seeding method proposed here is a universal metal deposition method on diamond surface. 3.2. Enhanced stability of Pt nanoparticles via thermal annealing Pt nanoparticles deposited via electrochemical methods are mechanically unstable. They are easily removed by rigorous water stream or cleaning process in an ultrasound bath [10]. Since carbon dissolves in platinum [22], thermal annealing is helpful to form an epitaxial lattice structure between diamond and Pt beneath the particles. Rapid thermal annealing (RTA) of Pt nanoparticles coated
Fig. 4. (a) AFM images of Pt coated diamond sample before (a, b) and after (c, d) rapid thermal annealing; images (a, b, and c) are recorded with tapping mode, while image (d) is recorded with contact mode; in image (b), contact mode removal of Pt particles was done in the center of the area.
F. Gao et al. / Electrochimica Acta 112 (2013) 493–499
diamond was then conducted at 700 ◦ C under N2 atmosphere for 5 min. Fig. 3(a) shows the SEM image of Pt nanoparticles fabricated by electrochemical overgrowth of Pt seeds shown in Fig. 1(b). Fig. 3(b) shows the SEM image of these Pt nanoparticles after thermal annealing. The density decreases from 1.4 ± 0.3 × 1011 cm−3 before annealing (Fig. 3a) to 0.9 ± 0.3 × 1011 cm−3 after annealing (Fig. 3b). The growth of the nanoparticles was analyzed from the histogram of these particles (Fig. 3c and d). The mean diameter of the particles grows from 16.1 ± 2.8 nm (Fig. 3c) to 22.3 ± 4.1 nm (Fig. 3d), showing the coalesced particles. AFM removal experiments were further applied to test the stability of Pt nanoparticles on diamond before and after thermal annealing. For convenience, polished nanocrystalline diamond films were used. It has a root mean square (RMS) surface roughness of 1.38 nm, measured over an area of 5 × 5 m2 . Fig. 4(a) shows a tapping mode AFM image of Pt particles deposited on diamond before thermal annealing process. Pt particles distribute homogeneously on diamond surface, the same as those seen from SEM images. Removal experiments were then applied in an area of 600 × 600 nm2 with a vertical deflection of 0.25 V, which is equivalent to a loading force of 0.15 N. Even with this small loading force, Pt nanoparticles were completely removed. One typical AFM image of such an experiment is shown in Fig. 4(b) where a clear diamond surface without any particles is seen. Although after thermal annealing Pt particles still distribute well on diamond surface (as seen from one typical AFM image in Fig. 4c), the AFM removal experiments show different results from Fig. 4(b). One typical contact mode AFM image is shown in Fig. 4(d) where a vertical deflection of 1.5 V was applied, which is equivalent to a loading force of 0.88 N. This loading force is 5 times higher than that
Fig. 5. Cyclic voltammograms in 0.1 M H2 SO4 at a scan rate of 100 mV/s on the Pt nanoparticles coated diamond electrodes (a) as-deposited, (b) after rapid thermal annealing, (c) after rapid thermal annealing and electrochemical activation.
497
applied in Fig. 4(b), but few of Pt particles were removed. Therefore thermal annealing process does enhance the adsorption force of Pt nanoparticles on diamond, namely the stability of Pt particles on diamond. According to Winterbottom’s theory on the equilibrium shape of supported small particles [23], in order to obtain stable particles on a substrate, the energy configuration must satisfy: SP < SV + PV
(3)
SP , SV , and PV are the interfacial energies of substrate–particle, substrate–vacuum, and particle–vacuum, respectively. The energy gain for the formation of a Pt (1 1 1)-diamond (1 1 1) interface is given by first-principal calculation as 3.61 J/m2 [24], which means the Eq. (3) is satisfied. Although this energy (∼60 kJ/mol carbon atom) is much smaller than common covalent bond on diamond surface such as C H 410 kJ/mol, C C 335 kJ/mol [25], it still indicates that Pt particles can be energetically stable on diamond surface. Then the experimental observation of an weak adsorption of the electrodeposited particles without annealing [26] is most likely to be a “dirty” interface. It includes surface atoms/terminations, adsorbed water layer, as well as other impurities. After annealing, however, first carbon layer dissolves in Pt and a clean Pt–diamond interface is then formed. Because the first-principal calculation shows no bonding between Pt and C, the dangling bonds at carbon surface probably induce a surface dipole
Fig. 6. (a) SEM image of overgrown Pt nanoparticles after rapid thermal annealing and electrochemical activation; (b) cyclic voltammogram in 0.1 M H2 SO4 at a scan rate of 100 mV/s on overgrown Pt nanoparticles after rapid thermal annealing and electrochemical activation.
498
F. Gao et al. / Electrochimica Acta 112 (2013) 493–499
at the Pt side of the interface. This dipole interaction contributes to the formation energy of the interface. 3.3. Refreshed activity of Pt nanoparticles via electrochemical activation and overgrowth Although the annealing process provides a convenient and effective route in stabilizing Pt nanoparticles on diamond surface, the surface passivation of the metal particles after high temperature treatment seems to be very serious. Therefore, electrochemical polishing was conducted as mentioned in experimental session before performing experiments in Figs. 5 and 6. Fig. 5(a) and (b) provides the CV measurement on Pt nanoparticles coated diamond electrodes in 0.1 M H2 SO4 before and after annealing, respectively. Before annealing, two couples of peaks are clearly seen for hydrogen adsorption/desorption, indicating that Pt nanoparticles are electrochemically active. After annealing, these peaks disappear and only background current similar to bare diamond is recorded, indicating the loss of reactivity of Pt nanoparticles toward hydrogen adsorption/desorption. This is probably due to the formation of a passivation film since carbon dissolves in platinum at high temperature and precipitates at the metal surface during cooling down [22]. In order to obtain active Pt nanoparticles, electrochemical reactivation of such a surface was conducted by cycling in 0.1 M H2 SO4 solution. The idea is to repeatedly oxidize and reduce the surface of Pt nanoparticles and then the surface will be refreshed
[27]. As shown in Fig. 5(c), the hydrogen adsorption/desorption peaks show up again after such a treatment. Comparing with those for the sample before annealing, the current scale however decreases. This might be due to the incomplete removal of the passivation layer, which leads to a smaller electrochemically active Pt area. An electrochemical overgrowth was then performed on the electrochemically activated particles. Fig. 6(a) shows one SEM image of a surface after such a growth. A rough surface is detected with voids and nanostructures. Hydrogen adsorption/desorption was then recorded on such a surface in 0.1 M H2 SO4 solution using cyclic voltammogram. As shown in Fig. 6(b), the responses of hydrogen desorption/adsorption are fully recovered (Fig. 5a). Please note that the current scales in cyclic voltammograms of Figs. 5 and 6 are slightly lower than the zero axes. This is probably due to notoptimized process for electrochemically polishing Pt nanoparticles. To conclude, the process introduced here provides an easy and efficient way to re-generate fresh Pt surface after thermal treatment. The stability was tested as well using ultrasound removal. An ultrasound probe was used to mechanically remove particles from electrode. After each removal, the hydrogen adsorption/desorption peaks were measured and the peak areas were used to calculate the removal rate of the particles. Fig. 7 summarizes normalized peak area as a function of the ultrasound power applied. These results suggest an improved physical adsorption for Pt particles after our annealing-activation-overgrowing process. At a power of 130 W, about 35% Pt particles are removed from the sample without annealing, but less than 1% of the particles are removed from the sample after annealing. AFM removal experiments (not shown) were conducted as well. It is found that Pt particles are hard to be removed even by a high vertical deflection of 1.5 V. As a result, Pt particles deposited with this new technique have higher adsorption stability and are thus more promising for catalytic reactions with a long lift-time. 4. Conclusion In summary, we developed a novel technology to form stable and active Pt nanoparticles on diamond surfaces. This process consists of steps of wet chemical seeding, electrochemical overgrowth, rapid thermal annealing, electrochemical surface activation and further overgrowth. Due to the negative formation energy of a Pt–diamond interface, the adsorption stability of the particle is enhanced, as confirmed by contact mode AFM removal and ultrasound removal tests. Electrochemical activities of these nanoparticles can be restored by electrochemical activation and subsequent overgrowth. Moreover, this technology can be potentially applied for seeding different kinds of metal nanoparticles. Before applying these systems for industrial applications, further experiments are important with respect to the stability of these electrode systems for electrocatalytic reactions such as oxidation of fuel liquids, reduction of carbon dioxide, and catalytic reactions during organic synthesis. Acknowledgements We thank Ms. Georgia Lewes-Malandrakis for the growth and laser cutting of diamond samples, and Dr. René Hoffmann for the fruitful discussions. The financial support by the European Commission under the Seventh Framework Program (MATCON, Grant No. 238201) is gratefully acknowledged.
Fig. 7. Normalized peak area as a function of the power of the ultrasound probe for Pt–diamond electrode after annealing-activation-overgrowth (black squares) and for seeded diamond electrode directly fabricated by seeding-overgrowth (red dots). The peak area is calculated from the hydrogen desorption peak shown in cyclic voltammograms and normalized with respect to the starting peak area before ultrasound treatment (0 W power). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
References [1] X. Sun, R. Li, D. Villers, J.P. Dodelet, S. Desilets, Composite electrodes made of Pt nanoparticles deposited on carbon nanotubes grown on fuel cell backings, Chemical Physics Letters 379 (2003) 99–104.
F. Gao et al. / Electrochimica Acta 112 (2013) 493–499 [2] S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Ordered nanoporous arrays of carbon supporting high dispersions of platinum nanoparticles (vol 412, p 169, 2001), Nature 414 (2001) 470. [3] U.A. Paulus, T.J. Schmidt, H.A. Gasteiger, R.J. Behm, Oxygen reduction on a highsurface area Pt/vulcan carbon catalyst: a thin-film rotating ring-disk electrode study, Journal of Electroanalytical Chemistry 495 (2001) 134–145. [4] S.A. Grigoriev, P. Millet, V.N. Fateev, Evaluation of carbon-supported Pt and Pd nanoparticles for the hydrogen evolution reaction in PEM water electrolysers, Journal of Power Sources 177 (2008) 281–285. [5] Z.L. Liu, X.Y. Ling, X.D. Su, J.Y. Lee, Carbon-supported Pt and PtRu nanoparticles as catalysts for a direct methanol fuel cell, Journal of Physical Chemistry B 108 (2004) 8234–8240. [6] S. Alayoglu, A.U. Nilekar, M. Mavrikakis, B. Eichhorn, Ru–Pt core–shell nanoparticles for preferential oxidation of carbon monoxide in hydrogen, Nature Materials 7 (2008) 333–338. [7] J. Wang, G.M. Swain, Fabrication and evaluation of platinum/diamond composite electrodes for electrocatalysis – preliminary studies of the oxygen-reduction reaction, Journal of the Electrochemical Society 150 (2003) E24–E32. [8] A. Kraft, Doped diamond: a compact review on a new, versatile electrode material, International Journal of Electrochemical Science 2 (2007) 355–385. [9] J. Hu, X. Lu, J.S. Foord, Nanodiamond pretreatment for the modification of diamond electrodes by platinum nanoparticles, Electrochemistry Communications 12 (2010) 676–679. [10] F. Gao, N. Yang, W. Smirnov, H. Obloh, C.E. Nebel, Size-controllable and homogeneous platinum nanoparticles on diamond using wet chemically assisted electrodeposition, Electrochimica Acta 90 (2013) 445–451. [11] R. Hoffmann, H. Obloh, N. Tokuda, N. Yang, C.E. Nebel, Fractional surface termination of diamond by electrochemical oxidation, Langmuir 28 (2012) 47–50. [12] R. Hoffmann, A. Kriele, H. Obloh, J. Hees, M. Wolfer, W. Smirnov, N. Yang, C.E. Nebel, Electrochemical hydrogen termination of boron-doped diamond, Applied Physics Letters 97 (2010), 052103–052103. [13] F. Gao, N. Yang, H. Obloh, C.E. Nebel, Shape-controlled platinum nanocrystals on boron-doped diamond, Electrochemistry Communications 30 (2013) 55–58. [14] D. Briggs, in: C.D. Wanger, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg (Eds.), Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp., Physical Electronics Division, Eden Prairie, MN, USA, 1979, p. 190, $195, Surface and Interface Analysis, 3 (1981) v-v.
499
[15] P. Kim, J.B. Joo, W. Kim, J. Kim, I.K. Song, J. Yi, NaBH4 -assisted ethylene glycol reduction for preparation of carbon-supported Pt catalyst for methanol electrooxidation, Journal of Power Sources 160 (2006) 987–990. [16] L. La-Torre-Riveros, E. Abel-Tatis, A.E. Méndez-Torres, D.A. Tryk, M. Prelas, C.R. Cabrera, Synthesis of platinum and platinum–ruthenium-modified diamond nanoparticles, Journal of Nanoparticle Research 13 (2011) 2997–3009. [17] L. La-Torre-Riveros, R. Guzman-Blas, A.E. Mendez-Torres, M. Prelas, D.A. Tryk, C.R. Cabrera, Diamond nanoparticles as a support for Pt and PtRu catalysts for direct methanol fuel cells, ACS Applied Materials & Interfaces 4 (2012) 1134–1147. [18] V.K. LaMer, R.H. Dinegar, Theory, production and mechanism of formation of monodispersed hydrosols, Journal of the American Chemical Society 72 (1950) 4847–4854. [19] E. Gyenge, Electrooxidation of borohydride on platinum and gold electrodes: implications for direct borohydride fuel cells, Electrochimica Acta 49 (2004) 965–978. [20] V.K.L. Mer, Nucleation in phase transitions, Industrial & Engineering Chemistry 44 (1952) 1270–1277. [21] L. Hutton, M.E. Newton, P.R. Unwin, J.V. Macpherson, Amperometric oxygen sensor based on a platinum nanoparticle-modified polycrystalline boron doped diamond disk electrode, Analytical Chemistry 81 (2009) 1023–1032. [22] J.C. Hamilton, J.M. Blakely, Carbon layer formation on the Pt (1 1 1) surface as a function of temperature, Journal of Vacuum Science and Technology 15 (1978) 559–562. [23] W.L. Winterbottom, Equilibrium shape of a small particle in contact with a foreign substrate, Acta Metallurgica 15 (1967) 303–310. [24] M. Shafiei, A.R. Riahi, F.G. Sen, A.T. Alpas, Improvement of platinum adhesion to carbon surfaces using PVD coatings, Surface and Coatings Technology 205 (2010) 306–311. [25] N. Yang, J. Yu, H. Uetsuka, C.E. Nebel, Characterization of diamond surface terminations using electrochemical grafting with diazonium salts, Electrochemistry Communications 11 (2009) 2237–2240. [26] J.A. Bennett, Y. Show, S. Wang, G.M. Swain, Pulsed galvanostatic deposition of Pt particles on microcrystalline and nanocrystalline diamond thin-film electrodes, Journal of the Electrochemical Society 152 (2005) E184–E192. [27] K. Yamamoto, D.M. Kolb, R. Kotz, G. Lehmpfuhl, Hydrogen adsorption and oxide formation on platinum single-crystal electrodes, Journal of Electroanalytical Chemistry 96 (1979) 233–239.