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Langmuir 2006, 22, 4480-4482
Evidence of a Unique Electron Donor-Acceptor Property for Platinum Nanoparticles as Studied by XPS Limei Qiu,† Fen Liu,*,† Liangzhong Zhao,† Wensheng Yang,‡ and Jiannian Yao*,† Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China, and College of Chemistry, Jilin UniVersity, Changchun 130023, PR China ReceiVed NoVember 14, 2005 In the interaction of PVP with Pt nanoparticles 25 nm in size, charge transfer is from Pt metal to the polymer side chain of PVP. There exists a critical nanoparticle size between 7 and 25 nm that would lead to a switch in the electron donor-acceptor property.
A fundamental question for polymer-stabilized metal nanoparticles is the interaction of the stabilizer with the nanoparticles. Another open question is whether the nanoparticles can possess a distinct property relative to the bulk metal in the interaction. However, to our knowledge, studies on the interaction of the stabilizer with Pt nanoparticles as well as the effect of the interaction on the core-level binding energies (BEs) of the nanoparticles are limited,1,2 although the interaction may affect the properties of the nanoparticles. In the present letter, the chemical interaction of Pt nanoparticles with the stabilizer poly(Nvinyl-2-pyrrolidone) (PVP) in the solid state is examined and compared with that of bulk Pt metal on the basis of core-level XPS measurements. It is found that Pt atoms in nanoparticles between 2 and 7 nm in size act as electron acceptors whereas Pt atoms in bulk Pt as well as in Pt nanoparticles >25 nm in size act as electron donors in interactions with PVP. The method of preparation of PVP-stabilized Pt (hereafter denoted as Pt-PVP) nanoparticles 25 nm in size were obtained from Aldrich Co. (99.9+%). Figure 1 shows TEM images of Pt-PVP nanoparticles. The particles are nearly monodisperse, and their shape is roughly spherical. XPS spectra were acquired using an ESCALab 220i-XL spectrometer (VG Scientific). The C 1s line of alkyl carbon in PVP molecules at 285.0 eV is used as the reference line.4 Figure 2 shows the N 1s XPS spectra as well as the corresponding peak positions of the samples. Besides the peak at 399.9 eV corresponding to that of PVP (see line g), an additional nitrogen signal 2.0 eV lower on the BE side is observed for PVP-filmcovered bulk Pt (see line a). Furthermore, the angular dependence of the N 1s spectra shows that the intensity of the lower BE peak decreases at the smaller information depth, indicating that the lower BE peak is mainly from the Pt-PVP interface. We attribute this chemical shift to the charge transfer from Pt metal to PVP (i.e., there is electron donation to the polymer side chain) in which Pt atoms act as electron donors and PVP molecules act as electron acceptors. In fact, similar N 1s chemical shifts to the * Corresponding authors. E-mail:
[email protected];
[email protected]. † Chinese Academy of Sciences. ‡ Jilin University. (1) Dassenoy, F.; Philippot, K.; Ely, T. O.; Amiens, C.; Lecante, P.; Snoeck, E.; Mosset, A.; Casanove M. J.; Chaudret, B. New J. Chem. 1998, 703-711. (2) Fu, X. Y.; Wang, Y.; Wu, N. Z.; Gui, L. L.; Tang, Y. Q. J. Colloid Interface Sci. 2001, 243, 326-330. (3) (a) Yu, W. Y.; Tu, W. X.; Liu, H. F. Langmuir 1999, 15, 6-9. (b) See Supporting Information for the detailed procedure. (4) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers; John Wiley and Sons Ltd.: Chichester, England, 1992; p 192.
Figure 1. TEM images of Pt nanoparticles. (A) Sample with a particle size of 2 to 3 nm. (B) Sample with a particle size of 6 to 7 nm. (C) Sample with a particle size of ca. 25 nm. These images were taken on a JEOL JEM-2010 electron microscope.
Figure 2. N 1s XPS spectra from the samples. Line a is from a PVP film on bulk Pt; lines b-d are from Pt-PVP nanoparticles with a starting mole ratio of PVP (as a monomeric unit) to Pt of 2:1, line b is from nanoparticles of size 2 to 3 nm on a Si wafer, line c is from nanoparticles of size 6 to 7 nm on a Si wafer, and line d is from a powder of nanoparticles of size 6 to 7 nm. Lines e and f are from Pt-PVP nanoparticles precipitated from Pt nanoparticles immersed in a 0.1% PVP aqueous solution for 48 h (line e is from Pt nanoparticles ca. 25 nm in size, and line f is from Pt nanoparticles ca. 300 nm in size), and line g is from pure PVP powder.
lower BE side have been observed for acetonitrile adsorbed on Pt surfaces5 and for thymine adsorbed on Au surfaces.6 These (5) Sexton, B. A.; Hughes, A. E. Surf. Sci. 1984, 140, 227-248. (6) Petrovykh, D. Y.; Kimura-Suda, H.; Whitman. L. J.; Tarlov, M. J. J. Am. Chem. Soc. 2003, 125, 5219-5226.
10.1021/la053071q CCC: $33.50 © 2006 American Chemical Society Published on Web 04/05/2006
Letters
Figure 3. Pt 4f XPS spectra from the samples. Notations for lines a-f are the same as those in Figure 2.
have been attributed to bond formation between the molecules and metal surfaces. In contrast, the N 1s spectra of Pt-PVP nanoparticles
