XPS Investigation of X-Ray-Induced Reduction of Metal Ions

Abstract

INTRO DUCTIONStructural inform ation derived by using X-rays is ever increasing and moving into all areas of pure and applied sciences, especially after wider utilization of synchrotron radiation facilities enabling easy access to the techniques such as X-ray absorption near-edge spectroscopy (XA-NES), extended X-ray absorption ne structure (EXAFS), and X-ray photoelectron spectroscopy (XPS) together with well-established analytical techniques such as X-ray uorescence (XRF), X-ray diffraction (XR D), photon-induced X-ray emission (PIX E), total re ection X-ray uorescence (TXRF), and synchrotron X-ray uorescence (SXRF). Prolonged exposure of the samples to X-rays is common to all these techniques and is known to cause various kinds of radiation damage.1 A particular kind of damage is the reduction of the m etal ions, which can cause severe interference in cases where determination of the formal oxidation state is important, as is the case in the majority of the applications of XANES, EXAFS, and XPS. 2-10 Electron, ion, and other energetic particle exposure, which is frequently utilized for cleaning and/or depth-pro ling, also causes damage, and similar to Xrays, this approach invariably gives rise to the reduction of the m etal ions to lower oxidation states and/or preferential removal of oxygen. 5,[11][12][13][14][15] The extent of reduction, however, varies drastically from one metal ion to another.In this contribution, we will demonstrate reduction by X-rays of Au, Hg, Bi, V, and W m etal ions during our XPS analysis of various kinds of samples we have carried out over the years. 16,17 We will also discuss the possibility of correlating the ability of the metal ions to undergo Xray-induced reduction with their electrochemical reduction potentials. EXPERIM ENTALAu 31 , Hg 21 , and Bi 31 were deposited from their corresponding aqueous solutions onto gold metal or silicon wafers. An impregnating solution of NH 4 VO 3 to zirconia powders was used for V 51 , which was calcined for 2 h at 723 K. 18 A glass sample coated (via magnetron sputtering) with a thin layer (; 100 nm) of W O 3 was used for W 61 . Similar experiments were also carried out with different salts and/or substrates, resulting in similar observations. A Kratos ES300 electron spectrometer with unmonochromatized M gKa X-rays (1253.6 eV) was used for XPS analysis as well as X-ray exposure in vacuum.Receiv ed 6 May 2000; accepted 28 July 2000.The power of the anode was kept at a minimum level (15 kV, 8 m A) to avoid severe radiation damage. The X-rays were unfocused and consisted mainly of Mg (Ka and b) lines as well as the Bremstrahlung (up to 15 kV and at a m aximum of about 5 kV), and they hit an approximately 1 cm 2 (7 m m 3 15 m m) area of the sample. Hence the power density can be estimated to be about 120 W/ cm 2 . RESULTS AND DISCUSSIO NThe penetration depth of the X-rays (.1000 nm) is much larger than the sampling depth of XPS, which is less than 10 nm for comm on m aterials, 16 so we can safely assume that the X-ray damage is uniform for the ...

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Characterization of gold nanoclusters synthesized on carbon nanotubes film: Evaluation of the size distributions by means of X-ray photoelectron spectroscopy

“…From data obtained from literature the BE of Au III Au 4f 7/2 is expected to be around 87-88 eV [20] (90-91 for the Au 4f 5/2 component). For Au I the Au 4f 7/2 is expected around 86 eV (89.6 for the 4f 5/2 component) 1 .…”

X-ray induced reduction of Au and Pt ions on silicon substrates

“…1, we display a set of spectra, recorded consecutively while the sample is exposed to X-rays. At the beginning the Au4f peak has a complex shape but can be decomposed into 2 different spin-orbit doublets corresponding to 2 different chemical gold species which can be assigned to Au 3+ and Au 0 on the basis of their 4f binding energies [35,36]. As time progresses the intensity of the Au 0 doublet increases at the expense of the other doublet, indicating clearly that reduction by X-rays is taking place.…”

“…The extent of reduction varies drastically from one metal ion to another and extensive efforts had been devoted to correlate this with certain chemical/ physical properties of them [24][25][26][27][28][29][30][31][32][33][34]. In one of our earlier work, we had claimed that the extent of X-ray induced reduction could be related with the electrochemical reduction potential of the metal ion [35,36]. Accordingly, gold, having a very high reduction potential of [ϵ 0 red (Au 3+ ) = 1.5 V] is one of the most amenable.…”