Elsevier

Applied Surface Science

Volume 252, Issue 12, 15 April 2006, Pages 4335-4339
Applied Surface Science

Experimental and theoretical studies on X-ray induced secondary electron yields in Ti and TiO2

https://doi.org/10.1016/j.apsusc.2005.09.045Get rights and content

Abstract

Generation of X-ray induced secondary electrons in Ti and TiO2 was studied from both experimental and theoretical approaches, using X-ray photoelectron spectroscopy (XPS) attached to a synchrotron radiation facility and Monte Carlo simulation, respectively.

The experiment revealed that the yields of secondary electrons induced by X-rays (electrons/photon) at photon energies to 4950 and 5000 eV for Ti and TiO2 are δTi(4950 eV) = 0.002 and δTi(5000 eV) = 0.014 while those for TiO2 are δTiO2(4950eV)=0.003 and δTiO2(5000eV)=0.018.

A novel approach to obtain the escape depth of secondary electrons has been proposed and applied to Ti and TiO2. The approach agreed very well with the experimental data reported so far. The Monte Carlo simulation predicted; δTi*(4950eV)=0.002 and δTi*(5000eV)=0.011 while δTiO2*(4950eV)=0.003 and δTiO2*(5000eV)=0.015.

An experimental examination on the contribution of X-ray induced secondary electrons to photocatalysis in TiO2 has also been proposed.

Introduction

Recently Okada et al. [1] reported that X-ray irradiation on a TiO2-coated surface induces surface excitation, leading to a photocatalysis reaction on the TiO2 as does ultraviolet light at energies over 3.2 eV. Since this result suggests the possibility that X-ray induced secondary electrons (XSE) are closely related to the photocatalysis reaction in TiO2, their paper has attracted renewed attention to XSE. However, to our knowledge, there has been little basic study on XSE, although XSE has been used as a powerful signal in X-ray photoemission microscopy (XPEEM) [2].

As far as secondary electrons (SE), which are generated by a primary electron beam, are concerned, systematic studies have been performed extensively, shedding intimate insight into the mechanism of SE generation. In the generation of XSE, the photoelectrons and Auger electrons associated with photoionization are considered primary projectiles that generate XSE just as primary electrons generate SE. Since the generation of photoelectrons induced by X-ray irradiation is well understood [3], XSE generation can be described theoretically by calculating the energy loss processes of the primary projectiles (X-ray induced photoelectrons and Auger electrons) as with the conventional theory of SE generation [4], [5], [6].

In view of the basic investigation of SE and XSE generations in metal oxide, TiO2 has long been regarded as one of the most appropriate materials because it is not insulative and free from charging from irradiation of primary electrons and/or X-rays. Furthermore, according to Okada et al., it is quite easy to get a clean TiO2 surface by simply irradiating the surface with X-rays.

Therefore, the primary purpose of the present work is to examine whether XSE is closely correlated to the photocatalysis reaction in TiO2 under X-ray irradiation. For this, we first have been involved in both experimental and theoretical studies of the generation of XSE at photon energies of 4950 and 5000 eV, just below and above the K-absorption edge of Ti, 4965 eV. Concerning the XSE yields for X-rays at photon energies above and below X-ray absorption edge, we have already reported [7] in a previous paper that the yield of XSE beyond the Ag L-absorption edge is nearly five times greater than that below the L-absorption edge. Hence, if XSE is closely correlated to the photocatalysis reaction in TiO2, the reaction should be different for X-rays at energies on both sides of the Ti–K absorption edge.

This paper reports both the experimental and theoretical studies on XSE generated in Ti and TiO2 to elucidate the mechanism of the XSE generation. The experiment was performed by X-ray photoelectron spectroscopy (XPS) attached to the synchrotron radiation (SR) facility (SR-XPS) [8] in the SPring-8. To describe the experiment we also performed Monte Carlo (MC) simulation based on the uses of the screened Rutherford scattering formula and Bethe's stopping power equation, combined with a photoelectron generation model [3]. Both the experimental and theoretical results have agreed very well with each other, demonstrating the significant contribution of photoionization in the K-shell to the generation of XSE.

On the possibility of the contribution of XSE to the photocatalysis on TiO2, we have proposed quantitative measurements of contaminant growths on a TiO2 surface under X-ray irradiation at energies of 4950 and 5000 eV by SR-XPS, shedding an intimate insight into the photocatalysis reaction in TiO2.

Section snippets

Model

Fig. 1 shows the schematic diagram of the simulation model. Photons of specific energy irradiate a specimen surface, penetrating into the specimen by causing photoionization along a straight path. This photoionization generates not only photoelectrons but also Auger electrons through the relaxation process of photoionization. The probability of the generation of Auger electrons in layer Δz at depth zi is given byp(zi)=(1ω)r1rμΔzcosθe(μz/cosθ)where ω, r, and μ are fluorescent yield,

Experiment

X-ray induced secondary electrons were measured by SR-XPS, the XPS system of which is based on the ULVAC-PHI model 10–360 Energy Analyzer. This SR-XPS enables us, in principle, not only to measure the X-ray induced secondary electrons by directly detecting specimen current but also to monitor contaminant growth by detecting X-ray photoelectron spectroscopy (XPS) signals of the contaminants during X-ray irradiation as was done by Okada et al. [1] by using a microscopic FT-IR.

The experimental

Results and discussion

Before applying the proposed approach, following the procedure described in Section 2, we verified the approach by applying it to pairs of metals and their oxides, i.e., Al and Al2O3 and Si and SiO2, for which experimental values of δm and Em have been published [12]. The results were γSi  1.0 nm and γSiO23.5nm and γAl  1.2 nm and γAl2O36.0nm, agreeing well with the values that have been estimated empirically [7]. The escape depth of Ti was then estimated by the proposed approach. First, once

Summaries

Experimental and theoretical studies on X-ray induced secondary electron yields for Ti and TiO2 were reported. A summary of the results is given in the following paragraphs:

  • (1)

    A practical approach to obtain the escape depth from the δ(Ep) curve has been proposed and applied to pairs of metals and their oxides, i.e., Al/Al2O3 and Si/SiO2. The results were γAl  1.2 nm and γAl2O36.0nm and γSi  1.0 nm and γAl2O33.5nm, revealing that this approach provides the escape depths by agreeing very well with

Acknowledgments

The authors are very grateful to Professor T. Koshikawa, Osaka Electra-Communication University, for stimulating discussion, particularly on generation of X-ray induced secondary electrons. Appreciation is also due to Professor H. Seiler, Tuebingen University, for drawing the authors attention to early experimental works on escape depths for metal oxides [12].

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