Bergaya F. Handbook of Clay Science

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References 787

Handbook of Clay Science

Edited by F. Bergaya, B.K.G. Theng and G. Lagaly

Developments in Clay Science, Vol. 1

r 2006 Published by Elsevier Ltd.

789

Chapter 12.3

X-RAY ABSORPTION SPECTROSCOPY

W.P. GATES

Centre for Green Chemistry, School of Chem istry, Monash University, Clayton, VIC

6800, Austrailia

X-ray absorption spectroscopy (XAS) is used here as a general term denoting a

range of X-ray spectroscopic techniques based on scattering, absorption and emis-

sion (ﬂuorescence, auger) of X-rays by matter. Since many of these techniques are

synchrotron-based, it is only in the past 20 years or so that they become generally

available for investigating the crystal structures and surface chemistry of clays and

clay minerals. Detailed information about synchrotrons and synchrotron radiation

together with their applications in clay mineralogy, geochemistry and environ-

mental science can be found in the books and reviews by Hodgson et al. (1984),

Koningsberger and Prins (1988), Hawthorne (1988), Decarreau (1990), Mottana and

Burragato (1990), Amonette and Zelazny (1994), Schulze et al. (1999) and Fenter

et al. (2002). The application of other X-ray techniques to mineral surface chemistry

was described by Vaughan and Pattrick (1995), Bertsch and Hunter (2001), de Groot

(2001) and Fenter et al. (2002) .

Synchrotron-based X-ray sources produce an intense X-r ay emission with total

ﬂuxes and ﬂux densities several orders of magnitude greater than conventional X-ray

tubes. As a result, a nearly ﬂat spectral distribution is produced over a large range of

mineralogically impor tant elements. This and recent advances in X-ray optics, de-

tectors, insertion and focusing devices (Fenter et al., 2002) as well as modelling of

scattering processes (e.g., Rehr et al., 1991, 1992; Rehr, 2003) made XAS a valuable

tool for obtainin g atomic-level information not previously attainable by conven-

tional X-ray sources. In other words, synchrotron-based X-ray techniques extend

diffraction methods beyond simply determining static atomic positions in minerals.

Besides allowing de tailed local structural information to be obtained, XAS is also

more versatile than other bulk and surface analytical techniques, such as Mo

ssbauer

and NMR spectroscopies.

Here we review synchrot ron-based studies of clays and clay minerals, focusing on

the application of XAS to elucidating the structure and reactivity of 2:1 phyllosil-

icates, with particular emphasis on smectites. Reference to other clay minerals and to

short-range order soil minerals will be made where appropriate.

DOI: 10.1016/S1572-4352(05)01029-9

12.3.1. SYNCHROTRON-BASED TECHNIQUES

Various synchrotron-based techniques are at the disposal of the modern clay min-

eralogists, including X-ray absorption/emission, X-ray scattering and X-ray micro-

probe techniques. The most important of these are absorption/emission methods,

notably extended X-ray absorption ﬁne structure (EXAFS), X-ray absorption near

edge structure (XANES) or near edge absorption ﬁne structure (NEXAFS) methods,

X-ray ﬂuorescence (XRF), X-ray photoelectron spectroscopy (XPS; see Chapter

12.4), photoelectron diffraction and the Auger effect. Scattering methods encompass

X-ray diffraction (XRD), surface sensitive X-ray reﬂectivity and X-ray standing wave

(XSW) techniques. Microprobe techniques enable X-ray absorption spectroscopic

and diffraction techniques to be used for chemical mapping and imaging.

Synchrotron-based X-ray studies were largely concerned with determ ining the

reactive sites on miner al surfaces that are involved in the sorption, dissolution and

co-precipitation of metal hydroxides and the epitaxial growth of new phases. These

studies provide new insights into the limits of low-temperature solid–solution and

solid–solid transformations. Other signiﬁcant areas of research include the interac-

tions between metals, minerals and organisms. Likewise, advances in the develop-

ment of specialised techni ques (e.g., polarised EXAFS) permitted detailed struc tural

studies of short-range order minerals. The capability of synchrotrons to deliver

picosecond (ps) pulses of X-rays of nanosecond (ns) duration made possible kinetic

methodologies which already contributed signiﬁcantly to molecular-level under-

standing of reactions of clays and clay minerals.

White light-based techniques, including XRF elemental mapping using micro-

probes (Sutton and Rivers, 1999 ; Bertsch an d Hunter, 2001), were prominent in soils

and geochemical studies. However, they are generally restricted to high-e nergy syn-

chrotron beamlines (hard X-rays, >4–4.5 keV, atomic number ðZÞ420). Photo-

electron microprobe or electron microscope-based techniques are often more suitable

for analysing elements of low Z (lighter than Ca) (Hochella, 1988), while X-ray

absorption ﬁne structure (XAFS) generally has greater sensitivity for elements of

Z420. Hard X-rays penetrate deeper into samples than soft X-rays, but are also

attenuated less by air in the X-ray path. Recent advances in the development of

‘environmental cells’ open the possibility of analysing samples (Z o20) under in situ

conditions using soft X-rays ( Meyer-Ilse and Attwood, 2000). Also, improvement s in

X-ray optics design and engineering of 3rd generation synchrotrons overcome many

limitations for low Z elements (Brown and Sturchio, 2002).

Since the X-ray source can be ‘‘tuned’’ to energies speciﬁc to the absorption

energies of many elements of interest, monochromatised X-rays are commonly ap-

plied to obtain information that is speciﬁc to the local environment of the targeted

element. Element-speciﬁc analys es were often used in conjunction with microprobe

techniques to study the relationships between minerals in aggregates, metals and

mineral surfaces, soil contaminants and the potential efﬁcacy of rehabilitation/

remediation methods. Crystallographic studies employing XAFS concentrated on

Chapter 12.3: X-ray Absorption Spectroscopy790

local structural reﬁnement of poorly crystalline nontronites, micas, chlorites, kaolins,

phyllomanganates as well as (hydr)oxide and double hydroxide structures. The high

intensity of monochromatic synchrotron light allows for in situ and element-speciﬁc

analysis of samples for most elements heavier than carbon (Z ¼ 6). XAFS can be

near-surface sensitive (to a depth of about 10 nm, depending on energy) if grazing

incidence techniques are employed (Fenter, 2002; Waychunas, 2002). However, when

transmission or ﬂuorescence methods are used XAFS is a bulk technique, providing

averaged local structural information around the target atom throughout the

solid sample. Techniques that speciﬁcally probe surfaces independently of the bulk

structure include standing wave (Bedzyk and Cheng, 2002) and grazing incidence

(Waychunas, 2002)XAS.

A. X-ray Absorption and Scattering Processes

The energies of most X-rays are similar to core-electron-binding energies of atoms,

which, in turn, are strongly dependent on atomic number (Z) and less strongly on

oxidation state and coordination environment. Electrons at core levels (e.g., 1 s or 2p

energy states) within atoms absorb energy from X-rays and are shifted to excited

energy states. If the X-ray energy exceeds these electronic transition energies, the

photoelectron is expelled from the atom into the surrounding atomic environment.

Thus, X-ray absorption (or emission) by matter can be used to probe core-electron-

binding energies of speciﬁc elements in substances with distinct chemistries and

structure. Since X-ray wavelengths of about 10 pm (0.1 A

) are comparable to inter-

atomic distances, X-ray scattering yields information about the surrounding local

environment in short-range order materials. Of course, if long-range periodic order

exists in a material, X-ray diffraction provides essential information on unit cell

parameters (see Chapter 2).

X-ray spectroscopy is characterised by features corresponding to the absorption

of an X-ray by an absorbing atom. The intensity, I, following an X-ray absorption

event is given by

I ¼ I

mt

ð1Þ

where I

is the incident X-ray intensity, t the sample thickness and m the absorption

coefﬁcient. Eq. (1) indicates that the probability of X-rays being absorbed by matter

is given by m. The value of m is averaged for all absorbing atoms, and is related to the

properties of the sample by

m ﬃ

ð2Þ

where r is the sample density, Z the atomic number, A the atomic mass and E the

X-ray energy. Eq. (2) shows that X-ray absorption depends on the atomic make-up

of matter (i.e., the type and number of atoms present) and the energy of the X-rays.

12.3.1. Synchrotron-Based Techniques 791

Generally, the magnitude of E is tuned such that a particular element is dominan-

tly involved in absorption, while the surrounding elements in physical space

are involved in scattering. When absorption processes are measured, m varies

smoothly with the X-ray energy, m(E), as the log of the ratio of measured to incident

intensities

mðEÞ¼log



ð3Þ

but in ﬂuorescence, m(E) becomes proportional to the ratio of the monitored inten-

sity of the ﬂuorescence line, (I

), to the incident intensity

mðEÞ/



ð4Þ

X-ray absorpt ion processes are the result of electronic transitions from core states

to unoccupied electronic states. Since these transitions occur at discrete energies

(quanta) unique to a particular element and its local environment, X-ray spectro-

scopy, especially pre-edg e and XANES, can provide information about the elec-

tronic conﬁguration of the absorbing atom (Bianconi et al., 1985; McKeown et al.,

1985) in its local chemical and bonding environments.

The excitation of a core state electron having orbital angular momentum l enables

one to probe the unﬁlled (l þ 1andl21) electronic states. The dipole selection rules

state that s-p and p-d transitions are dipole-allowed, but s-d transitions are

dipole-forbidden. How ever, symmetry-enhanced orbital mixing can occur in crystal

structures, and dipole-forbidden transitions are often observed (e.g., de Groot,

2001). The main absorption discontinuity of a 1st row transition metal K edge

absorption spectrum (Fig. 12.3.1A) is due to 1s-4p transitions and the edge features

are related to unoccupied states having p-like character. The weak pre-edge feature

of about 15 eV be low the absorption jump arises from dipole-forbidden 1s-3d

Fig. 12.3.1. (A) Example of Ni K EXAFS spectra of Ni-kerolite (after Manceau and Calas,

1986) showing traditional divisions into pre-edge, XANES and EXAFS. Also depicted is an

example of spline function used to background correct and normalise the XAFS spectrum so

that direct comparisons with other XAFS spectra can be made. Dm

(E) is deﬁned as the

difference between a point on the absorption edge extrapolated back from the EXAFS portion

of the spectrum and the pre-edge background. (B) Example of Fe K pre-edge spectra showing

1s-3d t

and e

transitions (after Manceau and Gates, 1997). (C) Typical Fe K XANES

spectrum of montmorillonite and possible decomposition of 1s - 3d, 1s-4p transitions and

multiple-scattering (m.s.) events. (D) Example of EXAFS spectra (Fe K of Garﬁeld nontronite

after Manceau et al. (1998)) showing the effects of k

o ¼ 1

and k

o ¼ 3

weighted Kaiser–Bessel

functions on the EXAFS amplitude (w(k)) as a function of reciprocal k space. (E) The resulting

Fourier transforms of the k

o ¼ 1

and k

o ¼ 3

weighted EXAFS spectra displayed in (D), un-

corrected for phase shift, showing the FT amplitude (w) variation (position of atomic shells) as

a function of distance from the target absorber.

Chapter 12.3: X-ray Absorption Spectroscopy792

Energy (eV)

XANES

7112 71307094 7148 7166

1s→3d

1s→4p

m.s.

Fourier

transform

R* (Å)

FT (k

χ) (normalised)

02 56

w = 1

w = 3

EXAFS

k (Å

-1

)

χ (

k) (normalised)

2468101214

w = 1

w = 3

7090 7094 7098

Energy (eV)

Absorbance (Normalized)

Pre-edge

XANES

EXAFS

µ(E)

Spline function

Energy (keV)

8.3 8.6 8.9 9.2

134

∆



(E)

12.3.1. Synchrotron-Based Techniques 793

transitions that are allowed due to p- and d-state mixing caused by distortion

(ﬂattening) of the octahedra (Calas and Petiau, 1983 ; Waychunas et al., 1983;

Waychunas, 1987; Manceau and Gates, 1997). Thus, the excitation energy required

for electronic transitions from core to excited states depends not only on the number

of unoccupied states availab le, but also on the amount of symmetry mixing between

the core and excited states.

The element-speciﬁc information regarding bound electronic states obtained from

pre-edge and XANES spectra can be calculated using molecular-orbital and crystal-

ﬁeld stabi lisation theories. For example, ‘density of states’ calculations of electrons

in the metal–oxygen bonds of 1st row transition metal oxides were found to be in

excellent agreement with the observed ligand (O) K XANES spectra (de Groot,

2001). For 2p elements (e.g., Al) the K-edge structure (amplitude and position) was

shown to depend on site symmetry, bond angles, interatomic distances and the

identity of nearest neighbours (Bianco ni et al., 1985; McKeown et al., 1985;

McKeown, 1989; Wu et al., 1996). All of these factors affect the crystal ﬁeld strength

and symmetry mixing within the bonding environment about the target atom, and

thus the XAS spectra.

X-ray absorption spectroscopies (NEXAFS, EXAFS) also involve the release of

photoelectrons into the ‘continuum’ (the surrounding atomic environment)

and subsequent scattering of the photoelectron. The EXAFS process can be thought

of as an in situ electron diffraction, in which the X-ray absorbing atom is the pho-

toelectron source. When the kinetic energy of the ejected photoelectron is great enough

to enable it to escape the bound state, it interacts with electrons in the bound states of

other atoms within (about 600 pm) the local chemical environment surrounding the

absorber. Energetically, the ‘continuum’ is up to several hundred electron volts above

the absorption edge. Interactions between the ejected photoelectron and other elec-

trons produce secondary sources of scattering and interference on return of the back-

scattering waves to the absorber (Fig. 12.3.2). Interferences between outgoing

scattering and incoming backscattering waves result in low frequency oscillations be-

tween 50 and 1000 eV above the absorption edge.

These EXAFS oscillations are of interest as they contain structural and chemical

information speciﬁc to the scattering atomic shells, including the number of co-

ordinating atoms (N), their distance from the absorber (R*), their identity (Z) and

the degree of disorder in the surrounding environment (s). An atomic shell is a group

of atoms of the same species at the same distance (7 a few picometer) from the

absorbing atom. Qualitatively, the EXAFS oscillations are of higher frequency for

long interatomic distances between the backscattering elements and the absorber,

and of lower frequency for shorter interatomic distances (Brown et al., 1988).

Quantitatively, the amplitudes of EXAFS oscillations can identify the type and

number of backscattering atoms as well as the distribution of these atoms about a

mean distance from the absorbing atom.

Single-scattering even ts generally probe shorter interatomic distances in cry-

stals than do multiple-scattering events. Single-scattering events (Fig. 12.3.2a)

Chapter 12.3: X-ray Absorption Spectroscopy794

predominate when the photoelectron wavelength is smaller than the interatomic

distance between back-scatterer and absorber, and probe interatomic distances to

about 500 pm from the absorber. When the photoelectron wavelength is greater

than the interatomic distance, multiple-scattering events (Fig. 12.3.2b ) predominate

X-Ray

~30 pm

Fig. 12.3.2. Schematic of X-ray absorption processes in crystals. (a) Outgoing photoelectron

wave (solid circles) from the absorbing (target) atom interferes with the photoelectron waves

backscattered (dashed circles) from electron clouds of neighbouring atoms. Constructive

(long-dashed circles) and destructive (short-dashed circles) interference results in oscillations

in the absorption at energies above the absorption edge. (b) Single (1) and collinear (2, 3)

multiple-scattering events are generally sufﬁcient to depict EXAFS and XANES spectra in

most minerals of interest to clay scientists.

12.3.1. Synchrotron-Based Techniques 795