Post on 12-Jun-2020
Métodos Físicos em Química Inorgânica
(119.229 e 314.889)Prof. José Alves Dias
Espectroscopia de fotoelétrons (XPS)
• O XPS está entre as técnicas maisfrequentemente usadas paracaracterizar a superfície demateriais (e.g., catalisadores).
Fornece informações sobre acomposição elementar, o estado deoxidação dos elementos e, em casosfavoráveis, sobre a dispersão de umafase sobre a outra
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• A fonte de fótons é monocromática e, dependendo
se uma radiação proveniente de raios X ou
ultravioleta é empregada, dois experimentos
resultam:
• XPS = ESCA (espectroscopia de fotoelétrons
excitados por raios X ou espectroscopia de elétrons
para análise química)
• UPS = PES (espectroscopia de fotoelétrons
excitados por ultravioleta ou espectroscopia de
fotoelétrons)
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• XPS é associado com elétrons dacamada de valência e do interior(core) do material.
• UPS é associado apenas com elétronsda camada de valência do material.
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• XPS é baseada no efeito fotoelétrico:
um átomo absorve um fóton de energia
(h), de forma que um elétron do
interior (core) ou de valência com uma
dada energia de ligação (Eb, biding
energy) é ejetado com com uma
energia cinética (Ek):
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• Ek = h - Eb - (Eq. 1)
onde:
Ek → energia cinética do fotoeletron
h → constante de Planck
→ frequência da radiação incidente
Eb → energia de ligação (binding energy)
do fotoeletron com respeito ao nível de
Fermi (EF) da amostra
→ função trabalho do espetrômetro
EF → energia do nível mais alto ocupado
por um sistema quântico a temperatura
de zero Kelvin. 6
• A função trabalho () é definida como
a diferença entre as energias de
vácuo EV e de Fermi EF para um
sistema.
= EV − EF
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Energias em estrutura de bandas para sólidos cristalinos
• Os conceitos de Função Trabalho,Energia de Vácuo, Energia de Fermi eLimiar de Fotoemissão sãoimportantes para a completa,correta e exata compreensão dasinformações obtidas em espectrosde XPS.
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I. Chorkendorff, J.W. Niemantsverdriet; Concepts of Modern Catalysisand Kinetics, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2003.
• Sólidos (incluindo soluções congeladas),
gases e líquidos tem sido estudados por
XPS e UPS.
• No experimento de XPS, as energias de
ligação são expressas relativamente a um
nível de referência.
• Como a amostra está em contato elétrico
com o espectrômetro, o nível de Fermi da
amostra e do espectrômetro é o mesmo
(função trabalho).
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• Fontes de raios X aplicadas rotineiramente
usam radiação Mg K (h = 1253,6 eV) e Al
K (h = 1486,3 eV), implicando que as
energias cinéticas dos fotoelétrons caem
aproximadamente na faixa de 0 a 1000 eV.
Nessas energias, os elétrons viajam não
mais do que algumas distâncias atômicas
através do sólido.
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A Figura 4.7 mostra que o caminho livre médio dos elétrons, , é
limitado a menos de 2 nm para essas energias. A sensibilidade
superficial ideal ( ~ 0,5 nm) é alcançada com elétrons com
energias cinéticas na faixa de 25 – 200 eV, onde quase metade
dos fotoelétrons vem da camada mais externa
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• Em XPS, mede-se a intensidade dos
fotoelétrons N(E) em função de sua energia
cinética. Usando a Eq. (1) converte-se a
energia cinética em energia de ligação, que
geralmente é a propriedade a ser obtida e
atribuída ao eixo X de um espectro.
As energias de ligação dos elétrons são
totalmente características do elemento do
qual o fotoelétron se origina.
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• Example: Figure 4.8 shows the XPS
spectrum of an alumina-supported rhodium
catalyst, prepared by impregnating the
support with RhCl3 in water.
• Picos devido ao Rh, Cl, Al, O e C (sempre
presente como contaminação de
hidrocarbonetos) são prontamente
atribuídos a partir de Tabelas de Energia de
Ligação.
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• In addition to the expected photoelectron peaks,
the spectrum in Fig. 4.8 also exhibits peaks due to
Auger electrons. These originate because the atom
from which the photoelectron has left is a highly
excited ion, with a hole in one of its inner shells.
This ion relaxes according to the scheme given on
the right of Fig. 4.6.
• One readily sees that Auger electrons have fixed
kinetic energies that are independent of the energy
that created the initial core hole. Nevertheless,
Auger peaks are plotted on the binding energy
scale, which has of course no physical significance.
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• The main peak of the O KVV Auger signal in Fig.
4.8 has a kinetic energy of about 500 eV, but
appears at a binding energy of about 986 eV,
because the spectrum was taken with Al K X-rays
of 1486 eV.
• Auger peaks can be recognized by recording the
spectrum at two different X-ray energies: XPS
peaks appear at the same binding energies, while
Auger peaks will shift on the binding energy scale.
This is the main reason why X-ray sources often
contain a dual anode of Mg and Al.
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• Photoelectron peaks are labeled according
to the quantum numbers of the level from
which the electron originates. An electron
with orbital momentum l (0, 1, 2, 3, ...
denoted s, p, d, f, ..) and spin momentum s
has a total momentum j = l + s. As the spin
may be up (s = +1/2) or down (s = –1/2),
each level with l 1 has two sublevels, with
an energy difference called the spin–orbit
splitting. Thus, the Rh 3d level gives two
photoemission peaks, 3d5/2 (with l = 2 and j
= 2 + 1/2) and 3d3/2 (l = 2 and j = 2 –1/2).
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• Auger electrons, however, are labeled
according to the terminology commonly
used in X-ray spectroscopy.
• An Auger electron labeled KLM
originates from a transition with the
initial core hole in the K shell, which is
filled by an electron from the L shell,
whereas the Auger electron is emitted
from the M shell.
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Table 4.1 summarizes the spectroscopic nomenclature.
• Spin–orbit splittings as well as binding energies of a
particular electron level increase with increasing
atomic number.
• The intensity ratio of the peaks from a spin–orbit
doublet is determined by the multiplicity of the
corresponding levels, equal to 2j + 1. Hence, the
intensity ratio of the j = 5/2 and j = 3/2 components
of the Rh 3d doublet is 6:4 or 3:2.
• Thus, photoelectron peaks from core levels come in
pairs–doublets – except for s levels, which normally
give a single peak.
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• Binding energies are not only element
specific but contain chemical information as
well, because the energy levels of core
electrons depend slightly on the chemical
state of the atom.
• Chemical shifts are typically in the range of
0–3 eV. In general, the binding energy
increases with increasing oxidation state,
and for a fixed oxidation state with the
electronegativity of the ligands.
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• Para apreciar o significado da energia de ligação, é
necessário considerar os efeitos finais do estado.
Na prática, usamos dados de XPS como se fossem
característicos para os átomos antes do evento de
fotoemissão.
• No entanto, isso não está correto pois os dados de
fotoemissão representam um estado do qual um
elétron acabou de sair. Consequentemente, a
energia de ligação de um fotoelétron contém
informações sobre o estado do átomo antes da
fotoionização (o estado inicial) e sobre o átomo
ionizado por núcleo deixado para trás após a
emissão de um elétron (o estado final).
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• Fortunately, it is often qualitatively
correct to interpret binding energy
shifts such as those in Fig. 4.8 in
terms of initial state effects.
• The charge potential modelelegantly explains the physicsbehind such binding energy shifts,by means of the formula:
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• in which Ebi is the binding energy of an
electron from an atom i, qi is thecharge on the atom, k is a constant, qj
the charge on a neighboring atom j, rij
is the distance between atoms i and j,and Eb
ref is a suitable energyreference.
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• The first term in Eq. (4) indicates that the
binding energy increases with increasing
positive charge on the atom from which the
photoelectron originates.
• In ionic solids, the second term counteracts
the first, because the charge on a
neighboring atom will have the opposite
sign.
• Because of its similarity to the lattice
potential in ionic solids, the second term is
often referred to as the Madelung sum.
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• The right-hand part of Fig. 4.8 illustrates
how the binding energy is sensitive to the
oxidation state of rhodium in Rh/Al2O3
catalysts.
• The Rh 3d XPS spectrum of the freshly
impregnated catalyst reveals that the Rh
3d5/2 binding energy is 310 eV, a value
characteristic for trivalent rhodium (as in
RhCl3).
• After reduction in hydrogen, the binding
energy of the rhodium has decreased to
307.4 eV, indicative of metallic rhodium.
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• Hence, XPS reveals both that rhodium
is present in the catalyst and the
oxidation state in which it occurs.
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• An unavoidable experimental problem with
techniques based on the detection of
charged particles such as electrons and ions
is that electrically insulating samples may
charge up during measurement.
• The potential the sample acquires is
determined by the balance of photoelectrons
leaving the sample, the current through the
sample holder towards the sample, and the
flow of Auger and secondary electrons from
the source window onto the sample.
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• Due to the positive charge on the sample, all
XPS peaks in the spectrum shift, by the
same amount, to higher binding energies.
• This is easily corrected if the sample
contains an element in a state for which the
binding energy is known. In SiO2-supported
catalysts, for example, one uses the binding
energy of the Si 2p electrons, which should
be 103.4 eV. If nothing else is available, one
can use the C 1s binding energy (284.6 eV)
of ever-present carbon contamination.
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• Whereas shifts due to homogenous
charging are usually readily corrected, the
broadening of peaks due to inhomogeneous
charging of samples is more problematic,
as it results in decreased resolution and a
lower signal-to-noise ratio.
• The use of a flood gun, which sprays the
sample with low energy electrons, and
sample mounting techniques in which
powders are pressed in indium foil may
alleviate the charging problems.
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• Because XPS is a surface sensitive technique, it
recognizes how well particles are dispersed over a
support. Figure 4.9 schematically shows two
catalysts with the same quantity of supported
particles but with different dispersions.
• When the particles are small, almost all atoms are
at the surface, and the support is largely covered. In
this case, XPS measures a high intensity IP from
the particles, but a relatively low intensity IS for the
support. Consequently, the ratio IP/IS is high. For
poorly dispersed particles, IP/IS is low. Thus, the
XPS intensity ratio IP/IS reflects the dispersion of a
catalyst on the support.
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• Several models have been reported that
derive particle dispersions from XPS
intensity ratios, frequently with success.
• Hence, XPS offers an alternative
determination of dispersion for catalysts
that are not accessible to investigate by the
usual techniques used for particle size
determination, such as electron microscopy
and hydrogen chemisorption.
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Alguns aspectos sobre instrumentação e análise de
espectros de XPS
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Diagrama com os componentes básicos de um XPS monocromático
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Equipamento de XPS
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um analisador de elétrons e uma fonte de raios X montados em geometria adequada no topo de uma câmara de vácuo. Ao fundo em azul, a unidade de controle
XPS - K-Alpha X-Ray Photoelectron Spectrometer (Thermo Scientific)
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• https://www.thermofisher.com/order/catalog/product/IQLAADGAAFFACVMAHV?SID=srch-srp-IQLAADGAAFFACVMAHV#/IQLAADGAAFFACVMAHV?SID=srch-srp-IQLAADGAAFFACVMAHV
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Fatores que determinam a intensidade (I) de um espectro de XPS