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