Definition of Chemical State
The term chemical state of any element in the periodic table, involves some description of the particular environment the atom is in, either on its own in elemental form, or in combination with different elements to make free molecules (gases), or in liquid or solid compounds.
There are several ways, or levels, of describing a “chemical state”. So, for instance, the chemical state of oxygen in water, H2O, can be described as that the oxygen atom, O, has a valency of 2, and is bonded to two H atoms, which each have a valency of 1. i.e. it gives you the formula of water. The physical state of water may be gas, liquid, or solid, but the chemical state is the same in each; two H atoms per O atom. H and O atoms only exist with the one valency (2 for O and 1 for H), but many elements can have different valencies in different compounds, eg FeO, where the Fe valency is 2, and Fe2O3, where it is 3.
The term “oxidation state” is often used as an alternative to valency. FeO has Fe in the Fe II oxidation state, whereas Fe2O3 the Fe is in the FeIII oxidation state. Though most commonly used for describing metals that can form positive ions (cations) in solution, the term oxidation state does not imply full ionicity in a compound, ie it does not imply that in solid FeO the Fe is an actual Fe2+ ion (with O as 2-) and in Fe2O3 it is a 3+ ion (with O as 2-). In fact all compounds have an admixture of covalent bonding, meaning the actual charge on the atom is less than indicated by the formal oxidation state. The term is much less used in organic solids, because they tend to be dominantly covalent, but it is still technically correct to describe, for instance, CH4 as having a C atom in the CIV oxidation state, but misleading because, since the electronegativity of C and H are very similar, there is little movement of charge from one atom to the other.
To further understand how a chemical state differs from a simple valency description, compare sodium fluoride (NaF) to polytetrafluoroethylene (PTFE, Teflon). Both contain fluorine, the most electronegative element, but only NaF dissolves in water to form separate ions, Na+ and F–. In PTFE the electronegativity though the fluorine strongly polarizes the electron density that exists between the carbon and the fluorine (the bond), it is not enough to produce ions which would allow it to dissolve in the water. So we describe NaF as being an ionic compound (though again in the solid there is not complete transfer of an electron from neutral Na to F, ie it is not 100% ionic), but PTFE as a covalent compound with a polarization of the bonding electrons towards the F atom.
So in actuality the oxidation state or valency of an atom describes only the formal gross differences of the chemical state of the atom concerned. The more nuanced description requires knowledge of the atoms to which the element concerned is bonded. The chemical state of C in CH4 is not the same as that in CF4, even though the C atom is formally in the C1V oxidation state in both. F is highly electronegative, meaning it likes to attract electrons from any species less electronegative in nature. So the amount of positive charge residing on the C atom in CF4 is far larger than on the C atom in CH4 (but not as much as 4+!). Likewise the actual positive charge on the Fe atom in FeF2 is greater than that on the Fe atom in FeO, which is more than on the Fe atom in FeS, but it is not as much as in an isolated 2+ ion in any of these. So a more complete chemical state description involves stating what the atoms are that are bonded to the atom concerned, or in some cases the group of atoms concerned, such as CO3, a carbonate grouping, or SO4, sulphate. These groups are often referred to as chemical moieties, chemical species, or functional groups. They tend to have well-defined chemical properties, for instance a group electronegativity.
A further useful descriptive for chemical states, is whether single, double, or triple bonds are involved between elements. Most prevalent in carbon chemistry, because C to C bonds can be single, double, or triple, they do occur also in some other situations.
Where do Chemical State Spectra come from? How do we make a chemical state spectrum?
A Chemical State spectrum is produced by focusing on one of the elements that are found in a Survey spectrum of the sample.
In this example, we want to measure the chemical states that exist for the Si (2p) signal. We have used a BE table to identify the various peaks in the Survey spectrum. Please find the peak labelled as Si (2p) printed on the survey spectrum near 100 eV BE. As we look at this signal we can barely see a shoulder at the upper BE region of the Si (2p) peak. From theory we know that Silicon should have a Si (2p3/2) peak and also a Si (2p1/2) peak that are separated by <1 eV. The peak area ratio for those two peaks is 2:1 based on theory. As we look at the survey spectrum, the ratio of the two peaks is not 2:1. Why?
The energy resolution of most survey spectra is low (~2 eV) because we use a large Pass Energy (150-200 eV) to Survey what elements are present or absent within the detection limit of XPS and our measurement settings. Such a poor energy resolution (2 eV) is almost never used when wanting to measure the presence or absence of different chemical states which are usually separated by only 1 eV.
We normally need much higher energy resolution (0.5-1.0 eV) to be able to see different chemical states. We use small Pass Energy values (50-90 eV) to measure normal everyday Chemical State spectra. We can use smaller pass energies (5-20 eV) but the signal is very very weak and the energy resolution is only a little bit better. It takes too much time to use very small pass energies and they are only useful in a few cases when measuring conductive materials, never insulators. It is best to use Pass Energies that produce a FWHM of ~1 eV for freshly, ion etched Ag (3d5) signal.
The image below shows a Survey Spectrum from a native oxide of Silicon (wafer). The survey spectrum is useful to measure elements, not chemical states. A survey spectrum is normally designed to run from -10 to 1,100 eV with a step size of 0.8-1.0 eV/data point (step) and using a large Pass Energy (150-200 eV).
To measure the chemical state spectrum for the Si (2p) signal we normally measure a much smaller spectrum window (range) – usually 10-20 eV wide spectrum. The step size should be 0.08-0.12 eV/data-point (step) and the Pass Energy should be between 50-90 eV for normal chemical state spectra.
The Chemical State Spectrum shown here is a rare special case. The energy separation between the Si (2p3/2) and the Si (2p1/2) peaks is only 0.602 eV. This sort of spectrum is used by instrument makers to show the highest performance of the XPS instrument.
To be able to resolve the two peaks at ~99.5 eV, we need to use a very small pass energy, PE=10-20 eV which is smaller than the everyday PE we discussed above. Special case.
The time needed to measure this ultra high resolution chemical state spectrum can be 20 minutes or 60 minutes. The time depends on the X-ray power being used.
And, then there is a small trick to get such nice data. Do NOT ion etch this native oxide of silicon. If you ion etch, then the surface becomes rough and the roughness causes FWHM to increase by ~2x.
And another trick is to measure the freshly cleave edge of the silicon wafer. Again, do not ion etch. Various single crystals of semiconductors will give nice looking data if your do NOT ion etch. If you ion etch the normal surface, then the peaks will broaden due to the roughness from ion etching.
This plot shows the various chemical states and chemical state labels used for the native oxide of silicon. If you study these sub-oxides, you will learn that the true chemical structure is much more complicated then this simple picture.
This plot shows three chemical states and chemical state labels used to identify the chemical states that exist for the C (1s) in PET (Mylar).
If you study this materials, you will learn that there is one more chemical state that you can peak-fit if you have the time and interest. That extra peak is part of the benzene ring and have a BE ~285.5 eV.
There is another peak at 292 eV which is due to a Shake-up of a pi electron in the benzene ring. To properly quantify PET you need to include the pi-pi* peak for the C (1s) peak area. The same is true for the O (1s) spectrum of PET.
Chemical States usually have a 1 eV difference between the different states in many, but not all, of the Chemical State spectra.
Note: A chemical shift is always made with reference to the pure element that is not ionized.