[MUSIC] Welcome, my name is Carrie Donley and I'm the director of the Chapel Hill Analytical and Nanofabrication Lab, or CHANL, at UNC. Today, we will be talking about X-ray photoelectron spectroscopy, or XPS, which can tell us about the surfaces of samples. If you've ever seen rust, you've seen evidence of surface chemistry. When iron is exposed to the atmosphere, the surface iron atoms will react with water and oxygen to form iron oxide or rust. Often, the surface of a material is very different from the bulk. Steel is a very strong, durable material but the rust that forms on its surface breaks very easily. Therefore, understanding surface chemistry is important especially for a nanoparticles. As the size of the material decreases, a larger percentage of the atoms are found at the surface. When working with nanoparticles, the properties of the material can be dominated by the surface properties. The nanoparticle shown here has a total of 147 atoms and 62% of them are at the surface. X-ray photoelectron spectroscopy, or XPS, can tell us about the surface chemistry of the top ten nanometers of a sample. The basic idea is that we will hit the sample with X-rays which will knock electrons out of the sample. We'll measure the energy of these electrons to determine the composition of the surface. The XPS experiment takes place in an ultra high vacuum chamber where pressures are typically in the ten to the minus nine torr range. One of the most common X-ray sources used an XPS has an energy of 1,486.6 eV. As a comparison, visible lights has energies of only 1.5 to 3 eV. So these X-rays have a lot of energy. In fact, they have enough energy to knock electrons out of the surface of the sample. These ejected electrons travels through an analyzer before being detected. XPS can detect all elements except for high hydrogen and helium. An atom consists of electrons that orbit a nucleus. This atom contains electrons in the 1s, 2s, and 2p orbitals. Remember, the electrons that are closer to the nucleus like the ones in the 1s orbital, are held more tightly than those in orbitals that are farther from the nucleus. When an X-ray enters the sample, it interacts with the electrons in that sample. These X-rays have enough energy to remove electrons from the sample. Here, an electron was removed from a 1s orbital. The XPS experiment measures how much energy was required to remove an electron from the sample which is called the electron binding energy. Since 1s electrons are held more tightly than 2p electrons, they will have a higher binding energy. The binding energy will also depend on the atom that the electron is bound to. Electrons in a carbon 1s orbital have a very different binding energy than electrons in an oxygen 1s orbital. When X-rays hit the sample, they penetrate fairly deep into the sample. Many of the electrons generated will scatter off other atoms in the sample, lose all of their energy, and become trapped in the sample. But electrons that are generated within the top 10 nanometers of the surface have a very good probability of escaping the sample and traveling towards the analyzer and detector. We call this the escape depth of the electrons. Only electrons that escape can be analyzed. And therefore XPS is used to characterize only the top 10 nanometers of the sample. If we wanted to analyze an even thinner portion of the surface, we could tilt the sample in a technique called angle resolved XPS. XPS gives us a spectrum, like the one shown here. Along the X axis is the electron binding energy which is a measure of how tightly the electron is bound to the atom its attached to. The binding energy tells you what element the electron was attached to, that is carbon or oxygen. On the Y axis is the intensity or the number of electrons submitted. The peak positions for various elements are well known and we can interpret the spectrum to identify the elements present on the surface of the sample. Polyethylene terephthalate, or PET, is a polymer often used to make beverage bottles. The peaks in the spectrum correspond to the positions for carbon and oxygen in the PET, consistent with its chemical structure. XPS can also tell us how much carbon and oxygen are present by comparing the areas under each peak. We can zoom into each of the peaks in the survey scan to see if these picks are really composed of multiple peaks, let's zoom in to the region outlined in red, in this graph. There are actually three peaks in the carbon 1s spectrum of PET, these peaks can tell something about what the carbon is bonded to. What affects the binding energy for an element like carbon? Here are two different environments that a carbon atom could be found in. Carbon bound to another carbon atom and carbon bound to an oxygen atom. The binding energy of an electron will depend on the atom that it's attached to, the carbon atom. And also the atoms would bound to the carbon. As you can see, the electron binding energy for carbon changes if that carbon atom is bound to another carbon atom or to an oxygen atom. Now, let's return to the carbon 1s spectrum of PET to assign the peaks. The component at the lowest binding energy is due to the carbon atoms bound to other carbon atoms labeled A. The peak at 286 eV corresponds to the carbon atoms bound to 1 oxygen atom marked B. And the peak just above 288 eV corresponds to the carbon atoms bound to 2 oxygen atoms marked C. The oxidation state of a metal can also effect the binding energy of an electron. The oxidation state refers to the electrical charge on the atom. Atoms are typically neutral but if they gain or lose an electron, they can become negatively or positively charged. Let's consider titanium as an example. Titanium metal exist in an oxidation state of 0 and its titanium to P peak appears at a binding energy of 453.9 eV. In the 2+ oxidation state, 2 electrons have already been removed from the titanium. It is therefore more difficult to remove an electron from titanium 2+ than from elemental titanium. And the binding energy is a bit higher. Finally, titanium dioxide, or TiO2, contains titanium in the 4+ oxidation stage. And the binding energy shifts to an even higher value. In general, as the oxidation state for an element increases, so will the binding energy. The titanium 2p data for TiO2 is shown here and we observed the expected doublet with peak positions consistent with titanium in the 4+ oxidation state. But if we have a very thin film of TiO2 on top of titanium metal, we may be able to observe the signal from both the TiO2 and the titanium metal. Remember, the depth of analysis for XPS is only about 10 nanometers. So if the TiO2 is less than 10 nanometers thick, we may be able to see some signals from the metal below it. In this case, we observe another peak at a much lower binding energy. This extra peak is part of a second titanium 2p doublet that partially overlaps with the TiO2 doublet and is due to elemental titanium. I hope this video helped you to understand that analysis by XPS can provide a lot of useful information about the surface of a sample. Thank you for joining me.
