Chapter 2 Experimental Methodology
2.2 Temperature-Programmed Desorption
Temperature-programmed desorption (TPD) is a method used to study surface reactions and kinetics of molecular desorption processes. The technique may be used to study surface coverage and the bonding energies of adsorbates to surfaces.
In a TPD experiment, a sample surface is exposed to one or more molecular species at a particular temperature in order to obtain a specific coverage. The sample is then heated in a controlled manner, usually a linear temperature ramp is desirable, to desorb species from the surface. The gas pressure above the surface is monitored as the surface is heated. Small quadrupole mass-spectrometers are the most commonly used detectors; a large number of evolved products can be monitored simultaneously. TPD experiments were carried out in Chamber 1, vacuum chamber with base pressure of 1 x 10-10 mbar. The chamber is fitted with
a quadrupole mass spectrometer, facilities for low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) and sample cleaning via argon ion sputtering. A schematic of the chamber is shown above in figure 2.1
Spectra can be generated that display the partial pressure variations of each recorded mass fragment as a function of temperature. The competition between molecules entering the gas phase volume and leaving the volume through pumping of the experimental chamber creates a TPD spectrum with shapes that are often characteristic of the nature of the absorbed species, the surface under investigation, adsorption temperature or surface coverage.
TPD data can also be used to estimate the activation energy of desorption, Ed, of a compound from a given surface1. The rate of desorption as the temperature ramp is applied to the sample can be expressed as follows:
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where θ is the surface coverage, N is the adsorbed number of molecules and m is the order of desorption.
Desorption is known to follow Arrhenius-type kinetics, thus the rate constant for the desorption process, kd may be expressed in terms of temperature, T, the pre- exponential factor, ν, the activation energy of desorption, Ed, and the gas constant,
R:
2.2
The rate of desorption can be re-written as:
2.3
where β (the heating rate). This can then be substituted into the equation 2.1 to give:
2.4
Subsequent substitution for kd yields:
2.5
The vacuum chamber in which the experiment takes place is pumped constantly, thus the temperature at which maximum desorption occurs (Tmax) must also correspond to the temperature of maximum desorption rate. At T = Tmax,
0. Therefore the following expression can be derived:
2.6
The method of analyzing TPD spectra depends on the information required by the investigation in question: the TPD might only be used to determine a temperature range at which an adsorbate may be stable on a certain surface or it may be used
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to extract more quantitative information such as activation energies of desorption. Desorption curves may be indicative of the order of the desorption, m.
Zero order desorption
If m = 0, Equation 2.1 and 2.6 may be written as:
2.7
Desorption rate in zero order processes is independent of coverage and rises exponentially with increasing temperature. Desorption peaks for these processes tend to have a common leading edge rising to the desorption maximum before a sharp decrease.
First order desorption
Molecules at submonolayer coverages commonly desorb via a first order process (m = 1). This occurs when the rate of desorption depends only on the concentration of one desorbing species.
2.8
First order desorption events typically produce an asymmetric peak shape, with a longer leading edge which sharply decreases after the desorption maximum. Based on the observed linear relationship between Ed and Tmax when is between 108 and 1013 K-1, the Redhead equation1,2 may be used to estimate Ed in first order
processes to good accuracy assuming a constant heating rate and ν = 1013 s-1:
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Second order desorption
Second order desorption (m = 2) typically arises from a recombinative event between two species before desorption. Peaks are characteristically broader and symmetric about Tmax. The desorption rate is given by:
2.10
As described, peak shapes may provide information on the order of the desorption process. However, this may be obscured by lateral interactions between adsorbate molecules, significant adsorbate-adsorbate interactions may change the peak shape or position. Repulsive interactions between molecules may be expected to lower the value of Ed with desorption occurring at lower temperature with increasing coverage. This study examines self-assembled systems which would otherwise not exist without attractive interactions (H-bonding), it is thought that this may significantly alter desorption spectra by raising the activation energy of desorption of molecules by the magnitude of the stabilization received from all lateral supramolecular interactions:
2.11
where Eads-subis the adsorption energy between a molecule and the surface, S1and
S2 are adsorbate-adsorbate interaction energies and x and y are the respective numbers of each interaction formed3. Depending on the system being analysed or the method, S1 and S2 might be used to represent the interaction of a molecule with its nearest neighbour and its next-nearest neighbour or in this case, the adsorbate-adsorbate interactions experienced from each of the two lateral dimensions.
The TPD spectra may also be made up of more than one peak or overlapping peaks4, this may occur if there are multiple adsorption sites or states. These may be related to different environments on the surface, different bonding configurations or different sites within the adsorbate overlayer.
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The speed of the pumping in the vacuum chamber and the sample heating rate may have a bearing upon the shape of TPD spectra, thus the method of analysis employed in Chapters 3 and 5 uses observed pumping speed plots (certain species may be pumped from the chamber quicker than others) and sample heating rate curves specific to the adsorbates and sample substrates used in the local vacuum chamber to gauge empirically activation energies of desorption for various adsorption states and better understand their nature.