An Atom Probe Investigation of Self-Assembled Mo - ACS Publications

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A new approach to understand the adsorption of thiophene on different surfaces: An atom probe investigation of self-assembled monolayers Katja Eder, Peter Johann Felfer, Baptiste Gault, Anna V. Ceguerra, Alexandre La Fontaine, Anthony F. Masters, Thomas Maschmeyer, and Julie M. Cairney Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01820 • Publication Date (Web): 22 Aug 2017 Downloaded from http://pubs.acs.org on August 27, 2017

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A new approach to understand the adsorption of thiophene on different surfaces: An atom probe investigation of self-assembled monolayers Katja Eder *, Peter J. Felfer *,†, Baptiste Gault ‡, Anna V. Ceguerra *, Alexandre La Fontaine *, Anthony F. Masters #, Thomas Maschmeyer #, Julie M. Cairney * * AMME and Australian Centre for Microscopy and Microanalysis, The University of Sydney, Australia † Department of Materials Science and Engineering, Friedrich-Alexander-Universität ErlangenNürnberg, Germany ‡ Max-Planck Institut für Eisenforschung, Max-Planck Str. 1, 40237 Düsseldorf, Germany # Laboratory of Advanced Catalysis for Sustainability, School of Chemistry, The University of Sydney, Australia KEYWORDS: Atom probe tomography (APT), bonding strength, Thiophene, Self-assembled monolayer (SAM)

ABSTRACT: Atom probe tomography was used to analyse self-assembled monolayers of thiophene on different surfaces, including tungsten, platinum and aluminium, where the tungsten

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was examined in both pristine and oxidised forms. A glove bag with controlled atmospheres was used to alter the level of oxidation for tungsten. It was shown that different substrates lead to substantial changes in the way thiophene adsorbs on the surface. Furthermore, the oxidation of the surface strongly influenced the adsorption behaviour of the thiophene molecules, leading to clear differences in the amounts and compositions of field evaporated ions and molecular ions.

INTRODUCTION: The interaction of aromatic molecules with metals plays a crucial role in the field of catalysis. The permanent adsorption of sulfur on a catalyst surface can cause the catalytic reaction to slow down or even lead to complete deactivation.1 In industry, this leads to severe costs, due to down time, and substantial maintenance costs, for the replacement or reactivation of the catalyst. Several studies have used thiophene as a model to investigate the effects of sulfur adsorption on metallic and catalytic surfaces.2, 3 Thiophene (C4H4S) is an organic liquid at room temperature that consists of a ring of four carbon atoms and one sulfur atom. When a substrate is immersed into the liquid, the thiophene molecules form a self-assembled monolayer (SAM) on the surface. The adsorption of thiophene molecules lowers the free energy of the interface, so the molecules readily attach to the surface, attempting to occupy every available site.4 SAMs are generally analysed with different spectroscopic methods 5, 6, 7, 8, such as X-ray photoelectron spectroscopy, Auger electron spectroscopy, etc., as well as by scanning tunnelling microscopy (STM) 9 and atomic force microscopy (AFM).10 These techniques provide information about the molecular orientation, growth processes and adsorption kinetics of the molecules. Although spectroscopic methods generally exhibit excellent sensitivity in characterizing the atomic composition and structure, they lack spatial resolution. For STM and AFM the opposite is true; atomic scale


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resolution is achieved, but with little chemical sensitivity.11, 12 Atomic scale characterization coupled with high chemical sensitivity would provide a better understanding of the mechanisms associated with the bonding strength, such as the catalytic activity and durability. This information would help to develop an understanding of the nature of the interface and how it is affected by sulfur adsorption. Atom probe tomography (APT) is a microscopy technique that enables the characterisation of solid, inorganic materials in three dimensions with near atomic resolution.13 It provides information on the position of each atom within the microstructure as well as their mass to charge state ratio, which is used to determine their elemental identity. In the experiment, atoms are field evaporated from the tip of a needle-shaped specimen and subjected to a high electrostatic potential upon application of a high voltage or laser pulse. By measuring the electrostatic field strength, information about the binding energy of an adatom can be obtained.14 Here we analyse SAMs of thiophene and their adhesion to the substrate via APT for both clean (non-oxidised) and oxidised tungsten substrates, as well as Pt and Al. Our goal is to use APT to determine surface coverage, desorption energy and dissociation properties. There have been only a small number of previous studies in which APT has been used to analyse SAMs.15, 16, 17 Unlike previous work, which was conducted on other systems or using different experimental conditions, here we have used the voltage pulsing mode for better control over the evaporation sequence and detected fragments, and, crucially, to avoid possible effects caused by the laser illumination on the specimen’s structure or its field evaporation behaviour.18 This is the first time the field evaporation behaviour of thiophene has been compared for different substrate types. EXPERIMENTAL


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In order to conduct atom probe tomography, needle shaped specimens are required with a tip diameter of less than 100 nm.19 These were produced via electrochemical polishing. The procedure and solutions used are described in the literature 20, 21 for W and Al respectively. For the Pt tips rough polishing 22 was performed, followed by annular milling with a focussed ion beam (FIB), conducted on a Zeiss Auriga FEG SEM. For imaging with the electron beam, an accelerating voltage of 10 kV was used, and the annular milling process was conducted at 10 kV and 50 pA, to reduce damage caused by Ga ions. Following the electropolishing/FIB process all specimens were “pre-evaporated” on a Cameca LEAP 3000X Si atom probe in voltage mode until 1 million atoms were detected. Preevaporation was performed to remove all contaminations on the surface, and to form a spherical shaped apex in order to avoid artefacts during the reconstruction of the datasets. Then the freshly evaporated tips were removed from the vacuum chamber of the atom probe and dipped in thiophene (purity ≥ 99 %) in air, for 5 minutes each. After this they were instantly returned into the vacuum chamber of the atom probe to avoid contamination. Dipping each needle for 5 minutes in thiophene produces a monolayer that covers the whole surface of the tip. This can be seen in the atom probe reconstructions in Figure 1. To study the differences of thiophene adsorption on oxidised and non-oxidised surfaces, a glove bag setup was designed that enabled the transfer and dipping process to be conducted in a controlled environment. Two separate glove bags had to be installed. The first bag was installed on top of the load lock chamber of the atom probe and filled with nitrogen. The dipping process of the pre-evaporated tips could not be conducted within this bag, due to safety and practicality reasons. Therefore, a second glove bag had to be installed inside a fume hood. Both bags were kept at positive pressure of nitrogen or argon to ensure that no oxygen could leak through any of the seals. For the transfer between the


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two glove bags a separate plastic bag filled with nitrogen or argon was used. For the W substrate, samples using three different substrate conditions were prepared: freshly field-evaporated W, with no oxide present on its surface, oxidised W and an intermediate oxidation stage, with only a small amount of WO being present on its surface compared to the oxidised W. This substrate will be herein referred to as ‘partially oxidised’ in this manuscript. The main APT experiments as well as the pre-evaporation were carried out on a Cameca LEAP 3000X Si atom probe in voltage pulsing mode with a pulse rate of 200 kHz, a pulse fraction of 20 %, a target evaporation rate of 0.5 % and the temperature ranging between 40- 50 K. This temperature range was chosen, to ensure evaporation of the W and Pt substrate. All datasets were reconstructed in IVAS, and corrected for time of flight (TOF), image compression factor (ICF) and ‘field reduction factor’ (kF). The ICF was determined by first indexing the poles, visible in the desorption map of the detector, followed by calculating the ratio between the actual and observed angle between two crystallographic directions.23 The charge-state-ratio (CSR) of W3+ and W4+ ions, throughout the atom probe run, was determined via Matlab, to see at what voltage a consistent CSR is reached. This CSR was then used to obtain the field strength, by using Kingham curves.24 For all W and WO substrates the field strength was determined to be around 50 – 52 V/nm, for Pt and Al the field strengths were 35 V/nm and 21 V/nm, respectively. For each dataset, the inter-planar spacing of the substrate was determined via spatial distribution maps along the z-direction (depth), for several different crystallographic orientations, using a 4 nm cylindrical region of interest (ROI), which was centred upon the corresponding pole within the atom probe dataset.25 To correct kF, the inter-planar spacing of the substrate was set to the correct value by adjusting the initial radius of curvature. The radius obtained could then be used to calculate the correct field factor according to Eq.1, as well as the tip radius evolution for the


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entire dataset. By knowing the radius at every point throughout the reconstruction and the field of evaporation, the evolution of the field evaporation could be determined by using the rule of proportion. To calculate the desorption energy, the change in field strength that the specimen was subjected to during the entire atom probe experiment was determined according to Eq.1. F= (Eq.1)

V k∙r

In this equation F is the field strength, V the applied voltage, k the field reduction factor (a constant that accounts for the tip shape and its electrostatic environment), and r the radius of curvature of the tip shaped specimen. The energy barrier of desorption in the presence of an electric field for carbon and sulfur were then calculated by using the following equation: nଷ eଷ QሺFሻ = Q଴ − ඨ ∙ F + ∆Λ 4πε଴ (Eq.2) In this equation ε0 is the vacuum permittivity, e the charge state of the element, n the ionisation energy, and ∆Λ is the increase in sublimation energy that arises in the presence of an electric field.26 An increase of the sublimation energy (∆Λ) can be accounted for with the following formula,

(Eq.3)

1 ΔΛ = ε଴ ΩF ଶ 2


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where Ω is the atomic volume of the surface atom. In Eq.2, Q0 is the activation barrier for the desorption of an n-fold charged ion under zero field conditions and can be calculated by: Q଴ = Λ଴ + ෍ I୬ − nΦ ୬

(Eq.4)

with Λ0 being the sublimation energy of a neutral atom, In being the n-th ionisation energy and Φ being the work function of the emitting surface. The values used for the calculation of the activation barrier (Λ0, In, Φ) can be found in appendix E of the book by Miller et al. 20 RESULTS AND DISCUSSION Tungsten (W) was initially chosen as a substrate, since it is used as a catalytic surface or additive, for example in hydrodesulfurisation processes.27 Oxidation of the substrate changes the surface interactions with thiophene molecules.28 For example, Preston et al. 28 found that thiophene molecules do not dissociate from an oxidised W (211) surface, but rather desorb molecularly, indicating that the bond that is formed between the oxide layer and the thiophene is weaker compared to the non-oxidised W (211) surface. To see how the thiophene adsorption is affected by the substrate, thiophene specimens on Al and Pt substrates were also prepared. Dipping each atom probe needle in thiophene produced a SAM that covered the whole surface of the tip, as can be seen in the example of a SAM on WO, shown in Figure 1. Similar results were obtained for Al and Pt substrates.


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Figure 1. 3D atom probe reconstruction of a thiophene SAM on oxidised W, with the SAM ions shown in red, the W ions in grey, and the WO ions in green. Figure 2A compares the voltage curves obtained from atom probe experiments conducted on the samples with the oxidised substrate (red curve, (e)), clean substrate (blue curve, (a & b)) and intermediate oxidation stage (green curve, (c & d)). Voltage curves illustrate how the specimen voltage changes throughout the experiment in order to reach the critical field necessary to induce field evaporation. The voltage is proportional to the applied electric field, which, in turn, depends on the specimen geometry. A large shank angle will cause the intensity of the field to be lowered. In the reconstruction of the atom probe dataset this is accounted for by a ‘field reduction factor’ (kF).29, 30 For a more accurate comparison, the curves in Figure 2A were normalised with respect to the voltage at which a consistent CSR of W is reached (which was 6195 V for pristine W). That is, curves have been adjusted so that the voltage at which all three curves reach a consistent CSR is 6195 V. The voltage curve of the clean, non-oxidised surface (blue) shows two distinct fields, and can therefore be divided into three different evaporation stages (Figure 2B). Information about the ionic species that evaporate throughout the experiment is available in the mass history (Figure 2C), which shows the mass-to-charge-state ratios detected during the experiment as trails, with


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the intensity of the trails representing the amount of detected ions. The mass to charge state ratios seen in Figure 2C, during Stage 1 of the experiment, correspond to fragments of organic molecules (e.g. S2H3+ at 69 Da) and sulfur (32 Da) that arise from the evaporation of thiophene. Stage 2 features ions with ratios that correspond to the evaporation of the SAM (carbon trail at 6 and 12 Da) together with trails from the W substrate (e.g. 62 Da), and Stage 3 to W only. Different organic molecules were detected from the SAM for all three samples (see suppl. Table S1), but the detected voltage curve patterns were the same (Figure 2), with the exception of the heavily oxidised substrate, where Stage 1 is absent, and only Stage 2 and Stage 3 occur. Figure 2A shows that, for all three voltage curves, evaporation begins at a very low voltage, around 500 – 1000 V. For the oxidised surface (red curve, (e)) a steep voltage increase is observed after just 2.000 evaporated atoms, caused by the difference in evaporation field between the SAM and the substrate. For the partially oxidised and non-oxidised substrate (green and blue curves, (c & d) and (a & b), respectively) the steep voltage increase starts at 0.3 × 106 hits and 2.7 × 106 hits, respectively, much later than the heavily oxidised surface. This shows that the amount of thiophene adsorbed on the surface, decreases as the substrate is oxidised. The general shape of the voltage curves for the individual substrates is reproducible, with only the stages varying in length.


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(A)

(B)

(C)

Figure 2. (A) Voltage curves for three different oxidation states, where blue (a & b) corresponds to the non-oxidised substrate, green (c & d) to the partially oxidised and red (e) to an oxidised substrate. The labels (a) to (e) mark different evaporation stages, which correspond to the mass spectra shown in Figure 3. The curves were normalised with respect to the voltage at which a consistent charge state ratio of W3+ and W4+ is reached. (B) Voltage curve of the non-oxidised W substrate, divided into three stages. (C) Mass history of the non-oxidised W substrate. During Stage 1 only the SAM evaporates, whereas in Stage 2 the mass evolution shows W3+ (61.33 Da) and W4+ (46 Da) together with C+ (12 Da), C2+ (6 Da) and S2+ (16 Da). By Stage 3 only W is evaporating.

Mass spectra for each evaporation stage (1-3) and all three oxidation states of the W substrate can be seen in Figure 3. The mass spectra from the 3rd evaporation stage (not shown) consist only of ions from the bulk substrate, W3+ and W4+. Figure 3 (a) and (b) show the mass spectra for


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the non-oxidised W substrate, (c) and (d) show the mass spectra of the partially oxidised substrate and (e) shows the mass spectrum of the heavily oxidised W substrate (no Stage 1 was seen for the heavily oxidised specimen).

Figure 3. (a) and (b) show the mass spectra of Stage 1 and 2 of the non-oxidised W substrate, respectively, (c) and (d) show the mass spectra of Stage 1 and 2 of the partially oxidised W substrate and (e) shows the mass spectrum of the heavily oxidised W substrate. No Stage 1 was seen for the heavily oxidised W. For easier comparison the red dashed lines show the peak position of S3+ (10.6 Da), S2+ (16 Da), S+ (32 Da). The peak position of W2+, ranging from 87.65 – 93.25 Da, is marked with a grey dashed line to highlight that the peaks ranging from 85.8 – 91.48 Da in both (d) and (e) are not W2+. The labels (a) to (e) also represent the partitioning of the runs into different evaporation stages and correspond to the labelling in Figure 2a. In the Stage 1 mass spectrum of the non-oxidised substrate (Figure 3a), S+ (32 Da) and S2+ (16 Da) peaks can be seen, together with several complex ions (Table S1 of the supplementary material elaborates on all the fragments found). Once Stage 2 is reached (Figure 3b), all of the


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complex ions that were visible during Stage 1 disappear and only S2+ (16 Da), S3+ (10.6 Da), C+ (12 Da), C2+ (6 Da), and W2,3,4+ (92, 61.3, 46 Da) are detected. No overlap of sulfur and oxygen is expected in the mass spectra, since the samples were produced in an oxygen-free environment (see experimental section). The Al3+ peak (9 Da) that is visible in the W mass spectrum is due to trace quantities of Al in the W substrate. By using thermal desorption spectroscopy, Whitten 31 previously revealed that several multilayers of thiophene are physisorbed on top of a chemisorbed monolayer, for a W (110) single crystal. It has also been shown that the thiophene molecules of the chemisorbed layer fully dissociate into hydrocarbon fragments and sulfur.28 After heating the specimen to 800K, Auger electron spectroscopy detected that residues of sulfur and carbon are still present on the surface, indicating strong adsorption. These findings correspond well with our APT results. The first stage of evaporation is thought to correspond to the physisorbed layer. Since this layer does not form any chemical bonds with the surface, but is bound by van der Waals forces, a lower field is required to remove it from the surface compared to the chemisorbed layer, where chemical bonds are formed between the substrate and the thiophene molecules. This is in agreement with the increase in voltage between Stage 1 and Stage 2, where the difference in bonding strength between the physisorbed and chemisorbed layer lead to the voltage increase observed. The carbon and sulfur atoms that are evaporated during Stage 2 are thought to belong to the dissociated, chemisorbed monolayer. The fact that all of the fragments disappear in the transition from Stage 1 to Stage 2 (see mass spectra (a) and (b) of Figure 3) indicates that the thiophene molecule must be completely dissociated, otherwise hydrocarbons or other complex ions would be expected in addition to the single carbon and sulfur peak. After the rapid voltage increase there is still sulfur and carbon present in the mass spectrum, evaporating at the same field of


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evaporation as tungsten, which indicates the presence of strong chemical bonds. Since S2+, C+ and W3+ evaporate simultaneously, it was assumed that they evaporate at the same field. The value for the field of evaporation of W3+ was determined to be 52 V/nm, by using the Kingham curves.24 This value was used to determine the change of field strength (Eq.1) to which the specimen was subjected during the entire atom probe experiment (see experimental section for more details). If the Stage 1 mass spectra of the partially oxidised (Figure 3c) and clean substrate (Figure 3a) are compared, it can be seen that, in the case of the partially oxidised substrate, fewer, and different complex ions (see suppl. Table S1) evaporate alongside S+ and S2+. Once again, as the physisorbed layer is evaporated, the complex ions disappear (Figure 3d), but in this case the mass spectrum looks similar to the mass spectrum of the oxidised surface (Figure 3e), where only carbon, sulfur and two fragment peaks (C4H43+ and C4H8S+) are present in addition to the tungsten and tungsten oxide peaks from the substrate. The peak at 17.22 Da was identified as C4H43+, as it is the only molecule that fits the observed peak, even though a charge state this high, for a molecule of this size, is rather unusual. For the oxidised substrates, an overlap of the sulfur and oxygen peaks at 16 Da is expected, as well as an overlap of the OH+ and C4H43+ peaks. No clear secondary isotopes are available to de-convolute the peaks. To test whether this species is affected by charge-hopping 32, its time of flight (TOF) was plotted against the square root of the voltage (√V), which yielded a proportional ratio and was therefore thought not to be affected by hopping. A comparison of the mass spectra illustrates that the oxidation state of the substrate influences the adsorption behaviour of thiophene. If this were not the case, the mass spectra for the different oxidation states would contain the same types of complex ions (see


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suppl. Table S1 for list of complex ions). A mass spectra comparison was possible, since the field conditions were found to be in the same range for all specimens. The differences observed in the mass spectra were reproducible and interpreted the following way: Where fragments of the thiophene molecule are present during the evaporation, rather than single atomic ions, the internal bonds between the atoms of the molecule are thought to be stronger than the bonds with the substrate, which suggests weak adsorption to the substrate. On the other hand, single hits of carbon and sulfur are believed to indicate strong bonding. Therefore, the physisorbed layers of both the clean and intermediate substrate are interpreted to be weakly adsorbed, whereas the chemisorbed layer of the clean substrate is more strongly adsorbed to the W substrate. The chemisorbed layer on the oxidised surface is believed to have a weaker adsorption to the substrate, since complex ion peaks are observed within the mass spectrum. We have considered the possibility that the oxide layer of the W substrate lowers the electric field and is thus too weak to cause fragmentation of the molecule. However, if this was the case, one would expect the mass spectrum of Stage 2 for the partially oxidised substrate to show a mix between the oxidised- and non-oxidised substrate, as the oxide layer would be much thinner compared to the oxidised one. This is not the case. For the partially oxidised substrate, the results show intermediate adsorption strength of the SAM, since fewer hits are detected for the physisorbed layer and complex ion peaks are observed during the evaporation of the chemisorbed layer. Therefore, the results show that the adsorption strength of the SAM decreases with increasing amount of substrate oxidation. Despite weaker adsorption on the oxidised substrate, the SAM is bound strongly enough to the substrate to not be pulled off all at once when an electric field is applied.


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As described earlier, we have found that the amount of thiophene present on the surface decreases with increasing substrate oxidation, which is explained by physisorbed layers being present in addition to the chemisorbed monolayer, consistent with the literature. To estimate how much sulfur is present on the surface, measurements of the surface coverage (Γ) were performed by dividing the number of detected sulfur atoms by the surface area. The surface area of the SAM was calculated by first creating a model of the SAMs surface via Blender, an open source 3D visualisation software, which was then exported into MATLAB, where the surface area was calculated. For a more detailed explanation the reader is referred to previous work.33 For the calculation of the coverage (Γ), the complex ions were ignored and only single and multiple sulfur hits were considered. The results shown in Figure 4 are therefore considered to be an underestimate. Including the complex ions would lead to an increase in the amount of S and C on the surface, especially in the case of the non-oxidised surface, which shows the most amount of complex ions, further increasing the trend that is observable. The large variation for the clean W is due to a large variation in the length of Stage 1 across all the atom probe runs that were performed. This variation is thought to be the result of different needle-shaped specimen geometries as well as slightly varying crystallographic orientations for each specimen (judged by the slight difference in distribution of the main poles within the field desorption map), which is known to influence the bonding strength of the SAM to the substrate to a small extent.14 However, even with those variations, the overall trend that the thickness of the SAM decreases with increasing oxidation of the surface is still observable. No evidence for a variation in thickness across the surface, depending on the crystallographic orientation of the substrate was noted. To test whether dipping times affected the results, an experiment was conducted in which


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the dipping time was increased from 5 minutes to 7.5 hours. The result did not vary compared to the samples that were dipped for 5 minutes only.

Figure 4. Comparison of surface coverage of sulfur, for the non-oxidised, partially oxidised and oxidised W substrate. The next question that arises is whether the electric field, which is generated at the end of the specimen during the atom probe experiment, influences the inclination of the adsorbed molecules and therefore influences the observed thickness. From the literature it is known that when an electric field is applied to a SAM the inclination of the molecules changes due to an induced polarisation of the molecules, which aligns them with the electric field.34 Since the chemisorbed monolayer on the clean substrate is dissociated and strongly bound to the substrate, no alignment is expected. On the other hand, for the oxidised surface, as well as for the physisorbed layers a change in inclination of the SAM is possible. If such a change in inclination occurred, one would expect to see a clear evaporation sequence where, for example, molecular ions, consisting of carbon and hydrogen, are evaporated before sulfur ions, which is not the case (see S.-Figure 1 for further details). Therefore, it is not believed that a significant change in inclination occurs.


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During the analysis of the SAM, a large fraction of multiple hits occurred (up to 85 %), where several atoms hit the detector within the timeframe following a single pulse.13 As soon as the substrate started to evaporate, the multiple hit value dropped to ~5 %. Additional atom probe experiments were conducted at 27 K to see whether the multiple hit value would change with a decrease in temperature, which was not the case. It is thought that the high proportion of multiple hits may be associated with clusters or molecular ions dissociating during field evaporation. Correlation histograms 35 were produced for the three evaporation stages. Only multiple hits were used for the analysis. A correlation histogram is a 2D histogram where all detected multiple hits are accumulated, with the x-axis displaying the mass to charge state ratio of the heavier ions produced by dissociation, and the y-axis displaying the mass-to-charge state ratio of the lighter ions that were produced. Correlation histograms can show whether dissociation occurred after field evaporation took place, and if the frequency of evaporated fragments within each multiple hit are correlated and cause ion-pairs to be evaporated in a certain sequence. If dissociation occurs, it is visible as a trail running towards the bottom-right or the top-left in the correlation histogram. For a more detailed explanation on correlation histograms the reader is referred to the work of F. De Geuser 36 and D. Saxey 35. A correlation histogram for the SAM on the heavily oxidised substrate is shown in Figure 5a containing about 20,000 atoms with up to 70 % multiple hits per pulse. The dissociation trail has been highlighted and, for better visibility, enlarged in subfigure 5b. Since the peak at 90.6 Da could not be unambiguously identified, several different dissociation scenarios are possible and their implications are discussed here. The dissociation trail could correspond to the dissociation of the following molecules: (1) WOC4H4S5+ → WOC4H43+ + S2+


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(2) WOS35+ → WOS23+ + S2+ (3) C4H8S23+ → C4H8S+ + S2+ The proposed dissociation reactions have nothing in common with regular dissociation reactions, but are rather a results of the influence of the high electric field during the analysis, which can induce a breakup of the evaporated molecules. Scenarios 1 and 2 would indicate strong bonding of the thiophene molecule to the substrate. However, a 5+ charge state of an initially evaporated molecule has never been reported to occur, and scenario 1 and 2 are therefore unlikely. In the case of scenario 3, it could be interpreted that thiophene desorbs molecularly from the substrate and therefore the bonds formed with the substrate are weaker than the internal bonds of the molecule. However, some adsorption to the substrate still occurs, since the SAM is not being pulled off the substrate all at once. The only aspect that cannot be fully explained with this model is that a thiophene molecule only contains one sulfur atom, but the initially evaporated molecule contains two sulfur atoms. Since no further dissociation reactions seemed plausible, scenario (3) is thought to be the most plausible one. In comparison, Figure 5c shows a dissociation track visible only in the correlation histogram of Stage 2 (chemisorbed SAM + W) of the non-oxidised W substrate, containing about 0.65 × 106 atoms and multiple hits ranging from 15 – 67 % per pulse. The dissociation trail belongs to the dissociation of: (4) WC3+ → W2+ + C+


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This dissociation shows that a strong chemical bond is formed between the clean W substrate and carbon. No dissociation trails are visible during Stage 1 or 3. Therefore the dissociation can only be correlated to the bonding of the chemisorbed layer to the substrate.

Figure 5. (a) Correlation histogram from SAM + WO dataset with its associated mass spectrum. The highlighted area corresponds to subfigure (b) which shows a dissociation trail. In comparison subfigure (c) show the dissociation trail associated with the evaporation of the chemisorbed layer of the non-oxidised W substrate. In order to see how the thiophene adsorption is affected by the substrate, Al and Pt specimens were also prepared. A comparison of the different mass spectra is shown in Figure 6. Each of the different substrates displays a mass spectrum that is quite different to the others. In particular, the Al mass spectrum shows almost no sulfur (S.-Figure 2), which corresponds well with the weak


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adsorption proposed in the literature.37, 38 The differences observed in the mass spectra are reproducible and therefore thought to be related to the metal substrate. It was not possible to accurately determine the stoichiometry of the SAM for each substrate, as some peaks in the individual mass spectra could not be unambiguously identified and, in all cases most of the peaks in the mass spectrum where those of complex ions, with several overlaps between them.

Figure 6. Mass spectra for SAM of thiophene on different substrates. (a) WO, (b) Pt, (c) Al. For easier comparison the dashed line shows the peak position of S2+ (16 Da) and S+ (32 Da). Crystallographic poles 39 could be identified within the reconstruction of the substrate allowing the identification of surfaces of different crystallographic orientation. Examination of surface coverage with respect to these poles revealed only slight differences in the distribution of some complex ions across the surface, as well as some differences in the distribution of chemisorbed C and S on W. Insufficient data was available to be able to make a reliable assumption about the


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influence of the crystallographic orientation of the substrate on the thickness or amount of thiophene. In order to obtain information about the desorption energy that is required to remove adsorbed species from the substrate, several calculations were performed. It was assumed, that chemisorbed sulfur and carbon are subject to the same evaporation field as the substrate, since they evaporate simultaneously. Although the local field can be different from the average field 40 and could be predicted by using DFT calculations 41, this process is not straightforward in conjunction with APT, since many assumptions have to be made to obtain an accurate model, and was therefore not attempted here. The values calculated for the desorption energy of sulfur and carbon, using the equations specified in the experimental section, are listed in Table 1. The desorption energy for carbon and sulfur on the oxidised W substrate was calculated by using a work function of a WO3 surface, determined via ultraviolet and inverse photoemission spectroscopy.42 Since the work function strongly depends on the chemical composition of the material, the actual work function for the WO substrate analysed in this study might be different to the value of WO3 42, used for the calculations, and therefore the values calculated for the desorption energy of WO could vary accordingly. Table 1. Desorption energies of chemisorbed carbon and sulfur calculated for different substrates Energy barrier of desorption [eV] C (chemisorbed) Subs trate

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Al

S (chemisorbed)

N.A. since C mainly evaporates as N.A. since S mainly evaporates as


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compounds

compounds

Pt

6.33 (± 0.00) – 9.99 (± 1.09)

6.13 (± 0.00) – 16.42 (± 3.00)

W

5.65 (± 0.00) – 7.72 (± 0.5)

3.51 (± 0.00) – 9.5 (± 1.3)

WO

3.48 (± 0.01) – 7.26 (± 1.22)

- 0.83 (± 0.03) – 9.95 (± 3.47)

It can be seen, that the values calculated for Pt are only slightly higher than the ones for W. Larger desorption values are interpreted as stronger bonds between the substrate and the adsorbed species, whereas for smaller values the opposite is the case. The large range listed arises from the fact that, with progression of the atom probe experiment, the voltage and radius of the needle shaped specimen increases, which causes the electric field to increase and therefore induces a drop of the desorption energy. The negative value calculated for the bonding strength of sulfur on oxidised tungsten, is interpreted as an indication that no bond is formed between S and WO, but rather a bond between C and WO. No comparison of the desorption energy of W and WO to the literature was possible, since no values were given. In the literature the theoretical adsorption energy calculated via DFT for S on Al(111) is 0.54 eV.38 For Pt(111) at low thiophene coverage and therefore flat adsorption geometry, an adsorption energy of 1.14 eV 43 and 1.51 eV 44 were given. At high thiophene coverage and thus inclined orientation, an adsorption energy of 0.86 eV 43 and 1.55 eV 44were calculated. For Pt(110) several different adsorption geometries were determined, with the theoretical adsorption energies ranging from 0.97 eV to 0.99 eV for the upright adsorption geometries, and 1.68 eV to 2.87 eV for the flat adsorption geometries.45 The values calculated for the desorption energy of S on Pt are much higher compared to the theoretical values stated in the literature. An explanation for the discrepancy between our values and the values in the literature could be the influence of the electric field, involved in the desorption process, different crystallographic facets being present


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on the atom probe substrate compared to the DFT calculations, and the choice of parameters for the DFT calculations. CONCLUSION In this work, the influence of the oxidation state of the substrate on the adsorption behaviour of thiophene has been studied via atom probe tomography for the first time. It was shown that the adsorption strength of the SAM decreases with increasing amount of oxidation of a tungsten substrate. We were able to show the existence of physisorbed layers on top of a dissociated, chemisorbed layer of thiophene for the clean W substrate and partially oxidised substrate. Furthermore a comparison of self-assembled monolayers on different substrates was undertaken. The results showed that different substrates lead to substantial changes in the way thiophene adsorbs on the surface. Therefore it can be concluded that it is possible to obtain comparative information about the surface coverage as well as quanti- and qualitative information on the desorption energy, by analysing SAMs of thiophene with the atom probe. This enables further investigations of other catalytic surfaces via atom probe tomography. A vacuum transfer system between the atom probe and a glove box would be advised for future experiments, especially for surfaces that easily oxidise under atmospheric conditions. ASSOCIATED CONTENT SUPPORTING INFORMATION

S.-Figure 1-2, and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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* [email protected] Author Contributions All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The work was funded by the Australian Research Council (ARC). Financial support for KE by the University of Sydney International Scholarship (USydIS) is gratefully acknowledged. The authors are also grateful for the scientific and technical input and support from the Australian Microscopy & Microanalysis Research Facility (AMMRF) at the University of Sydney. Especially Dr. Takanori Sato for his help in setting up the glove bag experiment and the interns Zelie Catherine, Florant Exertier and Michael Lord for their assistance during several glove bag transfers. We also thank Dr. Andrew Breen and Dr. Leigh Stephenson for helpful discussions. REFERENCES 1. Bartholomew, C. H. Mechanisms of catalyst deactivation. Appl. Catal., A 2001, 212 (1– 2), 17-60. 2. Matsuura, T.; Nakajima, M.; Shimoyama, Y. Growth of self-assembled monolayer of thiophene on gold surface: An infrared spectroscopic study. Jpn. J. Appl. Phys. 2001, 40 (12), 6945-6950. 3. Reyes, P.; Pecchi, G.; Morales, M.; Fierro, J. L. G. The nature of the support and the metal precursor on the resistance to sulphur poisoning of Pt supported catalysts. Appl. Catal., A 1997, 163 (1–2), 145-152. 4. Adamson, A. W.; Gast, A. P. Physical chemistry of surfaces; Wiley: New York, 1997; Vol. 6th. 5. Mrksich, M. Mass Spectrometry of Self-Assembled Monolayers: A New Tool for Molecular Surface Science. ACS Nano 2008, 2 (1), 7-18. 6. Hähner, G. Near edge X-ray absorption fine structure spectroscopy as a tool to probe electronic and structural properties of thin organic films and liquids. Chem. Soc. Rev. 2006, 35 (12), 1244-1255. 7. Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. Comparison of the structures and wetting properties of self-assembled monolayers of nalkanethiols on the coinage metal surfaces, copper, silver, and gold. J. Am. Chem. Soc. 1991, 113 (19), 7152-7167.


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45. Zhu, H.; Lu, X.; Guo, W.; Li, L.; Zhao, L.; Shan, H. Theoretical insight into the desulfurization of thiophene on Pt(110): A density functional investigation. J. Mol. Catal. A: Chem. 2012, 363–364 (0), 18-25. Table of Contents artwork


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