Anal. Chem. 2003, 75, 6571-6575
Application of Gas Microstrip Detectors for X-ray Absorption Spectroscopy in Common Process Gases Antje Vollmer,† John D. Lipp,‡ Jonathan R. I. Lee,† Gareth E. Derbyshire,‡ and Trevor Rayment*,†
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K., and CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, U.K.
We report upon the design of a new gas microstrip detector (GMSD) for use in X-ray absorption spectroscopy applied to the study of catalysis and material science. We show that GMSDs can operate not only with the gas mixtures normally used in proportional counters but also with the majority of gas mixtures used in common catalytic reactions. The detector functions well in the presence of water vapor. EXAFS investigations of a test system of NiO on Ni metal are discussed in which it is demonstrated that depth profiling using electron yield X-ray absorption spectroscopy is possible in a wide variety of gaseous environments. Electron detection of XAS using GMSDs is applicable to metals, semiconductors, and insulators presented in almost all forms of sample including films, pellets, powders, crystals, and liquids. X-ray absorption spectroscopy (XAS) has emerged as an indispensable tool in the study and development of heterogeneous catalysts, not least because of the lack of constraints upon the form of the material and its suitability for in situ studies. However, XAS is not an intrinsically surface-sensitive technique, and catalytic processes depend for the most part upon the structure and properties of the surface or near-surface region. Among surfacesensitive analytical techniques, those that depend on electron detection are prevalent because of the small inelastic mean free paths of electrons in solids; however, these usually operate in ultrahigh-vacuum conditions. Over the past few years, it has been established that electron detection is possible under nonvacuum conditions if the sample acts as the internal photocathode of a gas-flow ionization detector.1-5 In a series of extensive studies, we have shown that there are virtually no restrictions upon the composition of the gaseous * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: +44-1223-336469. Fax: +44-1223-336362. † University of Cambridge. ‡ CCLRC Rutherford Appleton Laboratory. (1) Moggridge, G. D.; Schroeder, S. L. M.; Lambert, R. M.; Rayment, T. Nucl. Instrum. Methods B 1995, 97, 28-32. (2) Schroeder, S. L. M.; Moggridge, G. D.; Ormerod, R. M.; Lambert, R. M.; Rayment, T. Physica B 1995, 209, 215-216, (3) Song, I. H.; Rickett, B.; Janavicius, P.; Payer, J. H.; Antonio, M. R. Nucl. Instrum. Methods A 1995, 360, 634. (4) Schroeder, S. L. M.; Moggridge, G. D.; Rayment, T.; Lambert, R. M. J. Mol. Catal. A 1997, 119, 357-365. (5) Schroeder, S. L. M.; Moggridge, G. D.; Chabala, E.; Ormerod, R. M.; Rayment, T.; Lambert, R. M. Faraday Discuss. 1996, 317-336. 10.1021/ac034288x CCC: $25.00 Published on Web 10/21/2003
© 2003 American Chemical Society
environment in total electron yield (TEY) experiments and that measurements using TEY can be made at both elevated temperatures and pressures.6-8 However, the surface sensitivity in TEY is not as great as would be expected at first since the important parameter is not the inelastic mean free path (IMFP), which is typically less than 10 nm, but rather the escape depth of the resultant Auger electrons, which is much larger, for example, 500 Å for Ni KR in Ni metal.9 Higher surface sensitivity can be achieved by selective detection of only elastic Auger electrons in the high-energy region of the TEY spectrum. (IMFPs, 5-100 Å). Energy-selective electron detection has so far been largely confined to measurements under high-vacuum conditions. Energy discriminating electron yield X-ray absorption fine structure (XAFS) at ambient pressure is feasible and was first described in 198510 using an ionization chamber with a wire anode, allowing the discrimination between electron energies via analysis of the charge pulse intensity. However, wire anodes are mechanically fragile, extremely sensitive to electrical interference, and, worst of all, prone to random breakdown due to chemical contamination. They have also a very low maximum count rate, typically ∼50 kHz, from which less than 10-20 kHz constitutes the desired spectral information (the remainder being background signals). The statistical accuracy required for acceptable XAFS data is 0.1%, which implies a minimum of 106 counts collected per data point. Considering that a full XAFS spectrum normally contains several hundred data points, the duration of the experiment would thus be unacceptably long. It appears that proportional mode detection had been abandoned mainly for these practical limitations. Most of these problems have now been overcome by the invention of gas microstrip detectors (GMSDs), which consist of a pattern of interleaved metallic strips deposited on a semiconducting glass plate by photolithography.10 One group of strips is held at a high positive potential with respect to wider intervening strips. In a suitable gas atmosphere, these strips function as the anodes of an avalanching gas counter, producing a gain of ∼103 charges per initially formed electron. With appropriate amplifier and (6) Schroeder, S. L. M.; Moggridge, G. D.; Lambert, R. M.; Rayment, T. J. Phys. IV 1997, 7, 91-96. (7) Schroeder, S. L. M. J. Phys. IV 1997, 7, 153-154. (8) Schroeder, S. L. M.; Moggridge, G. D.; Rayment, T.; Lambert, R. M. J. Mol. Catal. A 1997, 119, 357-365. (9) Stoehr, J.; Jaeger, R.; Brennan, S. Surf. Sci. 1982, 117, 503. (10) Oed, A. Nucl. Instrum. Methods A 1988, 263, 351.
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Figure 2. Cut away sketch of the assembled gas microstrip detector.
Figure 1. (a) Photograph of a GMSD detector plate. Overall size of the active plate is 50 mm × 50 mm. (b) Interleaved array of microstrip (cathodes and anodes) with sketch of the field lines in the drift field between the sample and the detector plate and in the gas avalanching region close to the anode strips.
counting electronics, a GMSD can accept electron impingement rates of 250 kHz/mm2 or 1 MHz/microstrip.10 Figure 1 shows a photograph of a GMSD plate and a sketch of the field lines that result from this particular geometry. The potential difference between the cathode and the anode strips is an important parameter, which defines the shape of the field lines and thus the electron gain in the electron avalanche.10 At low values of the potential difference between the two sets of strips, the gas gain is too low to distinguish signal pulses from electronic noise, while a too-high difference leads to electrical breakdown between the strips that invariably damages the plate. It is therefore important to find the optimum voltages under which to operate the detector for each individual gas mixture. A detailed description of a prototype GMSD for use in X-ray absorption spectroscopy is to be found elsewhere.11 (11) Rayment, T.; Schroeder, S. L. M.; Moggridge, G. D.; Bateman, J. E.; Derbyshire, G. E.; Stephenson, R. Rev. Sci. Instrum. 2000, 71, 3640-3645.
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However, before gas microstrip detectors can be deemed a viable technique for research in catalysis and material science, there are two hurdles that have to be overcome. The first hurdle is that, until now, gas microstrip detectors have only been used with well-understood, “standard” detector gas mixtures, typically small proportions of alkane quencher gases in a noble gas. The second is that they have not been incorporated into a system for the study of catalysts. The first steps toward the design of a reaction cell electron detector are described in the next section, and the following section presents details of the operation of the cell with reaction gas mixtures. Cell Design: Principles and Critique of Previous Work. In initial work on the application of microstrip detectors to electron yield detection in XAS, a prototype GMSD11 was described that consisted of a fixed sample-detector plate incorporated into a die cast box. This arrangement demonstrated the improved surface sensitivity of the detector under standard gas mixture conditions (He/isobutane), but the nature of the die cast box rendered it unsuitable for in situ investigations of chemical reactions in reactive gas mixtures. In addition to the fact that the inside of the box was painted, the detector contained a variety of conducting and insulating materials that could interfere with reactions. Therefore, a new design has been developed, using stainless steel only that also permits rapid sample changing. A schematic isometric view of the new detector assembly is shown in Figure 2. It consists of a stand for mounting the vessel onto a goniometer. A bottom flange holds the sample at the center of rotation. It also accommodates the gas inlet and outlet and eight electrical feed-throughs for sample bias, sample heating, and thermocouples. The detector chamber was machined from a monolithic stainless steel cuboid by spark erosion. Knife-edge sealed CF flanges were then machined into the sides and O-ring seals onto the top and bottom faces. The GMSD itself is mounted on the top flange facing down toward the sample, leaving a gap of 11 mm between the sample and the detector plate. All electrical connections to the GMSD come through this top plate from an electronics enclosure mounted on the outside of the flange. The
Figure 3. (a) Dependence of the pulse height (energy deposit in the counter gas) distribution upon cathode potential for the standard gas mixture of 75% He, 25% isobutane. The anodes are held at ground potential. As the cathode potential is increased, the field in the vicinity become stronger, which leads to increased gas gain. For optimum signal-to-noise ratio, the cathode voltage is increased to move the peak in the distribution above the noise. At the same time as the voltage is increased, there is an increased risk of uncontrolled gas gain at defects in the anode/cathode structures. This leads to arcing and ultimately destruction of the microstrip plate. (b) Gas gain of the standard gas mixture. Table 1 gas gain (%) cathode potential 500 V
chemical characteristic
hydrocarbon containing
standard mixture oxidizing oxidizing reducing reducing reducing wet wet
yes
75% He/25 IB
100
yes no yes no no yes no
90% He/5%O2/5%IB 95% He/5%O2 90% He/5%H2/5%IB 90% He/5%H2/5%CO2 90% He/10%H2 90% He/H2O/10%IB 97%He/3%H2O
50 0 100 30 0 100 100
gas mixture
Figure 4. Pulse height distributions obtained with Mn X-rays as a function of cathode potential for water-containing gas mixtures.
electronics enclosure is made of aluminum (alu-chrome plated) and houses and shields a set of preamplifiers, shaping amplifiers, and the necessary high-voltage and low-voltage power supply filtering. The design is modular to allow easy modification if required. Sample changing takes place though a side flange while the system remains mounted and aligned on the beam line. The sample assembly is mounted on a slide so that realignment is not necessary after changing samples. Samples may be presented in a wide variety of forms by deposition onto flat substrates or by pressing into a recessed plates of sizes between 7 and 25 mm wide and 3 and 80 mm long. Gas Mixtures: Selection Criteria. As stated above, the standard gas mixtures employed in gas proportional detectors are of limited interest for the vast majority of catalytic systems. While the range of possible gas mixtures used in catalysis is infinite, a small number may be identified that encompass a wide range of chemical processes. Oxidation of CO to CO2 and reduction of CO2 to methanol with H2 are two simple, but widely studied and important reactions. Furthermore, epoxidation and metathesis are important classes of reactions that require the presence of organic molecules. In material science, and particular corrosion studies, water is an essential component. Taking these into account, we have undertaken a screening program for suitable gas mixtures involving H2, O2, CO, CO2, H2O, C4H10, and various combinations of these. Table 1 shows the gas mixtures chosen to fit these criteria. The purpose of the program was not to investigate in depth the electron detector physics of any single system but to discover whether conditions could be identified in which a GMSD system
could operate and produce useful data. Although the system is used for the detection of Auger electrons, it is not feasible to test the resolution and gain of the detector using electrons since it is not possible to generate a monochromatic electron source at ambient pressures. Therefore, investigations were carried out using X-ray photons. These were generated using an 55Fe X-ray source (Mn K X-rays, 5.9 keV) and synchrotron radiation (CCLRC Daresbury, Station 9.3). The gas mixture was controlled by Brooks mass flow controllers with an absolute accuracy given as 1% and a repeatability of 0.25% of the flow; the flow rate through the detector was 100 mL/min at atmospheric pressure. RESULTS AND DISCUSSION Pulse Height Distribution and Gas Gain. The fundamental question that must be addressed is whether conditions can be found whereby if an ionizing particle or photon is incident on the detector, then an electronic response is produced that on average is proportional to the energy of the object. The physics of this process is complex and has been investigated in great detail for standard gas mixtures,12,13 but three stages can be identified. First (12) Franzen W.; L. W. Cochran, Pulse ionisation chambers and proportional counters. in Nuclear instruments and their uses; Snell, A. H., Ed.; Wiley: New York, 1956. Rice-Evans, P. Spark, streamer, proportional and drift chambers; Richelieu: London, 1974. (13) For possible side effects and reactions, compare a study of the operation of He filled proportional counters at low temperatures: Masaoka S.; Katano, R.; Kishimoto, S.; Isozumi, Y. Nucl. Instrum. Methods Phys. Res. 2000, B171, 360-372.
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Figure 5. Gas gainsnumber of detected electrons with respect to initially created electronssfor (a) oxidizing and (b) reducing gas mixtures. It is not possible to obtain satisfying results with only He/H2 or He/O2, respectively.
Figure 6. Gas gain for (a) gas mixtures containing different amounts of hydrocarbon and (b) gas mixtures with and without hydrocarbons, showing the significance of a gas with good quenching and stopping properties.
the incoming particle loses its energy by ionization of the detector gas producing electron/ion pairs in numbers proportional to the kinetic energy of the particle. Second, in the vicinity of the anode/ cathode strips, multiplication of the charge takes place via a fieldinduced electron cascade, and third, quenching of unwanted secondary processes takes place to prevent electrical breakdown within the detector. The behavior of the detector was assessed by measuring the distribution of pulse heights for a fixed photon energy input as a function of the potential difference between the anodes and cathodes strips and the drift voltage between the sample and the microstrip plate. The shape of the pulse height distribution (PHD) may be analyzed to calculate the energy resolution of the detector and the gain. A plot of the gain with applied voltage is also a useful tool in assessing the operation of the detector. Figure 3a shows a typical PHD for a standard gas mixture of 75% He, and 25% isobutane (IB), with an 55Fe X-ray source, measured under optimal conditions. This is the standard mixture against which other gaseous environments are measured, and as such, its maximum gas gain is considered to be 100%. It is seen in Figure 3 that as the voltage difference between the anodes and cathodes is increased the peak position of the pulse height distribution increases (more charges per pulse) and moves well clear of the noise of the detector system. Figure 3b shows a wellbehaved almost linear dependence of the logarithm of the gain upon the applied voltage. In comparison, Figure 4 shows the behavior of the GMSD in He gas with 3% water vapor. It can be clearly seen that the performance of the detector in saturated water vapor is comparable to the standard gas mixture condition. No isobutane is required, indicating that water satisfies the requirements for both avalanch6574
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ing and quenching. (The second peak visible at low voltages in the pulse height distributions displayed in Figures 3 and 4 is caused by Al KR auger electrons produced by stray photons from the 55Fe source interacting with the aluminum plate used to form the drift field.) The gain curves for reducing and oxidizing mixtures are shown in Figure 5a and b, respectively. It should be noted that neither of the simplest oxidizing and reducing environments He/H2 or He/O2 yields sufficient gas gain for the GMSD to operate. There are two distinct reasons for this. For the He/H2 mixture, the cross section for photoionization of 5.9-keV photons is simply too low to stop the X-rays within the path length of the detector. Therefore, the detection efficiency is too low. For He/O2, as electrons drift toward the anode they are captured by O2 to form O2- ions, which reduces signal and hence apparent gain. This is a general problem for electronegative atoms in gas counters and can be overcome to a certain extent by increasing the drift potential up to 3 kV to reduce the residence time of ions in the detector. Oxygen is the classic electronegative pollutant in gas counters. He-H2-IB, He-H2-CO2, and He-O2-IB are all suitable. Figure 6 shows the gain curves of hydrocarbon-containing and free mixtures. For carbon monoxide (a), operation was not possible using He/CO only (for the same reasons that apply for O2); gain curves are therefore given for different concentrations of CO and IB. Carbon dioxide, however, gives satisfactory electron amplification without isobutane, but a small amount of IB improves the gas gain significantly (b). Summary of Gas Gain Results. (1) Exchanging helium by hydrogen has very little effect on the gain. (2) CO will not quench. (3) CO and O are about equally electronegative but are workable
Figure 7. Pulse height distributions obtained from measurements of a sample of 15-nm NiO on Ni metal and energy selective windowing: (a) the full pulse height distribution and (b) distribution obtained by low-energy window and (c) high-energy window.
Figure 8. EXAFS spectra of the NiO/Ni sample using high- and low-energy windows referring to surface- and bulk-sensitive measurements in standard gas mixtures and water-saturated He as indicated in the key above.
Figure 9. Overview of the gas gain achieved in various gas mixtures normalized to the maximum gain obtained with the standard detector gas of 75% He and 25% isobutane.
at the 5% level if the drift potential is increased, (4) CO2 is a poor quencher. A small percentage of IB (5%) makes almost anything work! With low-noise electronics it is probably possible to work with any mixture that gives a gain of 50 or greater. Figure 9 gives an overview of the maximum gas gain performed by various gas mixtures with respect to the standard detector gas of 75% He and 25% isobutane. EXAFS Experiments. The XAFS measurements in electronyield mode were carried out at beamline 9.3 (double-crystal Si-
(111) monochromator) of the electron storage ring at the CCLRC Daresbury Laboratory (U.K.), which operated in single bunch mode with ring currents between 8 and 15 mA. The spot size of the X-ray beam at the sample was 100 µm (height) by 6 mm (width), illuminating an area of 12.7 mm2 with the sample inclined at 3° with respect to the incoming X-ray beam. To prove the validity of these experiments under experimental conditions used to perform EXAFS studies with synchrotron radiation, depth profiling experiments were carried out in a standard gas mixture of dry He and IB and also in water-saturated He. We have studied the EXAFS of 20-nm NiO films on a Ni substrate by measuring total electron yield and partial electron yield X-ray absorption spectra. The energy discrimination was done electronically by selecting the energy windows in the counter PHD shown in Figures 7 and 8. A difference in the EXAFS obtained from the electrons from the topmost layers (high-energy region) and from the bulk (low-energy region) can be clearly seen. This enables us to do depth-profiling experiments without destroying the sample as well as to look at the surface region alone while performing in situ measurements of chemical reactions, for example, corrosion in wet environment. CONCLUSIONS It has been found that the GMSD works with a broad variety of gas mixtures suitable for in situ studies of reactions. Significant changes of the gas composition lead to considerable variations in the gas gain and must be avoided during the course of an experiment; hence at present, experiments have to be conducted at steady state. For these experiments, we have used small molecules, but it may be predicted that the detector will work with all volatile hydrocarbons and alcohols, so that the range of applications will be large. Perhaps the most important result of this study is that measurements are possible even in water vapor. Spectra (not shown here) have been collected from wet slurried sampless analysis of these data is still at an early stage and will be reported later. From these measurements we can predict that it will be possible to study emmersed electrodes in contact with an equilibrium water vapor pressure. This would make the technique valuable for the study of hydrated surfaces. In situ studies of catalytic reactions on surfaces will be accessible by combining GMSD and mass spectrometry. The system is robust and facilitates rapid sample turnaround. At present, measurements are made at ambient temperature but work is ongoing to incorporate sample heating and cooling for the GMSD plate. This will be reported in a future publication. ACKNOWLEDGMENT This project has been funded by EPSRC by provision of equipment, a postdoctoral fellowship (A.V.), and a studentship (J.R.I.L.). We thank Dr. Eddie Bateman of Rutherford Appleton Laboratory for his indispensable contribution to this work.
Received for review March 21, 2003. Accepted June 13, 2003. AC034288X Analytical Chemistry, Vol. 75, No. 23, December 1, 2003
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