beamlines
SUT-NANOTEC-SLRI beamline for X-ray absorption spectroscopy
aSynchrotron Light Research Institute, 111 University Avenue, Muang, Nakhon Ratchasima 30000, Thailand, bFaculty of Science, Kasetsart University, 50 Ngam Wong Wan Road, Chatuchak, Bangkok 10900, Thailand, cSchool of Physics, Suranaree University of Technology, Muang, Nakhon Ratchasima 30000, Thailand, and dNational Nanotechnology Center, National Science and Technology Development Agency, 111 Thailand Science Park, Phahonyothin Road, Klong Luang, Pathum Thani 12120, Thailand
*Correspondence e-mail: wantana@slri.or.th
The SUT-NANOTEC-SLRI beamline was constructed in 2012 as the flagship of the SUT-NANOTEC-SLRI Joint Research Facility for Synchrotron Utilization, co-established by Suranaree University of Technology (SUT), National Nanotechnology Center (NANOTEC) and Synchrotron Light Research Institute (SLRI). It is an intermediate-energy 8 to 2 × 1010 photons s−1 (100 mA)−1 varying across photon energies. Details of the beamline and instrumentation are described. To demonstrate the beamline performance, K-edge XANES spectra of MgO, Al2O3, S8, FeS, FeSO4, Cu, Cu2O and CuO, and spectra of Cu and CuO are presented.
beamline at SLRI. The beamline delivers an unfocused monochromatic X-ray beam of tunable photon energy (1.25–10 keV). The maximum normal incident beam size is 13 mm (width) × 1 mm (height) with a of 3 × 10Keywords: XAS beamline; X-ray absorption spectroscopy; Mg K-edge XANES; Al K-edge XANES; S K-edge XANES; Cu K-edge XANES; intermediate-energy X-ray beamline.
1. Introduction
et al., 2012) of the Synchrotron Light Research Institute (SLRI) (Nakhon Ratchasima, Thailand), was successfully commissioned and opened publicly for academic and industrial users in 2006. After a few years of operation, the number of proposals requesting beam time rapidly increased, resulting in inadequate beam time allocation. In 2009, Suranaree University of Technology (SUT), National Nanotechnology Center (NANOTEC) and SLRI jointly supported and launched a program to establish a central laboratory inside SLRI called SUT-NANOTEC-SLRI Joint Research Facility for Synchrotron Utilization, to provide a collaborative platform to utilize synchrotron techniques for researchers of the three parties and their collaborators. The program plan was to construct another beamline at SLRI as the flagship under the name SUT-NANOTEC-SLRI beamline to be utilized in a time-sharing fashion among the three parties. The beamline was successfully constructed and commissioned in 2012. One-third of the beam time under SLRI quota is given to general users (both domestic and international) of SLRI via a regular beam time application, whereas one-third of the beam time is distributed by SUT for their faculties, researchers and students and another one-third of the beam time is distributed by NANOTEC for their in-house researchers and collaborators. During the past four years, research output from the beamline has already resulted in 64 publications in international scientific journals. For the last two years, most of the research carried out at the beamline has involved investigations on valence states and local atomic structures in magnetic materials (e.g. Wongsaprom et al., 2015; Sonsupap et al., 2016; Kidkhunthod et al., 2016; Luadthong et al., 2016), dielectric materials (Boonlakhorn et al., 2015; Jumpatam et al., 2016; Meeporn et al., 2016; Wongmaneerung et al., 2016) and catalysts (Itthibenchapong et al., 2015; Srifa et al., 2015; Sanetuntikul et al., 2015; Witoon et al., 2016; Ketwong et al., 2016).
emerged as a new advanced characterization technique for material research in Thailand when the first beamline, namely Beamline 8 (KlysubunThe SUT-NANOTEC-SLRI beamline is classified as an intermediate-energy X-ray beamline working in the photon energy range 1.25–10 keV. It allows one to study the gives examples of such beamlines with their technical specifications and references. Some of them have ceased operations. Similar to the DCM beamline at CSRF and the BL7A beamline at UVSOR, our beamline is equipped with a double-crystal monochromator (DCM) as the sole optical component for scanning photon energies while the other beamlines additionally employ X-ray mirrors for collimating and focusing their X-ray beams. The basic design of our DCM allowed in-house fabrication, use of different crystal types and a simple optical alignment. The installation of crystals and the fine-tuning process takes a shorter time (∼1 h) than ordinary commercial DCMs. In the soft X-ray regime, the measurement becomes more challenging due to a much shorter (compared with hard X-rays) through the instrumental windows and sample environment. Therefore, the station was necessarily optimized for low-energy (soft) X-rays. The following sections provide the technical details of our beamline and its station; beam characteristics (photon and energy resolution); the quality of XANES and data demonstrated at the K-edges of magnesium, aluminium, sulfur and copper; and conclusions.
of magnesium, aluminium, silicon, phosphorus, sulfur, chlorine, potassium, calcium and the first-row transition elements. There are only a small number of beamlines at other synchrotron facilities working in similar energy ranges. Table 1
‡Calculated at 10 keV. §Measured at 3.2 keV. ¶Ceased operation. |
2. Beamline
The SUT-NANOTEC-SLRI beamline is connected to the vacuum chamber of bending magnet number 5 (BM5) of the SLRI 1.2 GeV synchrotron storage ring. The longitudinal source position of BM5 observed by the beamline is 7 cm after the BM5 edge, where the electron beam is deflected by 1.46° in the maximum 1.44 T magnetic field of BM5. In a user-service operation, the stored electron-beam current decays from 150 mA to 70 mA in 11 h. Fig. 1 shows a schematic view of the beamline. The first vacuum chamber is the heat absorber chamber (ABS) housing a water-cooled mask and a water-cooled copper block. The aperture size of the mask is 15 mm (width) × 7 mm (height) for passing a white beam of 6.5 mrad (h) × 3 mrad (v). Behind the ABS chamber, an attached side branch at the zero degree line is reserved for utilizing future insertion-device radiation. A water-cooled horizontal slit is installed behind the ABS for adjusting the white-beam width for downstream components. The white beam and Bremsstrahlung radiation can be shut off by a pneumatic drive lead block in the beam shutter chamber (BS). Four fluorescence screen monitor (FSM) chambers and an X-ray beam position monitor (XBPM) chamber are used for beam position measurements.
The DCM was in-house fabricated according to a Lemonnier design (Lemonnier et al., 1978). The water-cooled first crystal and the uncooled second crystal are on a motorized rotation stage (Newport RV120HAHLT) driven by a motion controller (Newport MM4006). The center of rotation is on the surface of the first crystal. Several types of crystals are used in the DCM: KTiOPO4(011), KTP (2d = 10.955 Å) covering 1.25 keV to 4.78 keV; InSb(111) (2d = 7.481 Å), 1.83 eV to 7.00 keV; and Ge(220) (2d = 4.001 Å), 3.44 keV to 10.00 keV. The (θB) can be scanned from 13.7° to 65.0° with a minimum increment (δθB) of 0.001°. Using the differential form of Bragg's equation, δE/E = δθBcotθB, a minimum energy step (δE) of our DCM can be determined for a given photon energy. For example, Mn K-edge (6539 eV) spectra can be measured with actual energy steps of 0.2 eV, with good reproducibility. A mechanical cam, on which the second crystal travels, provides a vertical offset of 3.0 cm. Optical alignment of the two crystals is carried out with motorized manipulators adjusting pitch angles of both crystals and roll angle of the second crystal. Energy resolution of the monochromatic X-ray beam is partly controlled by an entrance slit in the DCM, which is normally opened at 0.77 mm, corresponding to the angular beam divergence of 82 µrad in the vertical direction.
The vacuum of the beamline is maintained with three −1) in the two front-end sections, an ion pump (300 l s−1) attached to the FSM3 chamber, and two turbomolecular pumps (250 l s−1) attached to the XBPM and DCM chambers. The differential pressure of four orders of magnitude between the front-end ultra-high vacuum (2.6 × 10−10 torr) and the DCM high vacuum (4.0 × 10−6 torr) is achieved using two small ducts of low conductance (LCD1 and LCD2). The lengths of LCD1 and LCD2 are 230 mm and inner dimensions are 17 mm (width) × 8 mm (height) and 19.6 mm (width) × 8.6 mm (height), respectively. LCD1 and LCD2, installed between the vacuum chambers (BS, FSM3 and XBPM), function as two differential pumping stages (Renier & Draperi, 1997; Klysubun et al., 2007), and thus no beryllium (Be) window, which strongly absorbs low-energy X-rays, is needed. A polyimide window (PW) (7.5 µm thick) is installed between the DCM chamber and the first of the station. The inner diameter of the window is 13 mm, thus giving an angular beam divergence of 1.3 mrad in the horizontal direction. The beamline is operated with an interlock system using a programmable logic controller (Omron CS1G), which allows only safe opening and closing sequences of the pneumatic valves (PVs), the lead shutter in the BS chamber and the water-cooled copper absorber in the ABS chamber. The self-closing sequence would automatically turn on in case of vacuum failure detected by four cold cathode gauges and a full range gauge.
(pumping speeds of 500 l s3. Experimental station
A photograph of the . measurements can be conducted in either transmission or fluorescence-yield modes. The intensities of the incident X-ray beam (I0) and the transmitted beam (It) are measured by the first (IC1) and second (IC2) gas-filled ionization chambers, respectively. The IC1 and IC2 chambers require 300 V DC to generate electrical currents when the X-ray beam passes through them. The full beam size at the sample chamber is 13 mm (width) × 1 mm (height). Polypropylene windows (PPWs) (6 µm thick) are used to seal the filled gas (nitrogen or argon) in the IC1 and IC2 chambers. Polypropylene is chosen because of its low X-ray absorption. The gas pressure is specified according to Klysubun et al. (2012). In the transmission mode, the absorption (μx) of a probed element in a sample is determined by μx = ln(I0/It), where μ is the and x is the sample thickness (Bunker, 2010). For measurements below 5 keV, the sample chamber needs to have helium gas flowing in it to reduce air absorption. The sample thickness is recommended to be around one absorption length (x = 1/μ). At this thickness, ln(I0/It) = 1 or It/I0 = 1/e, and thus the sample absorbs 63% of the incident X-ray beam intensity (Calvin, 2013).
station is shown in Fig. 2At 45° to the sample surface, and perpendicular to and in the same plane as the incoming X-ray beam, a four-element silicon drift (4SDD) detector (Vortex-ME4), with a 12.5 µm-thick Be window, is installed for fluorescence-mode ISCA, of the Kα, Lα or Mα emission lines from a probed element in a sample. The dead-time of the 4SDD detector is corrected using the ratio of input count rate (ICR) to output count rate (OCR) recorded by the digital X-ray processor (Woicik et al., 2010), If = ISCA × ICR/OCR. The absorption is given by μ = If/I0, which is only valid for diluted samples and thin concentrated samples (Calvin, 2013). For measurements of X-ray energy below 5 keV, the detector is attached to the sample chamber and operated under a steady flow of He gas.
The detector is integrated with a digital X-ray processor (XIA DXP-XMAP) and configured to selectively collect the fluorescence signal,The signal currents (I0 and I1) from each typically having a value around 0.1 to 100 nA, are fed to current amplifiers (Keithley 428). With a proper gain used (107 V A−1 to 109 V A−1), the output voltage is fed to a voltage-to-frequency converter (Nova N111VTF) to generate signal pulses at the rate of 0.1 MHz V−1. A counter/timer device (NI PCI-6602) is used to record the number of signal pulses in a given dwell time synchronized with counting of If. A LabVIEW data acquisition program has been developed in-house to control all of the electronics and DCM scanning during the data collection.
4. Performance for X-ray absorption spectroscopy
This section reports the Introduction, some test samples and standards were measured to demonstrate the quality of the XANES and data.
and photon energy resolution of the monochromatic X-ray beam delivered by the beamline. Besides research samples, data of which can be found in the literature given in the4.1. and photon energy resolution
We used the first ) to measure the experimental It was filled with nitrogen gas or argon gas at a pressure so as to absorb 10% of the and generate an ionization current (Klysubun et al., 2012). Based on the theory of (Knoll, 2010; Samson, 1999; Bunker, 2010), the relationship between the ionization current (I) and the (f) is expressed by
IC1 (Fig. 2where e is the σ is the absorption of the gas, ρ is the gas density, L is the absorption path length in the E is the photon energy, and w is the average energy needed to create an electron–ion pair. The σ data were reported by Henke et al. (1993) and are available online through the Center for X-ray Optics (https://henke.lbl.gov/optical_constants). The w values for nitrogen and argon gases are 34.8 eV and 26.4 eV, respectively (Thompson & Vaughan, 2001). The gas density can be determined by the law for a given gas pressure and gas temperature.
Fig. 3 shows the experimental of the full-sized monochromatic X-ray beam (13 mm × 1 mm) as measured by the IC1 chamber. In general, our beamline experimentally gives a of 3 × 108 to 2 × 1010 photons s−1 (100 mA)−1. At the magnesium K-edge (1303 eV) and aluminium K-edge (1560 eV), for which the KTP(011) crystals are employed, the beamline delivers an experimental of 9.1 × 108 and 1.5 × 109 photons s−1 (100 mA)−1, respectively. The maximum of 2.2 × 1010 photons s−1 (100 mA)−1, at the sulfur K-edge (2472 eV), is achieved with InSb(111) crystals. It should be noted that photon fluxes of the same order of magnitude are also available for studying phosphorus (2146 eV) and chlorine (2822 eV). Since InSb(111) crystals absorb X-rays at the L-edges of indium (3730, 3938 and 4238 eV) and antimony (4132, 4380 and 4698 eV), we alternatively use Ge(220) crystals for scanning photon energies from 3500 eV to 10000 eV. In this region, the experimental ranges from 3.3 × 108 to 8.2 × 109 photons s−1 (100 mA)−1 and decreases with increasing photon energies. The photon fluxes of our beamline are comparable with those of the beamlines DCM, BL1A and BL7A (Table 1). It is noteworthy that the performance of a beamline is limited by the of its X-ray source (bending magnet or insertion device). In general, the of a bending magnet or an insertion device is determined by its electron beam emittance and storage ring energy. In terms of our bending magnet BM5 is most comparable with the bending magnets of beamlines DCM and BL1A while the other beamlines take advantage of the brighter X-ray sources.
To determine the theoretical Shadow software (Sanchez del Rio et al., 2011). An X-ray source, consisting of 25000 rays, was simulated with spatial and angular distributions corresponding to the BM5 bending magnet and the SLRI storage ring specifications. Two optical components were defined as reflectors having perfect flat surfaces and diffraction properties of InSb(111) and Ge(220). It should be noted that Shadow does not implement a diffraction model for KTP crystals. The horizontal and vertical acceptances of the beamline (1.3 mrad × 82 µrad) and optical transmission of the polyimide window (thickness of 7.5 µm) were also included in the calculation. The theoretical as a function of photon energy is shown in Fig. 3. It is higher than the actual measurement by a factor of 1.1 to 1.3. This indicates some loss of in the real optical configuration. An imperfect crystal surface resulting from a polishing procedure and a slope error introduced by crystal mounting probably decrease the of our beamline, as is the case for the SXRMB beamline at CLS (Table 1) during its preliminary commissioning (Hu et al., 2010). Therefore, it will be beneficial to evaluate such possible influences for further improvement of our beamline in terms of the photon flux.
a ray-tracing calculation was carried out using theAt a given θB), a monochromatic X-ray beam diffracted by a beamline DCM has a certain photon energy distribution, E ± ΔE/2, and the percentage of the energy width (ΔE/E) represents a respective energy resolution of the beamline. To experimentally determine ΔE, a rocking curve measurement is commonly performed by finely scanning (rocking) pitch angles of one of the DCM crystals and recording the diffracted intensity of the monochromatic beam as a function of the rocking angle (Abbate et al., 1999; Kitamura et al., 2003). An observed angular width of the rocking curve (Δθ) is expressed as Δθ = (ΔE/E)tan(θB) in the differential form of the Bragg equation. Due to the fact that our DCM is incapable of measuring a rocking curve as the pitch angles of the second crystal are not calibrated, an alternative measurement, proposed by Lytle et al. (1984), was used and can be described as follows. A XANES spectrum of argon gas was measured as shown in Fig. 4. The white-line peak, seen at 3206.5 eV, is attributed to the electronic transition from the K (1s) level to the N (4p) level (Watanabe, 1965), thus its theoretical width (ΔEth) is the sum of the natural widths of the atomic K and N levels; ΓK + ΓN. For Ar gas, ΓK is 0.68 eV whereas ΓN is much smaller than ΓK and thus negligible (Krause & Oliver, 1979). Therefore, ΔEth of the Ar gas white line is approximately 0.68 eV. It can be seen that the observed width (ΔEobs) is larger than ΔEth. Fitting the white-line peak with a Lorentzian function (Watanabe, 1965), ΔEobs is equal to 1.3 eV. The observed width is approximately a convoluted width of ΔEth and the instrumental width (ΔE) of our beamline; = + ΔE2. Therefore, ΔE can be deducted and is equal to 1.1 eV (solid square in Fig. 6). As a result, the experimental energy resolution of our beamline at 3207 eV is approximately 3.4 × 10−4. It is noteworthy that our energy resolution is clearly better than that of the LNLS SXS beamline (8 × 10−4) at the same energy (Table 1).
(Likewise, a XANES spectrum of KMnO4 can be used to determine ΔE at the Mn K-edge. As shown in Fig. 5, the sharp pre-edge peak (6543 eV) is attributed to the 1s → 3d electronic transition (Tolentino et al., 2001). Its theoretical width is 2.66 eV, the sum of the natural width of the Mn K level (1.16 eV; Krause & Oliver, 1979) and the natural width of the 3d (1.5 eV; Tolentino et al., 2001). The observed width (FWHM) is 3.39 eV as shown in Fig. 5. The instrumental width of the beamline can be deconvoluted from the observed width and is equal to 2.10 eV (open square in Fig. 6). This corresponds to an energy resolution of 3.2 × 10−4 at 6543 eV. We expect the broadening effects on the K-edge XANES spectra to arise significantly from the instrumental energy widths of the beamline as they are larger than the natural widths of the atomic K levels (solid circles in Fig. 6). On the contrary, the natural widths of the atomic L3 levels (open circles in Fig. 6) are primarily responsible for the broadening of the L3-edge XANES spectra. The ray-tracing calculations also predict an instrumental width of the beamline as a function of photon energy, in the case of the perfect optical configuration as discussed previously. The ideal instrumental widths are shown in Fig. 6 by a dashed line and a solid line for InSb(111) and Ge(220), respectively. They are considerably smaller than the actual instrumental widths. A further investigation is needed to identify causes of error affecting the energy resolutions of the beamline and whether they relate to the reduction of the photon flux.
4.2. Magnesium and aluminium K-edge XANES
XANES measurements in the energy range of the K-edges of magnesium (1303 eV) and aluminium (1560 eV) are challenging because the low-energy photons are easily absorbed by a sample and its mounting materials. Preparing a transmission sample becomes difficult, if not impossible, since the thickness of one absorption length is about 1 µm or even smaller. For example, such thicknesses are 1.1 µm and 0.7 µm for Mg metal and MgO, respectively. When a powder sample is used, a supporting tape such as polyimide tape, on which the sample is spread, further reduces the photon intensity. The polypropylene window of the IC1 chamber also absorbs the by about 40% at 1303 eV. Therefore, such a XANES experiment at our beamline can only be carried out in the fluorescence-yield mode.
Fig. 7 shows Mg K-edge XANES spectra of pure MgO (Mg concentration of 600 mg Mg g−1) and diluted MgO (60 mg Mg g−1). Each spectrum was averaged from five scans. Each scan was collected with energy steps of 0.3 eV and a dwell time of 30 s for each energy. A photon energy calibration was performed using the white-line energy of MgO at 1311.3 eV (Li et al., 1999). The diluted MgO was prepared by mixing 10 mg of MgO with 90 mg of boron nitride (BN) using a mortar and pestle. The pure and diluted MgO samples were pressed into pellets for the XANES measurements. Denoted by A, B, C, D and E in Fig. 7, all characteristic peaks of MgO are clearly recognized in our XANES spectrum and correspond to those recorded by Aritani et al. (2001) and Takata et al. (2001) using the BL7A and BL1A beamlines at UVSOR, respectively (Table 1). It should be noted that their XANES measurements were carried out in a vacuum using electron-yield modes. The energies of the peaks A, B, C, D and E, in our MgO spectrum, are 1311.3 eV, 1313.8 eV, 1317.1 eV, 1319.3 eV and 1329.4 eV, respectively, whereas those of the BL7A spectrum are shifted by −13.7 eV ± 0.4 eV due to a different photon energy calibration used. In terms of signal-to-noise ratio, our MgO data are as noisy as the BL1A data and noisier than the BL7A data. However, we can correctly identify the main features of MgO.
Experimental noise appears as random fluctuations in the normalized spectrum of pure MgO (Fig. 7). The magnitudes can be estimated by the standard deviation (σ) of normalized μ in a pre-edge region of 1292 eV to 1305 eV. Consequently, σ was found to be 0.039 shown as an error bar in Fig. 7. The signal-to-noise ratios of the peaks A, B, C, D and E, as estimated by their μ/σ values, are 57, 47, 60, 59 and 39, respectively. Using the same evaluation for the normalized spectrum of the diluted MgO, σ is 0.076, and the signal-to-noise ratios of the peaks A, B, C, D and E are 31, 22, 31, 33 and 19, respectively. It is noteworthy that the signal-to-noise ratios are not linearly decreased with the decrease of Mg concentration.
Fig. 8 shows Al K-edge XANES spectra of diluted Al2O3 with Al concentrations of 5.3 mg Al g−1 and 53 mg Al g−1. Each spectrum was averaged from five scans. Each scan was collected with energy steps of 0.3 eV and a dwell time of 30 s for each energy. A photon energy calibration was performed using the K-edge energy of Al metal at 1560 eV (Li et al., 1995). The diluted Al2O3 samples were prepared, similar to the case of MgO. We mixed 1 mg of Al2O3 with 99 mg of BN to yield 5.3 mg Al g−1, and 10 mg of Al2O3 with 90 mg of BN to yield 53 mg Al g−1. Each diluted Al2O3 sample was pressed into a pellet for the XANES measurements. Signal-to-noise ratios of the two spectra of 5.3 mg Al g−1 and 53 mg Al g−1 are approximately 57–73 and 170–227, respectively. Our diluted Al2O3 spectra are qualitatively similar to the Al K-edge spectra of pure Al2O3 (530 mg Al g−1) previously published by Li et al. (1995), Takata et al. (2001) and Manuel et al. (2012). Their spectra were measured in electron-yield modes using the beamlines DCM, BL1A and LUCIA, respectively (Table 1). According to an interpretation by Li et al. (1995), two strong peaks seen in our spectra at 1568.7 eV and 1572.4 eV (denoted by A and B in Fig. 8) are attributed to the 1s → t1u and 1s → t2g electronic transitions, respectively, whereas a weak peak at 1576.8 eV (denoted by C) arises from a multiple-scattering effect. The energies of the peaks A, B and C reported by Li et al. (1995) are lower than ours by 0.3 eV, 0.8 eV and 0.9 eV, respectively. It is noteworthy that, at the lower Al concentration (53 mg Al g−1), we were able to record the Al2O3 spectrum slightly noisier than the Al2O3 spectra of higher Al concentration (530 mg Al g−1) from those beamlines. In addition, the signal-to-noise ratios of the Al2O3 (5.3 mg Al g−1) spectrum are close to those of the MgO (60 mg Mg g−1) spectrum, and therefore our beamline is more efficient for XANES measurements at the Al K-edge than it is for the Mg K-edge. This is basically due to the higher available at the Al K-edge (Fig. 3).
4.3. Sulfur K-edge XANES
Sulfur is one of the most abundant elements existing in a large variety of organic and inorganic species with formal oxidation states of sulfur ranging from −2 to +6 (Prietzel et al., 2011). The SUT-NANOTEC-SLRI beamline has the highest at the sulfur K-edge, and is thereby powerful for sulfur speciation. For pure standards and concentrated samples, S K-edge XANES spectra can be measured in the transmission mode, for which the ideal thickness of one absorption length (x = 1/μ) can be practically prepared using powder samples. Fig. 9 shows S K-edge XANES spectra of S8 and FeSO4.7H2O measured in transmission mode. Characteristic peaks of S8 and FeSO4.7H2O are clearly visible and their spectral shapes resemble those reported by Dann et al. (1998), Figueiredo & Silva (2009) and Prietzel et al. (2011). Their S K-edge XANES spectra were measured at the DCMSX beamline at SRRC, the ID21 beamline at ESRF and the BL8 beamline at SLRI (Klysubun et al., 2012), respectively.
To demonstrate the performance of our beamline for sulfur speciation, we used a test sample (T1) consisting of three oxidation states of sulfur (−2, 0 and +6). The sample T1 was prepared by mixing 6 mg of FeS, 2 mg of S8 and 2 mg of FeSO4.7H2O. Therefore, the nominal atomic percentages of S2−, S0 and S6+ in sample T1 are 0.50, 0.45 and 0.05, respectively. A four-digit balance was used for weighing, and a mortar and pestle were used for grinding the mixture. The sample T1 was diluted using 1 mg of T1 and 99 mg of BN and mixed with a mortar and pestle. Finally, the diluted mixture was pressed into a pellet for the XANES experiment in the mode. The three standards (FeS, S8 and FeSO4.7H2O) were diluted, prepared in the same way and measured under the same experimental conditions, as is the case for the diluted sample T1. We used a linear combination fitting (LCF) tool in the Athena software (Ravel & Newville, 2005) to determine the percentages of sulfur atoms in each present in sample T1. The S K-edge XANES spectra of the three standards were used as fitting components. Fig. 10 shows the best-fit curve of T1 as the sum of its LCF components scaled by the atomic percentages of sulfur (0.46FeS, 0.44S8 and 0.10FeSO4.7H2O). The best-fit curve reproduces very well the experimental T1 spectrum with a very small fractional misfit factor of 0.00035. The LCF speciation results are very close to nominal sulfur percentages. The slight discrepancy is probably due to inaccuracy of the four-digit balance and non-uniformity of the T1 sample. Particle size and concentration effects from the sample-preparation procedure can also introduce distortion of the XANES data (Morgan et al., 2009). However the accuracy of our S speciation is markedly better than that reported by Prietzel et al. (2011) for a large set of different mixtures.
4.4. Copper K-edge XANES and EXAFS
Fig. 11 shows Cu K-edge XANES spectra of Cu metal (5 µm-thick foil), Cu2O and CuO. The data were collected in the transmission mode. The fine structures are clearly observed in all samples and consistent with those previously reported (Du et al., 2015; Yalovega et al., 2016; Klysubun et al., 2015; Gaur et al., 2009). The insert of Fig. 11 shows the shoulder peaks (8981.3 eV) of Cu metal recorded by Du et al. (2015) at the XAFCA beamline (Table 1) and Ravel & Newville (2005) at the X23A2 beamline at the National Synchrotron Light Source (Golovchenko et al., 1981), in comparision with our data. Our peak is as broad as the XAFCA peak and considerably broader than the X23A2 peak. Therefore, the energy resolutions of our beamline given by Ge(220) crystals and the XAFCA beamline by Si(111) are approximately equal at the Cu K-edge. It should be noted that the better energy resolution of X23A is achieved by Si(311). However, the lower energy resolution of our beamline is clearly sufficient for detecting the sharper shoulder peak (8982.7 eV) of the 1s → 4p electronic transition in Cu2O. To demonstrate the performance of our beamline, K-edge spectra of Cu metal and CuO were measured in the transmission mode. Fig. 12 shows the Fourier-transformed spectra of the Cu metal and CuO in comparison with the data from beamlines XAFCA and X23A2. The Fourier transformation (FT) was carried out using the Athena software (Ravel & Newville, 2005) with the same FT parameters: Hanning window, k-range of 3 to 12 Å−1, k2 weight, dk = 2 Å−1 and Rbkg = 1. It can be seen that the data from the three beamlines are consistent in terms of observed peak position and magnitude in the R-space. For quantitative assessment, the fractional mismatch between our data, |χ(R)|, and those from the other two beamlines, |χ(R)|′, was calculated as ∑[|χ(R)| − |χ(R)|′]2/∑|χ(R)|2, where the summation was computed over all data points between R = 0 and R = 6 Å. In the case of the Cu metal the resultant fractional mismatch was 0.00075 and 0.0042, for the XAFCA and X23A2 data, respectively. In the case of the CuO the fractional mismatch was 0.0192 for the X23A data. Even though the of our beamline at the Cu K-edge is considerably lower than those of the XAFCA and X23A2 beamlines, we have demonstrated that our data are quantitatively comparable with the data from the two high-performance beamlines.
5. Conclusions
The design and specifications of the SUT-NANOTEC-SLRI beamline and its 8 to 2 × 1010 photons s−1 (100 mA)−1] and photon energy resolutions given by the KTP(011), InSb(111) and Ge(220) DCM are practical for conventional measurements as shown in Figs. 7–12. For the Mg and Al K-edges, the XANES data can be collected in fluorescence-yield mode with recommended concentrations higher than 60 mg Mg g−1 and 5.3 mg Al g−1. For measurements at the K-edges of Si, P, S, Cl, Ca and the first-row transition elements, both transmission and fluorescence-yield modes can be applied depending on sample characteristics. We have demonstrated that the SUT-NANOTEC-SLRI beamline and its station can provide XANES and data in good agreement with those from the other beamlines of comparable energy ranges (Table 1) and the hard X-ray beamline X23A2. Therefore, this paper can be used as a reference point and introduces a choice of intermediate-energy X-ray beamlines for the international users' community.
station have been reported. The working photon energy range is 1.25 to 10 keV. The available photon fluxes [3 × 10Acknowledgements
The authors acknowledge Denchay Bumrungkoh and Samrerng Duangnil for supervision of the in-house fabrication of the beamline and
components. Beamline control software has been developed by Nattawat Yachum and Narupon Wongprachanukul. Sompin Mahakot, Somboonsup Rodporn, and Dechmongkhon Keawsuwan are thanked for the measurements. Yonghua Du and Ping Yang are acknowledged for providing the data of Cu metal from the XAFCA beamline. Bruce Ravel is acknowledged for providing the data of Cu metal and CuO from the X23A2 beamline at NSLS, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-AC02-98CH10886. Garnet Hoyes and Professor Jörg Prietzel are acknowledged for grammar correction. This research was funded by SUT-NANOTEC-SLRI Joint Research Facility for Synchrotron Utilization.References
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