Light source

The upstream of a 12 m long-canted long straight section in the SSRF was selected as the light source for the tender energy spectroscopy beamline, and the downstream section (3.06 m long) was used for the fast X-ray imaging beamline (BL16U2) [32]. An U26 IVU with a 3.2 m length, 26 mm period, and 6 mm minimum gap was chosen as the light source. Detailed information regarding the undulator of the BL16U1 beamline is provided in Table 1. The maximum magnetic field strength exceeded 1.02 T, with a total power greater than 7.7 kW. By tuning the gap from 6 mm to 15 mm, 1–7^th harmonics and X-ray energy ranges from 2.1 keV to 16 keV can be generated.

Synchrotron radiation from an undulator source (planar mode) has high brilliance and a narrow spectral range, which is too narrow to be used in EXAFS experiments. The EXAFS technique requires a high photon flux and a spectral range of approximately 1000 eV. In the IVU design, the taper mode is typically used to expand the bandwidth of the undulator source for EXAFS detection [33]. In the taper mode, the two out-vacuum girders are tilted. In BL16U1, with a maximum gap taper adjustment range of 0.5 mm, indicating a reproducible mechanical gap difference between the exit and entrance gaps (±0.5 mm [34]), an EXAFS above 5 keV can be obtained. In SSRF, “Taper mode” is also used by the BL15U beamline [5]. However, the taper mode widens the spectrum at the expense of reducing the brilliance of the undulator [33]. We now use the taper mode at the non-focusing end-station for EXAFS detection and the planar mode (taper=0) at the microprobe and sub-microprobe end-stations for focusing. The gap-scan mode, in which the gaps of IVU are adjusted according to energy, will be used in the future.

Beamline optics

The primary optical layout of the beamline is shown in Fig. 1. A toroidal mirror, liquid-nitrogen-cooled double-crystal monochromator, high-harmonic rejection mirror, and two pairs of Kirkpatrick–Baez (KB) mirrors were installed on the beamline. The details of all beamline mirrors are listed in Table 2. The layout of the beamline is similar to that of the hard X-ray microfocusing beamline (BL15U1) at the SSRF [5] and the microfocus spectroscopy beamline (I18) at Diamond [24]. A horizontally deflecting toroidal mirror (FMB Oxford), achieved by mechanically bending a sagittal cylindrical mirror, was placed 35 m from the source. A set of water-cooled slits (Slit1, Fig. 1), placed 26 m from the source, were used to define the incoming beam on a toroidal mirror. Considering the effective length, reflectivity, and heat load, the toroidal mirror was water-cooled and operated at a grazing incidence angle of 3.5 mrad with an active area of 800 mm. The Rh coating on a single-crystal Si substrate was designed for high energies above 8 keV, and the Si coating was designed for photon energies below 8 keV. The two coatings could be switched using an in-vacuum translation mechanism. Typically, to simplify the beamline adjustment, only the Rh coating is used for the entire energy range from 2.1 keV to 16 keV. Using a toroidal mirror, the beam in the vertical plane is collimated, and the influence of vertical source divergence is removed. Thus, energy resolution is primarily a function of the bandpass of the crystals used in the monochromator. In the horizontal plane, the beam was focused onto a secondary source using a mechanical elliptical bend. The secondary slit (MS1 in Fig. 1), placed 48 m away from the light source, was served as the secondary source. The secondary source was used for the horizontal focusing optics of the KB mirrors after the monochromator.

Fig. 1Full-screen View

(Color online) Schematic showing the principal elements of the beamline

Owing to the high power density of the undulator, a monochromator was installed after the toroidal mirror. A fixed-exit double-crystal monochromator (DCM, TOYAMA) was placed approximately 38 m away from the light source. Photon energies between 2.1 keV to 16 keV with resolution below 1.64×10^-4 (ΔE/E @2.5 keV) can be obtained with Si (111) crystal sets. Another set of Si(220) crystal was also applied for better energy resolution, with photon energies ranging from 3.35 keV to 16 keV. The crystals were translated using an in-vacuum translation mechanism. Owing to the high power density of the undulator source, the first and second crystals were indirectly cooled using liquid nitrogen because of the high power density of the undulator source. The fixed-beam exit was maintained by vertically translating the second crystal. The final height difference was selected to be 25 mm. To cover the required energy range, the monochromator has a high angular range from 0° to 75°. To maintain the alignment of the first and second crystal lattice planes over this angular range, two coarse motors, ±12 mrad and ±8 mrad, were used for roll and pitch coarse adjustment, and two piezo actuators (±0.2 mrad) were used for fine adjustment of the roll and pitch motors.

Two sets of monochromatic four-knife slits without water-cooling were installed downstream of the monochromator. The layout of the beamline slit was similar to that of the microfocus spectroscopy beamline (I18) on Diamond [24]. The first monochromatic four-knife slit (MS1; Fig. 1), 10 m away from the monochromator, serves as the secondary source for focusing optics in the horizontal direction. At this point, the largest slit size is 350 μm×1400 μm (h × v). Another monochromatic four-knife slit (MS2; Fig. 1), 4 m away from MS1, was used to remove the scatter of the beam at the non-focusing end-station and limit the horizontal and vertical beam sizes onto the KB and smaller KB (SKB) mirrors. At this point, the largest slit size is 1600 μm×1400 μm (h × v). The slit position was fixed; however, the slit size was controlled using a parallelogram mechanism. The slit size can be changed according to the different spot sizes at the three end-stations.

A harmonic rejection mirror (HRM, TOYAMA) is placed at 51 m from the source. A pair of horizontally reflecting flat silicon mirrors was used to reject higher harmonics. The mirrors had three stripes of chrome (Cr), silicon (Si), and rhodium (Rh), which were vertically translated under vacuum. The Cr reflector can be used for 2.05 keV to 3.5 keV with a grazing incidence angle of 10 mrad. The Si reflector can be used for 3.5 keV to 7.5 keV with a grazing incidence angle of 3.5 mrad. The Rh reflector can be used for 7.5 keV–13 keV with a grazing incidence angle of 3.5 mrad. The grazing incidence angle was regulated by two horizontal vacuum motors installed upstream and downstream of the mirrors. In addition to the three coatings that reflect the X-ray beam, the mirrors can be moved out of the beam in vacuum translation to ensure that the incoming X-rays pass through without being reflected. In our beamline, owing to the use of Rh coating in the toroidal mirror, the HRM is usually moved out of the beam for energies above 8 keV.

Non-focusing end-station

The non-focusing end-station is housed in a vacuum vessel, enabling operation in vacuum (1 mbar to 1×10^-6 mbar) or He atmosphere. No load-lock system was used for sample replacement in the non-focusing end-station. Usually, only the dry pump is turned on, and a vacuum of 1 mbar is sufficient for this non-focusing end-station. He gas was purged into the vessel when water was present in the samples, and no vacuum was used. The dry pump and turbo pump (Pfeiffer, HiPace 700) were turned on when high-vacuum and KB systems were used. 20 min to 30 min are need for vacuum vent and samples replacement.

Figure 4 shows a photograph of the non-focusing end-station. A set of translation (X-Z) and rotation (R) motors (VACGEN) were used to adjust the sample position in the vacuum vessel. The sample holder was ~9 cm in total length, with a YAG crystal on top to assist with the beam location (inset in Fig. 4). Samples are usually smeared onto carbon or Kapton tapes or pressed into disks. Usually, 6–9 samples can be placed on a sample holder simultaneously. By indirectly cooling with liquid nitrogen, the sample in the non-focusing end-station can be operated in vacuum under cryogenic conditions to ~120 K. A graphene carbon window (KETEK, ~900 nm in thickness and ~10 mm in diameter) separated the vacuum of the non-focusing vessel from the beamline. Four photodiodes (AXUV300C) were installed at the four corners of a 5 mm hole to measure the fluorescence after a thin Al film with 2 μm thickness, which was used as the incident beam intensity (I₀). Owing to the tight space of the beamline, the I₀ detector was placed before the graphene carbon window. Before the I₀ detector, several Al foils with different thicknesses (25 μm to 500 μm) were used as attenuators. After samples, a photodiode (AXUV300C) was mounted in the same vacuum vessel to measure the transmitted beam intensity (I₁). The I₁ photodiode can be removed from the beamline by vacuum translation when a KB microprobe end-station is used. A three-channel silicon drift diode (SDD, RaySpec) with a collimated active area of 150 mm² was installed perpendicular to the beamline for XRF and partial fluorescence yield (PFY) detection of the sample. A photodiode was installed next to the SDD to measure the total fluorescence yield (TFY) of the samples. The total electron yield (TEY) mode was used to measure the sample current. Schematics of these three detection modes are shown in Fig. 2.

Fig. 4Full-screen View

(Color online) Photograph of the non-focusing end-station. (a) attenuators used by several Al foils with thickness from 25 μm to 500 μm, (b) Al foil for I₀ with thickness of 2 μm, (c) the I₀ detector by four photodiodes with a 5 mm pinhole, (d) graphene carbon window, (e) the I₁ detector, a normal photodiode, (f) valve and turbo pump for the vessel, (g) vacuum gauge of the vessel, (h) the tube for liquid nitrogen

Several XAS results obtained at the non-focusing end-station are shown in Fig. 5. Different XAS detection modes were used based on the morphology, conductivity, and absorption edges of the samples. For elements with absorption edges above 5 keV, the TFY, PFY, and transmission modes were used for XAS detection, according to their morphology and concentration. The TEY, TFY, and PFY modes were used for elements with absorption edges below 5 keV. For the PFY mode with low concentration and the transmission mode with high concentration, the samples should be pressed into disks of proper thickness. For the TEY and TFY modes, the samples are usually smeared onto carbon or Kapton tapes. The I₀ and I₁ photodiodes shown in Fig. 4, was used for the transmission mode. The P K-edge XANES spectrum of KH₂PO₄ in the TEY mode is shown in Fig. 5a. The P K-edge XANES is similar to that of ESRF-ID21 [35]. The max of “white-line” (s→ p electronic transition) of P K-edge of KH₂PO₄ is corrected to 2152.8 eV according to ID21 [35]. The Sr K-edge XANES of C₄H₆O₄Sr performed in the transmission mode is shown in Fig. 5b; the spectrum is similar to that of SrCO₃ in [36]. The test results showed that the photon energy of the beamline covers the design energy range from 2.1 keV to 16 keV. During the test, each energy integration time was one second with different undulator gaps. The undulator tapper was set to 0.45 mm and the beam current was 220 mA.

Fig. 5Full-screen View

(Color online) (a) The normalized P K-edge XANES of KH₂PO₄ done by TEY mode, (b) The normalized Sr K-edge XANES of C₄H₆O₄Sr done by transmission mode, (c) The normalized S K-edge XANES of CaSO₄ with different concentration done by TFY, TEY and PFY modes, (d) The normalized K-edge XAFS of Ni stand foil done by transmission mode, (e) the EXAFS k²χ data and (f) the FT spectra of Ni standard foil, (g) The normalized K-edge XAFS of Ti stand foil done by transmission mode and TiO₂-nano (rutile) powder with a mass concentration of 3% done by TFY mode, (h) the EXAFS k²χ data and (i) the FT spectra of Ti standard foil and TiO₂-nano (rutile) powder

The S K-edge XANES spectra of CaSO₄ obtained in the TFY, TEY, and PFY modes are shown in Fig. 5c. The S K-edge XANES spectrum is very similar to that of ESRF-ID21 [37]. The max of “white-line” (s→ p electronic transition) of S K-edge of CaSO₄ is corrected to 2482.5 eV according to ID21 [37]. High-purity CaSO₄ powder and CaSO₄ powder diluted with LiF to mass concentrations of 2.5% and 0.5% were used as samples. The CaSO₄ powder was smeared evenly onto very thin Kapton or carbon tapes. High-purity CaSO₄ powders obtained using the TEY and TFY modes are shown in Fig. 5c. Owing to the self-absorption of fluorescence, the fluorescence spectral signal intensity of TFY (red) was much lower than that of the TEY mode (blue) for samples with high purity. In Fig. 5c, CaSO₄ with a 0.5% concentration was performed in the PFY mode (green), and CaSO₄ with a 2.5% concentration was performed in the TFY mode (purple). The order of the normalized maximum values was 100% TEY mode, 0.5% PFY mode, 2.5% TFY mode, and 100% TFY mode. Typically, TEY is used for samples with high concentrations, TFY for samples with concentrations between 1% and 5%, and PFY for samples with concentrations of less than 1% [30]. CaSO₄ with 0.5% was performed in the TFY mode with a very close working distance between the sample and TFY photodiode (~10 mm distance), the spectrum was not very smooth. Thus, the PFY mode was suggested for samples with low concentrations (<1%).

The K-edge XAFS result of the Ni standard foil (in the transmission mode) is shown in Fig. 5d. The I₀ and I₁ photodiodes in Figs. 3 was used for the transmission mode. The EXAFS k²χ data and Fourier transform (FT) spectra of the Ni standard foil K-edge XAFS spectra are shown in Fig. 5e and f. For energy calibration, the energy and Bragg angle of the DCM were reset according to the first-derivative spectrum of the Ni foil from EXAFS Materials [38]. After energy calibration, the EXAFS k²χ data and FT spectra of the Ni standard foil K-edge XAFS spectrum were compared with those of X18B at the National Synchrotron Light Source [39]. The K-edge XAFS of the Ti standard foil in transmission mode is shown in Fig. 5g. The I₀ and I₁ photodiodes in Fig. 4 were also used in transmission mode. For comparison, the K-edge XAFS spectrum of TiO₂-nano (rutile) diluted with LiF to a mass concentration of 3% was also tested in the TFY mode (Fig. 5g. The energy was also calibrated according to the spectrum of Ti foil from EXAFS Materials. The EXAFS k²χ data and FT spectra of the Ti standard foil and TiO₂-nano (rutile) are shown in Fig. 5h and i. The EXAFS k²χ data and FT spectra of the Ti standard foil K-edge XAFS spectrum can be compared with those of the TPS 44A at the Taiwan Photon Source [40]. The EXAFS and FT spectra of TiO₂-nano (rutile) are similar to those obtained in the synchrotron laboratory HASYLAB/DESY, Hamburg [41].

These figures demonstrate that BL16U1 can collect the XAS spectrum across the entire target photon energy range from 2.1 keV to 16 keV. For the tender energy range of 2.1 keV to 5 keV, XANES spectra for phosphorus (P), sulfur (S), chlorine (Cl), potassium (K), and calcium (Ca) are usually collected in the TEY, TFY, and PFY modes. For energies above 5 keV, the XAFS spectra are usually collected in the transmission, TFY, and PFY modes. Although ion chambers are mainly used for synchrotron spectroscopy beamlines worldwide, our results showed that photodiodes can also be used to obtain XANES and XAFS spectra. The only drawback of a photodiode is the diffraction peaks resulting from the crystalline nature of photodiodes [42], which can be removed by the “deglitch” function in the Athena software.

Non-focusing end-station has been in operation for more than one year since the final acceptance test in July 2023. In 2024, this station received more than 85 users with a total user time of 2934 h. Important achievements have been made in many fields, including Co oxidation reactions [43], semi-hydrogenation of propylene [44], and flexible aqueous batteries [45]. This end-station is currently open to the users.

Microprobe and sub-microprobe end-stations

With the aim of analyzing materials at the microscopic scale, microprobe end-stations have been constructed among synchrotron facilities worldwide in recent years (Table 4). A spot size of ~2.1 μm×2.5 μm (h×v) on Daimond I18 [24], a spot size of ~2.5 μm × 2.5 μm (h×v) on SLS PHOENIX I [25], a spot size of ~2.5 μm × 2.5 μm (h×v), or even smaller than 180 nm on ESRF ID21 [27], have been achieved using the undulator source. Using a bending magnet source, the spot size in SXRMB beamline at CLS is approximately 10 μm [26] and the spot size in TES beamline at NSLS-II can be tuned from 2 μm to 25 μm [28]. For these microprobe beamlines, KB mirrors are used to focus the beam. Micro-XRF, micro-EXAFS, and micro-X-ray diffraction are the main methods used for these microprobe beamlines.

Here, we focus on the micro-XRF and micro-XANES techniques. Using a multichannel SDD detector, the elemental distribution and correlations of elements can be mapped on a micrometer scale. Using micro-XANES scans, one can obtain the chemical speciation of elements by recording XANES spectra of selected sample spots with grain sizes in the order of micrometers. Micro-XRF and micro-XANES can also be performed on the BL16U1 beamline at the microprobe and sub-microprobe end-stations using two sets of KB mirror systems. The microprobe and sub-microprobe end-stations were installed after the non-focusing end-station. Two sets of KB mirrors were placed in two vacuum vessels, as shown in Fig. 6. The vacuum of the two sets of KB systems was lower than 5×10^-7 using ion pumps, and two sets of load-lock systems were used for sample transfer.

Fig. 6Full-screen View

(Color online) The vacuum vessels for KB and SKB systems

3.2.1

Microprobe end-station

Using one pair of fixed-surface KB mirrors, the focal spot of the microprobe end-station is approximately 56.85 m from the source. The mirrors substrate were made of silicon and coated with 6 mm wide Ni, Si, and Rh stripes. The coating stripes were translated using an in-vacuum translation mechanism according to energy. A VFM and a HFM were aligned behind each other in orthogonal planes. The incident angles are 4 mrad and 4.7 mrad for VFM and HFM, respectively. Details of the KB mirrors are listed in Table 2.

For the first set of KB mirrors, the focal spot is located at 600 mm and 245 mm from the center of VFM and HFM, respectively, which gives a standard working distance of 75 mm from the end of the HFM to the sample focal plane. A photograph of the KB mirrors and sample stages is shown in Fig. 7a. The mirrors and sample holder were installed in the same vacuum vessel without a vacuum window used for vacuum separation between the KB mirrors and samples. A four-axis sample stage (Micronix) was used for the sample positioning, as shown in Fig. 7b. An angle of 45° exists between the sample’s horizontal motion and the beam. The XYZ stages had a scanning precision of 200 nm. A photodiode (AXUV300C) was mounted after the sample holder in the same vacuum vessel to measure the transmitted beam intensity (I_1KB).

Fig. 7Full-screen View

(Color online) The photograph of KB mirror system (a) and the design view of sample stages (b), the horizontal with a 45° angle (c) and vertical (d) focused beam profiles of KB system at 10 keV. (e) The μXRF mapping and (f) μXANES of a Cu net

To measure the focal spot size of the KB system, a knife-edge scan using a 50 μm gold wire was used. The knife-edge scan is similar to that performed by Ando et al. [46]. The beam profile was measured using the 50 μm gold wire scanned through the beam, with the intensity of the transmitted beam recorded by a photodiode (I_1KB) behind the gold wire. The smallest FWHM of the spot size obtained at 10 keV is 4.59 μm×1.22 μm (h×v), Fig. 7c and d. Because of the 45 angle between the horizontal motion of the sample and beam (Fig. 7b, the horizontal FWHM spot size is obtained by multiplying the Gaussian fitting result with 45°; thus, the smallest FWHM of the horizontal spot size at 10 keV is 3.25 μm. Considering the motor resolution, the focal spot size of the KB system should be 3.3 μm×1.3 μm (h×v). The photon flux at this station was recorded using a photodiode (I_1KB). The highest current recorded for I_1KB was 3.5×10^-4A@10 keV (Fig. 7c and d), the photon flux of the beamline at this station was above 2.48×10¹² photons/s @10 keV.

Using the same “I₀” mentioned in the non-focusing end-station, μXANES spectra can be done in the KB vessel. A four-channel SDD (Vortex, Hitachi, USA) with a collimated active area of 200 mm² was installed perpendicular to the beamline for μXRF and PFY detection. Micro-XRF mapping can also be performed in the KB vessel. Because of their windowless design, micro-XRF (μXRF) and micro-XANES (μXANES) can only be achieved under vacuum at the microprobe end-station. Fig. 7e and f show the μXRF mapping and μXANES spectra of the Cu net. The type of Cu net is GILDER G200-C3. The scan range was 200 μm×200 μm with a step size of 5 μm.

3.2.2

Sub-microprobe end-station

After the microprobe end-station, a pair of SKB system was employed to focus the beam to a spot size at the submicron level. When the X-ray is focused by the SKB system, the KB mirrors and photodiode in the microprobe end-station should be moved out of the beam by vacuum translation. Similar to the KB system in the microprobe end-station, fixed-surface SKB mirrors with Ni, Si, and Rh stripes were used in the SKB system. The coated stripes were translated using an in-vacuum translation mechanism. Details of the SKB mirrors are provided in Table 2.

For the SKB mirrors, the focal spot is located at 230 mm and 90 mm from the center of VFM and HFM, respectively, which gives a standard working distance of 60 mm from the end of HFM to the sample focal plane. Design drawings of the SKB mirrors and sample stages are shown in Fig. 8a and b. In contrast to the KB system, the mirrors and sample holder were installed in different vacuum vessels separated by a Be window (8 μm thickness and ~9.2 mm diameter). Compared to the KB system, the SKB system had a lower flux and smaller spot size. In situ measurements were performed under various conditions at the station. A four-axis sample stage (Micronix) was used to position the sample. An angle of 45° exists between the sample’s horizontal motion and the beam. The XYZ stages had a scanning precision of 50 nm. A photodiode (AXUV300C) was mounted after the sample in the vacuum vessel to measure the transmitted beam intensity (I_1SKB).

Fig. 8Full-screen View

(Color online) The design drawing of SKB mirror system (a) and the sample stages (b), the horizontal with a 45 angle (c) and vertical (d) focused beam profiles of SKB system at 2.5 keV

With the same incident angles for VFM and HFM, a spot size of 0.67 μm×0.21 μm can be obtained at 2.5 keV using the SKB system (Fig. 8c and d). An angle of 45° between the sample’s horizontal motion and the beam was also used in the SKB sample stages (Fig. 8b). The horizontal FWHM spot size is obtained by multiplying the Gaussian fitting result by 45°. Thus, the smallest FWHM of the horizontal spot size at 2.5 keV is 0.47 μm. Considering the motor resolution, the focal spot size of the SKB system should be ~0.5 μm×0.25 μm (h×v). The photon flux at this station was recorded using a photodiode (I_1SKB). The highest current recorded by I_1SKB was 7.5×10^-6 A@2.5 keV (Fig. 8c and d), the photon flux of the beamline at this station was above 7×10¹⁰ photons/s@2.5 keV.

μXAS and μXANES can be performed in the SKB end-station. A one-channel SDD (Vortex, Hitachi, USA) with a collimated active area of 50 mm² was installed perpendicular to the beamline for μXRF and PFY detection. In contrast to the microprobe end-station, a Be window (8 μm thickness) was used to separate the vacuum of the mirrors and samples. Thus, micro-XRF (μXRF) and micro-XANES (μXANES) under vacuum or He atmosphere can be achieved at the sub-microprobe end-station.