Article pubs.acs.org/Langmuir
Thin Water Film Formation on Metal Oxide Crystal Surfaces Benjamin Gilbert,*,† Jordan E. Katz,§ Bruce Rude,‡ T. E. Glover,‡ Marcus P. Hertlein,‡ Charles Kurz,∥ and Xiaoyi Zhang∥ †
Earth Sciences Division and ‡Advanced Light Source Division, Lawrence Berkeley National Laboratory, MS 74R316C, 1 Cyclotron Road, Berkeley, California 94720, United States § Department of Chemistry and Biochemistry, Denison University, Granville, Ohio 43023, United States ∥ X-ray Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ABSTRACT: Reactions taking place at hydrated metal oxide surfaces are of considerable environmental and technological importance. Surface-sensitive X-ray methods can provide structural and chemical information on stable interfacial species, but it is challenging to perform in situ studies of reaction kinetics in the presence of water. We have implemented a new approach to creating a micrometer-scale water film on a metal oxide surface by combining liquid and gas jets on a spinning crystal. The water films are stable indefinitely and sufficiently thin to allow grazing incidence Xray reflectivity and spectroscopy measurements. The approach will enable studies of a wide range of surface reactions and is compatible with interfacial optical-pump/X-ray-probe studies.
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INTRODUCTION In the natural environment and some technical settings, important chemical processes occur on hydrated mineral surfaces.1 Reactions that involve the mineral surface directly include the growth or dissolution of minerals and the adsorption of dissolved ions. Numerous reactions that can also occur in homogeneous solution are found to proceed faster on mineral surfaces. Examples include the surface nucleation of new mineral phases and reactions between dissolved species when one or both interact with the surface. All such interfacial reactions exert an important influence on the chemical composition of surface waters,2 and there is an ongoing need to understand their mechanisms. Surface-sensitive X-ray methods, including X-ray scattering and spectroscopy, are valuable methods for investigating the structure of hydrated surfaces,3,4 the coordination geometry of species bound to surfaces,5,6 and the products of interfacial reactions.7 X-ray studies of reactions at hydrated interfaces are challenging because surface water films strongly attenuate the X-ray beam, particularly at low angles of incidence. To date, the only method available for creating a sufficiently thin water film on a solid surface has been to confine the liquid layer beneath an X-ray-transparent membrane,8 which has several drawbacks. In particular, the membrane creates a stagnant surface layer, so changes in solution composition require cycles of mechanical repositioning and flushing. Furthermore, a motivation for this work is the study of interfacial reactions with picosecond to nanosecond temporal resolution using the optical-pump/X-rayprobe method.9,10 Confining membranes cannot be used in pump−probe experiments that use laser pulses to initiate © 2012 American Chemical Society
reactions because they absorb laser energy and eventually rupture. Here we describe an alternative, membrane-free approach to creating thin water films on spinning crystal surfaces. As shown in Figures 1 and 2, it is achieved by combining a liquid water jet (to hydrate part of the surface continuously) with a gas stream (to remove excess water from the X-ray analysis region), forming a micrometer-scale thin water film. This approach is closely related to the use of tailored gas streams to create flowing liquid jets and droplets with micrometer-scale diameters.11,12 In some industrial settings, high-pressure gas streams are employed to blow excess liquid off surfaces.13 However, to our knowledge this is the first experimental realization of indefinitely stable membrane-free micrometerscale water films on crystals for X-ray studies of interfacial chemistry. We demonstrate the formation of thin water films on single crystals of two oxides, hematite (α-Fe2O3) and zincite (ZnO), using X-ray reflectivity measurements to estimate the film thickness. We present X-ray absorption near-edge structure (XANES) spectroscopy at the transition-metal K edge, acquired in grazing incidence geometry to achieve surface sensitivity. The XANES data were acquired using the same configurations and X-ray flux that is typical of current pump−probe studies.9 Received: August 7, 2012 Revised: September 14, 2012 Published: September 17, 2012 14308
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could be manually aligned relative to the axis of the rotational stage such that the angular wobble during rotation was ∼0.4 mrad (0.026°), as estimated by the precession of a reflected laser beam on a wall or ceiling. This represents the finest alignment that could be achieved with a 1 in.2 mirror mount; a larger mount would be required for higher precision. Use of the magnetic kinematic base allowed the sample to be removed and remounted with a very small (but detectable) loss of crystal alignment. For X-ray studies, the crystal was subsequently aligned relative to the incident X-rays by finding the sample angle and vertical position to which the surface plane was parallel to and at the vertical center of the X-ray beam. Water Film Formation. The crystal was rotated at 2 to 3 Hz, and a 250-μm-diameter jet of water from PEEK tubing flowed onto the surface at a rate of approximately 10 mL/min using an Ismatec model ISM895A gear pump. Pressurized nitrogen gas streamed onto the sample through a gas wand: a 1/16 in. o.d. stainless steel tube with one sealed end and a 250-μm-wide, 6-mm-long slot that directed the gas flow onto the sample surface. The water flow rate, nitrogen pressure, and gas wand position were adjusted manually until a stable water film was achieved. As water dripped off the crystal surface, it was collected in a silicone canneles cake mold (Freshware) in which a central hole was cut to fit around the sample post. A cylinder of Kapton film was positioned around the sample to act as a splash shield. One important issue for ensuring long-term operation was the selection of a selfpriming pump that reliably extracted excess water from the silicone mold as it filled. The Welch model 3200 peristaltic pump with 1/4 in. i.d. tygon tubing was most effective. X-ray Studies. Hematite single crystals were studied at Advanced Light Source (ALS) beamline 6.0.1 with a 100 μm vertical beam size. In these experiments, the sample spinner was attached to a motorized manipulator with x, y, z, and θ degrees of motion. X-ray alignment and reflectivity measurements were performed using an unbiased pin diode with a 4 × 0.25 mm2 horizontal pinhole mounted on a 2θ rotational arm. The pin diode was ungated, but the total X-ray intensity was reduced by an X-ray chopper. Zincite single crystals were studied at Advanced Photon Source (APS) beamline 11-ID-D with a 20 μm vertical beam size. In these experiments, the sample spinner was mounted on stacked goniometer and translation stages. Sample alignment was performed using a largearea ionization gauge downstream from the sample. X-ray reflectivity measurements were performed by blocking the direct beam and recording the reflected intensity as a function of the sample angle within the stationary ionization gauge. Surface-sensitive X-ray absorption spectra were collected using an avalanche photodiode (APD). A Z-1 filter and Soller slit combination were positioned in front to the detector to minimize the background from scattered Xrays. The laser pump pulse was the third harmonic output of an Nd/ YLF regenerative amplified laser with a 1.6 kHz repetition rate, giving 351 nm laser pulses with a 5 ps fwhm.
Figure 1. Graphical representation of the experimental layout for grazing incidence X-ray reflectivity and spectroscopy analysis of a single-crystal surface beneath a thin water film. Surface-sensitive X-ray spectroscopy measurements are performed using an X-ray fluorescence detector assembly consisting of a Soller slit (not to scale) in front of an avalanche photodiode (APD). The detector is translated so that the focus of the Soller slit is coincident with the X-ray beam footprint.
Figure 2. Photograph of the experimental layout for creating thin liquid films on a single-crystal surface. A 1 cm2 hematite crystal is shown mounted on the vertical post. Tubing to remove water from the red liquid collector is visible behind the splash shield.
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MATERIALS AND METHODS
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Single-Crystal Source and Preparation. A natural hematite single crystal (Bahia, Brazil) was cut into 1-mm-thick plates yielding the (0001) surface that was polished to achieve a root-mean-squared (rms) surface roughness of less than 1 nm. Synthetic ZnO crystals that were 1 cm2 and 500 μm thick with one polished (0001) face were purchased from MTI Corporation (Richmond, CA). The crystals were mounted and repolished by hand with colloidal silica (Buehler, IL). Excess colloidal silica was removed by thoroughly wiping the crystal surface on a water-saturated polishing pad. Polished hematite crystals were soaked in a water solution of sulfuric acid and Nochromix (Godax Laboratories, Inc., MD) at a 10% concentration relative to the recommended formulation for glass cleaning. The surface roughness and cleanliness were measured by atomic force microscopy (AFM) using a Digital Instruments Nanoscope IV multimode AFM in tapping mode. Experimental Description and Crystal Alignment. A single crystal was mounted on an aluminum sample post using thermal epoxy (Buehler). The post was screwed into a modified optical mirror mount attached to one plate of a kinematic base (Thorlabs). The second plate was attached to the top surface of a Newmark Systems, Inc. model HR-3 rotary stage. Flat single crystals mounted on the sample post
RESULTS AND DISCUSSION As depicted in Figures 1 and 2, thin water films can be created by combining two fluidic flows. A liquid water jet is directed onto a spinning crystal so as to hydrate one part of the surface continuously. The crystal rotation moves the wetted surface beneath a gas stream that removes excess water, creating a thin layer of residual liquid with micrometer-scale thickness. We created thin water films on surfaces of hematite, zincite, quartz (SiO2), and fluorite (CaF). Film formation was visually evident because of the appearance of several Newton rings beneath the gas jet with colors and spacings that varied depending on the precise experimental geometry. To date, however, we have not been able to capture photographic images of the Newton rings. The thin film region covered between one-third and one-half the surface area of the 1 cm2 crystal. It is likely that the water films are not laterally uniform (i.e., they vary in thickness depending on the distance from the gas jet) and additional experiments are planned to characterize the film morphology. 14309
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Thin film creation is a dynamic phenomenona steady-state regime caused by several kinetic processesyet it was found to be very stable and reproducible. There was no visual evidence of any hydrodynamic instability, and the films could be sustained indefinitely. Changes to solution chemistry had little effect on film formation: stable free-standing films were made using ultrapure water and solutions of 1 M NaCl and 0.1 M HCl. With a fixed experimental geometry, the gas or water flow could be stopped and restarted and the water films recreated with high reproducibility. However, challenges remain with respect to constraining all of the experimental degrees of freedom, particularly the orientations of the water and gas jets. Consequently, for each new experiment, achieving a surface film of suitable area and thickness took manual adjustment but was normally achieved within 15 min. We performed X-ray reflectivity measurements of dry and wet single crystals to estimate liquid film thicknesses that could be achieved. Figure 3 displays calculated X-ray reflectivity
detected at the APD detector positioned as indicated in Figure 1. At 9.7 keV, the APD signal is derived from zinc fluorescence plus scattered photons; at 9.6 keV only the scattered background contributes. Below the critical angle, Zn fluorescence dominated the APD signal, but the scattered background contribution remained detectable. More complete background suppression could be achieved by reorienting the sample spinner and detector so that both the crystal rotational axis and the normal to the active area of the detector were horizontal and parallel to the X-ray electric field polarization vector (cf. Figure 1). Separate trials showed that thin water films could also be achieved in this configuration. We performed surface-sensitive X-ray absorption spectroscopy at the Zn K edge by setting the angle of incidence to 0.13° and collecting the fluorescence signal with the APD. As shown in Figure 3, at incident angles beneath the critical angle, the penetration of the electric field into the sample is less than 10 nm. By comparison, the X-ray attenuation lengths at normal incidence are orders of magnitude greater (e.g., 46 μm at 7 keV and 6.8 μm at 7.2 keV for hematite). Figure 6 presents surfacesensitive Zn K-edge spectra acquired from the same ZnO (0001) face following different surface treatments, compared with data for bulk ZnO acquired in transmission. Following the initial polishing and cleaning, the spectrum for the ZnO surface was similar to that of the bulk reference, with no evidence of self-absorption effects that can distort the line shapes of fluorescence spectra by preferentially reducing peak intensities. There was a distinct enhancement of the first peak at a threshold that could represent surface states. We illuminated the dry crystal surface with 351 nm laser pulses at a fluence of approximately 0.1 μJ cm−2 for 4 h. The laser photon energy is greater than the ZnO band gap, creating electron−hole pairs that either recombine within 50 ps or decay into long-lived (>1 ns) trap states.16 Band-gap excitation has been shown to enhance the rate of ZnO dissolution significantly,17 indicating that photogenerated electrons or holes are chemically reactive. As shown in Figure 6, the surface Zn K-edge spectrum following laser exposure has lost the eminent prepeak at 9.669 keV. We subsequently created a thin water film on the crystal and acquired Zn XANES of the hydrated surface after about 30 min. The spectrum exhibited even sharper near-edge features than observed for the aspolished surface. The data of Figure 6 are thus consistent with a model in which UV illumination breaks surface Zn−O bonds. Washing with water enables the dissolution of destabilized surface atoms, regenerating the surface. Thus, light-initiated surface chemical processes that occur in hydrated systems could be studied using the approach described here. Our observations of water film formation have enabled a preliminary evaluation of the physical and chemical factors controlling the film thickness and stability. Specifically, our studies suggest that the most important parameter is the strength of the interaction between water and the crystal surface (i.e., surface hydrophilicity). For example, thin film formation could not be visually observed on the surface of freshly polished pyrite, a hydrophobic sulfide mineral. Furthermore, the presence of carbonaceous contamination on nominally hydrophilic oxide surfaces likewise adversely affected the thin film formation. For a given crystal surface, the film thickness could be varied within a small range by simply adjusting the rate and direction of the N2 gas flow (Figure 4). Thin films could not be formed on stationary crystals; however, the sample rotational rate had little effect on the film thickness.
Figure 3. Calculated X-ray reflectivity and 1/e penetration depths for hematite (α-Fe2O3) and zincite (ZnO) at 7 and 9.6 keV, respectively.
curves for hematite and zincite at 7 and 9.6 keV, respectively (just below the corresponding metal K-edge absorption threshold).14,15 Figure 4a presents measured X-ray reflectivity curves at 7 keV for the (0001) surface of a hematite single crystal, spinning at 3 Hz and measured dry or with a water film of two thicknesses. The experimental critical angle matched the predicted value of 0.34°, with no detectable change following water addition. The loss of reflectivity at low angle was partially due to incident beam spill off, which was not corrected for. The major effect of water film formation was a loss of reflected intensity, which we used to estimate the film thickness. Figure 4b compares the reflectivity data with relative X-ray attenuation calculated for uniform water films with a range of thicknesses. Changing the gas flow rate by changing the nitrogen pressure in the gas wand caused the apparent film thickness to vary between 2 and 3 μm. Figure 5 shows X-ray reflectivity data at 9.6 keV for the ZnO (0001) surface with and without a water film. Both curves exhibit the expected critical angle of 0.26°. From the loss of reflected intensity, the estimated film thickness is 1 μm. Figure 5 also shows the angular dependence of the X-ray photon signal 14310
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Figure 4. Experimental X-ray reflectivity curves at 7 keV for a hematite single crystal spinning at 3 Hz with a 4 s dwell time per point, measured in air and with a thin film of water created by a gas jet using N2 at 37 or 50 psi. (a) Logarithmic plot of the X-ray reflectivity curves. (b) Linear plot of the same reflectivity curves, compared with X-ray attenuation factors (plotted against the right axis) calculated for surface water films from 250 nm to 3 μm thick.
Figure 6. Grazing incidence (GI) XANES spectra acquired from a ZnO (0001) surface at the Zn K edge following successive surface treatments (colored lines and markers), compared to the spectrum from bulk ZnO (black line). GI-XANES spectra are vertically displaced for clarity.
Figure 5. (Left axis) Experimental X-ray reflectivity curves at 9.6 keV for a ZnO single crystal spinning at 3 Hz, measured in air and with a thin film of water created by a gas jet at 40 psi. (Right axis) Sample angle dependence of the X-ray fluorescence signal measured on an avalanche photodiode (APD) detector.
solution and the crystal surface, continuous processes such as mineral dissolution could be studied, providing that substantial surface roughening does not occur. Reactions that are hosted by an interface, such as adsorbate reactions or heterogenerous nucleation, could also be studied. The solution chemistry in the thin water film can be changed in much less than 1 s, enabling surface reactions to be started or stopped simply by switching the source for the inlet to the pump. A goal of future work will be to determine the speed at which solution conditions can be changed. We anticipate that an important application of this method will be studies of reactions at hydrated surfaces on much shorter time scales using the methods of ultrafast X-ray science. When a chemical reaction can be initiated abruptly, typically
Even when applied to 1 in. crystals, the rotational rates are far too slow to create thin water films using the spin-casting method. The main purposes of spinning the sample were to separate the regions of water and gas flow spatially and to control the lateral spreading of the water drop that was formed by the liquid jet.
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CONCLUSIONS We have implemented a membrane-free approach to creating micrometer-scale water films on metal oxide crystals. The method is well suited for in situ X-ray studies of interfacial reactions because the liquid jet continuously refreshes the surface film. By permitting constant exchange between bulk 14311
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X-ray Spectroscopy and Long-Period X-ray Standing Waves. J. Electron Spectrosc. Relat. Phenom. 2006, 150, 66−85. (9) Bressler, C.; Chergui, M. Ultrafast X-ray Absorption Spectroscopy. Chem. Rev. 2004, 104, 1781−1812. (10) Chen, L. X. Probing Transient Molecular Structures in Photochemical Processes Using Laser-Initiated Time-Resolved X-ray Absorption Spectroscopy. Annu. Rev. Phys. Chem. 2005, 56, 221−254. (11) DePonte, D. P.; Weierstall, U.; Schmidt, K.; Warner, J.; Starodub, D.; Spence, J. C. H.; Doak, R. B. Gas Dynamic Virtual Nozzle for Generation of Microscopic Droplet Streams. J. Phys. D: Appl. Phys. 2008, 41, 195505-1−195505-7. (12) Weierstall, U.; Spence, J. C. H.; Doak, R. B. Injector for Scattering Measurements on Fully Solvated Biospecies. Rev. Sci. Instrum. 2012, 83, 035108-1−035108-12. (13) Lacanette, D.; Gosset, A.; Vincent, S.; Buchlin, J.-M.; Arquis, E., Macroscopic Analysis of Gas-Jet Wiping: Numerical Simulation and Experimental Approach. Phys. Fluids 2006, 18, 042103-1−042103-15. (14) CXRO X-ray Interactions with Matter. http://henke.lbl.gov/ optical_constants/mirror2.html. (15) Henke, B. L.; Gullikson, E. M.; Davis, J. C. X-ray Interactions: Photoabsorption, Scattering, Transmission, and Reflection at E=50− 30000 eV, Z=1−92. At. Data Nucl. Data Tables 1993, 54, 181−342. (16) Cavaleri, J. J.; Skinner, D. E.; Colombo, D. P., Jr.; Bowman, R. M. Femtosecond Study of the Size-Dependent Charge Carrier Dynamics in ZnO Nanocluster Solutions. J. Chem. Phys. 1995, 103, 5378−5386. (17) Han, J.; Qiu, W.; Gao, W. Potential Dissolution and PhotoDissolution of ZnO Thin Films. J. Hazard. Mater. 2010, 178, 115−122. (18) Chen, J.; Zhang, H. H.; Tomax, I. V.; Wolfsburg, M.; Ding, Z.; Rentzepis, P. M. Transient Structures and Kinetics of the Ferrioxalate Redox Reaction Studies by Time-Resolved EXAFS, Optical Spectroscopy, and DFT. J. Phys. Chem. A 2007, 111, 9326−9335. (19) Katz, J. E.; Gilbert, B.; Zhang, X. Y.; Attenkofer, K.; Chapman, K.; Frandsen, C.; Falcone, R. W.; Waychunas, G. A. Electron Small Polarons and Their Mobility in Iron (Oxyhydr)Oxide Nanoparticles. Science 2012, 337, 1200−1203. (20) Katz, J. E.; Gilbert, B.; Zhang, X. Y.; Attenkofer, K.; Falcone, R. W.; Waychunas, G. A. Observation of Transient Iron(II) Formation in Dye-Sensitized Iron Oxide Nanoparticles by Time-Resolved X-ray Spectroscopy. J. Phys. Chem. Lett. 2010, 1, 1372−1376.
with a fast laser pulse, pump−probe X-ray methods can be used to follow the reaction course with subnanosecond temporal resolution, which is much better than can be achieved by mixing reagents. Several geochemically relevant pump−probe studies have been performed. Chen et al. studied the photolysis of ferrioxalate in solution.18 Katz et al. studied the reduction of transition-metal oxides using dye-sensitized nanoparticles.19,20 However, to the best of our knowledge, no pump−probe studies have employed surface-sensitive X-ray methods to study fast interfacial phenomena on high-quality single-crystal surfaces.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Pupa Gilbert for preparing the graphic in Figure 1, Steve Ferriera for machining the sample holder components, Glenn Waychunas for providing the hematite single crystals and advice on grazing incidence X-ray methods, Sirine Fakra for the reference spectrum of bulk ZnO, Klaus Attenkofer for advice on the experimental setup at APS 11-ID-D, and John Spence for helpful discussions and encouragement. Grazing incidence Xray reflectivity and X-ray absorption spectroscopy were acquired at beamline 6.0.1 at the Advanced Light Source (ALS) and at beamline 11-ID-D at the Advanced Photon Source (APS). B.G. was supported by the Director, Office of Science, Office of Basic Energy Sciences, U.S. Department of Energy, hereby abbreviated to DOE-BES, under contract no. DE-AC02-05CH11231. Use of the ALS and the APS is supported by DOE-BES under contract nos. DE-AC0205CH11231 and W-31-109-ENG-38, respectively.
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REFERENCES
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