FEATURE
Scanning Probe Microscopy of Environmental Interfaces The method is producing atomic-scale views and data needed to strengthen models. CARRICK M. E G G L E S T O N , STEVEN R. H I G G I N S , AND PATRICIA A. MAURICE
M
odels of processes that occur at solidfluid interfaces are needed in many environmental applications, ranging from calculating optimum conditions for particle coagulation in wastewater treatment to predicting how fast a pollutant will migrate in groundwater. Unfortunately, die validity of the models is difficult to establish because the fundamental interfacial chemistry of complex particulate surfaces in fluids is poorly understood. This problem is being addressed through development and use of powerful surface analytical techniques. Among these is scanning probe microscopy (SPM), a revolutionary method uniquely capable of imaging surface shapes and structures down to atomic scale. SPM is a family of techniques in which digital images are made on the basis of interactions between a sharp tip and a surface. The capacity of SPM extends well beyond in situ imagery. Quantification of surface reaction rates, energies, and other physical and chemical properties is possible. SPM imaging enables measurement of key energetic parameters for reactive sites on mineral surfaces. It has been used to study interactions between nonaqueous-phase liquid (NAPL) constituents and mineral surfaces as well as to examine sulfide minerals whose oxidation produces acidity in acid-mine drainage settings. For example, at Virginia Polytechnic Institute and State University in Blacksburg, Michael F. Hochella, Jr., a pioneer in the application of SPM to mineral surfaces, has combined ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) observations with quantum mechanical calculations to study atomic-scale oxidation processes on galena and pyrite surfaces. At Kent State University in Kent, Ohio, SPM is being used by Patricia Maurice to image and study biofilms, an important step in expanding knowledge of the environmental role of microorganisms in pollutant cycling and fate 4 5 6 A • OCT. 1, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
SPM imaging has invigorated the study of surface atomic structure. "The advent of scanned probe techniques, and their ability to probe solid-liquid interfaces on an atomic scale in situ, represents one of the biggest opportunities for improving our understanding of the chemistry of environmentally important solid-liquid interfaces," said Robert J. Hamers, professor of chemistry at the University of Wisconsin, Madison. Already, some researchers have used SPM to manipulate individual sttoms cUid. molecules at room temperature, and environmental scientists may soon use SPM to study micron-scale Arr&ys of micro-experiments in SL combinatorial chemistry approach to ootimization of chemical treatment strategies. The Earth's surface is characterized by an abundance of solid-fluid interfaces (>1013 km2) whose behavior profoundly influences our chemical environment, from the partitioning of solutes between water and available surfaces in soils and aquifers to the role of mineral weathering in global climate control. Werner Stumm, to whom this issue is dedicated, has consistently emphasized the importance of these interfaces and the application of interface chemistry concepts to environmental problems. The use of SPM techniques is one way in which Stumm and others have gained a nf*w view of microscopic processes at environmental interfaces. Our relatively sophisticated chemical understanding of solid surfaces (such as silicon) used in many technological applications results in part from the extensive application of electron and other spectroscopies to well-defined single-crystal surfaces prepared and analyzed under UHV conditions. By comparison, models of complex particulate surfaces in fluid media are still largely based on wetchemical measurements that indicate, for example, how much and how fast a solute partitions to a solid surface, or how fast a mineral dissolves but little about the molecular structures involved or the sur0013-936X/98/0932-456A$15.00/0 © 1998 American Chemical Society
face heterogeneity of reactive areas. Because of this, a variety of in situ surface analytical techniques, such as X-ray scattering, vibrational spectroscopies, magnetic resonance techniques, and SPM are being applied to environmental interfaces. SPM works on the basis of interactions between a sharp tip and a surface—resolution thus depends on tip sharpness and the physics of interaction. In SIM, the interaction is an electrical current; in atomic force microscopy (AFM), the interaction is a force. Tip position can be controlled at the atomic scale quite easily, while the interaction magnitude is monitored, enabling formation of a digital interaction map (current or force). Recent AFM variants form images on the basis of the frequency shift, phase shift, or amplitude attenuation of an oscillating tip due to interactions with the surface. These methods provide images of surface properties other than simple topography and allow imaging of soft organic molecule layers or living microorganisms in situ without damage (see photo at right). Despite its drawbacks artifacts are common and atomic-resolution SPM is often ambiguous with regard to exact atomic locations—the profound utility of the technique was signaled with the award of a Nobel Prize in 1986 to Gerd Binnig and Heinrich Rohrer of IBM in Zurich Switzerland
The scanning probe microscope assembly shown stands about 1 foot high. Several of the instrument's features can be seen: (a) sample location, (b) housing for the piezoelectric scanner that micropositions the sample relative to the tip; (c) cable interfacing the SPM to an image acquisition and processing computer; and (d) a massive platform suspended from elastic cords and enclosed in a sound-insulated box for purposes of vibration isolation, which is extremely important for high-resolution imaging. The red dot is a laser spot used in monitoring forces on the tip, which is only about 4-pm high and thus invisible in this picture.
The view at the atomic scale SPM provides one of the few ways in which UHV and in situ observations can be directly linked. Such work is helping to confirm the validity of atomic structures assumed in surface chemistry models. For example, SPM images of iron atoms at an iron oxide (hematite) The transfer of electrons across interfaces is also surface in a UHV environment (2) and of the same type important in environmental chemistry. STM has been of hematite surface in air are virtually identical, im- used to follow, at the atomic scale, the spread of oxplying that the atomic structures are similar (see pho- idized areas during galena (PbS) and pyrite (FeS2) oxtograph on the left, p. 458A, parts (a) and (b)). idation in air and water and thus to constrain reacThe technique also provides information on the tion models. Electron transfer is important for socalled "iron-filings walls," in which metallic iron presence or lack of surface disorder as well as the distribution of structured and defective areas. Struc- shavings are used in reactive barriers that help break tural defects are often sites of preferential adsorp- down chlorinated solvents migrating in neartion, dissolution, or catalytic activity on mineral surface groundwater. Usually, electrochemical pasurfaces that are otherwise relatively unreactive. STM rameters for specific surface sites, which are needed has been used to analyze Ti02, which is being stud- to model long-term behavior of such barriers, are unied as a photocatalyst for breakdown of organic pol- known. STM has been used to perform an electrolutants. Imaging of a Ti02 surface (see photo on the chemical characterization of single iron atoms within left, p. 458A, part (c) (2)) reveals that a substantial por- protoporphyrin molecules (6) (see photo on the left p 458A part (f)) The general concept behind this tion of it is composed of several types of defect sites. work is relevant to environmental systems At an atomic scale, steps and kinks are discernible. These are sites of molecular attachment and detachment during crystal growth or dissolution, a pro- Sorption and microbiology cess that can be key to partitioning of pollutants such The technique has also been used to observe the moas heavy metals between solid and solution. AFM has tion of individual surface complexes formed by adbeen used to image a step with kink sites on a calcite sorption of aqueous chromium to iron oxide sursurface in water (see photo on the left, p. 458A, part (d)) faces (see photo on the right, p. 458A, parts (a) and (3) and with a kink site along a step on a gypsum sur(b)). Such partitioning of solutes between aqueous and face (see photo on the left, p. 458A, part (e)) (4). From solid phases is important in many environmental prosuch images, the energy of kink formation has been cal- cesses, for example, pollutant transport in groundwaculated (4). Liang and Baer (5) used the kinetics of step ter. From image sequences, useful energetic paramemotion during dissolution to calculate activation en- ters for these complexes can be estimated. ergies for kink motion in recent AFM studies perThe transport and remediation of NAPLs, usually formed at the Environmental Molecular Science Lab- spilled fuels and oils, depend in part on interactions oratory of the Richland, Wash., Pacific Northwest between NAPL constituents and mineral surfaces. ComNational Laboratory. plex organic mixtures can often be regarded as solu-
OCT. 1, 1998 /ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS » 4 S 7 A
Surface structure, defects, and oxidation-reduction at the atomic scale can be studied using SPM techniques, (a) An ultrahigh vacuum scanning tunneling microscope (STM) image shows iron atoms at an iron oxide (hematite) surface (/). (b) An STM image of the same type of hematite surface as in part (a) taken in air. Note that the unit cell in parts (a) and (b) are virtually identical. The cells are marked, and the edge length in both views is 0.5 nm. (c) An STM image of a Ti0 2 surface (2) shows areas of ordered surface structure (box A), steps (box B), and both point (box C), see also inset) and line (box D) defects (the scale bar represents 10 nm). (d) An atomic force microscopy (AFM) image taken in aqueous solution shows a step with kink sites on a calcite (CaC03) surface (3). (e) An AFM image of gypsum (4) shows one kink site along the step (arrow), (f) In the STM image of protoporphyrin molecules on a graphite surface, the bright spots are Fe atoms complexed within these molecules in a 1:4 mixture of Fe-containing to Fe-free molecules (The scale bar represents 5 nm.) (6). All images are in false color, (a): Reprinted with permission from Ref. (1). Copyright 1995 by the American Physical Society; (c): Reprinted from Ref. (2), Copyright 1995, with permission from Elsevier Science; (d): Reprinted with permission from. Ref. (3). Copyright 1993 American Association for the Advancement of Science; (e): Courtesy of Microscopy + Analysis, March 1994; (f): Reprinted with permission from Ref. (6). Copyright 1996 by the American Physical Society.
tions from which organic solutes adsorb to or precipitate on mineral surfaces, affecting the subsequent surface wettability. Models of the mobility or recoverability of NAPLs often depend on assumptions about wettability. SPM can investigate the degree to which such assumptions are realistic (see photo on right, part (c)). It is even becoming possible to chemically identify functional groups on different areas of a surface through chemical force microscopy (7), developed by Charles Lieber and colleagues at Columbia University in New York City. Hydrophilic flnd hydrophobic aress of 3. surfs.CC C3J1 be distinguished on the basis of local frictional and adhesion forces. In an example of precipitative partitioning, uptake of lead by phosphate minerals has been inves4 5 8 A • OCT. 1, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY/ NEWS
The two successive images (parts (a) and (b)) of chromium (Cr) atoms adsorbed on an iron oxide surface were completed 8.4 seconds apart (both images are 5 nm on a side). In (a), two Cr atoms appear just below center (the left atom is circled). In (b), two new Cr atoms appear (box), and the circled Cr atom has moved. An identical set of lines is superimposed over both images to show the relative motion, (c) Asphaltenes are sorbed to a quartz surface from Prudhoe Bay crude oil (the scale bar represents 500 nm). The surface is rougher, and the sorbed layer is thicker than models have assumed. (d) Lead phosphate crystals have formed upon reaction of hydroxyapatite (calcium phosphate hydroxide) with an aqueous lead solution. The image is about 350 nm on a side. Formation of the insoluble lead phosphates by application of apatite is used for remediation of highly lead-contaminated sites, (e) A 20 x 20 pm image shows iron-hydroxide granules coating a glass slide, (f) A 20 x 20 urn image indicates a surface similar to that in part (e) but inoculated with 5. puttsfaciens (note the bacterium at lower right) and then imaged 72 hours later. The bacteria have extensively dissolved the ferric hydroxide (3). All imaqes are in false color, (a) and (b): Reprinted from Eaaleston C M ; Stumm W GBOChim Cosmochim Acta 1993 57 4843-4850 Coovright 1993 with oermission from Elsevier Science; (c) Courtesy Carrick Eaaleston and Xina Xie; (d): Courtesy Mineraloaical Society of America; (e) and (f): Reprinted from Ref (0) Coovright 1997 with permission from Elsevier Science
tigated with SPM (see cover illustration and photo on right, part (d)) (8). Such observations, coupled with other chemical information, help constrain models of uptake mechanisms. Because SPM can provide real-time, in situ images not only of topography but also of different frictional, adhesive, and viscoelastic properties, it is uniquely useful for the study of organic molecules, molecular films, or living organisms that might be difficult to identify or preserve using other techniques. Patricia Dove and
colleagues at Georgia Institute of Technology in Atlanta, have imaged the consumption of ferric hydroxide solids by iron-reducing bacteria in situ (see photo on right, p. 458A, parts (e) and (f)) (9). At Kent State, Maurice is using new property-sensitive SPM imaging modes to emphasize features of different "softness" and adhesive properties. Mineral dissolution and growth SPM is often used to monitor surface microtopography during mineral dissolution and growth. Proposed reaction mechanisms can be constrained and quantified through observation of growth or dissolution spirals and rates of step motion. Results (see photo at right, part (a)) from the work of the late Andrew Gratz and colleagues illustrate a growth spiral on a calcite (CaC03) surface. This work and subsequent work (5) on calcite have improved the understanding of calcite dissolution and growth mechanisms. The growth and dissolution mechanisms of clay minerals, which can dominate the reactive mineral surface area in many hydrogeologic settings, have sometimes been controversial. SPM provides particle shape and microtopographic information that helps constrain the growth mechanisms in some cases. At die University of Colorado, Boulder, Kathy Nagy is using SPM to observe die growth modes of clay minerals. Processes diat occur at die atomic scale can influence the environment at the global scale. The dissolution of certain feldspars plays a part in controlling the C02 content of the atmosphere over long periods. SPM imaging of weadiered feldspar (see photo at right, part (b)) highlights a complicated texture stemming from preferential dissolution at mineral defects such as crystal twin planes and layers (lamellae) of different composition that intersect die surface. From such images, weathering rates can be estimated {10). The inability to pressurize an SPM has limited the temperatures at which imaging can be done in aqueous solution. Carrick Eggleston and Steven Higgins, working with Kevin Knauss and Carl Boro of Lawrence Livermore National Laboratory, Calif, have developed a pressurizable AFM capable of imaging in aqueous solution up to 150 °C. BaS04 dissolution by step retreat and pit expansion has been observed under hydrotiiermal conditions (see photo at right, parts (c) and (d)). This expansion of the rein 26 of SPM applicability to hydrotiiermal reactions is of interest in environmental arenas ranging from radioactive waste storage to steam-flooding as a means of organic pollutant breakdown or recovery and in the study of silicate mineral dissolution reactions in situ New opportunities The application of SPM to specific questions within environmental research is just beginning. The results so far are promising, and there are recent SPM advances that have not yet been used in environmental science. Among these are scanning near-field optical microscopy, in which the probe tip is fiber-optic, allowing high-resolution optical imaging. The technique can be coupled with optical and vibrational (such as Raman) spectroscopies to provide local chemical information. Another example is scanning electrochem-
(a) In the growth spiral on a calcite surrace, ,ndividual steps emanate from a screw dislocation (arrow). The image is 600 nm on a side, (b) Weathering texture is evident on a feldspar (peristerite) surface ( 1 x 1 pm). Two images (parts (c) and (d)) from a sequence, taken with a new hydrothermal scanning probe microscope, show single-molecular layers retreating across a barium sulfate (barite) surface (at 127 °C, taken in water; images are 4 pm high). Note the large changes in step position over time and the time-dependent increase in size of etch pits. All images are in false color, (a) Reprinted from Gratz, A. J.; Hillner, P. E.; Hansma, P. K. Geochim. Cosmochim. Acta1B93,57,491-495, Copyright 1993, with permission from Elsevier Science.
ical microscopy, developed by Allen Bard and colleagues at the University of Texas-Austin (see photo above, part (f)). SPM can also be used independently of imaging to measure local forces or other variables. For example, modeling of particle-particle interaction—important in flocculation and flotation for waste treatment and in many natural processes— depends on an understanding of interparticle forces as a function of aqueous conditions. By using chemically tailored tips, SPM will be used to study these forces in specific environmental systems. References (1) Condon, N. G.; Leibsle, F. M.; Lennie, A. R.; Murray, P W; Vaughan, D. J.; Thornton, G. Phys. Rev. Lett. .995, ,5,1961— 1964. (2) Fischer, S.; Munz, A. W.; Schierbaum, K-D.; Gopel, W. Surf. Sci. 1995, 337, 17-30. (3) Ohnesorge, E; Binnig, G. Science 1993, 260, 1451-1456. (4) Bosbach, D.; Rammensee, W. Geochim. Cosmochim. Acta 1994, 58, 843-849. (5) Liang, Y.; Baer, D. R. Surf. Sci. 1997, 373, 275-287. (6) Tao, N. J. Phys. Rev. Lett. 1996, 76, 4066^1069. (7) Noy, A.; Sanders, C. H.; Vezenov, D. V; Wong, S. S.; Lieber, C. M. Langmuir 1998, 14, 1508-1511. (8) Lower, S. K.; Maurice, P. A.; Traina, S. J.; Carlson, E. H. Am. Mineral. 1998, 83, 147-158. (9) Grantham, M. C.J Dove, R M.; DiChristina, T. J. Geochim. Cosmochim. Acta 1997, 61, 4467-4477. (10) Nugent, M.; Brantley, S.; Pantano, C ; Maurice, P. Nature 1998, in press.
Carrick Eggleston is an assistant professor at the University of Wyoming.StevenHiggins is a temporary research scientist at the University of Wyoming. Patricia Maurice is an assistant professor at Kent State University. OCT. 1, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 4 5 9 A
