Atomic force microscopy study of clay nanoplatelets and their impurities

The results of an in-depth study of 1 nm thick individual clay sheets by atomic force microscopy (AFM) are presented. Several techniques have been employed, including lateral force microscopy (LFM) and force modulation microscopy (FMM). The individual clay sheets, also referred to as clay nanoplatelets in the literature, were found to be extremely compliant and strongly adhered to a variety of substrates. In all cases, these nanoplatelets were accompanied by mobile impurities that could not be separated from the nanoplatelets. A detailed examination of these impurities using AFM suggests that they are comprised of low molecular weight silicates as well as the intercalating salt used to process the clay.Anunderstanding of the nature and chemistry of these impurities will be necessary as models of these clays are developed for various applications, such as reinforcement in composite materials or elements for molecular electronics.Further, these clay nanoplatelets and their impurities present a novel two-dimensional nanoscale system for future surface science studies using AFM and other methods.

The term clay refers to a class of materials generallyMade up of layered silicates that are similar to mica. Whilethese clay particles can span centimeters in lateraldimensions, the in-plane dimensions of the individual claylayers are on the order of a micron, and the thickness ofa single clay nanoplatelet is on the order of a nanometer.These layered clays are characterized by strong intralayercovalent bonds within the individual sheets comprisingthe clay, with only weak van der Waals interactionsbetween adjacent clay sheets. These strong in-planecovalent bonds have generated significant interest in theuse of these materials as a reinforcing element in compositematerials. However, because of the weak interactionbetween adjacent clay sheets, a significant effort on clayreinforcedpolymers has focused on methods of intercalatingthe polymer within the intralayer galleries, orcompletely exfoliating the clays so that individual claysheets are dispersed within the polymer, to maximize thecontact area (and hence the interaction) between thepolymer and the clay. Brillouin scattering measurementson bulk muscovite mica give in-plane moduli on the orderof 170 GPa, while the modulus perpendicular to the layersis given as about 60 GPa.1 Experimental measurementson clay particles give a much lower modulus value in theC-direction.2 However, we are unaware of experimentsthat have attempted to measure the modulus of anindividual clay nanoplatelet. Thus, the motivation of thecurrent work was to probe the mechanical properties ofindividual clay nanoplatelets using AFM.The interest in using clay as a reinforcing phase in apolymer matrix composite was spurred by the pioneeringwork by researchers at Toyota in the early 1990s.3-5 Sincethat time, research into clay nanocomposites has exploded;the reader is referred to recent review articles for an indepthdiscussion of the current state of the art.6-13 Ingeneral, the incorporation of the nanoclays into variouspolymers has been shown to result in increases in tensileproperties (elastic modulus, tensile strength), decreasedthermal expansion coefficients and improved thermalstability, increased swelling resistance, decreased gaspermeability, and improved flammability properties. Anew class of nanoclay-polymer composites, based on thelayer-by-layer assembly of individual clay nanoplateletsand appropriate polymers, is also under development.14-17Because of the large surface area of the clay nanoplateletswithin the nanocomposite, an understanding and controlof the clay surface are expected to play an important rolein the optimization of these material systems. Models ofthe mechanical properties of the individual clay nanoplatelets,as input parameters into models of effective claynanocomposite behavior, will also be critical.However, these clay nanoplatelets are interesting aboveand beyond their use as an element of composite materials.Because the individual clay platelet thicknesses are onthe order of 1 nm, with lateral dimensions on the micronscale, they are the two-dimensional analogue of zerodimensional(quantum dots, nanoclusters) and onedimensional(nanotube, nanowires) materials. In addition,depending on their thermal and electrical properties, thesilicate nanoplatelets are a potential building block fornanoscopic electrical devices. The presence of impuritiesassociated with the clay nanoplatelets, and the inabilityto separate these nanoplatelets from the impurities,complicate the surface chemistry of these materials.18,19Unless methods are found for their removal, theseimpurities will prevent accurate measurements of themechanical, electrical, and thermal properties of theindividual clay nanoplatelets.Experimental SectionReagent grade chemicals were used as delivered. Tetrahydrofuran(THF), acetonitrile (ACN), N,N-dimethylformamide(DMF), and dodecylamine were from Aldrich. 3-Aminopropyltriethoxysilanewas from Alfa-Aesar, and toluene was fromFisher. Waterwaspurified using a Barnstead modelD6431threecartridgefilter system. Mica substrates were from Ted Pella,and highly ordered pyrolytic graphite (HOPG) was from SPI.Gold films were deposited with an Edwards model 306 e-beamevaporation system. Ultrasonication was done using a Crestmodel 175HT, 50 W bath.Clay Characterization. All images were recorded on ParkScientific model CP Research AFMs. Contact mode images weremade with Park Scientific microlevers with silicon nitride tips.Tapping mode images weremadewith Park Scientific noncontactultralevers with silicon tips. (Note that Park Scientific is nowowned by Veeco.) Typical scan speeds were 1 or 2 Hz. The contactmode set point was for a normal force of 1 nN. In tapping mode,the resonant frequency was about 90 kHz, with small variationsfrom cantilever to cantilever. Time-of-flight secondary ion massspectrometry (TOF-SIMS) was performed using a PhysicalElectronics model Thrift III.Processing of Clays. A number of different clays sampleswere examined in this work (see Table 1). Other than details oflateral dimensions, all clays looked very similar by AFM. Theexfoliation, mechanical behavior, and presence of impurities alsoappeared to be similar for all samples. Thus, our efforts focusedon the SOMASIF ME-300 sample, a synthetic clay made fromnatural talc. According to the manufacturer (UniCoOP, Tokyo,Japan), the clay is processed by heating talc in the presence ofNa2SiF6, such that the salt decomposes and the Na ionsintercalate between the silicate layers that comprise the talc.Whenplaced in water and agitated, water can enter the galleriesand further swell the talc until it becomes completely exfoliated.To study individual clay nanoplatelets viaAFM,it is necessaryto first fully exfoliate the clay crystals, followed by deposition ona suitable substrate. Unless otherwise specified in the text, theindividual clay nanoplatelets were prepared in the followingmanner. First, a few micrograms of the clay powder was placedin 10 mL of pure water. The clays were exfoliated in a low-powerultrasonic bath for 8-16 h. After this, the resulting suspensionwas allowed to settle for a day or more. Sedimentation allows the nonexfoliated clays to sink, while the fully exfoliated individualclay sheets will stay in suspension for months. A drop of thesuspension is then placed on a substrate and allowed to standfor a few minutes. The excess water is then removed by a highvelocityjet of compressed gas or is wicked away. Clay nanoplateletcoverage on a substrate can be controlled by adjusting theconcentration of the suspension and the time it is exposed to thesubstrate.Results and DiscussionInitial Measurements of Individual Clay Nanoplatelets.InitialAFMimages were obtained by depositingthe clay nanoplatelets on mica. Mica is ideal for this initialimaging because it is atomically flat and adds nothing tothe topology of the system, making it easier to understandthe topological features of the clay nanoplatelets.Throughtrial and error, it was possible to get nanoplatelets evenlydistributed in submonolayer coverage on the mica surface.One can distinguish individual clay nanoplatelets by theirthickness, as they are 1 nm high in all AFM topologyimages as shown in Figure 1. Areas where multiple claynanoplatelets are stacked on top of each other are alsovisible in the image.For these initial images of the nanoplatelets, contactmode AFM was used in parallel with lateral forcemicroscopy (LFM). Because LFM is very sensitive todifferences in the chemistry of the surface, it would indicateany differences in chemistry between the clays and themica substrate. This is mostly due to the effects of capillary forces and the amount of adsorbed water on a sample surface.20-23 AFM and LFM images of a clay nanoplateletsample directly after deposition, and 5 days after deposition,on mica are shown in Figure 2.

While the surface topology of the clay samples remains unchanged, thelateral force contrast has reversed over a period of days(i.e., the clay nanoplatelets are light and dark respectivelyin parts B and D of Figure 1). We believe (see discussionbelow) that this change in lateral force contrast was dueto the migration of impurities. At room temperature theseimpurities are highly mobile and strongly interact withthe AFM tip. In later experiments, it was discovered thatthis change in contrast can be reversed by rinsing in water,such that the friction on the clay nanoplatelets returns tobeing lower than on the mica substrate. If the sample isthen dried by heating in an oven or by rinsing in ethanol,the contrast again reverses so that friction on thenanoplatelets is higher than the mica substrate. However,after repeating this process several times, the friction onthe clay nanoplatelets remained higher than on the micasubstrate. Currently, we believe that this behavior is dueto a redistribution of impurities and their effect on thelocal wetting properties of the sample. Rinsing does notremove the impurities, which seem to be strongly attractedto both the clay and the mica. However, the impuritiesare mobile and seem to prefer to locate on the surface ofthe clay nanoplatelets. After repeated rinsing they aremostly moved to the top of the clays. The presence of theseimpurities is significant, as they will alter the surfacechemistry of the nanoplatelets and are likely to impactthe use of fully exfoliated clays in applications.Compliance of Individual Clay Nanoplatelets.Disregarding the issue of impurities for the moment, oneof the long-term goals of our research is to understand themechanics of these clay nanoplatelets. Our first approachwas to place individual nanoplatelets across a very narrowgap cut into a glass substrate in order to measure themodulus via a force vs displacement curve on a suspendednanoplatelet. To test this capability, a clay nanoplateletwas deposited across a 100 nm wide scratch in a glasssubstrate, as shown in Figure 3. Rather than spanningthe trench, the clay sinks into the scratch and readilyconforms to it. This is consistent with molecular modelsthat show that the clay nanoplatelets should be verycompliant in the out-of-plane direction.24To test whether a smaller gap is necessary for the forcevs displacement drum-skin test, clay nanoplatelets weredeposited on a nanoporous alumina substrate with uniform60 nm pores spaced 100 nm apart.25 However, similar toour findings with the glass substrate, when placed on ananoporous alumina substrate the clay nanoplatelets sinkinto the pores and appear wrinkled, as shown in Figure4. The friction on the clay is lower than the aluminasubstrate (see Figure 4b), similar to the case for the micasubstrate.Returning to the issue of impurities, initial attempts toseparate the impurities from the clay nanoplatelets usedan electrophoretic technique. If the nanoplatelets arecharged, the application of an electric current might drivethe nanoplatelets to one of the gold electrodes and leaveany impurities in the water. The electrode was a 20 nmthick polycrystalline gold film evaporated onto mica. Thesegold electrodes were placed in an aqueous nanoplateletsuspension, and a voltage was applied until a current ofabout 1 mA could be detected. This method was notsuccessful, as very few nanoplatelets were found on thepositive electrode (and none on the negative electrode). Afew of the clay nanoplatelets that were found on thepositive electrode are shown in Figure 5. Because theindividual clay nanoplatelets are so compliant in the outof-plane direction, the grains of the gold substrate canclearly be seen below the clay plates. The friction is loweron the clay nanoplatelets than on the supporting gold film.On a few occasions, we observed large areas where theclays formed almost complete monolayers. This may berelated to surface charge on the clay nanoplatelets. Underan optical microscope, large rafts of the clays are sometimesseen floating on the surface of the water as a sampleis drying, possibly indicating that the nanoplatelets orderthemselves on the surface of the water and are then leftbehind as the water film dries. On mica, water dries asa more or less uniform film and does not break up intosmall droplets. A tapping mode AFM image on micademonstrating this tiling effect is shown in Figure 6.Force Modulation Microscopy on the Clay Nanoplatelets.As our previous results indicate, the extremeout-of-plane compliance of the individual clay nanoplateletssuggests that meaningful modulus values obtainedusing a force vs displacement drum-skin test are unlikely.However, there is a new AFM technique known as forcemodulation microscopy (FMM) that can, in principle,directly measure the modulus of a material at the surface.In FMM, the normal force between the tip and sample ismodulated. The amplitude and phase of the response canbe used to calculate the contact stiffness of the surface ofthe sample. In principle, these results can be used tocompute the modulus of the sample.To test whether this new technique is sensitive to themechanical properties of the clay nanoplatelets, it isnecessary to deposit the nanoplatelets on a clean, atomicallyflat substrate. Clearly, a textured substrate is notsuitable for such a technique, as the underlying substrateeffects will interfere with measurements on the systemunder study (see Figure 5). Because mica is a layeredsilicate and likely has mechanical properties similar tothe clays, highly oriented pyrolytic graphite (HOPG) wasselected as the substrate. A drop of clay suspension wasprepared as described earlier for the mica substrates.However, because HOPG is hydrophobic, the clays didnot settle out evenly over the substrate surface. Thedynamics of the drying suspension were very different,and the clays tended to pile into large clusters, makingit difficult (but not impossible) to find individual nanoplateletson the substrate. The difference between theindividual clay nanoplatelet and the HOPG substrate isquite obvious in the FMM image, as shown in Figure 7.However, Figure 7 also shows that additional materialwas adsorbed on the substrate. Because the FMMtechnique is very sensitive to the surface of the sample,any adsorbates could interfere with modulus measurementsusing this technique.AFMAnalysis of the Clay Impurities. In an attemptto drive off these adsorbates for subsequent FM Manalysis,the sample was heated to 100 C in air for an hour. Becausethe clays did not settle out evenly over the substratesurface, large clumps of clays were present (not shown)that seem to hold large amounts of the impurity. Overtime, the impurity covers the whole HOPG substrate witha thin film, as the impurity is much more mobile on HOPGthan on mica. This makes contact mode imaging of thebaked sample difficult, as shown in Figure 8. In the figurethere is one small clay nanoplatelet in the upper lef th and cornerof the image; however, the impurity dominatesthe image. This image was preceded by several 2 mscans,which swept the impurities into two parallel lines. Wefound that the more that the clays are exfoliated (byincreasing the amount of time the suspension is sonicated),the more impurity is found to be present in the water. InFigure 8, there are graphite step edges visible in theLFMimage, and the impurities seem to be attracted to theseedges and form lines.Atapping mode image of the impurityon HOPG, shown in Figure 9, shows that heating thesample drives the impurity to form a film that appears tobe ordered. As observed in the contact mode images inFigure 8, the impurity is highly mobile on a graphitesurface and tends to collect along step edges on the HOPGTo better understand the role that the impurity(ies) is(are) playing in ourAFMimages, a suspension of ME-300was prepared. The ME-300 sample was filtered througha 0.20 mdisk filter (Fisher #09-7190) so that most of theclays are removed, leaving just the impurity. Only a fewdrops of water passed through this filter before it wascompletely clogged. But this was enough that we couldget data on concentrated impurities by AFM and TOFSIMS.A drop of the filtrate was placed on a mica surfaceand allowed to dry. After drying, a film formed on themica substrate as shown in Figure 10. It was almostimpossible to scan in contact mode. However, after theAFM tip was treated with dodecylamine to reduce thecapillary force,21 amuchbetter image resulted (right imagein Figure 10). This is consistent with the impurity beinghydrophilic, as would be the case for a silicate or sodiumhydroxide/water complex. Some of this filtrate was placedon a gold foil and analyzed via time-of-flight secondaryion mass spectrometry (TOF-SIMS). The elements andmolecular fragments found by TOF-SIMS were consistentwith those in the starting material, namely, sodium,fluorine, magnesium, and silicates.The several different types of clay studied here werefound to be very similar. For example, AFM images of theME-100 sample suggest that the individual nanoplateletsin that sample are the same lateral size as those for theME-300 sample (see Figure 11).35 After the sample haddried for a day the lateral force image shows higher frictionon the clay nanoplatelets than the mica substrate,suggesting that the ME-100 sample has a very similarsurface chemistry, and comparable impurities, as theME-300 sample.Since impurities were found in all clay samples, testswere conducted to prove that the ultrasonic dispersionwas not the source of the impurity due to breaking off ofsmall parts of the clay nanoplatelets. Thus, a simple suspensionwasprepared by low-speed mechanical stirringusing a magnetic stir bar. After a week of gentle stirring,the clays were well dispersed in water, but the impuritywas also still present. A tapping mode image of a sampleprepared in this way is shown in Figure 12. Note theordering of the impurity. There is just one nanoplateletin this image, in the lower center. The humidity was 50%when this image was recorded. When tapping mode isproperly tuned, islands of water can be imaged;36 thus,the islands imaged in Figure 12 are suspected to be mostlywater. Also evident in the image are small spheres(diameters on the order of 5 nm) that are almost alwaysseen in tapping mode images of the clays and, as discussedbelow, are believed to be sodium hydroxide.At its core, AFM is a surface probe. Our results showthat these impurities can completely dominate AFMmeasurements of the clay nanoplatelets and their properties.As a further investigation into these small sphericalimpurities, a sample of ME-300 was prepared and driedin an oven at 100 C. Figure 13 is a tapping mode imagerecorded at very low humidity (less than 5%). Note thepresence of many of the small spherical impurities.Because these spheres look very similar to potassiumhydroxide particles noted by other researchers,37-39 webelieve they are sodium hydroxide formed by the reactionof the original sodium intercalation ion with water. Thishypothesis was tested by placing a drop of pure water on the surface in Figure 13. After a minute, the water wasflame tested and showed a bright orange flame indicativeof sodium. After rinsing and repeating the flame test, noevidence of sodium was visible, which suggests that thesemobile sodium ions were removed. After rinsing in water,the sample was checked again by AFM and the smallspheres were largely gone.Attempts To Separate the Clays from the Impurity.Numerous procedures to separate the nanoplateletsfrom the impurities have been attempted. While sedimentationis the obvious first choice, once exfoliated boththe nanoplatelets and the associated impurities will staysuspended in water for an indefinite amount of time.Attempts to filter this nanoplatelet-impurity suspensionwere not successful, as the pores of the filter were instantlyclogged by the nanoplatelets. Another method was todeposit the clays on a mica surface, followed by dryingand subsequent rinsing with clean water; however, boththe clays and the impurities remained on the substrate.Even when placed in boiling water for several hours, theimpurities were not removed, indicating that at least oneof the impurities is insoluble in water and cannot beremoved from the substrate once dried.Because the clay is stable to very high temperatures,attempts to drive off the impurities by heating thesubstrate were undertaken. Clays on mica were heatedas high as 500 C, but the impurities remained on thesubstrate. On glass, the clays were heated to a red heatin a propane flame; while the clays were burned off, theimpurity still remained. As discussed earlier, dc electrophoresisas a means to separate the impurities from theclays was also ineffective; thus, any surface charge on thenanoplatelets is either very weak or screened in someway.A chemical method to separate the impurities from theclay does not seem likely, as the water-insoluble impurityis probably a silicate and thus very similar to the claysthemselves. However, an amine-terminated surface wastested in the hopes that the clay nanoplatelets would stickto the amine. The surface could then be washed before itdries and the impurity flushed away, leaving the claybehind. Our first attempt at this was to modify a micasurface with 3-aminopropyltriethoxysilane by placingseveral drops of the 3-aminopropyltriethoxysilane invarious organic solvents and then dipping a freshly cleavedmica substrate into the solution for 1-5 min. Ourtechnique is a simplification of a solution-based silanemodification of silicon oxide surfaces reported by others.40,41Our results indicate that this method depends stronglyon the solvent used to deposit the silane on the micasubstrate. If toluene was used, a very flat (surfaceroughness less than 0.5 nm rms) hydophobic surfaceresulted on the mica substrate, but it had almost no affinityfor the clays. However, if ACN or DMF was used as thesolvent, the clays strongly adhered to the substrate surface.Vigorous rinsing with pure water before the sample coulddry did not remove the clay plates, suggesting that whenACN or DMF is used as solvent, the aminosilane is crosslinkingand creating a surface with active silane groupsexposed. Conceivably, these silanes then react with theclays and bind them to the surface through a covalentbond. Unfortunately, any excess silanes at the surfacealso tend to react strongly with the AFM tip. In Figure In summary, several different exfoliated clays werestudied. The individual clay nanoplatelets are approximately1 min the lateral direction and 1 nmthick. Theseclay nanoplatelets are extraordinarily compliant andreadily conform to nanoscale surface features. In theprocess of imaging these nanoplatelets, we found that theclay sheets have substantial quantities of mobile impuritiesthat could not be separated from the clay nanoplatelets.Because AFM is a surface probe, these impuritiesinterfere with precise AFM measurements, especiallyFMM, of the individual clay sheets. The impurities are oftwo types; one is water-soluble and appears to be sodiumand perhaps its counterions. The other impurity is silicatebasedand water-insoluble and is mobile on an atomicallysmooth surface. The precise nature and chemistry of theseimpurities are currently unknown and the focus ofadditional study in our lab.The presence of the impurities, and the inability toseparate them from the clay nanoplatelets, is significantand will impact the use of clay nanoplatelets in applications.For example, because of their nanometer size, theseimpurities will contribute a large surface area componentto a composite system as well as influence the interactionof the clay nanoplatelets with the matrix. The presenceof these impurities will also interfere with nanoscalemeasurements of the stiffness of a single clay sheet andis undesirable for applications where the nanoplateletsare to be used as a building block for molecular electronicsystems. It is our hope that this paper will stimulatefurther interest in clay nanoplatelets and that an awarenessof the presence of these impurities will lead to thedevelopment of rigorous clay purification methods

The results of an in-depth study of 1 nm thick individual clay sheets by atomic force microscopy (AFM) are presented. Se presentan los resultados de un estudio en profundidad de 1 nm gruesas lminas de arcilla individuales por microscopa de fuerza atmica (AFM). Several techniques have been employed, including lateral force microscopy (LFM) and force modulation microscopy (FMM). Varias tcnicas se han empleado, incluyendo la microscopa de fuerza lateral (LFM) y microscopa de fuerza de modulacin (FMM). The individual clay sheets, also referred to as clay nanoplatelets in the literature, were found to be extremely compliant and strongly adhered to a variety of substrates. Las lminas de arcilla individuales, tambin conocidos como nanoplatelets de arcilla en la literatura, se encontr que eran extremadamente compatible y fuertemente adherido a una variedad de sustratos. In all cases, these nanoplatelets were accompanied by mobile impurities that could not be separated from the nanoplatelets. En todos los casos, estos nanoplatelets fueron acompaadas por las impurezas mviles que no poda separarse de los nanoplatelets. A detailed examination of these impurities using AFM suggests that they are comprised of low molecular weight silicates as well as the intercalating salt used to process the clay.Anunderstanding of the nature and chemistry of these impurities will be necessary as models of these clays are developed for various applications, such as reinforcement in composite materials or elements for molecular electronics. Un examen detallado de estas impurezas utilizando AFM sugiere que se componen de silicatos de bajo peso molecular, as como la sal de intercalacin utilizados para procesar la clay.Anunderstanding de la naturaleza y de la qumica de estas impurezas ser necesario como se desarrollan modelos de estas arcillas para diversas aplicaciones, tales como refuerzo en materiales compuestos o elementos para la electrnica molecular. Further, these clay nanoplatelets and their impurities present a novel two-dimensional nanoscale system for future surface science studies using AFM and other methods. Adems, estos nanoplatelets arcilla y sus impurezas presentan un novedoso sistema nanoescala bidimensional para futuros estudios de la ciencia superficie utilizando AFM y otros mtodos.The term clay refers to a class of materials generally El trmino "arcilla" se refiere a una clase de materiales en general Made Hecho up of layered silicates that are similar to mica. de silicatos estratificados que son similares a la mica. While Mientras these clay particles can span centimeters in lateral estas partculas de arcilla pueden abarcar centmetros de lateral dimensions, the in-plane dimensions of the individual clay dimensiones, las dimensiones en el plano de la arcilla individuo layers are on the order of a micron, and the thickness of capas son del orden de una micra, y el espesor de a single clay nanoplatelet is on the order of a nanometer. una sola nanoplatelet arcilla es del orden de un nanmetro. These layered clays are characterized by strong intralayer Estas arcillas en capas se caracterizan por una fuerte intralayer covalent bonds within the individual sheets comprising enlaces covalentes dentro de las hojas individuales que comprenden the clay, with only weak van der Waals interactions la arcilla, con slo el dbil de van der Waals between adjacent clay sheets. entre lminas de arcilla adyacentes. These strong in-plane Estos fuertes en el plano covalent bonds have generated significant interest in the enlaces covalentes han generado un inters significativo en el use of these materials as a reinforcing element in composite El uso de estos materiales como un elemento de refuerzo en material compuesto materials. materiales. However, because of the weak interaction Sin embargo, debido a la interaccin dbil between adjacent clay sheets, a significant effort on clayreinforced entre hojas adyacentes de arcilla, un esfuerzo significativo en clayreinforced polymers has focused on methods of intercalating polmeros se ha centrado en mtodos de intercalante the polymer within the intralayer galleries, or el polmero dentro de las galeras intralayer o completely exfoliating the clays so that individual clay exfoliante completamente las arcillas de modo que la arcilla individuo sheets are dispersed within the polymer, to maximize the hojas se dispersan dentro del polmero, para maximizar la contact area (and hence the interaction) between the rea de contacto (y por tanto la interaccin) entre el polymer and the clay. polmero y la arcilla. Brillouin scattering measurements Mediciones de dispersin de Brillouin on bulk muscovite mica give in-plane moduli on the order sobre mica moscovita mayor dar mdulos en el plano del orden of 170 GPa, while the modulus perpendicular to the layers de 170 GPa, mientras que el mdulo perpendicular a las capas is given as about 60 GPa. 1 Experimental measurements se da como aproximadamente 60 GPa. 1 Mediciones experimentales on clay particles give a much lower modulus value in the en la arcilla partculas dan un valor de mdulo mucho menor en el C -direction. 2 However, we are unaware of experiments C -direccin. 2 Sin embargo, no somos conscientes de experimentos that have attempted to measure the modulus of an que han tratado de medir el mdulo de un individual clay nanoplatelet. nanoplatelet arcilla individual. Thus, the motivation of the Por lo tanto, la motivacin de la current work was to probe the mechanical properties of trabajo actual era sondear las propiedades mecnicas de individual clay nanoplatelets using AFM. arcilla individuo nanoplatelets utilizando AFM. The interest in using clay as a reinforcing phase in a El inters en el uso de arcilla como una fase de refuerzo en una polymer matrix composite was spurred by the pioneering compuesto de matriz polimrica fue estimulado por el pionero work by researchers at Toyota in the early 1990s. 3 - 5 Since trabajar por investigadores de Toyota a principios de 1990. 3 - 5 Desde that time, research into clay nanocomposites has exploded; ese momento, la investigacin de nanocompuestos de arcilla se ha disparado; the reader is referred to recent review articles for an indepth se remite al lector a los ltimos artculos de revisin para una indepth discussion of the current state of the art. 6 - 13 In discusin sobre el estado actual de la tcnica. 6 - 13 En general, the incorporation of the nanoclays into various en general, la incorporacin de las nanoarcillas en varias polymers has been shown to result in increases in tensile polmeros se ha demostrado que el resultado en un aumento de la traccin properties (elastic modulus, tensile strength), decreased propiedades (mdulo elstico, resistencia a la traccin), disminucin de la thermal expansion coefficients and improved thermal coeficientes de expansin trmica y trmica mejorada stability, increased swelling resistance, decreased gas estabilidad, aumento de la resistencia hinchazn, disminucin de gas permeability, and improved flammability properties. permeabilidad, y la mejora de las propiedades de inflamabilidad. A La new class of nanoclay - polymer composites, based on the nueva clase de nanoarcilla - compuestos de polmero, basado en el layer-by-layer assembly of individual clay nanoplatelets ensamblaje capa por capa de arcilla individuales nanoplatelets and appropriate polymers, is also under development. 14 - 17 y polmeros adecuados, tambin est en fase de desarrollo. 14 - 17 Because of the large surface area of the clay nanoplatelets Debido a la gran superficie de las nanoplatelets de arcilla within the nanocomposite, an understanding and control dentro del nanocompuesto, la comprensin y el control of the clay surface are expected to play an important role de la superficie de arcilla se espera que juegue un papel importante in the optimization of these material systems. en la optimizacin de estos sistemas de materiales. Models of Modelos de the mechanical properties of the individual clay nanoplatelets, las propiedades mecnicas de los nanoplatelets de arcilla individuales, as input parameters into models of effective clay como parmetros de entrada en los modelos de arcilla efectiva nanocomposite behavior, will also be critical. comportamiento nanocompuesto, tambin ser fundamental. However, these clay nanoplatelets are interesting above Sin embargo, estos nanoplatelets de arcilla son interesantes por encima and beyond their use as an element of composite materials. y ms all de su uso como un elemento de materiales compuestos. Because the individual clay platelet thicknesses are on Debido a que los espesores de plaquetas de arcilla individuales son en the order of 1 nm, with lateral dimensions on the micron del orden de 1 nm, con dimensiones laterales en el micras scale, they are the two-dimensional analogue of zerodimensional escala, son el anlogo bidimensional de zerodimensional (quantum dots, nanoclusters) and onedimensional (Puntos cunticos, nanoclusters) y unidimensional (nanotube, nanowires) materials. (nanotubos, nanocables) materiales. In addition, Adicionalmente, depending on their thermal and electrical properties, the en funcin de sus propiedades trmicas y elctricas, la silicate nanoplatelets are a potential building block for nanoplatelets silicato son un bloque potencial de construccin de nanoscopic electrical devices. dispositivos elctricos nanoscpicas. The presence of impurities La presencia de impurezas associated with the clay nanoplatelets, and the inability asociado con los nanoplatelets de arcilla, y la incapacidad to separate these nanoplatelets from the impurities, separar estos nanoplatelets de las impurezas, complicate the surface chemistry of these materials. 18,19 complicar la qumica de la superficie de estos materiales. 18,19 Unless methods are found for their removal, these A menos que los mtodos se encuentran para su eliminacin, stas impurities will prevent accurate measurements of the impurezas evitar mediciones precisas de la mechanical, electrical, and thermal properties of the propiedades mecnicas, elctricas y trmicas de la individual clay nanoplatelets. nanoplatelets de arcilla individuales. Experimental Section Seccion Experimental Reagent grade chemicals were used as delivered. Se utilizaron productos qumicos de grado reactivo como se entrega. Tetrahydrofuran El tetrahidrofurano (THF), acetonitrile (ACN), N , N -dimethylformamide (THF), acetonitrilo (ACN), N, N-dimetilformamida (DMF), and dodecylamine were from Aldrich. (DMF), y dodecilamina eran de Aldrich. 3-Aminopropyltriethoxysilane 3-aminopropiltrietoxisilano was from Alfa-Aesar, and toluene was from era de Alfa-Aesar, y tolueno fue de Fisher. Fisher. Waterwaspurified using a Barnstead modelD6431threecartridge Waterwaspurified usando un Barnstead modelD6431threecartridge filter system. sistema de filtro. Mica substrates were from Ted Pella, Sustratos de mica fueron de Ted Pella, and highly ordered pyrolytic graphite (HOPG) was from SPI. y altamente ordenada grafito piroltico (HOPG) era de SPI. Gold films were deposited with an Edwards model 306 e-beam Pelculas de oro fueron depositados en un modelo de Edwards 306 e-beam evaporation system. sistema de evaporacin. Ultrasonication was done using a Crest Ultrasonidos se realiz utilizando una cresta model 175HT, 50 W bath. modelo 175HT, 50 W bao. Clay Characterization . Caracterizacin de la arcilla. All images were recorded on Park Todas las imgenes fueron grabadas en el Parque Scientific model CP Research AFMs. Modelo CP Cientfico AFMs investigacin. Contact mode images were Imgenes en modo de contacto eran made with Park Scientific microlevers with silicon nitride tips. hecho con microlevers Parque Cientfico con puntas de nitruro de silicio. Tapping mode images weremadewith Park Scientific noncontact Tapping imgenes en modo sin contacto weremadewith Parque Cientfico ultralevers with silicon tips. ultralevers con puntas de silicio. (Note that Park Scientific is now (Tenga en cuenta que el Parque Cientfico es ahora owned by Veeco.) Typical scan speeds were 1 or 2 Hz. propiedad de Veeco.) velocidades de exploracin tpicos eran 1 o 2 Hz. The contact El contacto mode set point was for a normal force of 1 nN. punto de establecer el modo fuera por una fuerza normal de 1 nN. In tapping mode, En el modo de grabacin, the resonant frequency was about 90 kHz, with small variations la frecuencia resonante fue de aproximadamente 90 kHz, con pequeas variaciones from cantilever to cantilever. de voladizo a voladizo. Time-of-flight secondary ion mass Tiempo de vuelo de masas de iones secundarios spectrometry (TOF-SIMS) was performed using a Physical espectrometra (TOF-SIMS) se realiz con un fsico Electronics model Thrift III. Modelo de Electrnica Thrift III. Processing of Clays . Procesamiento de las arcillas. A number of different clays samples Un nmero de diferentes muestras de arcillas were examined in this work (see Table 1). fueron examinados en este trabajo (ver Tabla 1). Other than details of Aparte de los detalles de lateral dimensions, all clays looked very similar by AFM. dimensiones laterales, todas las arcillas se vean muy similares por AFM. The La exfoliation, mechanical behavior, and presence of impurities also exfoliacin, comportamiento mecnico, y la presencia de impurezas tambin appeared to be similar for all samples. pareci ser similar para todas las muestras. Thus, our efforts focused Por lo tanto, nuestros esfuerzos se centraron on the SOMASIF ME-300 sample, a synthetic clay made from en la ME-300 muestra SOMASIF, una arcilla "sinttico" hecha de natural talc. talco natural. According to the manufacturer (UniCoOP, Tokyo, Segn el fabricante (Unicoop, Tokio, Japan), the clay is processed by heating talc in the presence of Japn), la arcilla se procesa mediante el calentamiento de talco en presencia de Na 2 SiF 6 , such that the salt decomposes and the Na ions Na 2 SiF 6, de tal manera que la sal se descompone y los iones Na intercalate between the silicate layers that comprise the talc. intercalado entre las capas de silicato que comprenden el talco. Whenplaced in water and agitated, water can enter the galleries Whenplaced en agua y agitado, el agua puede entrar en las galeras and further swell the talc until it becomes completely exfoliated. y se hinchan an ms el talco hasta que se vuelve completamente exfoliada. To study individual clay nanoplatelets viaAFM,it is necessary Para estudiar arcilla individuo nanoplatelets viaAFM, es necesario to first fully exfoliate the clay crystals, followed by deposition on a primera exfoliar completamente los cristales de arcilla, seguido de la deposicin sobre a suitable substrate. un sustrato adecuado. Unless otherwise specified in the text, the A menos que se especifique lo contrario en el texto, el individual clay nanoplatelets were prepared in the following nanoplatelets de arcilla individuales se prepararon de la siguiente manner. forma. First, a few micrograms of the clay powder was placed En primer lugar, unos pocos microgramos de polvo de arcilla se coloc in 10 mL of pure water. en 10 ml de agua pura. The clays were exfoliated in a low-power Las arcillas se exfolian en un bajo consumo de energa ultrasonic bath for 8 - 16 h. bao de ultrasonidos durante 8-16 h. After this, the resulting suspension Despus de esto, la suspensin resultante was allowed to settle for a day or more. se deja reposar durante un da o ms. Sedimentation allows the nonexfoliated clays to sink, while the fully exfoliated individual La sedimentacin permite que las arcillas nonexfoliated a hundirse, mientras que el individuo totalmente exfoliada clay sheets will stay in suspension for months. lminas de arcilla se quedarn en suspensin durante meses. A drop of the Una gota de la suspension is then placed on a substrate and allowed to stand la suspensin se coloca entonces sobre un sustrato y se deja reposar for a few minutes. por unos pocos minutos. The excess water is then removed by a highvelocity El exceso de agua se elimin entonces por un highvelocity jet of compressed gas or is wicked away. chorro de gas comprimido o es malo de distancia. Clay nanoplatelet Nanoplatelet Arcilla coverage on a substrate can be controlled by adjusting the la cobertura sobre un sustrato se puede controlar mediante el ajuste de la concentration of the suspension and the time it is exposed to the concentracin de la suspensin y el tiempo que se expone a la substrate. sustrato. Results and Discussion Resultados y discusin Initial Measurements of Individual Clay Nanoplatelets . Las mediciones iniciales de arcilla Nanoplatelets individual. InitialAFMimages were obtained by depositing InitialAFMimages se obtuvieron mediante el depsito de the clay nanoplatelets on mica. la arcilla nanoplatelets sobre mica. Mica is ideal for this initial Mica es ideal para este inicial imaging because it is atomically flat and adds nothing to proyeccin de imagen, ya que es atmicamente plana y no aade nada a the topology of the system, making it easier to understand la topologa del sistema, por lo que es ms fcil de entender the topological features of the clay nanoplatelets.Through las caractersticas topolgicas de la arcilla nanoplatelets.Through trial and error, it was possible to get nanoplatelets evenly ensayo y error, fue posible conseguir nanoplatelets uniformemente distributed in submonolayer coverage on the mica surface. distribuido en la cobertura submonolayer en la superficie de la mica. One can distinguish individual clay nanoplatelets by their Se puede distinguir nanoplatelets arcilla individuales por su thickness, as they are 1 nm high in all AFM topology espesor, ya que son 1 nm altos en todas topologa AFM images as shown in Figure 1. Areas where multiple clay imgenes, como se muestra en la Figura 1. Las reas donde la arcilla mltiple nanoplatelets are stacked on top of each other are also nanoplatelets se apilan en la parte superior de cada otro son tambin visible in the image. visible en la imagen. For these initial images of the nanoplatelets, contact Por estas imgenes iniciales de los nanoplatelets, contactos mode AFM was used in parallel with lateral force modo de AFM se utiliz en paralelo con fuerza lateral microscopy (LFM). microscopa (LFM). Because LFM is very sensitive to Debido a LFM es muy sensible a differences in the chemistry of the surface, it would indicate las diferencias en la qumica de la superficie, se indicaran any differences in chemistry between the clays and the cualquier diferencia en la qumica entre las arcillas y el mica substrate. sustrato de mica. This is mostly due to the effects of capillary Esto se debe principalmente a los efectos de capilaridad forces and the amount of adsorbed water on a sample fuerzas y la cantidad de agua adsorbida sobre una muestra surface. 20 - 23 . superficie 20 - 23 de AFM and LFM images of a clay nanoplateletsample directly after deposition, and 5 days after deposition,on mica are shown in Figure 2. AFM y LFM imgenes de un nanoplateletsample arcilla directamente despus de la deposicin, y 5 das despus de la deposicin, en la mica se muestran en la Figura 2. While the surface Mientras que la superficie topology of the clay samples remains unchanged, the topologa de las muestras de arcilla permanece inalterada, el lateral force contrast has reversed over a period of days contraste fuerza lateral se ha invertido durante un perodo de das (ie, the clay nanoplatelets are light and dark respectively (Es decir, los nanoplatelets arcilla son claros y oscuros, respectivamente in parts B and D of Figure 1). en las partes B y D de la figura 1). We believe (see discussion Creemos que (vase la discusin below) that this change in lateral force contrast was due a continuacin) que este cambio en contraste fuerza lateral era debido to the migration of impurities. a la migracin de impurezas. At room temperature these A temperatura ambiente estos impurities are highly mobile and strongly interact with impurezas son altamente mviles e interactan fuertemente con the AFM tip. la punta del AFM. In later experiments, it was discovered that En experimentos posteriores, se descubri que this change in contrast can be reversed by rinsing in water, este cambio de contraste puede ser revertida mediante lavado en agua, such that the friction on the clay nanoplatelets returns to de tal manera que la friccin en la arcilla nanoplatelets vuelve a being lower than on the mica substrate. siendo ms bajo que en el sustrato de mica. If the sample is Si la muestra es then dried by heating in an oven or by rinsing in ethanol, luego se sec por calentamiento en un horno o mediante lavado en etanol, the contrast again reverses so that friction on the el contraste se invierte de nuevo para que la friccin en el nanoplatelets is higher than the mica substrate. nanoplatelets es ms alto que el sustrato de mica. However, Sin embargo, after repeating this process several times, the friction on despus de repetir este proceso varias veces, la friccin en the clay nanoplatelets remained higher than on the mica los nanoplatelets arcilla permanecieron ms altos que en la mica substrate. sustrato. Currently, we believe that this behavior is due Actualmente, creemos que este comportamiento es debido to a redistribution of impurities and their effect on the a una redistribucin de las impurezas y su efecto sobre la local wetting properties of the sample. propiedades de humectacin locales de la muestra. Rinsing does not El enjuague no lo hace remove the impurities, which seem to be strongly attracted eliminar las impurezas, que parecen estar fuertemente atrado to both the clay and the mica. tanto a la arcilla y la mica. However, the impurities Sin embargo, las impurezas are mobile and seem to prefer to locate on the surface of son mviles y parecen preferir para localizar en la superficie de the clay nanoplatelets. los nanoplatelets arcilla. After repeated rinsing they are Despus de enjuagar repetido que son mostly moved to the top of the clays. en su mayora se traslad a la parte superior de las arcillas. The presence of these La presencia de estos impurities is significant, as they will alter the surface impurezas es significativo, ya que alterar la superficie chemistry of the nanoplatelets and are likely to impact qumica de los nanoplatelets y puedan afectar the use of fully exfoliated clays in applications. el uso de arcillas exfoliadas plenamente en las aplicaciones. Compliance of Individual Clay Nanoplatelets . Cumplimiento de arcilla individuales Nanoplatelets. Disregarding the issue of impurities for the moment, one Sin tener en cuenta el tema de impurezas, por el momento, una of the long-term goals of our research is to understand the de los objetivos a largo plazo de nuestra investigacin es entender la mechanics of these clay nanoplatelets. mecnica de estos nanoplatelets arcilla. Our first approach Nuestro primer enfoque was to place individual nanoplatelets across a very narrow era colocar nanoplatelets individuales a travs de una muy estrecha gap cut into a glass substrate in order to measure the brecha cortado en un sustrato de vidrio con el fin de medir la modulus via a force vs displacement curve on a suspended mdulo a travs de una fuerza vs curva de desplazamiento en una suspendido nanoplatelet. nanoplatelet. To test this capability, a clay nanoplatelet Para probar esta capacidad, un nanoplatelet arcilla was deposited across a 100 nm wide scratch in a glass se deposit en una amplia cero 100 nm en un vaso substrate, as shown in Figure 3. Rather than spanning sustrato, como se muestra en la Figura 3. En lugar de que abarca the trench, the clay sinks into the scratch and readily la zanja, los sumideros de arcilla en el cero y fcilmente conforms to it. se ajusta a la misma. This is consistent with molecular models Esto es coherente con los modelos moleculares that show that the clay nanoplatelets should be very que muestran que los nanoplatelets arcilla deben ser muy compliant in the out-of-plane direction. 24 compatible en la direccin fuera del plano. 24 To test whether a smaller gap is necessary for the force Para probar si un hueco ms pequeo es necesario para la fuerza de vs displacement drum-skin test, clay nanoplatelets were vs desplazamiento "tambor de piel de" prueba, nanoplatelets arcilla eran deposited on a nanoporous alumina substrate with uniform depositado sobre un sustrato de almina nanoporoso con uniforme 60 nm pores spaced 100 nm apart. 25 However, similar to 60 poros espaciados nm 100 nm aparte. 25 Sin embargo, similar a our findings with the glass substrate, when placed on a nuestros hallazgos con el sustrato de vidrio, cuando se coloca en una nanoporous alumina substrate the clay nanoplatelets sink sustrato de almina nanoporoso los nanoplatelets de arcilla se hunden into the pores and appear wrinkled, as shown in Figure en los poros y aparecen "arrugada", como se muestra en la figura 4. The friction on the clay is lower than the alumina 4. La friccin en la arcilla es menor que la almina substrate (see Figure 4b), similar to the case for the mica sustrato (vase la figura 4b), similar al caso de la mica substrate. sustrato. Returning to the issue of impurities, initial attempts to Volviendo al tema de las impurezas, los intentos iniciales a separate the impurities from the clay nanoplatelets used separar las impurezas de los nanoplatelets de arcilla utilizados an electrophoretic technique. una tcnica electrofortica. If the nanoplatelets are Si los nanoplatelets son charged, the application of an electric current might drive cargada, la aplicacin de una corriente elctrica podra conducir the nanoplatelets to one of the gold electrodes and leave los nanoplatelets a uno de los electrodos de oro y dejar any impurities in the water. las impurezas en el agua. The electrode was a 20 nm El electrodo fue un 20 nm thick polycrystalline gold film evaporated onto mica. gruesa pelcula de oro policristalino evapor sobre mica. These Estos gold electrodes were placed in an aqueous nanoplatelet electrodos de oro se colocaron en un nanoplatelet acuosa suspension, and a voltage was applied until a current of suspensin, y una tensin se aplic hasta que una corriente de about 1 mA could be detected. aproximadamente 1 mA poda ser detectado. This method was not Este mtodo no era successful, as very few nanoplatelets were found on the xito, ya que muy pocos nanoplatelets se encontraron en el positive electrode (and none on the negative electrode). electrodo positivo (y ninguno en el electrodo negativo). A La few of the clay nanoplatelets that were found on the algunas de las nanoplatelets arcilla que se encontraron en el positive electrode are shown in Figure 5. Because the electrodo positivo se muestra en la Figura 5. Debido a que el individual clay nanoplatelets are so compliant in the outof- nanoplatelets arcilla individuales son tan compatibles en el outof- plane direction, the grains of the gold substrate can direccin del plano, los granos del sustrato de oro puede clearly be seen below the clay plates. verse claramente por debajo de las placas de la arcilla. The friction is lower La friccin es menor on the clay nanoplatelets than on the supporting gold film. en los nanoplatelets de arcilla que en la pelcula de oro de soporte. On a few occasions, we observed large areas where the En algunas ocasiones, observamos grandes reas en las que los clays formed almost complete monolayers. arcillas forman monocapas casi completos. This may be Esto puede ser related to surface charge on the clay nanoplatelets. en relacin con la superficie de carga en los nanoplatelets arcilla. Under Bajo an optical microscope, large rafts of the clays are sometimes un microscopio ptico, grandes balsas de las arcillas son a veces seen floating on the surface of the water as a sample visto flotando en la superficie del agua como una muestra is drying, possibly indicating that the nanoplatelets order est secando, posiblemente, lo que indica que los nanoplatelets orden themselves on the surface of the water and are then left a s mismos sobre la superficie del agua y luego se dejan behind as the water film dries. detrs, como la pelcula de agua se seca. On mica, water dries as En la mica, el agua se seca como a more or less uniform film and does not break up into una pelcula y ms o menos uniforme no se rompen en small droplets. pequeas gotitas. A tapping mode AFM image on mica A modo de imagen tocando AFM sobre mica demonstrating this tiling effect is shown in Figure 6. demostrar este efecto de mosaico se muestra en la Figura 6. Force Modulation Microscopy on the Clay Nanoplatelets . Fuerza Modulacin Microscopa en la arcilla Nanoplatelets. As our previous results indicate, the extreme A medida que nuestros resultados anteriores indican, la extrema out-of-plane compliance of the individual clay nanoplatelets fuera de plano cumplimiento de los nanoplatelets de arcilla individuales suggests that meaningful modulus values obtained sugiere que los valores de mdulo significativa obtuvieron using a force vs displacement drum-skin test are unlikely. utilizando una fuerza vs prueba de tambor de piel de desplazamiento es poco probable. However, there is a new AFM technique known as force Sin embargo, hay una nueva tcnica de AFM conocido como vigor modulation microscopy (FMM) that can, in principle, microscopa de modulacin (FMM) que puede, en principio, directly measure the modulus of a material at the surface. medir directamente el mdulo de un material en la superficie. In FMM, the normal force between the tip and sample is En FMM, la fuerza normal entre la punta y la muestra es modulated. modulada. The amplitude and phase of the response can La amplitud y fase de la respuesta puede be used to calculate the contact stiffness of the surface of ser utilizado para calcular la rigidez de contacto de la superficie de the sample. la muestra. In principle, these results can be used to En principio, estos resultados se pueden utilizar para compute the modulus of the sample. calcular el mdulo de la muestra. To test whether this new technique is sensitive to the Para probar si esta nueva tcnica es sensible a la mechanical properties of the clay nanoplatelets, it is propiedades mecnicas de los nanoplatelets de arcilla, es necessary to deposit the nanoplatelets on a clean, atomically necesario depositar los nanoplatelets en un lugar limpio, atmicamente flat substrate. sustrato plano. Clearly, a textured substrate is not Claramente, un sustrato texturizado no es suitable for such a technique, as the underlying substrate adecuado para una tcnica tal, como el sustrato subyacente effects will interfere with measurements on the system efectos interferirn con las mediciones en el sistema de under study (see Figure 5). en estudio (ver Figura 5). Because mica is a layered Debido a mica es un estratificado silicate and likely has mechanical properties similar to silicato y probablemente tiene propiedades mecnicas similares a the clays, highly oriented pyrolytic graphite (HOPG) was las arcillas, grafito piroltico altamente orientado (HOPG) era selected as the substrate. seleccionado como el sustrato. A drop of clay suspension was Una gota de suspensin de arcilla era prepared as described earlier for the mica substrates. preparado como se describe anteriormente para los sustratos de mica. However, because HOPG is hydrophobic, the clays did Sin embargo, debido HOPG es hidrfobo, las arcillas hicieron not settle out evenly over the substrate surface. No se conforme de manera uniforme sobre la superficie del sustrato. The La dynamics of the drying suspension were very different, dinmica de la suspensin de secado eran muy diferentes, and the clays tended to pile into large clusters, making y las arcillas tienden a acumularse en grandes grupos, por lo que it difficult (but not impossible) to find individual nanoplatelets difcil (pero no imposible) encontrar nanoplatelets individuales on the substrate. sobre el sustrato. The difference between the La diferencia entre el individual clay nanoplatelet and the HOPG substrate is nanoplatelet arcilla individuo y el sustrato es HOPG quite obvious in the FMM image, as shown in Figure 7. bastante obvio en la imagen FMM, como se muestra en la Figura 7. However, Figure 7 also shows that additional material Sin embargo, la Figura 7 tambin muestra que el material adicional was adsorbed on the substrate. se adsorbi sobre el sustrato. Because the FMM Debido a que el FMM technique is very sensitive to the surface of the sample, tcnica es muy sensible a la superficie de la muestra, any adsorbates could interfere with modulus measurements cualquier adsorbatos podran interferir con las mediciones de mdulo using this technique. utilizando esta tcnica. AFMAnalysis of the Clay Impurities . AFMAnalysis de las impurezas de la arcilla. In an attempt En un intento to drive off these adsorbates for subsequent FM Manalysis, para eliminar estos adsorbatos para su posterior FM Manalysis, the sample was heated to 100 C in air for an hour. la muestra se calent a 100 C en aire durante una hora. Because Porque the clays did not settle out evenly over the substrate las arcillas no se asientan de manera uniforme sobre el sustrato surface, large clumps of clays were present (not shown) superficie, grandes cmulos de arcillas estaban presentes (no mostrado) that seem to hold large amounts of the impurity. que parecen contener grandes cantidades de la impureza. Over Encima time, the impurity covers the whole HOPG substrate with tiempo, la impureza cubre todo el sustrato con HOPG a thin film, as the impurity is much more mobile on HOPG una pelcula delgada, como la impureza es mucho ms mvil en el HOPG than on mica. que en mica. This makes contact mode imaging of the Esto hace que el modo de contacto de formacin de imgenes de la baked sample difficult, as shown in Figure 8. In the figure muestra al horno difcil, como se muestra en la figura 8. En la figura there is one small clay nanoplatelet in the upper lef th and corner hay una pequea nanoplatelet arcilla en el XX lef superior y esquina of the image; de la imagen; however, the impurity dominates sin embargo, la impureza domina the image. la imagen. This image was preceded by several 2 mscans, Esta imagen fue precedido por varios 2 mscans, which swept the impurities into two parallel lines. que barrieron las impurezas en dos lneas paralelas. We Nosotros found that the more that the clays are exfoliated (by encontrado que el ms que las arcillas se exfoliadas (por increasing the amount of time the suspension is sonicated), aumentando la cantidad de tiempo que la suspensin se someti a ultrasonidos), the more impurity is found to be present in the water. el ms impureza se encuentra para estar presente en el agua. In En Figure 8, there are graphite step edges visible in theLFM La Figura 8, hay bordes de los escalones de grafito visibles en theLFM image, and the impurities seem to be attracted to these imagen, y las impurezas parecen ser atrados a estos edges and form lines.Atapping mode image of the impurity imagen modo de bordes y forma lines.Atapping de la impureza on HOPG, shown in Figure 9, shows that heating the en HOPG, que se muestra en la Figura 9, muestra que el calentamiento de la sample drives the impurity to form a film that appears to muestra impulsa la impureza para formar una pelcula que parece be ordered. ser ordenado. As observed in the contact mode images in Como se observa en las imgenes en modo de contacto en Figure 8, the impurity is highly mobile on a graphite La Figura 8, la impureza es altamente mvil en un grafito surface and tends to collect along step edges on the HOPG superficial y tiende a acumularse a lo largo de los bordes de paso en el HOPG To better understand the role that the impurity(ies) is Para entender mejor el papel que la impureza (es) es (are) playing in ourAFMimages, a suspension of ME-300 (Estn) jugando en ourAFMimages, una suspensin de ME-300 was prepared. se prepar. The ME-300 sample was filtered through La muestra ME-300 se filtr a travs a 0.20 mdisk filter (Fisher #09-7190) so that most of the un 0,20 filtro mdisk (Fisher # 09 a 7190), de modo que la mayor parte de la clays are removed, leaving just the impurity. arcillas se retiran, dejando slo la impureza. Only a few Slo unos pocos drops of water passed through this filter before it was gotas de agua pasan a travs de este filtro antes de que fuera completely clogged. completamente obstruido. But this was enough that we could Pero esto fue suficiente para que pudiramos get data on concentrated impurities by AFM and TOFSIMS. obtener datos sobre impurezas concentradas por AFM y TOFSIMS. A drop of the filtrate was placed on a mica surface Una gota del filtrado se coloca sobre una superficie de mica and allowed to dry. y se deja secar. After drying, a film formed on the Despus del secado, una pelcula formada sobre la mica substrate as shown in Figure 10. It was almost sustrato mica como se muestra en la Figura 10. Era casi impossible to scan in contact mode. imposible escanear en modo de contacto. However, after the Sin embargo, despus de la AFM tip was treated with dodecylamine to reduce the AFM punta fue tratado con dodecilamina para reducir el capillary force, 21 amuchbetter image resulted (right image fuerza capilar, 21 imagen amuchbetter resultado (imagen de la derecha in Figure 10). en la Figura 10). This is consistent with the impurity being Esto es consistente con ser la impureza hydrophilic, as would be the case for a silicate or sodium hidrfila, como sera el caso de un silicato de sodio o hydroxide/water complex. / complejo acutico de hidrxido. Some of this filtrate was placed Algunos de este filtrado se coloc on a gold foil and analyzed via time-of-flight secondary en una lmina de oro y se analizaron a travs de tiempo-de-vuelo secundaria ion mass spectrometry (TOF-SIMS). espectrometra de masas de iones (TOF-SIMS). The elements and Los elementos y molecular fragments found by TOF-SIMS were consistent fragmentos moleculares que han encontrado los TOF-SIMS fueron consistentes with those in the starting material, namely, sodium, con los del material de partida, a saber, sodio, fluorine, magnesium, and silicates. flor, magnesio y silicatos. The several different types of clay studied here were Los diferentes tipos de arcilla fueron estudiados aqu found to be very similar. encontrado para ser muy similar. For example, AFM images of the Por ejemplo, las imgenes de AFM de la ME-100 sample suggest that the individual nanoplatelets Muestra ME-100 sugieren que los nanoplatelets individuales in that sample are the same lateral size as those for the en esa muestra son del mismo tamao lateral que los de la ME-300 sample (see Figure 11). 35 After the sample had ME-300 de la muestra (vase la Figura 11). 35 Despus de la muestra tena dried for a day the lateral force image shows higher friction secado por un da la imagen de fuerza lateral muestra mayor friccin on the clay nanoplatelets than the mica substrate, en los nanoplatelets arcilla que el sustrato de mica, suggesting that the ME-100 sample has a very similar lo que sugiere que la muestra ME-100 tiene una muy similar surface chemistry, and comparable impurities, as the qumica de la superficie, y las impurezas comparables, como el ME-300 sample. Muestra de ME-300. Since impurities were found in all clay samples, tests Dado que se encontraron en todas las impurezas de arcilla muestras, pruebas were conducted to prove that the ultrasonic dispersion se llevaron a cabo para demostrar que la dispersin ultrasnica was not the source of the impurity due to breaking off of no era la fuente de la impureza debido a la ruptura de las small parts of the clay nanoplatelets. pequeas partes de los nanoplatelets arcilla. Thus, a simple suspensionwasprepared by low-speed mechanical stirring De este modo, un simple suspensionwasprepared por agitacin mecnica de baja velocidad using a magnetic stir bar. usando una barra de agitacin magntica. After a week of gentle stirring, Tras una semana de agitacin suave, the clays were well dispersed in water, but the impurity las arcillas se dispersan bien en agua, pero la impureza was also still present. tambin todava estaba presente. A tapping mode image of a sample Una imagen modo de aprovechar de una muestra prepared in this way is shown in Figure 12. Note the preparado de esta manera se muestra en la Figura 12. Nota del ordering of the impurity. ordenamiento de la impureza. There is just one nanoplatelet Slo hay una nanoplatelet in this image, in the lower center. en esta imagen, en el centro inferior. The humidity was 50% La humedad era 50% when this image was recorded. cuando se registr esta imagen. When tapping mode is Cuando el modo de tocar es properly tuned, islands of water can be imaged; 36 thus, bien afinadas, islas de agua se pueden obtener imgenes; 36 por tanto, the islands imaged in Figure 12 are suspected to be mostly las islas fotografiadas en la Figura 12 son sospechosos de ser en su mayora water. agua. Also evident in the image are small spheres Tambin evidente en la imagen son pequeas esferas (diameters on the order of 5 nm) that are almost always (dimetros del orden de 5 nm) que son casi siempre seen in tapping mode images of the clays and, as discussed visto en el aprovechamiento de las imgenes en modo de las arcillas y, como se discute below, are believed to be sodium hydroxide. a continuacin, se considera que es hidrxido de sodio. At its core, AFM is a surface probe. En su esencia, AFM es una sonda de superficie. Our results show Nuestros resultados muestran that these impurities can completely dominate AFM que estas impurezas pueden dominar por completo AFM measurements of the clay nanoplatelets and their properties. mediciones de las nanoplatelets de arcilla y sus propiedades. As a further investigation into these small spherical Como una investigacin ms a fondo en estos pequeos esfrica impurities, a sample of ME-300 was prepared and dried impurezas, se prepar una muestra de ME-300 y se secaron in an oven at 100 C. en un horno a 100 C. Figure 13 is a tapping mode image La Figura 13 es una imagen de modo tapping recorded at very low humidity (less than 5%). grabado en humedad muy baja (menos de 5%). Note the Tenga en cuenta la presence of many of the small spherical impurities. presencia de muchas de las pequeas impurezas esfricas. Because these spheres look very similar to potassium Debido a que estas esferas son muy similares al potasio hydroxide particles noted by other researchers, 37 - 39 we partculas de hidrxido observados por otros investigadores, 37-39 nos believe they are sodium hydroxide formed by the reaction Creo que son hidrxido de sodio formados por la reaccin of the original sodium intercalation ion with water. de la intercalacin de iones de sodio original con agua. This Este hypothesis was tested by placing a drop of pure water on the surface in Figure 13. After a minute, the water was hiptesis fue probada colocando una gota de agua pura en la superficie en la figura 13. Despus de un minuto, el agua estaba flame tested and showed a bright orange flame indicative llama a prueba y mostr una llama de color naranja brillante indicativa of sodium. de sodio. After rinsing and repeating the flame test, no Despus de enjuagar y repitiendo la prueba de la llama, sin evidence of sodium was visible, which suggests that these evidencia de sodio era visible, lo que sugiere que estos mobile sodium ions were removed. Se eliminaron los iones de sodio mviles. After rinsing in water, Despus de enjuagar en agua, the sample was checked again by AFM and the small la muestra se comprob de nuevo por AFM y la pequea spheres were largely gone. esferas haban desaparecido en gran parte. Attempts To Separate the Clays from the Impurity . Los intentos de separar la Arcillas de la impureza. Numerous procedures to separate the nanoplatelets Numerosos procedimientos para separar las nanoplatelets from the impurities have been attempted. de las impurezas se han intentado. While sedimentation Mientras que la sedimentacin is the obvious first choice, once exfoliated both es la primera opcin obvia, una vez exfoliada ambos the nanoplatelets and the associated impurities will stay los nanoplatelets y las impurezas asociadas permanecern suspended in water for an indefinite amount of time. suspendida en agua durante un perodo indefinido de tiempo. Attempts to filter this nanoplatelet-impurity suspension Los intentos de filtrar esta suspensin nanoplatelet-impureza were not successful, as the pores of the filter were instantly no tuvieron xito, ya que los poros del filtro fueron instantneamente clogged by the nanoplatelets. obstruido por los nanoplatelets. Another method was to Otro mtodo consista en deposit the clays on a mica surface, followed by drying depositar las arcillas en una superficie de mica, seguido de secado and subsequent rinsing with clean water; y posterior aclarado con agua limpia; however, both sin embargo, tanto the clays and the impurities remained on the substrate. las arcillas y las impurezas permanecieron sobre el sustrato. Even when placed in boiling water for several hours, the Incluso cuando se colocan en agua hirviendo durante varias horas, la impurities were not removed, indicating that at least one impurezas no fueron quitados, lo que indica que al menos una of the impurities is insoluble in water and cannot be de las impurezas es insoluble en agua y no puede ser removed from the substrate once dried. retirado del sustrato una vez seca. Because the clay is stable to very high temperatures, Debido a que la arcilla es estable a temperaturas muy altas, attempts to drive off the impurities by heating the los intentos para eliminar las impurezas por calentamiento de la substrate were undertaken. se llevaron a cabo sustrato. Clays on mica were heated Arcillas en mica se calentaron as high as 500 C, but the impurities remained on the tan alta como 500 C, pero las impurezas permanecieron en el substrate. sustrato. On glass, the clays were heated to a red heat En el vidrio, las arcillas se calentaron a un calor rojo in a propane flame; en una llama de propano; while the clays were burned off, the mientras que las arcillas se queman, la impurity still remained. impureza an permaneca. As discussed earlier, dc electrophoresis Como se seal anteriormente, la electroforesis dc as a means to separate the impurities from the como un medio para separar las impurezas de la clays was also ineffective; arcillas tambin fue ineficaz; thus, any surface charge on the Por lo tanto, cualquier carga de la superficie en el nanoplatelets is either very weak or screened in some nanoplatelets es o muy dbil o proyectado en algunos way. manera. A chemical method to separate the impurities from the Un mtodo qumico para separar las impurezas de la clay does not seem likely, as the water-insoluble impurity arcilla no parece probable, ya que la impureza insoluble en agua is probably a silicate and thus very similar to the clays es probablemente un silicato y por lo tanto muy similar a las arcillas themselves. s mismos. However, an amine-terminated surface was Sin embargo, una superficie terminado en amina era tested in the hopes that the clay nanoplatelets would stick probado con la esperanza de que los nanoplatelets arcilla se apegar to the amine. a la amina. The surface could then be washed before it La superficie podra entonces ser lavado antes de que se dries and the impurity flushed away, leaving the clay se seca y la impureza vacan, dejando la arcilla behind. detrs. Our first attempt at this was to modify a mica Nuestro primer intento en esta era modificar una mica surface with 3-aminopropyltriethoxysilane by placing la superficie con 3-aminopropiltrietoxisilano colocando several drops of the 3-aminopropyltriethoxysilane in varias gotas de la 3-aminopropiltrietoxisilano en various organic solvents and then dipping a freshly cleaved diversos disolventes orgnicos y luego sumergiendo un recin escindido mica substrate into the solution for 1 - 5 min. mica sustrato en la solucin durante 1 - 5 min. Our Nuestro technique is a simplification of a solution-based silane tcnica es una simplificacin de un silano basado en soluciones modification of silicon oxide surfaces reported by others. modificacin de superficies de xido de silicio reportados por otros. 40,41 40,41 Our results indicate that this method depends strongly Nuestros resultados indican que este mtodo depende en gran on the solvent used to deposit the silane on the mica en el disolvente utilizado para depositar el silano sobre la mica substrate. sustrato. If toluene was used, a very flat (surface Si se utiliz tolueno, un muy plana (superficie roughness less than 0.5 nm rms) hydophobic surface rugosidad de menos de 0,5 nm RMS) de superficie hydophobic resulted on the mica substrate, but it had almost no affinity resultado sobre el sustrato de mica, pero tena casi ninguna afinidad for the clays. para las arcillas. However, if ACN or DMF was used as the Sin embargo, si se utiliz ACN o DMF como el solvent, the clays strongly adhered to the substrate surface. disolvente, las arcillas fuertemente adheridas a la superficie del sustrato. Vigorous rinsing with pure water before the sample could Aclarado vigoroso con agua pura antes de la muestra pudo dry did not remove the clay plates, suggesting that when seca no quit las placas de arcilla, lo que sugiere que cuando ACN or DMF is used as solvent, the aminosilane is crosslinking ACN o DMF se utiliza como disolvente, el aminosilano es la reticulacin and creating a surface with active silane groups y la creacin de una superficie con grupos silano activos exposed. expuesta. Conceivably, these silanes then react with the Posiblemente, estos silanos entonces reaccionan con el clays and bind them to the surface through a covalent arcillas y se unen a la superficie a travs de un enlace covalente bond. bonos. Unfortunately, any excess silanes at the surface Desafortunadamente, cualquier exceso de silanos en la superficie also tend to react strongly with the AFM tip. tambin tienden a reaccionar fuertemente con la punta del AFM. In Figure En la figura In summary, several different exfoliated clays were En resumen, varios diferentes arcillas exfoliadas eran studied. estudiado. The individual clay nanoplatelets are approximately Los nanoplatelets de arcilla individuales son aproximadamente 1 min the lateral direction and 1 nmthick. 1 min la direccin lateral y 1 nmthick. These Estos clay nanoplatelets are extraordinarily compliant and nanoplatelets arcilla son extraordinariamente adaptables y readily conform to nanoscale surface features. adaptarn fcilmente a caractersticas de la superficie a nanoescala. In the En el process of imaging these nanoplatelets, we found that the proceso de obtencin de imgenes de estos nanoplatelets, encontramos que la clay sheets have substantial quantities of mobile impurities lminas de arcilla tienen cantidades sustanciales de impurezas mviles that could not be separated from the clay nanoplatelets. que no podan ser separados de los nanoplatelets arcilla. Because AFM is a surface probe, these impurities Debido AFM es una sonda de superficie, estas impurezas interfere with precise AFM measurements, especially interferir con las mediciones precisas de AFM, especialmente FMM, of the individual clay sheets. FMM, de las lminas de arcilla individuales. The impurities are of Las impurezas son de two types; dos tipos; one is water-soluble and appears to be sodium uno es soluble en agua y parece ser de sodio and perhaps its counterions. y tal vez sus contraiones. The other impurity is silicatebased La otra impureza se silicatebased and water-insoluble and is mobile on an atomically y agua insoluble y es mvil en una atmicamente smooth surface. superficie lisa. The precise nature and chemistry of these La naturaleza precisa y la qumica de stos impurities are currently unknown and the focus of Actualmente se desconocen las impurezas y el foco de additional study in our lab. estudio adicional en nuestro laboratorio. The presence of the impurities, and the inability to La presencia de las impurezas, y la incapacidad de separate them from the clay nanoplatelets, is significant separarlos de los nanoplatelets de arcilla, es significativo and will impact the use of clay nanoplatelets in applications. y tendr un impacto en el uso de nanoplatelets arcilla en las aplicaciones. For example, because of their nanometer size, these Por ejemplo, debido a su tamao de nanmetros, stos impurities will contribute a large surface area component impurezas contribuirn un gran componente de superficie to a composite system as well as influence the interaction a un sistema de material compuesto as como la influencia de la interaccin of the clay nanoplatelets with the matrix. de los nanoplatelets de arcilla con la matriz. The presence La presencia of these impurities will also interfere with nanoscale de estas impurezas tambin interferir con nanoescala measurements of the stiffness of a single clay sheet and mediciones de la rigidez de una hoja nica y arcilla is undesirable for applications where the nanoplatelets es indeseable para aplicaciones donde el nanoplatelets are to be used as a building block for molecular electronic son para ser utilizado como un bloque de construccin para la electrnica molecular systems. sistemas. It is our hope that this paper will stimulate Es nuestra esperanza que este trabajo estimule further interest in clay nanoplatelets and that an awareness ms inters en nanoplatelets arcilla y que la conciencia of the presence of these impurities will lead to the de la presencia de estas impurezas dar lugar a la development of rigorous clay purification methods desarrollo de mtodos de purificacin de arcilla riguroso

Texto original en ingls:It is our hope that this paper will stimulateProponer una traduccin mejor