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Driving Surface Chemistry at the Nanometer Scale Using Localized Heat and Stress Shivaranjan Raghuraman, Meagan B Elinski, James D. Batteas, and Jonathan R Felts Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b03457 • Publication Date (Web): 10 Mar 2017 Downloaded from http://pubs.acs.org on March 11, 2017
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Driving Surface Chemistry at the Nanometer Scale Using Localized Heat and Stress Shivaranjan Raghuraman§, Meagan B. Elinski†, James D. Batteas‡ and Jonathan R. Felts* §
*, Advanced Nano Manufacturing Laboratory, Department of Mechanical Engineering, Texas A&M University, 3123 TAMU, College Station, TX 77840, United States. †,‡
Department of Chemistry, Texas A&M University, 3123 TAMU, College Station, TX 77840, United States.
Keywords: Atomic Force Microscopy, Surface Chemistry, Mechanochemistry, single asperity friction, nanolithography ABSTRACT: Driving and measuring chemical reactions at the nanoscale is crucial for developing safer, more efficient and environment-friendly reactors and for surface engineering. Quantitative understanding of surface chemical reactions in real operating environments is challenging due to resolution and environmental limitations of existing techniques. Here we report an atomic force microscope technique that can measure reaction kinetics driven at the nanoscale by multi-physical stimuli in an ambient environment. We demonstrate the technique by measuring local reduction of graphene oxide as a function of both temperature and force at the sliding contact. Kinetic parameters measured with this technique reveal alternative reaction pathways of graphene oxide reduction previously unexplored with bulk processing techniques. This technique can be extended to understand and precisely tailor the nanoscale surface chemistry of any two-dimensional material in response to a wide range of external, multi-physical stimuli.
Surface properties influence many chemical and biological processes that hold the key to fundamental advancements in 1-3 diverse fields such as energy storage and conversion , 2D 4, 5 optoelectronics , friction and wear modification and miti6, 7 8, 9 10-12 gation , wetting and self-assembly , medicine and 13, 14 nanofabrication . The chemical reactivity of many surfaces in a specific process often depends on multiple factors, including local surface geometry and composition, environmental composition, temperature, pressure, and any additional external stimuli, such as light and electric field. Developing a comprehensive understanding of how to drive and measure surface reactions at the nanometer scale under a wide range of conditions is crucial for development of more efficient chemical reactors, fuel cells, batteries and superca15-21 pacitors, flexible electronics, lubricants and biomaterials . Presently, Atomic Force Microscopy (AFM) techniques have gained prominence in studying nanoscale surface chemistry and structures owing to their ultra-high resolution in ambient air or liquid. Functional AFM tips can drive adsorption and desorption from surfaces in a wide range of environments via local application of electric field, light, temperature, and force. AFM tips have previously demonstrated ox-
ide growth and reduction, deposition of self-assembled monolayers, thermochemical modification and phase change in organic films, optical lithography, and thermal deposition of 22-26 . polymers, metals, and composites While AFM proves well suited to drive small-scale surface reactions, obtaining quantitative, in situ information on the kinetics and thermodynamics of surface reactions remains a 27, 28 significant challenge . Recent studies of atomic scale wear have begun to directly measure reaction rates of the wear process of both sliding asperity and the counter 29 surface . The results show great promise in using AFM tips to drive and measure chemical reactions. However, it is difficult to interpret the results in the context of conventional chemical kinetic and thermodynamic descriptions, which rely on measures of surface concentration as a function of time and temperature in the absence of an applied load. There is a need to develop tip-based techniques that can directly assess chemical kinetics and thermodynamics at surfaces in situ, making it possible to measure reactions driven by arbitrary multi-physical sources in virtually any environment and interpret results in the context of conventional surface analysis techniques. Here we measure the kinetics and thermodynamics of oxygen group desorption from graphene oxide in ambient air by independently tuning both the force and temperature of a heated atomic force microscope (AFM) probe. Heated probes have previously demonstrated direct writing of conductive nanoribbons on GO sheets through local thermal 23 reduction . Figure 1A illustrates the experimental setup, where a heated tip locally cleaves oxygen groups from a graphene oxide (GO) sheet via the application of both temperature and force. Previous AFM mechanochemistry studies on functionalized graphene show that measured friction linearly relates to the chemical composition at the surface, and that 30 the GO sheet can be reduced solely through applied force . Specifically, oxygenated graphene has two to nine times 31, 32 higher friction than pristine graphene . Accordingly, friction force images of thermochemically fabricated lithographic features reveals sub 100 nm low friction lines as shown in fig. 1B. Measuring the friction of the lithographic features relative to surrounding unmodified GO (Figure 1C), termed “relative friction,” provides a measure of the extent of reduc-
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tion. We employed a two-step routine that includes a “drive scan” to cleave the functional groups at high temperature and/or force, and a subsequent “measure scan” at room temperature and low force to monitor the resulting change in relative friction (see supporting information “Bond Scission Dynamics Measurement” for the different experimental techniques constituting this study and for the calculation of relative friction). Measuring chemical kinetics with the same AFM tip that drives a chemical reaction enables in situ characterization of the evolving surface composition and quantitative understanding of the combined effect of heat and stress on a chemical reaction driven locally at the surface.
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20-800 ˚C with less than 10% error in the control of the heat33 er temperature . Placing the hot tip onto a substrate forms a nanometer scale hotspot used here to drive local reduction of graphene oxide. The surface temperature profile exhibits a maximum temperature within the tip-surface contact area, and rapidly decays with distance away from the tip (see supporting information “Temperature Profile around the Tip”). It is important to note that the tip-surface interface is often at a lower temperature than the tip heater (Figure 1A), and must be estimated based on the thermal resistance of the tip, contact resistances, and thermal properties of the sample and substrate. The estimated interface temperature used for activation energy calculations in the current experimental study is 66±10% of the heater temperature (see supporting information “Thermal Resistance Network”). The ability to quickly modulate the heat at the surface locally makes it possible to study reactions at much higher temperatures than global heating, where high temperatures would drive the reaction faster than a single AFM scan. Thermodynamics of oxygen removal from graphene was first studied by measuring the kinetics of reduction driven by the tip at various constant heater temperatures. Figure 2A shows changes in relative friction for constant heater temperatures between 300-375 ˚C as a function of cumulative tip dwell time (see supporting information “Bond Scission Dynamics Measurement”). Cumulative tip dwell time—or the total time the tip spends at any scan point over multiple scans spanning an entire reduction process—was calculated using measurements of the tip contact area and the scan parameters (see supporting information “Cumulative Tip Dwell time”). For a given temperature, the decrease in relative friction as a function of time is given by
Δ
(1)
where, Δ is the relative friction, is the reaction rate and t is the cumulative tip dwell time. Error in relative friction measurements that arose from surface topography and thermal noise was consistently within ±3% for all of the experimental data in this study. Figure 2B plots the fitted reaction rates as a function of heater temperature, showing a strong exponential dependence. This thermally activated process was fit using the classical Arrhenius activation model,
exp
Figure 1 (A) Schematic of reduction by a hot AFM tip, where heat and pressure locally remove oxygen groups. (B) Friction Force image of the reduced graphene oxide patterns. (C) A friction drop of 60% due to removal of oxygen groups. The AFM probe is single crystalline silicon with high-doped legs for efficient charge transfer, and a low-doped resistive Joule heater-thermometer at the cantilever free end. The heater electrical resistivity is a strong function of temperature, making it possible to measure and control heater temperatures in situ by correlating resistance to temperature via optical measurements (see supporting information “Temperature Calibration of tip”). The heater operates routinely from
(2)
where is the energy activation barrier, is Boltzmann’s constant, is absolute interface temperature and is the effective attempt frequency pre-factor (where reasonable attempt frequencies are based on atomic vibrations in the 13 15 -1 34 range 10 – 10 s ). . Applying this to the data as shown in figure 2B inset, the slope of the straight line yielded an activation energy of 0.8 ± 0.3 eV, which compares reasonably 30 well to previous AFM mechanochemistry measurements . Although the activation energy measured here compares reasonably to some previous experimental and theoretical works, the broad error in the activation energy resulting from uncertainty in the exponential fits, and an assumption of a first order reaction prevents any definitive conclusion regarding the chemical reactions under investigation. Gath-
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ering additional isothermal datasets for longer dwell times to bring down the measurement error would require prohibitively large amounts of data and an accordingly large sampling area, thereby making the experiment practically impossible. This necessitates an experiment compact in both time and sampling area to reduce the measurement error significantly.
tained were normalized for the purpose of analyses throughout this study. Friction profiles were obtained for 190, 380, 760 and 1500 ºC/min effective ramp rates (see supporting information “Effective Ramp Rates under the Tip”). The thermochemical reduction occurred between 50 ˚C and 450 ˚C heater temperature, where ramping beyond 450 ˚C did not cause any appreciable further reduction in the relative friction, and abrupt increases in friction due to catastrophic sheet tearing were observed commonly at temperatures exceeding 500 ˚C (see supporting information “Film Failure”). The derivative of the decomposition curve as shown in fig. 3B provides the rate of removal as a function of temperature.
Figure 2 (A) Relative Friction drop as a function of cumulative tip dwell time for various heater temperatures. (B) Reaction rate as a function of heater temperature - inset shows the Arrhenius plot. Estimated interface temperatures were used for activation energy calculations. To overcome the limitations of the isothermal approach, we developed a non-isothermal, nanoscale thermal desorption microscopy (ThDM) technique to quickly measure the temperature dependence of reaction rate during a tip temperature or force ramp, using principles from bulk thermal analysis techniques such as thermogravimetry and temperature 35 programmed desorption . Linearly increasing either the tip temperature or the tip force between each scan provides a measure of the reaction rate as a function of either tip temperature, force, or both. Figure 3A shows change in friction as a function of heater temperature for different tip heating rates, determined by dividing the temperature increase per scan by the tip dwell time per scan. The friction data ob-
Figure 3 (A) Normalized Relative friction drop as a function of heater temperature for various ramp rates. The friction data obtained were normalized for the purpose of analyses. (B) Normalized relative friction derivative showing a rightward shift in maximum removal with increasing ramp rates. (C) A typical constant heating rate curve – inset illustrates the calculation of activation energy. Estimated interface temperatures were used for activation energy calculations
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The rate of change of the oxygen concentration during a lin36 ear temperature ramp is governed by ,
d a!"# e d
(3)
where is the temperature ramp rate, ν is the exponential pre-factor, is the measured friction that linearly correlates 30 to oxygen concentration , is the order of reaction, is the activation energy for bond removal, !% is the applied normal tip force, is the Boltzmann’s constant and is the absolute interface temperature. The term !% models the nontrivial effect of applied load on the energy landscape, and depends on the geometry of the molecule under study, and the location and direction of the applied forces. For linear temperature ramps at constant load, the energy barrier is treated as a constant where the observed energy barrier includes the effect of the applied force Eeff = Ea - φ(FN). At constant force, integrating eq. (3) over temperature has similar form to solutions developed over the last few decades for 36 bulk thermal analysis, and an effective estimation is :
'
where,
()** e +, 0 11 03 .// .//
7 8 6 9 , ' 1 5 1 ln 1 , 4 2
(4)
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mathematical form for the function ϕ!% . A number of theoretical models describe stress assisted chemical reactions. 34 The Zhurkov model , originally developed to model fracture events, proposes a linear reduction of the activation energy in response to an applied stress over some activation volume. The model has been used previously to describe atomic wear at tip-substrate interfaces in AFM scanning experiments, clearly showing an exponential dependence between the 37 observed reaction rates and the applied stress. Bell’s theory (ABCDD Aϕ!% 0 ϕ!% , FGH ϕ!% !% ΔI) proposes a similar linear dependence between applied load and activation barrier, but in terms of an applied force over some reaction coordinate (I, where AI is the distance between reactant and transition state configurations and A is the activation energy barrier when no external force is applied. This model has been used to interpret the forces required to break chemical bonds and the change between 38, 39 reactant and transition states in CFM experiments . Unlike CFM, where a known load is applied to a single known bond, here it is difficult to relate the applied load on the tip to the loads applied to each molecule under the tip, but these two models are directly related with appropriate choice of contact mechanics model between tip and substrate.
;1 1
For a first order reaction, plotting ln' against 1/ yields a straight line with the slope proportional to activation energy (slope=-Ea/k). For higher order reactions, a computational approach could be taken to iterate on the reaction order that gives the best straight line fit. Applying this technique to the data in figure IIIA yielded an activation energy of 0.55 ± 0.08 eV with a reaction order of 1 (figure 3C). The nano-ThDM technique reduced the error in the activation energy by a factor of 4, requiring far less data than the isothermal technique, and allowed direct determination of reaction order from the experimental data. The ability to independently control applied localized temperature and force provides an effective means to quantitatively study the impact of force on the observed energy barrier Eeff. Figure 4A shows friction measured during linear temperature ramps from 50 ºC to 450 ºC for various tip loads between 40-320 nN. Increasing the load spreads the reaction over a wider temperature range as illustrated by the growing peak width of the derivative curves in fig 4B. Activation energies calculated by applying eq. (4) for decomposition curves as shown in fig. 4C decreased from 0.61 eV to 0.22 eV over a force range of 40-320 nN applied tip load, clearly demonstrating that the applied stress substantially modifies the activation barrier. The influence of applied load on observed activation barrier as shown in fig. 4C provides a basis to adopt an explicit
Figure 4 (A) 760 °C/min constant heating rate curves for various tip forces. The friction data obtained were normalized for the purpose of analyses. (B) Normalized relative friction derivative showing a shift in maximum removal towards lower temperatures for increasing tip force (C) Measured activation energy as a function of tip force, showing a non-linear
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reduction with increasing force. Estimated interface temperatures were used for activation energy calculations Many studies have shown the failure of linear dependence 40-42 suggested by Bell’s model . Bell’s model relies on an implicit assumption that the applied force does not distort the potential energy landscape and does not change the reactant or transition state structures. A study on disrotatory and conrotatory ring opening of cis-1,2-dimethylbenzocyclobutene (BCB) demonstrates that the Bell’s model fails to capture the non-linear stress dependence and disappearance of reaction pathway beyond a certain applied 43 force . A similar study on BCB shows that including higher order terms in Bell’s approximation accounts for the non44 linear stress dependence . An experimental study demonstrates rupture of Carbon-Carbon bonds at forces of ~2 nN, whereas theoretical studies show that the maximum force required to rupture a C-C bond is about 6 nN, possibly indi45 cating a modified reaction pathway . Further, as our observations (fig. 4C) point to a non-linear stress dependence of activation barrier, we adopt an extension of Bell’s model that 44 includes second order correction ,
ϕ!% AI!%
AJ 9 ! 2 %
(5)
where the term AI denotes the reaction coordinate between the initial and transition states and the term AJ denotes the change in mechanical compliance of the molecules on which force is applied. While the contact area and hence the contact volume would typically be load dependent for a parabol46 ic AFM tip , here pre-worn blunted tips were employed to avoid this issue (see supporting information “Influence of Tip Shape on Contact Mechanics”). Accordingly, we assumed that the applied load is proportional to applied stress without changes in the sampled reaction volume. While the instantaneous loads applied to each bond under the tip vary widely in both magnitude and direction, over long times and many molecules the average force felt by each bond should be linearly proportional to FN. Fitting data in fig 4C with a second order polynomial revealed a non-linear decrease in force dependence of activation. This phenomenon is commonly known as the Hammond effect, where increased applied force shifts the energy levels of the reaction state and transition state, thereby altering the minimum reaction trajectory through the transition state and reducing the force depend47, 48 ence of activation barrier. . The dependence of a chemical reaction to applied load was further assessed by monitoring the decomposition of oxygen groups as a function of increasing force (see supporting information “Bond Scission Dynamics Measurement”). Figure 5A shows relative Friction profiles as a function of linearly ramped force for 50, 100, 150, 200, and 250 ºC heater temperatures. Figure 5C-inset illustrates the non-linear rate dependence of activation energy. A molecular system with no change in mechanical compliance following Bell’s model would exhibit a linear rate dependence on applied load. On the contrary, including the Hammond effect in the definition of ϕ!% results in a non-linear plot indicated by the red curve. This result is consistent with the data in figure 4.
Useful interpretation of the observed trends requires a number of assumptions about the thermal and mechanical behavior of the contact. First, the applied load may lower the thermal contact resistance, causing a shift to higher temperatures at higher loads. Second, changes to tip contact area due to tip wear and mechanical deformation could also cause an overestimation of surface force at higher applied loads. Both of these phenomena would exaggerate the observed non-linear dependence of energy barrier on applied load. For the case of the multi-layer GO film studied here, variations in contact resistance significantly affect interface temperature error estimates, yet when propagated through to activation energy calculations, is dwarfed by the aggregate experimental uncertainty (see supporting information “Thermal Resistance Network” and “Variations in Activation Energy associated with Tip Temperature Uncertainties”). Measurements of contact area as a function of load, as well as pull off force measurements taken after every data point in the present study, show that tip contact radius indeed is independent of load and scan time, as expected for blunt, heavily pre-worn tips (see supporting information “Influence of Tip Shape on Contact Mechanics”). In general, this experimental method requires great care in evaluating the temperature and stress fields in the vicinity of the tip-sample interface.
Figure 5. (A) Normalized Relative friction drop as a function of tip force while a constant heater temperature was main-
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tained. The friction data obtained were normalized for the purpose of analyses. (B) Normalized relative friction derivative showing a shift in maximum removal towards lower forces for higher temperatures. (C) A typical linear force ramp curve with inset illustrating the failure of Bell’s model. The chemical kinetics and thermodynamics observed here differ significantly from previous bulk thermal reduction studies on graphene oxide which suggest a second order recombination reaction with an activation energy in excess of 49, 50 1.3 eV . The observed activation barrier of 0.55 eV is much closer to theoretical estimations of oxygen diffusion on gra51, 52 phene and silicon oxide , suggesting oxygen groups diffuse along either surface or hop between surfaces. The measured first order reaction further suggests diffusion or hopping. This is consistent with previous investigations of the stability of graphene oxide films, which found reduction was largely driven by diffusion (indeed, they must diffuse before they can recombine), with energy barriers of 0.81 and 0.35 eV for 53 epoxides and hydroxyls, respectively . Finally, non-linear stress dependence of activation energy suggests that higher loads increase stability of the oxygen groups. During sliding, 54 thin graphene sheets undergo out of plane deformation forming a convex region around the periphery of the tip, 55 where atoms bound are more stable . Strain of the tip material may also play a role in how the atoms migrate away from its original site if the pathway is along the tip material. This phenomenon is supported by bulk heating of locally reduced features, which showed no appreciable surface diffusion in the absence of a sliding counter-surface, indicating that the diffusion process observed relies on the local thermal and stress gradients at the sliding interface (see supporting information “Global Heating Experiments”). In summary, we have demonstrated that nano-ThDM is an effective approach to measure the kinetics of surface chemical reactions and to precisely tune the surface composition. The results showed that the sliding silicon tip removed oxyst gen groups via a 1 order process with an activation energy roughly 0.6 eV. The results of this study significantly deviate nd from bulk studies, where oxygen reduction is a 2 order process with an activation energy above 1.3 eV, indicating that the sliding silicon tip drives reduction via an alternative pathway. Higher loads begin to impede the reaction nonlinearly, showing that it is not appropriate to assume a linear relationship between applied load and energy barrier for surface reduction. Varying the tip material, functional group to be cleaved, environmental composition, and substrate material would provide chemical kinetics of virtually any rubbing interface, and appropriate passivation of the tip material would allow the tip to observe the reaction without significantly altering the reaction pathway. Incorporating both molecular dynamics simulations, and in situ electron microscopies will provide a more complete picture of the stoichiometric products, reaction pathway of the observed reactions, and more accurate contact mechanics with atomic resolution. Importantly, this technique has few limits on environmental conditions, making it ideal to study multiphysical reactions in extreme environments, closely resembling operation of real chemical systems. Material Preparation. GO/H2O was purchased from SigmaAldrich. The dispersion was further diluted in deionized H2O
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to obtain 0.5mg/ml solution. Single layered GO flakes on SiO2 were obtained by spin coating the solution at 4000 rpm for 120s. Experimental setup. AFM experiments were performed using an Asylum Research MFP-3D. A closed-cell heater was used to maintain an ambient temperature of 32 °C throughout the AFM experiments. Heated atomic force microscope tips were used to perform all the local reduction experiments. A relationship between the cantilever vertical deflection and tip force was obtained using force distance curves 56 and thermomechanical noise measurements . These relationships were used to calculate the deflection voltage necessary to apply a specified force. The raw data collected from the experiments were pull-off force and relative friction as a function of tip force, cantilever temperature and scans. The details of the experiments are described in the supporting information.
ASSOCIATED CONTENT Supporting Information Temperature Calibration of tip; Bond Scission Dynamics measurement; Cumulative tip dwell time; Thermal resistance network; Temperature profile around the tip; Effective Ramp rates under the tip; Global heating experiments; Film Failure; Influence of tip shape on Contact Mechanics. Variation in Activation Energy associated with Tip Temperature uncertainties and Pull-off Force measurements. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author ‡
Jonathan R. Felts, Advanced Nanomanufacturing Laboratory, 3123 TAMU, College Station, Texas – 77840 Email:
[email protected] Ph: 979.458.3580
Author Contributions ‡
§
Devised* and Designed experiments, performed experi§ ‡§ ments , developed experimental programs , interpret re§ ‡ § ‡ sults * , composed the written research work * and Raman † thermometry .
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT The authors acknowledge the Texas A&M Engineering Experiment Station for the funding. James D. Batteas also acknowledges partial support of this work from the National Science Foundation (CMMI-1436192). The authors gratefully acknowledge Dr. William P. King from University of Illinois at Urbana Champaign for providing the heated atomic force cantilevers used in this work.
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Figure 1 (A) Schematic of reduction by a hot AFM tip, where heat and pressure locally remove oxygen groups. (B) Fric-tion Force image of the reduced graphene oxide patterns. (C) A friction drop of 60% due to removal of oxygen groups. 130x205mm (600 x 600 DPI)
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Figure 2 (A) Relative Friction drop as a function of cumula-tive tip dwell time for various heater temperatures. (B) Reac-tion rate as a function of heater temperature - inset shows the Arrhenius plot. Estimated interface temperatures were used for activation energy calculations. 126x194mm (300 x 300 DPI)
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Figure 3 (A) Normalized Relative friction drop as a function of heater temperature for various ramp rates. The friction data obtained were normalized for the purpose of analyses. (B) Normalized relative friction derivative showing a rightward shift in maximum removal with increasing ramp rates. (C) A typical constant heating rate curve – inset illustrates the cal-culation of activation energy. Estimated interface tempera-tures were used for activation energy calculations. 139x236mm (300 x 300 DPI)
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Figure 4 (A) 760 °C/min constant heating rate curves for various tip forces. The friction data obtained were normal-ized for the purpose of analyses. (B) Normalized relative friction derivative showing a shift in maximum removal to-wards lower temperatures for increasing tip force (C) Meas-ured activation energy as a function of tip force, showing a non-linear reduction with increasing force. Estimated interface temperatures were used for activation energy calculations 114x158mm (300 x 300 DPI)
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Figure 5. (A) Normalized Relative friction drop as a function of tip force while a constant heater temperature was main-tained. The friction data obtained were normalized for the purpose of analyses. (B) Normalized relative friction deriva-tive showing a shift in maximum removal towards lower forces for higher temperatures. (C) A typical linear force ramp curve with inset illustrating the failure of Bell’s model. 126x197mm (300 x 300 DPI)
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TOC figure 39x19mm (300 x 300 DPI)
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