Investigation of the LigandâNanoparticle Interface - ACS Publications

Article Cite This: J. Phys. Chem. C 2018, 122, 3582−3590

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Investigation of the Ligand−Nanoparticle Interface: A Cryogenic Approach for Preserving Surface Chemistry Ajay S. Karakoti,*,†,‡ Ping Yang,*,∥ Weina Wang,⊥ Vaishwik Patel,‡ Abraham Martinez,# Vaithiyalingam Shutthanandan,# Sudipta Seal,§ and Suntharampillai Thevuthasan# †

School of Engineering and Applied Science, Ahmedabad University, Ahmedabad, Gujarat 380009, India Division of Biological and Life Sciences, School of Arts and Sciences, Ahmedabad University, Ahmedabad, Gujarat 380009 India § Nanoscience and Technology Centre, University of Central Florida, Orlando, Florida 32826, United States ⊥ School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an, Shaanxi China 710062 # EMSL, Pacific Northwest National Laboratory, Richland, Washington 99354, United States ∥ Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ‡

S Supporting Information *

ABSTRACT: Ligand-functionalized nanoparticles have replaced bare nanoparticles from most biological applications. These applications require tight control over size and stability of nanoparticles in aqueous medium. Understanding the mechanism of interaction of nanoparticle surfaces with functional groups of different organic ligands such as carboxylic acids is confounding despite the two decades of research on nanoparticles because of the inability to characterize their surfaces in their immediate environment. Often the surface interaction is understood by correlating the information available, in a piecemeal approach, from surface sensitive spectroscopic information on ligands and the bulk and surface information on nanoparticles. In present study we report the direct interaction of 5−7 nm cerium oxide nanoparticles surface with acetic acid. An in-situ XPS study was carried out by freezing the aqueous solution of nanoparticles to liquid nitrogen temperatures. Analysis of data collected concurrently from the ligands as well as functionalized frozen cerium oxide nanoparticles show that the acetic acid binds to the ceria surface in both dissociated and molecular state with equal population over the surface. The cerium oxide surface was populated predominantly with Ce4+ ions consistent with the thermal hydrolysis synthesis. DFT calculations reveal that the acetate ions bind more strongly to the cerium oxide nanoparticles as compared to the water molecules and can replace the hydration sphere of nanoparticles resulting in high acetate/acetic surface coverage. These findings reveal molecular level interaction between the nanoparticle surfaces and ligands, giving a better understanding of how materials behave in their immediate solution environment. This study also proposes a simple and elegant methodology to directly study the surface functional groups attached to nanoparticles in their immediate solution environment.

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INTRODUCTION Ligand functionalized nanoparticles are currently explored in various biomedical applications such as site specific drug delivery, treatment of cancers or tumors and as cellular imaging or contrast agents.1−3 A majority of synthesis and applications of these functional nanomaterials is realized by using ligands as functional groups for (a) passivation of surface defects, (b) tuning the size of nanoparticles, and (c) providing stability in various solvents and (d) as linkers for additional surface functionalization of nanoparticles.4−9 Specifically in biomedical applications surface ligands provide an important function of conjugating the surface of nanoparticles with selected biomolecules or fluorescent tags.6,7 The ligand system usually contains an anchoring ligand and a functional ligand. Scheme 1 illustrates such a conjugated molecule wherein the primary/ anchoring ligand (silane) acts as an anchoring group for the © 2018 American Chemical Society

attachment of a secondary functional ligand (fluorophore) to the surface of nanoparticles. The nature of bond between the anchoring ligand and nanoparticle surfaces determines the fate of conjugated functional biomolecules (chemistry, orientation and surface density) within the hostile environment of the body. For example, the strength of this bond determines whether the attached ligand will be stable or exchanged with a host of other ligands present within the cellular environment. The chemistry of conjugation of nanoparticles with selected biomolecules such as proteins, antibodies and nucleotides is making rapid progress based upon the principles of synthetic organic chemistry however; the existing knowledge of the Received: October 7, 2017 Revised: January 27, 2018 Published: January 29, 2018 3582

DOI: 10.1021/acs.jpcc.7b09930 J. Phys. Chem. C 2018, 122, 3582−3590

Article

The Journal of Physical Chemistry C

as and when necessary to avoid long-term storage related contamination. The nanoparticles samples were divided in several aliquots and stored separately to avoid the contamination of the entire batch from exposure to atmosphere and pipet tips during different characterization of nanoparticles. Synthesis and Functionalization of Nanoparticles. Ceria nanoparticles were prepared by thermal hydrolysis using previously established methods.19 Briefly, cerium(IV) ammonium nitrate (Sigma-Aldrich) was dissolved in DI water and heated to 70 °C. Stoichiometric amount of ammonium hydroxide (molar ratio Ce:NH4OH 1:2.5) was added dropwise with vigorous stirring to synthesize ceria nanoparticles. As synthesized ceria nanoparticles were further divided in two parts. One part was dialyzed as is against DI water until the pH of the solution was 4.5. The second part was dialyzed to reach a pH close to 2.5 to nearly match the pH of 0.3 M acetic acid solution. The concentration of ceria nanoparticles after dialysis was calculated from the UV−visible absorption of nanoparticle solution and comparing the same with the calibration curve of ceria nanoparticles. For functionalization, 10 mL (10 mg/mL) of ceria nanoparticles at pH 2.45 were mixed with 10 mL of 0.3 M acetic acid (pH 2.65) and stirred for 12 h. Functionalized nanoparticles were subsequently dialyzed against deionized water to get rid of excess carboxylic acid, salt and unwanted impurities using a 12000−14000 MWCO cellulose membrane. The cellulose membrane was precleaned with copious amounts of deionized water to remove residual glycerin and other contaminants. Dialyzed pure ceria and functionalized ceria solutions were divided in several aliquots and stored for further characterization. Entire synthesis and functionalization was carried out in pre cleaned hood. Aliquots were transferred to precleaned vials and stored in the glovebox under nitrogen environment. The final concentration of dialyzed and functionalized ceria nanoparticles was 3.8 mg/mL. Dynamic Light Scattering. The size of nanoparticles in solution before and after dialysis was characterized using DLS (BIC Zetasizer) using 639 nm laser. For size measurements the nanoparticle suspensions were diluted to a concentration of 1 mg/mL. All cuvettes were prerinsed with DI water prior to measurements and a total of five measurements of 3 min were recorded. The effective diameter that gives predominantly an intensity weighted number was reported. Transmission Electron Microscopy. High resolution transmission electron micrographs of the as synthesized nanoparticles and particles after functionalization with acetic acid were obtained by using a JEOL 1011 electron microscope. TEM samples were prepared by drop casting 10 μL of nanoparticle suspension on holey carbon coated copper grids. X-ray Photoelectron Spectroscopy. XPS measurements were recorded using an Kratos Axis Ultra spectrophotometer, using a monochromatic focused Al Kα X-ray (1486.7 eV) source, which was connected to a glovebox for sample preparation in a clean environment prior to loading for XPS measurements. The glovebox is also equipped with a load lock chamber wherein the sample can be cooled to liquid nitrogen temperature. Freezing the samples to liquid nitrogen temperatures preserves the surface chemistry of nanoparticles in solution and significantly reduces the sample damage by X-rays. For XPS measurements, 40 μL of the sample solution was dropped on a precleaned copper substrate and placed on a transfer stage in the load lock chamber inside the glovebox. The sample was cooled in the load lock chamber from room temperature to liquid nitrogen temperature resulting in slow

Scheme 1. Ligand-Functionalized Nanoparticle System Illustrating the Functional (Fluorescent Molecule) Ligand Attached to the Surface of Nanoparticle Using a SilaneBased Anchoring Ligand

nature of interaction between nanoparticle surface and anchoring ligands is very limited. The primary reason for this gap in our current understanding of the interaction between nanoparticles surface and anchoring ligands is the lack of convincing and adequate direct characterization of ligands on the nanoparticle surfaces.10−12 Several challenges associated with surface characterization of nanomaterials make this an extremely difficult task such as (i) the reactive and dynamic nature of the nanoparticles that changes with length scales and its immediate chemical and physical environment, (ii) the high surface area of nanomaterials, which makes them prone to surface contamination, and (iii) the fact that nanomaterials undergo probe and proximity induced damages during characterization.13 Because of these challenges, most of the fundamental studies to understand the ligand−nanoparticle surface interactions are limited to model thin film systems that are often criticized as not a true representation of the nanoparticles surface.14,15 Studies carried out by model thin film systems under high vacuum environment provide relevant information on the interaction between crystalline inorganic surfaces with organic anchoring ligands.15,16 Attempts have also been made to characterize long chain aliphatic hydrocarbons or larger ligands bonded to nanoparticle surfaces17,18 however, smaller ligands interaction have always been challenging. Herein, we report the direct characterization of the nature of bonding between nanoparticle surface and anchoring ligands by using cerium oxide (ceria) nanoparticles and acetic acid as the model nanoparticle and anchoring ligand, respectively. Special precautions were taken to preserve the surface of nanoparticles and avoid unwanted contamination of the surface. Using a simple technique of slow freezing of aqueous solution of nanoparticles, we could identify the surface chemistry of nanoparticles using X-ray photoelectron spectroscopy.

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EXPERIMENTAL DETAILS Glassware Cleaning. Rigorous care is required during the synthesis as well as storage of nanoparticles to preserve the surface of nanoparticles and avoid organic contamination. All the glassware were boiled in fresh piranha solution (70:30 H2SO4:H2O2 Caution! Piranha solution is extremely dangerous and oxidizing and must be handled caref ully with proper PPE and ventilation) for 30 min. Boiling in piranha solution turns the surface of glass hydrophilic that facilitates glassware cleaning. Glassware were washed thoroughly using 18.2 MΩ deionized water (Barnstead Nanopure with UV Lamp) to wash off any traces of piranha solution. The glassware were washed 3583


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initio simulation program (VASP) with the projectoraugmented wave (PAW) potential to describe core electron.26,27 The cerium 5s, 5p, 5d, 4f, and 6s electrons, the 2s and 2p electrons of oxygen and carbon, and the hydrogen 1s electron were treated as valence electrons. Plane-wave basis sets with a cutoff energy of 400 eV are used to expand the valence electronic wave functions. These calculation parameters were recently employed to investigate the ceria surface and nanoparticles.22,28−30

cooling of aqueous solution. The sample stage was cooled by passing liquid nitrogen through the stage. This was achieved by passing a nitrogen line through a dewar and then through the sample stage. The dewar was filled with liquid nitrogen after the sample was inserted in the load lock chamber. The nitrogen gas passing through the dewar was cooled to liquid nitrogen temperature through this arrangement. The sample was frozen within 5−7 min but the cooling was continued further for about 30 min or until a steady stream of liquid nitrogen was observed flowing out of the nitrogen line (passing through the stage). The load lock chamber was pumped to a vacuum of 1 × 10−7 Torr through a combination of backing and turbo pump and the sample stage was kept at liquid nitrogen temperatures for the entire duration of pumping and sample transfer. The sample was seamlessly transferred from the load lock chamber to sample transfer chamber (STC maintained at 5 × 10−10 Torr) and from STC to sample analysis chamber (SAC). The sample stage in SAC was precooled to liquid nitrogen temperature using a mechanism similar to that described for cooling the stage in load lock chamber. DI water and acetic acid were also frozen under similar conditions and used as control for XPS measurements. The charging shifts were referenced to 916.6 eV peak from cerium 3d3/2 core level peak. The survey spectra were collected at 160 eV pass energy and the high resolution scans for individual elements were collected at pass energy of 40 eV with 0.1 eV step size at a dwell time of 300 ms. Binding energy of the instrument was calibrated prior to sample characterization and referenced to an energy scale with binding energies for Cu 2p3/2 at 932.67 ± 0.05 eV and Au 4f at 84.0 ± 0.05 eV. The base pressure of the SAC was maintained at 5.2 × 10−9 Torr, and it increased to 1.8 × 10−8 Torr after inserting the frozen nanoparticles sample for XPS measurements. Atomic percentage quantification of elements was done using survey and high resolution spectra and both the methods yielded concentrations within 1 atomic percentage difference using only 3d5/2 portion of the Ce 3d spectra and O 1s and C 1s core level regions. Surface Area Measurements. As synthesized and functionalized nanoparticles were freeze-dried and stored in precleaned glass vials. The surface area of both as synthesized and acetic acid functionalized cerium oxide nanoparticles was measured using NOVAWIN from Quantachrome Instruments. The surface area was calculated from the nitrogen adsorption isotherm at various gas pressures using BET surface area measurement equation. Prior to surface area measurements the samples were degassed at 110 °C for 4 h.

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RESULTS AND DISCUSSION Ceria nanoparticles prepared by thermal hydrolysis method are stable in aqueous suspension. The hydrodynamic size of assynthesized and functionalized nanoparticles in solution before and after dialysis was analyzed using DLS and reported in Table 1. It can be observed that as-synthesized nanoparticles Table 1. Hydrodynamic Size and ζ Potential of Bare and Functionalized Nanoparticles ζ potentiial

particle size NPs

before dialysis

after dialysis

before dialysis

after dialysis

bare AcOH

12.4 12.1

148.0 30.9

30.3 22.2

0.05 0.07

aggregated upon dialysis as the pH of the solution increased from 1.5 to 4.5 whereas the acetic acid functionalized nanoparticles showed only slight aggregation. Absence of heavy agglomeration in functionalized nanoparticles indirectly points to the successful functionalization of nanoparticles with acetic acid. The pH of the solution was monitored before and after mixing acetic acid and ceria nanoparticles. It was found that the pH of the acetic acid solution changed from 2.65 to 1.9 upon mixing with equal volume of 10 mg/mL solution of ceria nanoparticles (at pH 2.45). The change in the pH after mixing acetic acid and ceriua nanoparticles directly corresponds to the deprotonation of acetic acid and release of H+ ions in the solution. pKa values were calculated for acetic acid before and after mixing with ceria nanoparticles (using Henderson−Hasselbalch equation) based on the changes in the pH of the solution and taking into account the change in the concentration of acetic acid upon mixing. The pKa value for acetic acid changed from 4.75 to 2.97 after mixing with ceria nanoparticles. The deprotonation of acetic acid increased from 0.76% before mixing with nanoparticles to 3.9% after mixing. The number of free acetate ions can be calculated from the pH of the solution and was found to be about 7.04 × 1018 ions/mL of the solution. The BET surface area of ceria nanoparticles before functionalization was measured to be 159 m2/g. Upon mixing with equal volume of acetic acid, the final concentration of ceria nanoparticles in the solution is 5 mg/mL. That corresponds to 8.8 × 1018 acetate ions/m2 of ceria nanoparticles. High resolution transmission electron microscopy images in Figure 1 from as-synthesized and functionalized nanoparticles show that the individual particle size of the nanoparticles (∼5 nm) did not change after functionalization. Both low and high magnification TEM images show that the nanoparticles were well dispersed and did not show presence of hard agglomerates. The size distribution from the TEM images show tight size distribution in functionalized nanoparticles. The hydrodynamic size from DLS points toward slight aggregation of nanoparticles in the aqueous solution. Aggregation was more for particles

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COMPUTATIONAL DETAILS Spin-polarized density functional theory (DFT) calculations were conducted to understand interactions of acetic acid or water molecule interacting with ceria nanoparticles. The electron exchange and correlation were treated within the generalized gradient approximation using the optimized Perdew−Burke−Ernzerhor functional (optPBE-vdW)20,21 that accounts for the on-site Coulomb interaction via a Hubbard term (DFT+U) and long-range dispersion correction. The value of U is set to be 5.0 eV according to earlier theoretical works to count for the on-site Coulomb interactions of Ce 4f electrons in the partially reduced ceria.22,23 The adsorption energies were corrected using a nonlocal van der Waals density functional recently developed by Langreth and Lundqvist and co-workers (vdW-DF)24 and implemented in the VASP by J. Klimeš.25 The calculations were carried out using the Vienna ab 3584


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Figure 1. Transmission electron microscopy (TEM) results from bare and functionalized nanoparticles depicting the 3−5 nm crystallite sizes of functionalized ceria nanoparticles: (a) sample before dialysis, (b) sample after dialysis, (c) low resolution, and (d) high resolution TEM image.

after dialysis as the pH approached closer to the reported isoelectric point of ceria nanoparticles. The aggregation is also evident from the lower ζ potential values of nanoparticles after dialysis. While the bare nanoparticles aggregated in absence of any charge or steric stabilization the acetate functionalized nanoparticles were still dispersed owing to the steric stabilization provided by the acetate ions on its surface. The surface of ceria nanoparticles before and after functionalization was probed with XPS to determine the surface stoichiometry, bonding environment, and the oxidation states of cerium, oxygen, and carbon. The XPS spectrum of deionized water as a reference sample was collected prior to measurements of nanoparticle samples. It is important to mention that cooling the sample from room temperature to liquid nitrogen temperature gradually is essential to accumulate nanoparticles at the top of the frozen sample. The sample is placed as a drop of aqueous nanoparticle solution on a precleaned copper stub. As the stub is cooled to liquid nitrogen temperatures the aqueous nanoparticle solution starts to cool in a bottom upward direction, i.e., upward from the point of contact at the copper stub. As the freezing front moves upward it rejects the nanoparticles from the freezing ice front. This pushes a majority of the nanoparticles in the upward direction and on complete freezing a major concentration of nanoparticles is accumulated at the top of the frozen ice drop. The accumulation of nanoparticles at the top of the frozen ice allows the nanoparticles to be analyzed in XPS without being buried under the ice as shown in Figure 2A. The liquid nitrogen temperature can also preserve the particles from any X-ray beam induced damage. Parts B and C of Figure 2 show the high resolution XPS spectra (water frozen as ice) of core level O 1s and C 1s photoelectrons. As expected the XPS spectra predominantly shows oxygen with trace amounts of carbon in water. Atomic composition analysis showed about 96 at. % O, 3 at. % C and only 1 at. % Na. The peak observed for C 1s (284.8 eV) from ice is typical of the usual hydrocarbon contamination observed for XPS analysis. The presence of sodium and carbon in the XPS spectra of deionized water could be ascribed to the impurities that may have accumulated during the sample preparation. The O 1s peak at 532.8 is 0.6 eV lower than those reported by Kreplova et al.31 The difference in the peak position of O 1s may arise from the difference in the way water was frozen. While in our study water drop was frozen from its liquid state, in previous studies ice films were deposited from water vapors at low pressure. XPS spectra were obtained from

Figure 2. X-ray photoelectron spectroscopy from frozen samples under high vacuum conditions, (A) sample inside the XPS chamber and core level photoelectron spectra from (B) C 1s and (C) oxygen 1s from water frozen as ice and (D) Ce 3d, (E) O 1s, and (F) C 1s from functionalized ceria nanoparticles frozen as ice.

ceria nanoparticles functionalized with acetic acid. Parts D−F of Figure 2 show the high resolution XPS spectra of Ce 3d, O 1s, and C 1s from functionalized ceria nanoparticles. Good signal intensity directly validates that nanoparticle sample was not buried under the ice and accumulated at the top of the frozen sample as expected. The survey spectrum did not show any other impurities, and only trace amounts of nitrogen (less than 1 at. %) were detected, which confirmed the efficient dialysis of nanoparticle solution to remove free ions and unwanted impurities from surface of nanoparticles. Ce 3d core level spectrum confirms that cerium is predominantly in Ce4+ oxidation state after functionalization with acetic acid (Figure 3d). The relative abundance of peaks at 882.3, 888.9, 898.3, 900.8, 907.5, and 916.6 eV characteristic of Ce4+ oxidation state and absence of peaks at 880, 885, 900, and 905 eV characteristic of Ce3+ oxidation state confirm that cerium ions are fully oxidized in functionalized ceria nanoparticles.15 This observation is consistent with the method of synthesis of nanoparticles as thermal hydrolysis is usually known to produce ceria nanoparticles in predominantly oxidized form. The XPS spectrum of bare nanoparticles confirms the same and has been reported in SI-1. Two distinct peaks were observed in O 1s spectrum corresponding to three different oxygen environments from cerium oxide, carboxyl group of the acetic acid and the water adsorbed as ice on the surface (Figure 2e). Higher intensity of peak corresponding to oxygen from cerium oxide (at 529.2 eV) as compared to the peak corresponding to carboxyl group (at 531.0 eV) and ice suggests that the concentration of cerium oxide was maximum in the depth analyzed by the XPS. The core level C 1s spectrum (Figure 2F) shows two peaks in a nearly 1:1 stoichiometry corresponding to carbon from CH3 (at 284.8 eV) and COO− groups (at 288.7 eV) from acetic acid confirming the functionalization of ceria nanoparticles with acetic acid. A small peak at high binding energy originates from the Ce 4s core level electrons and is unrelated to any hydrocarbon contamination. From the atomic composition analysis the atomic percent of various elements was calculated as Ce = 3585


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Figure 3. Most thermodynamically favorable configurations and corresponding energy values for adsorption of water and acetic acid on a Ce19O32 cluster surface. Cerium ions were in a mixed oxidation state in this cluster.

CH3CO2H binding Ce19O32 ceria cluster. This octahedral Ce19O32 ceria cluster model was constructed by the most stable (111) planes cut from the bulk fluorite lattice28,29,32 which is consistent with the preferential thermodynamic stability of the (111) surface of CeO2.33 This model has been applied earlier in our study of adsorption of acetic acid to ceria nanoparticles.34 Additionally, the application of this model also allows us to study the impact of cerium oxidation state on the interface using the same model (Ce3+ versus Ce4+). Six acetic acid molecules are attached to the nanoparticle in a symmetrical manner to form ligand-adsorbed complexes. Various binding conformations including both molecular state and dissociative state were sampled to find thermodynamically stable structures and their associated energies. The geometries of Ce19O32 and its ligand-adsorbed complexes were fully optimized. The average ligand binding energies for each ligand were calculated as BE = (ECe19O32L6 − ECe19O32 − 6EL)/6, L = acetic acid or water, where ECe19O32L6, ECe19O32, and EL are the total energies of the ligand−nanoparticle complex, nanoparticle, and ligand, respectively. The main computational results are depicted in Figure 3. It is found that the dissociative adsorption is more thermodynamically favorable than molecular state adsorption for both water and acetic acid molecules regardless of the oxidation state of Ce atom on the interface. The existence of Ce3+ on the interface do not alter the ligand chemical behavior. It must be noted that direct interaction between ligands and nanoparticles in undersaturated coverage was studied, i.e, the interaction between the ligands was not considered . In case of multi-layer coverage where multiple ligands could strong interact with same surface site and among themselves, the dynamics becomes an important factor to determine the configurations of ligands on the interface which is beyond the scope of the current study. It is noticeable that the binding energies for adsorption of acetic acid in various configurations are much stronger than those for adsorption of water on ceria nanoparticles surfaces. This directly validates our assumption that the acetic acid molecules can easily replace the water molecules from

18.6%, O = 60.0%, and C = 21.4%. Ce 3d core level spectra clearly show that cerium is predominantly in the +4 oxidation state and thus for maintaining 1:2 stoichiometry of Ce:O in cerium oxide (CeO2) at 18.6 at. %, cerium should correspond to 37.2 at. % oxygen. The carbon to oxygen ratio in acetic acid is 1:1 and ascribing 21.4% oxygen (OCH3COO) to match the 1:1 stoichiometry of carbon leaves 1.4% oxygen, corresponding to the contribution from water frozen as ice. An only 21.4% surface carbon concentration suggests that the ceria nanoparticles may be covered by less than a monolayer of acetic acid molecules on its surface though the actual coverage was not quantified. . In aqueous medium ceriua nanoparticles are surrounded by a shell of water molecules that passivate the under-coordinated cerium ions on the surface. However, due to the affinity of acetate ions for cerium oxide surface these water molecules are replaced by acetate ions and acetic acid molecules that bind directly to the cerium oxide surface. Thus, the resulting surface cerium ions may bind to more than one acetate ions resulting in a higher carbon and oxygen concentration in the first surface layer. High resolution cerium 3d, oxygen 1s, and carbon 1s spectra collected from bare ceria nanoparticles depicted that cerium ions are predominantly in Ce4+ oxidation state consistent with the observation from functionalized ceria nanoparticles (see Supporting Information, Figures S1−S3). The oxygen 1s spectra showed only two peaks corresponding to oxygen from cerium oxide and ice. The carbon 1s spectrum showed presence of very little adventitious carbon, confirming the clean nature of cerium oxide surfaces prior to functionalization. Theoretical calculations were done to evaluate the interaction of water and acetic acid molecule with cerium oxide surface. The structures and energetics of water and acetic acid bound to ceria nanoparticles were calculated using first-principle electronic structure method. Earlier work shows that the acetic acid functional group prefers dissociative adsorption on both stoichiometric (Ce4+) and oxygen-deficient (Ce3+) CeO2(111) surfaces.15,30 Herein, the approach of spin-polarized density functional theory (DFT) computation was employed on the 3586


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acid. The difference in the peak separation of carboxylate and aliphatic carbon as well as the low concentration of carboxylate carbon in our studies may arise from the pure form of acetic acid used in our studies that was directly frozen to liquid nitrogen temperatures. It can also be noted that despite fitting the peaks there was residual area representing the aliphatic carbon of acetic acid. The ratio of the intensity of two peaks is around 1.25:1 for CCH3:CCOOH in acetic acid frozen as ice. This observation is consistent with previous observation31 that there may be other orientation of acetic acid when frozen as ice that result in this unaccounted excess area. The presence of adventitious hydrocarbon could also give rise to excess aliphatic carbon in the XPS spectrum. The O 1s PE spectrum in Figure 4b shows a dominant peak at 532.8 eV that overlaps with the oxygen peak from water frozen as ice. This peak was fitted to reveal the oxygen contributions from ice (at 532.8 eV) and contributions from different oxygen environment of carboxylic acid at 533.9 and 532.3 eV. The peak separation obtained between the carbonyl and hydroxyl oxygen (1.6 eV) is consistent with the earlier reported values.31 Quantification of contributions from protonated and deprotonated form of carboxylic acid was not attempted for O 1s spectra (from both acetic acid and functionalized ceria nanoparticles) due to multiple peaks and complicated spectrum. The C 1s core level spectrum (Figure 4c) from acetic acid functionalized ceria nanoparticles show two distinct peaks at 284.8 and 288.6 eV with a peak separation of 3.8 eV as compared to 4.3 eV observed for pure acetic acid. The narrowing of peak separation shows that the bonding environment of the carboxylate and aliphatic carbons was affected differently. As the ceria nanoparticles bind to carboxylic acid through the oxygen in the carboxylate anion, the carbon in carboxylate anions is directly affected by this bonding while the aliphatic carbon environment is unaffected by this bonding. The carbon peaks in acetate functionalized ceria were fitted using the same parameters as described previously for fitting pure acetic acid. The ratio of peaks corresponding to the deprotonated:protonated form increased from 0.06 for pure frozen acetic acid to 0.87 for acetic acid functionalized ceriua, suggesting that contribution from deprotonated form of acetic acid increased significantly as compared to the pure frozen acetic acid. These findings strengthen our experimental observation where an immediate pH change was observed upon reaction of acetic acid with cerium oxide resulting from the deprotonation of acetic acid on ceria surface. The presence of two separate peaks also suggests that all the acetic acid molecules are not deprotonated on ceria surface and very strongly suggest that there may be more than one configuration of acetic acid (in both protonated (molecular) and deprotonated (dissociated) form) bonded to ceria surface. Our previous experimental work on adsorption of trimethyl acetic acid (TMAA) on single crystal cerium oxide thin films showed similar possibility of existence of more than one orientation of acetate ions on stoichiometric ceria surfaces.15 Recently Wang et al. studied the adsorption of TMAA on cerium oxide using plane wave DFT calculations and predicted that carboxylate group can bind to ceria surfaces in both dissociated (deprotonated) and molecular (protonated) states.30 It was predicted that the dissociated states are energetically more favorable than the molecular states while comparing the independent energies of adsorption of acetic acid on the surface. In our experiments, multiple acetic acid molecules can

nanoparticle surfaces in aqueous medium as suggested by highest thermodynamically stable adsorption energies of −50.96 kcal/mol for acetic acid adsorption compared to −32.18 kcal/mol for adsorption of water molecules. Our results are also consistent with previously reported theoretical and experimental study.14,35,36 H2O molecules can be either molecularly adsorbed or dissociated with proton close to OH to preserve local charge neutrality on stoichiometric and oxygen-deficient CeO2 surfaces. Theoretical calculations show that the dissociated mode of adsorption is more favorable than the molecular mode however based on the preliminary XPS data in Figure 2 it does not seem apparent and thus high resolution C 1s and O 1s core level spectra of acetic acid functionalized ceria nanoparticles were compared with the C 1s and O 1s spectra of pure acetic acid frozen as ice. It can be observed from C 1s PE spectrum of frozen acetic acid in Figure 4a that the there are two peaks

Figure 4. C 1s and O 1s core level PE spectra of (a, b) acetic acid and (c, d) acetic acid functionalized Ceria nanoparticles respectively depicting clear differences in the core level binding energies of oxygen and carbon corresponding to aliphatic and carboxylate carbons of acetic acid: (a and c) black, original data; red, fitted data; blue, carboxylate carbon; and green, aliphatic carbon corresponding to acetic acid; pink,carboxylate carbon; and purple, aliphatic carbon corresponding to acetate anion; orange, Ce 4s; (b) black, original data; pink, fitted data; blue, oxygen corresponding to hydroxyl; red, oxygen corresponding to carbonyl; and green, oxygen corresponding to water molecules in ice; (d) blue, original data; green, oxygen corresponding to water molecules in ice; red, oxygen from the carboxylic acid and carboxylate anion; and black, oxygen contribution from cerium oxide.

corresponding to carboxylate carbon (289.3 eV) and methyl carbon (285.0 eV) with a peak separation of 4.3 eV between the carboxyl and methyl carbon. This peak separation is larger than those reported in the literature for the gas phase as well as liquid phase acetic acid. The peaks were fitted following the reports from Ottoson et al.37 and also from Kreplova et al.31 who recently revealed the presence of protonated and deprotonated forms of acetic acid in both aliphatic and carboxylate carbons using XPS. We adopted similar method of fixing equal fwhm (1.35 eV) and keeping a peak separation of 1.2 and 0.7 eV between the two peaks corresponding to the carbon in acetic acid and acetate anion in the carboxylate and aliphatic carbon, respectively. As compared to earlier observation of Kreplovaet al.31 we observed a very low concentration of the free form of acetate anion in frozen acetic 3587


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thus far and gives an understanding of the binding of carboxylic acids on ceria nanoparticle surfaces.

bind to the surface, and thus our results show that both states are equally distributed over the ceria surfaces. On the basis of experimental observations of immediate change in pH and the favorable energy of adsorption, it is possible that initially the carboxylate group binds to almost half of the available sites for adsorption of carboxylate (or acetate ions in this case) ions along with deprotonation. This leads to a charge distribution on the ceria surface that may favor bonding of molecular states of carboxylate group. Both these processes of dissociative and molecular adsorption can occur concurrently. A comparison of peak separation between the protonated and deprotonated carbon peaks Figure 4, parts a and c, shows that the peak separation corresponding to aliphatic carbon remains unchanged at 0.7 eV, while for the carboxyl carbon it reduces to 0.6 eV (upon adsorption of acetateions on ceria surface) from 1.2 eV (observed for pure frozen acetic acid). This strongly supports our hypothesis that both protonated and deprotonated forms of carboxylate groups bind to ceria surface. O 1s PE spectra in Figure 4d depicts oxygen contributions from adsorbed acetate, cerium oxide, and ice. The contribution of oxygen corresponding to ice was minimal suggesting that this XPS technique is suitable for analyzing the functionalized nanoparticles in their pristine state within aqueous medium. While the peak from the oxygen corresponding to ice did not show any significant shift, the oxygen peaks from carboxylate ion shifted to low energies confirming their bonding to metal sites on ceria surface. The shift to lower energies also indicates that electron density over carboxylate group increased after bonding to ceria surface. A quantitative interpretation from the O 1s would be speculative and was not attempted due to many overlapping peak contributions. We have previously studied the bonding of acetic acid to cerium oxide surfaces using the sum frequency generation−vibrational spectroscopy (SFG−VS) technique and have reported that bidentate chelating as well as bridging modes coexists on reduced ceria surfaces while oxidized surfaces are dominated by bidentate bridging modes.34 In current study the XPS data shows that cerium is in Ce4+ oxidation state (oxidized state) suggesting that the acetate ions on the surface of ceria are in bidentate configuration. In SFG−VS studies, the ceria nanoparticles were initially dried on the hemispherical CaF2 prism and then exposed to a medium containing acetic acid as compared to the complete preservation of ceria nanoparticles surface in aqueous environment in the current study.34 Thus, we observed both intact acetic acid molecule as well as acetate ions on ceria surface. Our previous studies on thin ceria films also suggested that more than one form of binding is present on stoichiometric ceria surfaces while only one form is preferred over reduced ceria surface that is consistent with the current findings.15 In the earlier thin film and the theoretical work carried out by us and Wang et al.,30 the theoretical model chosen did not consider the effects of explicit solution or solvent molecules. It is evident from experimental observations that the pH decreases following the adsorption of acetic acid molecule on ceria nanoparticles; thus, the proton is lost to the aqueous environment around nanoparticles. However, this proton was usually coordinated with surface oxygen in our theoretical work due to the lack of surounding solvent molecules. Inclusion of more than one ligand and explicit solvent molecules in the theoretical model may improve the correlation between the theoretical and experimental findings. The studies carried out using frozen nanoparticles is the most accurate description of pristine nanoparticles surfaces obtained

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SUMMARY Semi in situ XPS data collected in this work allowed us to explore the pristine surfaces of functionalized nanoparticles in their immediate aqueous environment for the first time. The technique of freezing the nanoparticles in a bottom upward manner on a copper substrate allowed the segregation of nanoparticles at the top of the substrate following classical ice solidification principles. This resulted in collection of XPS spectra of functionalized ceria nanoparticles without them being trapped in ice. The XPS data clearly show that the acetate ions are bonded to the ceria surface through the carboxylate group in both molecular and dissociated states. The peak fitting (of C 1s) data clearly revealed contributions from aliphatic and carboxylate carbons of acetic acid. The peak shifts observed in carboxylate region of XPS spectrum confirmed the dissociation of acetic acid over cerium oxide surface followed by adsorption in dissociated state. XPS also showed that both molecular and dissociated states can be present over the ceria surface in agreement with theoretical calculations. The difference in the energy of adsorption of molecular and dissociated states and yet their presence over cerium oxide surfaces suggest that the adsorption of deprotonated acetic acid may change the local equilibrium making the adsorption of protonated acetic acid favorable on ceria surface. This technique strongly support the theoretical predictions and creates a case for better theoretical models for explaining the effect of local equilibrium (including pH) in understanding the competitive adsorption of protonated and deprotonated states of carboxylic acids over nanoparticle surfaces.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b09930. Comparison of the high resolution Ce 3d photoelectron scans of bare and functionalized ceria nanoparticles, comparison of high resolution O 1s photoelectron scans of ice and bare ceria nanoparticles, and a high resolution C 1s core level photoelectron spectrum (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(A.S.K.) E-mail: [email protected]. *(P.Y.) E-mail: [email protected]. ORCID

Ajay S. Karakoti: 0000-0001-9081-2325 Ping Yang: 0000-0003-4726-2860 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.S.K. would like to acknowledge Manjula Nandasiri and Mark Engelhard for their support in this work. The research was performed using Environmental Molecular Sciences Laboratory (EMSL), a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. This work was supported by the EMSL intramural program at PNNL. P.Y. 3588


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were supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Biosciences and Geosciences Division (CSGB), Heavy Element Chemistry Program, and the Laboratory Directed Research and Development program of Los Alamos National Laboratory (LANL) under project number 20160604ECR. Los Alamos National Laboratory is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of U.S. Department of Energy (Contract DEAC52-06NA25396). A.S.K. would also like to acknowledge Early Career Research grant (Grant No. ECR/2016/000055) from the Department of Science and Technology- Science and Engineering Research Board, India.

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