pH Dependent Electronic and Geometric Structures at the Water

Article pubs.acs.org/JPCC

pH Dependent Electronic and Geometric Structures at the Water− Silica Nanoparticle Interface Matthew A. Brown,*,† Marco Arrigoni,‡ Florent Héroguel,§ Amaia Beloqui Redondo,† Livia Giordano,‡ Jeroen A. van Bokhoven,†,∥ and Gianfranco Pacchioni*,‡ †

Institute for Chemical and Bioengineering, ETH Zürich, CH-8093 Zurich, Switzerland Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, 20125 Milan, Italy § Laboratory of Inorganic Chemistry, ETH Zürich, CH-8093 Zurich, Switzerland ∥ Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland ‡

S Supporting Information *

ABSTRACT: Electronic and geometric structures at the water-amorphous silica nanoparticle (NP) interface are determined as a function of suspension pH using a combination of X-ray photoelelectron spectroscopy (XPS) from a liquid microjet, solid-state nuclear magnetic resonance (NMR), and density functional theory (DFT). We provide direct spectroscopic evidence of the existence of (de)protonated silanol groups at the liquid−NP interface and give a microscopic description of the interface structure. The (de)protonated silanol groups, Si−OH2+ and Si−(OH)(OH2+) in acidic suspension and Si−O− and Si−(OH)(O−) in basic, give rise to wellresolved peaks in the Si 2p spectra that allow their identification and subsequent assignment by DFT. The change in surface potential at the silica NP surface as a function of pH can be directly measured by XPS and allows for an estimate of the fraction of silanol groups that become (de)protonated at the pH of the experiments. In agreement with DFT calculations, NMR is unable to directly identify the (de)protonated silanol species. DFT calculations, including solvent effects indicate that protonation of bridging O atoms can compete with protonation of silanol groups, and that (de)protonation strongly affects the local geometry and stability of the silica network.

1. INTRODUCTION

groups of the oxide material remain in equilibrium with neutral sites M−OH. Being the most abundant oxide material, silica (SiO2) has received particular attention over the years.16 The pHdependent equilibrium of silica surface groups (silanols) in aqueous solutions has stimulated a large amount of research, from both experiment and theory. From a fundamental standpoint, the water−silica interface is often viewed as an ideal system for testing various physicochemical models of solution−oxide interfaces. Of the surface complexation models17−24 (SCM) developed to describe the pH-dependent behavior of oxide (SiO2) interfaces in aqueous solutions, the 2pK model22,23,25 is the most common. This model assumes two sequential (de)protonation steps:

Many chemical and physical properties at water−mineral oxide interfaces are regulated by the charge state of surface hydroxyl (−OH) groups.1 The electric field that results from the protonation and deprotonation of these sites governs oxide dissolution,2−4 colloid suspension stability,5 natural geochemical, and biogeochemical cycles,6 a broad array of biomedical1 and technological applications,7 and the structure and hydrogen bonding of water and reactant molecules adsorbed to, and near, the aqueous−oxide interface.8−12 The speciation of surface hydroxyl groups can therefore be viewed as controlling the surface chemistry/reactivity of many aqueous−mineral oxide interfaces.13 Oxide materials are amphoteric in aqueous solutions, and the speciation of surface hydroxyl groups are controlled by solution pH.14 At pH below the point of zero charge (pHPZC), defined as the pH at which the positive and negative charges of a zwitterionic surface are balanced,15 excess protons adsorb from solution to the oxide surface and generate protonated sites, M− OH2+ (M represents the metal in the oxide), that result in an overall net positive charge at the interface. At a pH above pHPZC, protons desorb from hydroxyl groups into solution to create deprotonated sites, M−O−, and an overall negative surface charge. Across the entire pH range, the charged surface © XXXX American Chemical Society

SiOH 2+ ↔ SiOH + H+ ↔ SiO + 2H+ (1)

where Si represents a silicon atom coordinated to bulk sites. The 2-pK SCM has proved relatively robust over the years, and Special Issue: John C. Hemminger Festschrift Received: March 5, 2014 Revised: April 25, 2014

A

dx.doi.org/10.1021/jp502262f | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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electrostatic repulsion between particles and limits its use at the pH near pHPZC = 3.0.34 Some physical properties of the silica suspension provided by the manufacturer include the following: specific area = 345 m2/g, titratable Na2O = 0.56 wt % (for charge stabilization), and amorphous in structure. TEM. The pH-dependent size of Ludox SM-30 was measured with a FEI Tecnai F30 FEG transmission electron microscope (300 kV) by drying a drop of a 3 wt % suspension on a copper TEM grid with carbon film support. The diluted suspension pH was first adjusted to 10.0 or 0.3 before drying, using concentrated NaOH (Sigma-Aldrich, BioUltra 10 M in H2O) or HCl (Sigma-Aldrich). Representative TEM images and particle size distributions for both pH are shown in Figure 1. Particle size distributions were calculated at pH 10.0 using 116 particles, and with 111 particles at pH 0.3 using the ImageJ software package.

is viewed as successfully describing all titration data for the water−silica interface. Despite the success of the 2-pK SCM in accurately describing potentiometric titration data of water−silica interfaces,2 the model is based on a macroscopic, not a microscopic, description of the interface.26 The physical meaning of the charged surface groups Si−OH2+/Si−(OH)(OH2+) and Si−O−/Si−(OH)(O−) at the water−silica interface in the 2-pK SCM is poorly established because of the lack of direct in situ spectroscopic evidence that can validate their existence.3 In other words, (de)protonated silanol groups are only presumed to physically exist, as they are required to interpret potentiometric titration data based on the 2-pK SCM. The difficulty in unambiguously identifying the spectroscopic signatures of the charged surface species at the water−silica interface, and more generally hydroxyl groups and adsorbates at any water−oxide interface, is that the signal (chemical/ spectroscopic detail) of the relatively few atoms that make up the interface must be extracted and discriminated from the large signal that is generated by the bulk liquid and solid environments.27−29 To date, the explicit identification of (de)protonated silanol groups has only been realized for extended silica substrates in an ex situ XPS study by Duval and co-workers in 2002.26 Indirect approaches, again using XPS, have been used for the inference of ionized silanol sites at the frozen water−silica nanoparticle (NP) interface,30,31 but definitive proof of their existence either at the silica NP interface (in situ or ex situ), or in situ at the aqueous−silica substrate interface, has yet to be realized. In spite of the aforementioned difficulties in performing molecular level spectroscopy at the buried water−oxide interface, the in situ identification of charged surface sites is required (a) to validate the 2-pK SCM by proving their existence, and (b) in order to generate a molecular level description of the pH-dependent water−oxide interface structure. In situ studies are further justified for colloidal samples, where drying the sample likely changes the particles’ size, shape, and ionization state, or the local distribution of ions from the electrical double layer and are therefore not actual representations of the pH-dependent water−oxide NP interface.32,33 In this study, we use a combination of in situ X-ray photoelectron spectroscopy (XPS), solid-state nuclear magnetic resonance (NMR), and density functional theory (DFT) to study the pH-dependent electronic and geometric structures at the water−silica NP interface. The silica particles are amorphous in structure and on average 9 nm in diameter. Our study focuses primarily on conditions where the equilibrium of eq 1 is shifted far to the right (basic suspension) in favor of deprotonated silanol groups and far to the left (acidic suspension) in favor of protonated ones. We show that in situ XPS provides the most direct spectroscopic assignment of (de)protonated silanol groups from the Si 2p spectra. DFT calculations of SiO2 cluster models fully corroborate the XPS assignments and together provide the first unambiguous in situ spectroscopic confirmation of the existence of (de)protonated silanol groups at the water−silica NP interface.

Figure 1. Silica particle size distributions measured by TEM. The mean value and distribution for each NP suspension is shown. Results shown in blue are at low pH (0.3) and in red at high pH (10.0). Changes in solution pH do not affect the size of the particles.

The average particle sizes are 8.8 ± 1.4 nm at pH 10.0 and 8.0 ± 1.6 nm at pH 0.3. The mean diameter determined by TEM at both pH is slightly larger than that provided by the manufacturer (7 nm). The absolute particle diameter is, however, not overly important to the present study, but instead it is important to note that the size and distribution of particles remains constant at both pH values. There are no changes in particle size or the distribution of particle sizes between low and high pH, which effectively rules out changes in particle sizes with solution pH, influencing the results of the subsequent XPS and NMR experiments. For simplicity the particles will be referred to as having a 9 nm diameter throughout. In situ XPS. Experiments were performed using the LiquidJet endstation of beamline PGM-U41 at BESSY for XPS measurements at the water−silica NP interface.29,35,36 A liquid microjet37,38 of 28 μm, a liquid flow rate of 0.55 mL/min, and a temperature, set externally using a recirculating bath of 279 K was used. The measurement chamber pressure during the experiments was 3 × 10−4 mbar, while the detector pressure of the hemispherical energy analyzer was 2 × 10−8 mbar. The total resolution of the hemispherical energy analyzer and the beamline was increased, at the expense of total count rate, to better resolve the surface structures in the Si 2p spectra. Total

2. EXPERIMENTAL METHODS All experiments were performed using commercially available Ludox SM colloidal amorphous silica. The suspension is provided at pH 10.0 as a charge stabilized (i.e., surfactant-free), 30 wt % suspension. The stability of the suspension arises from B



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resolution was 100 meV. Suspensions of 10 wt % Ludox SM were prepared by diluting the stock 30 wt % suspension using Milli-Q water (the native pH of this suspension is 10.0). A second suspension was prepared where the pH was adjusted to 0.3 using HCl (Sigma-Aldrich, BioReagent). At pH 10.0 and 0.3, the stability of the colloid suspension is at a maximum.16 By contrast, intermediate pH results in limited colloid stability with respect to agglomeration (gel time) and prevents us from currently operating the liquid microjet under these conditions. The total electrolyte concentration was 0.05 M NaCl (SigmaAldrich, ACS reagent-grade). High-resolution spectra of the Si 2p, O 1s and valence band (VB) regions were collected. The VB spectra were collected using the same photon energies as the Si 2p and O 1s regions, and the positions of the spectra were charge-corrected using the 1b1 orbital of water, as described in detail previously.39 The Si 2p spectra were collected using a photon energy of 510 eV, corresponding to a photoelectron kinetic energy of ca. 400 eV. The O 1s spectra were recorded using a photon energy of 930 eV, corresponding to a pKE of ca. 395 eV. Because colloidal silica NPs are hydrophilic and reside several water layers below the vapor− liquid interface,36,40 while not surface-sensitive to the vapor− liquid interface,41 a pKE of 400 eV greatly increases the Si 2p count rate by generating photoelectrons from subsurface particles.35 The Si 2p spectral region was fit using Gaussian functions following a standard Shirley background subtraction. MAS NMR. The pH was adjusted by the addition of HCl (Sigma-Aldrich) to the stock 30 wt % Ludox SM suspension. The suspensions were concentrated under reduced pressure to form thick gels that contained approximately 25% water. The thick gels were packed into a 4 mm o.d. Zirconia rotor for solidstate 29Si single pulse-magic angle spinning (SP-MAS) NMR experiments using a Bruker 400 ultra shield spectrometer operating at 79.495 MHz. The spinning rate was 10 kHz. For a typical experiment 37000 scans were acquired with a recycle delay of 2 s. Chemical shifts (ppm) are with reference to tetramethylsilane (TMS), Si(CH3)4, and calibrated using a Q 8 M 8 {(octakis(trimethylsiloxy)silsesquioxane), [Si(CH3)3]8Si8O20} standard (δ = +12.6 from TMS).42

Figure 2. (a) Si6H8O16 model of an isolated silanol, Si−OH; (b) Si6H7O16− model of deprotonated silanol, Si−O−; (c) Si6H9O16+ model of protonated silanol, Si−OH2+; and (d) Si7H10O19 model of an isolated silanol that also includes a bulk Si atom (Si) with no hydroxyl group.

liquid solvent is described simply as a continuous unstructured dielectric. We refer to the cluster without solvent effects as model 0 and the cluster embedded in PCM as model 1. In a slightly more elaborate approach, model 2, two water molecules have been added to the cluster and their interaction with the neutral, deprotonated or protonated silanol groups has been considered explicitly. The position of the two water molecules has been optimized, and the rest of the solvent has been taken into account with the PCM approach. Core level binding energies, BEs, have been determined at the DFT level as the negative of the single particle energies, BE = −εi, where εi is the eigenvalue of the corresponding Kohn− Sham orbital.52 In Hartree−Fock theory this corresponds to Koopmans’ approach to the initial state BE of the core electrons and does not include final state relaxation effects following the creation of the core hole. Therefore, the absolute values of the core level BEs cannot be compared directly to the experiment. Core level shifts, ΔBE, are more significant in this respect and have been determined for Si (2p) levels of the silanol groups, taking the average of the Si 2p energy levels of the remaining Si atoms of the cluster as a reference. The NMR chemical shifts and other properties have been computed using perturbation theory and the Gauge-Invariant Atomic Orbitals (GIAO) method.53,54 Atomic charges have been evaluated according to the natural population analysis.55

3. THEORETICAL METHODS The geometric and electronic structure of silica NPs has been determined based on the DFT approach using the B3LYP hybrid functional.43,44 The Kohn−Sham orbitals have been constructed using Gaussian-type atomic orbital (AO) basis sets [6-31G* on Si, O, and H atoms45 for geometry optimization; 6311+G(2df,p) for NMR properties]. The local environment of the hydroxyl groups in silica has been modeled by Si6H8O16 nanoclusters derived from the edingtonite structure, Figure 2 (panels a−c). It has been shown that the structure of edingtonite has elements in common with amorphous silica surfaces.46 The broken bonds at the cluster periphery have been saturated by H atoms fixed at lattice positions,47−49 while the rest of the cluster has been fully reoptimized. The clusters shown in Figure 2 (panels a−c) do not contain bulklike coordinated Si atoms, as all the Si atoms are on the cluster “surface” and are bound to at least one OH group. In order to be able to identify the properties of bulkcoordinated Si atoms, in particular for XPS, we have considered a larger Si7H10O19 cluster (Figure 2d), where the bottom OH group is replaced by a O−Si(OH)3 unit. Solvent effects have been taken into account by means of the polarizable continuum model (PCM) approach,50,51 where the

4. RESULTS: EXPERIMENT A. XPS. Core Level Shifts: Experiment. Figure 3 shows the Si 2p XP spectra recorded from a liquid microjet of a 10 wt % suspension of 9 nm SiO2 at (Figure 3a) pH 10.0 and (Figure 3b) 0.3. O 1s spectra are shown in Figure S2 of the Supporting Information. The Si 2p spectra were collected with an incident photon energy of 510 eV, which corresponds to a Si 2p pKE of ca. 400 eV. The finite inelastic mean-free path (IMFP) of the photoelectrons in condensed matter, including aqueous C



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by DFT to arise from protonated silanol sites, accounts for 11% of the total intensity, is shifted to the high BE side of the main component (108.1 eV, fwhm = 1.4 eV), and well-fit by a Gaussian function centered at a BE of 109.1 eV and with a fwhm = 2.0 eV (blue) (Table 1). At both pH, the main components arise from both neutral silanol sites and from bulkcoordinated silicon atoms that do not interact with liquid water. The main peaks are shifted in BE from one another by 300 meV. This shift is a direct result of the change in surface potential at the water−silica NP interface between pH 10.0 and 0.3.56 B. Chemical Shifts in 29Si NMR: Experiment. Solid-state NMR requires that the sample be packed into a rotor, and therefore, the silica suspensions were concentrated under reduced pressure. The samples have the consistency of a thick gel that preserves the local hydrated environment of the NPs. Single pulse 29Si solid state NMR provides quantitative information about the relative abundance of different silicon sites based on changes in magnetic shielding. The nomenclature of silicon sites in silica follow a Qn system, where n represents the number of siloxane bridges around a given silicon site (Figure 4). Bulk sites in silica are denoted as Q4,  Si. Formally, there are three different sites for both Q3 and Q2, Si−OH, Si−O−, Si−OH2+, and Si−(OH)2,  Si−(OH)(O−), and Si−(OH)(OH2+), respectively. Figure 5a shows the single pulse 29Si NMR spectra recorded from the thick colloidal 9 nm SiO2 gel at pH 10.0, 6.5, 4.0, 1.0, and 0.1. The spectra have been normalized to the total area. Each spectrum contains three resolved peaks that are assigned to Q4 (−112 ppm), Q3 (−102 ppm), and Q2 (−92 ppm) sites57 (Table S1 of the Supporting Information). The dominant component at each pH is the bulk Q4, Si, site (Figure 5b). The relative abundance of the Q4 peak varies from 66% at pH 0.10 to 78% at pH 6.5 (Table S1 of the Supporting Information). The fwhm of the Q4 peak varies by 7% with pH and is consistent with values reported in the literature for fumed silica.58 The Q3 peak follows an inverse trend with a maximum of 28% at pH 0.1 and a minimum of 18% at pH 6.5. The Q2 peak changes very little with pH and represents 16− 18% of the total silanol (Q2 + Q3) sites at all pH and compares well with values reported in the literature for fully hydroxylated amorphous silica.13 With changes in suspension pH, there are no new peaks, no shifts in peak positions, or changes in fwhm of the Qn components that would allow for the simple assignment of deprotonated silanol at high pH or of protonated silanol at low pH. Measured variations in Qn population with suspension pH are a direct reflection on the stability of the silica suspension.16 Near a pH of 6.5, the suspension is least stable with respect to gelation (condensation of two silanols occurs to form a siloxane bridge and a molecule of water), and a maximum in Q4 population is observed.

Figure 3. Silicon 2p spectra collected at (a) pH 10.0 and (b) pH 0.3 from a liquid microjet of 10 wt % 9 nm colloidal SiO2. Both spectra are fit using two Gaussian functions (the fit residuals are found in the Supporting Information). At high pH, the shoulder component is on the low BE side of the main peak, whereas at low pH, the shoulder is on the high BE side. The electron-binding energy scale is relative to the vacuum level.

solutions,41 limits the probe depth of the experiment to the outermost ca. 5 nm of the solution interface. At 400 eV, approximately 20% of the Si 2p signal arises from the water− silica interface, with the remaining intensity from the subsurface and bulk of the NP that is not in contact with the solution. Both spectra are asymmetric in line shape with a low intensity shoulder that requires a second component to properly fit (residuals to the fits are shown in Figure S1 of the Supporting Information). At pH 10.0, the minor component, which is later shown by DFT to arise from deprotonated silanol sites, accounts for 13% of the total Si 2p intensity and arises from the surface of the NP based on pKE-dependent measurements.35 This peak is on the low BE side of the main component (107.8 eV, fwhm = 1.4 eV) and well-fit by a Gaussian function centered at a BE of 106.8 eV and with a fwhm = 2.0 eV (shown in red) (Table 1). At pH 0.3, the shoulder, which is later shown Table 1. Summary of the Measured XPS Binding Energies (BE), Full Width Half Maximum (fwhm), and Relative Signal Intensity for 10 wt % 9 nm SiO2 Suspensions

assignment

BE (eV)

fwhm (eV)

relative intensity (%)

sample

orbital

10 wt % SiO2, pH 0.3

Si 2p Si 2p O 1s

Si, Si−OH protonated silanol water (solvent)

108.1 109.1 538.1

1.4 2.0 1.8

89 11 100

10 wt % SiO2, pH 10.0

Si 2p Si 2p O 1s

Si, Si−OH deprotonated silanol water (solvent)

107.8 106.8 538.1

1.4 2.0 1.8

87 13 100

5. RESULTS: THEORY A. Structure and Stability. In acidic conditions, the protonation of silica NPs can involve either the (isolated) silanol, geminal silanol, or the siloxane groups:

D

SiOH + H+ → SiOH 2+

(2)

Si(OH)2 + H+ → Si(OH)(OH 2+)

(3)

SiOSi +H+ → SiO(H+)Si

(4)



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Figure 4. Qn classification of silcon sites in silica. Formally there are three unique silicon sites for both Q3 and Q2.

Figure 5. (a) Single pulse 29Si solid-state NMR spectra from a thick gel of colloidal 9 nm SiO2 as a function of suspension pH. Three well-resolved peaks are present in each spectrum that is labeled according to the Qn system (see text and Figure 4). (b) Relative abundance of the different Qn sites with suspension pH.

preferred by only 3 kcal/mol, for the H3Si−OH and H3Si−O− SiH3 gas phase molecules, the introduction of the solvent with the PCM approach (model 1) stabilizes the protonation of bridging oxygen sites and the geminal silanol unit compared to the Si−OH2+ one. The explicit inclusion of two water molecules reduces the difference to 2 kcal/mol, still in favor of siloxane protonation (Table 2). A substantial difference in protonation energy of the siloxane group is found, depending on the position of the Si atoms (Table 2), suggesting that the reactivity can largely depend on the local environment of a  Si−O−Si group. The calculations including solvent effects indicate that protonation of bridging O atoms can compete with protonation of silanol groups. Protonation of silica has some important geometrical consequences, in particular the Si−O distances increase considerably (Table 3), indicating a weakening of the Si−O bond upon proton addition with important consequences on

The proton affinity of (isolated) silanol and siloxane groups has been investigated in the past with accurate calculations for the simple gas-phase molecules H3Si−OH and H3Si−O−SiH3, and the results indicate that protonation of the bridging oxygen in the H3Si−O−SiH3 molecule is preferred by about 2 kcal/mol with respect to protonation of the OH group in H3Si−OH.59,60 Our DFT calculations confirm this result and indicate a protonation energy of 188 kcal/mol for H3Si−OH and 190 kcal/mol for H3Si−O−SiH3. The same process has been investigated on our silica nanoclusters (Figure 2, panels a−c), with and without the solvent effect (Table 2). For the protonation of the Si−O−Si bond, we have various cases, one where the O atom is between a Si of Q3 type and another of Q2 type, and two, where the O is between two Si atoms of Q3 type (Table 2). While for the bare silica cluster (model 0), the protonation of the Si−O−Si bond is Table 2. Protonation Energies, ΔE (kcal/mol) of Si−OH, Si−(OH)(OH), and Si−O−Si Bondsa Si Si−OH Si−(OH)(OH) (HO)−Si−O−Si−(OH)2−b (HO)−Si−O−Si−(OH)b (HO)−Si−O−Si−(OH)b

Q3 Q2 between Q3 and Q2 between Q3 and Q3 between Q3 and Q3

model 0

model 1

model 2

193 196 196

240 263 258

291 − 293

199

239

193

257

Table 3. Selected Geometrical Properties of Neutral, Protonated, and Deprotonated Groups in Silica NP (Model 1)a neutral

Si−OH

−

r(OH) (Å) r(Si−O) (Å) α(Si−O−H)°

0.97 1.63 116

−

protonated

Si−(OH)2

− 1.63 1.63 − Si−O(H+)− Si Si−(OH)(OH2+)

0.97 1.64 116

Si−OH2+

r(OH) (Å) 0.98 0.98 0.99 r(Si−O) (Å) 1.77 1.61 α(Si−O−H)° 120 120 119 deprotonated Si−O−

a Model 0, cluster without solvent effects; model 1, the solvent is represented by the polarizable continuum model (PCM); model 2, the solvent is represented by two water molecules plus PCM; ΔE values are given by the difference: E(A) − E(AH+), where A is the noncharged cluster and AH+ is the protonated cluster. bThe silica clusters used contain siloxane groups between a Q3 and a Q2 site and siloxane groups between two Q3 sites. Since these clusters have no symmetry, the siloxanes are not equivalent after protonation and have different protonation energies.

r(OH) (Å) r(Si−O) (Å) α(Si−O−H)°

Si−O−Si

− 1.55 −

0.97 1.64 116

1.00 1.78 120 0.98 1.68 112

1.00 0.98 − 1.80 1.79 120 115 115 Si−(OH)(O−) − 1.56 −

a

In some groups the same geometrical parameters occur more than once; when this happens the columns are split.

E



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the stability of the silica network. In particular, the protonated siloxane can further react in the presence of water molecules61 with the breaking of the Si−O(H+)−Si bond:

Table 4. Si 2p Core Level Binding Energies (BE) and BE Shifts (ΔBE) from DFT Calculations Si 2p BE (eV)

SiO(H+)Si + H 2O → SiOH 2 + SiOH +

silica cluster

model 1

neutral Si Si−OH Si(OH)2 deprotonated Si−O− Sia ΔBE Si-(OH)(O−) Sia ΔBE protonated Si−OH2+ Sia ΔBE Si−(OH)(OH2+) Sia ΔBE

(5)

This process, not investigated in detail by DFT, leads to the formation of new silanol groups that have Q3 character. Also the Si−OH2+ group is considerably distorted compared to the neutral Si−OH precursor. While the Si atom of the  Si−OH group has a tetrahedral structure, in Si−OH2+, Si has an almost trigonal planar structure and the complex can be seen as an H2O molecule coordinated to a Si+ group. The protonation of the geminal silanol leads to the Si− (OH)(OH2+) group, where the proton is bound to one of the hydroxyl groups. The Si−O bond length between the silicon and the oxygen of the protonated hydroxyl reaches the value of 1.78 Å, whereas for the other hydroxyl, this distance is only 1.61 Å (Table 3). As for the silanol group, also in this case we can consider the protonated group as a complex where a water molecule is coordinated to the Si+−OH species. Deprotonation at high pH results in the formation of a negatively charged Si−O− group (Figure 2b) with a shortening of the Si−O bond length, from 1.64 to 1.55 Å (Table 3) and an elongation of the other three Si−O bonds. The situation is similar for the geminal silanol, where the Si− O bond distance decreases to 1.56 Å between the Si atom and the oxygen of the deprotonated hydroxyl and increases to 1.68 Å between the silicon and the oxygen of the other hydroxyl (Table 3). B. Core Level Shifts: Theory. We first considered a cluster which contains at the same time a bulk coordinated Si atom (Q4), isolated silanols, Si−OH (Q3), and geminal silanols, Si(OH)2 (Q2) (Figure 2d). With this cluster, we have computed the Si 2p BE levels for Q4, Q3, and Q2 groups (Table 4). The Q4 species are calculated to have a peak at 101.4 eV, while Q3 is at 101.3 eV. Therefore, the two signals are separated by about 0.1 eV only and cannot be distinguished in the experimental spectrum (see Discussion). In the following, we use the average of the Si 2p core level BEs of Q3 groups as an internal reference for the calculation of the core level shifts of protonated and deprotonated species. In fact, the addition or removal of a proton results in a charged cluster with consequent shift of all the core levels, which requires defining an internal reference for the core level shifts. Since the Q3 Si atoms have 0.1 eV smaller core level BEs than bulk Q4 atoms, our computed core level shifts have been corrected by this amount. The Si core level BEs have been determined for neutral, deprotonated, and protonated silica clusters using the PCM approach (model 1, Table 4). In the solvated neutral clusters, the Si−OH and Si(OH)2 groups have the Si 2p levels at 101.3 and 101.0 eV, respectively (Table 4). Thus, there is a small shift of about 0.3 eV in the Si 2p states of single compared to geminal silanols. As a next step, we considered the removal of a proton from the top Si−OH group (Figure 2b). We observe a shift to smaller BE, 99.6 eV, of the corresponding Si 2p level (Table 4). The other Q3 Si 2p levels have an average BE of 100.9 ± 0.1 eV, which corresponds with the 0.1 eV correction (see above), to a bulk Q4 reference of 101.0 ± 0.1 eV (Table 4). The shift due to the deprotonation is thus of −1.4 eV; taking into account the

Q4 Q3 Q2

101.4 101.3 101.0

Q3 Q4

99.6 101.0 −1.4 99.0 100.8 −1.8

Q2 Q4

Q3 Q4 Q2 Q4

102.7 101.7 +1.0 102.4 101.5 +0.9

model 2 − − −

± 0.1 ± 0.2 ± 0.1 ± 0.3

± 0.1 ± 0.1 ± 0.1 ± 0.3

99.7 100.8 ± 0.1 −1.1 ± 0.1 − − − 101.9 101.4 ± 0.1 +0.5 ± 0.1 − − −

a Estimated from computed values for Si 2p levels in the Si−OH groups of the cluster corrected by the difference in core level of Si− OH and Si groups (0.1 eV).

standard deviation of the various data, we estimate with this model a shift of −1.4 ± 0.2 eV. As discussed above, the addition of a proton to the Si−OH group has important geometrical consequences. Formally, it is as having a water molecule interacting with a Si+ center. Here the core level shifts have an opposite sign compared to the deprotonated case. In particular, there is a shift of the Si 2p level to higher BEs, 102.7 eV (Table 4). The remaining Q3 Si atoms of the cluster have their Si 2p levels around 101.6 eV (Table 4), corresponding to 101.7 eV for the Si bulk reference. The computed shift due to the protonation is therefore around +1.0 eV. Adding a proton to a geminal group increases the Si 2p levels BE to 102.4 eV, 0.9 eV higher than the Q4 reference for the cluster (101.5 eV). On the other hand, when a proton is removed from one of the two hydroxyl groups of the geminal species, the energy of the Si 2p levels decreases to 99.0 eV, with a shift of −1.8 eV with respect to the reference Q4 value (100.8 eV). It is common to interpret core level shifts in terms of changes in electron density around a given atom.62 A larger valence electron density results in a more efficient screening of the core hole and in a shift of the corresponding BE to smaller values (and vice versa). Besides charge density arguments, however, electrostatic terms can also be very important.63 The natural atomic charge on the Si atoms of Q4 groups is +2.48|e| (model 1); this is also the charge on a neutral Si−OH group. Deprotonation with formation of Si−O− leads to a reduced net charge on Si of +2.36|e|, corresponding to an increase of the electron density by about 0.12|e|; on the other hand, protonation leads to a tiny increase of the net charge on Si which on Si−OH2+ becomes +2.50|e|, with a reduction of electron density on Si of only 0.02|e|. These charge differences are very similar to those reported in recent calculations.64 Despite the fact that the changes in charge density for deprotonated and protonated silanols differ by 5−6 times, the F



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that removing or adding a proton from/to the Si−OH group does not change the Q3 character of the signal, since the Si atom always has three Si second neighbors (Figure 4); for the same reasons, the Q2 character of germinal silanols is not affected by the addition or removal of a proton. The reference value is thus the shift of −106 ppm computed for a silanol in the neutral cluster (Table 5). Deprotonation leads to a decrease

shift in core levels has the same magnitude, ± 1 eV (Table 3); furthermore, it is difficult to justify a shift of about 1 eV with a change in charge density by 0.02|e|. The reason is that the origin of the shift is not so much in the change in charge density on Si but on the electrostatic potential generated by the extra charge created by deprotonation or protonation. To prove this, we have taken the neutral cluster representing the Si− OH group and we added a −1 or a +1 point charge at about 1 Å from the O atom of the silanol (model 1). The geometry has not been reoptimized, and the Si 2p core levels have been evaluated, as in previous cases. In [Si−OH]−, we observe a shift of −1.0 eV of the Si 2p levels; in [Si−OH]+, the shift is +1.2 eV. Notice that there are no basis functions associated with the point charge, so that no chemical bond can be formed. The only effect of the point charge is to polarize the electron density of the cluster and to generate a potential varying as 1/r that shifts all the core levels. However, the shift is more pronounced for the atoms close to the point charge (in this case the Si atom of the deprotonated or protonated group) than for the rest of the cluster, and the global effect is a shift similar in sign and in magnitude of that computed for the real system. This shows quite unambiguously that the origin of the shift is mostly electrostatic. In model 2, the deprotonated or protonated silanol group is directly bound to two water molecules, and the cluster-water complex is embedded in PCM. The results are qualitatively similar to those obtained with model 1. In particular, in model 2, the Si 2p core level BE of the Si−O− center increases slightly to 99.7 eV, while the reference level of bulk Si slightly decreases to 100.8 eV (Table 3). As a consequence, with model 2 we find for the deprotonated system (Si−O−) a shift in the Si 2p levels of −1.1 ± 0.1 eV. The results of model 2 for the protonated case show that the absolute value of the Si 2p BE of the Si−OH2+ group decreases to 101.9 eV (it was 102.7 in model 1); this is the consequence of the extra screening provided by the explicit treatment of the water molecules. However, the BE of the reference Si 2p levels also decrease with respect to model 1, and the final effect is a smaller final shift due to protonation, +0.5 ± 0.1 eV. This shows that the exact value of the shift depends on the level of treatment. C. Chemical Shifts in 29Si NMR: Theory. Chemical shifts, δ(29Si), in silica nanoclusters have been determined at the DFT-B3LYP level with respect to the isotropic shielding constant, σiso(Si) = 328 ppm, computed for TMS (tetramethylsilane), a usual standard. For the Q4 (Si) and Q3 ( Si−OH) groups of the neutral cluster (Figure 2d), δ(29Si) is −117 and −106 ppm, respectively, to be compared with the experimental shifts of −112 and −101 ppm (Figure 5). While the computed chemical shifts are quite accurate, the absolute values of the shielding computed at the DFT level with the present basis set are overestimated by about 4−5%. The chemical shift of geminal silanols, Si(OH)2, δ(29Si) = −95 ppm, is also about 3% larger than the experimental one, −92 ppm (Figure 5). This shows that the error is systematic. The nuclear magnetic shielding of 29Si is thus about 10 ppm smaller for a geminal than for an isolated silanol. These values are consistent with the assignment of the three signals in the NMR spectra of Figure 5 to Si bulk centers (Q4, −112 ppm),  Si−OH groups (Q3, −101 ppm), and to geminal silanols Si− (OH)2 (Q2, −92 ppm). Next we have considered the effect of the deprotonation and of the protonation on the NMR spectra. First of all, we notice

Table 5. NMR Properties Computed for Various Groups in Silica Nano-Clusters (model 1): Isotropic Shielding, σiso(29Si); Chemical Shift, δ(29Si) Si Si−OH Si−O− Si−OH2+ Si−(OH)2 Si−(OH)(O−) Si−(OH)(OH2+) a

Q4 Q3 Q3 Q3 Q2 Q2 Q2

σiso(29Si) (ppm)

δ(29Si)a (ppm)

445 433 438 435 423 423 428

−117 −106 −110 −107 −95 −95 −100

Reference value for TMS σiso(29Si) = 328 ppm.

of the shielding by about 4 ppm, δ(29Si) = −110 ppm, for silanol groups, while it does not affect the chemical shift of geminal silanols that remains constant to −95 ppm (Table 5). Furthermore, the shift in the shielding in silanols will bring the signal of the Si−O− group closer to the region of the dominating Q4 signal of bulk Si, making a detection of the silanolate group unlikely. Protonation on a silanol leads to the Si−OH2+ Q3 group with δ(29Si) = −107 ppm. This value is shifted by only 1 ppm with respect to the main Q3 line (−106 ppm) and is expected to result, at most, in a broadening of this signal. Adding a proton to a geminal silanol group gives a chemical shift of −100 ppm in a region dominated by the signal of Q3 groups, making a deconvolution of the signal a difficult task. Therefore, the changes in 29Si MAS spectra due to changes in pH of the solution are expected to result only in minor modifications of the peak positions, while it can possibly produce a change in the intensity of the respective features. These results are entirely consistent with the experimental measurements (Figure 5 and Table S1 of the Supporting Information).

6. DISCUSSION XPS. Based on DFT calculations, the main components of the Si 2p XP spectra at 107.8 eV (pH 10.0) and at 108.1 eV (pH 0.3) can be assigned to two overlapping features: bulk Q4 sites, Si, in the interior of the NP that do not interact with water and to neutral Q3 sites, Si−OH, at the surface of the NP that do not become (de)protonated at the pH of the experiments. The latter is consistent with several reports that show that a fraction of neutral Q3 sites do not become ionized at pH 10.0 and 0.3.2,26 In accordance with DFT calculations, these two silicon sites give nearly overlapping Si 2p BEs, 101.4 eV for Si and 101.3 eV for Si−OH (see Table 4, neutral cluster, model 1). The difference in BE of 0.1 eV between these two sites cannot be resolved in a Si 2p XPS experiment where the narrowest reported line width, in agreement with what we report here (Table 1), for room temperature silica is 1.4 eV.65 The 0.3 eV shift between the two main components of the spectra56 originates from a change in surface potential at the NP interface (vide infra). G



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reported earlier by Duval and co-workers.26 Further quantitative estimates of the fraction of (de)protonated silanol groups are currently not possible because of several experimental uncertainties, including not knowing the exact spatial distribution of the NPs relative to the vacuum−liquid interface. However, a qualitative estimate can be made based on the ΔBE = 0.3 eV of the main components in the Si 2p spectra and the known pH dependence of the surface charge density.36 From these two results, we can estimate that ca. 30% of silanol sites undergo protonation at pH 0.3 and a roughly equal amount become deprotonated at pH 10.0. These results are consistent with those obtained by ex situ XPS for quartz surfaces as a function of solution pH.26 NMR. Solid-state NMR does not allow for a trivial assignment of (de)protonated silanol groups, which is fully consistent with our DFT models. In basic suspension our DFT models predict that the chemical shift of Si−O− would bring the peak closer to the dominant Q4 signal, whereas no shift is expected for the deprotonated geminal species. In acidic suspension, our DFT models predict virtually no chemical shift for Si−OH2+ and a small shift for the protonated geminal species in the direction of the more abundant Q3 signal. At best, our DFT models predict a small broadening of the NMR peaks with changes in pH; however, our experiments do not show any broadening above experimental uncertainty.

The pH-dependent response in the BE of the minor components in the Si 2p spectra suggests that they originate from the surface of the NP that is in direct contact with water. In the case of the pH 10.0 suspension, previous pKE dependent measurements have shown that the minor component originates from the surface of the NP.35 At pH 10.0, the minor peak is shifted in BE relative to the main peak by −1.0 eV, whereas at pH 0.3, it is shifted by +1.0 eV. At high pH, the equilibrium of eq 1 is shifted to the right and the surface of the silica NP is expected to contain a significant fraction of deprotonated Q3 sites.16,26 We assign the peak at 106.8 eV in the pH 10.0 spectrum to a combination of Si−O− and Si− (OH)(O − ), which is qualitatively confirmed by DFT calculations that predict a shift of −1.4 eV for Si−O− and −1.8 eV for Si−(OH)(O−) from the main component of the spectrum. At low pH, a similar argument based on the equilibrium of eq 1 can be made, and we assign the peak at 109.1 eV in the pH 0.3 spectrum to a combination of Si− OH2+ and Si−(OH)(OH2+), which is again supported by DFT calculations that predict a shift of +1.0 and +0.9 eV, respectively. The increased spectral width (fwhm 2.0 eV) of these charged surface sites relative to the main components of the spectra (fwhm 1.4 eV) reflects distributions in (a) the Qn nature of the (de)protonated species, (b) the geometry of the ionized site, and (c) in the local hydrogen-bonding structure of the interface. DFT calculations have shown that ionization of silanol groups results in geometrical changes at the interface in bond distances and bond angles. The dynamic equilibrium at the interface (eq 1) will result in a distribution of bond lengths and angles that increases the width of the photoelectron peaks. (De)prontonated silanol groups are also believed to disrupt the local hydrogen-bonding environment at the interface3 that results in a distribution of chemical environments and an increased distribution of pKEs. The BEs of the main components in both Si 2p spectra, 107.8 eV at pH 10.0 and 108.1 eV at pH 0.3 are in good agreement with a BE of 108.0 (±0.3) eV (relative to the vacuum level) reported by Bianconi for extended solid films of silica.66 The ΔBE of 0.3 eV between the two main components of the spectra are a direct result of a change in surface potential at the NPs interface with suspension pH that affects the kinetic energy of the outgoing photoelectron.56 The change in surface potential results from the (de)protonation of silanol groups according to the equilibrium of eq 1. At high pH where the NP contains Si−O− and Si−(OH)(O−) sites, the surface is negatively charged and the BE is shifted to lower energies.67 At low pH, the NP surface contains Si−OH2+ and Si− (OH)(OH2+) and has a net positive charge that shifts the BE of the main component higher in BE. The magnitude of ΔBE is in qualitative agreement56 with the change in surface potential calculated using the Grahame68 and Ohshima69 equations for pH 0.3 and 10.0 based on reported surface charge densities of 9 nm silica particles.36 Our results show that not only can XPS be used to identify (de)protonated hydroxyl groups, but also can be used as a direct in situ probe of changes in surface potential at mineral oxide interfaces. The intensities of the (de)protonated silanol groups relative to the neutral species are 11% in the case of pH 0.3 and 13% at pH 10.0. The 2% difference is within our experimental uncertainty, and we can therefore conclude that protonated and deprotonated silanol sites have roughly equal populations at the surface of the particles in aqueous solutions at the pH of the experiments. This result is fully consistent with that

7. CONCLUSION Spectroscopic identification of (de)protonated silanol groups has been realized using XPS from a liquid microjet. In basic suspensions, the deprotonated silanol groups Si−O− and  Si−(OH)(O−) give rise to a shoulder in the Si 2p XP spectrum on the low BE side of the main neutral components (Si and Si−OH) that is well-resolved. In acidic suspensions, the protonated silanol groups Si−OH2+ and Si−(OH)(OH2+) are shifted to higher BE than the neutral species and our assignment is again supported by DFT models. DFT models provide qualitative support for these assignments. In both cases, DFT has shown that the origin of the shift in BE is mostly electrostatic. In agreement with DFT models, NMR is unable to unambiguously identify (de)protonated silanol groups at the water−silica NP interface. Calculations predict at best a broadening of the Q3 line in basic suspension and of the Q2 line in acidic media. In both cases, the small changes in predicted chemical shifts are in the direction of the more abundant species, making detection difficult. The protonation state of silanol groups is predicted to have pronounced influence on the local geometric structure surrounding the (de)protonated site with a shortening of the Si−O bond length in the case of deprotonated silanol and an increase in length for the protonated case. Finally, DFT models that include solvent (water) effects showed that the protonation of bridging O atoms can compete with the protonation of silanol groups, but the reactivity was predicted to largely depend on the local environment.

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ASSOCIATED CONTENT

S Supporting Information *

Residuals to the fits of the Si 2p XP spectra, O 1s XPS results and discussion, calculated chemical shift anisotropy, experimental details, and data fitting for the NMR 1H−29Si crosspolarization experiments. This material is available free of charge via the Internet at http://pubs.acs.org. H



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(17) Piasecki, W.; Rudzinski, W.; Charmas, R. 1-pK and 2-pK Protonation Models in the Theoretical Description of Simple Ion Adsorption at the Oxide/Electrolyte Interface: A Comparative Study of the Behavior of the Surface Charge, the Individual Isotherms of Ions, and the Accompanying Electrokinetic Effects. J. Phys. Chem. B 2001, 105, 9755−9771. (18) Piasecki, W. 1pK and 2pK Protonation Models in the Theoretical Description of Simple Ion Adsorption at the Oxide/ Electrolyte Interface: Studying of the Role of the Energetic Heterogeneity of Oxide Surfaces. Langmuir 2002, 18, 8079−8084. (19) Piasecki, W. 1pK and 2pK Protonation Models in the Theoretical Description of Simple Ion Adsorption at the Oxide/ Electrolyte Interface: The Analysis of Temperature Dependence of Potentiometric Titration Curves. J. Colloid Interface Sci. 2002, 254, 56−63. (20) Piasecki, W. 1pK and 2pK Protonation Models in the Theoretical Description of Simple Ion Adsorption at the Oxide/ Electrolyte Interface: A Comparative Study of the Predicted and Observed Enthalpic Effects Accompanying Adsorption of Simple Ions. Langmuir 2002, 18, 4809−4818. (21) Hiemstra, T.; Vanriemsdijk, W. H.; Bolt, G. H. Multisite Proton Adsorption Modeling at the Solid-Solution Interface of (Hydr)Oxides: A New Approach 0.1. Model Description and Evaluation of Intrinsic Reaction Constants. J. Colloid Interface Sci. 1989, 133, 91−104. (22) Yates, D. E.; Levine, S.; Healy, T. W. Site-Binding Model of Electrical Double-Layer at Oxide-Water Interface. J. Chem. Soc., Faraday Trans. 1 1974, 70, 1807−1818. (23) Davis, J. A.; James, R. O.; Leckie, J. O. Surface Ionization and Complexation at Oxide-Water Interface 0.1. Computation of Electrical Double-Layer Properties in Simple Electrolytes. J. Colloid Interface Sci. 1978, 63, 480−499. (24) Piasecki, W. Theoretical Description of the Kinetics of Proton Adsorption at the Oxide/Electrolyte Interface based on the Statistical Rate Theory of Interfacial Transport and the 1pK Model of Surface Charging. Langmuir 2003, 19, 9526−9533. (25) Chan, D.; Perram, J. W.; White, L. R.; Healy, T. W. Regulation of Surface-Potential at Amphoteric Surfaces during Particle-Particle Interaction. J. Chem. Soc., Faraday Trans. 1 1975, 71, 1046−1057. (26) Duval, Y.; Mielczarski, J. A.; Pokrovsky, O. S.; Mielczarski, E.; Ehrhardt, J. J. Evidence of the Existence of Three Types of Species at the Quartz-Aqueous Solution Interface at pH 0−10: XPS Surface Group Quantification and Surface Complexation Modeling. J. Phys. Chem. B 2002, 106, 2937−2945. (27) Zaera, F. Surface Chemistry at the Liquid/Solid Interface. Surf. Sci. 2011, 605, 1141−1145. (28) Zaera, F. Probing Liquid/Solid Interfaces at the Molecular Level. Chem. Rev. 2012, 112, 2920−2986. (29) Brown, M. A.; Jordan, I.; Beloqui Redondo, A.; Kleibert, A.; Wörner, H. J.; van Bokhoven, J. A. In Situ Photoelectron Spectroscopy at the Liquid/Nanoparticle Interface. Surf. Sci. 2013, 610, 1−6. (30) Shchukarev, A. V. A Study of the SiO2-Aqueous Electrolyte (NaCl, CsCl) Interface by X-Ray Photoelectron Spectroscopy. Colloid J. 2007, 69, 514−525. (31) Shchukarev, A. V.; Rosenqvist, J.; Sjöberg, S. XPS Study of the Silica-Water Interface. J. Electron Spectrosc. Relat. Phenom. 2004, 137− 140, 171−176. (32) Brown, M. A.; Huthwelker, T.; Beloqui Redondo, A.; Janousch, M.; Faubel, M.; Arrell, C. A.; Scarongella, M.; Chergui, M.; van Bokhoven, J. A. Changes in the Silanol Protonation State Measured In Situ at the Silica-Aqueous Interface. J. Phys. Chem. Lett. 2012, 3, 231− 235. (33) Brown, G. E.; Parks, G. A.; Bargar, J. R.; Towle, S. N. In MineralWater Interfacial Reactions: Kinetics and Mechanisms; Sparks, D. L., Grundl, T. J., Eds.; American Chemical Society: Washington, D.C., 1999. (34) Tadros, T. F.; Lyklema, J. Adsorption of Potential-Determining Ions at Silica-Aqueous Electrolyte Interface and Role Of Some Cations. J. Electroanal. Chem. 1968, 17, 267−275.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +41 44 632 3048. *E-mail: [email protected]. Tel: +39 02 6448 5219. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Christophe Copéret (ETH Zurich), Bernd Winter (BESSY), Martin Sterrer (FHI Berlin), Zareen Abbas (Gothenburg), and Luca Quaroni (PSI) for many fruitful discussions. This work was supported by an ETH Postdoctoral fellowship (M.A.B.) and by the Italian MIUR through the FIRB Project RBAP115AYN.

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