Characterization of Phosphate Species on Hydrated Anatase TiO2

Article pubs.acs.org/Langmuir

Characterization of Phosphate Species on Hydrated Anatase TiO2 Surfaces Frederik Tielens,*,† Christel Gervais,† Geraldine Deroy,‡ Maguy Jaber,§ Lorenzo Stievano,∥ Cristina Coelho Diogo,†,⊥ and Jean-François Lambert‡ †

Sorbonne Université, UPMC Univ Paris 06, CNRS, UMR 7574, Laboratoire de Chimie de la Matière Condensée de Paris, Collège de France, 11 place Marcelin Berthelot, 75231 Cedex 05 Paris, France ‡ Sorbonne Université, UPMC Univ Paris 06, UMR 7197, Laboratoire de Réactivité de Surface. 3 rue Galilée, F-94200 Ivry-Sur-Seine, France § Sorbonne Université, UPMC Univ Paris 06, UMR 8220 Laboratoire d’Archéologie Moléculaire et Structurale, 4 Place Jussieu, 75005 Paris, France ∥ Université Montpellier, ICGM, UMR 5253, 2 Place Eugène Bataillon - CC 1502, 34095 Montpellier CEDEX 5, France ⊥ IMPC, Institut des Matériaux de Paris Centre FR2482, 75252 cedex 05 Paris, France ABSTRACT: The adsorption/interaction of KH2PO4 with solvated (100) and (101) TiO2 anatase surfaces is investigated using periodic DFT calculations in combination with GIPAW NMR calculations and experimental IR and solid state 17O, and 31P NMR spectroscopies. A complete and realistic model has been used to simulate the solvent by individual water molecules. The most stable adsorption configurations are characterized theoretically at the atomic scale, and experimentally supported by NMR and IR spectroscopies. It is shown that H2PO4− chemisorbs on the (100) and (101) anatase surfaces, preferentially via a bidentate geometry. Dimer (H3P2O7−) and trimer (H4P3O10−) adsorption models are confronted with monomer adsorption models, in order to rationalize their occurrence.

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posteriori the chemical potential of species in gas34,35 or solution36 to bridge the temperature and pressure gap.37−39 Different mechanisms may indeed account for the adsorption in ultrahigh vacuum40 (UHV), at low temperature, and in solution, at room temperature,41 and the organization of the liquid at the interface with the solid is likely to play a pivotal role.42 One of the candidate molecules to promote biocompatibility is KH2PO4 (monopotassium phosphate). Other derivates of phosphoric acid such as phosphonic acids, methyl phosphonate,43 dimethylphosphonate,44 and so forth have been studied earlier.45,46 It is usually accepted that these phosphate derivates bind to titanium dioxide surfaces via Ti−O−P bonds; in other words, adsorption involves coordination of the phosphoryl oxygen to surface Ti4+ ions, which is also called adsorption by inner sphere complex formation in the parlance of colloidal chemistry. Different coordination modes can be envisaged such as mono-, bi-, or tridentate.47,48 Furthermore, bidentate phosphate groups can be bridging between two Ti4+, or chelating to a single Ti4+.

INTRODUCTION Nanostructured titania is a versatile material with unique optical,1 environmental,2−5 and photocatalytic6,7 properties used in various applications, such as in biomedicine8 and cosmetics.9,10 The large use of titanium in many industrial, biological, and medical applications has led to the interest in functionalizing superficially oxidized titanium, or titanium oxide surfaces. Indeed, anchoring organic molecules or monolayers to the surface is an efficient way to tailor surface properties.11 The best studied examples are thiol or amino acid monolayers on coinage metal surfaces,12,13 also studied in our group,14−19 and silane monolayers on silica surfaces.20−22 Titanium dioxide surfaces have been extensively studied using both experimental23 and theoretical methods24−28 in the scope to design biocompatible devices based on titanium. Since Ti surface exposed to air oxidizes and forms a TiO2 layer on top of the metal surface, it is more appropriate to investigate directly the TiO2 surface properties in relation to the interaction with biocompatible molecules. Most theoretical works describe the molecule−surface interaction very precisely,29 but the role of the solvent is not taken into account explicitly.30,31 In fact, the extrapolation of those results when considering the real solid−liquid interface at ambient temperature is not straightforward.32,33 Some works use the atomistic thermodynamics approach which includes a © 2016 American Chemical Society

Received: September 21, 2015 Revised: November 27, 2015 Published: January 6, 2016 997

DOI: 10.1021/acs.langmuir.5b03519 Langmuir 2016, 32, 997−1008

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The (100) and (101) anatase surfaces are modeled using slabs containing 32 TiO2 units. The dimensions of the unit cells are 7.619 Å × 9.515 Å × 40.000 and 7.376 Å × 10.884 Å × 40.000 Å for the (100) and (101) surfaces, respectively. On these surfaces, 12 (on the (100) surface) and 16 (on the (101) surface) water molecules are adsorbed following the literature data, that is, forming surface hydroxyl groups and physisorbed water molecules.59 On top of this formed water monolayer, the vacuum between the hydrated TiO2 slabs was filled up with water molecules. This liquid phase on top of the hydrated TiO2 surface is modeled by 50 and 60 H2Oliq molecules, corresponding to a total of 284 and 326 atoms, respectively, for the (100) and (101) surfaces. The dissociated KH2PO4 or the KH3P2O7 molecule was introduced in the slab in substitution to two water molecules in order to maintain a pressure of 1 atm. Indeed, the molecular volume of water is smaller than the molecular volume of KH2PO4 (see Figure 1).

Experimental confirmation of adsorption through Ti−O−P bond formation could be provided by solid-state NMR, especially of 17O. Indeed, this method shows a high potential for the description of the binding modes of adsorbed monolayers on inorganic surfaces, and 17O appears as a key nucleus to characterize inorganic materials49 but almost systematically requires an isotopic enrichment due to the poor sensitivity of this nucleus due to its low natural abundance (0.037%). Enriched derivatives can then be easily studied by MAS or MQ-MAS (multiple quantum magic angle spinning50) experiments (leading to the complete averaging of second order quadrupolar effects). The 17O isotropic chemical shift of oxo bridges in M-O-M′ fragments (M, M′ = Si, P, Ti, Zr···) is highly sensitive to the chemical nature of M and M′. 51,52 Experimentally, phosphonic acids interaction with TiO2 surface has been investigated using high field 17O MAS NMR.53 Brodard-Severac et al. identified by NMR the presence of several different binding modes, with a preference for the bridging bidentate adsorption mode.53 Recent density functional theory (DFT) calculations on phosphonic acid54 suggested that the most stable adsorption arrangement for this acid to a titania surface would be a monodentate binding mode involving the coordination of the PO group, stabilized by two hydrogen bonds between the remaining POH groups and surface oxo bridges.48 Performing periodic localized basis set B3LYP calculations, Bermudez predicted a bidentate interaction for the adsorption of dimethysphosponate on rutile (110) and (101) surfaces44 in vacuum. Knowledge of the binding modes of functionalizing molecules is of utmost importance in the understanding of the biocompatibility of titania, and other metal oxides in general. In the present study, the adsorption modes of KH2PO4 (including mono-, bi-, and tridentate phosphate species) on hydrated TiO2 anatase surfaces have been investigated theoretically using periodic DFT. Different surface models are proposed after a systematic theoretical study including solvation of the surface and introduction of KH2PO4, KH3P2O7, or KH4P3O10, and water molecules. The preferred structures are selected as a basis for calculating structural and NMR parameters using a unique model. The computed NMR results are then related to those obtained experimentally for KH2P17O4 adsorbed on anatase nanoparticules.

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Figure 1. Model system used for the investigation of hydrated phosphate adsorption. To explore the surface potential energy of the system, we performed, initially, an ab initio MD study within the microcanonical ensemble (NVE approach), at T = 400 K, to scan the possible conformations of the system. We considered several starting conformations (but no statistics was performed on them) and the run was stopped after t ≥ 2 ps. The local minima found from MD results were systematically reoptimized at 0 K, in order to achieve the absolute electronic minimum energy for each configuration. As shown by Du et al.,60 radial distribution functions (RDFs) of liquid water in these solid/liquid water models are found comparable to pure bulk liquid water, giving us confidence that the system size is large enough. The minimization of the total energy and geometry optimization were performed using the VASP code,61,62 in the periodic DFT framework. The Kohn−Sham equations are solved by means of the PW91 functional.63 The electron−ion interaction is described by the Projector Augmented-Wave method (PAW).62,64 All atomic positions of the model are relaxed without geometrical constraints. Optimizations are performed at Γ-point for the Brillouinzone integration. An energy cutoff for the plane waves of 500 eV is chosen. The full quantum mechanical electronic structure was obtained after the total energy differences between the loops became less than 10−4 eV. 2. NMR Calculations. The first-principles NMR calculations were performed within Kohn−Sham DFT using the QUANTUM ESPRESSO software.65 The PBE generalized gradient approximation63 was used, and the valence electrons were described by norm conserving pseudopotentials66 in the Kleinman−Bylander67 form. The core definition is 1s2 for O and 1s22s22p6 for P and Ti. The wave functions are expanded on a plane wave basis set with a kinetic energy cutoff of 816 eV. The integral over the first Brillouin zone is performed on the Γ-point68 for the charge density and chemical shift tensor

THEORY AND COMPUTATIONAL DETAILS

1. Model. We modeled KH2PO4 adsorption on the low index anatase TiO2 surfaces (100) and (101). Both bare and hydroxylated surfaces are well-known and described in the literature.15 Nevertheless, no ab initio model exists, representing solvated surfaces with real water molecules in interaction with the anatase surface. Until now, very few ab initio based models included explicitly water molecules to represent the solvent. In periodic plane wave DFT, one can fill up the space between the surface slabs with water molecules as has been already proposed for MgO55 and AlOOH56 surfaces. A complete solvated DFT level amorphous SiO2 surface was characterized and described by us recently.57,58 In order to obtain realistic configurations of the water molecules (taken from a standard liquid water box) on top of the anatase surfaces, a molecular dynamics (MD) simulation was performed as implemented in VASP, from which an equilibrium configuration was chosen and subsequently optimized. These models containing water molecules on top of a hydroxylated anatase surface constitute the starting structure of the study, on which the phosphate species are physisorbed or chemisorbed. 998


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Langmuir calculation. The shielding tensor is computed using the GIPAW69 approach which permits the reproduction of the results of a fully converged all-electron calculation. The isotropic chemical shift δiso is defined as δiso = −[σ − σref] where σ is the isotropic shielding and σref is the isotropic shielding of the same nucleus in a reference system. TiO2 anatase (δiso = 558 ppm)70 for 17O and α-Ti(HPO4)2·H2O71 (δiso = −18.4 ppm)72 for 31P were chosen, respectively. Calculations were performed on selected and size reduced models of adsorption on 100 and 101 surfaces obtained with the VASP code as previously described, keeping only three layers of water above the surface to obtain system sizes compatible with this type of computation.

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neutral. Physisorption as well as chemisorption with the formation of one, two, and three Ti−O−P linkages is investigated: the latter will also be called mono-, di-, and trigrafting, respectively (see below). For KH2PO4 the interaction energy is calculated as following: ΔE int = Ecomplex + 2E H2O,solvated − E KH2PO4 − E TiO2,solvated (1)

The factor 2 in eq 1 stems from the fact that two water molecules were replaced by one KH2PO4 molecule. The solvation energy of H2O was found to be overestimated by about 0.3 eV from the experimental ΔEsolvated = −0.56 eV for H2O. Although the geometric parameter distributions analysis reflects the correct structure of liquid water (vide infra), the experimental autosolvation energy of water is −0.28 eV or −27.0 kJ mol−1.73 This energy difference, characteristic of pure DFT calculations, is in line with the absolute error on the energies for the PW91 method. The inclusion of dispersion energy corrections might improve the autosolvation energy for water. Nevertheless, it is expected that pure electrostatic interactions will dominate in our systems due to the ionic character of H2PO4−, and determine the trends in the energy calculations. Similar calculations were performed in clay systems in which a water solvation energy of 0.63 eV was calculated.74 a. Physisorption. The energy associated with the physisorption process on the (100) plane is −2.95 eV (Table 1,

EXPERIMENTAL DETAILS

1. Synthesis of TiO2. A soluble derivative of formula Ti3O(OiPr)8(OOC−CH2−C(OH)(COO)−CH2−COOH) is obtained by reacting monohydrated citric acid HOOC−CH2−C(OH)(COOH)− CH2−COOH·H2O(CitH3) with 3 mol equiv of distilled Ti(OiPr)4 in anhydrous tetrahydrofuran (THF) at 20 °C. A clear THF− isopropanol solution, containing this freshly prepared titanium compound and titanium isopropoxide (Ti/cit = 20), is then added to an aqueous solution of tetrabutylammonium bromide. The resulting suspension is heated at 100 °C for 3 h. A white powder is obtained after centrifugation, washing with deionized water and ethanol and drying in air at 70 °C for 12 h. 2. Synthesis of KH2P17O4. Phosphoric acid (H3P17O4) was first obtained by reacting 0.415 g (2.0 mmol) of PCl5 (98% purity) with 0.200 g of 40%-labeled H217O to form H3P17O4. The reaction medium was stirred under argon in ice for 30 min and then heated at 90 °C for 2 h. Then 0.299 g (2.2 mmol) of KH2PO4 was added, together with 0.5 g of 20%-labeled H217O and 0.5 g of 40%-labeled H217O, and the reaction medium was stirred at room temperature. All reagents were purchased from Sigma-Aldrich except H217O (CortecNet). Incipient wetness impregnation was used to adsorb KH2PO4 on TiO2 powder calcined at 500 °C during 10 h with a heating rate of 5 °C/min. Typically 100 mg of TiO2 was impregnated with a volume of KH2PO4 solution just sufficient to wet the powder (1 mL), and no separation was conducted between solid and liquid phases. The paste obtained was dried at 60 °C under nitrogen for 12 h. X-ray powder diffraction (XRD) was carried out on the final solids with a Bruker D8 Avance diffractometer using Cu Kα radiation (wavelength λ = 1.5404 Å). XRD patterns were recorded between 3 and 70° with a step size of 0.05°. Thermogravimetric analysis (TGA) of the samples was carried out on a TA Instruments − Waters LLC SDT Q600 analyzer with a heating rate of 5 °C min−1 under a dry air flow (100 mL min−1). Transmission-mode IR spectra were recorded in KBr pellets using a Bruker Vector 22 with a DTGS detector and a resolution of 4 cm−1. 3. Solid-State NMR. 17O and 31P NMR experiments were performed at high magnetic field on a 700 MHz AVANCE III Bruker spectrometer operating at 94.89 and 283.36 MHz, respectively, using a 3.2 mm Bruker probe spinning at 20 kHz. For 31P, a single-pulse excitation with a flip angle of 45° and a recycle delay of 5 s was used, while for 17O a spin−echo θ−τ−2θ pulse sequence with θ = 90° was chosen to overcome problems of probe ringing and baseline distortions. Low power pulses, selective for the central transition were used. The τ delay was synchronized with the spinning frequency.

Table 1. Calculated Interaction Energies for the Different Monomer H3PO4− Model Structures Considered in This Worka ΔE structure

surface (100)

surface (101)

physisorption I IH IOH IIa IHIHa IHIa IIb IHIHb IHIb IOHIa IOHIOHa IOHIb IOHIOHb III

−2.95 −1.06 −0.65 −0.65 −1.47 −0.15 −0.83 −1.41 0.51 −0.33 −1.28 −0.20 −1.00 −0.84

−1.59 −1.89 −0.84 −0.93 −1.52 −1.98 −1.21

−1.54 −1.40

−1.25

a

The Roman number designates the number of P−O−Ti links, the H or OH indicates if a H or an OH was omitted to make the structure, a or b indicates the adsorption site (energies in eV).

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RESULTS AND DISCUSSION 1. Theoretical Investigation. The models described above are used to study the interaction of KH2PO4, KH3P2O7, and trimer KH4P3O10 with the anatase TiO2 (100) and (101) surfaces. The hydrogen phosphate mono-, di-, and trimers are dissociated into K+ and hydrogen phosphate moieties, and placed in the vicinity of the surface. The K+ ion is placed at one side of the surface, and the phosphate moiety at the other side, each ion replacing 1, 2, or 3 water molecules in the model for mono-, di-, and trimers, respectively. The system is electro-

Figure 2a). After relaxation of H2PO4− at the solvated anatase (100) surface, a proton of H2PO4− has been transferred to the surface, forming a solvated HPO42− ion. The proton is bonded to one of the bridging oxygens of the surface, forming a Ti− O(H+)−Ti species. As a consequence of this transfer, the adsorption energy is a reaction energy including, in addition to the interaction between the phosphate and the surface, the dissociation energy of a PO−H group and the formation of a surface hydroxyl. 999


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surface, and thus, the former surface is more reactive, and probably having more basic oxygens, than the latter surface. b. Chemisorption. Chemisorption is also called grafting or inner sphere complex formation in the geochemical literature. It involves the formation of at least one Ti−O−P bond. One must consider mono-, di-, and trigrafting of H2PO4− species, initially in aqueous solution (aq), on the TiO2 anatase surface. Actually, these may be considered as successive steps, with the first one (monografting) being: H 2PO4 − aq + K+ + Ti − OH 2surf → Ti − O − PO3H 2− surf + K+ + H 2Oaq

(2)

with Ti−OH2 surf being a surface Ti coordinatively bound to undissociated water. The optimized geometry of the singly chemisorbed (or monografted) species is shown in Figure 2b and 3b. A possible subsequent step is the formation of a second Ti− O−P linkage by dehydration, i.e. digrafting, (see Figure 2c and 3c). Ti−O−PO3H 2− surf + Ti−OHsurf → (Ti−O)2 −PO2 H− surf + H 2Oaq

(3)

which may be followed, at least on the (101) surface, by a third, and last, grafting step: (Ti−O)2 −PO2 H− surf + Ti−OHsurf → (Ti−O)3 −PO− surf + H 2Oaq

(4)

The overall reaction for the formation of the trigrafted species can be written as

Figure 2. Energetically most favorable physisorption (a) and chemisorption of (b) (structure I) and (c) (structure IIa) geometries of H2PO4− on solvated (100) TiO2 anatase surface.

H 2PO4 − aq + K+ + Ti−OH 2surf + 2Ti−OHsurf → (Ti−O)3 −PO− surf + K+ + 3H 2Oaq

On the (101) surface (Table 1, Figure 3a), H2PO4− is not deprotonated, which explains the large difference in interaction energy with the (100) surface (−1.59 against −2.95 eV). This result confirms the relative stability trend between the hydrated anatase surfaces: the (100) surface is less stable than the (101)

(5)

These relatively simple equations hide some further complexities that have been systematically investigated. First, the bridging oxygen may be protonated, that is, one might have Ti−OH−P links instead of Ti−O−P. Second, chemisorption might be accompanied by proton transfer from P−OH groups to the surface, and finally, the transferred proton could be adsorbed on different sites on the surface or form H3O+(aq). Ti−O−PO3H 2− surf + Ti−OHsurf → (Ti−O)(Ti−OH)PO2 Hsurf + OH−aq

(6)

or Ti−O−PO3H 2− surf + Ti−OH 2+ surf → (Ti−O)(Ti−OH)PO2 Hsurf + H 2Oaq

(7)

The different models investigated are named following a particular code. The number of covalent bonds between the chemisorbed phosphate group and the surface is indicated as I, II, or III. When H2PO4− is covalently linked via an oxygen (Ti− O−P linkage), no particular letter is added; when it is linked via an OH group (Ti−OH−P linkage), the model code contains OH. Finally, when the H2PO4− is dissociatively adsorbed (with deprotonation, i.e., protonating the anatase surface), the model code contains an H. If the proton can be adsorbed on different

Figure 3. Energetically most favorable physisorption (a) and chemisorption of (b) (structure I), (c) (structure IHIH), and (d) (structure III) geometries of H2PO4− on solvated (101) TiO2 anatase surface. 1000


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chemisorbed polymeric phosphates in order to determine if such species can be identified by NMR. We first explored chemisorbed pyrophosphates derived from H3P2O7− ions. Two adsorption situations were considered for the H3P2O7− derived species on the (100) surface: one similar to form I previously discussed for H2PO4− chemisorption, and a second one similar to form IIa (see Figure 5), that is,

neighboring oxygens, the letters a or b are added to the model code. The reaction energies calculated for monografting (eq 2) and listed in Table 1 show that, on both studied surfaces, (100) and (101), the most favorable interaction modes are those corresponding to a Ti−O−P bond, that is, to a Ti−O− PO3H2− surf species (denoted as structure I in the Table) rather than the alternate possibilities, Ti−OH−PO3H− surf (structure IOH), or the same with transfer of a proton to a neighboring surface site (structure IH). Concerning the digrafting mechanism (eq 3), the same result is found as for monografting on the (100) surface, namely, the most favorable interaction is via two Ti−O−P bonds without proton dissociation. For the (101) surface, in contrast, dissociative adsorption is found to result in the most stable model structure, with two H atoms bonded to an available Ti neighboring oxygen to form hydroxyls. This structure is called the IHIH model, according to our conventions. During the geometry optimization one of the PO− groups recombined with a surface proton, and resembles a IIH species (see Figure 4).

Figure 5. Energetically most favorable chemisorption geometries of polyphosphates on solvated (100) TiO2 anatase surface. (a) Monografted and (b) digrafted geometries similar to the (100) I and (100) IIa monomer geometries, respectively, and (c) digrafted H4P3O10−.

monografted and digrafted forms, respectively. The digrafted dimer chemisorption configuration is found to be 1.56 eV more stable than the monografted chemisorption configuration. Finally, we extended our study to triphosphate entities (derived from H4P3O10−) (see Figure 3c). The geometries of the selected species were further investigated on their NMR properties (vide ultra), in order to evidence the eventual presence of polymeric phosphate species. c. NMR Parameters. Very few publications report NMR investigations of the functionalization of TiO2 surfaces with phosphates,75−78 and these studies are essentially focused on 31 P spectra. The latter are experimentally observed to exhibit relatively large signals, ranging from 0 to −15 ppm and generally centered around −6 ppm. The 31P and selected 17O chemical shift calculations based on our models for the physisorbed and chemisorbed forms of the phosphates are reported in Tables 2−4, for monomers, dimers, and trimers, respectively. Concerning 31P, our theoretical results are consistent with the experimental literature, which reports that the chemical shift range of chemisorbed phosphates is clearly different from that bulk titanium phosphates; however it remains difficult to draw conclusions about the binding mode of the phosphate species based only on the changes in the chemical shift, considering for example the small difference calculated between the monografted and the trigrafted groups in the (100)-I and (101)-III models, respectively (Table 2), and the likely signal broadening in experimental spectra. This difficulty has also been observed in the case of the grafting of TiO2 particles by phosphonate species.78,79 Nonetheless, we can notice that the 31 P chemical shifts of grafted species are more negative than that of physisorbed phosphate, even though there is no

Figure 4. Reaction mechanism showing the PO− group recombining with a surface proton during geometry optimization.

Due to sterical constraints, the third dehydration reaction is only possible on the (101) surface, on which an adsorption energy of −1.25 eV is calculated. Since all models contain the same number of atoms, they are isomeric structures from each other; consequently their total energy can be used as a measure for their relative stability. From Table 1, it is clear that the most stable configurations overall on the anatase (100) surface are the ones with a digrafted phosphate species (−1.47 and −1.41 eV, for structures IIa and b, respectively). On the anatase (101) surface, the digrafted structure is also found to be the most favorable structure, nevertheless, with the hydroxyl protons bonded to the surface (−1.98 eV). The monografted species is found to be less stable, and on the (101) surface the trigrafted species is the least stable of all. For the sake of completeness, some calculations were performed on the adsorption of polymeric phosphates. Our aim was not to explore in depth all the possible coordination modes for these molecules, which would be a daunting task, but to compute NMR parameters for relevant structures of 1001


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Table 2. Selection of Calculated 17O and 31P Isotropic Chemical Shifts in Modeled Chemi- and Physisorbed Phosphates on the (100) or (101) Surfaces of Anatasea

a

Labeling of the atoms indicated on the right. Ti, O, P, and H atoms are in light blue, red, green, and yellow, respectively.

O in Ti−O−P and terminal O in phosphates, whether protonated or not. The picture is a little less clear for chemisorbed pyrophosphates (Table 3). Oxygens in Ti−O−P moieties still exhibit high (downfield) chemical shifts, between 157.5 and 222.3 ppm. Terminal oxygens (POH and PO) are calculated to resonate for the most part between 53.1 and 117.7 ppm, with the conspicuous exception of terminal oxygen 65 in structure IIa which is found at 176.4 ppm. As for bridging oxygens in the polyphosphate backbone, corresponding to P−O−P moieties, they lie between 97.3 and 118.8 ppm. This range overlaps the range of terminal oxygens: in summary, the Ti−O−P indicative of grafting should still be identifiable by NMR, but it will be more difficult to determine by this technique if the P−O−P backbone bonds are intact or have been broken (e.g., by hydrolysis to PO and P−OH). Similar results can be seen in Table 4 for the only form of chemisorbed triphosphate that we have investigated, although the separation between terminal O and P−O−P is better defined here, with P−O−P falling in a chemical shift range intermediate between terminal O and Ti− O−P.

monotonous trend as a function of the number of Ti−O−P bonds. The same tendency is observed for pyrophosphate chemisorption (Table 3), but it is striking that H-bonding and other noncovalent interactions can have effects of the same order of magnitude as covalent binding (through-bond connections): contrary to species in solution, it is not easy to deduce a chemical structure from the 31P chemical shift. Regarding 17O NMR, the chemisorption of phosphates on the anatase surface leads to the formation of Ti−O−P moieties. According to the literature, the corresponding oxygens should have a resonance between 150 and 240 ppm,80 possibly depending on the Ti−O−P angles. Our calculated values for chemisorbed monophosphate species lie between 157.5 and 273.9 ppm, with an average of 215.9 ppm; no clear correlation can be established between the oxygen-17 chemical shifts and the Ti−O−P angles.53 This is consistent with the range observed for phosphonates grafted on TiO2 nanoparticles. In contrast, the δiso(17O) for terminal oxygens in POH and PO groups show calculated values between 52.6 and 109 ppm in agreement with already reported for phospho(i)nic acids80,81 and titania nanoparticles modified with phosphonates.53 We conclude 17O NMR should clearly distinguish between bridging 1002


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Table 3. Selection of Calculated 17O and 31P Isotropic Chemical Shifts in Modeled Chemi- and Physisorbed Dimers on the 100 or 101 Surfaces of Anatasea

a


2. Experimental Results. X-ray diffraction patterns of the white powder before and after adsorption of phosphate or potassium dihydrogenophosphate exhibit the reflections corresponding to the anatase phase (Figure 6a). The average crystallite size as derived from the half-width maxima of the peaks using the Debye−Scherrer equation is about 6 nm, which is consistent with that obtained in previous works.82,83 It must be underlined that no new crystalline phases were formed upon phosphate deposition. Pure TiO2 exhibits a specific surface area of 250 m2/g which corresponds to the theoretical external surface of 6 nm TiO2 crystallites (assuming spheres with a density of 3.9 g·cm−3). Thermogravimetric analyses, carried out in flowing air up to 500 °C with a temperature ramp of 5 °C/min, exhibit weight losses within the 20−500 °C temperature range. The derivative (DTG) traces show one main event in the 20−140 °C range (endothermic) with 3% and 10% weight loss for TiO2 before and after adsorption of KH2PO4, respectively (Figure 6b),

which may be assigned to the elimination of physisorbed water. The second weight loss of 1.8% observed in the 140−300 °C range for TiO2 before adsorption is assigned to dehydroxylation (condensation of two Ti−OH groups yielding a bridging O, Ti−O−Ti, and eliminating one water molecule). The absence of this peak after adsorption of KH2PO4 is probably due to the formation of Ti−O−P bonds by reaction between the phosphate and hydroxyls at the surface. IR spectra (not shown) are not very informative due to the strong absorption by the TiO2 support. In the window of transparency of the support, two maxima are observed after adsorption of KH2PO4 at 1135 and 1045 cm−1. They are consistent with the stretching bands of the phosphate tetrahedron84 but do not provide more precise information regarding the phosphate environment. The 31P MAS NMR spectrum (Figure 7a) of the phosphate adsorbed TiO2 nanoparticles shows three major resonances centered on 0.8, −4.6, and −11.2 ppm, and a minor one around −19.0 ppm. Comparison with NMR calculations presented 1003


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Table 4. Selection of calculated 17O and 31P Isotropic Chemical Shifts in a Chemisorbed Triphosphate on the (100) Surface of Anatasea

a


Figure 7. (a) 31P and (b) 17O MAS NMR spectra of 17O- enriched KH2PO4 adsorbed on TiO2: experimental spectra, simulation using the parameters in Table 5, and individual components of the simulation. Figure 6. (a) X-ray diffraction pattern of the KH2PO4/TiO2 system, showing the reflections of the TiO2 anatase phase. (b) DTG curves for TiO2 and KH2PO4/TiO2 system, between 20−600 °C.

relatively narrow signal at 16 ppm possibly corresponding to water. A simulation of the spectrum was carried out and yielded the parameters and proportions summarized in Table 5. The presence of P−O−Ti, PO, and P−OH sites indicates that grafted surface phosphate units are present in this system. A precise quantification remains difficult, but the high proportion of P−O−Ti related to terminal O suggests a predominance of di- and trigrafted species. Indeed, the experimental P−O−Ti/ terminal O ratio is about 2:1, as compared to 3:1 for trigrafted species and 1:1 for digrafted species. As the reader can ascertain

previously suggests the presence of both physisorbed and chemisorbed species; nevertheless, the assignment of the different components remains ambiguous. Therefore, a 17O MAS spectrum was recorded (Figure 7b). It displays a main signal around 180 ppm consistent with the presence of a majority of Ti−O−P bonds, a second resonance centered at 90 ppm assigned to terminal PO and POH groups, and a minor 1004


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Langmuir

the same species on two different oxides is different. The same difference has been noticed, for example, for glutamic acid, which is grafted on the surface of titania (inner-sphere complex formation,86), but only forms H-bonds with the surface of silica87). It is also noteworthy that, on silica, a minor amount of triphosphate species were formed from KH2PO4. This was assigned to a displacement of the phosphate polymerization equilibrium upon solution concentration during the drying step, and the same phenomenon might occur on anatase titania, explaining the minor 31P signal at −19 ppm. Since phosphates underwent a much larger degree of polymerization on silica upon moderate thermal activation, it would be interesting to determine if the same phenomenon also occurs on titania, or if it is prevented by the “tighter” adsorption mode on the latter support. The initial study described in the present paper already illustrates the possibility of a precise molecular identification of adsorbed species using a combination of modeling and spectroscopic investigations, which will help us refine the adsorption mechanisms proposed by colloid chemists in early adsorption work.

Table 5. NMR Parameters Extracted from the Simulation of the 17O and 31P MAS NMR Spectra of 17O-Enriched KH2PO4 Adsorbed on TiO2 31

δiso (ppm)

P

assignment

intensity (%)

0.8 −4.6 −11.2 −19.0

16 45 32 7 17

O

δiso (ppm)

CQ (MHz)

η

assignment

intensity (%)

180 105 90 15

4.7 5 7.5

0.3 0.3 0.6

P−O−Ti PO P−O−H H2O

60 6 27 7

from Table 2, this conclusion is not incompatible with 31P NMR. Indeed, identifying the three main 31P resonances, in the order of decreasing chemical shift, with digrafted, trigrafted, and monografted species, respectively, would result in an average P−O−Ti/terminal O ratio of 1.15:1. However, this assignment remains temptative and the real situation must be more complex, as witnessed by the minor 31P resonance at −19.0 ppm which suggests a minor degree of phosphate polymerization upon adsorption.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed using HPC resources from GENCI[CCRT/CINES/IDRIS] (Grant 2013-[x2013082022]) and the CCRE of Université Pierre et Marie Curie. Dr. B. Diawara from LCPS ENS Paris is kindly acknowledged for providing us with ModelView used in the visualization of the structures. The French Rég ion Ile de France - SESAME program is acknowledged for financial support (700 MHz spectrometer).

CONCLUSIONS Our modeling results show that phosphate grafting is definitely favored on the surface of (anatase) titania in the presence of a water solution. This result in itself is not unexpected, since many instances are known in which small molecules coordinate to surface Ti4+ by displacing a water molecule. We went further than this straightforward conclusion, by comparing different grafting modes. We conclude that digrafting and trigrafting are not only geometrically possible, but thermodynamically favored; the predominant form depending on both the exposed crystal face and the temperature, so that on well-dispersed anatase several different grafted species are expected to coexist. In order to check these predictions, we recorded the 31P and 17 O NMR spectra of a sample prepared by KH2PO4 deposition on an anatase powder from aqueous solution and compared their parameters with those calculated for the theoretical models using the GIPAW method. A good agreement is obtained between the theory and the experiment. 17O NMR unequivocally shows the existence of a large number of Ti−O− P moieties, providing direct experimental evidence for grafting. This confirms the potential of 17O NMR to study adsorption mechanisms at the molecular level, although the application of this technique is limited by the necessity for isotopic enrichment. Quantification of the observed signals suggests the coexistence of di- and trigrafted species in conformity with theoretical predictions. These quantifications could be put on a firmer basis by combining 31P and 17O NMR, although more experimental work is necessary to confirm signal assignments. We have previously studied experimentally the adsorption of KH2PO4 on silica.85 17O NMR was not available, but we used both 31P and 29Si NMR, with the latter being potentially sensitive to the formation of Si−O−P that would occur upon grafting. Deposition of KH2PO4 in the same conditions as the present study did not result in grafting but in weaker interactions with the surface: the adsorption mechanism of

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Characterization of Phosphate Species on Hydrated Anatase TiO2

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