The Role of TiO2 Anatase Surface Morphology on Organophosphorus

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

The Role of TiO Anatase Surface Morphology on Organophosphorus Interfacial Chemistry Marco M. Allard, Stephanie N Merlos, Brittney N. Springer, Jamey Cooper, Guangyu Zhang, Danilo S. Boskovic, So Ran Kwon, Kevin E. Nick, and Christopher C. Perry J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08641 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

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The Journal of Physical Chemistry

The Role of TiO2 Anatase Surface Morphology on Organophosphorus Interfacial Chemistry

Marco M. Allard1*, Stephanie N. Merlos1,2, Brittney N. Springer3, Jamey Cooper3, Guangyu Zhang4, Danilo S. Boskovic4, So Ran Kwon5, Kevin E. Nick3, Christopher C. Perry2,4*

1

Department of Chemistry and Biochemistry, La Sierra University, Riverside, CA 92515

2

School of Pharmacy, Loma Linda University, Loma Linda, CA 92350

3

Department of Earth and Biological Sciences, School of Medicine, Loma Linda University, Loma Linda, CA 92350 4

Division of Biochemistry, Department of Basic Sciences, School of Medicine, Loma Linda University, Loma Linda, CA 92350 5

Center for Dental Research, School of Dentistry, Loma Linda University, Loma Linda, CA 92350 (Corresponding Authors: [email protected]; [email protected])

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ABSTRACT Optimization of physicochemical properties of TiO 2 anatase for organophosphorus remediation remains challenging. One approach is to use anatase nanofibers, prepared from hydrothermally synthesized titanates. Charge densities and potentials of anatase nanofibers were determined from atomic force microscopy force–curve measurements using modified Derjaguin, Landau, Verwey, Overbeek (DLVO) theory, which includes roughness and hydration forces, in the pH range 4–9. Calculated values were -0.007 to -0.03 C m-2 and -70 to -150 mV, respectively. In contrast, at neutral pH, the magnitudes of diffuse layer surface charge densities and potentials have a minimum in anatase nanoparticles. These observations and zeta–potential results suggest that nanofiber surfaces are more acidic compared with nanoparticles. This is consistent with nanofibers having ~3–fold higher adsorption for organophosphorus methyl parathion (pH ≈7). The resulting adsorption includes contributions from (1) charge accumulation at local coordination sites caused by the morphological roughness; (2) greater crystallographic nanoscale variations; and (3) active site competition between parent and daughter species. The structural features of nanofibers have potential applications in catalysis and sequestration of organophosphorus compounds.


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INTRODUCTION Organophosphorus compounds (OPCs) comprise a class of environmental pollutants found in flame-retardants, plasticizers, pesticides, and remaining stockpiles of nerve agents present in military arsenals.1-2 Intense efforts are directed in developing environmentally benign methods for water purification, with an emphasis on the removal of hazardous OPCs. Titania (TiO 2 ), a readily available low-cost material, is known to principally adsorb and decompose OPCs.3-5, 6 Further optimizing the physicochemical properties of TiO 2 may improve its sorption efficiency and reactivity. Nanostructures, derived from titanates, are good candidates for optimized titania because of their enhanced surface areas and porosities compared with commercially available TiO 2 nanoparticles. Precursor titanate structures include A 2 Ti 3 O 7 , A 2 Ti 2 O 4 (OH) 2 and A 2 Ti 2-x/4 ⊗x/4O4 (x ~0.7, ⊗: vacancy, A = Li, K, Na, H).7 Moreover, these titanate structures are metastable transforming into titania polymorphs upon annealing. For example, annealing lepidocrocite8-9 and hydrogen titanate 10-12 transforms these structures to the most photocatalytic active TiO2 anatase polymorph. Alkaline hydrothermal treatment of TiO2 at elevated temperatures (> 100 °C) and pressures (> 1 atm) produces titanate precursor nanostructures, via a sequence involving the dissolution of the initial TiO2 and the crystallization of the final product.13-14 Kasuga and coworkers15 first reported that a reaction between concentrated NaOH and TiO2 particles produces nanotubes ~ 8 nm by 100 nm in size. This hydrothermal method was used to produce other nanostructure morphologies, including nanosheets,16 nanowires/fibers,17 18 and nanoribbons.19-20 Morphologically different titanate nanostructures are made by varying the amounts of TiO2 (starting material), NaOH concentration, filling fraction (reaction pressure), hydrothermal temperature, reaction time, and cooling rate. Typical conditions for nanotubular growth involves concentrated NaOH (2 to 10 M), under moderately high pressure, generated at temperatures 110– 150 °C.6, 12, 15, 21-23 Mechanical stirring at high temperature induces the formation of multi-wall titanate nanotubes upon hydrothermal treatment.24 Nanofibers are produced at temperatures above 170 °C or when KOH is used in place of NaOH.14, 25-26 After hydrothermal treatment, the x-ray diffraction spectra of titanate products contain sub 10° 2-theta peaks, which are assigned to the inter spacing of layered titanate phases.20, 26 Nanofibers are thermodynamically more stable



than nanotubes, since the latter have larger surface areas and higher stresses within the crystal lattice.27 Strong binding to metal oxide surfaces by organic pollutants is necessary for efficient environmental remediation. Enhanced binding of environmental toxins can be due to increased surface areas of the adsorbent, surfaces having high defect densities with uncoordinated atoms, and the wide range of Lewis acid/base properties of these materials, with respect to atom composition and pH.28 Organophosphorus binding to metal oxides at room temperature occurs primarily through phosphoryl oxygen to Lewis acid metal sites, which are under-coordinated metal atoms inherent in surface structure.29-36 Adsorbate binding to TiO2 will vary as a function of solution pH, electrolyte ionic strength, and organic pollutant speciation (uncharged or charged forms).37 In turn, surface TiO2 acid-base properties are determined by its crystallinity (long-range order), size (surface-to-volume ratios), aqueous electrolyte concentration, and surface morphology (i.e. exposed crystal planes). Ahmad and co-workers38 showed that surface acid– base activities of different shaped anatase nanocrystals depend on the type of surface planes present (i.e., {001}, {101}, {100}), and their relative proportions. Bullard and Cima used atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS) to probe the pH dependent surface potential of rutile (110), (001), and (100) surfaces.39 The (100) surface is the most acidic, caused likely by its reduced coordination number and the higher density of acidic and basic sites. Selmani and co-workers37 investigated the acid-base properties of titanates, demonstrating the order of acidity to be nanowires > nanotubes > nanoparticles. These differences in acidity were attributed to differences in nanomaterial structure and morphology. Borghi and co-workers40 using TiO2 nanostructures, systematically compared the effect of nanoscale roughness on acidbase behavior, while controlling for size and crystallinity. They concluded that morphology contributes to surface acidity, by causing spatial overlap of the electric double layers, resulting in more heterogeneous charge density gradients on rough surfaces. Our objective is to determine the mechanisms by which hydrothermally synthesized titania nanofibers are superior to nanoparticles for organophosphorus binding and degradation even in the absence of light. The fate of aqueous methyl parathion (MP) was investigated with and without anatase TiO2 through liquid chromatography-mass spectrometry (LC-MS) and computational modeling. We demonstrate that hydrothermally prepared anatase TiO2 nanofibers are more efficient sorbents than TiO2 nanoparticles for aqueous MP. Moreover, the presented 4 ACS Paragon Plus Environment

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experimental evidence demonstrates that morphological differences determine the acid-base properties of titania.

METHODS AND MATERIALS Materials. Methyl parathion (MP; analytical standard, ≥99.6 % purity) (100 mg), 4nitrothiophenol (4-NTP), internal standard triphenyl phosphate (TPP), 4-nitrophenol, and anatase titanium (IV) oxide (637254–50G, Lot#: MK13K2771V) were purchased from Sigma Aldrich. The nominal TiO2 nanopowder crystallite size is ≈ 25 nm with surface area 45–55 m2 g-1. Ultrapure water (18.2 MΩ cm−1 resistivity) was used. Stock solutions of 2 mg mL-1 MP and 1 mg mL-1 of 4-nitrophenol were prepared in methanol and stored at – 4 °C. Prepared stock aqueous solutions were ammonium formate (AF, pH 4, 10-1 M) buffer, saline (0.2 M NaCl), and DI water pH adjusted to 7, 9, and 11 (using 1 M NaOH). Synthesis of Titania Nanofibers. One gram of TiO2 anatase nanoparticle powder in 5 M NaOH (20 mL) was placed in a stainless-steel enclosed Teflon autoclave (100 mL volume size) containing a Teflon coated stir-bar. The total volume filled was ≈35%. This Teflon autoclave was ≈ 2/3 submersed in an oil–bath at ~120-130 °C on a magnetic hot-plate with continuous stirring for approximately 48 to 72 hours. We note that 48 hours was sufficient to form the nanofibers as observed by the absence of anatase diffraction peaks. Because the autoclave was in direct contact with the heating element, the internal temperature of the Teflon autoclave calibrated by temperature strips was ~ 170 °C. Following heating, the autoclave in the oil–bath was allowed to cool to room temperature (≈5 hours). Excess base was removed by centrifugation, then vacuum filtration with aqueous HCl (1 M HCl ≈ 3 x 30 mL, followed by 4 M HCl ≈ 30 mL), followed by water to remove excess chloride. Complete removal of sodium was confirmed by energy-dispersive spectroscopy (EDS). Then, the sample was oven dried at 110 °C overnight (≈ 8 hours), followed by annealing at 400 °C, to restore the anatase crystalline phase. Characterization of Titania Nanofibers. Powder x-ray diffraction measurements were performed using a Bruker D8 Advance diffractometer irradiating Cu Kα x-rays (λ = 1.5418 Å) with an applied power of 1.5 kVA (30 kV x 50 mA). The scans were performed in continuous mode from 5 to 70° (θ−2θ geometry). TiO2 nanofibers were characterized by scanning electron 5 ACS Paragon Plus Environment


(SEM) (FEI nanoSEM 450 operating at 15 kV) and atomic force (AFM) (Bruker multimode 8) microscopies. Digital transmission electron microscopy (TEM) was carried out on a Philips Tecnai 12 instrument operating at 120 kV fitted with a Gatan camera. Samples were prepared for electron microscopy measurements by dropping 5 to 10 µL of a dilute nanofiber suspension onto a Formvar/carbon-coated Cu grid (Ted Pella 200 mesh) and allowing the samples to air dry. The surface–to–volume ratios (S/V) of TiO2 nanomaterials were determined using 4-NTP as the adsorbate, where the Langmuir adsorption isotherm model was applied as described previously.41 The Brunauer-Emmett-Teller (BET) method was applied to determine specific surface areas from nitrogen physisorption at -196 °C using a Micromeritics Tristar 3000 instrument. Samples were degassed at 130 °C under vacuum for 12 h prior to N2 exposure. Total pore volume and pore size distributions were obtained according to the Barret, Joyner, and Halenda (BJH) method. The Lewis or Brønsted active sites were determined using the molecular probe pyridine. Using custom built equipment, TiO2 nanomaterials were heated to 400 °C and, after cooling, placed in a bell-jar which was evacuated to 1 Torr. Pyridine was subjected to freeze-pump-thaw cycles using a methanol/dry-ice cooling bath. The prepared TiO2 (≈ 10 mg) samples were exposed to pyridine vapor for 30 minutes to allow the surfaces to be saturated. After venting, the samples were analyzed using attenuated total reflectance FTIR (Jasco 400, Ge crystal). Difference spectra were recorded, by subtracting spectra of exposed from non-exposed surfaces. Zeta (ζ) potentials were evaluated as a function of pH change, by means of dynamic light scattering (ZLS Z3000, Nicomp, Port Richey, FL). We define the ζ−potential as the potential at the boundary between the compact and diffuse solvation layers around the TiO2 surface.40 The isoelectric point (IEP) is where the ζ−potential is zero. For ζ−potential measurements, TiO2 NPs and NFs were suspended in a 10-3 M NaCl (supporting electrolyte) solution at a concentration of 50 µg mL-1 and sonicated for 15 minutes prior to measurements. Isoelectric points were obtained by interpolating the curves of measured ζ−potentials at different pH levels (4–8) by adding 10-3 M HCl or NaOH to the suspension. Each measurement was performed in triplicate. Determination of the Surface Charge and Potential by AFM. Titania was immobilized on freshly cleaved mica by dropping 20 µL (100 µg mL-1 titania solution, adjusted to pH ≈ 3 with 0.1 M HNO3) and allowing the sample to air-dry. To remove organic contamination, the samples 6 ACS Paragon Plus Environment

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were plasma cleaned for five minutes (5 mA, 100 mTorr). The same AFM probe was used for nanoparticle and nanofiber experiments. Force measurements were made in 10-3 M NaCl solutions at pH between 4 and 9 (adjusted with 0.1 M HCl or NaOH) using the Multimode 8 AFM in Peak-Force tapping mode equipped with a fluid cell. The pH was ramped down from 9 to 4 using the same TiO2 nanoparticle or nanofiber sample. TiO2 nanoparticles or nanofibers were incubated in solution for 15 minutes, prior to measurements, to equilibrate the pH. Before and after AFM measurements at each pH, the fluid cell was flushed with appropriate pH solutions. Spring constants of ScanAsyst fluid probes (≈ 0.7 N m-1, R ≈ 20–30 nm) were determined using the thermal tuning method provided in the Nanoscope v 8 software. The raw data of deflection of the silicon nitride tips in volts, versus distance in nanometers, were converted to force versus separation. Cantilevers used for the surface force measurements have pyramidal-shaped silicon nitride tips. The shape of the tips can be reasonably approximated as conical with a spherical cap. Angles α and β are the geometrical angles for the spherical cap at the tip apex and the conical tip, with α + β = 90° (Fig. S1). The nominal radius was determined from tip size estimation using the Bruker Nanoscope Analysis “Tip Qualification” software feature. The nominal angle, α, determined from geometrical constraints is ≈ 45°. Approximately 50 to 120 force measurements extracted from the force–volume images (2 x 2 µm2) were made on titania substrates, for at least five random locations, at each pH. All the force measurements were performed at scan rates of 0.5–0.8 Hz, and captured at a resolution of 256 points/measurement. Averaged approach force curves (when the cantilever is approaching the substrate or the particles) were analyzed to determine the colloidal interaction forces between the silicon nitride tip and the substrate. The recorded force-distance curves were analyzed using the Derjaguin, Landau, Verwey, Overbeek (DLVO) theoretical model,42 modified to include effects of surface roughness and hydration forces (Supplemental A; Fig S2).43-44 Surface-charge density and surface potentials were determined between pH 4 to 9 for anatase nanoparticle and nanofiber surfaces. Degradation Quantification of Methyl Parathion. Quantification of MP degradation was done by UV-vis (Agilent Cary 300 and Thermo Nanodrop), high-performance liquid-chromatography (HPLC) (Thermo 3000 UHPLC; 35 °C), and LC-MS (Agilent 1200 LC (~25 °C) coupled with a 6410b triple quadrupole mass spectrometer (QQQ)). Total ion counts in positive and negative



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scan and multiple reaction-monitoring (MRM) modes were done (Table S1). Precursor-toproduct ion transitions of (M+H)+/z 264 → 124.5, (M-H)-/z 138 → 108, (M+H)+/z 327 → 152 were used to monitor for MP, 4-nitrophenol, and TPP internal standard quantifications, respectively. The mobile phases were aqueous formic acid (0.1%) and acetonitrile (gradient profiles 1 and 2) or methanol (gradient profile 3) (see Supplemental B for description of HPLC gradient methods and sample preparation protocols). Computational Modeling. All geometry optimizations and electronic structure calculations were performed using Gaussian 09, software revision D.01.45,55 Density functional theory (DFT) was adopted for optimizing the geometries of all the reactants and transition states (TSs). Normal convergence criteria were used throughout, for all geometry optimization calculations, along with full vibrational frequency calculations, performed on all optimized structures, to verify that an energy minimum had been attained using standard methods. 46-47 Gibbs free energies were calculated using the standard methods of

∑

o G298 K −

∑

o G298 K , with the zero-point energy

reactants

products

corrected free energies. The nature of stationary points was assessed in all cases by computation of analytic vibrational frequencies, also used to compute the molecular partition functions necessary to predict accurate 298 K thermochemistry. Transition states (TS) were found using quadratic synchronous transit (QST) protocol, and TS structures were identified by their single imaginary frequencies. Intrinsic reaction coordinates48 (IRC) were computed to confirm their full reaction paths. We employed B3LYP49 and M06-2X50 with 6-311++G(d,p) basis set for main group elements, and the LANL2DZ basis set for the (TiO2)n cluster. For computational 51 efficiency, we decided to use (TiO2)n and (TiO2 )− 𝑛𝑛 clusters used by Qu and co-workers, with n

= 10 using the polarizable continuum solvation model (PCM).

The energy of adsorption (Eads) was determined from the relationship, 𝐸𝐸𝑎𝑎𝑎𝑎𝑎𝑎 = 𝐸𝐸𝑇𝑇𝑇𝑇𝑂𝑂2 +𝑎𝑎𝑎𝑎𝑎𝑎 − 𝐸𝐸𝑇𝑇𝑇𝑇𝑂𝑂2 − 𝐸𝐸𝑂𝑂𝑂𝑂𝑂𝑂 ,

(1)

where 𝐸𝐸𝑇𝑇𝑇𝑇𝑂𝑂2+𝑎𝑎𝑎𝑎𝑎𝑎 is total energy of TiO2 clusters and adsorbates, 𝐸𝐸𝑇𝑇𝑇𝑇𝑂𝑂2 is the initial optimized

energy of (TiO2)10 clusters, and 𝐸𝐸𝑂𝑂𝑂𝑂𝑂𝑂 represents initial optimized energies of free solvated organophosphates.


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RESULTS TiO2 Nanofiber Characterization. We characterized nanofibers at steps of synthesis, purification, and transformation to their final TiO2 anatase form. The starting TiO2 anatase nanoparticles were processed using a minimal base concentration and temperature (5 M NaOH, ≈170 °C), and reaction time of ~48 hours to produce sodium titanate nanofibers. Figure 1A shows the XRD patterns of TiO2 anatase starting material, and acid-washed synthesized nanofibers, before and after annealing. Acid and water washing, followed by filtration, exchanged sodium ions with protons, leaving hydrogen titanate nanofibers. The low angle peaks at ≈8 and ≈10°, observed after initial synthesis, are characteristic of layered titanates.37 Calcination at 400 °C of acid washed fibers converted them to TiO2 anatase polymorph, without changing the shape, as observed by electron microscopy. Washed and annealed TiO2 nanofibers are corrugated and segmented (Fig. 1B, C), which originates from stress relief during contraction, while hydrogen and water are released from titanate defect sites, during drying and annealing.26 Electron microscopy shows that TiO2 nanofibers consist of crystallites (Fig. 1D, E), which are estimated to be ≈13 nm. Upon annealing, the H+ ions can desorb as either H2 or H2O, allowing the amorphous layered titanate to become restructured to an energetically favored anatase phase. In contrast, for samples washed only in water, trapped sodium in the lattice prevents titanate reconstruction to generate long-range ordered crystallite domains, as demonstrated by persistent low–angle sub ≈10° 2-theta peaks associated with trititanate phases,52-53 and the absence of peaks associated with the anatase phase (Fig. S3). Sonicating fibers in water at pH 9 resulted in fragmentation, giving two DLS hydrodynamic size populations, centered at ≈200 and ≈1000 nm, respectively (Fig. S4). For NPs, the two populations were ≈100 and ≈400 nm, respectively.



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B

C

D

E

Fig. 1. Titania fibers were synthesized hydrothermally in 5 M NaOH for 48 hours, and then washed with dilute HCl characterized by (A) X-ray diffraction, (B)-(C) scanning electron microscopy (SEM) and, (D)-(E) transmission electron microscopy (TEM). (A) From bottom to top, diffraction traces show the starting nanoparticle anatase, the synthesized product, and product after annealing at 300 and 400 °C, respectively. The peaks shown in the XRD patterns above 400 °C correspond to the (101), (004), (200), (105), and (211) planes of the TiO2 tetragonal anatase phase (JCPDS no. 21-1272). (B) – (E) Electron microscopy images of the nanofibers after annealing at 400 °C.

Adsorption experiments determined relative surface areas of commercial anatase NP starting material, or of synthesized anatase nanofibers which were annealed at 400 °C. Nitrogen adsorption–desorption isotherms are classified type II for nanoparticles and type IV for nanofibers, respectively, the latter characteristic of mesoporous materials (Fig. S5). The BET specific surface areas of starting TiO2 NPs, TiO2 nanofibers annealed at 120 °C and TiO2 nanofibers annealed at 400 °C are 93, 173 and 45 m2 g-1, respectively. Comparable pores sizes are 13, 94, and 22 nm, respectively (Table S2). Annealing above 300 °C restores the crystallinity and reduces the surface area by changing the structure from layered titanate to anatase crystallites.


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Using 4-NTP as an adsorbing molecular probe, we compared the surface-to-volume (S/V) ratio of TiO2 nanoparticles versus nanofibers using the Langmuir model of adsorption. On 100 µg mL-1 nanoparticles and nanofibers the estimated monolayer coverages of 4-NTP are ≈ 7 µM. Similar coverages are expected for MP. The S/V ratios are comparable (≈ 0.8 m2 L-1 (Fig. S6)), and consistent with BET areas. This confirms the formation of nanofibers rather than nanotubes, which are known to have surface areas and pore volumes that are 2 to 5-fold higher.26 Molecular probe pyridine was used to detect the presence of intrinsic Brønsted and Lewis acid sites on TiO2 nanoparticles and nanofibers (Fig. S7) before the sites were passivated by water. Direct correspondence is not expected between surface pKa values at solid–gas versus solid–liquid interfaces. However, in the pyridine experiments, the intrinsic surface acidity is measured at the solid–gas interface, without the shielding of water and solvated ions. Thus, the pyridine experiments provide insight into how surface acidity is influenced by morphology on the nanoscale without aqueous effects. All spectra have the diagnostic signals for pyridine molecules coordinated at Lewis sites at 1606, 1578, 1490, and 1445 cm-1. The bands at ~1450 and 1540 cm-1 are assigned to Lewis and Brønsted acid sites, respectively.54 No attempt was made to quantify and compare the (1540/1450) ratios because of low S/N for the nanoparticles. However, from the IR adsorption signal we can infer that 400 °C annealed nanofibers have additional bands at 1635 and 1652 cm-1, which are associated with protonated pyridine at Brønsted sites. This is consistent with presence of larger numbers of hydroxyl groups on nanofiber surfaces. In aqueous solution, TiO2 surfaces are expected to be hydroxylated and will have available Lewis or Brønsted active sites. Under acidic conditions, the surface hydroxyl groups will be partially protonated forming surface bound waters. These findings were further supported by the zeta potential measurements made for nanofibers as well as nanoparticles, under aqueous conditions. The interpolated isoelectric point for nanofibers is more acidic at 5.5 compared with 6.1 for nanoparticles (Fig. S8). The lower isoelectric point for nanofibers implies ~4-fold more numerous acidic sites. Colloidal Forces and Calculated Surface Charges. AFM was used to determine spatial variations of charge and potential at the electric double layer for TiO2 nanomaterials. Calculated charge densities and potentials obtained from AFM measurements are then correlated with surface acidity. Using DLVO theory, interfacial charge densities and potentials are derived from 11 ACS Paragon Plus Environment


approach force versus distance measurements. DLVO theory describes only non–contact forces at distances greater than ≈ 2 nm. At shorter distances, hydration and elastic contact forces become important. We used the DLVO model with roughness (DLVO-R), accounting for the roughened non-contact forces and the elastic contact of asperities.44 Accordingly, the surface position is obtained by the average of surface heights. This DLVO-R model has two elements, first is the roughened non-contact interaction, in which we model using a Gaussian roughness distribution. The second aspect accounts for contact compression forces. Figure 2 shows the DLVO-R analysis between pH 4 to 9 superimposed on force versus distance approach curves. Panel A shows representative averaged force curves for nanofibers. Clearly, at pH 7 the maximum forces are 4-fold higher for nanofibers (≈2.9 nN) compared to those for nanoparticles (≈0.7 nN) (Figure 2B). Under acidic conditions nanofibers are pH 4 pH 7 pH 8.6 pH 7 NP

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-2 -3

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Fig. 2. (A) Average normalized approach force–distance (Radius = 20 nm) plots with the DLVO-R fitted model for TiO2 anatase nanofibers between pH 4 to 9 (pH 4, N = 96; pH 5, N = 119; pH 6, N = 53; pH 7, N = 62; pH 8.6, N = 67). The approach curve for TiO2 nanoparticles at pH 7 is included for comparison (N = 87). (B) Maximum force versus pH for nanoparticles and nanofibers. Plots of calculated surface charge density (C) and surface potentials (D) from the DLVO-R model. Panels C and D reflect Panel B. The lines are guides to the eye; error bars are standard deviations. 12 ACS Paragon Plus Environment

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characterized by lower maximum surface forces compared to nanoparticles. At neutral pH the forces are consistent with calculated surface magnitudes of charge densities (≈-0.03 vs ≈-0.01 Cm-2) and potentials (≈-150 vs ≈-100 mV). For nanofibers at acidic pH the charge densities (≈0.03 to -0.007 Cm-2) and potentials (≈-150 to -70 mV) monotonically decrease with pH. In contrast, for nanoparticles both properties vary weakly with respect to pH. Indeed, the important observation is that both the surface charge density and potential absolute magnitudes pass through a minimum for TiO2 NPs. The combined results of AFM and Zeta potential measurements show that nanofibers are more acidic than nanoparticles. Moreover, the minimum in the charge and potential at neutral pH correlates with a higher proportion of weaker acidic sites on nanoparticles. This suggests that the adsorption of MP can be modulated by pH, so that at neutral pH, MP binding to nanofibers is favored. Surface adhesion forces on the tip were estimated by analyzing the force versus distance retract curves (Fig. 3). The average adhesion forces were determined from the minimum in the retract curve. Panels A and B show the effect of pH on adhesion forces, which are higher on the Si3N4 tip for nanoparticles, except at neutral pH. Panels C and D show AFM adhesion force channel images of nanoparticles and nanofibers with adhesion force cross-sections 1 and 2 at pH 7. Panel E is a cross-sectional representation of the adhesion forces at pH 7, consistent with the average adhesion forces being lower for adsorbed nanoparticles.



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0.5

0.6

Fig. 3. (A) Average representative nanofiber retract force-distance curves at pH 4 (N = 96), pH 7 (N = 62), and pH 9 (N = 67). The average retract force curve (N = 87) at pH 7 for TiO2 nanoparticles is shown for comparison (solid black). (B) Average adhesion forces determined from the minimum in the retract force curves against pH. The lines are guides to the eye; and errors bars are standard deviations. Representative AFM 2 x 2 µm2 adhesion topography force images for TiO2 anatase (C) nanoparticles and (D) nanofibers at pH 7. (E) Adhesion force cross-section line scans taken from images C and D for nanoparticles (1) and nanofibers (2); average adhesion forces are lower for nanoparticles.

Methyl Parathion Degradation in the Aqueous Phase. The degradation products in aqueous solution, with and without TiO2, were detected by LC-MS. MP degradation pathways consist of competing reactions that include hydrolysis, oxidation and isomerization.55-56 MP degrades in aqueous media with half-lives varying from days to weeks depending on ionic strength, pH, and co-analytes.55-56 We observed that isomerization occurs readily in solutions made from pH 4 ammonium formate (0.1 M) stock, even after subsequent pH adjustment (Supplemental D; Fig. S9–S10). Consequently, preparing stable solutions of MP is challenging, because basic conditions favor hydrolysis (Supplemental Section E), but acidic conditions while inhibiting hydrolysis favor isomerization (Supplemental Section F). Both MP isomerization and hydrolysis 14 ACS Paragon Plus Environment

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were reduced substantially by using saline at neutral pH (0.2 M NaCl(aq), pH 7) (Supplemental D). The high concentration of ions compete for water molecules, so that rate of MP hydrolysis is reduced. Scheme 1 and Table 1 show the identified products between pH 4 to 9. Product identification in aqueous solution was aided by DFT calculations of total energies and molecular dipole moments, where molecules with higher dipole moments elute preferentially from C-18 column (Supplemental D; Fig. S12–S13). Hydrolysis of OPCs such as MP and parathion can proceed via isomer intermediates.55, 57 We determined that isomer conversion can occur in solution, with or without the presence of nanofibers, from the (M+H)+/z = 264 ion signal using LC-MS (Fig. S11). Three peaks were observed: parent, A, the O, S (O, Sdimethyl-O-(4-nitrophenyl) thiophosphate) B, and S-aromatic H isomers (scheme 1). Isomerization has been observed to occur in the presence of dealkylation-alkylation agents58 and certain clay materials.55, 57, 59 Experimentally, the intermediate isomer B hydrolyzes more rapidly than MP at all pH values.55 The proposed base–catalyzed mechanisms on modified clays are associated with cleavage of oxygen–methyl bonds in the MP molecules as a step in the production of intermediate B.55 Mingelgrin and Saltzman showed that the hydrolysis of adsorbed parathion on clay surfaces occurs directly, or through a rearrangement step.57 We propose that the isomerization mechanisms of parathion and MP are acid catalyzed. The surface cation site (Lewis acid site) coordinates to water at the interfacial region, which protonates the oxygen on the alkyl or aryl group prior to rearrangement and hydrolysis.



Scheme 1. Organophosphorus compounds detected by LC-MS. The numbers 1 and 2 refer to cleavage at the alkyl O–C and S–C bonds, respectively. The hydrolysis of the parent, A, and isomer B can occur via SN2 mechanisms at the methoxy (A→D, B→F), alkyl thiol (B→G), and at the phosphorus groups (A→C, B→C, B→E).


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Table 1. Observed hydrolysis and isomerization products by LC-MS of methyl parathion in water. 2HPLC gradient profile 2 (acetonitrile); 3HPLC gradient profile 3 (methanol); Otherwise HPLC gradient profile 1 used; ***weak; a,bIsomers; ϒCalculations were done at the (M06-2X/6311+g(d,p)) level of theory.

Retention Time (min)

pH 7 14.5 (9.6)2 (11.2)3 18.5 (11.2)2 (14)3 9.9 (7.5)2 (8.7)3

pH 9

Dipole Momentϒ (Debye)

14.4

8.2

0

18.5

2.4

-2.9

9.7

7

-

5.3

6.2

6.8

7.9

-

232.0016

4.5

-

-

5.2

-

(M-H)-

247.9788

17.5***

17.9

17.9

2.8

-

C7H8NO5PS

(M-H)-

247.9788

16.2***

16

16

3.8

-

C8H10NO5PSb

(M+H)+

264.009

8.5

1

Calculated m/z g mol-1

Label

Molecular Formula

A

C8H10NO5PS

(M+H)+

264.009

13.7

B

C8H10NO5PSa

(M+H)+

264.009

18.5

C

C6H5NO3

(M-H)-

138.0197

4.7

D

C7H8NO5PS

(M-H)-

247.9788

E

C7H8NO6P

(M-H)-

F

C7H8NO5PS

G H

pH 4

(8.2)2 (9.0)3


∆G-Isomerϒ (kcal mol-1)


Adsorption of Methyl Parathion on TiO2. Adsorption experiments on 100 µg mL-1 TiO2 were performed with limiting regimes of low (2 µg mL-1) or high (20 µg mL-1) MP concentrations. Degradation experiments were performed in saline (0.2 M NaCl(aq), pH 7), rather than in DIwater, because the starting MP degradation was lower in saline (< 5% 4-nitrophenol). Methyl parathion stock solution (20 µg mL-1 or ≈ 76 µM) does not degrade appreciably within experimental time frame, as shown by UV-vis, demonstrated by lack of absorbance loss at 270 nm. The relevant environmental MP concentrations are in the µg L-1 (ng mL-1) range. However, we used 2 µg mL-1 (≈ 7.6 µΜ) for our analyses, which is within the saturated monolayer coverage range of 100 µg mL-1 (≈ 7 µΜ; Supplemental Fig. S6) titania nanoparticles and nanofibers. Methyl parathion sorption was characterized on commercial and on 400 °C annealed TiO2 nanofibers by HPLC and LC-MS. For TiO2 nanoparticles there was a ≈ 30% reduction in 270 nm absorbance after two hours, as measured by HPLC. However, with TiO2 nanofibers this signal was below the detection limits. Approximately 80 % reduction of MP was observed within 10 minutes by LC-MS MRM in 100 µg mL-1 TiO2 saline solution (Fig. 4; Supplemental D; Fig. S14). An extraction with acetonitrile was attempted to explore the condition of the adsorbed organophosphates. Points 1 and 2 in Fig. 4 represent signals for extracted MP and 4-nitrophenol, respectively. Methyl parathion is strongly adsorbed to the TiO2 nanofibers. In addition, the hydrolysis product, 4-nitrophenol, after ≈ 10 minutes adsorption to the TiO2 nanofibers, was largely bound to the surface (~ 3-fold greater amount adsorbed compared to the supernatant). In contrast to solution hydrolysis, only parent MP and its isomers B, H and hydrolysis product C (4-nitrophenol) were detected in the MS. In addition to molecular adsorption, degradation products of (m+H)+/z 336, 352, and 453 (not shown) were also extracted with acetonitrile from the TiO2 nanofibers. These high molecular weight products suggest possible bimolecular coupling or surface associated polymerization reactions.


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7

2.0 -1

Concentration (µg mL )

2 6 1 3

Intensity (cps x 10 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5 4

1.5 1.0 0.5 0.0 40 80 Time (min)

0

3

120

2 1 0 0

20

40

60 Time (min)

80

100

120

Fig. 4. Adsorption of methyl parathion (MP) (2 µg mL-1) on TiO2 anatase (100 µg mL-1) nanofibers in 0.2 M NaCl, pH 7 solution. LC-MS (M+H)+/z signals for 264 → 124.9 conversion in supernatant over time of exposure. The black ( ) and white ( ) triangles represent methyl parathion and 4-nitrophenol signals, respectively. The black ( ) 1 and white ( ) 2 squares represent signals from MP and 4-nitrophenol extracted with acetonitrile from TiO2 nanofibers, respectively. Inset: Representative results of methyl parathion supernatant concentrations when incubated with TiO2 nanoparticles ( ) and nanofibers ( ), as determined by HPLC. After thirty minutes of incubation with TiO2 nanofibers (vertical dotted line), the concentration of MP in supernatant was below the lower limit of detection.

DISCUSSION Annealing titanate nanofibers transforms them to the anatase polymorph (Fig. 1). However, even though the short–range crystalline order is restored, with the collapse of the interlayers forming anatase domains, the long–range morphological structure is preserved. Under our experimental conditions the fibers are segmented with large voids intertwined between fiber segments. Similarly, electrospun fibers, after annealing to the anatase phase, have grain sizes



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comparable to hydrothermally prepared nanofibers (~ 10 nm). However, the electrospun fibers are fully crystalline in their transverse directions, rather than bundled into segmented domains.60 Regarding the morphologically different titanate nanostructures,

28-29, 61

alternative

mechanisms were proposed for nanotube and nanofiber formation. First, it was postulated that anisotropic growth occurs forming sheets, which either roll to form nanotubes or stack producing nanofibers.23, 26, 33, 43-47 Second, crystalline domains of metal oxides can grow anisotropically by an oriented attachment mechanism under heating in a strongly basic environment.62-64 A study by Horvath and co-workers65 is an example of the orientated attachment mechanism that could account for experimental observations. They concluded that solution agitation by stirring during hydrothermal synthesis facilitated the formation of nanofibers microns in length, disrupting the random orientation of TiO2.65 Our observation of larger pore sizes for titanate nanofibers, compared to nanoparticles, could be rationalized by the condensation of TiO6 octahedral monomers along the growth orientation,11 so that crystalline domains of metal oxide grow anisotropically by an oriented attachment mechanism.62-63, 66 Force curve measurements against pH indicate that surface charge densities and diffuse layer potentials behave differently in TiO2 anatase nanoparticle and nanofiber polymorphs (Fig. 2). Notably, except at pH 7, the magnitudes of these quantities are greater for nanoparticles. Such differences are rationalized by the variation in relative crystallinity of the TiO2 nanomaterials. Ahmad and coworkers38 synthesized TiO2 anatase nanoparticles of various morphologies, each having different proportions of exposed crystalline faces, and measured points of zero net proton charge. They identified five distinct domains, derived from proton affinity distributions, at pKa 4, 5.5, 7.0, 8.3, and 10, consistent with earlier work by Contescu and coworkers.67 By applying the refined MUSIC model68 they determined that {101} faces are populated by more extreme proton affinities (pKa 5.5, 10), while {001} (pKa 7.0) and {100) (pKa 8.3) faces are dominated by intermediate proton affinities. Even though the crystallinity of the nanofibers faces was not determined in this study, we infer that the relative proportions of acidic sites are different from those of nanoparticles. We anticipate that relative proportions of {101} sites will be greater for nanofibers than for nanoparticles, while {001} and {100} sites will favor nanoparticles rather than nanofibers. Both FTIR pyridine and ζ−potential measurements support this conclusion. The pyridine probe experiments show more acidic sites for nanofibers, consistent with the lower interpolated isoelectric point of 5.5, versus 6.1 for nanoparticles (Fig. S8). 20 ACS Paragon Plus Environment

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The electrical double layer surface charge and diffuse layer potentials are modulated by adsorption of electrolytes and by electrical double layer overlap as surfaces approach each other. This process is referred to as “charge regulation”.69 Sodium ions can adsorb onto the surface of TiO2 modulating the electric double layer. Thus, considering the acidic domains on crystallographic faces of TiO2, charge regulation effects of adsorbed cationic electrolytes will contribute to surface charge densities differently between the nanoparticles and nanofibers. In the electrical three layer model that includes the double layer model, the represented charge densities include: the surface charge density σ0, the charge density at the inner Helmholtz plane (IHP) σi, and the charge density at the outer Helmholtz plane (OHP) σd. Considering cationic species C+, and anionic species A-, charge densities are represented by: σ0 = F([M-OH2 +] + [M-OH2+⋅A-] - [M-O-] - [M-O-·C+]),

(2)

σi = F ([M-O-·C+] - [M-OH2+⋅A-],

(3)

σd = −(σ0+σi) = -F([M-OH2+] - [M-O-]),

(4)

where F is the Faraday’s constant (Coulombs/mole).40 Moreover, AFM probe is sensitive to the total charges at the Stern and the diffuse layers, σd. Calculated nanofiber magnitudes of surface charges and potentials monotonically decrease with acidic pH. In contrast, nanoparticles have weak dependence in surface charges and potentials except with a minimum around neutral pH. We postulate that nanoparticles have a greater number of moderately acidic sites of pKa ~ 7. Because of this, at neutral pH, more sodium cations will accumulate at the OHP, or penetrate into the IHP, of the Stern layer on nanoparticles compared to nanofibers. Two shielding effects occur because of greater adsorption of sodium that will influence surface charge density and potential behavior at acidic pH: (i) reduction of repulsive interactions between ionizable sites on the surface, and (ii) reduction of repulsions between the surface and protonated electrolytes (hydronium and hydroxyl ions). This results in larger surface charge densities and potentials for nanoparticles than for nanofibers, at lower pH. Because of this, in acidic pH, TiO2 anatase polymorphs of nanoparticles and nanofibers diverge in their ζ−potentials, with larger ζ−potentials for nanoparticles (Fig. S8). Adsorbate Binding. In an aqueous environment, organophosphorus species can either interact with the surface as a Lewis base against the protons of a surface hydroxide, or as a Lewis acid 21 ACS Paragon Plus Environment


accepting electrons from the oxygen of a hydroxyl group. Interfacial water molecules and solvated ions can result in local pH gradients, which can affect chemical binding strength, rates of reactions, and adsorption/desorption. Observations of enhanced surface binding of MP to TiO2 nanofibers are linked to acid-base characteristics of TiO2 surfaces. This would suggest a role for surface morphology or charge density in determining acid-base properties. Surface bound waters can form hydrogen bonds with MP, protonating the oxygen–methyl bond, to stabilize transition states, toward formation of the isomer B and H intermediates. Both TiO2 nanofibers and nanoparticles are formed as the same anatase polymorph, and measured IEP values represent spatial averages of their crystallographic faces. The heterogeneity of the TiO2 nanofiber surface, reduces the electric field at its protrusions even though the overall charge density is higher compared to a flat surface. The decrease in observed IEPs of TiO2 was ascribed to increased nanoscale roughness that exposes multiple facets, changing the local chemical environment via the spatial arrangement of the active groups (i.e. surface bound water, TiOH groups, and adsorbed electrolytes). 37,40 As the approaching species (water, MP, or its derivatives) come to within a proximal distance, comparable to cavity or pore sizes, the augmented local charge density and the dispersion forces are felt directly and favor binding. We suggest that the variation in TiO2 charge behavior is due to different acidic domain densities on each of the exposed crystallographic faces. These acidic domains include functional groups (e.g. M-OH) on metal oxide surfaces. Numerous studies attempted to correlate catalytic activity and binding with acidity and basicity of oxide surfaces.70-73 Simple empirical rules were formulated:71 (i) the presence of a Lewis acid on an oxide enhances the binding of a Lewis base; (ii) the oxide itself has certain properties as a Lewis base, which can be modified by Lewis acid adsorption; and (iii) the presence of a Lewis base on the oxide surface reduces the subsequent binding of another Lewis base. Methyl parathion is weakly basic, with a pKa ~ 7.2. It can interact with Lewis acid sites at or near the surface. The number of Ti-OH dangling bonds is pH dependent, so that (Fig. S18) the most favorable MP interactions occur at those moieties. Thus, MP binding will be weaker on nanoparticle surfaces with fewer surface charges than nanofibers. Organophosphorus species on mineral oxides are strongly dependent on spatial orientation in the active site, the nature of the Lewis-acid sites, and the outer sphere dielectric environment. In aqueous solution, the micro-environment at the TiO2 – water interface is dominated by Brønsted


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acid sites. Thus, the rate of MP hydrolysis is likely facilitated by surface–mediated protonation and isomerization. We used small (TiO2)n, n =10, nanoclusters to model the possible localized binding interactions as the precursor steps to surface-mediated isomerization and hydrolysis. Discrete molecular orbitals from DFT were used for cluster modeling rather than the plane waves method.74 Plane waves are delocalized and as a result are not sufficiently specific to a particular site in the crystal lattice. Moreover, the localized effects of surface modifiers, dopants, and defects on bonding need to be investigated.75 The large surface-to-volume ratios of nanoclusters caused by corners, surfaces and edges, absent in the bulk of the structures, require saturation of the under-coordinated atoms. Thus, to saturate the free valence at the surface of (TiO2)10, and to keep the coordination of the surface atoms similar to the bulk, one bridging oxygen atom was removed, simulating an oxygen vacancy. Nearby water molecules would readily dissociatively bind to the vacancy resulting in terminal hydroxyl groups,76 which was the focus of modeling. We applied frontier molecular orbital (FMO) theory, in which the frontier orbitals of two different molecules interact together, namely the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs). The HOMO and LUMO are mainly responsible for chemical reactions and can be applied to most Lewis acid/base reactions, such as the ones considered here. The energy of the HOMO is a measure of the ease of oxidation (basicity), while the LUMO energy shows the ease of reduction (acidity). These clusters were studied for their photo-chemical properties and have bandgaps (HOMO-LUMO) of ≈ 3.5 eV, in the range observed experimentally for bulk anatase.51 This suggests that these nanoclusters are credible models for localized acid-base interactions. Our modified (-OH capped) cluster calculations show that the HOMOs are dominated by O 2p orbitals, while LUMOs are dominated by empty Ti 3d orbitals. This is consistent with the intrinsic acidity of the surface. Non-crystalline nanostructures of up to n ≈ 38 are the most energetically stable compared with anatase nanocrystals.77 Small bulk-mimicking nanocrystals are metastable against the most energetically stable nanoclusters below n ~ 125, which is around ~2–3 nm diameter.77 All electron relativistic DFT calculations show anatase nanoparticles acquire bulk properties around ~ 20 nm diameter.78 For (TiO2)10 nanoclusters, the global minima structures consist of tetrahedral rather than octahedral titanium.77, 79 As the size of n increases, the average Ti (4 to 6) and O (2 to 3) coordination converges slowly, because of the high proportion of under23 ACS Paragon Plus Environment


coordinated surface atoms. The local bond polarizations in our cluster are expected to be similar to bulk properties, as evidenced by the bandgap. However, the subtle differences may be stronger especially for direct Ti interactions. An important caveat is that (TiO2)n nanoclusters have energetically low lying isomers. For each isomer the physicochemical and electronic properties will be unique. Thus, the characteristics of non-crystalline nanoclusters cannot be extrapolated to bulk crystalline polymorphs.77, 79 Nonetheless, they may be useful in predicting some properties including adsorption energy trends. Possible binding sites on TiO2, relevant to MP adsorption energetics, were calculated by DFT (Supplemental F). The binding is more exergonic when MP interacts directly to Ti. This is consistent with previous gas-phase experimental and computational studies with strongest organophosphorus binding to Lewis acid (Ti) sites. 29-32, 34-36, 71, 80-81 Other types of binding interactions, including hydrogen bonding between the surface and MP, ranging from -8.0 (for S-H-O-Ti) to +13.3 kcal mol-1 (for five–coordinate Ti-O-P) can be considered (Supplemental Figs. S17-18), and may become important in aqueous solutions. We have used a polarizable solvent continuum model in all our DFT calculations, but we expect that binding energies and pKa values will be dependent on the number of surface water molecules. The n = 10 cluster has four possible binding sites. A single water molecule preferentially binds to the cluster Ti atom through its oxygen atom with average binding energy ~ 1 eV (≈ 23 kcal mol-1).79 This agrees with our calculations where the addition of discrete water will result in a binding energy difference of ≈ 20 kcal mol-1 (Supplemental Fig. 18 PB and PC). Previous work showed a maximum of four bound water molecules leading to decreased average binding energy (~ 1 to 0.9 eV).79 However, only two of the four potential waters bind to Ti. The other two water molecules are H-bonded. We attempted to include up to four water molecules in our calculations, but the geometry optimization for these did not converge readily, due to the many possible interactions between the cluster, MP and other water molecules. In aqueous solution, the presence of water molecules and solvated ions can result in local pH gradients near the surface which in turn can affect chemical binding strength, rates of reactions, and adsorption/desorption.79 Complete surface catalyzed hydrolysis of MP requires cleavage of phosphorus–methoxy, phosphorus–aromatic, and P=S(O) bonds in discrete steps ultimately leading to phosphate (Scheme 1). It is well-known that phosphate groups are strongly bound to TiO2 surface.82-83 Our 24 ACS Paragon Plus Environment

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DFT calculations confirm that the conjugate base degradation products including phosphate containing species and 4-nitrophenolate bind stronger than parent MP. The binding is strongest with phosphate product species, such as 4-nitrophenol phosphate, with negative binding energies -13.1 and -1.3 kcal mol-1, for double hydrogen bonding interactions (Fig. 5 A, B). Species 4nitrophenol phosphate could be formed from the hydrolysis of E. Similarly, the surface hydrogen bonding interaction with phosphate species for HPO42- (-19 kcal mol-1) and H2PO4- (-0.4 kcal mol-1) is exergonic, rationalizing the strong pH dependent binding of phosphate species to TiO2 surface (Fig. 5). In comparison, the binding energy for 4-nitrophenolate and 4-nitrophenol are 4.6 kcal mol-1 and +3.7 kcal mol-1, respectively (Fig. S19). There is strong binding of partially hydrolyzed MP to TiO2 (Fig. 4), thus making the gathering of kinetic experimental evidence for all those species involved in the hydrolysis challenging.

Fig. 5. Selected daughter species (A)-(B) 4-nitrophenol phosphate, (C) HPO42-, (D) H2PO4-, with adsorption energetics calculated on (TiO2)10 clusters at B3LYP/LANL2DZ level of theory.



CONCLUSIONS Hydrothermally synthesized TiO2 nanofibers display ~3–fold higher adsorption for archetypal organophosphorus methyl parathion when compared to nanoparticles at neutral pH. This is a clear indication that surface morphological and crystallographic changes can dictate binding and sorption chemistry. Methyl parathion adsorption on TiO2 nanoparticles in aqueous solutions should depend on MP’s efficient coordination with water molecules in the outer sphere dielectric region of active sites. Under aqueous conditions, it is expected that defect sites will no longer be available as sorbent sites since they would react with water molecules and become inactive (passivated). Moreover, nanofiber surface shielding by electrolytes is attenuated compared to that of nanoparticles. This increases interfacial charges and potentials facilitating the binding of methyl parathion. If surface proximity of MP favors hydrolysis (partial or complete) it is expected that daughter products and phosphate species will bind strongly to surface sites through hydrogen bonding. The interface of hydrothermally prepared TiO2 nanofibers presents greater numbers of acidic sites for adsorption, and ultimately more opportunity for MP degradation through hydrolysis.

Supporting Information Available: derivation of DLVO-R modeling with fitted parameters; HPLC methods and sample preparation protocols; discussion of hydrolysis and isomerization; Tables; and Cartesian coordinates of optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS This work was supported in part by the Loma Linda University Grants for Research and School Partnerships (GRASP #2170315). Access to the SEM and TEM was provided by the Central Facility for Advanced Microscopy and Microanalysis (CFAMM) at the University of California Riverside (UCR). MMA and SNM are grateful for summer support from the Chemistry and Biochemistry Department at La Sierra University.

Conflict of Interest Disclosure. The authors declare no competing financial interest.


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Solution Concentration

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NP

NF Time

TOC Graphic


The Role of TiO2 Anatase Surface Morphology on Organophosphorus

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