Article pubs.acs.org/JPCC
Pyroelectrically Driven •OH Generation by Barium Titanate and Palladium Nanoparticles Annegret Benke,*,∥,† Erik Mehner,∥,‡ Marco Rosenkranz,§ Evgenia Dmitrieva,§ Tilmann Leisegang,‡ Hartmut Stöcker,‡ Wolfgang Pompe,† and Dirk C. Meyer‡ †
Institute of Materials Science and Max Bergmann Center of Biomaterials, TU Dresden, D-01062 Dresden, Germany Institute of Experimental Physics, TU Bergakademie Freiberg, Leipziger Straße 23, D-09596 Freiberg, Germany § Leibniz Institute for Solid State and Materials Research Dresden (IFW Dresden), Helmholtzstraße 20, D-01069 Dresden, Germany ‡
ABSTRACT: The disinfection of bacteria by thermally excited pyroelectric materials in aqueous environments provides opportunities for the development of new means of sanitization. However, little is known about the formation of reactive oxygen species (ROS) at the surface of the thermally excited pyroelectric materials. To investigate the pyroelectrically driven ROS generation we performed OH radical specific measurements of thermally stimulated barium titanate nanoparticles in contact with palladium nanoparticles. Through electron spin resonance measurements with the spin trap BMPO (5-tert-butoxycarbonyl 5-methyl-1-pyrroline n-oxide) and fluorescence spectroscopy of 7-hydroxycoumarin, OH radical generation was detected, which confirms the hypothesis of pyroelectric ROS production. Since pyroelectric potential changes are insufficient for direct electrochemical OH radical generation, we propose a two-step chargetransfer model facilitated by intermittent contact between the palladium and the pyroelectric nanoparticles and the pyroelectric effect as the driving force for charge transfer.
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INTRODUCTION Commercial water disinfection currently relies on chemical methods using chlorine- or ozone-based chemicals, whereas physical methods like thermal disinfection or ultraviolet radiation are less often employed. Due to their high oxidative potential, reactive oxygen species (ROS) are well suited as a physical means of disinfection. A completely new approach for creating ROS is the utilization of the pyroelectric effect,1 which seems favorable when naturally occurring temperature changes can be employed for the excitation of the pyroelectric materials and, thus, offer an environmentally friendly method of water disinfection. In an aqueous solution the spontaneous polarization at the surface of a ferroelectric is screened, for example, by dissolved ions or dissociated water molecules. Changes in temperature trigger the pyroelectric effect. The imbalance of polarization and screening charges changes the effective surface potential. It was shown that these potential changes whether they stem from changes in temperature or strain can be used to drive electrochemistry between physisorbed molecular species.1,2 For example Hong et al. demonstrated water splitting on mechanically excited surfaces of BaTiO3 and ZnO. Gutmann et al. proposed that the observed water disinfection with thermally stimulated LiNbO3 and LiTaO3 is facilitated by production of ROS at the surface of the pyroelectric materials. Free radicals have high oxidation potentials, especially the OH radical whose oxidation potential is twice that of chlorine which is commonly used for disinfection. It is known that OH radicals can pull H atoms from C−H and S−H bonds and split © XXXX American Chemical Society
aromatic rings. Living cells are damaged by radicals reacting with amino acids and DNA molecules.3 Photocatalytic E. coli inactivation with TiO2 showed cell damage caused by various ROS, such as OH radicals, hyperoxide radicals, and H2O2.4 Basically ROS react immediately at the place of their origin. Their reaction rate with biomolecules is very high being 107 to 1010 mol−1 s−1 in the diffusion-limited regime.5 As they are short-lived on the time scale of 70 ns,6 only short diffusion lengths of 3−20 nm result. Consequently, all methods for ROS detection function indirectly, for example, degradation of dyes or other organics in aqueous solutions,7,8 fluorescence spectroscopy of marker molecules,9 like 2′,7′-dichlorodihydrofluorescin (DCFH)10,11 or 7-hydroxycoumarin,12 or oxidation of Jtriiodide to J 3 -triiodide 13 or para-chlorbenzoic acid (pCBA).14,15 ROS detection by oxidation of DCFH appears to be nonspecific for ROS because it was shown that not only ROS contribute to the reaction.16 The reaction mechanism itself proceeds over several stages and is not understood entirely. Several substances have been identified which oxidize DCFH directly, whereas others catalyze the reaction.17 The established ROS detection methods are mainly applied for biochemically and photocatalytically generated radicals. To the best of our knowledge their viability for detection in the vicinity of thermally stimulated pyroelectric materials has not been examined. In this article we report detection of pyroelectrically Received: May 13, 2015 Revised: July 10, 2015
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DOI: 10.1021/acs.jpcc.5b04589 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C generated ROS with two independent methods: fluorescence spectroscopy of 7-hydroxycoumarin and an electron spin resonance (ESR) based approach. We investigated the pyroelectric generation of ROS using pyroelectric, thermally excited barium titanate powder in combination with palladium nanoparticles. Noble metal nanoparticles are of great importance as catalysts and are often used in photocatalytic water disinfection. Catalytic activity depends on particle size and interactions between carrier and metal nanoparticles. 18 These parameters influence the electronic structure of the nanoparticles and can enhance the reactivity of the carrier. Examples of reactions that are typically analyzed with metal cocatalysts are water splitting19,20 and the oxidation of carbon monoxide (CO).21−23 Inoue et al. investigated differently polarized surfaces of lithium niobate decorated with small palladium particles. An increase of CO oxidation on the positive polarized surface was observed. The electron transfer from palladium into the pyroelectric material results in electron depletion of the metal and weakening of chemisorption between metal and CO. It is the intention of this study to reveal the basic mechanism of ROS production in the BaTiO3−Pd nanoparticle system. Besides the evidence of pyroelectrically generated ROS, we propose a model for the ROS generation reaction. It explains how the pyroeletric effect acts as the driving force for the exchange of charge carriers between pyroelectric and metal nanoparticles. Due to the impact of particle size on the crystal structure of barium titanate and the fact that pyroelectricity requires the tetragonal phase of barium titanate we put special emphasis on the structural analysis of the barium titanate employed.
Transmission Electron Microscopy (TEM). The palladium nanoparticles were investigated by TEM using a LIBRA 200 transmission electron microscope (Zeiss) in order to visualize their shape and to measure the size distribution. Samples were prepared on a carbon-coated copper grid by mounting a 10 μL drop of palladium nanoparticle solution, with a settling time of this drop of 10 min, and a final rinsing with ultrapure water. X-ray Diffraction and Fluorescence (XRD/XRF). The asreceived and poled crystalline BaTiO3 powder materials were characterized using XRD. Diffraction patterns were recorded in reflective Bragg−Brentano geometry with Cu-Kα radiation on a θ−θ goniometer (Bruker D8 Advance). The diffractometer employs primary and secondary 2.3° axial Soller collimators and a Johansson-type secondary graphite monochromator. The equatorial beam divergence was limited to 2°, whereas the focal point was constrained to 0.025°. Samples were continuously rotated during the measurement at 60 rpm. The instrumental broadening and shapes of reflection profiles were calibrated and fitted with program TOPAS26 and a fundamental parameter approach27 using the diffraction pattern of NIST SRM 640d silicon standard powder. Accordingly, crystallite sizes can be extracted from a cos θ convolution using Scherrer’s formula. Xray fluorescence spectra were recorded with a wavelengthdispersive spectrometer (Bruker S8 Tiger) and evaluated with the programs SpectraPlus and QuantExpress (Bruker). Fluorescence Spectroscopy of Coumarin/7-Hydroxycoumarin. Coumarin, a well-known probe molecule for specific detection of photocatalytically generated OH radicals,12 was used first for the detection of such radicals in the context of thermally stimulated pyroelectric materials. Reacting with OH radicals coumarin forms the highly fluorescent 7-hydroxycoumarin with a specific fluorescence emission maximum at wavelength of 455 nm. Coumarin works as a qualitative specific test method for OH radicals. By measuring the fluorescence intensity the amount of radicals can be quantified. For preparing the samples, 30 mg of as-received or poled barium titanate powder was weighed in a reaction cap and mixed with 50 μL of palladium nanoparticle solution. Palladium particles were not immobilized on the barium titanate surface; instead all particles are free in the solution and can form a temporary contact. Then, 150 μL of a solution with 1 mmol of coumarin (Sigma-Aldrich) in ultrapure water was added. The samples were heated from 20 to 70 °C (temperature stability of the coumarin solution was verified up to 80 °C) and then cooled to 20 °C in a thermoshaker (Thermomixer comfort, Eppendorf) at a heating and cooling rate of 5 K/min. In intervals of 3 min, the samples were mixed at 600 rpm for a period of 9 s. This procedure was carried out 5 times in total. Finally, the samples were centrifuged (14 000 min−1, 20 min), and the specific fluorescence intensities of the supernatants were measured with a fluorescence spectrometer (Nanodrop ND 3300, ThermoScientific) at wavelengths of 360 and 455 nm for excitation and emission, respectively. Control samples without thermal excitation, without palladium nanoparticles, and containing only coumarin or only palladium nanoparticles were measured for all samples. A calibration curve for the concentration of 7-hydroxycoumarin/•OH was captured by measuring the fluorescence intensity of 0.1, 0.25, 0.5, and 1 μmol of 7-hydroxycoumarin (Sigma-Aldrich) solution in 1 mmol of coumarin (Figure 5b). All experiments were protected from light to exclude photo effects.
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MATERIALS AND METHODS Pyroelectric BaTiO3 Powder. BaTiO3 was purchased from IoLiTec as a nanopowder material (nominal particle size 100 nm) with a purity of 99.9% and a relative permittivity of 2500− 2800. The pyroelectric coefficient of bulk barium titanate is approximately 200 μC/m2 K.24 It was used as received and after poling in an electric field, respectively. Powders were poled with a constant high voltage (6 MV/m) for 1 h as a dielectric in a parallel plate capacitor placed in a high vacuum chamber at 1 × 10−5 mbar pressure. By poling an alignment of domains is expected and therefore an enhancement of polarization uniformity. Preparation of Palladium Nanoparticles. Palladium nanoparticles were synthesized by the route of Bigall,25 albeit with slight modifications. An amount of 4.4 mg of palladium chloride K2PdCl4 (Sigma-Aldrich) was dissolved in 1 μL of concentrated hydrochloric acid and then injected in 50 mL of ultrapure boiling water through a filtering syringe with a pore size of 0.22 μm resulting in a final concentration of the metal precursor of 0.27 mmol. After 1 min 1.1 mL of the mild reducing agent containing 1% sodium citrate and 0.05% citric acid was injected. After another half minute 0.55 mL of a freshly prepared strong reducing agent with 0.08% sodium borohydrate, 1% sodium citrate, and 0.05% citric acid were added, and the solution was left to boil for 10 min before cooling. Scanning Electron Microscopy (SEM). The powder sample morphology of BaTiO3 was investigated by SEM using a DSM 982 Gemini electron microscope (Zeiss). The powder samples were mixed in a drop of water, mounted on carbon pads, dried, and carbon-coated before insertion into the microscope. B
DOI: 10.1021/acs.jpcc.5b04589 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Figure 1. SEM micrograph of barium titanate powder. Individual particles (a) are aggregated forming spheres of different sizes (b).
Figure 2. Powder diffraction pattern of the commercial barium titanate powder. Measured (Yobs) and calculated intensities (Ycalc) are given on a logarithmic scale, whereas the difference (Ydiff = Yobs − Ycalc) is given on a linear scale. The solid line (red) shows the best Rietveld fit (see results in Table 1). Insets show composition of 111 and 002/020 reflections with respect to cubic and tetragonal fractions on a linear scale (au: arbitrary units).
A first long thermal excitation step (4 h, seven cycles between 20 and 70 °C) of the pyroelectric barium titanate in combination with the palladium nanoclusters has been applied in water to remove adsorbed gases in the Stern bilayer and equilibrate screening charges on the barium titanate powder surface. Then, 20 μL of aqueous solution of the spin trap molecule was added to each sample so that a final concentration of 50 mmol is achieved followed immediately by the thermal excitation for trapping the free radicals by the spin trap. Thermal excitation took 15 min (seven cycles) between 5 °C (ice water) and 30 or 40 °C (thermoshaker) while mixing (protected from light to exclude photo effects). BMPO (5-tert-butoxycarbonyl 5-methyl-1-pyrroline N-oxide, high purity, Enzo Life Science) was employed as a spin trap to detect specifically short-lived O-, C-, S-, and N-centered free radicals by formation of stable radical adducts. The concentration of BMPO was 50 mM. BMPO radical adducts are much more stable compared to other spin traps like DMPO (5,5-dimethyl-1-pyrroline N-oxide).30 The half-life time is 23 min (Enzo Life Science) at room temperature; therefore, thermal excitation was only 15 min. Finally, the samples were centrifuged (14 000 min−1, 5 min) to separate all barium
Electron Spin Resonance (ESR) Spectroscopy. ESR (also known as electron paramagnetic resonance spectroscopy) is a method for studying chemical species that have at least one unpaired electron leading to absorption of microwave radiation in an external magnetic field. Beyond its many applications, ESR has already been used for detection of ROS and especially free radicals.28,29 Spin trap molecules reacting with free radicals in solution generate stable products, which in turn are directly observable by ESR. Spin trapping is therefore a valuable tool for studying the very short-lived free radicals. To measure electron spin resonance, a sample volume of about 1300 μL was used. This volume has been split into six parts for sample preparation in order to get virtually the same sample volume and powder mass for thermal excitation in the thermoshaker like in the fluorescence spectroscopy experiments. For each sample fraction, 32.5 mg of as-received or poled barium titanate powder was weighed in a reaction cap and mixed with 54.2 μL of palladium nanoparticle solution. Palladium particles were not immobilized on the barium titanate surface, but all particles are free in the solution and can form temporary contacts. Then, 142.5 μL of ultrapure water was added. C
DOI: 10.1021/acs.jpcc.5b04589 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C Table 1. Summary of Rietveld Refinementa refinement model single cubic
cubic BaTiO3 lattice constant acubic (Å)
tetragonal BaTiO3 lattice constants atetra, ctetra (Å)
4.012(3) − −
single tetragonal
− 4.006(1) 4.025(3)
“simple” linear combination cubic + tetragonal
4.006(6)
“best fit” linear combination cubic + tetragonal
4.007(4)
4.006(9) 4.025(4) 4.006(7) 4.024(9)
crystallite size (nm)
mass fraction (wt %)
residual weighted profile RWP (%)
goodness of fit
188 ± 12 − 57 ± 27 − 155 ± 5 48 ± 16 51 ± 6 465 ± 91 46 ± 12 55 ± 6 500 ± 102 46 ± 12
98.4 ± 0.6 − 1.7 ± 0.3 − 98.5 ± 0.4 1.5 ± 0.2 20.5 ± 1.7 77.7 ± 1.8 1.7 ± 0.2 22.9 ± 1.6 75.4 ± 1.7 1.7 ± 0.2
10.9
3.25
7.3
2.19
6.5
1.96
5.7
1.73
a
Crystallite size and mass fraction refer to cubic BaTiO3, tetragonal BaTiO3, and BaCO3 impurity, respectively. Given errors are at a 3σ confidence level, without respect to serial error correlation.
The diffraction pattern (Figure 2) shows no clear splitting of the 020 and 002 reflections of BaTiO3 hinting to the cubic phase. However, the 111 reflection is considerably sharper than the 020 and 002 reflections, which is a distinct indication of the tetragonal phase. Hence, refinements using both cubic and tetragonal BaTiO3 structures with space groups Pm3̅m32,33 and P4mm33 were attempted. Structure models34,35 as well as structures for orthorhombic and rhombohedral BaTiO335,36 were not pursued since initial tests yielded low R-factors. For all refinements a fourth-order polynomial background was employed. Barium occupancy factors were adopted from XRF. The higher indexed reflections are significantly broader than lower ones, which is typically caused by microstrain. Although isotropic strain models are usually an oversimplified ansatz, we used a tan θ convolution to model the reflection broadening at higher angles because elastic properties of BaTiO3 are not of interest here. The refinements for a single cubic or tetragonal phase agree considerably better with the tetragonal phase since the different widths of the 111, 020, and 002 reflections are better reproduced by the tetragonal structure model. However, intensities and shapes of the reflections are not satisfactorily matched and require further improvement (RBragg). Following the surface reconstruction model by Hoshina et al. a refinement with a linear combination of cubic and tetragonal phases was attempted and yields a better refinement.37 On the basis of the average particle size found by SEM the Hoshina model predicts radii for its cubic surface layer and gradient lattice strain layer of approximately half the crystallite size of the refined cubic phase (see Table 1). Comparing the obtained crystallite size for the tetragonal phase with the SEM micrographs suggests at least partially oriented lattice intergrowth between the small particles (Figure 1a). The best conformity with the measured pattern was realized by incorporating platy textures along [111] and [011] directions which may stem from compaction of intergrown particles (Figure 1) during sample preparation. Since powder diffraction is unable to spatially localize the cubic fraction, investigations with TEM were conducted. The TEM micrographs exhibited strong strain-related contrast, although no indication for a separated core−shell structure was found. Consequently, we assume the cubic phase as a modification of the tetragonal phase that is likely to be located at the surface of the particles. Defining the characteristics of this cubic phase more precisely by the Rietveld refinement is hindered by the
titanate and palladium particles. Supernatants of all samples were collected for injection into the ESR flat cell. Control samples without palladium nanoparticles were also investigated. The ESR spectra were recorded by an EMX plus X-band CW spectrometer (Bruker) using an optical cavity (ER 4104OR, Bruker) and the Xenon software package (Bruker). A special ESR flat cell (Quarzglastechnik Ltd.) was used for the measurements in aqueous solution. The ESR spectra were measured at a modulation amplitude of 2 G (at 100 kHz) and microwave power of 5 mW. To determine the number of spins, the ER213ASC alanine spin concentration sample provided by Bruker BioSpin GmbH was used. The determined spin concentration of this standard was 2.00 × 1017 spins. Using this recalibration system and a special polynomial-sensitivity pattern for the used resonator, the absolute number of spins of an unknown sample can be determined with an accuracy of ∼20%.
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RESULTS Morphological and Structural Characterization of BaTiO3 Powder. The SEM micrograph shows single nanoparticles with a size of approximately 150 nm similar to the supplier’s declaration (Figure 1a), and particles are aggregated in random spheres of micrometer size (Figure 1b). In aqueous solution these aggregated spheres are difficult to deagglomerate, even by ultrasonic treatment. Therefore, it has to be assumed that nanoparticles are aggregated at least partially during the radical generation experiments. X-ray fluorescence analysis confirms the commercial BaTiO3 powder to be nearly stoichiometric Ba0.988±0.002Ti1.000±0.005O3.2±0.2, notwithstanding elements lighter than oxygen. Expected impurities like calcium or strontium were found to be below (
