Characterization of manufactured TiO2 nanoparticles€¦ · The technological advances in nanomaterials during the last twenty years have allowed the development of new applications

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Characterization of manufactured TiO2 nanoparticles

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2013 J. Phys.: Conf. Ser. 429 012012

(http://iopscience.iop.org/1742-6596/429/1/012012)

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Characterization of manufactured TiO2 nanoparticles

C Motzkus1, T Macé1, S. Vaslin-Reimann1, P. Ausset2 and M. Maillé2 1 Laboratoire National de métrologie et d’Essais (LNE), 1 rue Gaston Boissier, 75724 Paris Cedex 15, France 2°Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), UMR CNRS 7583, Université Paris-Est Créteil et Université Paris-Diderot, 61 Avenue du Général de Gaulle, 94010 Créteil. E-mail: [email protected] Abstract. Technological advances in nanomaterials have allowed the development of new applications in industry, increasing the probability of finding airborne manufactured and engineered nano-objects in the workplace, as well as in ambient air. Scientific studies on health and environmental risks have indicated that airborne nano-objects in ambient air have potential adverse effects on the health of exposed workers and the general population. For regulatory purposes, ambient measurements of particulate matter are based on the determination of mass concentrations for PM10 and PM2.5, as regulated in the European Directive 2008/50/EC. However, this legislation is not suitable for airborne manufactured and engineered nano-objects. Parameters characterising ultrafine particles, such as particle number concentration and size distribution, are under consideration for future health-based legislation, to monitor workplaces and to control industrial processes. Currently, there are no existing regulations covering manufactured airborne nano-objects. There is therefore a clear, unaddressed need to focus on the toxicology and exposure assessment of nano-objects such as titanium dioxide (TiO2), which are manufactured and engineered in large quantities in industry. To perform reliable toxicology studies it is necessary to determine the relevant characteristics of nano-objects, such as morphology, surface area, agglomeration, chemical composition, particle size and concentration, by applying traceable methods. Manufacturing of nanomaterials, and their use in industrial applications, also require traceable characterisation of the nanomaterials, particularly for quality control of the process. The present study arises from the OECD WPMN sponsorship programme, supported by the French Agency for Environmental and Occupational Health Safety (ANSES), in order to develop analytical methods for the characterization of TiO2 nanoparticles in size and count size distribution, based on different techniques to characterize five different manufactured TiO2 nanoparticles. In this study, different measurement techniques have been implemented: Transmission Electron Microscopy (TEM), Scanning Mobility Particle Sizer (SMPS) and Aerodynamic Particle Sizer (APS). The TEM results lead to a relatively good agreement between data from the manufacturer and our characterizations of primary particle size. With regard to the dustiness, the results show a strong presence of agglomerates / aggregates of primary particles and a significant presence of emitted airborne nanoparticles with a diameter below 100 nm (composed of isolated primary particles and small aggregates / agglomerates formed from a few primary particles): the number proportion of these particles varies from 0 to 44 % in the measurement range 14-360 nm depending on the types of powders and corrections of measurements.

Nanosafe 2012: International Conferences on Safe Production and Use of Nanomaterials IOP PublishingJournal of Physics: Conference Series 429 (2013) 012012 doi:10.1088/1742-6596/429/1/012012

Published under licence by IOP Publishing Ltd 1

1. Introduction The technological advances in nanomaterials during the last twenty years have allowed the development of new applications in various fields such as medicine (for therapeutic and diagnostic purposes), environment (water treatment), food (membranes for purification), industry (electronics, energy), etc.

The scientific knowledge on health and environmental risks associated to nanoparticles shows that airborne nanoparticles in ambient air have potential adverse effects on the health of exposed workers and on the general population (Witschger et al., 2005 [1]). Nowadays, only particles smaller than 2.5 microns are considered and measured by air quality monitoring networks in France. However, studies have shown that excessive amounts of solid and poorly soluble aerosols are able to penetrate deeply into the lungs by inhalation, to stay there permanently and to create a pulmonary overload weakening the ability of the body’s defense (Witschger et al., 2005 [1]; Hervé-Bazin B, 2007 [2]). These effects are all the more important since the particles are fine.

In the context of exposed workers, a report by the French Agency for Environmental and Occupational Health Safety (ANSES), entitled "Nanomaterials and safety" [3], indicates a production of several thousands tons of manufactured TiO2 nanoparticles in french industry. Therefore, in order to fulfil its aim of protecting human health, ANSES began to focus on the toxicology of nanoparticles such as titanium dioxide (Vincent, 2008 [4]) in order to protect the health of the workers involved in their production process. However, to perform reliable toxicology studies, it is necessary to determine the relevant characteristics of TiO 2 nanoparticles given by the International Organisation of Standardardization TC 229 “Nanotechnologies” and the OECD, such as the particle size/size distribution, shape, purity/impurity, solubility, surface chemistry, surface charge, crystallinity, surface area, chemical composition, aggregation/agglomeration state, concentration.

Consequently, we have developed an analytical strategy to characterize some key parameters of TiO2 nanoparticles such as size and count size distribution, based on different techniques to characterize five different manufactured TiO2 nanoparticles. The characterization results have been obtained after having developed traceable measurement methods (to the international system) and measurement uncertainty estimation (repeatability and reproducibility) to ensure reliability of the characterization of nanoparticles. In this study, different measurement techniques have been implemented: Transmission Electron Microscopy (TEM), Scanning Mobility Particle Sizer (SMPS) and Aerodynamic Particle Sizer (APS). The two measuring instruments SMPS (ISO 15900 [5]) and APS allow measuring and comparing the count size distribution, over a large range between 15 nm to 20 µm on the airborne particles generated with different manufactured TiO2 powders in the same conditions of dispersion ; this work allows to determine different dustiness of the nano-powders (Hamelmann et Schmidt, 2003 [6]). This study deals with the physico-chemical characterization of different manufactured TiO2 powders containing primary nanoparticles: P25 (Evonik), PC105 and Tiona AT1 (Cristal global – Milennium), and UV TITAN M262 and M212 (Sachtleben). In this paper, we present only the results for the P25 (Evonik) taking as an example.

This work arises from the OECD WPMN sponsorship programme.

2. Experimental methods 2.1. TEM measurement method The objective of this protocol is to set up the size distribution of TiO2 particles from powder by Transmission Electron Microscopy (TEM) after their deposition on a TEM grids. For each type of TiO2 powder, a very small amount (2 mg) was introduced in 6 ml of filtered ethanol, then the mixture was stirred and sonicated for 1 min (ultrasonic bath of BIOBLOCK 88155 type 460 with Freq 35 KHz).


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A drop of this suspension was deposited on copper TEM grids, and dried within a laminar flow in a fume cupboard and then observed. Two hundred mesh copper grids covered with Formvar/Carbon film (reference S162 of Agar Scientific) were used for transmission electron microscope measurements. A Jeol 100 CXII with magnifications between x100,000 and x320,000 and an acceleration voltage of 100 kV was coupled to a CCD camera (Dualvision 3000W model 780, Gatan Inc.) allowing digital images acquisition for the particles size and morphology determinations. The camera calibration was performed, at low magnification, with a Carbon grating replica grid (type 10020, 2160 lines/mm = 0.463 micron /line, E.F. FULLAM) and at high magnification, with Polystyrene Latex Spheres of (102 ± 3 nm with a standard deviation of 7.6 nm from the particle size distribution calibrated, Duke Scientific Corporation) certified by National Institute of Standards and Technology (NIST). The images were taken at different magnifications, depending on the size of the particles encountered: x 270 000.

For each powder, 20 to 30 images were performed at different random positions on the grid (in different meshes and different positions in the mesh): a maximum of five images were taken in the same mesh.

The obtained images are mostly composed of aggregates of primary particles with a size between 10 and 50 nm excepted for TIONA AT1. The primary particle size distribution was determined from 15 images of aggregates measured by TEM in order to obtain at least 100 primary particles measured.

A recent work of Song et al. (2009 [7]) confirms the importance of a statistical point of view to have at least 100 particles to correctly describe the size distribution when the expected size distribution is either normal or lognormal. In the case of a population with a log-normal or normal distribution, taking into account the measurement of 200, 500 or 1000 particles doesn’t give a significant contribution to the statistical level, see negligible ([7] and [8]).

The average primary particle size was calculated according the formula:

( )

2

+=∑ l sd d

dn

Where, - dl is the long diameter which is the length of the longest axis of the counted particle, - ds is the short diameter which is the length of the axis perpendicular to and through the center of the longest axis of each counted particle, - n is the total number of particles.

The diameter of primary particles is determined by the average length (l) and width (s) of the ellipse circumscribed with the particles using the software Image J. This software is currently used to process images produced by TEM (Song et al. 2009 [7]).

2.2. Dustiness measurement method In the first time, a bibliographic research has been performed to determine the actual methods of the nano-powder dispersion and shows that there is a few industrial systems that are able to disperse the nanoparticles starting from the nano-powders (Ma-Hock et al., 2007 [9]).

Although the most used and reproducible technique is atomisation (using a liquid suspension containing the powder), we decided to produce an aerosol with a powder disperser based on the vortex principle. This disperser has the advantage to produce an aerosol with a size distribution of the potentially particles close to the original powder (aggregates/agglomerates and primary particles) and the chemical nature unmodified by the use of a liquid suspension, ultrasonic


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protocols or thermic treatment. Furthermore, dispersion directly made by a vortex air allows to determine the potential or the maximum emission particles during an hypotethic accident or incident during industrial process and storage.

2.2.1. Generation method In a first step, we selected a commercial generator to study the dustiness of the nano-powders of TiO2 nanoparticles. The chosen generator was the commercial disperser "model PA100” of Naneum based on the principle of a vortex. This system generates aerosol from powder by concentrating high velocity vibrating jets of clean air at the powder surface, disturbing and separating particles from the powder to create an aerosol.

Vortices are formed by the turbulent air flow inside the aerosolisation chamber which acts to further break apart the aerosol particles. The aerosol is then passed through a 10 liter gravitational chamber which allows to homogenize the concentration and to remove the larger aerodynamic particles sizes, above a few ten micrometers on the basis of gravitational settling. The concentration of aerosol particles and the separation of the particles constituting an agglomerate can be controlled by adjusting the position of the jet nozzle in relation to the surface of the aerosolising powder. A generation protocol was developed to optimize the protocol for determining the dustiness of nanoparticles from the primary particles constituting the agglomerates/aggregates of nanostructured powders.

The goal of this protocol is to achieve the most stable and reproducible aerosol in size and number concentration including the following steps.

- The aerosol dispersion state from powders; dispersion of the powder in aerosol phase by injection of filtered air (choice of the jet nozzle, distance between the surface of the powder and the nozzle, flow rate of the disperser and powder mass used).

- The aerolic conditions of the dispersed material; this addresses mainly the dilution step with clean dry air. - The change of some plastic parts with stainless steel parts to reduce the loss of particles by electrostatic effects. In addition, a cleaning protocol to be implemented before each measurement has been developed

and validated. With our experimental conditions (geometric dimensions, flow rates and concentrations), this system allows to homogenize the aerosol while minimizing losses by diffusion and sedimentation of the generated aerosol in the measurement range based on calculations of deposit particles in the settling and homogenization chamber (Hinds, 1999 [10]).

2.2.2. Experimental facilities

Figure 1 presents the facilities developed to measure the count size distribution of an aerosol containing the nanometer primary particles and aggregates/agglomerates over a large range from 15 nm to 20 µm. This set-up is composed of an air filtration system, a generator of aerosol from a powder, a flow splitter, a diluter and two systems of parallel measurements: Scanning Mobility Particle Sizer (SMPS) and Aerodynamic Particle Sizer (APS). In this study, the results are expressed in aerodynamic diameter (dae) and electrical mobility diameter (dm) respectively for the results obtained by the APS and SMPS.

We observed that the concentration was above the APS concentration limit (>> at 1000 particles /cm3), thus it was necessary to use a diluter (VKL of PALAS) with clean dry air in order to measure the aerosol emitted by the generator. The used SMPS is composed of the impactor to remove the larger particles above 1 µm and thus measure the submicrometer particles, an aerosol neutralizer with a Kr85 source (model TSI 3077), a Long Differential Mobility Analyzer (model TSI 3081) and a Condensation Nuclei Counter (CNC). The used CNC (TSI 3022) detects particles


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from 7 nm (minimum particle size corresponding to an efficiency of 50%) at concentrations up to 107 particles/cm3 (Sem, 2002 [11]).

Figure 1. Scheme of the experimental set-up used to study the dustiness of nanostructured powders

Experimental conditions for SMPS

The flow rate of the polydisperse aerosol inlet was 0.3 L/ min. The sheath flow rate used in the Electrostatic Classifier (DMA 3080) was equal to 3 L/min. In these conditions, the electrical mobility diameter dm range was between 14 and 673 nm. The cut-off sizes (d50) of the impactor were respectively of 812 and 360 nm for the particles with a density of 1 and 4.2 g/cm 3 (TiO2 density). It is very important to underline that the cut-off size of 360 nm was lower than the maximum electrical mobility diameter dm of 673 nm. Thus, the larger particles above the d50 , with a strong inertia, will be removed by the impactor. We decided to take into account only the particles with a dm below 360 nm. We used the Aerosol Instrument Manager Software of TSI to perform the diffusion and charge corrections for the following densities: 4.2 g/cm3 for the particle and 0.0012 g/cm3 for gas.

Experimental conditions for APS and the dilutor

To compare the APS and SMPS measurements, it was decided to use the results of the APS on the same scanning time as the SMPS. With regard to the diluter VKL, its flow rate input was checked before each measurement to ensure that it was not clogged by a too high concentration of particles. The results presented in this paper take into account the dilution factor. Experimental conditions for the generator AERO PA100 The flow rate of dispersion is 6 L/ min and the mass of powder in the sample holder was fixed at 2.5 g. However, it was necessary to reduce this amount to 1.5 g when the dustiness was too high for some nanostructured powders, and thus to produce a "puff" of aerosol that can block up the jet nozzle. Two distances between the orifice and the surface of the powder were used in the experiments.

For each distance, the results of the count size distribution of twenty scans have been taken into account to determine the average and the standard deviation of the following parameters: mean diameter, median diameter, geometric diameter, modal diameter and geometric standard deviation.


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3. Experimental results for the powder AEROXIDE P25 (Evonik)

3.1. Dustiness results Preliminary work has been performed to develop measurement methods to ensure the

traceability of measurements (connecting instruments to the international system) and to estimate the uncertainties of measurements (repeatability and reproducibility) of the nanoparticles characterization.

On the same batch of powder, repeatability (several experiments in one day) and reproducibility (experiments over several days) experiments have been performed to have a better understanding of the count size distribution and the associated uncertainties.

A slow decrease in number concentrations measured by the APS and the SMPS during the time dispersion has been observed. But this decrease did not affect the stability of particles size distribution. Figure 2 shows the count size distribution for a measurement realized by the APS in the case of the powder P25 of Evonik taking as an example.

Figure 2. Count size distribution of the aerosol obtained for one sample with APS in the case of the powder

AEROXIDE P25

Table 1. Parameters characteristics of size distribution obtained by APS in the case of the powder AEROXIDE P25

Experiment 1st day

Experiment 2nd day

Experiment 3rd day

Statistical diameters

Average on 20 samples



Total Average

Repetability Standard deviation

Reproducibility Standard deviation

Median (µm) 0.77 0.78 0.76 0.77 0.03 0.03 Mean (µm) 0.87 0.89 0.92 0.90 0.04 0.05 Geometric Mean (µm) 0.81 0.83 0.82 0.82 0.03 0.03 modal (µm) 0.65 0.64 0.57 0.62 0.10 0.11


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The average parameters characteristics of size distribution obtained by APS were determined from several experiments (table 1) and twenty samples (scan measurements).

The observed size distributions have statistical diameters (median, arithmetic mean, geometric mean and modal) which are repeatable and reproducible (table 1). We obtained low repeatability standard deviations (<0.10 µm) and low reproducibility standard deviations (<0.11 µm). The obtained statistical average diameters are in a range between 0.6 and 0.9 µm. It is important to highlight the low presence of particles (agglomerates) above a few microns (see Figure 2). Most of the particles measured in the range 0.5 to 20 µm have an aerodynamic diameter between 0.5 µm and 1 µm.

Figure 3 presents respectively the count size distributions for a measurement performed with SMPS by taking into account both the diffusion and charge corrections (a) and with only the diffusion correction (b) in the case of the powder P25 (Evonik). This figure show that size distributions are influenced by taking into account the charge correction, as demonstrated by Ogura et al. [12] which studied NTC, fullerenes, ZnO and also TiO2 dustiness powders.

Figure 3. Count size distribution of the aerosol obtained for one sample with SMPS in the case of the powder Aeroxide P25: (a) both the diffusion and charge corrections and (b)with only the diffusion correction


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Table 2. Parameters characteristics of size distribution obtained by SMPS with both diffusion and charge

corrections in the case of the powder AEROXIDE P25

Experiment 1st day

Experiment 2nd day

Experiment 3rd day





Total Average



Median (nm) 303 300 287 297 30 31 Mean (nm) 254 256 244 251 21 21 modal (nm) 357 354 351 354 9 9

Table 3. Parameters characteristics of size distribution obtained by SMPS with both with only the diffusion

correction in the case of the powder AEROXIDE P25

Experiment 1st day

Experiment 2nd day

Experiment 3rd day





Total Average



Median (nm) 257 254 253 255 11 11 Mean (nm) 249 248 246 248 10 10 modal (nm) 351 352 343 349 14 14

We observed that the parameters of the size distribution (tables 2 and 3) are repeatable and reproducible (low standard deviations of repeatability and reproducibility less than 14 and 31 nm with only the diffusion correction and with both the diffusion and charge corrections respectively). The average statistical diameters (mean, median and mode) obtained are between 240 nm and 360 nm. In the case of charge correction, we observed that the total average of these statistical diameters were slightly higher until 14 % compared to the case without charge correction. It is important to notice that a high presence of agglomerates / aggregates of primary particles of a few hundred of nanometers is observed. Most of the particles measured in the range 14-360 nm have electric mobility diameter between 100 and 360 nm. Nevertheless, the results show a significant presence of emitted airborne nanoparticles with a diameter below 100 nm. Taking into account only the diffusion correction, we obtained a concentration of ultra fine particles (< 100nm) that could vary between 1 and 12% compared to the total concentration in the measurement range 14 -360 nm. In contrast, with both the diffusion and charge corrections, this concentration can vary between 1 and 20%, involving a greater proportion of emitted particles with a diameter below 100 nm. It is therefore very important to turn attention to the used measurement methods and the employed corrections because they can lead to different conclusions.


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3.2. Size distribution of primary particles Figure 4 shows aggregates of TiO2 nanoparticles from powder AEROXIDE P25 (Evonik).

Figure 4. TEM image of TiO2 particles from powder Aeroxide P25

The aggregates are composed of primary particles with diameters between 9 and 53 nm. The primary particle size distribution was performed from 13 images of aggregates measured by TEM (see Figure 5). This size distribution was established with measurements of 138 primary particles. We found a size distribution with a modal diameter of 20 nm and a mean diameter of 23 nm with a standard deviation of 7 nm.

Figure 5. Count size distribution of the primary particles of TiO2 from powder AEROXIDE P25 determined

by TEM

We observed that the obtained standard deviations are very high compared to the measurement uncertainty associated with the microscopy method (1 pixel corresponds to 0.42 nm for magnifications of x 270 000 and 0.78 nm for x 140 000).


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4. Conclusion This study allows us to develop analytical methods for the characterization of TiO2 nanoparticles in size and count size distribution, based on different techniques. The characterization results include the development of traceable measurement methods (to the international system) and measurement uncertainty estimation (repeatability and reproducibility) to ensure the reliability of the characterization of nanoparticles. In this study, different measurement techniques have been implemented: Transmission Electron Microscopy (TEM), Scanning Mobility Particle Sizer (SMPS) and Aerodynamic Particle Sizer (APS). The two measuring instruments SMPS (ISO 15900) and APS allow the measurement and the comparison of count size distribution, over a large range from 15 nm to 20 µm. This work has been focused on airborne particles generated with different manufactured TiO2 powders in the same conditions of dispersion.

This study was performed on different TiO2 manufactured powders: P25 of Evonik, PC105 and Tiona AT1 of Cristal global – Milennium, and UV TITAN M262 and M212 of Sachtleben. But we have presented in this paper only the results for the P25 (Evonik) taking as an example. The TEM results allow to observe a good agreement between data coming from the manufacturer and our characterizations of primary particle size. With regard to the dustiness, the results show a strong presence of agglomerates / aggregates of primary particles and show a significant presence of emitted airborne nanoparticles with a diameter below 100 nm (composed of isolated primary particles and small aggregates / agglomerates formed from a few primary particles): the number proportion of these particles varies from 0 to 44 % in the measurement range 14-360 nm depending on the types of powders and corrections of measurements. 5. References [1] Witschger O and Fabriès J F 2005 " Ultra-fine particles and occupational health. 1 - Characterization

of the potential effects on health " INRS, Hygiène et sécurité au travail ND 2227 199 21-35 [2] Hervé-Bazin B, 2007 Nanoparticles: a major challenge for occupational health ? 1ère édition Les Ulis

EDP Sciences [3] Agence Française de Sécurité Sanitaire de l’Environnement et du Travail (AFSSET) 2008

Nanomaterials - work safety [4] Vincent J M, 2008 Toxicology of titanium dioxide nanoparticles , INERIS, report DRC-08-94464-

11281A [5] ISO 15900, 2009 "Determination of particle size distribution-Differential electrical mobility analysis

for aerosol particles" 57p [6] Hamelmann F, Schmidt E, 2003 Methods for Estimating the Dustiness of Industrial Powders- A

review. KONA 21 pp7-18 [7] Song N W, Park K M, Lee I-H and Huh H, 2009 Uncertainty estimation of nanoparticle size

distribution from a finite number of data obtained by microscopic analysis Metrologia 46 480-488 [8] ISO 13322-1, 2004 Particle Size Analysis - Image Analysis Methods - Part 1: Static Image Analysis

Methods [9] Ma-Hock L, Gamer A O, Landsiedel R, Leibold E, Frechen T, Sens B, Linsenbuehler M et van

Ravenzwaay B, 2007 Generation and characterization of test atmospheres with nanomaterials. Inhalation Toxicology 19 no 10 833-848

[10] Hinds W 1999 Aerosol Technology- Properties, behavior, and measurement of airborne particles (Wiley-interscience, second edition)

[11] Sem GJ, 2002 Design and performance characteristics of three continuous-flow condensation particle counters: a summary. Atmos. Res. 62 267–294

[12] Ogura I, Sakurai H and Gamo M, 2009 Dustiness testing of engineered nanomaterials Journal of Physics: Conference Series 170 p. 012003


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Characterization of manufactured TiO2 nanoparticles€¦ · The technological advances in nanomaterials during the last twenty years have allowed the development of new applications