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A high resolution STEM-EELS study of silver nanoparticles exposed to light and humic substances. Isabella Romer, Zhiwei Wang, Ruth Corrin Merrifield, Richard E. Palmer, and Jamie R Lead Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b04088 • Publication Date (Web): 20 Jan 2016 Downloaded from http://pubs.acs.org on January 26, 2016

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Environmental Science & Technology

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A high resolution STEM-EELS study of silver nanoparticles exposed to light

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and humic substances.

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Isabella Römer1, Zhi Wei Wang2,3, Ruth C. Merrifield3,4, Richard E. Palmer2 and Jamie Lead1,4*

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Birmingham B15 2TT, UK.

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Birmingham, Edgbaston, Birmingham B15 2TT, UK.

School of Geography Earth and Environmental Sciences, University of Birmingham, Edgbaston,

Nanoscale Physics Research Laboratory, School of Physics and Astronomy, University of

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China

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of South Carolina, Columbia, South Carolina 29208, United States

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083,

Center for Environmental Nanoscience and Risk (CENR), Arnold School of Public Health, University

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*Corresponding author: [email protected], Telephone number: +1 803 777 0091

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ACS Paragon Plus Environment


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ABSTRACT

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Nanoparticles (NPs) are defined as particles with at least one dimension between 1 and 100 nm or

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with properties that differ from its bulk material, which possess unique properties. The extensive use

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of NPs means that discharge to the environment is likely increasing, but fate, behaviour and effects

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under environmentally relevant conditions are insufficiently studied. This paper focuses on the

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transformations of silver nanoparticles (AgNPs) under simulated but realistic environmental

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conditions. High resolution aberration-corrected scanning transmission electron microscopy (HAADF

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STEM) coupled with electron energy loss spectroscopy (EELS) and UV-vis were used within a multi-

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method approach to study morphology, surface chemistry transformations and corona formation.

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Although loss, most likely by dissolution, was observed, there was no direct evidence of oxidation

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from the STEM-EELS. However, in the presence of fulvic acid (FA), a 1.3nm oxygen-containing

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corona was observed around the AgNPs in water; modelled data based on the HAADF signal at near

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atomic resolution suggest this was an FA corona was formed and was not silver oxide, which was

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coherent (i.e. fully coated in FA), where observed. The corona further colloidally stabilized the NPs for

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periods of weeks to months, dependent of the solution conditions.

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INTRODUCTION

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Nanoparticles (NPs) are defined as particles with at least one dimension between 1 and 100 nm or

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with properties that differ from its bulk material, which are of great fundamental scientific interest as

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well as of interest to industry because of their unique properties.

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environment is increasing, but it can be noted that NPs at environmentally relevant conditions have

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not been widely studied.

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and because of their increasing use in commercial products they might pose a hazard to humans and

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other organisms due to potential release.

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invertebrates although somewhat less so to fish and humans.

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The discharge of NPs to the

Silver nanoparticles (AgNPs) have powerful anti-microbial properties,

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AgNPs have shown to be toxic to microbes and 8, 9

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It is well known that NPs including AgNPs are transformed in the environment,

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are poorly understood. Under environmental conditions, NPs will be exposed to the presence of a

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variety of major ions and natural organic macromolecules (NOM) at spatially and temporally variable

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concentrations, to sunlight etc., which will likely have an effect on particle persistence, morphology

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and chemistry. Changes in these properties will potentially affect their toxicity,

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fate

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and transport.

but these changes

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environmental

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Several papers have been published on the influence of environmentally relevant conditions on NPs.

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It has been shown that NOM can improve colloidal stability of AgNPs in media with low ionic strength.

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14,15

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either electrostatically or sterically after being adsorbed to the surface and replacing weakly bound

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coatings.

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for at least 48h due to the presence of NOM.

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water in high concentrations, and it has been shown that the presence of this ion decreases short

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term AgNP dissolution.

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MgCl2 and MgSO4) have a strong influence (ca. 50-65 fold) on aggregation of citrate capped AgNPs

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and FA can enhance the stability of the particles.

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influence their stability via dissolution.

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the first step in particle dissolution when present in an oxygenated solution and in the presence of

Fulvic acid (FA) and humic acid (HA) are types of NOM which generally enhance NP stability

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Other studies have shown that citrate capped AgNPs in natural freshwaters can be stable

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Chloride ions15 are present in natural and synthetic

It has been demonstrated that divalent cations (CaCl2, Ca(NO3)2, CaSO4,

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It is also believed that oxidation of AgNPs will

Surface oxidation at a nanometer level is believed to be

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electrolytes. 21,

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(2011)

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aggregated to different degrees depending on the surface coating. Li et al. (2012)

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morphological changes on citrate capped AgNPs exposed to sunlight, but found no impact on Ag+

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solubility. Yin et al. (2015)

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on NP aggregation and that sunlight could accelerate the morphology change, aggregation, and

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further sedimentation of AgNPs.

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Sunlight has also shown changes in NPs morphology and stability. 23, 24 Cheng et al.

observed that under sunlight irradiation, gum arabic and PVP coated AgNPs irreversibly

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found

found that both the presence of electrolytes and NOM had an influence

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This paper focuses on the quantification of the physico-chemistry (crystallinity, oxidation state, shape

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and eco-corona formation) of citrate-coated AgNPs after being exposed to natural sunlight, in the

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presence of FA and in two synthetic freshwaters with two different ionic strengths, at extremely high

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resolution using the aberration-corrected STEM

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spectroscopy (EELS), giving detailed understanding on a single particle basis of such behavior. UV-

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vis was used to assess particle stability over time.

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combined with electron energy loss

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EXPERIMENTAL SECTION

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Chemicals

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Chemicals and solvents used were purchased from Sigma Aldrich and Fisher Scientific. Ultra high

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purity water, UHP, (maximum resistivity of 18.2 MΩcm-1) was used for all experiments. All glass- and

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plastic-ware were washed before and after use with 10% nitric acid (HNO3) and thoroughly rinsed with

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UHP water ensuring no traces of acid remained. Suwannee River fulvic acid (FA) was purchased from

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the International Humic Substances Society (IHSS) and aqueous solutions were prepared by

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dissolving the appropriate amount of solid in ultrapure water to make a stock solution of 400 mg L-1,

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and left for 24 hours to hydrate.

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concentration. Synthetic freshwater, very soft water (VSW) and soft water (SW), were prepared based

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on the method recommended by the USEPA (United States Environmental Protection Agency).

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VSW had a calculated ionic strength (IS) of 0.633mM, and SW of 2.53mM.

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Dilutions of this stock solution were used to obtain the desired

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Particle synthesis and characterization

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Citrate stabilized AgNPs (12mg L ) of 20nm in size were prepared and characterized as previously

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published

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on aberration-corrected scanning transmission electron microscopy (STEM) combined with electron

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energy-loss spectroscopy (EELS) and surface plasmon data (UV-vis). Particle properties as

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measured by DLS, FFF,

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Particles are always used fresh and are stored at 4oC.

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using a standard borohydride reduction of AgNO3 and a multi-method approach based

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AFM and TEM

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are shown in Table S1 in supporting information (SI).

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Preparation of the AgNPs samples in the media

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AgNPs were added to the synthetic water to have a final concentration of 2.2mg L-1, higher than

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environmental concentrations but sufficient to allow easy STEM imaging. FA was added from a stock

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solution to the synthetic water before adding the particles to have a final concentration of 8mg L-1.

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When FA was added to the UHP water it had a final concentration of 20mg L-1. The high FA

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concentrations means that the FA:AgNP ratio is not unrealistic compared to environmental values.

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The particles were left either under natural sunlight or in the dark for periods between 24 hours and 6

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months.

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UV-vis measurements

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Absorbance was measured by a 6800 Jenway double beam UV-Vis spectrophotometer, with a 1 cm

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path length plastic cuvette. The UV-Vis absorption spectra were measured daily and collected over a

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wavelength range of 300-800nm. Suitable blanks and controls were measured.

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STEM – EELS measurements.

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STEM images were recorded by high angle annular dark field (HAADF) detectors in a JEOL

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JEM2100F coupled with a CEOS spherical-aberration probe corrector and a Gatan Enfina EELS.

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Elemental analysis was carried out by recording spectrum images from a region defined by a

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rectangle or a line through the edge of the particles that are suspended on the holes of the graphene

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carbon films. The power law fitting model from the Digital Micrograph software was used for

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background subtraction.

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particle solution on a graphene enhanced lacey carbon copper grids at room temperature. The grid

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was washed thoroughly with UHP water and re-dried.

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STEM- EELS samples were prepared by partially drying a drop of the

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STEM intensity model for corona thickness calculation.

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The STEM intensity model was used assuming the simple relationship I ~ tZα for thin samples (where

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αis close to 2),

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specimen thickness (t) and the thickness of the shell (Pa-Pb), and was calculated from (see Figure 4):

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where Z is the atomic number, the intensity (I) is approximately proportional to the

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Pa- Pb=PoPb-PoPa

Eq.(1)

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Where Po is the center of the core, Pa the interface between the core and the shell (corona) and Pb is

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the edge of the shell. PoPb stands for the distance between Po and Pb, i.e., the diameter of the whole

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nanoparticle, and PoPa is the distance between Po and Pa, i.e., the diameter of the core.

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Assuming that the particle features a uniform core-shell structure whose core/shell size is equal to

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the experimental value measured along the line PoPaPb.

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RESULTS AND DISCUSSION

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Characterization of the pristine particles

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A high-angle annular dark field (HAADF)-STEM image for the freshly synthesized AgNPs is shown in

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Table S1 (SI) and Figures 1 and S1 (SI), lattice fringes are visible in Figure S1 and marked with an

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arrow on particle B. An EELS line spectra analysis for the pristine AgNPs that show the presence of

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Ag is shown in Figure 1. The silver M4,5 absorption edge with onset energy of 367 eV and an

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intensity maximum delayed by larger than 50 eV beyond the threshold, can be observed in Fig.1(b),

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no oxygen K peak was present, which should be observed at 532 eV, this suggests that no oxidation

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was observed on the pristine particles. No oxygen K peak was observed on the carbon grid, as seen

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Fig.1(c). Pristine 12mg L 20nm AgNPs in 0.15mM citrate solution and kept at 4ºC and in the dark

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have shown to be stable for at least 1 year, the UV spectra obtained for these AgNPs is shown in

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Figure S2, it can be observed that after 2 years there was a 10% loss of the UV-vis signal, which

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shows that storage does not have a significant effect on particle stability.

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Influence of sunlight on the pristine particles

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Upon exposure to sunlight for 25 days the as-prepared AgNPs turn green and after 31 days a

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precipitate was observed (shown in supporting information (SI), Figure S3), which has been associate

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with changes in particle shape due to light irradiation in the past.

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confirms the observed color changes and shoulders on the maximum absorbance signal suggest a

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mixture of aggregation and shape change, after dissolution.

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be due to a slow sharp transformation or a growth in size distribution, the appearance of a second peak can be

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due to shape changes into rods, and the appearance of two more peaks due to the formation of triangles. STEM

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images shown in Figure 2 confirm this behavior, showing a change in shape and size with rods,

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triangles and hexagons, present in sizes of 1 nm upwards. Both aggregates and individual NPs were

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observed.

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UV-vis data shown in Figure S4

In general, the broadening of peaks can

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Inorganic media effects

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Pristine AgNPs were added to VSW and SW and were exposed to sunlight over a period of one

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month. UV-vis data for the particles in freshwater are shown in Table 1 and Figure 3 (A and C). In the

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case of the AgNPs in VSW (Figure 3A), a reduction of 36% in the UV signal was observed after 4

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weeks. The maximum wavelength of the peak remained stable at 398 nm and the full width at half

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maximum (FWHM) did not change after 4 weeks. The loss of the signal should be due to a loss in the

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particles, either by sticking to the container or maybe precipitation, but not due to dissolution, where a

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blue shift should be observed. Dynamic light scattering (DLS) data did not show aggregation (data not

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shown). In high ionic strength (SW) (Figure 3C) it can be observed that the FWHM for the AgNPs

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increased after 4 weeks and a 78% reduction in the UV signal was observed. The FWHM of the

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peaks determines the dispersity of the nanoparticles, where a large FWHM is attributed to peak

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broadening and hence, polydispersity.

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presence of an oxidized layer on the particle surface, although no oxidized layer was observed by

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STEM (data not shown), or due to loss through dissolution. Amorphous layers were found when

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STEM grids were accidentally allowed to dry, shown in Figure S5, which indicates a formation of AgO

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shell. It is believed that when AgNPs are in an oxygenated solution and in the presence of

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electrolytes, oxidation of the surface will be the first step in particle dissolution.

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note that SW has an IS only four times higher than that of VSW, and very small changes in conditions

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can have an impact on particle stability.

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A blue shift in the signal was observed, which suggests the

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It is important to

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Quantification of the eco-corona

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The term eco-corona has been used recently to describe the interaction between NOM and

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nanoparticles.

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aggregation and result in a complex mixture of transformed AgNPs that inevitably have different fates

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due to the disparity in their physical, chemical, and surface properties. In our work, FA was added to

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the pristine and spherical AgNPs to mimic environmentally relevant conditions. The AgNPs with FA in

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DI water after being exposed to sunlight or kept in the dark showed no significant changes in

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absorption when measured with UV-vis after 6 months (Figure S6). It has been shown that the

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Adsorption of NOM can displace smaller ligands and stabilize the NPs against

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presence of FA will promote the formation of a corona around the NP, which will enhance stability and

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was observed for these particles.

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substitution of the small citrate anions by the polyanionic FA is likely to occur. Aggregation is retarded

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by the steric stabilization offered by the FA and the longitudinal plasmon band may be absent from

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the UV–visible spectra of a dispersion containing FA. 36

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When FA is added to the citrate-stabilized NPs dispersion,

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Figure 4 shows the presence of an FA corona, with Figures 4(a) and (b) showing a STEM image

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obtained for the AgNPs in 20 mg L-1 FA and DI water after being exposed to sunlight for 10 weeks.

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The rectangular frame on Figure 4(a) marks the region from which the maps shown on Figure 4(c)-(e)

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were recorded. The middle images show the elemental maps for the particle. Figures 4(f) and (g)

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show EELS spectra taken from the two pixels shown on Figure 4(c). Pixel 1 in Fig. 4(c) was placed

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inside the particle and its EELS spectrum is shown in (f), a Ag-M4,5 core-loss edge was observed

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while no obvious oxygen signal (O-K) was found. The map for Ag is shown on Fig. 4(d). Pixel 2 in Fig.

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4(c) shows an EELS spectrum, Figure 4(g), which was taken near the edge of the same particle. No

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Ag signal was observed but an O-K edge is visible and the map is shown on Fig. 4(e), indicating the

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presence of oxygen (O) around the particle. The position of this spectrum is at 1-2 nm from the Ag NP

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edge, where it is expected that a FA corona would be present.

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containing coronas contained FA and were formed during the sample exposure. This corona was

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observed repeatedly in samples with FA both kept in the dark and exposed to sunlight and over time

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periods of up to 6 months; it was never observed when FA was not added (see Figures 1 and 2). Fig.

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4(b) shows the HAADF image retaken after the EELS spectrum imaging, in which an increase in the

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thickness of the corona is clearly observed, indicating a build-up of a carbon-rich substance as a

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result of electron bombardment. It is known that electron beam irradiation especially at high electron

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dose that is normally required for EELS spectrum imaging can induce specimen contamination by

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formation of a carbonaceous layer on the sample surface.

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formed by the specimen contamination do not contain a detectable O constituent, so it is clear that the

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corona does not come from a build-up of carbon contamination from electron beam damage. This can

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be easily identified by comparing the O map in Fig. 4(e) with the STEM images in Fig. 4(a) and (b),

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which show the HAADF images taken before and after the spectrum image 4(c) was recorded. The O

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map in Fig. 4(e) is consistent with the shell region in Fig. 4(a), and not with Fig. 4(b), so there was no

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oxygen contamination coming from the electron beam damage. It should be noted that in the inner

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region of the AgNPs, the O signal could be buried within the Ag background noise and would become

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invisible. A similar phenomenon has been observed in previous work, for example, van Schooneveld

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et al (2010)

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was mainly seen at a small shell region around the nanoparticles through EELS spectrum imaging.

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37, 38

15, 36

We suggest that the weak O-

However, these carbon-rich deposits

analysed lipid-coated silica nanoparticles and found that the carbon signal from lipids

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The O containing signal could have been due to the formation of AgO layers, as mentioned before.

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The observation of the corona and the unambiguous identification that it is composed of FA rather

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than silver oxides, was quantified using a modelling procedure to estimate the STEM intensity shown

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in Figure 5. Due to the incoherent nature of high-angle electron scattering, the HAADF-STEM

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intensity (I) is approximately proportional to the specimen thickness (t) and atomic number Zα for thin

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samples, where αis close to 2.

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core-shell structure whose core/shell size is equal to the experimental value measured along the line

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PoPaPb from the particle in Fig. 5(a), where PoPb = 4.8nm and PoPa = 3.5 nm. We then performed the

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STEM intensity model following the simple relationship I ~ tZ (α=1.70 is used). Three kinds of shells

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were considered in the modeling work: carbon shells and two types of silver oxide (AgO, Ag2O). The

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simulated intensities are shown in Fig. 5(c), together with the experimental one. It can be seen that

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the intensity plots of AgO and Ag2O almost overlap each other, because the volume density of Ag is

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nearly identical in AgO and Ag2O. While the density of oxygen is different to Ag, this does not cause a

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significant intensity variation since it has a much smaller z value than Ag. Fig. 5(c) shows that the

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experimental intensity matches very well with the simulated intensity obtained using carbon as a shell,

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while deviates very far from the AgO and Ag2O shells. This clearly suggests that the O signal

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observed was not caused by the AgxO, but from FA, which contains C and O. The thickness of the

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shell (Pa-Pb), was calculated from Eq.(1) as follows:

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For the model, we assumed the particle features a uniform

α

301 Pa- Pb=PoPb-PoPa=4.8-3.5=1.3nm

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The FA corona thickness obtained in this model was consistent with the data on FA thickness

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observed on other surfaces.

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with different techniques (FFF, fluorescence,45 AFM44) and at different conditions and obtained a size

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range from 1 to 2nm. These results indicate that by using STEM-EELS we were able to quantify

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corona formation, which may have a significant effect on environmental behavior, aggregation, and

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particle transformation.

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Lead et al (2000)

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measured the hydrodynamic diameter of FA

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AgNPs were added to VSW and SW in the presence of 8mg L of FA and exposed to sunlight. When

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FA was added to the VSW a reduction in the UV-vis signal of 27% was observed after 4 weeks (Table

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1 and Figure 3B), no shift was observed and the FWHM decreased, as shown in Table 1, which would

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mean more stable particles. In the case of SW, which has a higher ionic strength of 2.53 mM, there

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was a 51% reduction of the UV-vis signal when FA was present after 4 weeks. A red shift in the signal

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was observed and a second peak was observed at ~580nm, which is consistent with previous

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observations of small clusters or agglomerates of AgNPs.

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reduces the stabilizing effect of FA but there is still extra stability associated with the FA addition;

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compared to the AgNPs without FA present there was a smaller reduction of the UV-vis signal in all

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media. The STEM-EELS data for the AgNPs with FA in SW exposed to light for 24 hours is shown in

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Figure S7. No O-K peak was observed, which was also the case for the particles with FA in VSW

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(data not shown), although a corona was observed in the HAADF-STEM images in both cases. That a

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corona was not observed by EELS (but was observed by HAADF) in SW and VSW is likely due

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reduction in thickness of the corona caused by charge shielding from the added counterions, since

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the presence of salt should not affect the EELS analysis directly. It is important to add that the stress

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produced during the sample drying process on the carbon grid may harm the integrity of the FA

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corona to some degree. As a result, the identification of the corona becomes more difficult by STEM

328

imaging and especially by EELS, as reflected in our systematic experimental investigations. However,

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the preparation procedures used helped to mitigate the worst effects of drying.

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It is clear that a higher salt content

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ACKNOWLEDGEMENTS

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We acknowledge the UK Natural Environment Research Council (NE/H008764/1; NE/I0083/1;

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NE/G010641/1; NE/H013148/1; and the Facility for Environmental Nanoscience Analysis and

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Characterisation, FENAC), the Centre for Environment, Fisheries and Aquaculture Science (Cefas;

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Seedcorn funding through project DP247) and the Center for Environmental Nanoscience and Risk

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(CENR) for financial support.

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ASSOCIATED CONTENT

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Supporting information

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Size measurements for the pristine AgNPs (Table S1). Typical image of the AgNPs measured by

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STEM (Figure S1). UV-vis spectra for the pristine AgNPs over time (Figure S2). Picture of the AgNPs

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exposed after being exposed to light over time (Figure S3). UV-vis data for the AgNPs exposed to

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light (Figure S4). STEM images of oxidized AgNPs (Figure S5). UV-vis spectra for the AgNPs with FA

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(Figure S6). STEM-EELS data for the AgNPs with FA exposed to light after 24h (Figure S7). This

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material is available free of charge via the internet at http://pubs.acs.org.

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46. Zook, J. M.; MacCuspie, R. I.; Locascio, L. E.; Halter, M. D.; Elliott, J. T., Stable nanoparticle aggregates/agglomerates of different sizes and the effect of their size on hemolytic cytotoxicity. Nanotoxicology 2011, 5 (4), 517-530.

492 493 494 495

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496

Page 16 of 23

Table 1. UV-vis data for the AgNPs in synthetic freshwater with and without FA, exposed to sunlight.

Condition of the exposure

Maximum at time 0h (nm)

FWHM at time 0h (nm)

Maximum after 1 month (nm)

FWHM at after 1 month (nm)

VSW VSW + FA SW SW + FA

398 + 1 398 + 1 396.5 + 0.5 396.5 + 0.5

65 + 1 70 + 1 58 + 0.9 64 + 0.6

398 + 1 398 + 1 398 + 0.5 402 + 1

65 + 1 63 + 1 80 + 0.7 72 + 0.5

497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522

16


Reduction of the UV signal after 4 weeks (%) 36 + 1 27 + 1 78 + 0.7 51 + 0.6

Page 17 of 23


523 524

LIST OF FIGURES

525 526

Figure 1. EELS study of the pristine AgNPs. (a) shows a HAADF-STEM image of the nanoparticles,

527

where series of EELS spectra were taken along the green line. (b) and (c) show the spectra extracted

528

from P1 (located at the right end of the line and on the particle) and P2 (near the edge of the

529

nanoparticle on the left), respectively. (b) reveals that no oxidization occurred on the AgNPs and (c)

530

indicates that the carbon grid was clean and no O signal was observed (O K edge: 532 eV).

531 532

Figure 2. HAADF-STEM images of AgNPs exposed to sunlight for 25 days. Some particles were

533

found to have non-spherical shapes. We could also find spherical particles, as seen at the bottom

534

right image.

535 536

Figure 3. UV data for AgNPs in VSW (A and B) and in SW (C and D) after being exposed to sunlight,

537

without FA (A and C) and with FA (B and D).

538 -1

539

Figure 4. EELS study of AgNP in 20 mg L fulvic acid after being exposed to light for 10 weeks (a)

540

HAADF-STEM image. Contrast was adjusted to see the shell on the particle. (a) and (b), HAADF-

541

STEM images taken before and after the spectrum image (c) was recorded, respectively. (f) and (g),

542

EELS spectra extracted from the pixels 1 and 2 in (c), respectively. (d) and (e), Ag and O elemental

543

maps, respectively.

544 545

Figure 5. (a) Shows a contrast-enhanced display of a STEM image for a particle from AgNPs + FA,

546

which makes a weak shell visible. (b) shows a sketch of modelled cluster for (a) where a spherical

547

shape of the cluster is assumed. (c) Intensity plotting along the line from the center of the core (P0) to

548

the edge of the shell (Pb) for both the experimental image and the simulations assuming the shell

549

consisting of pure C, AgO and Ag2O, respectively.

550 551 17



Page 18 of 23

552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573

(b) (a) Ag

P2 P1

(c)

574

575

Figure 1. EELS study of the pristine AgNPs. (a) shows a HAADF-STEM image of the nanoparticles,

576

where series of EELS spectra were taken along the green line. (b) and (c) show the spectra extracted

577

from P1 (located at the right end of the line and on the particle) and P2 (near the edge of the

578

nanoparticle on the left), respectively. (b) reveals that no oxidization occurred on the AgNPs and (c)

579

indicates that the carbon grid was clean and no O signal was observed (O K edge: 532 eV).

580 581 582 583 584 585 586 587 588

18


Page 19 of 23


589 590 591 592 593 594 595

25 nm

25 nm

25 nm

25 nm

25 nm

25 nm

596 597 598 599 600 601 602 603 604 605 606 607 608 609 610

137x137nm

130x130nm

611

Figure 2. HAADF-STEM images of AgNPs exposed to sunlight for 25 days. Some particles were

612

found to have non-spherical shapes. We could also find spherical particles, as seen at the bottom

613

right image. 19



Page 20 of 23

A

B

C

D

Figure 3. UV data for AgNPs in VSW (A and B) and in SW (C and D) after being exposed to sunlight, without FA (A and C) and with FA (B and D). 20


Page 21 of 23


(a)

(b)

(c)

(d)

(e)

1! 2!

(f)

(g)

Figure 4. EELS study of AgNP in 20 mg L-1 fulvic acid after being exposed to light for 10 weeks (a) HAADF-STEM image. Contrast was adjusted to see the shell on the particle. (a) and (b), HAADFSTEM images taken before and after the spectrum image (c) was recorded, respectively. (f) and (g), EELS spectra extracted from the pixels 1 and 2 in (c), respectively. (d) and (e), Ag and O elemental maps, respectively.

21



Figure 5. (a) Shows a contrast-enhanced display of a STEM image for a particle from AgNPs + FA, which makes a weak shell visible. (b) shows a sketch of modelled cluster for (a) where a spherical shape of the cluster is assumed. (c) Intensity plotting along the line from the center of the core (P0) to the edge of the shell (Pb) for both the experimental image and the simulations assuming the shell consisting of pure C, AgO and Ag2O, respectively.

22


Page 22 of 23

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Ag-M4,5


O-K

High Resolution STEM-EELS Study of Silver ... - ACS Publications

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