RESEARCH PAPER

Recovery of iron/iron oxide nanoparticles from solution:comparison of methods and their effects

James T. Nurmi • Vaishnavi Sarathy •

Paul G. Tratnyek • Donald R. Baer •

James E. Amonette • Abhi Karkamkar

Received: 23 June 2009 / Accepted: 26 April 2010 / Published online: 15 May 2010

� Springer Science+Business Media B.V. 2010

Abstract Most methods currently being used to

recover Fe0-core/oxide-shell nanoparticles from solu-

tions (including the solvents they are synthesized or

stored in) are potentially problematic because they

may alter the particle composition (e.g., depositing

salts formed from solutes) or leave the particles prone

to transformations during subsequent storage and

handling (e.g., due to residual moisture). In this

study, several methods for recovery of nanoparticles

from aqueous solution were studied to determine how

they affect the structure and reactivity of the recov-

ered materials. Simple washing of the nanoparticles

during vacuum filtration (i.e., ‘‘flash drying’’) can

leave up to *17 wt% residual moisture. Modeling

calculations suggest this moisture is mostly capillary

or matric water held between particles and particle

aggregates, which can be removed by drying for short

periods at relative vapor pressures below 0.9. Flash

drying followed by vacuum drying, all under N2,

leaves no detectable residue from precipitation of

solutes (detectable by X-ray photoelectron spectros-

copy, XPS), no significant changes in overall particle

composition or structure (determined by transmission

electron microscopy, TEM), and negligible residual

moisture (by thermogravimetric analysis, TGA).

While this improved flash-drying protocol may be

the preferred method for recovering nanoparticles for

many purposes, we found that Fe0-core/oxide-shell

nanoparticles still exhibit gradual aging during stor-

age when characterized electrochemically with

voltammetry.

Keywords Recovery � Flash drying �Weight loss � Colloids � Thermogravimetric analysis �Transmission electron microscopy �X-ray diffraction � X-ray photoelectron

spectroscopy � Linear sweep voltammetry

Introduction

Many nanomaterials consist of, or are made from,

freely dispersible (i.e., unattached) nanoparticles

(NPs). During the manufacture use, and/or character-

ization of these NPs, it is often necessary to recover

them from suspension in solvents. The methods

J. T. Nurmi � V. Sarathy � P. G. Tratnyek (&)

Division of Environmental and Biomolecular Systems,

Oregon Health & Science University, 20000 NW Walker

Road, Portland, OR 97006, USA

e-mail: tratnyek@ebs.ogi.edu

D. R. Baer (&)

Environmental Molecular Sciences Laboratory, Pacific

Northwest National Laboratory, Box 999, Richland,

WA 99352, USA

e-mail: don.baer@pnl.gov

J. E. Amonette � A. Karkamkar

Fundamental and Computational Sciences Directorate,

Pacific Northwest National Laboratory, Box 999,

Richland, WA 99352, USA

123

J Nanopart Res (2011) 13:1937–1952

DOI 10.1007/s11051-010-9946-x

currently being used for recovery of NPs are numer-

ous, but—with a few exceptions (e.g., Lienemann

et al. 1998; Perret et al. 1991; Sweeney et al. 2006)—

are not standardized, generally not well validated, and

sometimes not even well documented. These factors

likely contribute to variations in the apparent structure

and properties of NPs (Baer et al. 2008; Burleson et al.

2004; Grainger and Castner 2008), but the scope of

this problem is unclear because very few studies have

made an effort to systematically assess the effects of

NP recovery methods. As a result of these challenges,

methods of NP recovery from solution are among the

issues included in several on-going initiatives to

standardize materials and protocols for use in research

related to NP-based technologies (European Network

on the Health and Environmental Impact of Nanom-

aterials 2008; National Institute of Standards and

Technology 2008).

The degree to which recovery procedures affect

NP structure and properties will vary with the

stability of the material and the property of concern.

Modest (but potentially significant) effects are to be

expected with NPs whose reactivity is largely cata-

lytic—leaving little or no change in the overall NP

composition and structure—as is the case with Ag,

Au, ZnO, ZnS, TiO2, carbon nanotubes, and fuller-

enes (Klabunde 2001; Obare and Meyer 2004; Zhang

et al. 2003). In contrast, larger effects of recovery

procedures are to be expected with NPs that are

substantially transformed in reactions with their

medium. A prominent example of the latter type is

the Fe0-core FeII/III-oxide shell NPs (herein, referred

to as nano-Fe0) that are widely studied and used for

remediation of contaminated groundwater (Li et al.

2006a, b; Rose et al. 2007; Tratnyek and Johnson

2006). In addition to a considerable amount of work

on the reactivity of nano-Fe0 with contaminants (e.g.,

Kim et al. 2008; Liu et al. 2005; Lowry and Johnson

2004; Song and Carraway 2005, 2006, 2008), recent

studies have described changes in the structure and

properties of these NPs due to processes associated

with handling and storage (Baer et al. 2008; Liu and

Lowry 2006; Nurmi et al. 2005; Sarathy et al. 2008).

Also, the dynamic characteristics of these NPs add to

the challenges in studying nano-Fe0 transport, fate,

and toxicity, which together determine what risk

these materials might pose when introduced into the

environment (Wiesner et al. 2006).

In the majority of studies of nano-Fe0, the original

material was a slurry, either because the NPs were

freshly precipitated from solutions of Fe2? or because

the NPs were made into a slurry for stabilization prior

to shipping. The solution phase of the slurries may

contain residual counter anions (e.g., Cl-, SO42-,

B(OH)4-), surfactants, and co-solvents (because of

the way these materials typically are prepared); the

pH may be high and dissolved oxygen low (because

of corrosion by nano-Fe0); and surface layers on the

NPs will include significant quantities of amorphous

and hydrated iron oxide phases. Some of these

characteristics are interrelated, so any procedure for

separating the solid and liquid phases of these slurries

is likely to alter composition and structure of the

recovered NPs. In addition to changes in structure

and composition of individual particles, the bulk

material may be altered during recovery due to

selective recovery, size fractionation, aggregation,

sintering, and cementation.

While the criteria for an optimal recovery process

depend somewhat on the objectives of study, the

potentially important characteristics of the recovery

process usually include: (i) removing residual solutes

to avoid redeposition of salts, (ii) removing solvent

and co-solvents in a manner that minimizes aggrega-

tion, (iii) eliminating non-structural water without

significantly transforming hydrated phases, (iv) min-

imizing erosion of original surface coatings by

dissolution or abrasion, and (v) avoiding reactions

with the medium or its contaminants by excluding

oxygen and other potentially reactive species. The

degree to which these requirements are met by any

particular recovery protocol depends on how the steps

(washing, dialyzing, decanting, filtering, magnetic

retention, centrifuging, passive/evaporative drying,

active drying with vacuum and/or heat, freeze drying,

etc.) are integrated and implemented. Actual prac-

tices to date have been highly variable, and in most

cases it is unlikely that the protocols used would

meet all, or even most, of the five criteria given

above.

In this study, we examine how the methods used to

recover nano-Fe0 from solution influence the struc-

ture and properties of the resulting materials, focus-

ing primarily on the retention of solvent. One

outcome of this study is an improved protocol that

we recommend for recovery of reactive NPs from

1938 J Nanopart Res (2011) 13:1937–1952

123

aqueous solution. We also provide analysis of how

selected properties measured on recovered materials

are affected by the commonly used recovery meth-

ods. Some of the general and specific results obtained

here for nano-Fe0 should be relevant to other types of

NPs.

Experimental

Materials

The nano-Fe0 used in this study (RNIP-10DS,

obtained from Toda Americas Corp., Schaumberg,

IL) is produced by reduction of goethite and hematite

with H2 at 200–600� C (Uegami et al. 2007). Here,

we continue to designate the material as FeH2, to

distinguish it from nano-Fe0 synthesized by other

methods (Baer et al. 2008; Nurmi et al. 2005; Sarathy

et al. 2008). We worked with FeH2 received as a dry

powder (FeH2(D)), which had minimal exposure to

moisture or oxygen during production, and FeH2

received as a wet slurry (FeH2(W)), which we recov-

ered 6 months after receipt using ‘‘flash-drying’’

method 1 (described below). Both FeH2(D) and

flash-dried FeH2(W) were stored under dry, anoxic

conditions. The specific surface areas (as) of these

materials—measured by BET gas adsorption—were

29 ± 2 m2 g-1 for FeH2(D) and 5 ± 2 m2 g-1 for

flash-dried FeH2(W), which agree well with previously

reported values (Nurmi et al. 2005; Reardon et al.

2008). Prior to testing, the dry materials were made

into solutions using deoxygenated/deionized (DO/DI)

water, generally at 1 g/L. The pH of these suspen-

sions varied from 8 to 10, depending on the aging

time.

The solvents (acetone and methanol) used in the

recovery methods described below were all HPLC

grade from Fisher Scientific and were used without

drying or other purification.

Recovery methods

All steps of both flash-drying procedures were

performed in high quality glove boxes containing

anoxic and low-humidity atmospheres. In one case,

recirculation of H2 (5% in N2) in the atmosphere over

a heated Pd catalyst maintained O2 at \1 ppm and a

desiccant filter kept the relative humidity \50%. In

the other system, continuous exchange with pure N2

(from boil-off of liquid N2) gave an atmosphere with

\3 ppm O2, \12–15 ppm H2O, and no H2.

Original flash dry method (FDv1)

A standard vacuum filtration apparatus was assem-

bled with a 0.02-lm PTFE filter (Whatman) and a

vacuum pump capable of producing -20 mm Hg

(Gast Manufacturing, Inc, MI). After pouring the NP

suspension onto the filter, about 30 s was typically

needed to remove the original solvent. Then the

filtrate was washed three times with acetone (or other

hygroscopic solvent), each time using just enough to

completely cover the filtered NPs. After washing, the

vacuum was maintained until the particle layer

appeared dry (typically 5–10 min). At this stage,

most types of nano-Fe0 gave a loose powder that was

readily transferred to a container for storage or

testing. Variations on FDv1 that were tested include:

rinsing with acetone, methanol, and DO/DI water.

New flash dry method (FDv2)

This method is similar to FDv1, except for several

modifications designed to give more controlled and

complete removal of solvent. After the third and final

wash with solvent, the top of the Buchner funnel was

sealed with a silicone stopper, and a vacuum gauge

was used to adjust the pressure inside the funnel to

-20 mm Hg. This condition was maintained for

30 min [because it was determined that this was

sufficient time for removal of most of the residual

solvent (see below)]. Finally, the vacuum was

relieved without disrupting the layer of NPs on the

filter by slowly opening a needle valve on a stainless

steel tube that pierced the silicone stopper. In this

case, the flash-dried powder was usually transferred

to an amber vial and stored (loosely capped) in a

vacuum desiccator containing a 50/50 mix of Drie-

rite� and activated charcoal.

Other recovery methods

The ‘‘passive drying’’ (PD) method used here

consisted of decanting most of the solvent and then

allowing the residual to evaporate under the glove

box atmosphere for 12 or more hours until the sample

J Nanopart Res (2011) 13:1937–1952 1939

123

appeared dry. This method did not include a sample-

washing step.

Thermal analyses

Differential scanning calorimetry (DSC), thermo-

gravimetric analysis (TGA), and mass spectrometry

(MS) were performed on a TG/DSC STA 449

Netzsch instrument equipped with an Aeolos QMS

300 MS by heating the samples under Ar. The MS

uses a standard electron impact ionization detector.

Within 30 min of their recovery, the NPs were

transferred to the thermal analysis laboratory where

they were stored, processed, and loaded in pre-

weighed alumina crucibles in a pure N2 atmosphere

glove box. In a typical procedure, a sample of 10–

20 mg was transferred from a N2 atmosphere glove

box to an alumina sample pan and quickly placed

under Ar stream at a flow rate of 25 mL/min in the

TGA. Experiments were performed starting at 25� C

and ramping the temperature up to 250� C at a rate of

1� C/min. The sampling rate for TGA was 10 data

points per min.

BET measurements

Batch weight-loss measurements were done by

measuring the difference in weight before and after

drying. Drying of the samples was done using the

VacPrep 061 Sample Degas System (Micromeritics).

Typically, 100–200 mg of sample was put in a sealed

BET vial in the glove box and weighed (pre-drying

weight). The sample in the BET vial was then put in

the VacPrep 061 in which the samples were heated

under flowing UHP N2 at 200� C. At selected times,

the samples were allowed to cool and then weighed

(after-drying weight).

X-ray diffraction

X-ray diffraction (XRD) measurements included

conventional ex situ XRD, which was used to

compare NP samples recovered after exposure to

water for various amounts of time. The samples were

loaded onto off-axis quartz substrates in a N2

atmosphere glove box, then protected from exposure

to oxygen during transfer to the XRD by coating the

samples with 5% glycerol (in methanol) and allowing

the excess methanol to evaporate overnight.

Additional details on this method have been reported

previously (Sarathy et al. 2008; Nurmi et al. 2005).

X-ray diffraction data were also obtained for

selected samples in solution (in situ) using a recently

installed Rigaku D/MAX RAPID II microdiffractom-

eter with a curved imaging plate and a rotating Cr

anode operating at 35 kV and 25 mA. For these

measurements, powder samples were loaded in

0.3-mm dia. quartz capillary tubes. Whole pattern

fitting was used for quantitation of the phases.

X-ray photoelectron spectrometry

X-ray photoelectron spectrometry (XPS) measure-

ments were performed using methods, calibration,

and apparatus that we have described previously

(Baer et al. 2007; Nurmi et al. 2005; Sarathy et al.

2008). This study involved the use of Physical

Electronics Quantum 2000 Scanning ESCA Micro-

probe with a focused monochromatic Al Ka X-ray

(1486.7 eV) source and a spherical section analyzer.

The X-ray beam was typically operated near 100 W

power, focused to 100 lm diameter, and rastered

over a 1.4-mm by 0.2-mm rectangle on the sample.

The NP samples for XPS were mounted using

*5 mm2 pieces of double sided Scotch tape (#34-

8505-86354) fixed to the surfaces of clean *1 cm2

Si(100). In order to avoid reaction of the particles

with O2 from air, samples were mounted onto the

tape and transferred into the spectrometer under a N2

atmosphere.

Charging differences in the XPS data were

corrected by aligning the binding energy scales at

the main oxygen peak using 529.8 eV. Reported

binding energies for the O 1s photoelectron peak of

Fe2O3 range from 529.8 to 530.3 (Anderson et al.

1998; Chambers et al. 1998; Moulder et al. 1992),

and the corresponding values for well-characterized

Fe2O3 and Fe3O4 are essentially identical (Chambers

et al. 1998).

Transmission electron microscopy

High-resolution transmission electron microscopy

(TEM) images were collected using a JEOL JEM

2010 operated at 200 kV, as described previously

(Baer et al. 2007; Nurmi et al. 2005; Sarathy et al.

2008). The point-to-point resolution of the micro-

scope is 0.194 nm. All images were digitally

1940 J Nanopart Res (2011) 13:1937–1952

123

recorded using a charge-coupled device (CCD)

camera and were analyzed using Gatan Digital

Micrograph 3.3.1. Dry or flash-dried samples of

nano-Fe0 were mounted onto the carbon-coated TEM

grids inside a recirculated N2 atmosphere glove box

maintained at \0.5 ppm oxygen. The grid was then

mounted in an airtight TEM sample holder (Gatan

UK Model VTST4006-003) while still inside the

glove box to minimize exposure to atmospheric O2.

TEM images were collected from at least five

different locations on the grid.

Electrochemistry

Linear sweep voltamograms (LSVs) were obtained in

three-electrode cells, containing a powder disk elec-

trode (PDE) made with FeH2, a Pt wire counter

electrode, and a Ag/AgCl reference. Details of the

design and electrochemical properties of the PDE

used in this study have been published previously

(Nurmi et al. 2004, 2005; Nurmi and Tratnyek 2008;

Sarathy et al. 2008). All potentials are reported

relative to the Ag/AgCl reference, and currents are

reported in accord with IUPAC convention (anodic

currents are positive and cathodic currents are

negative).

Results

In the study where we first introduced FDv1 (Nurmi

et al. 2005), it was shown with preliminary electro-

chemical data (LSVs obtained using PDEs) that this

recovery method produced nano-Fe0 that was more

stable during storage and gave more consistent

corrosion kinetics than nano-Fe0 recovered by pas-

sive, evaporative drying in an anaerobic glovebox.

However, in a subsequent study of nano-Fe0 aging

using FDv1, we found that rather subtle changes in

NP composition and structure could have significant

impacts on reactivity. Replotting some of the data

from our earlier work (Nurmi et al. 2005) in Fig. 1

reveals that the corrosion potential (Ecorr, the negative

peak in these LSVs) shifts to more anodic values

when the material recovered by FDv1 was stored

under glovebox conditions for significant time peri-

ods (e.g., 20 days) or when the material was recov-

ered by the more gradual process of passive drying

(PD under N2/H2). The observed anodic shift

indicates oxidation, in this case of the aged nano-

Fe0 that comprises the electro-active surface area of

the electrode.

Additional evidence that the method of NP recov-

ery from solution can affect the stability of the

resulting material is available from XRD data.

Analysis of XRD data from our study of FeH2 aging

in solution (Sarathy et al. 2008) showed a gradual

shift in composition from Fe0 to Fe3O4 for freshly

recovered material (by FDv1), but characterization of

similarly aged FeH2 samples that were stored for ca.

2 months between recovery and XRD analysis

showed significantly more oxidation of Fe0 to

Fe3O4 (Fig. 2a). Preliminary data from XRD analysis

of FeH2 in situ (i.e., without recovery) agrees well

with the data obtained when XRD was performed

immediately (several hours) after recovery by FDv1

(Fig. 2a); therefore, supporting the conclusion that

FeH2 recovered by FDv1 is susceptible to significant

changes in composition during storage under anoxic

conditions over several months. In addition to

causing systematic differences between properties

measured on recovered NPs versus unrecovered NPs

(i.e., NPs in situ), the instability of recovered NPs

during storage is likely to contribute to indeterminant

variability between samples. This could be responsi-

ble for the relatively large degree of scatter in oxide

-8.0

-7.5

-7.0

-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

log

|Cur

rent

(A

)|

-0.85 -0.80 -0.75 -0.70 -0.65 -0.60 -0.55

Potential (V) vs. Ag/AgCl

t = 5 min after FDv1 (Fig 3 + S7) t = 24 hrs after FDv1 (Fig S7) t = 20 days after flash-drying (Fig S7) PD under N2/H2 (Fig 3)

Fig. 1 Log absolute current versus potential plot of anodic

polarization curves for stationary powder disk electrodes

packed with flash-dried FeH2(W) in anaerobic aqueous borate

(pH = 8.4). Scan rate = 0.1 mV/s. Legend refers to figures in

(Nurmi et al. 2005)

J Nanopart Res (2011) 13:1937–1952 1941

123

content of samples of FeH2 that were aged in solution

for more than 5 days (Fig. 2b).

Both the electrochemical and XRD results sug-

gested to us that a more extensive characterization of

the stability of flash-dried nano-Fe0 was needed. For

this purpose, there are many additional characteriza-

tion methods that might reveal changes due to—or

influenced by—the process of recovering NPs from

solution (Burleson et al. 2004; Rose et al. 2007; Sun

et al. 2006). Here, we chose to focus on four

approaches: (i) weight-loss measurements to assess

the amount and type of residual solvent retained by

the NPs after recovery, (ii) TEM to look for changes

in NP size and morphology, (iii) XPS to determine

the surface composition chemical composition of the

particles, and (iv) electrochemistry to characterize

reactivity. These methods were applied to character-

ize the two types of nano-Fe0 (FeH2(D) and FeH2(W)),

with and without pretreatment (e.g., exposing FeH2(D)

to H2O), after recovery using the two flash-drying

methods with several variations (e.g., rinsing with

methanol instead of acetone). All of the combinations

of material, treatment, and recovery method for

which results are reported here are summarized in

Table 1.

Each combination of material, treatment, and

recovery method listed in Table 1 is represented by

a label where the starting material is represented by D

and W for FeH2(D) and FeH2(W), respectively; the

exposure step described by 0 or W, for no exposure or

water; the exposure time is represented with m, h, or

d, for min, hours, or days; the recovery method is

represented as 0, 1, or 2, for nothing, FDv1, or FDv2;

and the washing solvent during recovery is repre-

sented by a, m, or w, for acetone, methanol, or water.

For example, D/Wd/2a specifies FeH2(D) that was

exposed to water for 1 day and recovered by FDv2

with acetone.

Weight loss

The weight-loss experiments were of two types:

batch measurements of weight loss under typical

degassing conditions used for sample preparation

prior to surface area analysis by BET gas adsorption

80

60

40

20

0

Oxi

de W

eigh

t %

302520151050

Preexposure Time (days)

D/Wd/0 then stored 2 months

B

80

60

40

20

0

Oxi

de W

eigh

t %

543210Preexposure Time (days)

D/0/1a then stored 2 months D/Wd/1a analyzed immediately D/Wd/1a analyzed immediately, replicate D/Wd/0 ("In situ", i.e., w/o recovery)

A

Fig. 2 Effect of recovery and storage methods on XRD

determined oxidation (aging) after immersion of FeH2(D) in

dilute aqueous solutions for up to 5 days (a) and for up to a

month (b). The measurement is the ratio of iron as oxide

(magnetite) versus iron as Fe0. A sample series processed by

FDv1 and stored for more than 2 months prior to analysis

showed both a faster initial rate of oxidation at shorter time and

a large variability for the longer time measurements

Table 1 Treatments tested

Label Treatment (aging) Recovery method

D/0/0 FeH2(D) None None

D/0/1a FeH2(D) None FDv1 w/Acetone

D/0/1m FeH2(D) None FDv1 w/Methanol

D/Wh/1a FeH2(D) DO/DI 30 min FDv1 w/Acetone

D/Wh/2a FeH2(D) DO/DI 30 min FDv2 w/Acetone

D/Wd/0 FeH2(D) DO/DI 1 day None

D/Wd/0 FeH2(D) DO/DI 1 day PD under N2

D/Wd/1a FeH2(D) DO/DI 1 day FDv1 w/Acetone

D/Wd1-5/1a FeH2(D) DO/DI 1–5 day FDv1 w/Acetone

D/Wd/1m FeH2(D) DO/DI 1 day FDv1 w/Methanol

D/Wd/2a FeH2(D) DO/DI 1 day FDv2 w/Acetone

D/Wd/2m FeH2(D) DO/DI 1 day FDv2 w/Methanol

D/Wd/1w FeH2(D) DO/DI 1 day FDv1 w/Water

D/Wd/2w FeH2(D) DO/DI 1 day FDv2 w/Water

W/0/1a FeH2(W) None FDv1 w/Acetone

W/0/2a FeH2(W) None FDv2 w/Acetone

W/Wh/1a FeH2(W) DO/DI 30 min FDv1 w/Acetone

W/Wh/2a FeH2(W) DO/DI 30 min FDv2 w/Acetone

DO/DI deoxygenated deionized water, FDv1 flash dry method

version 1, FDv2 flash dry method version 2, PD passive drying

1942 J Nanopart Res (2011) 13:1937–1952

123

and continuous measurement of weight loss versus

time and temperature by TGA. Results from the batch

experiments are shown in Figs. 3 and 6 and from

TGA in Figs. 4, 5, and 7.

Using samples of FeH2(D) and FeH2(W) that were

recovered from solution using FDv1, we dried

batches of the samples—for time periods from 0 to

35 min—at 200� C under a constant flow of N2 using

the sample preparation station of the BET instrument.

We observed a weight loss of 15 and 9% using

FeH2(D) that had received no treatment other than

rinsing with methanol or acetone, respectively, during

FDv1 (Fig. 3, D/0/1m and D/0/1a). When acetone

was used to rinse during FDv1 on the FeH2(W) sample

(W/0/1a), approximately 13% weight loss was

observed. The greater weight loss for FeH2(W) versus

FeH2(D) could be because long-term storage of FeH2 in

water leads to transformations of the oxide shell to

phases that incorporate more structural water (that

would not be removed by washing with acetone). In

contrast to the three sets of results for FDv1, the

sample that was recovered with FDv2 showed much

less weight loss (*2%) after drying at 200� C

(Fig. 3, D/Wh/2a), indicating that FDv2 is more

effective at removing residual solvent. However, for

the experiments shown in Fig. 3, heating of the

sample to 200� C was essentially immediate and the

weight loss was complete within a few minutes;

therefore, these data are not conducive to more

detailed interpretation of the drying process.

In order to better characterize these weight-loss

effects, TGA was performed on samples of FeH2

prepared and recovered by various methods (specified

in Table 1). For samples recovered by FDv1, the TGA

data from 25 to 80� C are shown in Fig. 4, along with

the first derivative of each time series. The data for the

samples that were exposed to water (W/0/1a, D/Wh/

1a, and W/Wh/1a) have two prominent features—a

sharp transition at 26–29� C and a broader one

centered around 35–45� C—that are not seen in the

samples that had little or no exposure to water (D/0/0

and D/0/1a). These features represent loss of acetone

and water, respectively, as demonstrated by the MS

data (Fig. 5a, b) for atomic masses 42, 43, and 58

(acetone), and 17 and 18 (water). The amount of

weight loss due to acetone was negligible for FeH2

with no exposure to water prior to flash drying (D/0/

1a), but is substantial and roughly constant (*5%) for

all the materials that were exposed to water (W/0/1a,

D/Wh/1a, and W/Wh/1a). We assume that a primary

role of the solvent wash is to fully or partially replace

water loosely associated with the sample. When water

100

95

90

85

Wei

ght (

%)

403020100

Drying Time (min)

D/0/1a

W/0/1a

D/0/1m

D/Wh/2a

Fig. 3 Sample weights measured after drying for different

periods (at 200� C) for FeH2 obtained by a variety of recovery

methods. Treatment and recovery conditions are given in

Table 1

Fig. 4 Thermogravimetric analysis and the first derivative of the

weight-loss data for samples that were recovered by FDv1.

Treatment and recovery conditions are summarized in Table 1.

Markers are shown on every 3–5 data points. TGA analyses of

replicate preparations of W/Wh/1a yielded the same distinctive

profile and comparatively large overall weight loss (data not shown)

J Nanopart Res (2011) 13:1937–1952 1943

123

is retained within a sample, the removal of the acetone

is less rapid, possibly because dilution with water

increases the boiling point of acetone. Only the three

samples that had been preexposed to water (D/Wh/1a,

W/0/1a, and W/Wh/1a) showed notable water loss

after FDv1 with TGA (Fig. 4). The W/Wh/1a data

show a large (ca. 16%) and broad (over 20�) water loss

peak, suggesting this moisture includes a variety of

types (interfacial, capillary, etc., as defined in the

discussion section) of weakly bound structural water

and/or occluded pore water as well as adsorbed water.

The data in Figs. 4 and 5 indicate that FDv1 can

leave variable but significant residues of water—or

other solvents—on samples of recovered FeH2. After

testing a variety of alternative recovery methods, we

settled on FDv2 as a protocol that preserves the

advantages of flash drying, is more aggressive about

removing residual moisture, and yet is still practical

for a wide variety of materials and applications. The

key difference between FDv1 and FDv2 is a vacuum

drying stage after the washing steps are complete,

which greatly decreases the amount of residual

moisture on the recovered material, as shown by the

weight-loss data in Fig. 6. The experiments repre-

sented in Fig. 6 were performed with FeH2(W),

because—as concluded from Fig. 3—it provided

higher resistance to drying than did the FeH2(D)

sample. The conditions that apply to the first point in

Fig. 6 (t = 0) correspond to the data series for

sample W/0/1a in Fig. 3 and both give about the

same weight loss (17 and 15%, respectively), as

expected. The last point in Fig. 6 corresponds to

holding the sample under -20 mm Hg (*1 bar) for

30 min, which produced a residual moisture level of

2–3% (by weight). This treatment became the

standard protocol for FDv2. Figure 6 also shows that

the 2–3% moisture that is left on the sample after

FDv2 can be removed by storage for 24 h in

desiccator filled with 50:50 anhydrous calcium sul-

fate and activated charcoal.

For comparison with the batch weight-loss data for

FDv2 (Fig. 3, 6), we did TGA on several samples

20

15

10

5

0

% W

eigh

t Los

s

302520151050

Time (min)

Under Desiccant for 24 hr

Fig. 6 Percent weight loss of W/0/1a as a function of drying

time under -20 mm Hg (*1 bar) vacuum (open circles) and

under a desiccant for 24 h (horizontal line)

A B C

Fig. 5 Mass spectral data for three of the TGA runs shown in

Figs. 4 and 7. a FeH2(W) without treatment and recovered by

FDv1 with acetone, b FeH2(D) with 30 min in DO/DI water and

recovery by FDv1 with acetone, c FeH2(D) with 30 min in DO/

DI water and recovery by FDv2 with acetone. Treatment

details in Table 1

1944 J Nanopart Res (2011) 13:1937–1952

123

recovered by FDv2 in order to clarify the nature of

moisture retained under the various treatments. These

data (Fig. 7) again show transitions at 26 and 40� C

although the changes are much smaller than observed

with FDv1. The corresponding mass spectral data

(Fig. 5c) show that FDv2 was particularly effective at

removing residual acetone, as almost all the weight

loss for samples obtained by FDv2 was due to water.

In Fig. 7, the weight loss shown with D/Wh/2a is

greater than for W/Wh/2a, although this is the reverse

of what was expected from the data for FDv1. The

significance this comparison is uncertain, however,

because all of the weight losses obtained for FDv2

are quite small, especially when compared to those

obtained with FDv1 (c.f., Fig. 4 vs. Fig. 7).

Composition and structure

Differences in recovery methods—as exemplified by

the weight-loss data showing variability in quantity of

water and rinsing solvents that are retained by

recovered FeH2—have implications for the interpre-

tation of data on the composition and structure of the

recovered materials. Of particular interest are the

possibilities that (i) incomplete drying facilitates

aging (by oxidation or other solid-state

transformations) of the material during storage, (ii)

incomplete rinsing or concentration of solutes during

evaporation of solvents leads to precipitation of

residues on the particle surfaces, and (iii) incomplete

removal of solvents leaves behind species that can

alter the kinetics and mechanisms of reaction

between the NPs and target solutes (e.g., probe

compounds or model contaminants). In order to

investigate some of these possibilities, TEM, XRD,

and XPS were performed on several types of FeH2,

aged in water for various time periods, and recovered

by flash drying and other methods.

Baseline characterization of FeH2 particle size and

structure is available from TEM data reported in

numerous earlier studies (e.g., Nurmi et al. 2005): the

material consists of irregularly rounded, variably

aggregated, particles with size generally between 30

and 50 nm. Furthermore, TEM data obtained over the

course of our previously reported work (Baer et al.

2008, Fig. 3; Sarathy et al. 2008, Fig. S2) show that

exposing FeH2(D) to water for various time periods

followed with recovery by flash drying (FDv1)

resulted in structural changes in the primary particles

and formation of new secondary phases due to

solution-phase aging processes. Similar but less

pronounced aging effects were observed with FeH2(W)

that had been aged as a concentrated alkaline slurry

and then recovered with FDv1 (Sarathy et al. 2008,

Fig. S3). Additional TEM data obtained for this study

(Fig. 8) are consistent with those earlier results. In

particular, micrographs show no differences in the

apparent composition or structure of the recovered

material due to differences in the method of flash

drying. In contrast, FeH2 recovered by the less

controlled method of passive drying in a glove box

(PD) showed extensive structural changes and accu-

mulation of authigenic precipitates. These results

suggest that both flash-drying methods tested are

sufficient to minimize the first two of the three

possible recovery method effects noted above. How-

ever, possible effect (iii) (i.e., affects of residual

solvent on reactivity) and more subtle manifestations

of (i) and (ii) are not likely to be evident in TEM,

whereas they might be detected by more surface-

selective methods such as XPS.

In preliminary work for this study, XPS was

performed on many samples of FeH2, treated by

various ways, and recovered by FDv1. The data for

the OH and H2O portions of the O 1s photoelectron

Fig. 7 Thermogravimetric analysis and the first derivative of

the weight-loss data for samples that were recovered using

FDv2. Treatment and recovery conditions are summarized in

Table 1. Markers are shown on every 3–5 data points

J Nanopart Res (2011) 13:1937–1952 1945

123

peaks (not shown) revealed significant variability but

no significant trends. These observations further

encouraged the development of FDv2, which pro-

duced more consistent results by XPS.

Figure 9 shows XPS data for the Fe 2p peaks and

the C 1s and O 1s photoelectron peaks obtained with

FeH2(D) exposed to DO/DI water for various time

periods, and recovered several ways. In the Fe 2p

region (Fig. 9a), there is little variation between the

treated samples, all of which show notable broaden-

ing of the main iron oxide peak at *710.7 eV

relative to that for the D/0/0 sample. This broadening

mainly occurs on the lower binding energy side of the

peak and reflects increased Fe(II) content in the oxide

film on the particles due to autooxidation of Fe0

during exposure to water (Sarathy et al. 2008). The

data also suggest a small shoulder at *706.7 eV,

which is consistent with the influence of Fe0 in the

core of these particles, as we have discussed previ-

ously (Nurmi et al. 2005; Sarathy et al. 2008). In the

C 1s region (Fig. 9b), there does not appear to be a

systematic trend in C=O (peak at *289 eV) or C–O

(shoulder at *286.5 eV) relative to C–H and C–C

(peak at *0.8 eV). This contrasts with what we

Passive Dried (PD) FDv1 FDv2 0 hr

0.5 hr

24 hr

Fig. 8 TEM images of FeH2(D) before (0 h) and after 0.5- and

24-h exposure to DO/DI water, followed by recovery with

passive drying in an N2/H2 glove box, with FDv1 or with

FDv2. Because the passive drying process involved no solvent

exposure, the 0-h PD sample is that of the initial, as received

material

1946 J Nanopart Res (2011) 13:1937–1952

123

observed previously when natural organic matter is

adsorbed to FeH2(D) NPs (Johnson et al. 2009). In this

case, we conclude that the variation in the C 1s data is

negligible, and that no significant residue of organic

solvent (acetone or methanol) from flash-drying

survives the sample handling and preparation for

XPS. In the O 1s region (Fig. 9b), however, a

significant effect of the recovery procedure is appar-

ent: both samples obtained by flash drying without

the final steps of rinsing with methanol or acetone

show much greater intensity around 532 eV. This

suggests the oxide shell on these samples retains

some adsorbed H2O and perhaps contains a high

portion of relatively hydrous iron oxides like FeOOH

(*533 and *531.8–532.1 eV; Grosvenor et al.

2004) relative to Fe2O3 (*530.2 eV; Chambers

et al. 1998).

Electrochemistry

At the outset of this study, it was shown that FeH2

recovered by FDv1 underwent substantial changes in

electrochemical properties during storage under con-

ditions typical of N2/H2 glove box (Fig. 1). Since

FDv2 is more effective at removing solvent from

FeH2 recovered from solution (Figs. 3, 4, 5, 6, 7), and

the resulting material is stable with respect to changes

that might be detected by TEM, XPS, or XRD

(Figs. 3, 7, 9), increased stability vis-a-vis electro-

chemical properties was also expected. Figure 10

shows LSVs for D/W/2 as a function of time the

sample was stored in a sealed vessel. The data show

that Ecorr shifts anodically with time, but at a much

smaller rate (2.6 X) than for samples obtained by

FDv1 (Fig. 1). The inset shows that the change in

Ecorr was not entirely linear, but further investigation

of the kinetics of this process was not performed.

Discussion

A fundamental issue that arises from the measure-

ments reported in this article concerns the magnitude

and source of the weight losses reported during the

drying of material that had been flash dried. In

principle, the varying amounts of solvent associated

with the NPs studied here might include ‘‘structural’’

A

B

C

Fig. 9 Comparison of XPS data for FeH2(D) after various

exposure and recovery regimes. a Fe 2p region, b C 1s

photoelectron region, and c O 1s photoelectron region. Binding

energies were all aligned to the main oxygen peak at 529.8 eV.

Details for each treatment are given in Table 1

J Nanopart Res (2011) 13:1937–1952 1947

123

water (water chemically bonded to the nanoparticle

surfaces or incorporated into the oxide shell), but as

this is not likely to be significantly affected by our

treatments or analyses (transitions among iron oxide

phases due to changes in structural water are typically

observed at temperatures [100� C; Cao et al. 1995;

Chin and Yaacob 2006), we will focus solely on

solvent associated in less energetic ways with the

surface of the NPs. We classify this surface-associ-

ated solvent into three categories: (i) interfacial

(retained by molecular-scale forces and located

within two atomic layers of the solid surface); (ii)

capillary (retained by surface tension in pores and

other void spaces at matric potentials of -0.05 bar or

more negative); and (iii) bulk (retained very loosely

at matric potentials more positive than -0.05 bar).

The potential significance of interfacial, capillary,

and bulk water is addressed by the calculations

below.

The solvent content (hgrav), here defined as the

ratio of the mass of retained fluid (water and other

solvents) to the dry mass of the particles, can be

calculated as a function of spherical-particle packing

efficiency using the densities of the particles and the

solvent. In order to determine mean particle density,

we analyzed the untreated FeH2(D) NPs by XRD

(n = 8), from which a mean Fe3O4 content of

16.6 wt%, and a corresponding mean particle density

of 7.42 g cm-3 were estimated. The relationships

between particle packing efficiency and maximum

hgrav, which assumes no gas-filled porosity, are

shown in Fig. 11 for cases where the retained

solvents are water or acetone. For reference, we have

annoted the graph with lines showing the measured

packing efficiency for the dry untreated FeH2(D) NPs

(0.14) and for the calculated maximum packing

efficiencies associated with loose random packing

(0.60; Bordia 1984) and crystalline close packing

(0.74; Bordia 1984) of monodisperse spheres. The

very low packing efficiency (0.14) observed for the

dry untreated FeH2(D) NPs would be expected for

extremely large particle aspect ratios (e.g., Jia et al.

2007, Fig. 3a), which are not seen in the TEM images

we collected (Fig. 8). This result suggests that some

sort of magnetic ordering might be occurring in the

dry powder to prevent the NPs from settling into a

random packing configuration. Possible evidence for

magnetic ordering can be seen in Fig. 8, where only

clusters or aggregates of primary particles are

observed.

The maximum weight losses we observed (e.g.,

Figs. 3, 4) are equivalent to hgrav values of about 0.22

for FeH2(W) and 0.10 for FeH2(D). These values of hgrav

are greater than the maximum values calculated for

-9.0

-8.5

-8.0

-7.5

-7.0

-6.5

-6.0

-5.5

-5.0

log

|Cur

rent

(A

)|

-0.85 -0.80 -0.75 -0.70 -0.65 -0.60 -0.55

Potential (V) vs. Ag/AgCl

t = 0 day t = 0.5 t = 1 t = 10 t = 30

-0.772

-0.770

-0.768

-0.766

-0.764

-0.762

Eco

rr (

V)

3020100

Time (days)

Fig. 10 Anodic polarization curves obtained with FeH2(D)

recovered by FDv2 after 30 min exposure to DO/DI water (D/

Wh/2a) and then stored from 0 to 30 days in sealed evacuated

bottles in an anoxic glove box at 50% RH. Obtained by linear

sweep voltammetry at 0.1 mV/s using powder disk electrodes

immersed in deoxygenated aqueous borate (pH = 8.4)

Fig. 11 Calculated solvent content (hgrav) for FeH2(D) NPs

over the range of possible packing efficiencies

1948 J Nanopart Res (2011) 13:1937–1952

123

the loose random packing limits with acetone and

water solvents (0.07 and 0.09, respectively), but

obviously could be as low as zero if no solvent was

retained. More importantly, they are substantially less

than the maximum possible values of 0.65–0.83

calculated for the untreated FeH2(D) indicating that a

substantial fraction of the solvent is removed by the

flash-drying treatments, as expected, but that ener-

getic or kinetic factors could be preventing complete

removal. To help resolve this, we will focus the next

portion of the discussion on the energetic factors

responsible for solvent retention.

Of the three categories of surface-associated

solvent, the interfacial solvent is held the most tightly

and therefore would be the least easily removed by

flash drying. In order to estimate the maximum

portion of the solvent that can be considered inter-

facial, we first calculated the relationship between

spherical particle radius and mass of solvent held in

the first two atomic layers (two Langmuirs) assuming

the bulk molar volumes for each solvent were

approximately correct for the interfacial region.

Because the primary NPs that comprise FeH2(D) are

not smooth spheres, we estimated an effective

spherical particle radius, using the surface area

reported in (Nurmi et al. 2005) and the mean particle

density to calculate a volume-normalized geometric

surface area that could be compared with one

calculated directly from spherical particle geometry.

We obtained an effective radius of about 14 nm,

which by comparison with the mean particle radius of

22 nm reported by (Nurmi et al. 2005) yields a

surface roughness factor of 1.58 for the FeH2(D) NPs.

The general relationship between effective NP radius

and interfacial solvent content (Fig. 12) shows the

expected exponential decrease with increasing radius

and a slightly larger mass fraction for acetone relative

to water or methanol. Our estimates of the maximum

interfacial solvent content for FeH2(D) using the

effective radius of 14 nm range from 0.018 to

0.022, which is a small, but not insignificant, portion

of the solvent retained after the FDv1 treatment and is

comparable to the solvent contents measured after the

FDv2 treatment (e.g., Fig. 3, 6, 7).

It seems, therefore, that most of the solvent

retained after the FDv1 treatment is held by capillary

forces, as the bulk solvent should be easily removed

by normal vacuum filtration. The energy with which

capillary solvent is retained (under saturation vapor

pressures) can be calculated as matric potential

(wmatric) using the Young–LaPlace equation and has

units of negative pressure. It is inversely proportional

to the surface tension of the solvent and directly

proportional to the radius of the capillary. One might

expect, therefore, that as particle size decreases—and

with it the dimensions of capillary spaces between the

particles—that wmatric would become more negative.

Indeed, a significant impact of capillary forces on

enhancing solution retention is not a major surprise

and has been observed for nanoporous materials

(Gaspar et al. 2005).

The relation between capillary radius and wmatric is

shown in Fig. 13 for several solvents. Also shown is

the typical vacuum applied by laboratory vacuum

pumps (0.8–1 bar). From the figure, it is clear that for

removal of water under saturated vapor pressure

conditions (i.e., 100% relative humidity) the typical

laboratory vacuum pump cannot access capillary

pores below about 2000 nm in radius. This barrier is

lowered to about 500 nm by exchange of other

solvents for water, but even this level is only part way

into the NP regime, where capillaries having radii on

the order of tens of nm may dominate. This situation

is made even more challenging by the loss of

connectivity between pores as the larger pores drain,

which prevents further increases in vacuum from

removing solvent by the forces of surface tension.

Removal of solvent by vacuum filtration thus needs a

way around the loss of connectivity and the eventual

0.06

0.05

0.04

0.03

0.02

0.01

0.00

Inte

rfac

ial S

olve

nt (

mas

s fr

actio

n)

5040302010

Effective Particle Radius (nm)

100

80

60

40

20

0

Effective S

SA

(m2/g)

RNIP

Acetone (mass fraction) Methanol (mass fraction) Water (mass fraction) Effective Surface Area

Fig. 12 Estimated values for interfacial solvent content for the

FeH2(D) particles assuming two Langmuirs of adsorbed solvent

J Nanopart Res (2011) 13:1937–1952 1949

123

500–1500 bar energetic barrier exerted by capillary

forces in the NP regime.

Fortunately, the connectivity and energy barriers

to solvent removal can be diminished by decreasing

the vapor pressure of the atmosphere in the vicinity of

the capillaries. The calculated impact of vapor

pressure ratio on wmatric for water, the most tenacious

solvent, is shown in Fig. 14. A decrease in vapor

pressure ratio to about 33% of saturation is sufficient

to offset a wmatric of -1500 bar induced by capillary

forces. With lower vapor pressure ratios, a strong

driving force is created for removal of water by

evaporative processes. For example, the water vapor

pressure ratio maintained by CaSO4, the active

ingredient in Drierite� a common desiccant, is about

2.3 9 10-4, well below the 0.33 threshold. The

comparable radii of capillaries that can be emptied at

each vapor pressure ratio are also plotted in Fig. 14.

These confirm that nanometer-sized pores can be

emptied when vapor pressure ratios below the 0.33

threshold are used.

In summary, our analysis shows that because the

energetics of solvent retention by capillary forces are

much higher for NPs than for micron-sized powders,

the driving force required to remove the solvent at a

comparable rate is higher. The FDv1 approach did

not fully appreciate the importance of this factor. The

FDv2 modification, however, expressly considers the

kinetics and energetics of the process. Our calcula-

tions suggest that for best results, the relative

humidity in glove boxes should be maintained at

33% relative humidity or less, and sufficient vacuum

Fig. 13 Relation between solvent matric potential and capil-

lary radius for water and several organic solvents at saturated

vapor pressure. The matric suction exerted by typical

laboratory diaphragm vacuum pumps is also shown. Solvent

held at matric potentials more negative than this level of

suction will not be removed by pumping under these conditions

Fig. 14 Calculated

decrease in water matric

potential achieved by

application of vapor

pressure ratios below

saturation during vacuum

filtration. The effective

capillary radius for each

vapor pressure ratio is also

shown. Capillaries having

radii larger than this value

will release their water to

the atmosphere given

sufficient time

1950 J Nanopart Res (2011) 13:1937–1952

123

filtration time (30–60 min) must be allowed, to

completely dry the NPs. Replacing water in the NP

sample with a higher vapor pressure solvent such as

acetone before drying speeds the process by increas-

ing wmatric. And, storage of samples over a desiccant

is essential to removing vestigial water and main-

taining the surfaces of the NPs in a completely dry

state, if further reaction is to be avoided. Finally,

verification of the moisture content of any nominally

dry sample is essential in order to avoid artifacts in

subsequent reactivity studies.

Conclusions

The recovery of reactive nanoparticles by washing

during vacuum filtration under anoxic conditions

(FDv1) has various advantages over more commonly

employed methods such as freeze drying or decanting

and evaporation under an anoxic glove box atmo-

sphere. Most conspicuously, FDv1 avoids most of the

deposition of salts, etching, and cementation that is

visible in TEMs of nano-Fe0 recovered by evapora-

tion; however, further investigation revealed addi-

tional, more subtle effects such as shifts in redox state

(by voltammetry) and altered rates of NP aging (by

XRD). Furthermore, thermal analysis showed that

FDv1 left as much as 17 wt% residual solvent

associated with the recovered material, and modeling

calculations show that most of this moisture must be

capillary or matric water, which provides a medium

for continued aging and other reactions. A further

consequence of aging and related transformations of

NPs during storage is variability in the results of

subsequent characterizations (e.g., reactivity with

contaminants).

The residual moisture that is left behind by FDv1

was largely eliminated by a refined recovery proce-

dure involving flash drying followed by vacuum

drying (FDv2). Fe0 NPs recovered by FDv2 were

found to be essentially free of secondary precipitates,

erosion or etching, and surface oxidation. Immedi-

ately after vacuum drying, they retained \3 wt%

water, and this residual was completely removed by

storage overnight in an anoxic desiccator. By a range

of characterization methods, Fe0 NPs recovered with

FDv2 appear to be stable and reproducible, although

gradual aging (by autooxidation) was still detectable

electrochemically. For most purposes, FDv2 would

be a considerable improvement over the recovery

protocols that are most commonly used. For some

purposes, FDv2 might not be ideal, or even adequate,

but alternative methods should be evaluated critical,

as we have done here. The approach and general

considerations used here to evaluate FDv1 versus

FDv2 may provide guidance for evaluation of other

recovery methods for nanoparticles from solution.

Acknowledgments We acknowledge and thank C.-M. Wang,

P. Nachimuthu, M.H. Engelhard, J. Kwak, and C.K. Russell for

their assistance with TEM, XRD, XPS, and BET measurements

and sample preparation. Samples of nano-Fe0 were donated by

the Toda Kogyo Corp. We would also like to thank students

Abram J. Ledbetter and Jharana Dhal who conducted some

exploratory work on particle recovery during a Nanotechnology

course hosted by the Pacific Northwest National Laboratory

(PNNL) and the William R. Wiley Environmental Molecular

Sciences Laboratory (EMSL). This work was supported by the

U.S. Department of Energy (DOE) Division of Chemical

Sciences, Geosciences, and Biosciences. Parts of the work were

conducted at the EMSL, which is located at PNNL. EMSL is a

DOE User Facility operated by Battelle for the DOE Office of

Biological and Environmental Research. PNNL is operated for

the DOE under Contract DE-AC06-76RLO 1830.

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