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View Article OnlineView Journal | View Issue

Structures of Pd–

State Key Laboratory for Pollution Contro

Engineering, Tongji University, 1239 Siping

zhangwx@tongji.edu.cn

† Electronic supplementary informationTable S1 and Fig. S1–S5. See DOI: 10.1039

Cite this: RSC Adv., 2014, 4, 33861

Received 9th May 2014Accepted 23rd July 2014

DOI: 10.1039/c4ra04311a

www.rsc.org/advances

This journal is © The Royal Society of C

Fe(0) bimetallic nanoparticlesnear 0.1 nm resolution†

Lan Ling and Wei-Xian Zhang*

Palladized nanoscale zero-valent iron (Pd-nZVI) has attracted signifi-

cant attention due to its enhanced capability to transform halogenated

hydrocarbons, which are among the most frequently detected

contaminants in groundwater. In this study, the structural evolution of

Pd-nZVI (1.0% Pd) was examined using spherical aberration-corrected

scanning transmission electronmicroscopy (Cs-STEM) in combination

with X-ray energy dispersive spectroscopy (EDS). This method with

sub-nano spatial resolution was applied to map the location and

distribution of Pdon the nZVI surface, and generated themost detailed

images of Pd-nZVI. Results show that fresh Pd-nZVI consists of a

metallic iron core and a thin oxide shell, with Pd scattered as discrete

clusters of 2–4 nm particles on the nZVI surface. High-magnification

images further confirm that the Pd nanoparticles have a highly

ordered core enclosed by a disordered outer layer. With the progress

of iron oxidation, the Pd clusters disappear from the surface while new

deposits of iron oxides with varied morphologies appear. The bright

cores in the HAADF images, which correspond to metallic iron,

become smaller while the outer shell expands with time. After aging in

water for 120 h, Pd-nZVI particles are depleted with metallic iron,

totally oxidized and recrystallized. A physical model of Pd-nZVI is

presented.

Doping noble metals such as Pd, Pt, Ni, Ag or Cu has beenknown to enhance the reactivity of nanoscale zero-valent iron(nZVI) for degradation of halogenated hydrocarbons.1–3 Amongthese transition metals, Pd-doped nZVI (Pd-nZVI) is known toexhibit the highest reactivity.1–7 Studies have shown that Pd-nZVI is particularly effective for dechlorination of PCBs, chlo-rinated benzenes and chlorinated aliphatic compounds (TableS1, ESI†). It has been successfully applied in both in situ and exsitu applications (Table S1†).3–19 Palladium metal, a well-knownheterogeneous catalyst for dehalogenation and hydrogenation

l, School of Environmental Science and

Road, Shanghai, 200092, China. E-mail:

(ESI) available: Experimental details,/c4ra04311a

hemistry 2014

is generally assumed to catalyze the activation of halogenatedhydrocarbon compounds, formation and activation ofhydrogen.3,4 A major advantage of Pd-nZVI bimetallic particlesis less toxic by-products than those associated with mono-metallic iron,1–3 such as the formation of much less chlor-oethene (C2H3Cl) in the degradation of trichloroethene(C2HCl3).

While signicant (>30%) reduction of reaction activationenergies of the dechlorination reactions has been observed,3–8

studies also showed that Pd-nZVI in water exhibited rapiddecrease in reactivity. For example, an 80% reduction of reac-tion rate was observed during the dechlorination of TCE aer areaction time of just 24 h and the Pd-nZVI was almostcompletely deactivated aer 120 h.7 Considering that watermolecules are present in a far greater number compared tothose of contaminants, the interactions between the Pd–Feparticles and water (i.e., aging), to a certain extent, determinethe longevity of nZVI and its long-term effectiveness forcontaminant remediation. An improved understanding on theeffects of the aging mechanisms can provide insights into thecatalytic activity of bimetallic nanoparticles in environmentalapplications and also its transport and fate in the environment.

Based on the redox chemistry of Fe(0) and Pd(II), Pd(II) can bequickly reduced by metallic iron and deposit on the surface ofnZVI. A fast conversion of Pd(II) to Pd(0) was observed as thebrown palladium solution quickly turned colorless during thecourse of nZVI reactions with palladium acetate. Previousstudies suggested that rapid loss in reactivity of Pd-nZVI parti-cles is owing to the passivation or poisoning of the Pd surfaces.7

Nevertheless, direct and denite evidence on the location anddistribution of palladium has yet been published. A methodwith high spatial resolution and chemical sensitivity is thusdesired, a technical challenge until the recent development ofatomic-resolution spherical aberration-corrected scanningtransmission electron microscopy (Cs-STEM).20–22

In this work, a state-of-the-art Cs-STEM integrated withhigh-sensitivity X-ray energy dispersive spectroscopy (EDS) isapplied for high-resolution structural and morphological

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characterizations and elemental mapping of nZVI and Pd-nZVI,and the results were correlated with the analysis of X-raydiffraction (XRD). In addition to the direct visualization ofnanoscale structural and morphological changes of individualnZVI particles both before and aer palladization and theevolution of Pd-nZVI induced by aging in water, super-sensitiveEDS permits the elemental distributions and translocations tobe mapped with near atomic-scale precision. Results obtainedcan deepen our understanding on the reactivity and stability ofbimetallic Pd–Fe nanoparticles. The techniques explored in thiswork may also be extended to the investigations of otherchemical reactions in nanomaterials.

Fig. 1 presents STEM images of nZVI and Pd-nZVI. nZVIparticles are spherical with sizes in the range of 20–100 nm. Thesecondary electron (SE) images (Fig. 1a and d) present well-dened, three-dimensional appearances with steep surfacesand edges being brighter than at surfaces.20 An image resolu-tion of less than 0.15 nm can be achieved in the SE imaging withthe Hitachi HD-2700 system.20,21 While fresh nZVI (Fig. 1a) hasvery smooth surface, groups or lumps of much smaller particles(2–4 nm) are found on the surfaces of Pd-nZVI particles. Asattested in the following sections with additional experimentalevidence, the smaller units are identied as Pd clusters.

The core–shell structure of nZVI is clearly observed from thebright eld (BF) images (Fig. 1b and e). The shell layer appearsto be thin (2–4 nm) and more transparent than particle corearea. High-angle annular dark eld (HAADF) images of freshly

Fig. 1 STEM images of fresh nZVI (a–c) and Pd-nZVI (d–f) nanoparticles. ((f) high-angle annular dark field (HAADF) images.

33862 | RSC Adv., 2014, 4, 33861–33865

prepared nZVI and Pd-nZVI are showed in Fig. 1c and f,respectively. The images obtained from a HAADF detector arebased on the mass-thickness contrast arising from the inco-herent elastic scattering (i.e., Rutherford scattering) of elec-trons.20,23 The high-angle, incoherently Rutherford-scatteredelectrons are highly sensitive to variations in the atomicnumbers of the atoms in the test sample (i.e., the Z-contrast)and are minimally dependent on specimen thickness andmicroscope defocus. In practice, HAADF image can be used toeffectively differentiate the compositions of materials based onthe atomic weight. In this work, Pd and Fe atoms appearbrighter than O in a HAADF image.

Fresh nZVI particles consist of a bright core, correspondingto metallic iron [Fe(0)], which is surrounded by a dark shell oflower intensity (a mixture of Fe and O) (Fig. 1c). Freshlyprepared Pd-nZVI particles retain the core–shell structure(Fig. 1f). However, discrete and brighter (even brighter than thenZVI core) kernels spread over the surfaces of the nZVI particles(Fig. 2d–f). From the STEM observations, doping Pd on nZVIdoes not have a profound impact on the basic core-shell struc-ture and size of the nZVI particles, nonetheless, it does makethe nanoparticle surface rougher. The BF images (Fig. 1b and e)provide complementary views of the nanoparticles and furtherconrm the basic core–shell structure of nZVI. More images ofthe freshly prepared nZVI and Pd-nZVI particles are included inFig. S1.†

a) and (d) are secondary electron (SE), (b) and (e) bright-field (BF), (c) and

This journal is © The Royal Society of Chemistry 2014

Fig. 2 High magnification STEM images of Pd-nZVI nanoparticles. (a) and (d) are SE, (b) and (e) BF, (c) and (f) HAADF images.

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Higher-magnication images of the Pd-nZVI particles arepresented in Fig. 2. These smaller particles undoubtedly coa-lesce to form groups or clusters. The images also show thatthese smaller particles have a texture that is different from thatof the pure nZVI particles. The smaller particles with diameterson the order of 2–4 nm comprise a highly ordered inner core,the lattice structure of pure metallic palladium (Fig. 2e). Thedisordered surface layer is likely a result of highly curvedsurface and nature of very small nanoparticles.20,22

Detailed structures and morphologies of the nZVI andPd-nZVI particles can be interpreted more fully in conjunctionwith their corresponding EDS-STEM-based elemental mapping.The mapping of Fe, O, and Pd (Fig. 3a–c) and the correspondingoverlays of the elements (Fig. 3d–f) provide unambiguousevidence on the spatial distribution of each element in the Pd-nZVI particles. The Fe (Fig. 3a), O (Fig. 3b) footprints and theiroverlay (Fig. 3d), clearly indicate that O is concentrated in theouter layer while the Fe intensity is high in the core area and lowin the shell layer, proving again that the Pd-nZVI particlespreserve the core–shell structure of the nZVI particles. The Pdmapping and the corresponding SE, BF and HAADF imagesshow that palladium is deposited randomly on the surface,suggesting the lack of specic chemical bonding between Feand Pd. For dechlorination reactions, the much smaller Pdclusters on the surfaces of the iron nanoparticles provide largesurface area and active sites for electron transfer, hydrogen

This journal is © The Royal Society of Chemistry 2014

formation and activation as well as the subsequent formation ofdissociated hydrogen species.7

SE images (Fig. S2a–S2c†) and the corresponding HAADFimages (Fig. S2d–S2f†) of typical spent (aged for 24 h) Pd-nZVIparticles show that the brighter Pd clusters disappeared fromthe surfaces of the spent Pd-nZVI particles. Compared to thefreshly prepared Pd-nZVI particles, the spent Pd-nZVI exhibitssome distinctive morphologic features such as ower-like andcoral-shaped ones (Fig. S2b, S2c and S2e, S2f†). EDS mapping(Fig. S3†) and the results of XRD analysis (Fig. S4†) show thatthe new materials are iron oxides/hydroxides. A few spentPd-nZVI particles still retain their original core–shell structureand possess the decreased bright metallic iron cores and thickershells of lower intensity in the HAADF images (Fig. S2d†). Thesurfaces of the particles become rougher and the particlespresent a denser and more compact structure. Palladium isthen unevenly distributed among these iron oxide/hydroxideparticles (Fig. S3d†). Aer being aged for 120 h, the Pd-nZVIparticles exhibit signicant oxidization (Fig. S5†). Flake- andspicule-like crystals, which are iron (hydro) oxides, such asFe3O4 (magnetite), Fe2O3 (maghemite), goethite (a-FeOOH), andlepidocrocite (g-FeOOH) or a mixture of them, are formed;24,25

this is conrmed by EDS mapping (Fig. S3†) and XRD analysis(Fig. S4†). It has been previously reported that Pd-nZVI is almostcompletely deactivated aer 120 h reactions.7

The above results suggest further the oxidization andrecrystallization of the metallic iron, and disappearance of

RSC Adv., 2014, 4, 33861–33865 | 33863

Fig. 3 STEM-EDS-based elemental maps of (a) Fe, (b) O, (c) Pd and (d–f) the corresponding overlays showing the spatial distributions of theelements (Fe (blue), Pd (green), and O(red)). The signals were collected from freshly Pd-nZVI (shown in Fig. 1d).

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palladium from the particle surface. As a result, the agednanoparticles undergo substantial restructuring, with theremoval or rearrangement of the defects in their crystal struc-ture. This change in structure drastically decreases the reactivityof the particles with respect to hydrodechlorination as observedin numerous studies.7,17,25

Additional analysis was also done with XRD. The XRDpatterns of nZVI and spent Pd-nZVI are shown in Fig. S4.† Fromthe fresh Pd-nZVI sample, peaks corresponding to body-centered cubic a-Fe(0), especially the strong peak at 44.49�

ascribable to the (110) plane are observed. The weak peaks at35.59� and 62.93� are indicative of small amounts of magnetite/maghemite in the shell. The formation of iron oxides in thespent Pd-nZVI sample is clearly obtained. Pd is not identiedboth in the fresh and aged Pd-nZVI likely because of its insig-nicant weight fraction in the samples. As reported previously,peaks corresponding to metallic iron disappeared in spent

Fig. 4 Physical model of the palladization of nZVI, its use in dehalogena

33864 | RSC Adv., 2014, 4, 33861–33865

samples. Strong peaks at 35.29�/35.23�, 29.88�, and 62.47� andweak peaks at 42.98� and 56.90�, attributable to Fe3O4

(magnetite) or Fe2O3 (maghemite) or a mixture of the two areidentied.16 Moreover, all the peaks of iron oxides in 120 houraged sample are stronger and sharper than their counterparts infresh nZVI and 24 hour aged one. This is consistent with theiron corrosion observed in the high-resolution STEM images(Fig. 3, S3and S5†).

A conceptual model on the Pd-nZVI nanoparticles is pre-sented in Fig. 4, which highlights the experimental resultsdiscussed above. Fresh nZVI particles are spherical, havesmooth surface and the characteristic core–shell structure. Aerreacting with palladium(II) acetate, the palladium cations arequickly reduced to metallic Pd nanoparticles, which coalesceand form discrete clusters of 2–4 nm in size on the surfaces ofthe nZVI. Palladium then acts as the catalyst and provides activesurfaces for hydrogen formation and activation, and

tion, and its aging/deactivation.

This journal is © The Royal Society of Chemistry 2014

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subsequent formation of dissociated, reactive hydrogen species.The dissolution of water by Fe(0) leads to the hydrogen evolu-tion and is followed with the formation of atomic hydrogen atthe palladium surface. Atomic hydrogen then reduces chlori-nated hydrocarbons such as PCE through a surface-mediatedhydrodechlorination process.26 Palladium may also acceleratethe aging or oxidation of nZVI through the galvanic reactionsbetween iron and palladium and promotes water reduction andhydrogen generation, owing to its catalytic nature. Further,nZVI particles are continuously oxidized and recrystallized toform various forms of iron oxides with the Pd nanoparticlesburied by the oxide deposits. Future work on the design ofbimetallic nZVI systems should focus on improving the selec-tivity of the catalytic properties of Pd with respect to dechlori-nation and on slowing the aging of nZVI through structural andcompositional adjustments. Cs-STEM combined with advancedimaging and spectroscopy functions appears to be an effectivetechnique to directly probe the atomic-scale structural andchemical information of very specic catalytically active sitesand to identify structure–function relationship existing inheterogeneous environmental systems.

Acknowledgements

Financial support from the National Natural Science Founda-tion of China (NSFC Grants 21277102 and 21307094) and theFundamental Research Funds for the Central Universities(0400219189) are gratefully acknowledged. The authors thankHitachi High-Technologies Corporation for the use of HD-2700STEM at its Mito (Japan) laboratory.

Notes and references

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