# Successful Oxidation Induced Doping Of Nanoparticles Revealed By In Situ XAS

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The ﬁrst real-time in situ XANES/EXAFS study of the formation kinetics of Mo-doped and undoped fe oxide NPs.

Doping has led to a revolutionary breakthrough in the design of semicondutor devices. Yet, it is still challenging to introduce dopants in nanoparticles (NPs) in order to access properties unavailable in the undoped materials. Diﬀerent types of interactions between host and dopant are responsible for the properties of doped NPs. The new properties in doped NPs may originate from spin exchange interactions between the dopant and host, presence of long-lived highly isolated electronic states, or simply from high concentration of structural defects.

Recently, it has been demonstrated that doping of bulk $\rm \gamma\!-\!Fe_2 O_3$, traditionally considered as an anode material in lithium-ion batteries, with high-valent metal cations such as $\rm Mo^{6+}$ results in more than ﬁve-times increase in Li+ ion capacity in the cathode range from 4.1 to 2.0 V. Charge compensation in $\rm Mo^{6+}$doped $\rm \gamma\!-\!Fe_2 O_3$ assumes the formation of an extra cation vacancy per each $\rm Mo^{6+}$ in iron oxide matrix. As the concentration of cation vacancy increases, metal oxide can accommodate insertion of more Li+ions leading to higher capacities in the cathode range that tunes $\rm \gamma\!-\!Fe_2 O_3$ into cathode material.

Despite the growing importance of the doped NPs for a broad range of applications, their synthesis is facing challenges since doping NPs is in general more diﬃcult than their bulk counterpart. As a result, diﬀerent synthetic strategies based on control over reaction kinetics via adjustment of the reactivity of the precursors, reaction temperature, and diﬀusion have been proposed:

• When the dopant and host precursors have similar reactivity, the dopant atoms can participate in the nucleation and growth of the host material and direct in situ doping is possible.
• When doping of NPs is achieved by trapping of the dopant atoms at the surface of nucleated core, the low reaction temperature is found to prevent the dopant from diﬀusing out of the NPs during their further growth.
• Also, doping in NPs can occur as a result of inward diﬀusion of the dopant from the solution into the lattice of undoped NPs followed by cation exchange reactions between the dopant and host. This method exploits the fact that the ionic diﬀusion rate in NPs is higher than that in bulk.

There is a substantial progress in synthesis of doped NPs but our understanding of the doping mechanisms is still based on ex situ techniques including optical spectroscopy, electron paramagnetic resonance, and TEM analyses that provide only limited information on the doping reaction kinetics. Successful synthesis of doped NPs requires correlation of various kinetic parameters, and hence, time-resolved studies on the doping processes are critical to resolve the doping mechanism. Being inspired by the enhanced electrochemical properties of bulk $\rm \gamma\!-\!Fe_2 O_3$ doped with $\rm Mo^{6+}$ cations mentioned above, S. G. Kwon et al (1) conjectured that high-valent cation doping of metal oxide NPs, that already have high level cation vacancy, could be a promising strategy to further increase the $\rm Li^{+}$ ion capacity. Thus, they selected synthesis of Mo-dope hollow iron oxide NPs.

In order to access the reaction kinetics and to study the doping mechanism, they conducted real-time in situ synchrotron quick-scanning X-ray absorption spectroscopy (XAS) technique that allows simultaneous monitoring of reaction kinetics of the precursors, compositional change of the NPs, and the chemical state (valency and coordination geometry) and position (whether it is in the solution or in the host lattice) of the dopant. The time-resolved XAS study on the synthesis of Mo-doped hollow iron oxide NPs revealed an oxidation induced doping mechanism by which the mass transport of the host (iron and oxygen) induces the internalization of dopant atoms (Mo) into the lattice of oxidized NPs($\rm \gamma\!-\!Fe_2 O_3$).

Synthesis

In a typical reaction, iron pentacarbonyl [$\rm Fe(CO)_{5}$] is introduced into a reaction mixture containing the molybdenum precursor [$\rm MoO_{2}(acac)_{2}$] and oleyamine at elevated temperature that results in the thermal decomposition of iron precursor and formation of metallic NPs. Then bubbling of dry air into the mixture leads to the formation of Mo-doped hollow iron oxide NPs via Kirkendall eﬀect.

Experimental

In situ X-ray absorption spectroscopy measurements at the Fe K-edge (7112 eV) and Mo K-edge (20000 eV) for the samples were measured at 10-ID-B beamline at the Advanced Photon Source, Argonne National Laboratory. Ex situ synchrotron X-ray absorption spectroscopy at the Mo L3-edge (2520 eV) and Fe K-edge (7112 eV) were measured at the SXS beamline and the XAFS2 beamline, respectively, at the Laboratorio Nacional de Luz Sincrotron (LNLS, Campinas, SP, Brazil). X-ray diﬀraction (XRD) measurements were performed at GSECARS Sector 13-ID-D at the Advanced Photon Source, Argonne National Laboratory. The photon energy was 40 keV (0.3100 Å) for the iron samples and 37 keV (0.3344 Å) for cobalt samples. Transmission electron microscopy (TEM), energy-ﬁltered TEM, and energy dispersive X-ray spectroscopy (EDS) data were obtained by using a Jeol JEM-2100F equipped with a Gatan GIF Quantum Energy Filters and an Oxford X-MaxN 80 TLE detector and operated at 200 kV.

Doping Mechanism

The in situ XANES/EXAFS measurements at the Mo K edge :

• Rule out the possibility of the trapping as the process responsible for the internalization of Mo in hollow iron oxide NPs
• Confirm that the ion diffusion through oxide lattice of NPs has no significant contribution into the doping process.

The authors derived a new doping mechanism that takes place during the synthesis of Mo-doped hollow shell iron oxide NPs (Figure 1). The decomposition of $\rm Fe(CO)_{5}$ leads to the formation of iron NPs with $\rm Mo^{4+}$ ions adsorbed at their surface. After the air is introduced, metallic iron NPs are transformed to iron oxide hollow shell NPs by the Kirkendall eﬀect. In this eﬀect, oxidation of metal ions ($\rm Fe^0 \rightarrow Fe^{2+/3+}$ and $\rm Mo^{4+} \rightarrow Mo^{6+}$) induces the outward mass transport from the core of the NPs. As the structure of the NP is changed from metallic iron to hollow shell iron oxide, $\rm Mo^{6+}$ ions migrate into the cation sites of the iron oxide lattice. In this process the mass transport of the host materials (Fe and O), and not the diﬀusion of the dopant, is responsible for doping.

Figure 1. Description of the oxidation induced doping mechanism. A metallic NP(Fe in black) with dopant ions(Mo in purple)adsorbed on the surface (left). Early stage of the oxidation of metallic NP indicating the capturing of dopant ions inside of the oxidized layer (middle); doped hollow shell NP after full oxidation (right).

Conclusion

In conclusion, the authors conducted for the ﬁrst time real-time in situ XANES/EXAFS study of the formation kinetics of Mo-doped and undoped fe oxide NPs. This revealed a new oxidation induced doping mechanism by which the mass ﬂow of the host material triggered by oxidation can lead to fast doping of the NPs, and the doping level is controlled by varying the amount of the dopant precursor. This process was also validated for oxidized forms of cobalt (oxide and sulﬁde). Moreover, time-resolved XANES/EXAFS study revealed that the reaction kinetics of the nucleation, growth, and oxidation of the host NPs are signiﬁcantly aﬀected by the dopant precursor.

Source: Soon Gu Kwon, Soma Chattopadhyay, Bonil Koo, Paula Cecilia dos Santos Claro, Tomohiro Shibata, Félix G. Requejo, Lisandro J. Giovanetti, Yuzi Liu, Christopher Johnson, Vitali Prakapenka, Byeongdu Lee, and Elena V. Shevchenko, Nano Lett. 2016, 16, 3738. doi: 10.1021/acs.nanolett.6b01072.