Importance of XRPD for chemical synthesis of oxide nanomaterials
J. Bárta, V. Čuba, T. Pavelková,
L. Procházková
Department for Nuclear Chemistry, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Břehová 7, 115 19 Prague 1, Czech Republic
jan.barta@fjfi.cvut.cz
In the recent decade, significant attention has been dedicated to nano-scale materials owing to their interesting physico-chemical or optical properties and possible application as sorbents, catalysts or specialized nanocomposite materials [1]. Chemical approach used to synthesize such materials, very often oxides, may result in the formation of intermediate precursors, whose properties and composition may not be completely understood. In this regard, X-ray powder diffraction (XRPD) serves as very convenient tool for phase identification of prepared materials, determination of crystallite size or other structural properties related to nano-scale materials. A relatively novel approach to synthesize nano-oxides utilizing ionizing or UV radiation has been used by our research group for the preparation of various oxides [2-3]. Recently we also started to investigate mixed oxide systems (AOx – BOy), where the possibility of solid solution between both terminal oxides significantly complicates the description of the produced nanomaterial. In this regard, XRPD very simply identifies the presence of solid solution and when coupled with any analytical method capable of elemental analysis it can yield exact composition and weight fraction of all phases present in the sample.
The first and also most important purpose of XRPD in analysis of nano-oxide materials is the phase identification with respect to different preparative conditions. Irradiation of e.g. aqueous solution containing zinc nitrate and various other additives by UV light, electron beam or gamma rays can lead to formation of many different products. In the presence of propan-2-ol, hexagonal zinc oxide ZnO, orthorhombic zinc hydroxide Zn(OH)2 (wulfingite) or monoclinic layered hydroxide-nitrate Zn5(OH)8(NO3)2 may be formed (often also a mixture of two phases is produced). Involved mechanism seems to be rather complex and phase identification may thus indicate some possible explanations or hints to reactions involved. When hydrogen peroxide (·OH radical source and efficient UV sensitizer) is added to the solution, cubic zinc peroxide ZnO2 (pyrite structure type) with very small particle size is produced by the irradiation. Addition of formate anion HCOO– (·OH radical scavenger and photo-active compound) to zinc nitrate promotes the formation of rather small amount of layered hydroxide-carbonate Zn5(OH)6(CO3)2 (hydrozincite), probably due to radiation-induced CO2 formation from the formate ion. Mild thermal treatment of all produced non-oxide compounds (200 – 400 °C) leads to their decomposition into zinc oxide while retaining their nano-scale character.
Crystallite size of the prepared nanomaterials l can be also determined from powder diffractogram due to the diffraction line broadening. The simplest method involves determination of peak width (FWHM) and using Scherrer equation:
, (1)
where K is shape factor (0.89 for spherical particles), λ is radiation wavelength and βhkl is the FWHM of selected diffraction line θhkl corrected for instrumental broadening. Such a simple size determination is justified by the fact that many oxides produced by radiation method are true nanoparticles with sizes in the range 10 – 50 nm, which was confirmed by other methods such as electron microscopy. Very small spherical nanoparticles with ~ 10 nm diameter are produced in the case of ZnO2 – consequently, its diffraction lines are very broad and even after its decomposition to ZnO above circa 200 °C the particle size increases only slightly. Most produced compounds such as e.g. synthetic garnets retain their nanoparticle character even after calcination at very high temperatures [3].
Solid solutions of many oxides may be easily obtained when their structure types (or just their lattice systems) are identical – a typical example involves fluorite-type UO2 and ThO2. Due to the different size of U4+ and Th4+ ions, their lattice constants a differ; when the solid solution is formed, resulting fluorite-type phase has lattice constant a inbetween UO2 and ThO2. Thus, precise determination of diffraction lines position can be used to estimate the amount of U and Th in the fluorite-type dioxide. Such approach was used for the determination of composition in the radiation-induced preparation of mixed oxide (U,Th)O2 nuclear fuels, where irradiation of aqueous solution containing uranyl nitrate, thorium nitrate and formate ion causes the formation of an amorphous phase. After calcination at 400 °C or higher in reducing atmosphere, single-phase fluorite-type oxide is formed; diffraction lines positions shift with increasing Th content in aqueous phase indicating efficient incorporation of Th into the mixed oxide.
Solid solutions of oxides with different lattice systems are less common, but can be obtained in specific systems. Irradiation of solutions containing nickel nitrate, zinc nitrate and formate ion induces formation of amorphous solid phase, most probably consisting of mixed (basic) carbonates of Ni and Zn. After mild calcination, halite-type cubic NiO and wurtzite-type hexagonal ZnO are formed, both with very high specific surface area (> 50 m2.g-1) and small crystallite size. When the Zn content in the aqueous solution is low, only a single phase with cubic lattice is observed by XRPD – the lattice constant a is larger than in the case of pure NiO due to incorporation of larger Zn2+ ions into NiO lattice. When the zinc concentration is too high, two separate phases of pure NiO and ZnO are formed. Similar effect may be observed in radiation-induced preparation of ZnO-CdO mixed oxides (CdO has halite structure), where Cd2+ may be partially incorporated into hexagonal ZnO (shift of diffraction lines to lower angles 2θ) and simultaneously, Zn2+ is present in CdO (shift of CdO diffraction lines to higher angles 2θ). This is more surprising than (Ni,Zn)O solid solution, because CdO has no hexagonal wurtzite-type modification, whereas for ZnO a high-pressure halite-type modification is known.
To conclude, XRPD is an almost irreplaceable and extremely useful analytical method in the field of oxide nanomaterial synthesis, enabling the determination of not only the composition, but also the size of particles and presence of interesting structural effects, most notably solid solutions.
References
1. A.Z.
Moshfegh, J.
Phys. D: Appl. Phys., 42 (2009) 233001-233030.
2. V.
3. J.
Bárta, V.
Acknowledgements.
This research has been funded by CTU
(grant SGS11/163/OHK4/3T/14) and