TEM, EDX AND RAMAN STUDY OF NICKEL OXIDE MICRO – AND [610279]

TEM, EDX AND RAMAN STUDY OF NICKEL OXIDE MICRO – AND
NANOPHASES OBTAINED BY THERMAL DECOMPOSITION OF AN
ORGANOMETALLIC PRECURSOR

Mircea Niculescu1*, Andrei Racu2, Petrică Linul2,
Bogdan Țăranu2, Mihai -Cosmin Pascariu2,3*

1University Politehnica Timișoara, Faculty of Industrial Chemistry and Environmental
Engineering, Department of Applied Chemistry and Engineering of Inorganic Compounds
and Environment, 6 Vasile Pârvan Blvd., RO -300223, Timișoara, Romania
2National Institute of Research and Development for Electrochemistry and Condensed Matter
– INCEMC Timișoara, Renewable Energies – Photovoltaic Laboratory, 144 Dr. Aurel
Păunescu Podeanu, RO -300569, Timișoara, Romania
3“Vasile Goldiș” Western University of Arad, Faculty of Pharmacy, Department of
Pharmaceu tical Sciences, 86 Liviu Rebreanu, RO -310045, Arad, Romania
e-mail: [anonimizat], [anonimizat]

Abstract
Nickel(II) polyoxalate was thermally decomposed to nickel oxide in both oxidative and inert
atmospheres and the products w ere investigated using TEM -EDX and Raman spectroscopy.
The resulting micro – and nanoparticles were compared regarding their size, morphology and
composition. In dynamic aerobic conditions, the product obtained at 1000 °C shows better
crystallinity and pres ents more well defined shapes than the one obtained at 400 °C. The 1000
°C product obtained in argon is a mixture of NiO and metallic nickel.

Introduction
The thermal conversion of coordination compounds to metal oxides is a convenient way to
produce micro – and nanoparticles with defined properties (degree of crystallinity, size,
morphology and surface area) , which make them useful for a variety of applications. For
example, nanosized nickel oxide exhibits anomalous electronic and magnetic properties and
can be used for catalysis, electrochromic windows and sensors. In this paper we have
investigated the nickel oxide obtained through the thermal conversion of a complex
compound, namely nickel(II) polyoxalate hydrate [1], in both aerobic (at 400 and 1000 °C)
and inert (at 1000 °C) atmospheres.

Experimental
The nickel(II) polyoxalate h ydrate was prepared starting from nickel(II) nitrate and ethylene
glycol by using an original method, as described in a previous paper [ 1].
The Raman spectra were measured at room temperature using a MultiView 1000 system
(Nanonics Imaging, Israel) , which incorporates the Shamrock 500i Spectrograph (Andor,
UK). A laser wavelength of 514.5 nm was used as the excitation source, with a 20 s exposure
time and a 300 l mm-1 grating.
For the TEM analyses, the material was deposited from ethanol on a 200 mesh TEM c opper
grid covered with lacey carbon film. We used a Titan G2 80 -200 TEM/STEM (FEI Company,
the Netherlands) instrument with image correction. The images were registered at 200 kV
accelerating voltage. A Digital Micrograph v.2.12.1579.0 software was used f or images
recorded in TEM mode, while a TEM Imaging & Analysis v.4.7 software was used for
recording the EDX spectrum. The STEM -EDX elemental distribution maps were recorded
with the Esprit v.1.9 software.

Results and discussion
The global decomposition process , which takes place during both aerobic (+ O 2) and inert ( –
CO) heating of the homopolynuclear coordination compound , can be illustrated as shown
below :

The particles obtained in aerobic conditions at 400 °C ( Fig. 1) exhibit irregular shapes with
microporous structure and with dimensions dispersed between 15 nm and 2 .1 μm; the
monocrystalline areas are rather small (14 nm at most) and have the crystalline planes
distanced at about 2.27 Å, as seen using HRTEM. On the other hand, the aerobic samples
produced at 1000 °C ( Fig. 2 ) are c omposed of 50 -750 nm polyhedrons , with extended
monocrystalline areas composed of crystal planes distanced at about 2.15 Å. Lastly, the
particles obtained at 1000 °C in argon ( Fig. 3 ) consi st of 60 -220 nm parallelepipeds which
present wide monocrystalline areas (2.47 Å between the crystalline planes ) that are composed
mostly of NiO, but also seem to contain metallic nickel particles around 2 .6 μm in diameter
(Fig. 4), as shown by EDX.

Figure 1. TEM images of the 400 °C aerobic decomposition product (mostly NiO)

Figure 2. TEM images of the 1000 °C aerobic decomposition product (NiO)

Figure 3. TEM images of th e 1000 °C argon decomposition product , which show NiO
nanocrystals

Figure 4. STEM images of the 1000 °C argon decomposition product ; EDX analysis revealed
mostly NiO in the two areas from the left image and mostly metallic Ni in the area from the
right image

The EDX profile obtained for the aerobic 400 °C product is shown in Fig. 5 , and is similar in
shape to the one obtained in aerobic conditions at 1000 °C. The determined atomic ratio s
suggest a NiO 1.37 formula in the former case, and a NiO 0.82 formula in the latter.

Figure 5. EDX profile in a surface area of the product obtained by aerobic thermal conversion
of the complex compound at 400 °C (C and Cu peaks belong to the grid)

The ED X profiles for two distinct particles (Fig. 4 ) of the product obtained by the
coordination compound’s annealing in argon at 1000 °C are shown in Fig. 6. The analysis of
the two selected surface areas reveals that this product is a mixture of nickel oxide and
metallic nickel .

Figure 6. EDX profile in two different surface area s (C and Cu peaks belong to the grid): left
NiO 0.87, right metallic nickel (Ni > 97 atomic % )

The Raman spectrum of the final argon decomposition product is shown in Fig. 7. Peaks due
to one -phonon (553 cm-1), two -phonon ( 709, 892 an d 1086 cm-1) and two -magnon ( 1369 cm-
1) scattering are all present [2].

Figure 7 . RAMAN spectrum of the 1000 °C argon decomposition product

Conclusions
The thermal conversion of the nickel(II) polyoxalate hydrate gave mostly nickel oxide, which
also contains some metallic nickel when inert (argon) dynamic atmosphere is used. The
samples obtained at 1000 °C show more well defined edges and larger monocrystalline areas
than the ones obtained at 400 °C.

Acknowledgements
Part of this rese arch was done at the Center of Genomic Medicine of the “Victor Babeș”
University of Medicine and Pharmacy of Timișoara, POSCCE 185/48749, contract
677/09.04.2015.

References
[1] M. Niculescu, M.C. Pascariu, C. Muntean, J. Therm. Anal. Calorim. (2016 ), in press .
[2] N. Mironova -Ulmane, A. Kuzmin, I. Steins, J. Grabis, I. Sildos , M. Pärs, J. Phys.: Conf.
Ser. 93 (2007) 012039, doi:10.1088/1742 -6596/93/1/012039.