A low temperature composite-hydroxide approach to NiO nanocrystals

Article abstract of DOI:10.1016/j.materresbull.2007.10.042

Nanocrystalline NiO has been successfully synthesized via a low temperature molten composite hydroxide route using nickel chloride hydrate as nickel source. The as-obtained oxide nanocrystals were characterized by X-ray powder diffraction and transmission electronic microscopy. It is found that the ratio of reagent to molten solvent has a great influence on the quality of the products. The effect of temperature and time was also investigated. The present route may have a good prospect to prepare many functional nanomaterials.

Full text of DOI:10.1016/j.materresbull.2007.10.042

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Materials Research Bulletin 43 (2008) 2563–2568
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A low temperature composite-hydroxide approach
to NiO nanocrystals
*
Changhua An , Ranran Wang, Shutao Wang, Yunqi Liu
College of Chemistry and Chemical Engineering, China University of Petroleum, Dongying 257061, People’s Republic of China
Received 18 September 2007; received in revised form 29 October 2007; accepted 31 October 2007
Available online 20 February 2008
Abstract
Nanocrystalline NiO has been successfully synthesized via a low temperature molten composite hydroxide route using nickel
chloride hydrate as nickel source. The as-obtained oxide nanocrystals were characterized by X-ray powder diffraction and
transmission electronic microscopy. It is found that the ratio of reagent to molten solvent has a great influence on the quality of the
products. The effect of temperature and time was also investigated. The present route may have a good prospect to prepare many
functional nanomaterials.
# 2007 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides; B. Chemical synthesis; C. X-ray diffraction
1. Introduction
Nanomaterials have attracted much attention because of their unusual chemical and physical properties different
from those of bulk materials, based on size-quantization effect and extremely large specific surface area [1,2]. In
particular, nanosized nickel oxides are of great interest because they exhibit particular catalytic [3], anomalous
electronic [4,5] and magnetic [6,7] properties. The characteristic properties of NiO nanoparticles enable one to tailor
materials for a variety of applications including catalysis [3], electrochromic windows [8] and sensors [9].
So far, many synthesis methods have been developed for fabricating nickel oxide nanocrystals. For example, Wang
and co-workers prepared NiO nanowires by annealing the freshly prepared precursor [10]. Deki et al. prepared highly
concentrated nanosized NiO by means of vacuum evaporation of Ni on NH2-terminated poly(ethylene oxide) film
[11]. Qian and co-workers synthesized NiO nanoplatelets by thermal decomposition of the newly prepared nickel
hydroxide precursor [12]. Hyeon and co-workers synthesized NiO and Ni nanoparticles by thermal decomposition of
Ni-alkylamine complexes in trioctylphosphine (TOP) [13]. However, the existing techniques mainly rely on high
pressure, salt–solvent-mediated high temperature, surface-capping agent or organometallic precursor mediated
growth process. Seeking a simple approach for low-cost, lower-temperature, large-scale and controlled growth of
oxide nanostructures at atmospheric pressure is highly desired.
Molten salts have been well studied and exhibited a wide range of temperatures at which they remain liquid state
and thus are appropriate media for synthetic applications [14,15]. NiO nanowires were synthesized by annealing the
* Corresponding author. Fax: +86 546 8391971.
E-mail address: anchh@hdpu.edu.cn (C. An).
0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2007.10.042

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homogeneously grinded the mixture of freshly prepared NiS nanoparticles and NaCl at high temperature (810 8C)
[16]. Zheng and Zhang reported a molten-salt synthesis method using NaCl as a flux at 1000 8C to prepare NiO
nanoparticles with uniform morphology of hexagon plates [17]. Complex oxides, such as LiCoO2, LaAlO3 and
Ni0.5Zn0.5Fe2O4 powders, were also prepared by a molten-salt method in the temperature range of 700–1000 8C
[18–20]. Phase diagram of NaOH–KOH indicates that potassium hydroxide and sodium hydroxide are 323 and
360 8C, respectively [15]. However, the eutectic point at ratio of NaOH:KOH = 51.5:48.5 is only about 165 8C.
Therefore, molten solution of mixed KOH/NaOH was adopted as a reaction medium to prepare functional oxides
including BaTiO3, Ba(Sr)TiO3 nanocrystals [15]. Herein, we demonstrate an alternative low temperature molten
composite hydroxides approach to fabricate nickel oxide nanocrystals in attempting to find new applications or
improve the existing performances. The method is based on a reaction between a nickel chloride hydrate and molten
solution of mixed potassium hydroxide and sodium hydroxide with the eutectic point at 165 8C and at normal
atmosphere without using organic dispersant or capping agent. This methodology may provide a one-step, convenient,
nontoxic and mass-production route for the synthesis of nanostructures of functional oxide materials.
2. Experimental
All the chemicals were of analytical grade and used as received without further purification. The synthesis is
performed in the following steps: (1) appropriate amount of mixed hydroxides (MHO, NaOH:KOH = 51.5:48.5) and
NiCl2ꢀ6H2O was grinded and placed in a 50 mL Teflon-lined autoclave with a cover for preventing dust. (2) The
autoclave was put in a furnace, which was preheated to 200–250 8C. (3) After reacting for a period of time, the
autoclave was taken out and cooled to room temperature naturally. The product was filtered and washed by deionized
water, hot water and ethanol to remove hydroxides on the surface of the particles. The obtained black powders were
collected for characterization. It is noted that the molten KOH/NaOH solution is extremely corrosive, and dangerous
for operation. When performing experiments, we put on the gloves and protecting-glasses to do manipulation.
X-ray powder diffraction patterns (XRD) of the products were performed on a Japan Rigaku DMax-çA rotation
anode X-ray diffractometer equipped with graphite monochromatized Cu Ka radiation (l = 1.54178 Å). Transmission
electron microscopy (TEM) and electronic diffraction (ED) were taken on a Hitachi model H-800 transmission
electron microscope at an accelerating voltage of 150 kV.
3. Results and discussion
Fig. 1a is the XRD pattern of the as-synthesized product obtained at 200 8C for 24 h with the ratio of 1% of
NiCl2ꢀ6H2O to MHO. In the pattern, all the diffraction peaks can be indexed to pure phase of cubic NiO (JCPDS Card
02-1216). The broaden diffraction peaks indicate that the sample is on the nanometer scale. The size estimated from
Scherrer equation for the product is about 55 nm. TEM image (Fig. 1b) shows the products consisting of a lot of
irregular nanoparticles, and a little amount of small particles exists. The size distribution diagram (Fig. 1c)
programmed from 100 nanoparticles, indicated the even diameter of 55 nm, closing to the result from XRD. The
standard deviation value is in a range of 55 ꢁ 17 nm.
The initial mass ratio of reagent to MHO was found to have a great effect on the quality of the products. Fig. 2 gives
XRD patterns of the as-synthesized products at 250 8C for 2 h with different ratios of reagent to MHO. It can be seen
that with the amount of nickel chloride increased, the diffraction peaks became narrower, indicating that the crystallite
size became larger. The corresponding TEM images as shown in Fig. 3 demonstrate that the samples are dispersed
irregular particles. With the increase of reactants from 2, 10 to 30%, the diameter of the products also increased from
50, 150 to 450 nm, accordingly. The ED pattern shown in Fig. 3d can be assigned to polycrystalline NiO with cubic
phase, consisting with the results of XRD. The effect of ratio of reactant to MHO can be explained as follows. When
the ratio of nickel chloride to MHO is very little (1–2%), Ni2+ would be surrounded by a large amount of OHꢂ and the
nucleation of Ni(OH)2 would be very fast, whereas the particle growth will be restricted by surrounding OHꢂ to result
in small Ni(OH)2 crystallites. The freshly formed Ni(OH)2 would decompose rapidly and thus NiO nanocrystals be
produced. If the ratio became larger, the OHꢂ concentration around Ni2+ decreased, the nucleation of Ni(OH)2 became
slow. As a result, the produced NiO is more regular in shape and much larger in size.
In the present work, the influence of temperature and time on the formation of the products was also investigated. It
can be observed, from Fig. 4 and Table 1, that when the temperature was decreased from 220 to 170 8C, the diffraction

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Fig. 1. XRD (bottom is standard diffraction pattern) patterns (a), TEM image (b) and size distribution diagram (c) of the product obtained at 200 8C
for 24 h with the ratio of nickel chloride to hydroxides of 1%.
peaks would be broadened. This implied much smaller NiO crystallites produced. However, a trace amount of nickel
hydroxide occurred at 170 8C. This can be an evidence that NiO is resulted from decomposition of Ni(OH)2 formed by
a reaction between nickel chloride and mixed hydroxides. It is interesting that metallic nickel appeared when the
reaction was performed at 220 8C, although it is far lower than the decomposition temperature of NiO. This might be
Fig. 2. XRD patterns of the products obtained with different ratios (from 2, 10 to 30%) of nickel chloride to hydroxides at 250 8C for 2 h.

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Fig. 3. TEM images of the NiO obtained at 250 8C for 2 h with ratios of nickel chloride to mixed hydroxides from 2% (a), 10% (b) and to 30% (c);
(d) is the ED pattern of the sample (c). The TEM images show that the ratio has influence on the size and shape on the products.
Fig. 4. The XRD pattern of the samples obtained at different temperatures (a) 220 8C, (b) 200 8C and (c) 170 8C, indicating that the temperature has
an obvious effect on the phase purity of the products.