According to the theory of free volume, the most effective process of nanoparticle formation is the obtaining method in the liquid media, instead of in solid or gaseous environment. Metals nanoparticle electrochemical obtaining in the liquid media provides qualitative formation and process efficiency from the point of view of productivity and power inputs. The offered electrochemical method is based on reduction reactions of cobalt ions in water solutions. For cobalt nanoparticle obtaining, water solutions of cobalt salts are used;
aluminum microparticles are used as fluidizated electrode.
The experiments have shown that cobalt aquacations are restored on an aluminum substrate from solutions with high speed (Fig. 1). Reaction of cobalt separation can be adequately described the equation (Eq. 1)
D r
r
A
k t t
A
A
1 0 0
1
,
(1)A0 – reaction index at time point t0; t0 – reaction beginning time;
Ar - reaction index at the end of experiment;
k-1 – constant of reaction rate;
D – fractal dimension index.
Table 1. Values of equation parameters
Parameter A0 t0 Ar k-1, с D
Value 0 0 96 61 2,87
It is possible to assume with sufficient basis that cobalt discharge process on aluminium microparticles proceeds as follows. At the first stage, accompanied by surface oxide film transformation and simultaneous metal nucleation process begins and proceeds on an outer side of a particle surface. For short time, the aluminium surface becomes covered by a layer of metal nuclear. Across metal hydrogen is allocated. Hydrogen evolution in the process beginning occurs on cathode sites of an aluminium surface, but restoration of protons donors goes mainly on them in deposition process on cobalt surface. Further, after metal ions solution exhaustion, hydrogen deposition process proceeds only. High speed of cobalt sedimentation and intensive hydrogen deposition promote formation nucleation of nano-sized metal and interfere with formation of deposit dense layer.
For the purpose of reduction of sizes of particles obtained, metals deposit the solutions containing ions of a palladium (Pd2 +) or platinum ([PtCl6] 2-) are added to the initial solution containing metal ions. Using foreign metal, which intensively absorbs hydrogen (platinide family elements) in an element condition, as an ion, hydride dispersion of system containing this metal occurs [4]. It is also known [3] that bringing in a solution of electropositive metal
ion can lead to dispersion of metal deposit (in quantities, ten times smaller in comparison with quantity of deposited metal).
Figure 1. Kinetic curves of cobalt deposition from 1 M CoCl2 water solution. Points - experiment result, a line - calculation result (Eq. 1).
At all investigated samples obtained from cobalt (II) solutions, the element cobalt presented by two crystal polymorphic modifications (α-Co - hexagonal and β-Co - cubic) is found out as the basic phase. On diffraction patterns of these samples (Fig. 2), reflexes 2.17;
2.04; 1.91; 1.254; 1.069 Å correspond hexagonal cobalt modifications; reflexes 2.04; 1.78;
1.253; 1.068 Å correspond cubic cobalt modifications (some reflexes of these crystal phases are blocked). The halo-liked background raising on diffraction pattern in the rate approximately 10-35 ° 2 θ allows assuming presence phases amorphous to X-rays.
On diffraction patterns of the sample 1 (Fig. 2а), cobalt hydroxide and oxihydroxide reflexes are observed; they obviously formed at the set process mode owing to local cathode region alkalinization. Hydroxide and oxihydroxide colloidal particles formed in cathode region layer are deposited together with metal. In samples 2 and 3 (Fig. 2b, c) on diffraction patterns, palladium reflexes absence is possibly caused by a condition amorphous to X-ray.
On diffraction patterns of samples 4 and 5 (Fig. 2 d, f), fine-dispersed platinum reflexes are shown.
According to electronic microscopy data, cobalt deposits contain nanoparticles in the size of 30-50 nm. The sample 1 (Fig. 3а) represents nanoparticles in the size ~30-50 nm located separately. They tend to dark aggregates formation of the various size (to 0.5-1 µm). The sample 2 (Fig. 3b) also consists of nanoparticles in the size ~30-50 nm, located separately forming a continuous field. On this background (approximately to 5% from total of particles), larger dark quasi-hexagonal particles in the size to 100-150 nm are located at the sizes. Their sides are outlined by a light aura that is inherent in the objects incorporating a hydrated component (for example, OH − or Н2О). The sample 3 (Fig. 3c) differs from the sample 2 by
presence of the big number of nanoparticle aggregates forming conglomerates of the various forms. The sample 4 (Fig. 3d) represents nanoparticles in the size ~30-50 nm. Hexagonal separate particles with the linear sizes 100-200 nm are accurately visible. They are absolutely opaque, their thickness more than 1 µm. They also form aggregates of any form. In the sample 5 (Fig. 3d), except the separate nanoparticles in the size ~30-50 nm, "rash" from sub- individuals in the size of ≤10 nm is noted on some sample fragments. The similar picture is characteristic for fine-dispersed iron hydroxide present at natural mineral objects. Here, dark hexagonal particles in the size of ~150 nm and aggregates to 1 µm are located, and the last have the needle shape.
Figure 2. Sample diffraction patterns: а - 1; b - 2; c - 3; d - 4; e – 5.
Figure 3. (Continued).
Figure 3. (Continued).
Figure 3. Sample microphotographies: а - 1, ×65000; b - 2, ×26000; c - 3, ×29000; d - 4, ×16000; e - 5, ×40000.
It is necessary to notice that using magnetic separation for branch of cobalt particles from mother solution leads to occurrence of residual magnetisation owing to this action particles aggregate in large conglomerates. It is also connected with aggregation as nanoparticles are always characterised by very high value of the relation "surface-volume,” and aggregation process is thermodynamically favourable [1].
By results of X-ray diffractometry, it is established that in the investigated samples, the basic phase is cobalt. Management possibility by phase particle structure by using electrolyte of corresponding structure is established. It is revealed that the obtained particles practically do not contain oxide phase. It is possibly connected with the interfaced reaction of hydrogen separation during process of metal deposit formation. Saturation of electrolyte and deposit by hydrogen creates the reducing media, allowing performing synthesis without additional operation on removal of the dissolved oxygen from a solution.
Thus, it is possible to conclude that cobalt deposits obtained by electrochemical method consist basically from sphere- shaped or oval nanoparticles in the size of 50-100 nm, but it is observed the strong tendency to formation of large aggregates of the various sizes (to 0.5-6 µm), which it is possible to explain both the developed surface and residual magnetisation of powder samples. For the purpose of aggregate destruction, additional processing of the obtaining deposits by the HF-discharge or their crushing (mechanical, electro erosive) is necessary.
Results
At the base of nano-sized metal synthesis electrochemical process of cobalt ion restoration on slurrying substrate in aluminium solution lays.
Experiments spent with aluminium samples (purity not less than 99.0%). CoCl2
(qualifications «pure for analysis») without additional purification and also PdCl2
(qualifications « pure for analysis») and H2PtCl6 (qualifications «pure») are used as the basic reactants.
Restoration kinetics is studied by sampling method through the fixed time intervals with the subsequent definition by X-ray fluorescence (VRA20L) and atomic absorption (AAS-1N) analyses.
The obtained deposit separated by magnetic separation from mother solution washed out bidistillated before neutral reaction and dried under vacuum at 60ºС.
The X-ray analysis is spent by a powder method on D8 ADVANCE diffractometer (Bruker) using monochrome CuK α-radiations in a mode of step-by-step scanning. A scanning step is 0.02 ° 2 θ, exposition time in a point is 1 sec, an interval of shooting 2 θ is 3-95 °.
Samples prepared by press fitting of investigated material powder in standard disk ditch from quartz glass; during shooting, the preparation rotated in its own plane with a speed of 60 rpm. An operating mode of an X-ray tube is 40 kV, 30 mA.
Calculation of interplanar space values of diffraction reflexes was made automatically under EVA programmer entering into the complete set of device software.
Identification of crystal phases was carried out by standard way by comparison of the obtained experimental values of interplanar space and relative intensity with reference.
Investigations were spent by a method of transmission electronic microscopy (TEM) for detailed definition of dimension and morphological features of particles of studied samples according to methodical recommendations [5].
The synthesised sample particle size defined by the method of transmission electronic microscopy using of microscope-microanalyzer EMMA-4 at accelerating pressure of 75 kV.
Preparations were prepared by suspension method, with preliminary preparation on ultrasonic dispergator UZDN-2Т and the subsequent drawing on collodion film-substrate or a carbon dusting on vacuum apparatus VUP-4. Microphotographies are obtained by means of digital camera OLYMPUS C-8080.
References
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[2] Nanotechnology in nearest decade. Prognosis of investigation direction. / Edit. by М.К.
Roko, R.S. William, P. Alivisatosa. Trans. From eng. М.: Мir, 2002. 292 p.
[3] Dresvyannikov A.F., Grigor‘eva I.O., Kolpakov M.E. Physical chemistry of nanostructured aluminium containing materials. Kazan: Pub. house «Fәn» SA of TR, 2007. 358 p.
[4] Semenenko К.N., Yakovleva N.А., Burnasheva V.V. To a question on the reaction mechanism of hydride dispersion// Russian journal of General chemistry, 1994. v.64,
№ 4. P.529-534.
[5] Methodological recommendation № 137. Electronic microscopical analysis of minerals.
М.: NSOMMI, 2000. 36 p.
Editors: A. K. Haghi and G. E. Zaikov © 2013 Nova Science Publishers, Inc.