Growth of Nanowires on the Surfaces of Multicomponent Oxide Coatings on Titanium

Abstract

The thermal behavior of Ni- and Cu-containing coatings on titanium formed by plasma electrolytic oxidation and additionally modified with nickel and copper oxides is studied. Annealing of the produced multiphase coatings in air at a temperature of 750°C or higher is shown to result in the growth of surface nanowires, the main components of which are nickel, oxygen, and titanium. The sizes of nanowires depend on the temperature of annealing, and the diameters can be as large as tens or hundreds of nanometers at a length of several to tens of microns. Experimental and literature data show that the combination of plasma electrolytic oxidation with impregnation and annealing is promising for the production of both nanowires bound to metal-oxide substrates and individual nanostructures of certain compositions.

Full text

ISSN 20702051, Protection of Metals and Physical Chemistry of Surfaces, 2014, Vol. 50, No. 2, pp. 191–194. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © V.S. Rudnev, S. Wybornov, I.V. Lukiyanchuk, I.V. Chernykh, 2014, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2014, Vol. 50,
No. 2, pp. 174–177.
191
INTRODUCTION
In the last few decades, plasma–electrolytic oxida
tion (PEO), which is anodizing in electrolytes at volt
ages of spark and arc electric discharges, has been suc
cessfully used for the surface formation of complex
oxide systems on metals and alloys [1–12], such as
biocompatible layers that involve calcium phosphate
[1, 2]; ironcontaining coatings that absorb electro
magnetic radiation in certain spectral ranges [3] or are
ferromagnetic [4]; catalytically active layers that
involve nickel, copper, and molybdenum oxides
[5
⎯
8], etc.
In particular, Ni and Cucontaining PEO layers
on aluminum and titanium, which are active with
respect to CO oxidation to
СО
2
at temperatures above
300
°
С
, were obtained in a
Na
3
PO
4
+ Na
2
B
4
O
7
+
Na
2
WO
4
+Ni(CH
3
COO)
2
+ Cu(CH
3
COO)
2
electro
lyte [6, 7]. As was shown in the same works, additional
impregnation of the coatings in an aqueous
Cu(NO
3
)
2
+ Ni(NO
3
)
2
solution followed by annealing
in air at
500
°
С
results in a decrease in the content of
oxygencontaining copper and nickel compounds in
the modified coatings and the substantial increase in
their catalytic activity. In the latter case, oxidation of
CO to
СО
2
proceeds at a temperature above
150
°
С
.
Functional properties of such complex oxide systems,
which involve not only oxides of the metal treated, but
also oxides of other metals, on the metals depend on
many factors, including the surface structure and
composition.
The object of this work was to study temperature
induced changes in air in the surface structure and
composition of systems that involve nickel and copper
oxides on titanium, since the systems are considered
to be promising in catalysis.
EXPERIMENTAL
Similarly to as in [6], plasma–electrolytic treat
ment of titanium specimens was carried out under gal
vanostatic conditions (at current density
i
= 0.1 A/cm
2
for 10 min) in an aqueous electrolyte of the following
composition (M): 0.066
Na
3
PO
4
+ 0.034
Na
2
B
4
O
7
+
0.006 Na
2
WO
4
+ 0.1 Ni(CH
3
COO)
2
+0.025
Cu(CH
3
COO)
2
. Coatings were formed on specimens
made of sheet technical titanium of VT10 grade
(0.2 Fe, 0.1 Si, 0.07 C, 0.04 N, 0.12 O, 0.01 H, >99.6%
Ti at an admissible Al content of up to 0.7%) with a
size of
20
×
20
×
1
mm or on a wire made of the same
titanium with a diameter of about 1.5 mm. The set for
the plasma electrolytic treatment, conditions of pretreat
ment and oxidation of specimens, the current source,
and other experimental details can be found in [6].
Additional modification of PEO coatings was car
ried out similarly to as in [6] by exposing the speci
mens for an hour to an aqueous solution that con
tained 1 mol/L
Cu(NO
3
)
2
and 1 mol/L
Ni(NO
3
)
2
.
Impregnated specimens were dried over an electric
range and annealed in a furnace (SNOL 7.2/1100) at
500
°
C
for 4 h in air.
Growth of Nanowires on the Surfaces
of Multicomponent Oxide Coatings on Titanium
V. S. Rudnev
a,b
, S. Wybornov
c
, I. V. Lukiyanchuk
a
, and I. V. Chernykh
a
a
Institute of Chemistry, Far Eastern Branch, Russian Academy of Sciences,
Prosp. 100letiya Vladivostoka 159, Vladivostok, 690022 Russia
b
Far Eastern Federal University, Sukhanova str. 8, Vladivostok, 690950 Russia
c
Chair of Surface and Materials Technology, Institute of Materials Engineering,
University of Siegen, PaulBonatz str. 911, Siegen, 57076 Germany
email: rudnevvs@ich.dvo.ru
Received January 18, 2013
Abstract
—The thermal behavior of Ni and Cucontaining coatings on titanium formed by plasma electro
lytic oxidation and additionally modified with nickel and copper oxides is studied. Annealing of the produced
multiphase coatings in air at a temperature of 750°C or higher is shown to result in the growth of surface
nanowires, the main components of which are nickel, oxygen, and titanium. The sizes of nanowires depend
on the temperature of annealing, and the diameters can be as large as tens or hundreds of nanometers at a
length of several to tens of microns. Experimental and literature data show that the combination of plasma
electrolytic oxidation with impregnation and annealing is promising for the production of both nanowires
bound to metaloxide substrates and individual nanostructures of certain compositions.
DOI:
10.1134/S2070205114020130
NANOSCALE AND NANOSTRUCTURED
MATERIALS AND COATINGS

192
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 50 No. 2 2014
RUDNEV et al.
The modified PEO coatings obtained were addi
tionally annealed in air for an hour at a temperature in
a range from 650 to
950
°
С
. In this case, specimens
were placed in a cold furnace and taken away only
upon natural cooling of the furnace to
100–150
°
С
.
Highresolution surface images of the coatings
were obtained and elemental analysis of the composi
tion of coatings and threadlike crystals was carried out
with an ULTRA 55 electron microscope (Carl Zeiss
NTS GmbH, Switzerland) equipped with a special
detector.
Xray patterns of the coatings were recorded with a
D8 ADVANCE Xray diffractometer (Germany) in
Сu
К
α
radiation. For phase analysis, the EVA search
program and PDF2 database were used.
RESULTS AND DISCUSSION
Figure 1 shows the surface morphology of original
PEO coatings (Fig. 1a), as well as modified Ni and
Cucontaining coatings annealed in air at
500
°
С
(Fig. 1b) and others additionally annealed at 650, 750,
850, or
950
°
С
(Figs. 1c–f). On the surfaces of original
coatings (immediately after PEO), one can see alter
nating protuberances with pores with a diameter of up
to 10
µ
m on their tops and deeper areas (valleys)
between them. Pores are chaotically arranged in val
leys. Upon modification (impregnation and annealing
at
500
°
С
, Fig. 1b), valleys are filled with compounds
based on the components of the impregnating solution
and the surface relief becomes smoother.
Elemental and phase compositions, as well as
thicknesses of the original and modified coatings, are
listed in Table 1. According to the data, the mean
thicknesses of the original and modified coatings are
the same. The result confirms the above statement that
components of the impregnating solution fill chiefly
large pores and valleys. In contrast to the original coat
ings, the content of nickel in the modified coatings is
nearly twice as large, while that of copper is four times
as large. Moreover, crystalline NiO and CuO phases
are present in the modified coatings. This means that
the surface parts formed upon impregnation and
annealing consist chiefly of nickel and copper oxides.
Note that both original and modified Ni and
Cucontaining PEO coatings are active as catalysts
with respect to CO oxidation at temperatures above
300–350 and
150–200
°
С
, respectively [6, 7].
The modified wire specimens were additionally
annealed in air at temperatures of 650, 700, 750, 800,
850, 900, or
950
°
С
for an hour.
Upon additional annealing in air at temperatures of
650 and
700
°
С
, a shell composed of components of
the impregnating solution and containing nickel and
copper oxides that was formed in valleys of the original
coating becomes less pronounced (Fig. 1c). Probably
due to the diffusion processes, components of the shell
penetrate deeply into the original coating to form the
corresponding alloys. In this case, the surface becomes
cracked (Fig. 1c).
Starting from an annealing temperature equal to
~750
°
С
, nanowires begin to grow on the surfaces of
modified coatings (Fig. 1d). The initial growth of
nanowires often takes place in the vicinity of pores and
cracks. However, when the temperature of annealing is
further increased, nanowires cover the whole surface
of the coating (Figs. 1e, 1f). The surfaces of coatings
(b)
(c)
(а)
20
µ
m
30
µ
m
10
µ
m
200 nm
1
µ
m
(d)
(e) (f)
1
µ
m
Fig. 1.
Surface of (a) original PEO coating, (b) PEO coat
ing modified by impregnation and annealing at 500°C, and
(c–f) modified coating additionally annealed at (c) 650,
(d) 750, (e) 850, and (f) 950°C.
Table 1.
Thickness and elemental (Xray spectrum analysis data) and phase compositions of coatings
Composite
h
,
µ
Phase
composition
Elemental composition, at %
Ni Cu P Ti O W Na
PEO/Ti 40 ± 2 TiO
2
(r)
TiO
2
(a) 11.9 3.2 8.3 9.5 62.4 1.0 3.7
*PEO/Ti 40 ± 2 TiO
2
(r)
TiO
2
(a)
NiO, CuO
20.8 12.4 4.5 6.2 55.6 0.5 –
* PEO coatings modified by additional impregnation and annealing at 500
°
C in air, (r) rutile, (a)  anatase.

PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 50 No. 2 2014
GROWTH OF NANOWIRES ON THE SURFACES 193
that were annealed at
950
о
С
are nearly continuously
covered with a nanowire brush. Depending on the
temperature of annealing, nanowires have diameters
of tens to hundreds of nanometers at lengths of several
to tens of microns.
Note that, upon treatment at a temperature above
850
°
С
, one can see splits and spalls of the coating,
which partially exfoliates from the titanium substrate.
The latter peculiarity seems to be caused both by the
different coefficients of thermal expansion of the coat
ing and titanium and by intense oxidation of titanium
by oxygen from air that penetrates deeply toward the
base substrate via cracks formed. The oxidation was
confirmed in the case of
ZrO
2
+ TiO
2
PEO coatings on
titanium [10].
Analysis of the composition of nanowires was car
ried out on surface spots of 50
×
50
nm
2
(Fig. 2) with
the use of the energydispersive detector with which
the ULTRA 55 microscope was equipped. The com
position of the base coating was also determined (site
3
in Fig. 2a). The data obtained are listed in Table 2. The
results of the elemental analysis enable us to conclude
that nanowires are composed chiefly of nickel oxide
with admixtures of titanium, phosphorus, carbon, and
aluminum compounds. Nanowires have nearly the
same composition as the base coating material (base
coating, Table 2) except for titanium. Similarly to the
original surface, titanium is absent on the annealed
surface. At the same time, the titanium content in
nanowires is substantial (about 5.1–9.6 wt %), seem
ingly due to the diffusion from the depth of the coating
to the growing nanowires.
Note that the analysis provides the composition of
the bulk, which includes both nanowires and, partially,
the coating material. This means that the results
obtained in this work can by no means be related to the
composition of nanowires solely. The relation is only
qualitative.
As follows from the data (Table 2), copper is absent
in nanowires and on the surfaces of specimens
annealed at
850
°
С
. We can suppose that copper dif
fuses from the surface deeply into the oxide coating at
increased temperatures.
According to the results of Xray phase analysis
(XPA) (Table 1), specimens that were impregnated
and annealed at
500
°
С
contain crystalline nickel, tita
nium, and copper oxides. Further annealing at still
higher temperatures results in a decrease in signals of
crystalline nickel and copper oxides and an increase in
the intensity of signals of crystalline
TiO
2
in rutile and
anatase modifications.
The XPA data correspond to the whole nanowires
oncoating system, and we can scarcely formulate a
definite conclusion about the phase composition of
individual nanowires. Analysis of the elemental com
position (Fig. 2, Table 2) shows that the main compo
nents of nanowires are oxygen, nickel, and titanium,
with the mean atomic ratios (calculated from the data
of Table 2) being Ni/Ti
≈
4.2 and O/Ti
≈
16.8
. Nanow
1
µ
m
(а)
1
µ
m
(b)
13883 36467
13883 27551
2
3
3
2
1
Fig. 2.
Surface images of specimens covered with modified
coatings annealed at 850°C for 1 h and sites where the
composition of nanowires was determined.
Table 2.
Composition of nanocrystals and base coating
Element, wt % Nanocrystals Base coating
Site
1
in Fig. 2a Site
2
in Fig. 2a Site
2
in Fig. 2b Site
3
in Fig. 2b Site
3
in Fig. 2a
C 1.9 2.1 3.0 3.0 3.5
O 42.9 44.4 41.4 39.6 45.9
Al 2.3 3.0 2.2 1.5 2.2
Si ––– 0.3–
P4.69.43.91.510.6
Ti 8.4 5.1 9.2 9.6 –
Ni 39.9 36.1 40.3 44.5 37.9

194
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 50 No. 2 2014
RUDNEV et al.
ires probably involve a complex oxygencontaining
compound of nickel and titanium.
Thus, annealing a PEO coating, which was formed
on titanium and modified with nickel and copper
oxides, at a temperature of
750
°
С
or higher in air
results in the surface growth of nanowires, the main
components of which are nickel, oxygen, and tita
nium. The sizes of nanowires depend on the tempera
ture of annealing. The diameters of nanowires may be
from tens to hundreds of nanometers at lengths of sev
eral to tens of microns.
Covering the surface, nanowires substantially
increase the active surface of specimens, which is
especially important for catalysis. Insofar as nanowires
involve nickel (predominantly) and titanium oxides or
complex binary oxides of nickel and titanium, they
can be catalytically active with respect to redox pro
cesses. Catalysts based on nickeloxide nanowires
bound to a titanium oxide substrate are promising for
the conversion of organic compounds. In particular,
German researchers have shown recently that the
modified NiO +
CuO/TiO
2
/Ti
systems studied in this
work are catalytically active in the conversion of naph
thalene [13].
Judging from the thermal stability of the systems
produced, they can be used at temperatures up to
800
º
С
. At a higher temperature, some surface parts
begin to exfoliate from the titanium substrate.
At the same time, the discovered growth of nanow
ires on the surfaces of modified PEO coatings can be
used for production of individual nanowires by means
of their mechanical separation from an oxide sub
strate.
Note that no sign of the growth of nanowires was
noticed during the investigation of the thermal behav
ior of Ni and Cucontaining PEO layers, which were
formed [9] in accordance with the same technique
(electrolyte composition and conditions of forma
tion), but without additional impregnation, and then
annealed in air at temperatures up to
900
°
С
. On the
other hand, individual nanoribbons composed chiefly
of zirconium and titanium were observed on the sur
faces of coatings formed on titanium in an electrolyte
containing
Zr(SO
4
)
2
upon plasma electrolytic oxida
tion [10]. Nanowires were also present on the surfaces
of PEO coatings formed on titanium in a
Na
2
SiO
3

containing electrolyte and then impregnated in a
manganesenitrate solution and annealed in air at a
temperature of
500
°
С
[11]. At the same time, there
were no nanowires upon annealing at
800–900
°
С
. This
means that nanowires composed seemingly of manga
nese oxides can be formed and are stable only within a
certain temperature range. The formation of nanorods
was observed also on the surfaces of PEO coatings
formed in an electrolyte suspension based on
Na
2
SiO
3
+ NiO either individual or containing
C
18
H
33
NaO
2
surfactant additive upon annealing in air
for a 24 day [12].
Thus, the results obtained in this work and inde
pendent literature data confirm the idea that PEO
technique, including its combination with impregna
tion and annealing, can be used for creating nano
structures bound to metaloxide substrates or for pro
ducing individual nanosystems of certain composi
tions. The conditions of the formation of
nanostructures with a desirable structure and compo
sition with the use of the method described may be
clarified in forthcoming investigations.
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Translated by Y.V. Novakovskaya