Synthesis and photocatalytic properties of low-dimensional cobalt-doped zinc oxide with different crystal shapes

Article abstract of DOI:10.1134/S0036023611020136

The glycoxide complexes Zn_1-x Co _x (HCOO)(HOCH ₂CH₂O)_1/2 and Zn_1-x Co _x (OCH₂CH₂O) (0 ≥ x ≥ 0.3) have been synthesized by heating ethylene glycol solutions of zinc formate Zn(HCOO)₂ ? 2H₂O or its mixtures with cobalt formate Co(HCOO)₂ ? 2H₂O. The crystals of these complexes have the shape of filaments (needles, bars) and distorted octahedra, respectively. A new method in which these complexes are used as the precursor is suggested for the synthesis of low-dimensional wurtzite-like Zn_1-x Co _x O. The shape of the precursor crystals is fully inherited by Zn_1-x Co _x O resulting from their heat treatment. The Zn_1-x Co _x O solid solutions show high photocatalytic activity in hydroquinone oxidation in aqueous solution under UV or blue light irradiation, and their activity increases as their cobalt content is increased.

Full text of DOI:10.1134/S0036023611020136

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2011, Vol. 56, No. 2, pp. 145–151. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © V.N. Krasil’nikov, O.I. Gyrdasova, L.Yu. Buldakova, M.Yu. Yanchenko, 2011, published in Zhurnal Neorganicheskoi Khimii, 2011, Vol. 56, No. 2,
pp. 179–186.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Synthesis and Photocatalytic Properties of LowꢀDimensional
CobaltꢀDoped Zinc Oxide with Different Crystal Shapes
V. N. Krasil’nikov, O. I. Gyrdasova, L. Yu. Buldakova, and M. Yu. Yanchenko
Institute of Solid State Chemistry, Ural Branch, Russian Academy of Sciences,
Pervomaiskaya ul. 91, Yekaterinburg, 620041 Russia
Received June 19, 2009
Abstract—The glycoxide complexes Zn1 – xCox(HCOO)(HOCH2CH2O)1/2 and Zn1 – xCox(OCH2CH2O) (0
≤
x
≤
0.3) have been synthesized by heating ethylene glycol solutions of zinc formate Zn(HCOO)2 · 2H2O or
its mixtures with cobalt formate Co(HCOO)2 · 2H2O. The crystals of these complexes have the shape of filꢀ
aments (needles, bars) and distorted octahedra, respectively. A new method in which these complexes are
used as the precursor is suggested for the synthesis of lowꢀdimensional wurtziteꢀlike Zn1 – xCoxO. The shape
of the precursor crystals is fully inherited by Zn1 – xCoxO resulting from their heat treatment. The Zn1 – xCoxO
solid solutions show high photocatalytic activity in hydroquinone oxidation in aqueous solution under UV or
blue light irradiation, and their activity increases as their cobalt content is increased.
DOI: 10.1134/S0036023611020136
Zinc oxide belongs to the family of multifunctional
In recent years, preparative nanochemistry has
wideꢀbandgap semiconductors, which have been the been increasingly using precursor techniques in the
subject of extensive studies in recent years [1, 2]. Parꢀ synthesis of quasiꢀ1D oxides. The precursors
employed in these techniques are metal carboxylates
and glycoxides crystallizing as sticks orfilaments
whose length is tens or even hundreds of times larger
than their diameter. On heating, these compounds
turn into oxides, whose particles retain the shape of
the precursor crystals [16–18]. In this study, fine ZnO
and Zn1 – xCoxO powders consisting of extended partiꢀ
cles were synthesized by the thermolysis of glycoxide
complexes obtained by heat treatment of Zn(HCOO)2 ·
ticular attention have been focused on the synthesis of
micronꢀ and nanometerꢀsized oxides consisting of
quasiꢀoneꢀdimensional (1D), extended particles
(rods, needles, whiskers, filaments, tubes). It is anticꢀ
ipated that these oxides will possess new, better funcꢀ
tional properties [3–5]. A very important appropriateꢀ
ness criterion for these methods is whether the morꢀ
phology of the resulting oxide particles is controllable
[6]. One of the currently central and most promising
applications of zinc oxide is photocatalytic oxidation
of organic compounds and disinfection of water and
air [7–12]. The bandgap width of zinc oxide is 3.37 eV,
so undoped zinc oxide is active only in the UV region
of the spectrum. Its functional characteristics are
2H2O + ethylene glycol and (1 – x)Zn(HCOO)2 ·
2H2O +
xCo(HCOO)2 · 2H2O + ethylene glycol mixꢀ
tures [19].
EXPERIMENTAL
modified by doping it with dꢀ and fꢀelements, and this
doping has afforded materials and devices with very
interesting and valuable properties, including roomꢀ
temperature ferromagnets, gas and optical sensors,
phosphors, transistors, lightꢀemitting diodes, and
even catalysts for oxidation of toxic organic comꢀ
pounds [13–15]. However, the relevant literature deals
mainly with oxidative photocatalysis on highꢀporosity
and grained materials. There are only scarce data conꢀ
cerning the photocatalytic activity of zinc oxide conꢀ
sisting of extended particles [11–13], and we failed to
find any photocatalytic data for zinc oxide doped
with other transition elements. This prompted us to
study the formation conditions for zinc oxide and
Zn1 – xCoxO solid solutions as extended micronꢀ and
The existing methods of synthesis of metal glycoxꢀ
ide complexes are based on hydrothermal treatment of
alkoxides in ethylene glycol. The latter is taken in
excess and serves both as the solvent and as a reactant
[16–19]. The syntheses of the zinc glycoxide precurꢀ
sors for obtaining fineꢀparticle zinc oxide included the
following operations. HOCH2CH2OH (analytical
grade) was poured into four 150ꢀmL heatꢀresistant
glass beakers, 50 mL in each, and 2.5 g of zinc formate
Zn(HCOO)2 · 2H2O was added into each beaker. Three
of the resulting mixtures were heatꢀtreated at 50, 100,
and 150
fourth mixture was heated to 200
was held at this temperature for 1 h. The solid products
°
C
for 12 h with intermittent stirring. The
°
C
under stirring and
nanometerꢀsized particles and to test the photocataꢀ were separated from the liquid phase by vacuum filtraꢀ
lytic activity of these materials in hydroquinone oxidaꢀ tion, washed with absolute acetone, dried at 40
for
tion in water under UV and visible radiation. removing the acetone, and placed in weighing bottles
°
C
145

146
KRASIL’NIKOV et al.
with groundꢀglass stoppers to rule out the effect of Anodic voltammograms were recorded in the 0.0–1.0 V
atmospheric moisture. The (1 – )Zn(HCOO)2 · 2H2O + potential range; cathodic voltammograms, between
Co(HCOO)2 · 2H2O mixtures, with varied up to 0.3, 1.0 and –0.5 V. The indicator electrode was a cylindriꢀ
x
x
x
were heatꢀtreated in the same way. Zinc and cobalt forꢀ cal glassy carbon electrode with a working surface area
mates were synthesized by reacting formic acid (anaꢀ of 0.44 cm2. The reference and auxiliary electrodes
lytical grade) with zinc oxide (specialꢀpurity grade) were saturated Ag/AgCl electrodes (EVLꢀ1M3). The
and cobalt carbonate (analytical grade), respectively:
supporting electrolyte was 0.03 M sodium sulfate (speꢀ
cialꢀpurity grade).
(
)
(1)
(2)
ZnO + 2HCOOH + H O = Zn HCOO ⋅ 2H O,
2
2
2
CoCO3 + 2HCOOH + 2H2 O
RESULTS AND DISCUSSION
(
)
= C o HCOO ⋅ 2H O + CO .
2
According to the phase, elemental, and physicoꢀ
chemical analyses of the heatꢀtreatment products of
Zn(HCOO)2 · 2H2O mixed with ethylene glycol, the
interaction between the components takes place in
three steps according to the following chemical equaꢀ
tions:
2
2
The phase composition of the precursors and their
thermolysis products was determined by Xꢀray powder
diffraction (DRONꢀ2 diffractometer, Cu
K radiation)
α
and transmission light microscopy (POLAM Sꢀ112
polarizing microscope). Refractive indexes were meaꢀ
sured using a set of standard immersion liquids. Thermoꢀ
gravimetric analysis was carried out on a Qꢀ1500D therꢀ
moanalytical system in air at a heating rate of 10 K/min.
The IR spectra of powders were recorded on a Specꢀ
trumꢀOne spectrometer (PerkinElmer) in the 400–
4000 cm–1 range. Particle shapes and sizes were deterꢀ
mined by scanning electron microscopy (SEM) using
a Tesla BSꢀ301 instrument. The zinc and cobalt conꢀ
tents were determined by atomic absorption spectrosꢀ
copy (PerkinElmer spectrometer, acetylene–air
flame) and by inductively coupled plasma–atomic
emission spectroscopy (JYꢀ48 spectrometer). To
determine the total carbon content, the sample to be
Zn(HCOO)2 ⋅ 2H2O + HOCH2CH2OH
(3)
(
)
→ Zn(HCOO)2 HOCH2CH2OH + 2H2O ↑,
(
)
Zn(HCOO)2 HOCH2CH2OH
(4)
(
)
→ Zn(HCOO) OCH2CH2O
1/2
+1/2HOCH2CH2OH + HCOOH ↑,
(
)
Zn(HCOO) OCH2CH2O
1/2
(5)
+1/2HOCH2CH2OH
(
)
→ Zn OCH2CH2O + HCOOH ↑ .
As is clear from chemical equation (3), the first step
, yields the
. In the solvate Zn(HCOO)2(HOCH2CH2OH) through the
quantification of free carbon, we made use of the fact replacement of the two water molecules in zinc forꢀ
that free carbon is inert toward the hydrofluoric acid + mate by one ethylene glycol molecule. At ~100
analyzed (0.02–0.20 g) was covered with a CuO flux of this interaction, which occurs at ~50
and was burned in flowing oxygen at 1200
°
С
°
С
°
С,
nitric acid mixture, as distinct from carbon bound to a the Zn(HCOO)2(HOCH2CH2OH) solvate dissolves
metal. A 0.20ꢀg sample was placed in a platinum bowl, in ethylene glycol to yield a colorless homogeneous
HF (50 mL) and HNO3 (specialꢀpurity grade, 1–2 mL) solution, from which filamentous crystals of zinc forꢀ
were added, and the mixture was heated on a stove mate glycoxide, Zn(HCOO)(OCH2CH2O)1/2 or
until the complete dissolution of the solid. Next, the Zn2(HCOO)2(OCH2CH2O), precipitate. At 150 С
°
contents of the bowl were evaporated to wet salts, and above, the replacement of the formate ion in
20 mL of water was added, and the mixture was evapꢀ Zn(HCOO)(OCH2CH2O)1/2 followed by the removal
orated again. This evaporation–water addition proceꢀ of this ion as formic acid from the solution yields the
dure was repeated three times. The resulting solid was totally substituted zinc glycoxide Zn(OCH2CH2O)
collected on an asbestos filter, washed with water until whose crystals have the shape of a distorted octaheꢀ
pH 7, and dried at 110–120 . Its carbon content was dron. Therefore, the major factor determining the
,
°
С
determined as the amount of carbon dioxide resulting degree of substitution of the formate ion in
from the heating of the sample in flowing oxygen using Zn(HCOO)2 · 2H2O is the reaction temperature.
a MetavakꢀCS express analyzer. The specific surface Reaction (3) is completely reversible because, when
area of the thermolysis products of the glycoxide comꢀ Zn(HCOO)2(HOCH2CH2OH) is in contact with
plexes was estimated by lowꢀtemperature nitrogen water vapor at room temperature, water replaces ethꢀ
adsorption (BET method) on a TriStar 3000 automated ylene glycol in the solvate to yield Zn(HCOO)2 · 2H2O
.
analyzer (Micromeritics, United States). The soluꢀ A similar situation is observed in the water vapor
tions of hydroquinone (analytical grade, 0.01 mol/L), hydrolysis of MC2O4(HOCH2CH2OH) solvates (M =
in 50ꢀmL quartz beakers, were UVꢀirradiated using a Mn, Fe, Co, Zn) obtained by heating ethylene glycol +
BUFꢀ15 lamp (λmax = 253 nm). Visible light irradiaꢀ MC2O4 · 2H2O mixtures [20–22]. In this case, raising
tion was carried out using a blue luminescent lamp the synthesis temperature does not lead to the formaꢀ
(λmax = 440–460 nm). Current–voltage measureꢀ tion of the partially or totally substituted glycoxide
ments in the solutions were performed with a PUꢀ1 complex; that is, the process ends in the formation of
polarograph at a potential scan rate of 0.030 V/s. the MC2O4(HOCH2CH2OH) solvate.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 2 2011

SYNTHESIS AND PHOTOCATALYTIC PROPERTIES
147
Depending on the reaction conditions and x, the
Zn1–хCox(HCOO)(OCH2CH2O)1/2 solid solutions can
be obtained both as filaments with a large lengthꢀtoꢀ
diameter ratio and as long rods. Slowly heating a mixꢀ
ture of zinc and cobalt formates with ethylene glycol to
100–120 С followed by holding the mixture at this
°
temperature for a certain time yields extended crystals
of a solid solution:
(1 − x) Z n (HCOO) 2 ⋅ 2H2O
+x Co(HCOO)2 ⋅ 2H2O + HOCH2CH2OH
(6)
(
)
= Zn1− x Cox(HCOO) OCH2CH2O
1/2
+1/2HOCH2CH2OH + HCOOH ↑ +2H2O ↑ .
The thinnest needleꢀlike crystals of Zn1 – хCox
0.5 μm
(а)
(HCOO)(OCH2CH2O)1/2
the solution as it is evaporated slowly. The filamentous
Zn1 – хCox(HCOO)(OCH2CH2O)1/2 (0.1 0.3)
(х = 0.05) precipitate from
≤
x
≤
crystals resulting from the first step of the process tend
to longitudinal intergrowth, which results in the forꢀ
mation of intergrowths with a diameter of 0.5
above. As is increased, the Zn1 – хCox
(HCOO)(OCH2CH2O)1/2 crystals become shorter and
their diameter increases to become 0.5 m at = 0.3.
μm and
x
μ
х
The partial replacement of zinc with cobalt in the
Zn1 – хCox(HCOO)(OCH2CH2O)1/2 structure imparts
color to the crystals. As
increases from light pink (
0.3). As the synthesis temperature is raised to 180
х is increased, the color density
х
= 0.005) to intense lilac (
x
С
=
,
°
2 μm
(b)
the filamentous lilac phase of Zn1 – хCox
(HCOO)(OCH2CH2O)1/2 gradually disappears, giving
way to intensely blue crystals of the Zn1 – хCox
(OCH2CH2O) solid solution. In outline, the formation
of the cobaltꢀsubstituted glycoxide and cobalt glycoxꢀ
ide upon heating the formate + ethylene glycol mixꢀ
ture can be described by the following overall reacꢀ
tions:
Fig. 1. SEM images of the Zn(HCOO)(OCH CH O)
1/2
2
2
(a) filaments and (b) bars and (insets) the products of their
thermolysis at 450 in air.
°С
glycoxide Zn(HCOO)(OCH2CH2O)1/2 crystallizes as
thin colorless, needles. The crystals are very hygroꢀ
scopic and are characterized by extinction parallel to
their length. It seems impossible to determine the refracꢀ
tive indexes of filamentous Zn(HCOO)(OCH2CH2O)1/2
because of its high hygroscopicity. As a supersaturated
solution of Zn(HCOO)(OCH2CH2O)1/2 is heated rapꢀ
idly in ethylene glycol, the zinc salt can crystallize as
(1 − x) Z n (HCOO) 2 ⋅ 2H2O
(7)
+x Co(HCOO)2 ⋅ 2H2O + HOCH2CH2OH
(
)
= Zn1− x Cox OCH2CH2O + 2HCOOH ↑ +2H2O ↑,
Co(HCOO)2 ⋅ 2H2O + HOCH2CH2OH
bars, some of which having a diameter of about 2 m.
This allowed us to measure the refractive indexes of
this compound with a sufficient degree of accuracy:
μ
(8)
(
)
= Co OCH2CH2O + 2HCOOH ↑ +2H2O ↑ .
Ng = 1.609, Np = 1.543, (Ng – Np) = 0.066. The
Δ
Cobalt glycoxide crystallizes as very thin, optically
nearꢀisotropic flakes 0.5–1 m in diameter and ~100 nm
morphology and size of the filamentous and barlike
crystals of Zn(HCOO)(OCH2CH2O)1/2 before and
after their heat treatment are shown in Fig. 1. The filꢀ
amentous crystals aggregate by forming longitudinal
intergrowths (Fig. 1a). Upon heat treatment, the
filamentous shape of the precursor crystals is inherꢀ
ited by the resulting zinc oxide particles. Similar
transformations are observed for the barlike crystals
of Zn(HCOO)(OCH2CH2O)1/2, whose thermolysis
product (ZnO) consists of barlike particles. These parꢀ
μ
in thickness. The color of the powder is light violet.
Chakroune et al. [23] synthesized cobalt glycoxide
Co(OCH2CH2O) as discs by reacting cobalt acetate with
ethylene glycol and determined its unit cell parameters to
be a = 3.09 Å and c = 8.27 Å. In polarized light, the crysꢀ
tals of the solvate Zn(HCOO)2(HOCH2CH2OH) are
colorless and appear as rhombic plates with low bireꢀ
fringence and refractive indexes of Ng = 1.519 and
Np = 1.492. For comparison, Zn(HCOO)2 · 2H2O has
Ng = 1.546, Nm = 1.519, and Np = 1.506. Zinc formate ticles are porous or spongy and have a specific surface
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 2 2011

148
KRASIL’NIKOV et al.
Zn1 – хCox(OCH2CH2O) solid solution does not exert
any significant effect on the position of the reflections
in its Xꢀray diffraction patter. This pseudomorphic
transformation into the oxide phase is also typical of
Zn1 – хCox(OCH2CH2O) with
shows the SEM images of Zn0.9Co0.1(OCH2CH2O)
crystals and the product of their thermolysis at 450
0
≤
x
≤
0.3. Figure 2
°
С
(inset). In this case, the pseudocrystals of the thermolꢀ
ysis product are built of minute zinc oxide grains and
have a large number of cavities. The specific surface
area of Zn1 – хCoxO (0
≤
x
0.3) is 32 m2/g.
≤
The thermal decomposition of filamentous
Zn(HCOO)(OCH2CH2O)1/2 and Zn1 – xCox(HCOO)
5 μm
(OCH2CH2O)1/2 (0 <
cally in two steps at ~300–390 and 400–480
The weight loss for Zn(HCOO) (OCH2CH2O)1/2 at
450 is 41.5 wt %, approximately 0.5 wt % less
x
≤
0.2) in air occurs exothermiꢀ
°
C
(Fig. 3a).
Fig. 2. SEM images of Zn Co (OCH CH O) crystals
and (inset) the products of their thermolysis at 450°С in
0.9 0.1
2
2
°
С
air.
than is expected for the decomposition of this comꢀ
pound to ZnO (42.03 wt %). For the thermolysis of
the Zn1 – xCox(HCOO)(OCH2CH2O)1/2 solid soluꢀ
tions, the actual weight loss differs from the
expected weight loss by an average of 0.65 wt %. The
observed discrepancy between the theoretical and experiꢀ
mental weight loss values is due to the presence of bound
carbon in the samples, whose content is 0.53–0.76 wt %
area of 23.5 m2/g. Zinc glycoxide Zn(OCH2CH2O)
forms colorless crystals as distorted octahedra with
very imperfect cleavages. The refractive indexes of its
single crystals are Ng = 1.581, Nm = 1.564, Np = 1.548,
and (Ng – Np) = 0.033. The substitution of cobalt for
Δ
zinc raises the maximum refractive index and birefrinꢀ
gence; for example, for Zn0.95Co0.05(OCH2CH2O):
for
thermolysis of the Zn1 – xCox (OCH2CH2O) (0
glycoxides occurs exothermically in a single step (Fig. 3b).
0.039. Note that increasing the cobalt content of the The weight loss for Zn(OCH2CH2O) at 450 is 34.6 wt %,
0
≤
x
≤
0.1, according to elemental analysis data. The
≤ ≤
x
0.2)
Ng = 1.585, Nm = 1.567, Np = 1.546, and (Ng – Np) =
Δ
°
C
(а)
(b)
Δ
m, mg
TG
TG
10
20
30
40
10
20
30
40
DTA
DTA
420
365
320
200
400
600
T, °C
200
400
600
Fig. 3. TG and DTA profiles for (a) Zn(HCOO)(OCH CH O) and (b) Zn(OCH CH O)
.
2
2
1/2
2
2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 2 2011

SYNTHESIS AND PHOTOCATALYTIC PROPERTIES
(b)
149
(a)
1
1
2
2
3
3
4
10
20
30
40
50
60
, deg
20
30
40
50
60
, deg
2θ
2θ
Fig. 4. (a) Xꢀray diffraction patterns of (
1) Zn(HCOO) (HOCH CH OH), (2) Zn(HCOO)(OCH CH O) , and (3)
2 2 2 2 2 1/2
Zn(OCH CH O); (b) those of the products of the thermolysis of Zn
Co (HCOO)(OCH CH O) in (1–3) air (
х
= (
1) 0.05,
2
2
1 –
x
x
2
2
1/2
(2
) 0.2, and (
3) 0.3) and (4) helium (x = 0.2) at 450°С. The reflections from the minor phase with a cubic spinel structure are
starred.
whichis0.5wt % below the value expected for the comꢀ causes no significant changes in their diffraction patꢀ
plete decomposition of the compound to zinc oxide terns. The thermolysis product of Zn1 –xCox
(35.1 wt %). Again, elemental analysis data indicate (HCOO)(OCH2CH2O)1/2 with
the incomplete removal of carbon from all Zn1 – хCoxO CoxO with a wurtzite structure as the major phase and
solid solutions obtained at 450 . For the glycoxide a minor amount of a cubic spinel, probably Co3 –
complexes heatꢀtreated in a helium atmosphere, the ZnyO4. According to Xꢀray diffraction data, the latter
thermolysis product has a higher carbon content of 2– phase is present in the thermolysis products of the
x = 0.3 contains Zn1 –
х
°
С
y
x
>
5 wt %, depending on the nature of the complex. While 0.20 glycoxides. As was demonstrated by transmitted
the product of Zn0.95Co0.05(HCOO)(НOCH2CH2O)1/2 polarized light microscopy, the samples heatꢀtreated
thermolysis in the inert gas medium contains 2.1 wt % in the inert atmosphere consist of black, nontransparꢀ
carbon,
the
carbon
content
of
the ent objects pseudomorphic to the crystals of the preꢀ
Zn0.95Co0.05(OCH2CH2O) thermolysis product is cursor—zinc formate glycoxide or zinc glycoxide. The
4.62 wt %. Note that the weight loss of the samples black color of the pseudocrystals is due to the presence
heated in air to 750оС at a rate of 10 K/min is nearly of elemental carbon. The carbon is almost entirely
equal to the value expected for the formation of ZnO removed as the sample is heatꢀtreated in air at 450оС
,
or Zn1 – хCoxO. Based on the results of the thermoꢀ and the sample turns greenish.
gravimetric and elemental analyses, the composition
of the glycoxide thermolysis products can be repreꢀ
The IR spectrum of Zn(HCOO)(OCH2CH2O)1/2
(Fig. 5) can be viewed as the superposition of the specꢀ
tra of the HCOO– and OCH2CH2O2– anions. The very
sented as Zn1 – хCoxO1 – yСy
.
Figure 4 presents the Xꢀray diffraction patterns of strong absorption band at 1574 cm–1 and the strong
the solvate Zn(HCOO)2(HOCH2CH2OH), zinc forꢀ band at 1361 cm–1 are due to the asymmetric and symꢀ
mate glycoxide Zn(HCOO)(OCH2CH2O)1/2, zinc metric vibrations of the carboxyl group (COO) of the
glycoxide Zn(OCH2CH2O), and the products of the formate ion [24]. The C–O bond vibrations in the
thermolysis of Zn1 – xCox(HCOO)(OCH2CH2O)1/2 in OCH2CH2O2– ion coordinated to zinc give rise to two
air (x = 0.2, 0.3) and in helium (x
= 0.2) at 450°С. The very strong bands at 1084 and 1043 cm–1, which are
substitution of cobalt for zinc in these compounds close to the corresponding IR bands of liquid ethylene
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 2 2011

150
KRASIL’NIKOV et al.
1
2
4
(а)
1
3
2
1
2
3
4
0
4
2
4
6
8
, h
τ
(b)
1
2000
1500
1000
, cm–1
500
ν
3
Fig. 5. IR spectra of (
1
)
Zn(OCH CH O) and (
.
2)
2
2
Zn(HCOO)(OCH CH O)
2
2
1/2
2
1
glycol (1087 and 1043 cm–1) [25], vanadyl glycoxide
VO(OCH2CH2O) (1060 and 1012 cm–1), and titanium
glycoxide Ti(OCH2CH2O)2 (1055 and 1033 cm–1) [18].
The twisting vibrations of the C–C bonds show themselves
as very strong absorption bands at 899 and 871 cm–1,
which are shifted to higher frequencies relative to the
same bands of liquid ethylene glycol (883 and 862 cm–1)
and to lower frequencies relative to the corresponding
bands of vanadyl glycoxide (925 and 887 cm–1) and
titanium glycoxide (916 and 878 cm–1). The IR specꢀ
trum of Zn(OCH2CH2O) differs markedly from that of
Zn(HCOO)(OCH2CH2O)1/2 (Fig. 5): all of its bands
can be assigned with confidence to vibrations of the
OCH2CH2O2– ion and Zn–O bonds. The absorption
bands of the C–O bonds (1068 and 1049 cm–1) are
strongly shifted toward one another as compared to
the corresponding bands in the IR spectra of liquid
ethylene glycol and Zn(HCOO)(OCH2CH2O)1/2. At
the same time, the frequencies of the twisting vibrations
of the C–C bonds are increased to 898 and 881 cm–1.
The substitution of cobalt for zinc yielding Zn1 – xCox
4
5
2
3
0
2
4
6
8
10
12 14
, h
τ
Fig. 6. Hydroquinone (HQ) concentration as a function of
time: (a) UV irradiation of the solution ( ) in the absence
of a catalyst and ( ) in the presence of ( ) ZnO whisꢀ
kers, ( Zn Co whiskers, and ( Zn Co
pseudocrystals. (b) The same for blue lightꢀirradiated soluꢀ
tion ( ) in the absence of a catalyst and (2–5) in the presꢀ
ence of whiskers of ( Zn Co , ( Zn Co
Zn Co , and (
1
2
–
4
O
2
3
)
4
)
O
0.8 0.2
0.8 0.2
1
2
)
O
3
)
O,
0.95 0.05
0.99 0.01
5) Zn Co O.
(
4)
O
0.8 0.2
0.7 0.3
of a standard organic compound in water under UV
irradiation. The organic standard was hydroquinone,
which is among the strongest organic toxicants in natꢀ
ural water. Catalyst samples were prepared by heating
the precursors to 450
holding them at this temperature for 2 h. (These heatꢀ
(HCOO)(OCH2CH2O)1/2 or Zn1 xCox(OCH2CH2O)
᎐
solid solutions causes no noticeable changes in the IR
spectra.
For estimating the photocatalytic efficiency of
the wurtziteꢀtype Zn1 – xCoxO solid solutions
°
С at a rate of 10 K/min and
obtained by Zn1 – xCox(HCOO)(OCH2CH2O)1/2 ing conditions were found to be optimal.) These
and Zn1 xCox(OCH2CH2O) (0
≤
x
≤
0.3) thermolyꢀ experiments demonstrated that, for all Zn1 – xCoxO
᎐
sis, we studied their effect on the decomposition rate solid solutions, the rate of hydroquinone oxidation
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 2 2011

SYNTHESIS AND PHOTOCATALYTIC PROPERTIES
151
2. J. Xie, P. Li, W. Yanji, and Y. Wei, J. Phys. Chem. Solids
under UV irradiation (λmax = 253 nm) increases defiꢀ
nitely with an increasing cobalt content. The highest
photocatalytic activity is shown by Zn0.7Co0.3O, which
contains a minor amount of a cubic spinel phase,
probably Co3 – yZnyO4. These experimental data are in
agreement with the high photocatalytic performance
of Co3O4 in methyl orange oxidation [26]. The slightly
higher catalytic activity of the pseudocrystalline versus
quasiꢀ1D Zn1 – xCoxO samples at the early stages of the
process is likely due to the difference between their
specific surface areas (32 m2/g for the former against
25 m2/g for the latter). These catalytic activity data are
presented in Fig. 6a, which shows how the hydroꢀ
quinone concentration in the solution varies with
time under UV irradiation in the absence of a cataꢀ
lyst and in the presence of ZnO, Zn0.8Co0.2O, and
pseudocrystals of Zn0.8Co0.2O. The photocatalytic
activity of Zn1 – xCoxO in blue light depends on the
cobalt content of the catalyst in a similar way. Figure
6b demonstrates how the hydroquinone concenꢀ
tration in the solution varies with time in blue light
in the absence of a catalyst and in the presence of
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Zn1 – xCoxO (x = 0.01, 0.05, 0.2, 0.3) whiskers. It is
impossible to determine the effect of bound carbon
on the photocatalytic properties of Zn1 – xCoxO and
Zn1 – хCoxO1 – yСy because the carbon contents of the
samples are nearly equal. However, it was reported that
the photocatalytic activity of pure zinc oxide can be
enhanced by introducing carbon [27]. Therefore, the
residual carbon staying in the oxide after the thermolꢀ
ysis of the glycoxide precursor at least does not exert an
adverse effect on the catalyst.
14. L. Chen, J. Zhang, X. Zhang, et al., Optics Express 16
,
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101 (2004).
,
18. V. N. Krasil’nikov, A. P. Shtin, O. I. Gyrdasova, et al.,
Ros. Nanotekhnol. , 107 (2008).
3
Thus, we have devised and carried out the targeted
synthesis of micronꢀ and nanometerꢀsized Zn1 – xCoxO
19. O. I. Gyrdasova, V. N. Krasil’nikov, G. V. Bazuev, et al.,
Proceedings of the 11th International Symposium “Order,
Disorder, and Properties of Oxides” (pos. Loo, Rostovꢀ
onꢀDon, Russia) [in Russian], Vol. 1, p. 128.
(0
als with
≤
x
≤
0.2) solid solutions and heterogeneous materiꢀ
> 0.2 in two morphological types. These
x
materials are promising catalysts for the photocataꢀ
lytic oxidation of hydroquinone in UV and blue light.
A correlation has been established between the photoꢀ
20. O. I. Gyrdasova, V. N. Krasil’nikov, I. G. Grigorov, and
G. V. Bazuev, Zh. Neorg. Khim. 51, 1020 (2006).
21. V. N. Krasil’nikov, O. I. Gyrdasova, and G. V. Bazuev,
catalytic activity of Zn1 – xCoxO and
x. The photocataꢀ
Zh. Neorg. Khim. 53, 1984 (2006).
lytic activity of the catalysts depends more strongly on
22. V. N. Krasil’nikov, O. I. Gyrdasova, and G. V. Bazuev,
x
than on their particle size or specific surface area.
Zh. Neorg. Khim. 54, 1097 (2008).
23. N. Chakroune, G. Viau, S. Ammar, et al., New
J. Chem. 29, 355 (2005).
ACKNOWLEDGMENTS
24. K. Nakamoto, Infrared and Raman Spectra of Inorganic
and Coordination Compounds (Wiley, New York, 1986;
Mir, Moscow, 1991).
This study was supported by the Russian Foundaꢀ
tion for Basic Research, project no. 09ꢀ03ꢀ00252ꢀa.
25. H. Matsuura and T. Miyazawa, Bull. Chem. Soc. Jpn.
40, 85 (1967).
26. Y. Chen, L. Hu, M. Wang, et al., Colloids Surf. 336, 64
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RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 2 2011