Surface-Modified CdS/ZnO Material: Single-Reactor Synthesis and Mechanism of Formation in Aqueous Solution

Article abstract of DOI:10.1134/S1070427218030187

Method of chemical precipitation from aqueous solutions was used to cover the surface of polycrystalline ZnO nanotubes with a nanostructured CdS layer. The thus synthesized CdS/ZnO composite material was studied by the methods of X-ray diffraction analysis, electron microscopy, and optical spectroscopy. The fundamental time-related aspects of the process of CdS formation on the ZnO surface were examined. It was found that the amount of deposited CdS nanoparticles is independent of the deposition duration. The morphological specific features of ZnO nanotubes are preserved upon a prolonged keeping of ZnO in solution. The photocatalytic activity of CdS/ZnO under visible and UV light was examined in the reaction of hydroquinone oxidation. A possible mechanism of how the CdS/ZnO composite is formed in an aqueous solution in the course of growth of a layer constituted by CdS nanoparticles on the surface of ZnO nanotubes is suggested on the basis of the experimental data. It is demonstrated that the chemical-precipitation method can be used to obtain surface-active composite materials that are photoactive in the visible spectral range.

Full text of DOI:10.1134/S1070427218030187

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2018, Vol. 91, No. 3, pp. 454−462. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © N.S. Kozhevnikova, O.I. Gyrdasova, I.V. Baklanova, L.Yu. Buldakova, M.Yu. Yanchenko, A.S. Vorokh, 2018, published in Zhurnal
Prikladnoi Khimii, 2018, Vol. 91, No. 3, pp. 417−425.
SPECIFIC
TECHNOLOGICAL SOLUTIONS
Surface-Modified CdS/ZnO Material: Single-Reactor
Synthesis and Mechanism of Formation
in Aqueous Solution
N. S. Kozhevnikova*, O. I. Gyrdasova, I. V. Baklanova,
L. Yu. Buldakova, M. Yu. Yanchenko, and A. S. Vorokh
Institute of Solid-State Chemistry, Ural Branch, Russian Academy of Sciences,
ul. Pervomaiskaya 91, Yekaterinburg, 620990 Russia
*e-mail: kozhevnikova@ihim.uran.ru
Received January 15, 2018
Abstract—Method of chemical precipitation from aqueous solutions was used to cover the surface of polycrys-
talline ZnO nanotubes with a nanostructured CdS layer. The thus synthesized CdS/ZnO composite material was
studied by the methods of X-ray diffraction analysis, electron microscopy, and optical spectroscopy. The funda-
mental time-related aspects of the process of CdS formation on the ZnO surface were examined. It was found that
the amount of deposited CdS nanoparticles is independent of the deposition duration. The morphological specific
features of ZnO nanotubes are preserved upon a prolonged keeping of ZnO in solution. The photocatalytic activ-
ity of CdS/ZnO under visible and UV light was examined in the reaction of hydroquinone oxidation. A possible
mechanism of how the CdS/ZnO composite is formed in an aqueous solution in the course of growth of a layer
constituted by CdS nanoparticles on the surface of ZnO nanotubes is suggested on the basis of the experimental
data. It is demonstrated that the chemical-precipitation method can be used to obtain surface-active composite
materials that are photoactive in the visible spectral range.
DOI: 10.1134/S1070427218030187
ZincoxideZnObelongstothegroupofmultifunctional energy gap widths of ZnO (3.4 eV [3]) and CdS (2.4 eV
[4]) makes the CdS/ZnO composite a promising material
for application as a photocatalyst in reactions of oxidation
of organic compounds and as a photoabsorbing layer in
solar cells.
wide-bandgap semiconductors and is used in oxidation
photocatalysts, gas sensors, voltage-controlled resistors,
planar field displays, etc. However, the potential
applications of ZnO as a photoactive material are
limited to the UV spectral range because of its wide
bandgap. It is commonly believed that one of the main
ways to extend the spectral range of ZnO sensitivity to
longer wavelengths is via its chemical modification
(sensitization) with optically active compounds, e.g.,
chalcogenide nanoparticles (quantum dots, QDs) of
CdS, CdSe, CdTe, and PbS [1]. If ZnO has a developed
surface, the most effective sensitization variant is to form
a thin chalcogenide layer on the surface of the oxide
The goal of our study was to describe processes
occurring when the CdS/ZnO composite is formed in
an aqueous solution. The composite was synthesized
via precipitation of CdS nanoparticles from an aqueous
solution onto preliminarily synthesized ZnO nanotubes.
The samples were obtained at different synthesis
durations and their phase composition, crystal structure,
morphology, and optical properties were studied. The
mechanism by which CdS nanoparticles are formed on
matrix. In contrast to a bulk composite, such a surface- the ZnO surface is suggested on the basis of the data
modified material can provide a close and continuous obtained. The photocatalytic activity of the CdS/ZnO
contact of two materials with different energy gap widths composite under visible light was examined in the model
(heterojunction). The substantial difference between the reaction of hydroquinone (HQ) oxidation.
454

SURFACE-MODIFIED CdS/ZnO MATERIAL
455
EXPERIMENTAL
a 150MI pH-meter (Russia) with a combined electrode
and measurement accuracy of 0.02 on the pH scale.
ZnO nanotubes were synthesized via thermolysis
of organic precursors (or by the precursor method) [5.
In the first stage, zinc formate Zn(HCOO)2·2H2O was
produced by the action of formic acid on spectral-purity
ZnO under heating:
To determine the solubility of ZnO nanotubes
upon deposition of CdS onto their surface and also to
monitor the extent of reaction (3), an elemental analysis
of aqueous solutions over the CdS/ZnO deposits for
the content of zinc and cadmium atoms was made by
the method of atomic-adsorption spectroscopy in an
acetylene–air flame on a Perkin Elmer atomic-absorption
spectrometer (United States).
ZnO + 2HCOOH + H2O = Zn(HCOO)2·2H2O.
(1)
Then,
zinc
formatoglycolate
The theoretical values of the equilibrium
concentration [Cd2+]aq and [OH–]aq in solution over
the Cd(OH)2 and CdS deposits were calculated on the
following assumptions: for poorly soluble compounds
(Ksp < 10–7), the ion concentrations in its saturated
solution are very low; (b) the saturated solution
concentration is such that the activity coefficients of the
ions are close to unity, and, consequently, the activities
of the ions are nearly equal to their concentrations.
Zn(HCOO)·(OCH2CH2O)0.5 was synthesized at 100°C
by the brutto-reaction
Zn(HCOO)2·2H2O + 0.5HOCH2CH2OH
= Zn(HCOO)(OCH2CH2O)0.5 + HCOOH↑ + 2H2O. (2)
The resulting Zn(HCOO(OCH2CH2O)0.5 was an-
nealed in air at 500°C for 2 h. The thermolysis of
Zn(HCOO(OCH2CH2O)0.5 yielded ZnO with a qua-
si-1D morphology of tubular aggregates (nanotubes)
and HCP (face-centered close-packed) structure of the
wurtzite type (B4) [5].
An X-ray diffraction analysis of the composites was
made with STADI-P (STOE, Germany) and Shimadzu
MAXima-X XRD-7000 Japan) automated X-ray
diffractometers with CuKα radiation, 2θ angle step of
0.03°, and keeping at a point for 10 s. The diffraction
patterns were analyzed via full-profile description of a
diffraction curve with Multipeak Fit 2 software package
with IgorPro 6.1. The phase composition of the samples
was evaluated by the partial intensities of reflections
associated with separate phases. The particles sizes of
various phases were determined by the Scherrer formula
D = λ/B cos θ, where D is the size of particles (coherent
scattering regions); λ, wavelength of the X-ray radiation;
B, full width of a reflection at its half maximum; and θ,
radiation incidence angle.
The ZnO nanotubes were sensitized with CdS
nanoparticles, and the surface-modified CdS/ZnO
material was produced by the method of chemical
precipitation from aqueous solutions. For this purpose,
a 0.3-g weighed portion of ZnO was placed in a 250 mL
of a solution containing 1.25 × 10–3 mol of cadmium
chloride CdCl2, 0.375 mol of aqueous ammonia, and
12.5 × 10–3 mol of thiocarbonic acid diamide N2H4CS at
initial pH value of 12.73 ± 0.02.
The process of CdS formation in the CdCl2–NH4OH–
(NH2)2CS can be represented by the overall equation
[4, 6]:
Themorphologicalandmicrostructuralcharacteristics
and the elemental composition of the samples were
determined by scanning electron microscopy (SEM)
on a JEOL JSM 6390LA electron microscope equipped
with a JED-2300 elemental-analysis attachment (Japan).
Absorption spectra of pure CdS, ZnO, and CdS/ZnO
composites in the visible and UV spectral ranges were
obtained with a Shimadzu V-3600 spectrophotometer at
wavelengths of 300–700 nm with an integrating sphere
and BaSO4 as reference.
Cd(NH3)n2+ + (NH2)2CS + 2OH–
= CdS↓ + nNH3 + H2NCN + 2H2O.
(3)
CdS/ZnO-1, CdS/ZnO-2, and CdS/ZnO-3 samples
were precipitated at a temperature of 325 K in the course
of 30, 120, and 240 min, respectively. The CdS/ZnO-
4 sample was produced by precipitation first at 325 K
in the course of 180 min and then at 295 K for 24 h.
The equilibrium pH value of the solutions was 13.46 ±
0.04 for all the samples. The constant temperature in
Having an energy gap width of 3.4 eV, ZnO has
the course of reaction (3) was maintained with an LOIP a photocatalytic activity under UV irradiation at
LT-105 liquid thermostat (Russia), with temperature
stability within 0.1°. The pH value was monitored with
wavelengths of less than 370 nm. The tight binding
of ZnO and CdS at a surface modification of ZnO
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KOZHEVNIKOVA et al.
Table 1. Phase composition and particle size of composite samples CdS/ZnO-1–4
CdS/ZnO-1
30
CdS/ZnO-2
120
CdS/ZnO-3
240
CdS/ZnO-4
Parameter
Synthesis duration, min
Phase composition, wt %
1620 (27 h)
CdS
52.1
47.9
0.0
–
52.9
46.4
0.7
–
47.4
45.8
6.8
–
57.7
13.7
20.6
8.0
ZnO
Cd(OH)2
ZnS
Particle size, nm:
CdS
5.5
23.4
–
6.4
24.1
16.9
–
7.7
25.8
73.0
–
5.2
32.6
52.1
1.5
ZnO
Cd(OH)2
ZnS
–
Element ratio on the ZnO surface:
Cd : S
1 : 1
1.6 : 1
5.6 : 1
1.9 : 1
7.6 : 1
0.8 : 1
Zn : Cd
8.7 : 1
83.3 : 1
nanotubes with CdS nanoparticles must give rise to a
photocatalytic activity under exposure to visible light. To
verify this statement and determine whether there is an
electronic contact between CdS nanoparticles and a ZnO
nanotube, we evaluated the photocatalytic properties of
the CdS/ZnO composite in the visible spectral range by
using quinone (component of the hydroquinone/quinone
electrochemical pair) with opening of the benzene ring
as the argument of the quinone oxidation reaction.
Thus, we performed a set of experiments on HQ
oxidation under visible and UV irradiation in the
presence of the CdS/ZnO composite. In addition, we
performed a set of comparison experiments with pure
CdS as a HQ oxidation catalyst under exposure to
visible light.
RESULTS AND DISCUSSION
According to XRD data, all the samples contain CdS
and ZnO phases, and the Cd(OH)2 phase was found in
sample nos. 2–4 (Table 1). The ZnO phase corresponds
to the hexagonal structure of wurtzite (space group
N196 P63mc) with cell parameters a = 0.325 nm and c =
0.521 nm (PDF N 00-036-1451). Table 1 lists the partial
fractions of phases found in the samples and the particle
sizes. For example, the fraction of ZnO in sample nos.
1–3 is less than 50% of the total mass. The dependence
of the synthesis duration appears only after 27 h, when
the content of ZnO substantially falls and reaches 14%
in sample no. 4. The size of coherent scattering regions
in ZnO varies between 23 and 33 nm. This means that
oxide nanotubes are agglomerates of nanoparticles.
For this purpose, we introduced a 50-mg portion
of a sample studied as a photocatalyst into 0.4 mM
HQ solutions and exposed the mixture to a UV or
visible radiation. The UV irradiation of hydroquinone
solutions was performed in quartz electrochemical cells
with a BUF-15 lamp (λmax = 253 nm). The oxidation
under exposure to a blue lamp (λmax = 440–460 nm)
was performed in glass electrochemical cells. The
concentration of the substrate being oxidized was
measured in discrete intervals of time (1–2 h) by the
height of the quinone oxidation peak at 0 V. The amount
of HQ in solution was found from the calibration plot by
the voltammetric method on a PU-1 polarograph with
a three-electrode measurement scheme (glassy carbon
indicator electrode, and silver chloride reference and
auxiliary electrodes). The potential variation rate in the
measurements was constant (0.030 V s–1).
The mass fraction of nanostructured CdS in sample
nos. 1–3 is about a half of the whole composite and in-
creases to 58% in sample no. 4. The CdS particle size is
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SURFACE-MODIFIED CdS/ZnO MATERIAL
457
2θ, deg
Fig. 1. X-ray diffraction patterns of CdS/ZnO-1–4 samples. (I) Intensity and (2θ) Bragg angle.
actually independent of time and constitutes about 5–8
nm. The crystal structure of CdS is determined from
three reflections at 2θ angles in the range from 10 to 55°
at 26.7, 44, and 52° and has the form of a random close
packing. This CdS structure is described by an “aver-
aged lattice” with hexagonal (space group N 168 P6)
symmetry and unit cell parameters a = 0.236 nm and c =
0.334 nm. Here, a is the distance between projections
of all atoms on the packing plane, and c is the distance
between the nearest close-packed layers with like atoms
(Cd or S) [7].
sare bound together by van der Waals bonds. The [001]
direction is perpendicular to the packing layers. The rise
in the intensity of the [001] reflection in the first stage
reflects the growth of particles predominantly along this
axis, i.e., via formation of new Cd(OH)2 layers. In the
course of time, the fraction of Cd(OH)2 grows, other re-
flections appear, and the relative intensities of various
reflections become level. These data suggest that, in 4 h,
hydroxide particles grow in all the crystallographic di-
rections.
In addition, the diffraction pattern of sample no. 4
shows a halo at angles 2θ = 15–30°, which indicates that
the amorphous phase is formed after 24 h of synthesis.
The inset of Fig. 1 shows a fragment of the difference
spectrum between the diffraction patterns of sample
nos. 3 and 4 and its decomposition into separate reflec-
tions. The narrow reflections correspond to coarsely
crystalline Cd(OH)2, and the broad peak can be attrib-
uted to the ZnS phase. The formation of the ZnS phase
is due to the decomposition of ZnO, which is confirmed
The third phase Cd(OH)2 appears only after 30 min
of synthesis (sample no. 2), being formed in an insignifi-
cant amount (< 1%). In this stage, the phase is identified
by the reflection [001], 2θ = 18.9°. After 4 h of synthesis
(sample nos. 3 and 4), there appear rest of the reflec-
tions characteristic of Cd(OH)2. The layered structure
of Cd(OH)2 is described by a hexagonal lattice (space
group N 164 P3m1) and cell parameters a = 0.349 nm
and c = 0.471 nm (PDF N 00-031-0228). Cd(OH)2 layer-
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KOZHEVNIKOVA et al.
A
(a)
(b)
λ, nm
E, eV
Fig. 2. (a) Absorption (A) spectra in the UV and visible spectral ranges and (b) determination of the effective energy gap width of
composite CdS/ZnO-1–4 samples produced in (1) 30, (2) 120, (3) 240 min, and (4) 24 h after the beginning of synthesis. (λ) Wavelength,
[(αhν)2] squared product of the absorption coefficient α by the photon energy hν, and (E) energy.
by the decrease in the mass fraction of ZnO according to
XRD data (Table 1). According to these data, the partial
contribution of ZnS in the composite is 8%. The size of
the coherent scattering regions of the ZnS phase is 1.5 nm.
show two absorption edges, from ZnO nanotubes
and CdS nanoparticles, which confirms the presence
of these two phases in the samples synthesized. The
presence of the third phase, Cd(OH)2, could not be
confirmed by optical measurements because the energy
gap of Cd(OH)2 is about 4.5 eV [8], and, accordingly,
Figure 2 shows absorption spectra in the UV and
visible ranges for CdS/ZnO composites. The spectra
(a)
(b)
5 μm
100 μm
(c)
10 μm
Fig. 3. SEM micrographs of (a) and the change of the microstructure of CdS/ZnO composite samples obtained after (b) 240 min and
(c) 24 h of synthesis.
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SURFACE-MODIFIED CdS/ZnO MATERIAL
459
Table 2. Solubility of ZnO nanotubes and extent of the reaction in which CdS particles are produced in formation of CdS/ZnO
composite samples
Metal concentration in solution after the end
Amount of ZnO that
Conversion of cadmium
ions to a solid phase
of synthesis, M
Sample
passed into solution,
mol %
Zn
Cd
CdS/ZnO-1
CdS/ZnO-2
CdS/ZnO-3
CdS/ZnO-4
0.0021
0.0024
0.0024
0.0012
3.5 × 10–6
9 × 10–8
14
16
17
16
0.9993
0.999994
→ 1
9 × 10–8
≤ 9 × 10–8
→ 1
the absorption edge of Cd(OH)2 at around 270 nm lies
outside the range of our measurements.
regular shape of a hexagonal prism, with the number of
these growing in the course of time. It has been shown
[4] that this shape is inherent in Cd(OH)2 crystals. The
results of XRD and chemical analyses of the composites
and a quantitative analysis of the ionic composition
of the solution over the precipitates also indicate that
agglomerates of regular shape belong to the Cd(OH)2
crystal phase with minor content of CdS. The CdS
phase is poorly seen in the micrographs (Figs. 3b and
3c) because of the small size of CdS particles. A study of
the surface morphology of CdS/ZnO composites, with
the elemental composition determined by SEM, made
it possible to trace how the Cd : S and Zn : Cd element
ratios vary in the course of time (Table 1).
To determine the effective energy gap width of
CdSZnO, the experimental spectra were brought to
the form (αhν)2 = f(hν). The frequency dependence of
the absorption coefficient near the absorption edge is
described by the following equation:
α(ν) = [A(hν – Eg)1/2]/hν,
where α is the absorption coefficient; hν, photon energy;
Eg, optical gap width; and A, constant independent of
the frequency ν [8].
The extrapolation of the linear portions of the
resulting curves to intersection with the abscissa axis was
used to determine the energy gap widths. Eg determined
for ZnO by the procedure described in [9] remains
nearly constant (3.10 eV) in sample nos. 1–4. However,
a long-wavelength shift of the absorption edge of CdS
nanoparticles is observed in CdS/ZnO composites
relative to the coarsely crystalline CdS (2.40 eV). The
energy gap width of CdS nanoparticles in sample nos.
1–4 slightly decreases with increasing precipitation
duration and is 2.36, 2.35, 2.33, and 2.30 eV, respectively
(the Eg measurement error does not exceed 5 × 10–3 eV).
According to the XRD data (Table 1), the size of CdS
nanoparticles is nearly independent of the synthesis
duration, and, consequently, does not affect the value of
Eg. The variation of Eg is probably due to the decrease
in the fraction of surface CdS atoms in the course of
interaction with the oxide ZnO matrix [4, 10].
An elemental analysis of aqueous solutions over
the CdS/ZnO deposits demonstrated that the solubility
of ZnO nanotubes upon the deposition of CdS on their
surface does not exceed 17 mol %, and the conversion
of CdCl2 to the solid phase reaches a value of 0.9993
already by 30th minute of precipitation and then remains
nearly constant (Table 2). (The degree of conversion is
the ratio between the number of moles of ith substance
that entered into the reaction and the initial number of
moles of this substance.)
The results of a photocatalytic oxidation of HQ
in the presence of the CdS/ZnO composite (for the
example of CdS/ZnO-1 sample) are presented in Fig. 4.
The photocatalytic data demonstrate that the HQ
concentration in solution in the presence of both CdS/
ZnO and pure CdS as catalysts falls in the course of
time in comparison with a pure HQ solution without a
catalyst.
According to SEM data (Fig. 3), ZnO retains in
the course of sensitization the initial size and shape of
tubular aggregates. The micrographs of sample nos. 3
and 4 (Figs. 2b and 2c) show agglomerates having the
The activity of the CdS/ZnO composite during the
first hours is higher under the UV irradiation, compared
with the exposure to visible light. The reason is that,
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KOZHEVNIKOVA et al.
cHQ × 104, M
remains nearly constant in sample nos. 2–4. The Cd : S
element ratio in sample no. 1 is 1 : 1 (Table 1), which
corresponds to the CdS phase and agrees with the XRD
data.
Without a catalyst
The experimentally determined concentration of free
and complex ions, Cd2+ and Cd(NH3)n2+, in solution over
the CdS/ZnO composite, cCD,aq, was 3.5 × 10–6 M by the
30th minute of synthesis (Table 2), which substantially
exceeds the theoretical equilibrium value [Cd2+]aq/CdS =
8.89 × 10–14 M calculated from the solubility product
Ksp(CdS) = 7.9 × 10–27 [12]. This means that, theoreti-
cally, CdS nanoparticles must continue to be formed
and(or) grow in solution.
τ, h
According to the XRD data, the mass fraction and
size of CdS particles do not vary with time, which shows
that the CdS concentration in sample nos. 2–4 is constant
and reactions (3) and (4) stop after 30 min of synthesis.
Because the value of cCd,aq remains comparatively
high by 30th minute of synthesis and does not hinder
the formation of CdS, the termination of reaction (3) is
due to the absence of a source of S2–, thiourea N2H4CS.
Probably, the large specific surface area of ZnO
nanotubes results in that, after 30 min of synthesis, part
of N2H4CS molecules starts to be sorbed on the tubes
to give surface complexes of the type Zn[N2H4CS]n
[13], with the concentration of N2H4CS molecules and,
accordingly, S2– ions in solution dramatically reduced.
Fig. 4. Results of a photocatalytic oxidation of HQ in the
presence of CdS/ZnO (sample 1) undervisible (curve Vis) and
UV irradiation (curve UV). For comparison, the figure shows
HQ oxidation curves without a catalyst and in the presence of
pure CdS under visible irradiation at wavelengths in the range
440–460 nm. (cHQ) Content of hydroquinone and (τ) time.
having a wide energy gap (3.4 eV), the ZnO matrix
exhibits photoactivity under the action of UV light,
whereas in the visible light, only CdS is active. The
difference between the HQ decomposition curves under
visible and UV irradiation in the presence of CdS/ZnO
confirms the existence of an electronic contact between
sulfide nanoparticles and the oxide matrix. In turn, it
can be seen in Fig. 4 that using the CdS/ZnO composite
instead of the pure optically active CdS makes
substantially larger the extent of the reaction of quinone
oxidation under irradiation with visible light. Thus,
the deposition of photoactive CdS nanoparticles onto
the semiconducting matrix can raise the photocatalytic
activity of the composite material in the visible light.
Thus, a solid nanocrystalline CdS phase is formed
during the first 30 min of synthesis [Eq. (3)] and
deposited onto the ZnO surface, with the thiourea
adsorption beginning (Fig. 5)
(2) Formation of Cd(OH)2 particles. It can be seen
in Table 2, that the concentration cCd,aq decreases by
120th minute of synthesis and one more solid phase,
Cd(OH)2, starts to be formed (less than 1 wt % in
sample CdS/ZnO-2) (Table 1). This process continues
during a long time and the amount of Cd(OH)2 reaches a
significant value (21 wt %) in sample no. 4. It is important
to note that, during the first hours, the mass fraction of
Cd(OH)2 grows simultaneously with the particle size,
which increases from 17 to 73 nm, but this correlation
disappears after 24 h. It can be stated that, during the
first hours, the Cd(OH)2 phase predominantly grows due
to the growth of the already formed crystals, whereas
after 4 h, the number of nuclei starts to increase.
Based on the experimental data, we can distinguish
three processes occurring in solution when the CdS/ZnO
composite is formed.
(1) Formation of CdS nanoparticles in solution
and deposition of CdS onto ZnO nanotubes. During
the first 30 min of the ZnO sensitization process, N2H4CS
molecules are hydrolyzed by a reversible reaction to
give S2– ions [4, 6, 11]:
←
SC(NH2)2
H S + H CN ,
(4)
→
2
2
2
which are directly involved in the formation of CdS by
Eq. 3). During this time, the amount of CdS in the CdS/
ZnO composite reaches ~50 wt % (sample no. 1) and
To explain the observed formation of a new
solid phase Cd(OH)2, we calculated the theoretical
concentration [Cd2+]aq/Cd(OH) in a saturated solution
2
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SURFACE-MODIFIED CdS/ZnO MATERIAL
461
Disintegration of CdS/ZnO
Formation of CdS/ZnO Growth of Cd(OH)2
0.5 h 2 h
4 h
24 h
Fig. 5. Mechanism by which surface-modified CdS/ZnO material is formed in an aqueous solution.
ZnOsolid + Cd2+/Cd(NH3)n2+ → [Cd2+/Cd(NH3)n
]
,
ads/ZnO
2+
over the Cd(OH)2 deposit at which Cd(OH)2 can be
formed. [Cd2+]aq/Cd(OH) was calculated from data
2+
2+
[Cd2+/Cd(NH3)n
]
[Cd2+/Cd(NH3)n ]aq,
2
ads/ZnO
on the solubility product Ksp[Cd(OH)2] = 2.2 × 10–14
[11] with consideration for the presence in solution of
a strong NaOH electrolyte containing the same anion.
[Cd2+/Cd(NH3)n
]
aq + 2ОН– = Cd(OH)2↓ + nNH3.
2+
Thus, by the 120th minute of synthesis of the
CdS/ZnO composite (Fig. 5), the formation of CdS
nanoparticles in solution ends due to the sharp decrease
in the concentration of sulfidizer, N2H4CS, and particles
of the Cd(OH)2 solid phase start to be formed and grow,
which is accompanied by the sorption of free Cd2+ and
complex Cd(NH3)n2+ ions on the ZnO surface.
At pH 12.73, [Cd2+]aq/Cd(OH) is 7.6 × 10–14 10–12 M,
2
which is several orders of magnitude smaller than the
experimental value cCd,aq = 9 × 10–1410–8 M (Table 2).
Thus, the concentration of hydroxide ions at pH ≥ 12.7
is sufficient for reaching Ksp[Cd(OH)2], and Cd2+ and
Cd(NH3)n2+ ions start to interact with OH– ions, which
leads to the nucleation and the subsequent crystallization
of the Cd(OH)2 phase directly in solution:
(3) Disintegration of the CdS/ZnO composite
and formation of the amorphous ZnS phase. Upon a
prolonged keeping of the CdS/ZnO composite (sample
no. 4) in an alkaline solution, the mass fraction of
ZnO in the composite becomes three times smaller
(14 wt %), and the quantitative element ratio Cd :
S becomes nearly stoichiometric (Table 1). This is
indicative of the degradation onset of the composite:
ZnO starts to dissolve, the surface of a ZnO nanotube
disintegrates, and CdS nanoparticles are desorbed from
the ZnO surface. In the process, the morphology of the
tubular ZnO aggregates is preserved (Fig. 2c). Analysis
of the diffraction patterns of sample no. 4 (Fig. 1)
demonstrates that one more phase appears, amorphous
zinc sulfide ZnS. The reaction in which ZnS is formed
in an aqueous solution containing thiourea and ammonia
can be represented as
Cd2+/Cd(NH3)n2+ + 2ОН− = Cd(OH)2↓ + nNH3.
(5)
However, reaction (5) does not reduce the
concentration cCd,aq (Table 2). The constant-in-time
value of cCd,aq (Table 2) in a solution over the sample
nos. 2–4, rather slow (during 24 h) growth of Cd(OH)2,
and appearance of an excess amount of cadmium
atoms (relative to sulfur atoms) on the surface of ZnO
nanotubes (Table 1) in sample nos. 2 and 3 suggest that
also possible in sensitization of ZnO, together with the
chemical reactions in which CdS [reaction (3)] and
Cd(OH)2 [reaction (5)] are formed, is the sorption of
free Cd2+ and complex Cd(NH3)n2+ ions on the developed
surface of ZnO nanotubes. It is the sorption that leads
to a decrease in cCd,aq after 30 min of synthesis, and its
dynamic nature maintains cCd,aq constant after 120 min
of synthesis and supports the slow growth of Cd(OH)2
particles. Taking into account the aforesaid, we can
represent the process of Cd(OH)2 formation as follows:
ZnOsolid → Zn ZnOsol·H2O Zn2+ + 2ОН–,
Zn(NH3)n2+ + (NH2)2CS + 2OH–
= ZnS↓ + nNH3 + H2NCN + 2H2O
(6)
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462
KOZHEVNIKOVA et al.
Khimiyaprivitykhpoverkhnostnykhsoedinenii(Chemistry
CONCLUSIONS
of Grafted Surface Compounds), Lisichkin, G.V., Ed.,
Moscow: Fizmatlit, 2003.
The sensitization of ZnO nanotubes with CdS
3. Park, C.H., Zhang, S.B, and Wei, S.-H., Phys. Rev. B,
nanoparticles by the method of chemical precipitation
from solution yields a CdS/ZnO composite material.
The amount of deposited CdS nanoparticles is
independent of the precipitation duration. In the case of
a prolonged keeping of ZnO nanotubes in solution, the
morphological features of the polycrystalline nanotubes
are preserved. At the same time, a prolonged keeping in
solutioncausesagradualdegradationoftheoxidematrix,
which changes the phase composition of the composite
and leads to nucleation and growth of particles of the
Cd(OH)2 phase. The results obtained make it possible
to synthesize a CdS/ZnO composite with prescribed
characteristics related to the composition and optical
properties. The composite exhibits a catalytic activity in
the visible spectral range, which confirms the possibility
of its effective use as a photocatalyst and absorbing
layer of a solar cell.
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ACKNOWLEDGMENTS
11. Markov, V.F., Maskaeva, L.N., and Ivanov, P.N.,
Gidrokhimicheskoe osazhdenie plenok sul’fidov metallov:
modelirovanie i eksperiment (Hydrochemical Deposition
of Metal Sulfide Films: Simulation and Experiment),
Yekaterinburg: Izd. Ural. Otd. Ross. Akad. Nauk, 2006.
The study was supported by the Russian Science
Foundation (project no. 17-79-20165).
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