622
NAUMOV et al.
EXPERIMENTAL
To elucidate the role of thiourea complexes in forꢀ
mation of solid solutions of the CdS–ZnS system,
films were obtained by aerosol pyrolysis through
thus measured in a series of samples of variable comꢀ
positions reflected the relative tendency of the xO
dependence ( xZn is the zinc cation fraction).
(x)
x
≡
In addition, the semiconducting and optical propꢀ
erties of CdS–ZnS films were measured. The electriꢀ
cal conductivity was determined from dc current–
voltage characteristics (CVCs). Film silver contacts
were deposited by the vacuum evaporation method
onto the sulfide films 1500–3000 nm thick deposited
onto pyroceramic glass substrates. The cd CVCs of the
highꢀresistance Ag/Cd1 – xZnxS/Ag structure were
measured at voltages up to 20 V. The current was meaꢀ
sured by means of a U5ꢀ6 electrometric amplifier
within the range from 10–6 to 10–12 A. If CVCs were
nonlinear at high voltages, the conductivity was deterꢀ
mined from the initial linear segment or from the conꢀ
spraying solutions of the
[
M(thio)2Cl2
]
and
[
M(thio)2(ac)2] complexes (M = Cd, Zn). The thioꢀ
urea complexes were synthesized in aqueous solutions
from metal chlorides and acetates in the presence of a
fourfold molar excess of thiourea. The metal concenꢀ
tration in a solution was 0.1 mol/L. To obtain mixed
films of the CdS–ZnS system, solutions containing
the metal salts taken in specified molar ratios were preꢀ
pared. Previously, it was shown that the metal ratio in
the film, with a good accuracy, remained the same as
the ratio in the initial solution [6]. This was confirmed
by electron probe Xꢀray microanalysis of the layers
deposited in the present work. Previous calculations of
ionic equilibria and studies of aqueous solution of
thiourea complexes showed that, under the specified
ductivity
S
and
U
= 0:
Simultaꢀ
S = (dI dU)U = 0.
neously, the spectral distribution of the photocurrent
was measured under illumination of a small (free of the
contacts, ~20 mm2) area of the film by monochroꢀ
matic radiation. To this end, a VSUꢀ2P monochromaꢀ
tor was used, which allowed one to cut radiation from
the intrinsic absorption edge. The absorption spectra
of the films grown on quartz substrates were recorded
on SFꢀ56 and SFꢀ2000 spectrophotometers with
respect to the pure substrate (glass SiO2).
conditions (
= 0.1 mol/L, cM : cthio = 1 : 4), neutral
c
MX 2
mixed chloride–acetate complexes were dominating,
notwithstanding the fact that, in the case of zinc,
n
)
hydroxide species [Zn(thio)m(OH)n](2 – formed in
comparable concentrations, since zinc hydroxo comꢀ
plexes are more stable than cadmium hydroxo comꢀ
plexes [7]. The thiourea complexes that crystallized
from solutions were identified on the basis of IR specꢀ
tra in the range 500–4000 cm–1. The spectra were
recorded by the attenuated total reflection method on
a VERTEX 70 spectrophotometer.
The aim of this study was to elucidate how the
properties of the Cd1 – xZnxS layers depend on their
composition since the character of such dependences
allowed one to indirectly judge the formation of solid
solutions and their homogeneity ranges.
For deposition of films of the CdS–ZnS system,
mixed solutions of cadmium and zinc thiourea comꢀ
plexes of a specified composition were sprayed through a
pneumatic sprayer onto quartz or pyroceramic glass subꢀ
STRUCTURE OF THIOUREA COMPLEXES
AND ITS RELATION TO THE SULFIDE PHASE
strates heated to (350–400)
near the substrate surface was 0.2
1
°
C
. The aerosol density
×
10–8 L/(mm2 s). The
The thiourea molecule in metal complexes is coorꢀ
dinated mainly through the sulfur atom, except some
cases of bidentate S,N coordination [9]. Most likely, it
is precisely the specific features of the structure of the
ligand environment of the metal that are responsible
for the formation of a certain polymorph. This follows
from the fact that thermal destruction of the cadmium
thiourea complexes with different acido ligands natuꢀ
spraying time was varied from 2 to 10 min.
The phase composition of the deposited layers was
determined by Xꢀray powder diffraction on a DRONꢀ
3M diffractometer (Cu
K radiation, Ni filter). Xꢀray
α
powder diffraction patterns were recorded in the
2
θ
range 10 –70 . Films were deposited onto thin quartz
°
°
substrates. The phase composition was studied withꢀ
out preliminary annealing in order to elucidate the
character of the solidꢀphase interaction of sulfides
formed directly during the decomposition of thiourea
complexes. The interplanar spacings determined from
the Xꢀray powder diffraction patterns were compared
with reference data for polymorphs of cadmium and
zinc sulfides and solid solutions taken from the dataꢀ
base [8]. Electron probe Xꢀray microanalysis of the
layers was carried out on a JSMꢀ6380 LV electron
microscope. The results were averaged over the sample
surface. Inasmuch as atmospheric oxygen was not
completely evacuated from the chamber of the instruꢀ
rally leads to either
the substrate material and almost independently of the
deposition temperature within 350–400 . Glassꢀlike
wꢀCdS or sꢀCdS, independently of
°
C
substrates (glass SiO2, pyroceramic glass) have no
effect on the crystal structure of untextured polycrysꢀ
talline films. At the same time, it turns out that
is deposited as a result of thermal destruction of the
Cd(thio)2X2] compounds (X is the coordinated halide
ion Cl– or Br–).
ꢀCdS is deposited when
Cd(thio)2(ac)2], [Cd(thio)2(biꢀSO4)], and cationic
complexes [Cd(thio)4]Y2 (Y is an outerꢀsphere counterꢀ
wꢀCdS
[
s
[
−
ment and water was adsorbed on the sample surface, ion, such as F–,
and
2− ). It is worth noting
NO3,
1 2 SO4
the oxygen content in the films should be overestiꢀ that analogous pattern was described for precipitation
mated. However, the change in the oxygen content of cadmium sulfide from cadmium salt solutions
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 4 2011