Novel method for the preparation of inorganic sulfides and selenides. I. Binary materials and group II-VI phosphors

Article abstract of DOI:10.1149/1.1393268

A novel method for the synthesis of a wide range of metal sulfides and selenides is described. Polysulfide solutions formed by the dissolution of sulfur in hydrazine monohydrate have been shown to contain the hexasulfide and tetrasulfide anions. The actio

Full text of DOI:10.1149/1.1393268

Journal of The Electrochemical Society, 147 (2) 765-771 (2000)
765
S0013-4651(99)05-096-X CCC: $7.00 © The Electrochemical Society, Inc.
A Novel Method for the Preparation of Inorganic Sulfides and Selenides
I. Binary Materials and Group II-VI Phosphors
Dominic A. Davies,z Aron Vecht,* Jack Silver,* Paul J. Marsh, and John A. Rose
Centre for Phosphor and Display Materials, University of Greenwich, Woolwich Campus, London, United Kingdom SE18 6PF
A novel method for the synthesis of a wide range of metal sulfides and selenides is described. Polysulfide solutions formed by the
dissolution of sulfur in hydrazine monohydrate have been shown to contain the hexasulfide and tetrasulfide anions. The action of
these solutions, or their selenium analogues, with a range of transition and main group metal salt solutions yields a precipitate,
which after firing at an elevated temperature, forms a crystalline metal sulfide or selenide. This method of preparing metal chalco-
genides has been extended to some group II-VI phosphors with promising luminescent properties.
© 2000 The Electrochemical Society. S0013-4651(99)05-096-X. All rights reserved.
Manuscript submitted May 24, 1999; revised manuscript received September 9, 1999.
fluorescent screens,2e scintillation,2f vacuum fluorescent displays,2g
field emission devices (FEDs),2h radioluminescent paints,2i and
phosphorescent pigments.2i Activated cadmium sulfide and zinc and
cadmium selenide (and mixtures thereof) have also been used com-
monly as phosphors,1,2c though their continued application is now
limited by the toxicities of cadmium and selenium, they are still used
in radar displays and high resolution storage tubes.2c
In a preliminary study it was shown that the action of sulfur dis-
solved in hydrazine monohydrate on a zinc salt solution could be used
to prepare ZnS:Ag phosphors which luminesced at slightly longer
wavelengths than commercial materials and had higher efficiencies.11
Here we present more detailed and wider ranging results which
demonstrate that the reaction of a solution of sulfur or selenium dis-
solved in hydrazine monohydrate with metal salt solutions provides
a versatile and simple method for the preparation of a range of p-
block and transition metal sulfides and selenides.
Metal sulfides and selenides are currently used, or have potential
use, for a wide range of technological products in the modern world.
Examples include phosphors (in televisions, display devices, and
scintillation counters),1,2 semiconductors,3a ceramics,3b pigments,3c,d
IR transmitting windows,3e lubrication,3f hydrogenation catalysis,3g
sulfite process wood pulp bleaching,3h photochemical solar energy
conversion,3i photodetection,3j high temperature batteries,3k photo-
voltaic cells,3l and nonlinear optics.3m
Such metal sulfides and selenides are presently prepared by a
range of methods including the interaction of sulfur or selenium with
metals,4a the action of a sulfurizing atmosphere at elevated tempera-
ture on a metal salt,5 and wet chemical methods. Examples of the lat-
ter are precipitation with carbon disulfide or ammonium sulfide,1
reduction of metal sulfates/selenates,6 decomposition of organosul-
fur compounds such as selenourea,7 or thioacetamide,3c or by sol-gel
methods utilizing organic thiols rather than alcohols.8
However it has always been very difficult to precipitate pure and
stoichiometric metal sulfides, due to anionic impurities particularly
hydroxides (which form oxides at high temperatures).1 For some
uses, such as phosphors, contaminants, either anionic or cationic,
can drastically alter the performance and possibly the color of the
emission, therefore the preparation of highly pure materials from
readily available reagents is particularly important in this field.1 In
addition, the current ecological demands necessitate that future pre-
paration of metal sulfides should not produce high volumes of waste
or side products, such as sulfur/selenium containing gases,9 unlike
many of the methods mentioned above, particularly those utilizing a
sulfurizing atmosphere (such as H2S or CS2 to convert a precursor to
a sulfide at elevated temperatures).
Inorganic powder phosphors are highly pure crystalline materials
which have been purposely doped with small quantities of other ions
(0.01 mol % silver in cathode ray tube (CRT) ZnS:Ag),1 the precise
concentration of the dopant affects the luminescent properties and
thus the applications the material is suited for.2 The presence of other
ions, additional to the desired activator, can affect both the color and
efficiency of the phosphor produced.1,2 Examples of these effects are
(i) the addition of 0.015 mol % aluminum to ZnS:Ag results in a
shorter wavelength blue light being emitted (which is desirable), but
there is a concomitant decrease in the efficiency of the phosphor10
(which is not), and (ii) the presence of low quantities of iron in
ZnS:Ag results in a quenching of the visible luminescence which
becomes more dramatic as the iron concentration increases.2c
Zinc sulfide activated with silver (ZnS:Ag) has a blue lumines-
cence and is one of the most efficient cathodoluminescent phosphors
known.1,2c Changing the activator to copper (ZnS:Cu) and other co-
dopants gives a green luminescence.2c ZnS phosphors are very wide-
ly used. Applications include: CRTs,2c electroluminescence,2d X-ray
To investigate the nature of the reactive species present in sulfur
solutions of hydrazine monohydrate electronic absorption spectro-
scopic investigations were undertaken, and are discussed with re-
spect to previously reported investigations of electrochemically re-
duced sulfur in dimethylformamide. The dominant anionic sulfur
species in the solutions in hydrazine monohydrate has been identi-
fied and the implications of the nature of this species to the synthe-
sis of metal sulfides are discussed.
We have further investigated the use of this method for the prepa-
ration of ZnS:Ag and improved on our previous results. In addition
we have extended the method to the preparation of other II-VI phos-
phor materials including ZnS:Cu, zinc sulfoselenide activated with
copper (Zn{S,Se}1:Cu), and zinc cadmium sulfide activated with
copper ({Zn,Cd}1S:Cu).
A major advantage of this method is that it limits the quantities
of sulfur-containing gaseous by-products evolved during firing, and
it is therefore more environmentally friendly than current industrial
methods. The method is also quick to use, and (with suitable safety
precautions) can be scaled up easily.
Experimental
Reagents were used as purchased, for the preparation of lumi-
nescent materials all reagents were of at least AnalaR grade, dopants
used were usually of 99.99% purity or higher. 99.999% sulfur
(Aldrich) and/or 99.5% selenium (Aldrich) were used. Scanning
electron microscopy (SEM) images were recorded using Cambridge
Instruments Stereoscan 90, Coulter Counter particle size measure-
ments were recorded using a Coulter Multisizer II attached to a PC,
X-ray powder diffraction (XRD) patterns were recorded using a
Philips PW1710 diffractometer. A Perkin Elmer ␭3 spectrometer
was used to record the electronic absorption spectra. Cathodolumi-
nescent excitation was achieved under vacuum at around 10Ϫ6 Torr
and a Kimball Physics, Inc. (Walton, NH) model EGPS-7 electron
gun. The E-beam used was 1.4 mm in diam and the current 8.5 ␮A.
* Electrochemical Society Active Member.
z
E-mail: dd05@greenwich.ac.uk
Downloaded on 2014-07-08 to IP 132.203.227.62 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

766
Journal of The Electrochemical Society, 147 (2) 765-771 (2000)
S0013-4651(99)05-096-X CCC: $7.00 © The Electrochemical Society, Inc.
Photoluminescent and cathodoluminescent emission spectra were
measured using a Bentham Tel301D detector, DMC3B programma-
ble monochromator control, M300 monochromator, and a 228A pro-
grammable current amplifier linked to a PC. Photoluminescent exci-
tation was achieved using a 366 nm wavelength light. Cathodolumi-
nescent emission intensity was measured using a Photo Research
Pritchard PR880 photometer.
Stubs coated in the samples for luminescence measurements
were prepared by electrophoretic coating. The phosphor (Ϸ0.5 g)
was suspended in 5 ϫ 10Ϫ4 M Mg(NO3)2 in IPA (50 mL) and dis-
persed in an ultrasonic bath. The voltages used to coat the stubs
ranged from 2 to 300 V; times used varied to allow coats of similar
mass to be deposited for each sample.
drate in dimethylformamide were prepared, and these were mixed and
diluted with known quantities of dimethylformamide, to the required
concentrations, under dry nitrogen. The diluted solutions were al-
lowed to stand for 1 min while color developed prior to being exam-
ined by electronic absorption spectroscopy.
Results and Discussion
Synthesis of metal sulfides and selenides.—Table I summarizes
the range of main group and transition metal compounds prepared in
this work, the firing temperature (chosen to give a good crystalline
product for powder XRD studies),14 the precursor metal salt and the
phase composition of the fired product. The synthetic method gives
an essentially quantitative yield of the metal chalcogenide. The pow-
der XRD patterns of the fired materials were in good agreement with
the literature data.15 For the materials prepared which contain one or
two phases of the same compound there is no XRD pattern evidence
for the presence of crystalline impurities. Scanning electron micro-
graphs (SEMs), presented in Fig. 1, of two of the materials prepared
in this work clearly show their crystalline nature.
For cobalt, iron and copper, mixed metal oxidation state and/or
nonstoichiometric sulfide phases always resulted (see Table I). This
is not surprising in view of the number of nonstoichiometric sulfide
phases reported for these metals,15,16a (determination of the exact
composition of these was beyond the scope of this work). For alu-
minum and chromium(III) the material prepared after firing was
identified as M2O3 (M ϭ Al or Cr).15 In these cases precipitates may
have oxidized before firing, or may have decomposed to oxides dur-
ing firing. Attempts to prepare aluminum sulfide by dissolving the
metal precursor in dry polar organic solvents (such as alcohols, di-
methylformamide and dimethylsulfoxide, but not reducible solvents,
which would react with hydrazine) under anerobic conditions using
a hydrazine monohydrate/sulfur solution also resulted in the forma-
tion of aluminum oxide on firing.
Synthetic methods.—The precipitation of metal sulfides and
selenides was carried out by an extension of our previously detailed
method.11 Solutions of metal salts were used in concentrations of
0.25 mol dmϪ3 or less. It must be noted that nitrates, perchlorates,
carbonates, and other oxidizing anions are incompatible with the
strongly reducing hydrazine hydrate, therefore acetates and chlo-
rides were used. Sulfur and/or selenium solutions were prepared by
the slow addition of the chalcogenide to cooled (Ϸ0ЊC) hydrazine
monohydrate in air (sealed systems under N2 atmosphere were
avoided to prevent the danger in containment of a rapid gas evolu-
tion). In the case of a metal sulfide preparation an excess of the sul-
fur solution (calculated on sulfur concentration) was then filtered
under gravity into the metal salt solution (again open to the air). The
resulting precipitate was filtered off, washed with water, then iso-
propyl alcohol (IPA), and dried. The dry precipitate was then fired
under an inert atmosphere in a tube furnace. Table I shows the metal
salts used, the firing temperatures, and the phase composition of the
fired material.
Luminescent materials were prepared by a similar method, with
the addition of known quantities of dopants to the zinc and/or cad-
mium solutions. The precipitate was intimately mixed with a known
quantity of flux, such as NaCl (typically 2 wt %), and sulfur (typi-
cally 5 wt %). The mixture was then firmly packed into a silica cru-
cible which was placed inside a larger crucible filled with sacrificial
ZnS; the latter ZnS used must be of reasonable purity as metal ions
can migrate from this to the inner crucible. The crucibles were heat-
ed in a furnace at 900ЊC for 1 h and then allowed to cool. When cool
the material was washed first with 1:1 glacial acetic acid, then water
followed by IPA before being dried overnight at 80ЊC.
The mixed metal sulfide ZnCdS and the sulfoselenide ZnSSe
formed single-phase materials. The powder XRD pattern of Zn(S,Se)1
is typical of the cubic phase; the cell size of 5.54 Å is evidence that
the formula of the material is approximately Zn(S0.4иSe0.6). 17 X-ray
fluorescence (XRF) analysis and comparison with standard samples
gave the precise formula as ZnS0.47Se0.53. Since the chalcogen solu-
tion contained equimolar quantities of S and Se this indicates that to
achieve the desired S to Se ratio, in the fired product, an excess of sul-
fur must be used in the parent hydrazine solution. The powder XRD
pattern of (Zn,Cd)1S is typical of a hexagonal cell. The cell size was
calculated to be a ϭ 3.98 Å and c ϭ 6.49 Å which is evidence that
Preparation of solutions for electronic absorption spectrosco-
py.—Solutions were prepared using dry spectroscopic grade dimethyl-
formamide. The dimethylformamide was degassed under vacuum
while being agitated by an ultrasonic bath and then flushed with and
stored under dry nitrogen before use. All solutions were prepared and
manipulated in darkness to avoid photochemical decomposition.12,13
Solutions containing known quantities of sulfur or hydrazine monohy-
18
the approximate Zn to Cd ratio was (Zn0.6иCd0.4)S. XRF analysis
and comparison with standard samples gave the precise formula as
Zn0.68Cd0.32S. This suggests that, to prepare material of a given Zn to
Cd ratio, a slight excess of cadmium should be used.
Electronic absorption spectroscopic studies of the reduction of
sulfur by hydrazine hydrate to investigate the nature of the species
Table I. Metal sulfide and selenide materials prepared using solutions of sulfur and/or selenium in hydrazine monohydrate.
Material
Color
Comment and ICPDS pattern number14
Firing temperature (ЊC)
Starting metal salt
MnS
MnSe
Fe
Co
NiS
Cu2S
ZnS
ZnS0.47Se0.53
Zn0.68Cd0.32
ZnSe
CdS
Green
Brown
Black
Brown
Gold
␣ phase 6-0158
Cubic 11-683
Mixed phase of Fe1Ϫx
Mixed phases of cobalt sulfides
Hexagonal 2-1280
Mixed phases of copper(I) sulfides
␣ and ␤ phases, 36-1450 and 5-556
Cubic, a ϭ 5.56 Å.
Hexagonal, a ϭ 3.98 Å, c ϭ 6.49 Å.
Cubic 15-105
800
800
800
700
800
800
800
800
900
800
800
700
570
800
Mn(OOCCH3)2
Mn(OOCCH3)2
Fe(SO4)
Co(OOCCH3)3
Ni(SO4)
S
Blue / black
White
Cu(OOCCH3)2
Zn(OOCCH3)2
Zn(OOCCH3)2
1:1, Zn(OOCCH3)2:Cd(OOCCH3)2
Zn(OOCCH3)2
Cd(OOCCH3)2
In2(SO4)3
Tan
S
Orange
Yellow
Yellow
Red
Orange
Silver
Greenocktite 6-0314
Orthorhombic 25-390
Hexagonal 23-677
Galena syn 5-592
In2S3
SnS2
PbS
SnCl4
PbO/CH3COOH
Downloaded on 2014-07-08 to IP 132.203.227.62 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 147 (2) 765-771 (2000)
767
S0013-4651(99)05-096-X CCC: $7.00 © The Electrochemical Society, Inc.
The strong blue color of the solution arises from a combination of all
three absorption bands (with that of the S26Ϫ being dominant).20a
In Fig. 2 the spectrum (a) of a low concentration (compared to the
concentration used to precipitate metal sulfides) of sulfur in
hydrazine monohydrate is presented. Comparison of this spectrum
with those of electrochemically reduced sulfur in dimethylfor-
mamide,20a provides evidence for the presence of polysulfide anions,
and it can be seen that the concentration of S26Ϫ is very low (little
absorbance fron 600 to 650 nm) compared to that of S24Ϫ (400 to
450 nm). Progressively increasing the sulfur concentration causes
the solution to darken through red to a dull red/brown color. Initial-
ly this is due to an increase in the absorption around 400 nm attrib-
uted to the S24Ϫ ion concentration. However the intensity of the elec-
tronic absorption bands of more concentrated solutions is extremely
high (due to the high extinction coefficient of the species in solu-
tion). Accurate measurement is impossible as the minimum extinc-
tion coefficient ϭ 9.94 ϫ 104 (calculated from sulfur in hydrazine
hydrate assuming all S8 has been converted to S24Ϫ). Hence the pre-
cise nature of the polysulfide species present in concentrated solu-
tions remain unknown.
Electronic absorption spectra of the polysulfide ions, formed in
DMF, by the reduction of sulfur by hydrazine at much lower con-
centrations (Fig. 2b–d) were recorded for comparison with those of
electrochemically reduced sulfur (initial hydrazine concentration
5.13 mol dmϪ3, initial sulfur concentrations shown 4.9, 7.3, and
9.7 mmol dmϪ3). These spectra show that S24Ϫ (400–450 nm)and
S26Ϫ (600–650 nm) species are present in the solution. The relative
areas of the absorption bands show that the S24Ϫ ion’s absorption
band is now dominant.
From these findings we suggest that S24Ϫ is in the highest con-
centration in the solution, which appears dull brown in color, and is
of the same appearance as the solutions used for material preparation
in this work.
Discussion of the precipitation of metal sulfides using sulfur
solutions in hydrazine hydrate.—Hydrazine is a well-known reduc-
ing agent. Previous studies have shown that the products of the reac-
tion of hydrazine with an oxidant are affected by the pH of the solu-
tion, equations for both acidic and basic conditions are given in
Fig. 3.4b Dissolution of sulfur into hydrazine to form polysulfide
ions will occur according to the reaction given in Fig. 3 for basic
conditions (since hydrazine monohydrate is strongly basic).
The addition of the hydrazine/sulfur solution to that of the metal
(which results in the polysulfide ions being in a much more acidic
environment and in intimate contact with the metal cations) causes
immediate precipitation of the metal sulfide. Measuring the pH of
the solution during the hydrazine/sulfur addition yielded the results
Figure 1. SEMs of (a) ZnS:Cu and (b) ZnS:Cu,Al,Au.
formed.—Hydrazine has been previously used in the reduction of
metal sulfites,6b and in a quantitative test for selenium,19 but until
our recent work there had been no report on the preparation of metal
sulfides and selenides using chalcogenide solutions in hydrazine
monohydrate.11 Sulfur-containing solutions are well established and
may be formed by either dissolving the element in a suitable solvent,
or by reacting a suitable solvent with the element to form soluble
species.16b Sulfur solutions in dimethylformamide,16b,20 aqueous
base,20,21 and molten alkali metal halide,22 are blue in color.
Hydrazine monohydrate is a strongly reducing transparent liq-
uid.13 The addition of sulfur to this liquid results both in the evolution
of a clear gas (not H2S or acidic but N2, as shown later) and the for-
mation of an orange colored solution which turns red/brown on fur-
ther addition of sulfur.13 The decomposition products of these solu-
tions are known to be hydrogen sulfide, ammonia, and nitrogen.13
Polarographic studies of sulfur and selenium solutions in hydrazine
hydrate have led to the suggestion that they contain polysulfide or
polyselenide ions (S2nϪ or Sen2Ϫ).23 Electronic absorption spec-
troscopic studies of electrochemically reduced sulfur solutions in
dimethylformamide (which are blue ) show the presence of an
absorption band at 618 nm, that has been ascribed to the hexasulfide
ion (S26Ϫ).20a Other species which exist in equilibrium with this one
are the tetrasulfide (S24Ϫ) and the octasulfide (S28Ϫ) these have asso-
ciated electronic absorption bands at 435 and 505 nm, respectively.20a
Figure 2. Electronic absorption spectra of (a) 1 mol dmϪ3 sulfur in hydrazine
monohydrate; (b) 4.9 mmol dmϪ3, (c) 7.3 mmol dmϪ3, (d) 9.7 mmol dmϪ3
sulfur in hydrazine monohydrate (5.13 mol dmϪ3)/dimethylformamide.
Downloaded on 2014-07-08 to IP 132.203.227.62 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

768
Journal of The Electrochemical Society, 147 (2) 765-771 (2000)
S0013-4651(99)05-096-X CCC: $7.00 © The Electrochemical Society, Inc.
Figure 3. Reactions involved in the preparation of metal sulfides using sul-
fur solutions in hydrazine monohydrate.
shown in Fig. 4. It can be seen that for 400 mL of 0.5 M ZnOAc
solution, the pH starts in the acidic region but becomes progressive-
ly more basic on the addition of the hydrazine sulfur solution. This
is not surprising in view of the excess hydrazine used in these solu-
tions. We suggest that this is not evidence that both reactions, shown
in Fig. 3, occur as the precipitation is so fast that it takes place in
localized basic areas before homogeneous mixing has been
achieved. This can be inferred as no tendency for the alkalinity of the
solution to decrease was seen on standing after homogeneous mix-
ing had been achieved. If discernible quantities of hydrazine were
still reducing sulfur a lowering of the pH would have been observed.
X-ray powder diffraction studies of the unfired zinc sulfide pre-
cipitates confirmed the presence of small amounts of sulfur as an
impurity. The crystalline elemental sulfur is seen clearly in the pow-
der diffraction pattern as are broad X-ray peaks arising from semi-
crystalline unfired zinc sulfide. Soxhlet extraction with carbon tetra-
chloride was used to measure the amount of sulfur in the precipitate.
This was almost exactly the excess of sulfur used in the reaction, and
accounted for the excess yield over the theoretical maximum. This
confirms that the unfired precipitate is zinc sulfide and indicates that
the polysulfide ions have been reduced to sulfide ions in the reaction
vessel. The excess sulfur used reverts to S8 under nonreducing con-
ditions, explaining the observation of S8 in the powder XRD pattern.
A working, balanced overall reaction is presented in Fig. 3. A
more complete reaction scheme truly describing the reaction in full
is impossible to write until all the polysulfides and their concentra-
tions in the hydrazine hydrate solution are known, and such a study
is beyond the scope of this work. However we have shown that the
dissolution of sulfur in hydrazine monohydrate results in the forma-
tion of a range of polysulfide ions, including predominently tetrasul-
Figure 4. Variation of the pH of a zinc salt solution during the addition of a
sulfur solution in hydrazine monohydrate.
fide and some hexasulfides. When the polysulfide solution is added
to a metal salt solution the precipitation of metal sulfide occurs.
Preparation of metal sulfide and selenide phosphors.—We have
previously demonstrated that this method is useful for the prepara-
tion of ZnS:Ag phosphors with high emission efficiencies.11 How-
ever the emission spectra have a slightly more red-shifted wave-
length than commercial phosphors and initially manifested a tail into
the green region of the spectra which is not present in the emission
spectra of a commercial material.11 Such a tail is caused by the pres-
ence of oxidation products of zinc sulfide, formed during the firing
process. Tight packing of the unfired materials into a silica crucible,
and rigorous washing with acetic or sulfurous acid after firing have
been used to remove this tail from the emission spectra.1 The red-
shifted emission spectra (compared to that of the commercial mate-
rial) resulted in a ZnS:Ag with CIE y coordinate considerably larger
than that of the commercial material (see Table II). The effect of the
initial pH of the zinc salt solution, before addition of the sulfur solu-
tion, on the properties of the final phosphor were examined over the
range pH 1–6. No visible effects on particle morphology or particle
size were found (see Table II), and the samples phase composition
(containing both cubic and hexagonal ZnS)15 was also unaffected.
Constant current cathodoluminescent emission testing revealed no
Table II. Particle sizes and CIE coordinates for (ZnS:Ag) phosphors.
(ZnS:Ag)
Mean diam (␮m)
SD (␮m)
Dominant (␭/nm)
CIE-x
CIE-y
Previous work
Commercial
7.640
6.383
5.413
5.141
5.072
5.098
5.204
5.342
5.049
4.531
5.212
5.354
5.354
4.71
2.87
4.52
3.71
4.44
4.82
4.61
3.98
3.44
3.31
4.28
3.31
6.92
475.07
465.21
472.95
472.54
472.15
473.05
472.67
472.09
472.04
465.12
462.00
466.35
462.66
0.1704
0.1499
0.1578
0.1592
0.1577
0.1584
0.1597
0.1595
0.1593
0.1745
0.1563
0.1480
0.1554
0.1466
0.0574
0.1120
0.1117
0.1121
0.1118
0.1124
0.1118
0.1113
0.0819
0.0666
0.0604
0.0545
Aqueous HCl pH 1 a
Aqueous HCl pH 2 a
Aqueous HCl pH 3 a
Aqueous HCl pH 4 a
Aqueous HCl pH 5 a
Aqueous HCl pH 6 a
Water a
Methanol a
Ethanol a
Propan-2-ol a
Butan-2-ol a
a Solvent from which the unfired ZnS was precipitated.
Downloaded on 2014-07-08 to IP 132.203.227.62 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 147 (2) 765-771 (2000)
769
S0013-4651(99)05-096-X CCC: $7.00 © The Electrochemical Society, Inc.
Alcohols were used in place of water as they are suitable solvents
in which to dissolve metal salts, and they are not reduced by hy-
drazine monohydrate (reducible solvents, such as ketones, are total-
ly unsuited to this method as they react with the hydrazine). Scan-
ning electron micrographs (SEM) and Coulter counter particle size
measurement provide evidence that the particle size and distribution
of all the fired materials were similar to those of the commercial
material. The commercial sample contains predominately cubic ZnS
(with a minor hexagonal component). The powder XRD patterns of
samples prepared from a range of alcohols in this work showed that
they contained both cubic and hexagonal phases. It appears that the
solvent from which the ZnS is precipitated has no discernible effect
on the phase composition or particle size of the final material. The
CL emission spectra and a plot of constant current emission intensi-
ty against voltage for the materials precipitated from various alco-
hols are shown in Fig. 6 and 7, respectively. Table II presents the CIE
coordinates and particle sizes of both our materials and and com-
mercial ZnS:Ag materials. It can be seen that the effect of the solvent
is to reduce the dominant wavelength when the longer chain alcohols
are used. From this observation it appears that it is possible to con-
trol the chromaticity of the emission spectrum by changing the sol-
vent in which the metal salt solution is prepared. The most saturated
blue emission from ZnS:Ag is obtained by precipitating the material
from butan-2-ol. In addition, careful selection of the solvent, facili-
tates the preparation of ZnS:Ag phosphors manifesting higher effi-
ciencies in the 0-5000 V range, with comparable particle sizes, and
CIE coordinates (under CL excitation) to commercial material.
In parallel studies samples of ZnS:Cu, ZnS:Cu,Al, ZnS:Cu,Al,Au,
Zn(S,Se):Cu, and (Zn,Cd)S:Cu have been prepared to confirm the
general application of this method for the preparation of group II-VI
phosphors. In the preparation of ZnS:Cu materials and related sys-
tems no significant difference between the chromaticity of the mate-
rials prepared from an aqueous solution using a sulfur solution in
hydrazine monohydrate to those of commercial materials was found.
The chromaticity and emission spectra of the ZnS:Cu phosphor pre-
pared are in excellent agreement with those of the commercial mate-
rial (presented in Fig. 8 and Table III) while the intensity of the con-
stant current CL emission is significantly better than that of the com-
mercial sample used over the voltage range examined (0-4000 V)
(see Fig. 9).
Figure 5. Constant current CL emission intensity against voltage for ZnS:Ag
phosphors precipitated from various pH solutions.
discernible variation in the chromaticity of the blue emission of the
phosphors, and that the CIE coordinates were not improved with
respect to those of the commercial material (see Table II). The plot
of the emission intensity against voltage, presented in Fig. 5, does,
however, reveal evidence of a decreased emission intensity for the
samples which were prepared from the more acidic solutions. From
this we infer that suitable zinc salts which form less acidic solutions
are the favored starting materials. In an attempt to produce materials
with CIE coordinates comparable to commercial ZnS:Ag the effect
of the solvent in which the zinc and silver salts were dissolved was
examined. The alcohols used were methanol, ethanol, propan-2-ol,
and butan-2-ol.
Figure 6. CL emission spectra of ZnS:Ag phosphors precipitated from
Figure 7. Constant current CL emission intensity against voltage for ZnS:Ag
organic alcohols.
phosphors precipitated from organic alcohols.
Downloaded on 2014-07-08 to IP 132.203.227.62 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

770
Journal of The Electrochemical Society, 147 (2) 765-771 (2000)
S0013-4651(99)05-096-X CCC: $7.00 © The Electrochemical Society, Inc.
Figure 9. Constant current CL emission intensity against voltage for ZnS:Cu
phosphors.
Figure 8. CL emission spectra of ZnS:Cu phosphors.
the ZnS:Cu, however, it reduces the efficiency of the material.2c This
suggests that incomplete precipitation of the aluminum from the
stock solution is not the cause of the low efficiency of the materials
and therefore backs up the hypothesis that the copper concentration
is lower than ideal.
The preparation of (Zn,Cd)S:Cu and Zn(S,Se):Cu were also car-
ried out from aqueous solution, by the standard method. The major
stoichiometries of the materials formed were simillar to those dis-
cussed earlier in this work. Both phosphors luminesced red under
366 nm UV light. Photoluminescent emission spectra were recorded
and are shown in Fig. 12. Although no CL investigation of the latter
phosphors was undertaken, the PL emission spectra show that the
use of a sulfur and or selenium solution in hydrazine monohydrate
offers a suitable method for the preparation of group II-VI phospho-
rs other than ZnS materials.
ZnS:Cu,Al and ZnS:Cu,Al,Au phosphors were prepared from
aqueous solutions of metal salts using the standard method. Table III
lists the chromaticity and particle sizes of the materials, and Fig. 10
presents the CL emission spectra of the materials. The CL emission
spectra of the materials prepared in this work have a shoulder at low
wavelengths not present in the commercial material. This shoulder
alters the chromaticity of the materials, but the maxima of the main
peaks are in good agreement with those of the commercial materials.
It is probable that the shoulder is caused by a slight deficiency in the
desired copper content, similar shoulders are observed in ZnS:Cu
with decreasing copper concentration.2c The constant current CL
emission intensity plotted against voltage is given in Fig. 11; it re-
veals these materials are less efficient than the commercial materi-
als. The study of the precipitation of aluminum salts using this
method as discussed earlier showed that it was unsuitable for the
preparation of Al2S3. Aluminum is added to alter the chromaticity of
Conclusions
We have demonstrated that a polysulfide solution is formed by
the dissolution of sulfur in hydrazine monohydrate. This solution
Table III. Particle sizes and CIE coordinates for (ZnS:Cu)
phosphors and related materials.
Mean diam SD Dominant
Phosphor
(␮m)
(␮m)
(␭/nm)
CIE-xa CIE-ya
(ZnS:Cu)
This work
(ZnS:Cu)
8.458
4.731
9.218
6.080
7.355
8.273
8.55
4.65
5.46
6.22
3.36
7.97
1.30
2.23
546.25
547.23
536.81
550.99
542.77
549.11
595.51
588.87
0.2863 0.5494
0.2983 0.5615
0.2698 0.5424
0.3087 0.6108
0.2996 0.6094
0.2819 0.5655
0.5894 0.3962
0.5572 0.4342
Commercial
(ZnS:Cu,Al)
This work
(ZnS:Cu,Al)
Commercial
(ZnS:Cu,Al,Au)
This work
(ZnS:Cu,Al,Au)
Commerical
({Zn,Cd}1S:Cu) 4.554
This work
(Zn{S,Se}1:Cu)
This work
4.641
a Cathodoluminescent CIE coordinates, except for (Zn,Cd)S and
Zn(S,Se) which are photoluminescent CIE coordinates.
Figure 10. CL emission spectra of ZnS:Cu,Al and ZnS:Cu,Al,Au phosphors.
Downloaded on 2014-07-08 to IP 132.203.227.62 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 147 (2) 765-771 (2000)
771
S0013-4651(99)05-096-X CCC: $7.00 © The Electrochemical Society, Inc.
Figure 12. PL emission spectra of (ZnCd)1S,Cu and Zn(SSe)1:Cu phosphors.
Figure 11. Constant current CL emission intensity against voltage for
ZnS:Cu,Al and ZnS:Cu,Al,Au phosphors.
Compd., 225, 520 (1995); (e) S. Pratrap, J. Prasad, R. Kimur, K. Murari, and S. S.
Singh, Defence Sci. J., 46, 215 (1996); (f) E. R. Braithwaite, in Speciality Inorganic
Chemicals, R. Thompson, Editor, p. 346, Royal Society of Chemistry, London
(1981); (g) E. R. Braithwaite, in Speciality Inorganic Chemicals, R. Thompson,
Editor, p. 375, Royal Society of Chemistry, London (1981); (h) T. W. Swaddle,
Appl. Inorg. Chem., p. 52, University of Calgary Press, Calgary, Canada (1990); (i)
H. C. Youn, S. Baral, and J. H. Fendler, J. Phys. Chem., 92, 6320 (1988); (j) P. J.
Sebastian, Adv. Mater. Opt. Electron., 5, 269 (1995); (k) J. L. Sudworth, Philos.
Trans. R. Soc. London Ser. A, 354, 1595 (1996); (l) R. W. Birkmire and E. Eser,
Ann. Rev. Mater. Sci., 27, 625 (1997); (m) G. C. Catella and D. Burlage, MRS Bull.,
23, 28 (1998).
can be used to precipitate many metal sulfides from solution, and the
precipitate, after firing, is a crystalline metal sulfide. It has also been
shown that an analogous method, using selenium instead of sulfur in
hydrazine monohydrate, is suitable for the preparation of metal
selenides. The method is rapid and limits the amount of sulfurous
waste gases formed during preparation of the metal sulfide. The
range of materials that can be prepared has now been demonstrated
to be much more general than previously thought. The application of
this work to the synthesis of group II-VI phosphors has yielded
many materials with interesting luminescent properties.
4. (a) F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 3rd ed., p. 430,
Interscience, New York (1972); (b) F. A. Cotton and G. Wilkinson, Advanced Inor-
ganic Chemistry, 3rd ed., p. 350, Interscience, New York (1972).
5. R. Hoppe, Angew. Chemie, 71, 457 (1959).
6. E. Grillot, Bull. Soc. Chim. France, 1-2, 39 (1951).
7. A. Vecht, Br. Pat. 1,302.537 (1973).
8. E. J. Donahue, A. Roxburgh, and M. Yurchenko, Mater. Res. Bull., 33, 323 (1998).
9. V. Dalaine, J. M. Cormier, and P. Lefaucheux, J. Appl. Phys., 83, 2435 (1998).
10. R. Raue, M. Shiki, H. Matsukiyo, H. Toyama, and H. Yamamoto, J. Appl. Phys.,
75, 481 (1994).
11. A. Vecht, D. A. Davies, and D. Smith, Mater. Res. Innovat., 2, 176 (1998).
12. J. L. Occhipini, A. C. Bevilacqua, and J. E. Kenny, Abst. Amer. Chem. Soc., 201,
195 (1991).
Acknowledgments
We would like to acknowledge DARPA for funding the work, and
other members of this research group, particularly R. Ellis, D. Smith,
D. Nicholas, and C. Gibbons, for their help in preparing this work.
The University of Greenwich assisted in meeting the publication costs of
this article.
13. E. W. Schmidt, Hydrazine and Its Derivatives, John Wiley & Sons, New York
(1984).
14. Handbook of Chemistry and Physics, D. R. Lide, Editor. p. 8-20, CRC, London
(1997).
References
15. W. F. McClune, Powder Diffraction File. Inorganic Phases, ICPDS, Swarthmore,
PA (1983).
1. H. W. Leverenz, An Introduction to Luminescence of Solids, Dover Publications
Inc., New York (1968).
16. (a) F. Jellinek, in Inorganic Sulphur Chemistry, G. Nickless, Editor, p. 670, Elsevi-
er Press, Amsterdam (1968); (b) B. Meyer, in Inorganic Sulphur Chemistry, G.
Nickless, Editor, p. 255, Elsevier Press, Amsterdam (1968).
17. N. Kn. Abrikosov, V. F. Bankina, L. V. Poretskaya, L. E. Sherimova, and E. V.
Skudnova, Semiconducting II-VI, IV-VI and V-VI Compounds, p. 43, Plenum Press,
New York (1969).
18. W. M. Kane, J. P. Spratt, L. W. Hershinger, and I. H. Khan, J. Electrochem. Soc.,
113, 136 (1966).
19. A. L. Vogel, A Textbook of Qualitative Chemical Analysis, p. 437, Longmans,
Green and Co., London (1945).
2. (a) H. Yamamoto, in Phosphor Handbook, S. Shinoya and W. M. Yen, Editors,
p. 217, CRC Press, London (1998); (b) S. Shinoya, in Phosphor Handbook, S. Shi-
noya and W. M. Yen, Editors, p. 231, CRC Press, London (1998); (c) S. Inaho and
T. Hase, in Phosphor Handbook, S. Shinoya and W. M. Yen, Editors, p. 459, CRC
Press, London (1998); (d) M. Yoshida, A. Mikami, and T. Inoguchi, in Phosphor
Handbook, S. Shinoya and W. M. Yen, Editors, p. 581, CRC Press, London (1998);
(e) N. Miura, in Phosphor Handbook, S. Shinoya and W. M. Yen, Editors, p. 521,
CRC Press, London (1998); (f) N. Miura, in Phosphor Handbook, S. Shinoya and
W. M. Yen, Editors, p. 531, CRC Press, London (1998); (g) K. Morimoto, in Phos-
phor Handbook, S. Shinoya and W. M. Yen, Editors, p. 561, CRC Press, London
(1998); (h) K. Morimoto, in Phosphor Handbook, S. Shinoya and W. M. Yen, Edi-
tors, p. 573, CRC Press, London (1998); (i)Y. Muriayama, in Phosphor Handbook,
S. Shinoya and W. M. Yen, p. 651, CRC Press, Editors, London (1998).
20. (a) R. Bonaterre and G. Cauqis, J. Chem. Soc., Chem. Commun., 293 (1972); (b) E.
Levillan, F. Gaillard, P. Leghie, A. Demortier, and J. P. Lelieur, J. Electroanal.
Chem., 420,167 (1997).
21. W. Giggenbach, Inorg. Chem., 10, 1306 (1971).
22. W. Giggenbach, Inorg. Chem., 10, 1309 (1971).
23. S. K. Srivastiava, K. S. C. Babu, O. N. Srivastiava, and D. Singh, Bull. Chem. Soc.
Jpn., 64, 3662 (1991).
3. (a) J. C. Bean, in High Speed Semiconductor Devices, S. M. Sze, Editor, p. 24,
Wiley Interscience, New York (1990); (b) A. Celikkaya and M. Akinc, J. Amer.
Ceram. Soc., 73, 2360 (1990); (c) R. L. M. Allen, Colour Chemistry, p. 242,
Thomas Nelson, London (1971); (d) P. Maestro and D. Huguenin, J. Alloys
Downloaded on 2014-07-08 to IP 132.203.227.62 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).