Synthesis, morphology, and optical properties of cui microcrystals

Article abstract of DOI:10.1134/S0020168512060155

We have studied the effect of synthesis conditions on the luminescence spectra of ultrafine CuI powders. The results demonstrate that synthesis conditions (the reductant of Cu²⁺, the anion of the copper(II) salt, initial solution concentrations, and the presence of a stabilizer) influence the size and shape of the forming CuI particles and, accordingly, their luminescence spectrum. The highest luminescence intensity near λ_max ?720 nm (λ_ex ?370 nm) is offered by regularly shaped tetrahedral particles 1.1-1.2 μm in average size. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S0020168512060155

ISSN 0020ꢀ1685, Inorganic Materials, 2012, Vol. 48, No. 6, pp. 630–634. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © G.P. Shevchenko, I.Yu. Piskunovich, V.A. Zhuravkov, Yu.V. Bokshits, 2012, published in Neorganicheskie Materialy, 2012, Vol. 48, No. 6, pp. 725–730.
Synthesis, Morphology, and Optical Properties of CuI Microcrystals
G. P. Shevchenko, I. Yu. Piskunovich, V. A. Zhuravkov, and Yu. V. Bokshits
Research Institute of Physicochemical Problems, Belarussian State University, Leningradskaya ul. 14, Minsk, 220050 Belarus
eꢀmail: shevchenko@bsu.by
Received July 25, 2011
Abstract—We have studied the effect of synthesis conditions on the luminescence spectra of ultrafine
CuI powders. The results demonstrate that synthesis conditions (the reductant of Cu2+, the anion of the copꢀ
per(II) salt, initial solution concentrations, and the presence of a stabilizer) influence the size and shape of the
forming CuI particles and, accordingly, their luminescence spectrum. The highest luminescence intensity near
λmax 720 nm (λex 370 nm) is offered by regularly shaped tetrahedral particles 1.1–1.2
µ
m in average size.
DOI: 10.1134/S0020168512060155
INTRODUCTION
EXPERIMENTAL
In our experiments, we used synthesized and offꢀ
theꢀshelf (pure grade) CuI. We employed several synꢀ
thetic procedures based on the reduction of Cu2+ to
Cu+ by sodium sulfite (procedure 1); potassium iodide
(2); and potassium iodide, with polyethylene glycol
(PEG) as a stabilizer (3). In addition, colloidal copper
produced by reducing Cu2+ with sodium borohydride
was oxidized by atmospheric oxygen in the presence of
potassium iodide (4).
One important issue in modern optoelectronics is a
search for nanosystems capable of efficiently convertꢀ
ing UV radiation to the visible range.
CuIꢀcontaining films possess interband absorption
at λ ≤ 410 nm and rather efficient luminescence at
λ
420 nm. In addition to the “edge” luminescence
420 nm), they may exhibit luminescence at
(
λ
λmax 720 and 820 nm, due to electron–hole recomꢀ
bination at surface defects of CuI [1, 2], but the effiꢀ
ciency of this luminescence is low.
As shown by Malashkevich et al. [3], the quantum
yield of CuI luminescence in sol–gel derived oxide
films activated with CuI microcrystals exceeds 80%,
Procedure 1. CuI was synthesized according to the
scheme [4]
2CuSO4 + Na2SO3 + 2KI + H2O
→ 2CuI + K2SO4 + Na2SO4 + H2SO4.
and the fraction of photons emitted in the band at
λ
ꢀ
A
Na2SO3 solution (0.1 M) was added with vigorꢀ
720 nm approaches 100% at λex = 370 nm. For this
reason, such films can be regarded as potential conꢀ
verters of UV radiation, harmful to silicon solar cells,
to the spectral region of their highest sensitivity, which
would improve their efficiency. Oxide–CuI xerogels
are potentially attractive as luminescent markers for
security and authentication applications.
The preparation of copper(I) iodide has been the
subject of extensive studies. Most approaches use the
reduction of Cu2+ to Cu+ by various agents (sodium
sulfite or thiosulfate, sulfurous acid, acetone, sodium
borohydride, and others), followed by CuI precipitaꢀ
tion with a potassium iodide solution [4–6]. There are
also techniques that employ reactions of iodine with
ultrafine copper [7–9] and copper(II) sulfate with
potassium iodide [10].
The purpose of this work is to study the morpholꢀ
ogy, dimensions, and luminescence spectra of CuI
microcrystals in relation to synthesis conditions,
which will subsequently make it possible to assess the
influence of these factors on the luminescence spectra
of CuIꢀcontaining oxide films and xerogels as the most
promising materials for practical application.
ous stirring to a CuSO4 solution (0.5 M) in the molar
ratio CuSO4 : Na2SO3 = 1 : 3. To the resultant greenꢀ
yellow suspension was added dropwise a KI solution
(0.1 M) according to stoichiometry, which led to preꢀ
cipitation of whitish substance. After the completion
of the reaction, the precipitate was separated by
decantation, washed with a small amount of ethanol
and acetone, and dried in air at 90–95 С.
°
The properties of this sample were studied after
storage in air for several months.
Procedure 2. A copper(II) salt was precipitated
with potassium iodide:
2Cu2+ + 4I–
→
2CuI↓ + I2.
To a potassium iodide solution of predetermined
concentration was added dropwise with vigorous stirꢀ
ring a copper(II) salt solution. The resultant precipiꢀ
tate was separated by decantation; washed with a
0.02 M potassium iodide solution, distilled water, and
ethanol; and dried at 20 С.
°
To optimize the CuI synthesis conditions and
ensure a high product yield, we prepared a number of
630

SYNTHESIS, MORPHOLOGY, AND OPTICAL PROPERTIES OF CuI MICROCRYSTALS
631
samples at different concentrations of the starting
reagents, which were varied from 0.005 to 0.015 M for
the copper salt and from 0.01 to 0.05 M for potassium
iodide. At a CuSO4 : KI molar ratio of 1 : 2 and 1 : 10,
the product yield was 12–18 and 85%, respectively. An
excess of KI relative to stoichiometry facilitated dissoꢀ
lution of precipitating iodine, displacing the reaction
equilibrium toward CuI formation.
In this procedure, the divalent copper sources were
copper sulfate (pentahydrate) and copper nitrate (triꢀ
hydrate).
Procedure 3. Direct precipitation was carried out
as follows [11]: First, we prepared KI (0.38 g) and
CuCl2 (0.18 g) solutions in PEGꢀ400 (26.5 g) with
constant stirring. After complete dissolution, the soluꢀ
tions were poured together. The resultant solution was
added dropwise to a 0.05 M NaNO3 solution (80 mL)
RESULTS AND DISCUSSION
XRD characterization showed that the offꢀtheꢀ
shelf CuI and all the synthesized CuI samples had a
cubic structure [13]. The synthesized samples had
identical XRD patterns, so Fig. 1 shows the XRD patꢀ
terns of the offꢀtheꢀshelf CuI sample and the sample
prepared by procedure 4 using Cu(NO3)2. The syntheꢀ
sized samples had sharp, narrow diffraction peaks,
indicative of good crystallinity.
According to SEM data (Fig. 2), the CuI samples
consisted of polyhedral (procedure 1), tetrahedral
(procedures 2, 4), or platelike (procedure 3) particles
ranging in average size from 0.45 to 1.9 m. The offꢀ
μ
theꢀshelf CuI consisted of wellꢀfaceted, irregularly
shaped tetrahedral or polyhedral particles with an
average size of 1.8 m and had a broader particle size
μ
distribution in comparison with the synthesized samꢀ
ples.
The CuI sample prepared through direct precipitaꢀ
tion, using PEG as a stabilizer (procedure 3), conꢀ
at 20
centrifugation; washed with a 0.01 M KI solution; disꢀ
tilled water, and ethanol; and dried at 20
°С with stirring. The precipitate was separated by
°С.
Procedure 4. Colloidal copper prepared by reducꢀ
ing Cu2+ with sodium borohydride was oxidized by
atmospheric oxygen in the presence of potassium
iodide [6]:
sisted of the smallest particles (dav
0.45
μ
m)
ꢀ
(Fig. 2d).
In the case of direct CuI precipitation, the CuSO4
:
KI ratio influenced the shape and dimensions of the
forming particles (Fig. 3). The CuI particles prepared
at a CuSO4 : KI molar ratio of 1 : 2 from more dilute
solutions (Fig. 3a) had the most regular tetrahedral
shape. With increasing initial solution concentrations
at a constant CuSO4 : KI molar ratio, the particle size
varies little and the particle shape slightly distorts but
remains almost tetrahedral (Fig. 3c). At a CuSO4 : KI
molar ratio of 1 : 10, there are large agglomerates conꢀ
sisting of finer, faceꢀsharing tetrahedral particles
(Fig. 3b). At CuSO4 : KI = 1 : 1, we obtained irreguꢀ
larly shaped, elongated particles ranging in size up to
−
−
Cu2+ + 3I + BH4 + 3H2O
−
→ CuI↓ +H2BO3 + 4H2↑ + I2↓.
The divalent copper sources used were copper sulꢀ
fate (pentahydrate), copper nitrate (trihydrate), copꢀ
per acetate, and copper bromide. In our experiments,
the highest reaction rate and product yield were
achieved at pH 2.8–3.0.
To a Сu2+ salt solution was added a potassium
iodide solution with constant stirring. Next, argon was
bubbled through the solution for 15 min and a freshly
prepared sodium borohydride solution was added
dropwise. The reaction mixture was then stirred for
2.5–3 h and left to stand for 6–12 h. This led to preꢀ
cipitation of a white or cream white dense substance,
which accumulated on the bottom of the reaction vesꢀ
sel. The precipitate was separated by decantation,
washed with distilled water and ethanol, and dried at
4.0 m (Fig. 3d). Using Cu(NO3)2 instead of CuSO4 in
this procedure had no effect on the morphology or size
of the forming CuI particles.
μ
The CuI samples prepared by procedure 4 using
sodium borohydride consisted largely of tetrahedral
particles (Fig. 4), independent of the anion of the copꢀ
2−
−
per(II) salt (
SO4 , NO3,
CH COO–, or Br–), but difꢀ
3
40–60°С.
fered in particle size and polydispersity index (table).
The sample prepared using CuSO4 was the most
uniform in shape and dimensions (Fig. 4a, table).
All of the CuI samples had similar luminescence
excitation spectra, which differed only in intensity and
contained a broad luminescence excitation region,
from 250 to 410 nm (λem = 720 nm), with two weak
features at 340 and 410 nm, due to electron excitation
from inner levels and directly from the CuI valence
band top, respectively, to the conduction band [11].
Note that the intensity of the excitation band varied
little from sample to sample in the above spectral
region (Fig. 5a).
The initial solution concentrations were calculated
so as to obtain the following final concentrations:
×
10–3 M Cu2+, 10–2 M KI, and
product yield was 67–74%.
6
10–2 M NaBH4. The
The CuI samples were characterized by Xꢀray difꢀ
fracting (XRD) on a DRONꢀ2.0 powder diffractomeꢀ
ter with Сu
К radiation. The dimensions and shape of
α
particles were determined by scanning electron
microscopy (SEM) on a LEO 1420. Luminescence
excitation and luminescence spectra were measured in
the range 250–850 nm on an SM 2203 spectrofluoꢀ
rimeter.
The average particle size and polydispersity index
were evaluated as described in Voyutskii and
Panich [12].
The luminescence spectra of the CuI samples
(
λex 370 nm) showed two bands: one narrow, at
INORGANIC MATERIALS Vol. 48
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632
SHEVCHENKO et al.
(а)
(b)
111
222
111
311
222
311
200
30
200
30
222
222
10
20
40
50
60
10
20
40
, deg
50
60
2θ
, deg
2θ
Fig. 1. XRD patterns of CuI samples: (a) procedure 4, (b) offꢀtheꢀshelf CuI.
(а)
(b)
(c)
(d)
(e)
2
μ
m
2
μ
m
1
μ
m
1
μ
m
1 μm
n
, %
n
, %
n
, %
n
, %
n
50
40
30
20
10
0
, %
50
40
30
20
10
0
50
40
30
20
10
0
50
40
30
20
10
0
50
40
30
20
10
0
0.5 2.3 4.1
m
0.5
1.9
μ
3.3
0.4
1.2
2.0
0.1
0.5
μ
0.9
0.5 0.7 0.9
d
d
,
μ
d,
m
d,
μ
m
d
,
m
, μm
Fig. 2. SEM images and particle size distributions of CuI samples: (a) offꢀtheꢀshelf CuI, (b) procedure 1, (c) 2, (d) 3, (e) 4.
λmах = 420 nm, and the other broad, at λmах = 720 nm
(Fig. 5b). The intensity of the luminescence at λmах
720 nm correlates with that of the corresponding lumiꢀ
nescence excitation spectra, and the intensity of the
420ꢀnm luminescence varies in a more complex manꢀ
ner, which calls for further investigation. It should be
emphasized that, in what follows, we consider the
effect of synthesis conditions on the luminescence
around λmах = 720 nm.
According to our experimental data, the brightest
luminescence at
λ
720 nm was offered by the CuI
ꢀ
samples prepared by procedure 4, independent of the
anion of the copper(II) salt used. In those samples, the
majority of the particles had a regular tetrahedral
shape, with an average size in the range 1.1–1.5 m.
μ
The only exception was the sample prepared from
CuSO4, which showed very weak luminescence.
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SYNTHESIS, MORPHOLOGY, AND OPTICAL PROPERTIES OF CuI MICROCRYSTALS
633
2
μ
m
2
μ
m
2
μ
m
2 μm
(а)
(b)
(c)
(d)
Fig. 3. SEM images of CuI samples (procedure 2) obtained at different CuSO : KI ratios: (a) 1 : 2 (0.005 M CuSO , 0.01 M KI),
4
4
(b) 1 : 2 (0.015 M CuSO , 0.03 M KI), (c) 1 : 10 (0.005 M CuSO , 0.05 M KI), (d) 1 : 1 (0.015 M CuSO , 0.015 M KI).
4
4
4
(а)
(b)
(c)
(d)
1
μ
m
1
μ
m
1
μ
m
1 μm
n
, %
n
, %
n
, %
n
, %
50
50
50
50
40
30
20
10
0
40
30
20
10
0
40
30
20
10
0
40
30
20
10
0
0.5 0.7 0.9
m
0.8 1.6 2.4
μ
0.5 0.9 1.3
m
0.7 1.1 1.5
d
d
,
μ
d,
m
d
,
μ
, μm
Fig. 4. SEM images and particle size distributions of CuI samples (procedure 4) prepared using different precursors: (a) CuSO ,
4
(b) Cu(CH COO) , (c) CuBr , (d) Cu(NO ) .
3
2
2
3 2
(а)
3
(b)
1000
800
600
400
200
0
1000
800
600
400
200
0
1
5
6
2
6
3
1
7
8
3
2
2
5
4
6
5
7
9
4
8
1, 7
9
4
9
250
300
350
400
450
500
400
500
600
700
800
900
Wavelength, nm
Wavelength, nm
Fig. 5. (a) Luminescence excitation spectra (
λ
= 720 nm) and (b) luminescence spectra (
λ
= 370 nm) of CuI samples prepared
ex
em
by (1
–4
) procedure 4 using (
1
) copper bromide, (
2) copper acetate, (3) copper nitrate, and ( ) copper sulfate; (
4
5, 6) by procedure
2 using (
5) copper sulfate and (
6
) copper nitrate; (
8
) by procedure 1; and (9) by procedure 3; (7) offꢀtheꢀshelf CuI.
INORGANIC MATERIALS Vol. 48
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634
SHEVCHENKO et al.
Effect of the anion of the copper(II) salt on the average parꢀ
ticle size and polydispersity index of CuI samples prepared
by procedure 4
nescence is offered by regularly shaped tetrahedral
particles.
The present results will be used subsequently to
produce oxide films and xerogels based on CuI samꢀ
ples offering the brightest luminescence, with the posꢀ
sibility of using them in silicon solar cells in order to
improve their efficiency.
Anion
Sulfate Acetate Bromide Nitrate
Average particle diꢀ 0.8
1.5
1.1
1.2
ameter,
µ
m
Polydispersity index
1.1
1.3
1.2
1.4
REFERENCES
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Properties of CuI Nanocrystallites Embedded in a
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drei Bänden, von Brauer, G., Ed., Stuttgart: Ferdinand
Enke, 1978, 3rd ed., vol. 4.
The offꢀtheꢀshelf CuI, the samples synthesized
using sodium sulfite as a Cu2+ reductant (procedure 1),
and those prepared by direct precipitation of CuSO4
and Cu(NO3)2 with potassium iodide (procedure 2),
which consisted of large, irregularly shaped particles
with an average size of 1.9–2.0
μ
m, had weaker lumiꢀ
nescence (by a factor of 3–5).
The sample prepared by procedure 3, using PEG,
which consisted of small particles ( av= 0.45 m) with
d
μ
a nontetragonal shape, showed weak luminescence.
The sample synthesized from CuSO4 using sodium
borohydride as a reductant had a small particle size
(
dav= 0.8
μ
m) and almost zero luminescence intensity.
5. Ming Yang, JinꢀZhong Xu, Shu Xu, et al., Preparation
of Porous Spherical CuI Nanoparticles, Inorg. Chem.
Commun., 2004, no. 7, pp. 628–630.
CONCLUSIONS
6. Bernard, C.H. and Fan, W.Y., Facile Synthesis of Sinꢀ
gleꢀCrystalline
γꢀCuI Nanotetrahedrons and Their
We have synthesized CuI by various procedures.
Synthesis conditions (the reductant of Cu2+, the
anion of the copper(II) salt, initial solution concenꢀ
trations, and the presence of a stabilizer) have been
shown to influence the size and shape of the forming
CuI particles.
Induced Transformation to Tetrahedral CuO Nanoꢀ
cages, J. Phys. Chem. C, 2007, no. 111, pp. 9166–9171.
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Phase Transitions in MechanoꢀChemically Syntheꢀ
sized CuI Nanocrystals, Mater. Phys. Mech., 2001,
vol. 4, pp. 71–75.
The smallest CuI particles were produced through
8. Wagner, J.B. and Wagner, C., Electrical Conductivity
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9. Popolitov, V.I. and Lobachev, A.N., Chemical Syntheꢀ
sis and Properties of Copper Iodide Single Crystals, Izv.
Akad. Nauk SSSR, Neorg. Mater., 1973, vol. 9, no. 6,
pp. 1062–1063.
direct precipitation, using PEG as a stabilizer (dav
0.45 m). The other procedures yielded particles
=
μ
ranging in size from 0.8 to 2.0
fered in morphology:
μ
m. The particles difꢀ
tetrahedral (colloidal copper prepared by reducing
Cu2+ with sodium borohydride was oxidized by atmoꢀ
spheric oxygen in the presence of potassium iodide),
distorted tetrahedral (Cu2+ was reduced to Cu+ by
potassium iodide),
irregular polyhedral (Cu2+ was reduced to Cu+ by
sodium sulfite), and
10. Ripan, R. and Ceteanu, I., Chimia metalelor, Bucharꢀ
ٚ
ٚ
est: Editura Didactica si Pedagogica, 1969, vol. 2.
11. Xu, D.Y., Chen, X. Jiao, and Ba, L., PEGꢀAssisted
Fabrication of SingleꢀCrystalline CuI Nanosheets: A
General Route to TwoꢀDimensional Nanostructured
Materials, J. Phys. Chem. C, 2007, vol. 111, no. 1,
pp. 6–9.
12. Voyutskii, S.S. and Panich, R.M., Praktikum po kolloidꢀ
noi khimii i elektronnoi mikroskopii (A Practical Course
in Colloid Chemistry and Electron Microscopy), Mosꢀ
cow: Khimiya, 1974, pp. 196–197.
platelike (Cu2+ was reduced to Cu+ by potassium
iodide, with PEG as a stabilizer).
The morphology and size of CuI particles influence
their luminescence spectra. The optimal size of CuI
particles, ensuring the highest luminescence intensity
13. Powder Diffraction File, Swarthmore: JCPDS–Int.
Centre for Diffraction Data, 1989, nos. 6ꢀ246,
35ꢀ1091, 45ꢀ937.
near λmax 720 nm, is 1.1–1.2
μm. An important
point is that, at a given CuI particle size, brighter lumiꢀ
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2012