Rapid sonochemical preparation of shape-selective lead iodide

Article abstract of DOI:10.1016/j.materresbull.2012.04.083

Lead iodide (PbI₂) films/crystals with various nano/micro morphologies (e.g., Nanoflake, block and microrod) were rapidly synthesized by taking advantage of a simple sonochemical method. The PbI₂ crystals with uniform nanoflake structures could be fabricated directly on lead foils with the irradiation time as short as 36 s via interfacial reaction between lead foils and elemental iodine in ethanol at ambient temperature. It was found experimentally that the morphologies of the resulting thin films/crystals could be well controlled by the adjustment of several parameters including irradiation time, reaction solvents, iodine concentration, ultrasonic power, and reaction temperature. Most importantly, the resultant PbI₂ films are stable enough to resist rolling under the drastic ultrasound irradiation in a liquid media. This method is believed to be the fastest way for in situ fabrication of morphology-controlled semiconductor films on various metal substrates for subsequent applications related to the other metal iodide or metal sulfide semiconductor films.

Full text of DOI:10.1016/j.materresbull.2012.04.083

Materials Research Bulletin 47 (2012) 2599–2604
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Rapid sonochemical preparation of shape-selective lead iodide
Baojun Huang a, Qin He b, Wenjun Fa a, Pinjiang Li a, Yange Zhang a, Zhi Zheng a,
*
a Key Laboratory of Micro-Nano Materials for Energy Storage and Conversion of Henan Province, Institute of Surface Micro and Nano Materials,
Xuchang University, Xuchang, Henan 461000, China
b College of Chemistry and Chemical Engineering, Xuchang University, Xuchang, Henan 461000, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Lead iodide (PbI2) films/crystals with various nano/micro morphologies (e.g., Nanoflake, block and
microrod) were rapidly synthesized by taking advantage of a simple sonochemical method. The PbI2
crystals with uniform nanoflake structures could be fabricated directly on lead foils with the irradiation
time as short as 36 s via interfacial reaction between lead foils and elemental iodine in ethanol at ambient
temperature. It was found experimentally that the morphologies of the resulting thin films/crystals could
be well controlled by the adjustment of several parameters including irradiation time, reaction solvents,
iodine concentration, ultrasonic power, and reaction temperature. Most importantly, the resultant PbI2
films are stable enough to resist rolling under the drastic ultrasound irradiation in a liquid media. This
method is believed to be the fastest way for in situ fabrication of morphology-controlled semiconductor
films on various metal substrates for subsequent applications related to the other metal iodide or metal
sulfide semiconductor films.
Received 1 August 2011
Received in revised form 27 March 2012
Accepted 18 April 2012
Available online 26 April 2012
Keywords:
A. Halides
A. Thin films
B. Crystal growth
C. X-ray diffraction
D. Microstructure
ß 2012 Elsevier Ltd. All rights reserved.
1. Introduction
fabricate PbI2 films/crystals with nano/microstructures for the
above applications. Also, in our previous work, the hydrothermal/
Architectural control of semiconductor crystals with well-
defined nano/micro structures has attracted considerable atten-
tion for potential applications due to their physical and chemical
properties which are strongly influenced by their shapes and
dimensions. Thus, morphology-controlled synthesis of semicon-
ductor nano/micro structures has been the topic of intensive
investigation in recent years [1]. Sonochemical method, which
allows the rapid synthesis of many inorganic materials under
ambient conditions, has been recently investigated as a promising
alternative technique for the fabrication of inorganic materials that
exhibited controllable nano/micro structures [2–6]. It depends on
the cavitation effects which are produced by collapsing bubbles in
liquids [7–9]. Different from other means, the sonochemical route
always has some special advantages: straightforward, fast,
surfactant/template free, low-cost, and low temperature (always
at room temperature) [10].
solvothermal approaches were successfully introduced to grow
epitaxial PbI2 films composed of perfect hexagonal single crystals
[24,25]. However, long reaction time and rigorous experimental
conditions such as high temperature or high vacuum were always
needed in such approaches. Herein, we employ a rapid in situ
liquid-solid reaction route for the growth of PbI2 films/crystals on
lead foils at room temperature by the sonochemical technique. And
the effects of different reaction parameters including irradiation
time, reaction solvents, iodine concentration, ultrasonic power,
and reaction temperature on the morphologies of various PbI2
products were systematically studied. The understanding of these
parameters is very important for us to further exploit the rapid and
morphology-controlled growth of PbI2 films/crystals for their
following use for photocell, X-ray, and g-ray detectors.
2. Experimental
Lead iodide (PbI2) has been paid particular attention for a long
time owning to its attractive optical and electrical properties as
2.1. Synthesis
well as technological application for photocell, X-ray and
g-ray
detectors, and medical imaging arrays [11–15] because of wide
band gap energy (Eg = 2.3–2.5 eV) [16], high resistivity
(r
> 1013 V cm), and good chemical stability [17]. There are so
All reagents were of analytical grade and used without further
purification. In a typical procedure, a certain dose of elemental
iodine and 45 mL absolute ethanol were loaded into a 50 mL
beaker to form a transparent solution. A piece of lead foil
(Sinopharm Chemical Reagent Co., purity: >99.9%, thickness:
0.1 mm) was placed into the bottom of the above beaker. The lead
foil was ultrasonically cleaned in advance with a 1 M HNO3
solution for several seconds and rinsed with distilled water for
far many synthetic approaches [18–23], which have been used to
* Corresponding author. Tel.: +86 374 2968783; fax: +86 374 4369210.
E-mail address: zzheng@xcu.edu.cn (Z. Zheng).
0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.materresbull.2012.04.083

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B. Huang et al. / Materials Research Bulletin 47 (2012) 2599–2604
several times. Then the metal foil was exposed to ultrasound
irradiation under ambient condition for shorter than 3 min. The
face parallel to the lead foil. Similarly, samples obtained in this
paper with nanoflake structures show the similar XRD pattern.
However, in spite of the sample generated from the I2/ethanol
reaction system at 15 8C (Fig. 1b) or the sample generated from
mixed mediums of ethanol and water (volume ratio of 1:1) at room
temperature (Fig. 1c), polycrystalline characteristics can be
deduced by the appearance of peaks originated from (0 0 1),
(1 0 1), (1 0 2), and (1 1 0) faces.
The chemical composition of the sample generated from the I2/
ethanol reaction system at 35 8C was also examined by X-ray
energy dispersive spectrometer (EDS), as shown in Fig. 2. Only lead
and iodine signals have been detected, suggesting that the product
was indeed made up of Pb and I. The molar ratio of Pb:I obtained
from the peak area is about 1:1.9, which is less than the
stoichiometric proportion of PbI2, and the high ratio of Pb maybe
originate from lead foil.
ultrasound irradiation was performed with
a high-intensity
ultrasound probe (Xinzhi Co., China, JY92-II, 0.6 cm-diameter,
Ti-horn, 20–25 kHz) directly immersed in the reaction solution and
the horn tip kept consistently to the same depth (20 mm) from the
bottom of the beaker. After the sonication was completed, the
metal foil was taken out of the solution and washed with ethanol
and finally dried in air at room temperature (28 8C) for
characterization.
2.2. Characterization
The powder X-ray diffraction (XRD) patterns were recorded
using a Bruker D8 Advance XRD diffractometer using Cu K
a
radiation. Scanning electron microscopy measurements were
performed using a Zeiss EVO LS-15 SEM equipped with X-ray
energy dispersive spectrometer (EDS).
As we expected, both the XRD patterns and EDS result did not
show any reflection or signal originated from other byproducts
such as PbO or PbO2, confirming that the products generated from
these simple I2/lead reaction systems were pure PbI2 crystals.
3. Results and discussion
3.2. Growth of PbI2 crystals with different irradiation time
3.1. Phase and chemical composition of the resulting PbI2 films/
crystals
Since the sonochemical reactions are directly and readily
accomplished within a very short irradiation time, the reaction
mainly occurred on the surface of lead foils to form thin PbI2 layers.
Fig. 3 shows the morphologies of the resulting PbI2 crystals with
the iodine concentration of 6.7 g/L under irradiation time of 3 min,
1 min, and 36 s at room temperature, respectively. After irradiation
for 3 min, a dense and uniform monolayer PbI2 film is present
(Fig. 3a). The magnified SEM image (Fig. 3b) reveals that this film is
composed of flake-like crystals with the thickness and size about
100 nm and 1000 nm, respectively. It is interesting to note that
each nanoflake grows perpendicular to the lead foil, which is
different from other flake crystals lying horizontally on the
corresponding substrates [24,25]. When the irradiation time
was decreased to 1 min, the size of PbI2 crystals almost kept
unchanged except that the film is appreciably sparser (Fig. 3c and
d). Considering that we could still obtain PbI2 thick film in such
short irradiation time of 1 min, we tried to use a much lower
irradiation time of only 36 s in the same reaction system. There
were still abundant PbI2 crystals with well-defined shape of
nanoflake formed on the lead foil, and the thickness and size of a
single PbI2 crystal almost did not change a little except for the
farther sparsity of PbI2 crystals (Fig. 3e and f).
In the present study, we firstly used XRD to characterize the
resulting metal iodide films. Fig. 1 shows the typical XRD patterns
of the PbI2 films/crystals with various nano/micro morphologies in
different reaction medium/systems. Three strong peaks at 31.308,
36.258, and 52.218 are respectively correlated to the (1 1 1), (2 0 0),
and (2 2 0) planes of the pure lead foil (JCPDS file No. 4-686),
respectively. While the other weaker peaks can be readily indexed
to hexagonal phase of PbI2 (JCPDS file No. 7-235, space group:
PÀ3m1(164)). The as-prepared PbI2 films were very thin due to the
short irradiation time of 1 minute, and the corresponding PbI2
reflections were much weaker in contrast to that of lead foil. After
carefully comparing these XRD patterns, we found that there were
a few differences on the strongest diffraction peaks of PbI2 crystals.
For the sample generated from the I2/ethanol reaction system at
35 8C (Fig. 1a), there is only one main sharp peak corresponding to
the (0 0 1) face, which indicated that the PbI2 crystals generated in
this reaction systems are pronouncedly oriented with the (0 0 1)
Fig. 1. The representative XRD patterns of the PbI2 crystals obtained in different
reaction medium/systems: I2/ethanol reaction systems at the reaction temperature
of 35 8C (a) and 15 8C (b), mixed reaction medium of ethanol and water (volume
ratio of 1:1) at room temperature (c).
Fig. 2. EDS spectrum of PbI2 crystals generated from the I2/ethanol system with the
iodine concentration of 6.7 g/L under ultrasound irradiation for 1 min at the
reaction temperature of 35 8C.

B. Huang et al. / Materials Research Bulletin 47 (2012) 2599–2604
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Fig. 3. SEM surface morphologies of PbI2 crystals obtained from the I2/ethanol system with the iodine concentration of 6.7 g/L under ultrasound irradiation for 3 min (a and b),
1 min (c and d), and 36 s (e and f).
Fig. 4. SEM morphologies of various PbI2 products obtained in I2/ethanol (a), I2/ethyl acetate (b), I2/mixed medium of ethanol and water (volume ratio of 1:1) (c), and I2/glycol
(d) systems under ultrasound irradiation for 1 min with the iodine concentration of 2.22 g/L.

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B. Huang et al. / Materials Research Bulletin 47 (2012) 2599–2604
In our present work, it is important to note that the formation of
system and only a relatively bare surface was present. Therefore,
we draw a conclusion that the solvent with higher polarity and
lower viscosity was beneficial for growing massive PbI2 crystals,
and proper solvent such as ethanol was able to assist the growth of
the PbI2 crystals with perfect structure such as nanoflake.
PbI2 film could be accomplished under ultrasound irradiation even
within such a short irradiation time of 36 s. It is so far the fastest
way for forming such a thin PbI2 film. Particularly, the resulting
PbI2 films consisting of flake-like crystals prepared by the present
ultrasound approach were so hard that they could be well
preserved even after being vigorously turned over in the reaction
solution under ultrasound irradiation, which makes them promis-
ing candidates for advanced optical and electrical devices.
3.4. Effect of iodine concentration and ultrasonic power
Fig. 5 shows the morphologies of PbI2 crystals after ultrasound
irradiation for 1 min at ambient temperature with different iodine
concentrations. When the amount of iodine was 2.22 g/L, well-
defined hexagonal PbI2 plates with the thickness and size about
100 nm and 2000–3000 nm were generated in the whole region of
the foil surface (Fig. 5a). With an increase in the iodine
concentration to 6.7 g/L, irregular PbI2 plates with the less size
about 1000 nm were obtained (Fig. 5b). Contrasting Fig. 5a with
Fig. 5b, it can be seen that PbI2 crystals generated with lower iodine
concentration were distinctly sparser than those with higher
iodine concentration. We deduced that the increased number of
crystallization centers in solution with higher iodine concentration
and consequent blocking effect on crystal growth due to the
simultaneous crystallization of more crystallites resulted in
3.3. Effect of reaction medium
It is well known that the reaction mediums play important roles
in the crystal growth. To control the surface morphology, we
selected ethanol, ethyl acetate, mixed solvents of ethanol and
water (volume ratio of 1:1), and glycol as the reaction media for the
sonochemical reaction. Considering the solubility of iodine in the
above different solvents, we chose the iodine concentration as
2.22 g/L in the case. Fig. 4 shows SEM morphologies of various PbI2
crystals obtained in different reaction systems. In the condition
that ethanol was used as the reaction medium, we could get the
PbI2 crystals with perfect hexagonal flake structure, which
stretched from the lead foil at random directions (Fig. 4a). When
ethyl acetate was transferred into the reaction system, the shape of
obtained PbI2 crystals turned to irregular flake along with the
obvious size shrinkage of the single slice (Fig. 4b). However, when
the higher-polar mixed solvents of ethanol and water (volume
ratio of 1:1) were employed into the reaction system, the structure
of resultant PbI2 crystals changed from flake to block (Fig. 4c), and
the size of every crystal was enlarged to micrometers. Solvents
with high polarity seemed to accelerate the ionization of lead and
iodine and therefore facilitated the iodination process, resulting in
the formation of massive PbI2 crystals. To increase the viscosity of
the reaction medium, we chose glycol as the reaction medium.
Fig. 4d displays that almost no product was formed in the I2/glycol
Fig. 5. SEM morphologies of PbI2 crystals obtained from the I2/ethanol system under
ultrasound irradiation for 1 min with the iodine concentration of 2.22 g/L (a) and
6.7 g/L (b).
Fig. 6. SEM morphologies of PbI2 crystals obtained from the I2/ethanol system with
the iodine concentration of 6.7 g/L under the ultrasonic power of 400 W (a), 200 W
(b), and 0 W (c).

B. Huang et al. / Materials Research Bulletin 47 (2012) 2599–2604
2603
Fig. 7. SEM morphologies of various PbI2 products obtained with the iodine concentration of 6.7 g/L and irradiation time of 1 minute at the reaction temperatures of 35 8C (a
andb), 25 8C (c and d), and 15 8C (e and f).
abundant PbI2 crystals with lesser size, which is similar to the
explanation that higher concentration produced crystals with
smaller size [26].
When the reaction temperature was 35 8C, the morphology of the
as-prepared sample is shown in Fig. 7a and b. From these images, it
can be seen that the hexagonal PbI2 nanoplates laid radically on the
surface of lead foil. The sizes of PbI2 crystals are about 1000–
3000 nm, which is suitable for use in the direct X-ray imaging [27].
The thickness relating to the size of an individual crystal may
change from several tens of nm to 100 nm and the average
thickness/size aspect ratio is about 1/30, which is much less than
that of other flake crystals [25]. When we decreased reaction
temperature to 25 8C, the plate structure of as-synthesized PbI2
crystals almost kept changeless except for disappearance of perfect
hexagonal shape (Fig. 7c and d). Interestingly, under a lower
temperature of only 15 8C, the morphologies of the as-prepared
products altered prodigiously from the nanoplates to microrods
with around 500–3000 nm in length and 200–700 nm in diameter
(Fig. 7e and f), respectively.
Generally speaking, ultrasonic power provided the energy to
inspire the reaction between lead and iodine molecules, we also
investigated the effect of ultrasonic power on the formation of PbI2
crystals. The difference in the surface morphologies indicated that
reaction under lower ultrasonic power (200 W) resulted in denser
film and lesser crystals (Fig. 6b) in comparison with the sparser
film and bigger crystals (Fig. 6a) obtained at high ultrasonic power
(400 W). In contrast, only a little isolated crystals formed without
ultrasound irradiation (Fig. 6c). We speculated that higher
ultrasonic power supplied the higher energy to increase the
growth rate of PbI2 to form the bigger PbI2 crystals on the surface of
lead foil. However, when the ultrasonic power was extremely
violent, the newborn PbI2 crystals were likely stripped from the
lead foil by the ultrasonic.
Obviously, the reaction temperature greatly affected the
growth of PbI2 crystals and we can realize an easy control of the
surface morphology including the shape and dimension of PbI2
crystals by simply adjusting the reaction temperature.
3.5. Growth of PbI2 films/crystals with various morphologies
In order to find the role of reaction temperature on the resulting
products, we controlled the reaction temperatures in the water
bath. Fig. 7 shows the morphologies of the resulting PbI2 films/
crystals with the iodine concentration of 6.7 g/L and irradiation
time of 1 min at the reaction temperatures of 35 8C, 25 8C, and
15 8C, which is corresponding to the XRD results shown in Fig. 1.
4. Conclusions
In this study, lead iodide films/crystals with various nano/micro
morphologies (Nanoflake, block, and microrod) were rapidly and
directly prepared on lead foils by the simple sonochemical

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B. Huang et al. / Materials Research Bulletin 47 (2012) 2599–2604
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This work was supported by National Natural Science Founda-
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