Vibrational spectra of copper sulfate hydrates investigated with low-temperature raman spectroscopy and terahertz time domain spectroscopy

Article abstract of DOI:10.1021/jp302997h

In this paper, the vibrational spectra of copper sulfate hydrates (CuSO₄xH₂O, x = 5, 3, 1, 0) have been investigated with low-temperature Raman spectroscopy and terahertz time domain spectroscopy (THz-TDS). It is found that the four groups of Raman bands between 90 and 4000 cm^-1 can be assigned to lattice vibration as well as intramolecular vibrations of a copper complex, sulfate group, and water molecules. The variation of vibrational spectra during the dehydrated process are discussed in detail considering the transformation of the crystal structure, especially the bands between 3000 and 3500 cm^-1, which are attributed to the v ₁ and v₃ modes of water molecules. In addition, as a complement of Raman spectra, the THz spectra at 0.1-3 THz indicate the absorption due to the low-frequency lattice vibration and hydrogen bond.

Full text of DOI:10.1021/jp302997h

Article
pubs.acs.org/JPCA
Vibrational Spectra of Copper Sulfate Hydrates Investigated with
Low-Temperature Raman Spectroscopy and Terahertz Time Domain
Spectroscopy
Xiaojian Fu, Guang Yang, Jingbo Sun, and Ji Zhou*
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University,
Beijing 100084, People’s Republic of China
ABSTRACT: In this paper, the vibrational spectra of copper sulfate
hydrates (CuSO ·xH O, x = 5, 3, 1, 0) have been investigated with low-
4
2
temperature Raman spectroscopy and terahertz time domain spectros-
copy (THz-TDS). It is found that the four groups of Raman bands
−1
between 90 and 4000 cm can be assigned to lattice vibration as well as
intramolecular vibrations of a copper complex, sulfate group, and water
molecules. The variation of vibrational spectra during the dehydrated
process are discussed in detail considering the transformation of the
−1
crystal structure, especially the bands between 3000 and 3500 cm ,
which are attributed to the ν and ν modes of water molecules. In
1
3
addition, as a complement of Raman spectra, the THz spectra at 0.1−3
THz indicate the absorption due to the low-frequency lattice vibration and hydrogen bond.
INTRODUCTION
related research on inorganic hydrates is rarely reported, to our
knowledge.
■
Copper sulfate pentahydrate, commonly called chalcanthite or
blue vitriol, is a naturally occurring mineral. When heating
CuSO ·5H O, it will dehydrate in three steps, losing two water
In this paper, we present the study of Raman and THz
spectra of copper sulfate hydrates and analyze the vibrational
modes and hydrogen bonds in these compounds in detail.
Compared to previous research, our Raman spectra were
collected at low temperature (−175 °C) with a high frequency
resolution, and the assignment of the bands that belong to
water molecules is clarified. Furthermore, with THz spectra, we
studied the response of the hydrogen bond and lattice vibration
4
2
molecules at 50 °C, then another two at 90 °C, and the last one
1
,2
at 230 °C. In the hydrates, water molecules either coordinate
with copper ions or form hydrogen bonds, both of which have
important impacts on the structure and chemical properties of
the hydrates. Therefore, the chemical environment of water
molecules has been intensively investigated by X-ray diffraction
and vibrational spectroscopy in copper sulfate hydrates and
−1
in the low-frequency region (<100 cm ), which has not been
reported in these hydrates.
3
,4
sulfates with similar structure. Structure tests show that
different hydrates have different crystal symmetry and
5
−9
coordination.
Thermo-Raman and IR spectroscopy have
EXPERIMENTAL PROCEDURES
■
been exploited to study the dehydrated process and the change
of vibrational models due to phase transition.
Sample Preparation. Powder of copper sulfate pentahy-
drate with high purity was used as the raw material in our
experiments. In order to get the trihydrate, monohydrate, and
anhydrate, the pentahydrate was heated to 50, 90, and 230 °C,
respectively, and kept for 30 min to complete the phase
transition.
2
,10,11
However,
the resolution of Raman spectra is lowered at high temperature,
especially for the detection of water molecules whose
vibrational modes have weak Raman response. In addition,
the hydrogen bond is a kind of weak interaction, whose
response frequency is usually lower than 100 cm . In such a
frequency range, Raman and IR spectra are of poor frequency
resolution.
Recently, with the development of the generation and
detection technologies of terahertz rays, terahertz time domain
spectra have been applied in many fields, such as materials
−1
Raman Experiments. Raman spectra were obtained on a
LabRAM HR800 Raman spectrometer (HORIBA Jobin-Yvon
Ltd.) with a temperature control accessory (Dlink). The Raman
scattering was excited by the 632.8 nm line of a He−Ne laser
and detected with a CCD camera in frequency ranges of 90−
−1
12,13
1500 and 2500−4000 cm . The spectra at 25 and −175 °C
analysis, medical diagnosis, and security inspection.
THz
were collected for comparison.
measurements are highly sensitive to water molecules, and
therefore, it is an effective method to study the rotational
modes of water vapor and the hydrogen bonds in condensed
Received: March 29, 2012
Revised: June 10, 2012
Published: June 15, 2012
14
water and hydrates. Hydrogen bonds in some organic
1
5,16
molecules have been reported in the literature;
however,
©
2012 American Chemical Society
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The Journal of Physical Chemistry A
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Terahertz Spectra Experiments. Terahertz time domain
spectra were measured with Z3 THz time domain spectrometer
system (Zomega Terahertz Corp., U.S.A.), which is equipped
with a photoconductive antenna for the generation of THz
radiation and an electro-optical crystal for detection. The pump
laser is a mode-locked Ti:sapphire laser (Spectra Physics,
U.S.A.) with the center wavelength of 800 nm and a pulse
duration of 100 fs. The equipment was filled with high-purity
Table 1. Crystal Structure Information of Copper Sulfate
and Its Hydrates
coordination
of Cu
2+
formula
crystal structure
triclinic
lattice parameters
18
CuSO
4
·5H
2
O
a = 6.141, b = 10.736, c 4H
2
O +
2−]
=
5.986(Å), α =
82°16′, β = 107°26′, γ
102°40′
a = 5.592, b = 13.029, c 3H O +
2O[SO
4
=
7
CuSO ·3H O
monoclinic
4
2
2
(
99.999%) nitrogen when collecting the time domain signal.
3O[SO42−]
=
7.341(Å), β = 97°3′
Corresponding frequency domain spectra in the range of 0.1−3
THz were obtained through Fourier transform. We repeated
the measurements three times, and the average values were
adopted.
8
CuSO ·H O
pseudomonoclinic a = 5.037, b = 5.170, c = 2H O +
4
2
2
7.578, α = 108°37′, β
108°23′, γ = 90°56′,
a = 8.409, b = 6.709, c = 6O[SO
.839(Å)
4O[SO
2−]
2−]
4
=
19
CuSO
4
orthorhombic
4
4
RESULTS AND DISCUSSION
■
The Raman spectra of hydrates and anhydrates of copper
attributed to intermolecular vibration of copper and sulfate ions
−1
−1
sulfate between 90 and 1500 cm are shown in Figure 1. It is
(external modes). The 280 cm band splits into three, 248,
−1
2
68, and 283 cm , which can be assigned to the internal
modes of the complex. In CuSO ·5H O copper ion is six-
4
2
coordinate, in which four of ligands are water molecules and the
other two are oxygen atoms from two sulfate groups. The
−1
bands lying in 400−1500 cm are generated by the internal
−
1
vibration of sulfate ions. The peaks at 425, 441, and 468 cm
2−
are related to the symmetric bending vibration of SO4 (ν
2
1
modes). In addition, the peaks at 611, 984, and 1148 cm−
correspond to ν4 (antisymmetric bending), ν (symmetric
1
stretching), and ν3 modes (antisymmetric stretching),
respectively. The results are in good agreement with the ones
in ref 17, except for several weak bands.
During the heating process, CuSO ·5H O transforms into
4
2
CuSO ·3H O first, and the corresponding changes in the
4
2
Raman spectrum are observed. The peak of ν shows a blue
1
−
1
shift, from 984 to 1010 cm , and the ν mode shows a red
shift, from 1148 to 1127 cm . The band of the ν mode splits
3
−1
4
−
1
−1
to two, 586.5 and 618 cm , and the bands below 400 cm also
differ a lot (see Table 2). It can be explained by the change in
crystal structure. The trihydrate has a monoclinic structure, and
the six ligands of copper are three water molecules and three
oxygen atoms. Therefore, both internal modes of the sulfate
group and the lattice modes vary. At higher temperature,
another two water molecules are lost to form monohydrate,
which has pseudomonoclinic symmetry, and the ligands are
made up of two water molecules and four oxygen atoms.
Because sulfate ions have two kinds of crystal environments,
obvious splitting appears in the Raman spectrum. For instance,
−
1
the ν mode at 1010 cm splits into two at 1011.5 and 1045
1
−1
−1
cm , and the ν mode at 1127 cm splits into the doublet at
100 and 1209 cm . The bands of the ν and ν modes also
3
−1
1
2 4
exhibit remarkable changes; the former (423, 435, and 482)
−1
combine into two, 420.5 and 510 cm , while the latter (586.5,
−
1
−1
Figure 1. Raman spectra of CuSO ·xH O below 1500 cm . For all
618) split into three, 607, 620, and 665.5 cm . Moreover, the
4
2
modes below 400 cm− become more intense in the
1
four kinds of hydrates, the Raman spectra at temperatures of 25 and
175 °C are collected.
−
1
−
monohydrate. A typical example is that the band at 387 cm
shifts to 345.5 cm− and becomes much stronger. From the
monohydrate to the anhydrous one, the new phase possesses
orthorhombic symmetry and different lattice parameters. As a
1
found that the spectra are well-resolved at low temperature and
that the Raman bands can be divided into three groups. The
modes below 400 cm are related to the external modes of the
ions (lattice phonons) and internal modes of the copper
complex, while the modes above 400 cm are associated with
the intramolecular vibration of sulfate. The frequencies of
Raman bands are listed in Tables 2 and 3. According to the
spectra of pentahydrate, the broad band located at 125 cm
−1
result, the two bands of the ν mode merge into one band at
1
−1
1059.5 cm , and the peaks of the ν , ν , and ν modes also
2
3
4
−1
show some shift.
17
Figure 2 presents the vibrational spectra of H O molecules in
2
hydrates. According to symmetry analysis, free water molecules
have three nondegenerate normal modes, the symmetric stretch
−1
−1
−1
−1
consists of three modes, 122, 131, and 140 cm , which are
mode ν (3825 cm ), the bend mode ν (1654 cm ), as well
1
2
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−1
Table 2. Raman Bands of CuSO ·xH O below 400 cm
4
2
x = 5
x = 3
x = 1
x = 0
25 °C
−175 °C
25 °C
−175 °C
25 °C
−175 °C
25 °C
−175 °C
mode
125
122
123
160
127
145
153
105
130
109.5
137
110
164
186
116
170
192
lattice vibration
1
1
31
40
1
64
280
248
193.5
250
207.5
244
213
268
273
internal modes of complex
2
2
68
83
250
247.5
273
276
268
3
86
387
345
347.5
−1
Table 3. Raman Bands of CuSO ·xH O between 400 and 1500 cm
4
2
x = 5
x = 3
x = 1
x = 0
25 °C
−175 °C
25 °C
−175 °C
25 °C
−175 °C
25 °C
−175 °C
mode
4
55
425
429
423
435
482
586.5
618
419
515
420.5
510
458.5
497
459
482.5
500
ν2[SO42−]
4
4
41
68
481
6
11
611
586.5
607
607
583
605
579
ν4[SO42−]
6
20
620.5
620
587
6
69
665.5
607
9
32
930.5
1059.5
9
84
984
1009
1126
1010
1127
1014
1011.5
1045
1100
1209
1057
ν1[SO42−]
ν3[SO42−]
1
043.5
1
146
1148
1097
1162.5
1174
1154
1173
1203
1
204
1
197
−1
Table 4. Raman Bands of CuSO ·xH O above 3000 cm at
4
2
−
175 °C
x = 5
x = 3
x = 1
3
3
3
3
110
190
355
475
3100
3155
3360
3410
3105
3370
keep the structure stable. The aforementioned five H O
2
molecules can be divided into three groups; two of them
weakly bond to Cu2+ (denoted by I-H O), the other two have
2
stronger bond strength (marked as II-H O), and the last one is
2
denoted by III-H O. As shown in Figure 2, the resolution of the
2
spectra is poor due to noise and spectral broadening at room
temperature, whereas the results at −175 °C are much better.
The pentahydrate is detected with four bands, and their center
Figure 2. Raman spectra of CuSO ·xH O between 2500 and 4000
cm .
4
2
−1
−1
frequencies are 3110, 3190, 3355, and 3475 cm . After losing
two I-H O molecules, the trihydrate is formed, in which all
2
−1
20
2+
as the asymmetric stretch mode ν (3936 cm ). However, in
three H O molecules coordinate to Cu , but they are still
3
2
the condensed state, such as in inorganic hydrates, the H O
molecules coordinate with a cation. Thus, the vibrational
divided into two groups because of different bonding strengths.
Figure 2 and Table 4 indicate that CuSO ·3H O possesses four
2
4
2
−1
frequencies may be different from the ones in free H O
Raman bands above 3000 cm , whose frequencies are 3100,
2
−1
molecules due to the interaction with the center cation.
3155, 3360, and 3410 cm . When monohydrate is obtained at
In the previous research, the vibrational spectrum of H O in
about 90 °C, only two bands are investigated at 3105 and 3370
2
−1
CuSO ·5H O has been reported, and the observed bands are
cm . Anhydrous copper sulfate has no Raman peak above
4
2
17
−1
assigned to corresponding modes. In the present paper, we
show the Raman spectra of water molecules in the other copper
sulfate hydrates, which are listed in Table 4. As mentioned
above, in CuSO ·5H O, the copper ion is coordinated by four
3000 cm ; therefore, its spectrum is not given. According to
ref 17, in the pentahydrate, the peaks around 3110 and 3190
−
1
cm can be assigned to the ν mode of I-H O and II-H O,
1
2
2
−1
respectively, and the bands at 3355 and 3475 cm can be
4
2
H O molecules and two oxygen atoms (from the sulfate group)
2
ascribed to the ν and ν modes of III-H O, respectively.
1
3
2
to form an octahedron, whereas another H O molecule
connects the adjacent octahedral through a hydrogen bond to
However, through comparing three types of hydrates, we may
draw some different results. The monohydrates have only one
2
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Article
type of H O; thus, the two bands at 3105 and 3370 cm−1
2
Table 5. Absorption Bands of CuSO ·xH O between 0.1 and
4
2
should be assigned to its ν and ν modes. We also find that the
3 THz
1
3
frequency difference between the two modes reaches up to 265
−1
x = 5
x = 3
x = 1
x = 0
cm , which may be common in the other two hydrates. Then,
in the pentahydrate, the bands located at 3110 and 3355 cm−1
cm−1
cm−1
cm−1
cm−1
THz
THz
THz
THz
are more likely generated by the ν and ν modes of the water
molecules coordinated with copper ions (including I- and II-
1.34
1.46
1.61
1.76
1.93
2.00
2.13
44.5
48.7
53.7
58.7
64.3
66.7
71.1
75.3
80.4
84.9
94.0
1.60
1.88
2.18
2.34
2.50
2.86
53.4
62.7
72.5
78.0
83.3
95.4
1.76
1.90
2.24
2.39
2.80
58.6
63.4
74.5
79.7
93.2
1.87
2.35
2.76
2.86
62.3
78.3
92.0
95.3
1
3
H O, whose Raman frequency may be too close to be
2
−1
distinguished), respectively, while bands at 3190 and 3475 cm
can be ascribed to the ν and ν modes of III-H O, respectively.
Similarly, in the trihydrate, peaks at 3100 and 3360 cm are
1
3
2
−1
attributed to the ν and ν modes of two H O molecules with
1
3
2
−1
2
2
2
2
.26
.41
.55
.82
the same environment, and those at 3155 and 3410 cm are
generated by corresponding modes of another H O. From the
2
preceding discussion, we may conclude that the water
2
+
molecules coordinated to Cu have approximate vibrational
frequency in all three hydrates despite the fact that all six
ligands are not the same (see Table 1). Additionally, the water
molecule bridging the coordination octahedron through a
hydrogen bond has very different Raman bands.
−1
range is between 0.1 and 3 THz (about 3−100 cm );
therefore, the absorption mainly originates from lattice
vibration and the hydrogen bond. As shown in Table 5, the
number of absorption bands decreases with the decrease of
water molecule number. For x = 0, there are four peaks that are
generated by low-frequency lattice phonons because there is no
A hydrogen bond is a kind of weak interaction whose
−
1
frequency occurs below 100 cm in some compounds.
Research shows that the Raman frequency of a hydrogen
−1
bond in liquid water is about 75 cm and shifts suddenly to
H O in it. For x = 5, 3, and 1, the hydrogen bond will
20 cm−1 when ice is formed and that in some organic
2
2
contribute to the absorption of the THz wave, and their peaks
may lie in the relatively lower frequency region. However, it is
not easy to assign the bands to a certain mode because low-
frequency vibrations are usually intermolecular modes and
many atoms or atomic groups may be involved. Moreover, the
hydrogen bond is a kind of many-body interaction. Never-
theless, the study of lattice vibration and the hydrogen bond
with THz-TDS reveals more information about the crystal
structure and chemical properties of hydrates. Some theoretical
calculation based on structural chemistry may help to
distinguish and assign the absorption peaks.
−1
16,21
hydrates, their frequencies are usually below 100 cm .
In
such a frequency range, compared with the limited application
of Raman spectroscopy and IR spectroscopy, terahertz time
domain spectroscopy exhibits its advantages with high-
frequency resolution. Apart from the hydrogen bond, low-
frequency lattice phonons also respond to THz waves and
cause obvious absorptions. In this work, the THz time domain
spectra of copper sulfate hydrates have been measured, and
corresponding frequency domain spectra (FDS) are also
obtained though fast Fourier transformation (FFT). As an
example, we show the transmission spectra of the reference
(
without any sample) and the pentahydrate in Figure 3.
CONCLUSIONS
■
Compared to the reference spectrum, many absorption lines are
observed in the sample spectra. In order to decrease the
interference of scattering and other noises, we repeat the
measurements three times, and the average values are utilized.
For four kinds of compounds, the absorption peaks in FDS
have been extracted and are listed in Table 5. The frequency
In this work, we utilize low-temperature Raman spectra and
THz-TDS to study the vibrational modes of copper sulfate
hydrates. It is found that some broadened Raman bands are
well-resolved at low temperature, and fine spectral structure is
obtained. On the basis of crystal structure and previous work in
this field, we analyze the variation in Raman bands of these four
components, especially the bands corresponding to water
molecules, which have been rarely concerned about. What is
more, the terahertz spectra show that there are some absorption
−1
bands at 3−100 cm that can be attributed to lattice vibration
and a hydrogen bond. In conclusion, with Raman and THz
spectroscopies, we can investigate effectively the intramolecular
modes, lattice vibration, and hydrogen bond in condensed-state
matters.
AUTHOR INFORMATION
Corresponding Author
E-mail: zhouji@mail.tsinghua.edu.cn. Phone/Fax: 86-10-
2772975.
■
*
6
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China under Grants 90922025, 51032003,
■
Figure 3. THz spectra of the reference (without any sample) and
copper sulfate pentahydrate.
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The Journal of Physical Chemistry A
0921061, and 51102148, and the National High Technology
Article
5
Research and Development Program of China under Grant
012AA030403. We also thank Mr. Xiaoqing Xi (State Key
2
Laboratory of New Ceramics and Fine Processing, Tsinghua
University) for the assistance in the measurements of Raman
and THz spectra.
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