773
Figure 3. Raman spectra of various morphologies of Cu2(OH)PO4
crystals using different morphology-controlling agents: (a) ethylenedi-
amine at 150 °C, (b) tris(hydroxymethyl)aminomethane at 100 °C,
(c) tris(hydroxymethyl)aminomethane at 150 °C, and (d) 4,4¤-bipyridyl
at 150 °C.
Figure 2. XRD patterns of various morphologies of Cu2(OH)PO4
crystals using different morphology-controlling agents: (a) ethylenedi-
amine at 150 °C, (b) tris(hydroxymethyl)aminomethane at 100 °C,
(c) tris(hydroxymethyl)aminomethane at 150 °C, and (d) 4,4¤-bipyridyl
at 150 °C.
Table 2. The calculated cell volumes and lattice parameters of
Cu2(OH)PO4 with various morphologies
to 4,4¤-bipyridyl with the same hydrothermal method. Based on
these SEM results, different organic amines can be the reason
for the different morphologies of Cu2(OH)PO4 crystals. Adding
the same number of moles of different organic amines can
change the pH value of the reaction system and generate
Sample
a/¡
b/¡
c/¡
V/¡3
a
b
c
d
8.074
8.060
8.045
8.060
8.410
8.389
8.389
8.385
5.889
5.889
5.898
5.881
399.89
398.19
398.04
397.43
¹
different organic ammonium ions. The surface-adsorbed OH
and all kinds of ammonium ions can lead to linkage by
electrostatic attractions and repulsions.9 It can affect the form of
growth units and can then drastically affect the final morphol-
ogy. Under the condition of addition of tris(hydroxymethyl)-
aminomethane, changing the temperature of reaction resulted in
different morphologies, but others were not. It was the reason
that the pH of tris(hydroxymethyl)aminomethane’s buffer solu-
tion changed with the temperature. However, the pH exerts an
important influence on the synthesis of Cu2(OH)PO4 crystals
with various morphologies.3b
The XRD pattern (Figure 2) of as-synthesized copper
hydroxyphosphate crystals with different morphologies showed
several obvious peaks at 15.2, 18.5, 31.0, 34.2, and 37.3° in the
range of 4-40°. Based on above XRD result, we can see that
various morphologies of copper hydroxyphosphate crystals the
exhibited by the same phase. No diffraction peaks for other
phases or materials or copper phosphate hydrate are observed in
XRD patterns, indicating a high purity and crystallinity of the
final products. However, the relative intensities of the diffraction
peaks for one sample are obviously different from those for
other samples, which is attributed to the variations in the
morphology. For example, higher intensity for the (011) plane
as compared with other planes in Figure 2d is attributed to
the sheet morphology. According to the XRD analysis, the
percentage of (011) plane of Cu2(OH)PO4 crystals is 4.51% (a),
18.85% (b), 14.42% (c), and 21.97% (d). Therefore, from these
XRD results, it is expected that changing the organic amines
in this reaction system involves variations of the morphology
and crystal structure. Moreover, these variations of the crystal
structure, which correspond with a change in the morphology,
significantly influenced the catalytic activity in hydroxylation of
phenol.
observed at 976 and 453 cm¹1, respectively. The Raman bands
¹1
at 1020 cm was attributed to the v3 mode. The v4 modes were
¹
found at 557, 627, and 646 cm¹1. The lattice OH vibration
bands for all the crystals were observed at 816 cm¹1. However,
we can see that the peaks corresponding to 627, 976, and
¹1
1020 cm of Figure 3a were shifted to lower frequencies in
comparison with Figure 3b, Figure 3c, and Figure 3d, which
provided important information on the variation of the local
crystal structure with changes in the morphology. The lower
wavenumbers for the Raman stretching bands correspond to the
longer P-O bond lengths.5b,10 That coincided with the cell
volume determined by XRD analysis, as shown in Table 2.
Therefore, the result of Raman spectra demonstrated that the
crystal structure of Cu2(OH)PO4 changed as the morphology
was varied by changing the morphology-controlling agents. The
catalytic activity in hydroxylation of phenol will also change
with the crystal structure.
Figure 4 shows the electron paramagnetic resonance (EPR)
spectroscopy of as-prepared Cu2(OH)PO4 crystals with different
morphologies. As displayed in Figure 4, the g value (ga = 2.08,
gb = 2.10, gc =2.13, gd = 2.06) of each sample is different. That
is caused by the asymmetric crystal structures, such as the
changes in the P-O bond lengths (shown in Figure 3). The
distinctions of signal intensity are attributed to the difference in
the long-range dipolar interactions between neighboring Cu(II)
sites within Cu2(OH)PO4 crystals. The above results also
indicate the variations of crystal structure and morphology.
As shown in Table 1, various morphologies of Cu2(OH)PO4
crystals have the small BET surface areas. Therefore, the BET
surface areas cannot determine the catalytic activity in hydrox-
ylation of phenol. It has been reported that the Cu2(OH)PO4
catalyst was very active for phenol hydroxylation by H2O2, and
hydroxyl radicals were major active intermediates, which could
To further understand the variations in crystal structure
with morphology, Raman spectra of various morphologies of
Cu2(OH)PO4 crystals were also obtained and are shown in
Figure 3. The v1 and v2 mode of the phosphate vibration were
Chem. Lett. 2013, 42, 772-774
© 2013 The Chemical Society of Japan