Photosynthesis and characterization of Prussian blue nanocubes on surfaces of TiO2 colloids

Article abstract of DOI:10.1063/1.2169909

Prussian blue (PB) nanocubes were synthesized on the surface of titania (Ti O2) colloids using two-step process with ultraviolet light illumination. The formation of PB nanocubes starts with its nucleation under strong ultraviolet light illumination and followed by a slow growth of the nuclei under low intensity natural light illumination. This kind of PB nanocube has a very low Curie temperature.

Full text of DOI:10.1063/1.2169909

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Photosynthesis and characterization of Prussian blue nanocubes on surfaces of Ti O 2
colloids
Yan-Yan Song, Ke Zhang, and Xing-Hua Xia
Citation: Applied Physics Letters 88, 053112 (2006); doi: 10.1063/1.2169909
View online: http://dx.doi.org/10.1063/1.2169909
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/88/5?ver=pdfcov
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APPLIED PHYSICS LETTERS 88, 053112 ͑2006͒
Photosynthesis and characterization of Prussian blue nanocubes on
surfaces of TiO2 colloids
Yan-Yan Song, Ke Zhang, and Xing-Hua Xiaa͒
Key Lab of Analytical Chemistry for Life Science, Department of Chemistry, Nanjing University, Nanjing
210093, China
͑Received 25 September 2005; accepted 8 January 2006; published online 31 January 2006͒
Prussian blue ͑PB͒ nanocubes were synthesized on the surface of titania ͑TiO2͒ colloids using
two-step process with ultraviolet light illumination. The formation of PB nanocubes starts with its
nucleation under strong ultraviolet light illumination and followed by a slow growth of the nuclei
under low intensity natural light illumination. This kind of PB nanocube has a very low Curie
temperature. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2169909͔
Prussian blue ͑PB͒, formula FeIII͓FeII͑CN͒ ͔ , and its re-
solution is weakly acidic, and the surface of TiO2 colloid is
positively charged. Therefore, ferricyanide anions adsorb on
surfaces of TiO2 colloids.17,18 With illumination, electron of
TiO2 is excited to the conduction band, and subsequently
driven to the surface of TiO2 colloid under interior field.
6 3
4
lated metal hexacyanates have aroused intensive interest be-
cause of their potential application as molecule-based mag-
nets. Many methods, including microemulsion,1,2 polymer
protection,3 biomolecular matrix as a chemically and spa-
tially confined environment,4 have been advocated for the
synthesis of PB and its analogs. Recently, deposition of fer-
romagnetic PB Langmuir films was reported.5–9 We prepared
compacted ultrathin PB films on electrode surfaces from an
acidic ferricyanide solution.10–12
Although the thermodynamic potential of Fe3+/2+ couple ͑
3−/4−
0.771 V versus NHE͒ is higher than that for Fe͑CN͒
6
couple ͑0.356 V versus NHE͒, the excited electrons on the
colloidal surface are preferential to reduce the adsorbed
3−
4−
Fe͑CN͒ anions to Fe͑CN͒ which in turn couple with fer-
6
6
ric ions in the solution leading to PB formation. The left
holes in the valence band of TiO2 also transfer to the surface
resulting in the oxidation of H2O to O2.17,18 Under illumina-
tion with UV light, PB nuclei are formed on the surface of
TiO2 colloids. Density of the PB nuclei is determined by the
light intensity, illumination time, and the nature of TiO2
phase.22–24 Obviously, the growth process of the nuclei is
important for the resulted PB structures. In our case, after the
formation of PB nuclei under UV light illumination, we used
natural light with much lower intensity to allow the slow
growth of the PB nuclei. This process resulted in the growth
of regular PB nanocrystals around the TiO2 colloids ͓Figs.
2͑a͒ and 2͑b͔͒. All nanocrystals have regular cubic shape.
High- resolution transmission electron microscopy ͑HR-
TEM͒ image shows that the PB crystal has perfect nanocube
form with a mean edge length of 40 nm. Electron diffraction
Titania ͑TiO2͒ is the most studied semiconductor photo-
catalyst for various applications due to its high photocata-
lytic efficiency, chemical inertness, surface amphotericity,
and nontoxicity. Photodeposition of metals such as Ag, Pt,
and Pd on titania has been realized.13–16 Reports on photo-
sysnthesis of materials other than metals are limited.17–19
Tada et al.17 and Uchicla18 reported the photodeposition of
PB film on TiO2 but not cubic crystals since they used con-
tinuous strong ultraviolet light illumination. The unique mag-
netic behavior of PB depends on the constituents and ratios
of the transition metal ions in the compound.19–21 Here, we
report on the photosynthesis of monodisperse PB nanocubes
on surfaces of TiO2 colloids by UV light irradiation of a
TiO2 suspension containing FeCl3 and K3͓Fe͑CN͒ ͔ for
6
nucleation followed by weak intensity UV light illumination
for crystal growth; thereby regular PB nanocubes are formed.
The unique properties of the well-crystallized PB nanocubes
were characterized.
For the photosynthesis of PB on TiO2 colloids, equimo-
lar amounts of aqueous FeCl3 and K3͓Fe͑CN͒ ͔ were mixed
6
in the presence of 200 nm TiO2 colloids ͓amorphous as in-
dicated by the x-ray diffraction ͑XRD͒ pattern shown in Fig.
1, 5 mg/ml͔. The suspension was purged with a stream of
nitrogen for 15 min and then irradiated for 0.5 h using an
indium lamp ͑250 W with the major emission wavelength at
ϳ435 nm͒ for PB nucleation and followed by crystal growth
under the sunlight illumination for 10 h at room temperature.
The suspension was then centrifuged, and washed with
deionized water three times to remove the supernatants.
Transmission electron microscopy images of the air-dried
sample revealed layers of regular PB nanocubes enwrapped
the TiO2 colloids ͓Figs. 1͑a͒–1͑c͔͒. In our experiment, the
a͒
Author to whom correspondence should be addressed; electronic mail:
FIG. 1. XRD patterns of PB nanocubes deposited on surfaces of TiO2
colloids.
xhxia@nju.edu.cn
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053112-2 Song, Zhang, and Xia
Appl. Phys. Lett. 88, 053112 ͑2006͒
FIG. 3. ͑Color online͒ Field-cooled magnetization at 10 G as a function of
temperature and hysteresis loop ͑inset͒ of the PB nanocubes deposited on
surfaces of TiO2 colloids.
The magnetic properties of the resulted nanocubes using
two-step photosynthesis method were investigated by a mag-
netic property measurement system ͑Quantum Design
MPMS-XL, USA͒. The magnetic response of the PB
nanocubes on surfaces of TiO2 colloids was measured as a
function of temperature and applied field. The field-cooled
͑FC͒ magnetization versus temperature of PB nanocubes at
H=10 G shows a break at TC=4.6 K ͓the crossing point of
Mfc and Mzfc, Fig. 3 ͑zfc=zero field cooled͔͒ that is lower
than in the bulk materials ͑5.6 K͒.27–29 For PB analogs, Curie
temperature TC is expressed as30
FIG. 2. TEM images of ͑a͒ and ͑b͒ TiO2 colloid enwrapped by Prussian blue
nanocubes, ͑c͒ HRTEM graph of Prussian blue nanocube, and ͑d͒ electron
diffraction patterns of a single PB nanocube.
͑ED͒ pattern ͓Fig. 2͑d͔͒ of this single-crystal PB reveals a
typical fcc structure of PB. The averaged d spacings mea-
sured from ED patterns from several measurements from dif-
ferent PB nanocubes ͑5.8 Å ͓200͔; 4.0 Å ͓220͔; 2.8 Å ͓400͔;
2.1 Å ͓420͔͒ are in agreement with the x-ray powder diffrac-
tion analysis. The TiO2 used in our experiment is amor-
phous, as indicated by the XRD pattern ͑Fig. 1͒. The nature
of TiO2 phase ͑amorphous, anatase, and rutile͒ will affect the
formation of small nuclei.23,24 As we know, TiO2 nanocrys-
tallites with different crystal structures have different elec-
tron diffusion coefficients. The electron diffusion coefficient
of the anatase TiO2 is one order of magnitude larger than
those of the rutile ones, and the electron diffusion coefficient
of amorphous ones is the smallest, which means that the
electron diffusion length of the amorphous TiO2 is the short-
est. Thus, the photogenerated electrons and holes will recom-
bine more quickly than the others, resulting in fewer electron
reaching the surface of TiO2 colloids. In the first step of our
method, compared with the anatase and rutile TiO2, fewer
PB nuclei will be formed on surfaces of amorphous TiO2
colloids. For the same reason, the crystal growth of PB will
certainly be affected in the second step. Therefore, we can
expect that different morphology of PB will be formed on
TiO2 with different phase.
X-ray powder diffraction analysis was obtained by dry-
ing the precipitate on an indium tin oxide surface in the air
͑Fig. 2͒. It shows peaks at 17.6° ͑200͒, 24.9° ͑220͒, 35.5°
͑400͒, 39.8° ͑420͒, which can be indexed as the PB cubic
space group Fm3m.25 Infrared spectrum ͑not shown͒ of the
sample exhibits a absorption band at 2088 cm−1 due to the
CN stretching in the Fe2+–CN–Fe3+ group of PB. The bands
at 3443 and 1634 cm−1 are due to the O–H stretching and
H–O–H bending modes, respectively, indicating the presence
of interstitial water in the compound.10–12,26
ͱ
2 ZijZji͉Jij͉
ͱ
Tc =
Si͑Si + 1͒Sj͑Sj + 1͒,
͑1͒
3kB
where i= j=Fe3+, Si=Sj =5/2, Zij or Ziij is the number of the
nearest-neighbor i͑j͒-site ions surrounding a j͑i͒-site ion. kB
is the Boltzmann constant, and J is the magnetic interaction
constant. As the lattice parameter of nanocrystals does not
change, we suppose that the magnetic interaction constant Jij
between the Fe3+ ions of PB nanocubes is almost the same as
that of bulk. Therefore, the decrease in the Curie temperature
TC of the PB nanocubes is perhaps due to the diminution of
the average number of nearest magnetic interaction neigh-
bors in PB nanocubes with perfect crystal structure. Besides,
defects could be present on the surface or internal of the
Prussian blue cubes, which may effect the TC. A clear hys-
teresis loop of the nanocubes is observed in the field-
dependent magnetization at T=1.8 K, with a coercive field
͑Hc͒ of 67 G ͑inset in Fig. 3͒. This behavior is similar to PB
bulk obtained from neutron scattering measurement.31 Our
results demonstrate that the PB nanocubes are magnetic mo-
lecular crystals.
In conclusion, we report on the photosynthesis of PB
nanocubes on surfaces of TiO2 colloids in an aqueous solu-
tion of ferric-ferricyanide solution at room temperature and
under ambient pressure, in which PB nuclei on TiO2 surfaces
were formed using strong UV light illumination followed by
a slow growth process using low intensity natural light illu-
mination. The resulted PB nanocubes are regular in size and
highly crystal with a fcc structure. The observed decrease in
the Curie temperature demonstrates the average number of
The fcc structure of PB allows three-dimensional long-
range superexchange interactions between the neighboring
Fe3+ ions ͑S=5/2͒ through the NC–Fe–CN linkages, leading
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to a ferromagnetic ordering property at low temperature.25–27
magnetic interaction neighbors is limited due to its perfected
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053112-3 Song, Zhang, and Xia
Appl. Phys. Lett. 88, 053112 ͑2006͒
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