Journal of Nanoparticle Research (2005) 7: 691–705 Ó Springer 2005

DOI 10.1007/s11051-005-7520-8

Studies on the preparation of magnetic photocatalysts

S. Watson, J. Scott, D. Beydoun and R. Amal*

ARC Centre for functional Nanomaterials, The School of Chemical Engineering and Industrial Chemistry,
The University of New South Wales, Sydney 2052, Australia; *Author for correspondence (E-mail: r.amal@
unsw.edu.au )

Received 10 December 2004; accepted in revised form 13 May 2005

Key words: nanoparticles, photocatalysts, TiO2, magnetite, magnetic photocatalysts, preparation

Abstract

A crystalline titanium dioxide coating was deposited onto silica insulated magnetite particles to prepare a
stable magnetic photocatalyst. The direct deposition of crystalline titanium dioxide was conducted by aging
dispersions of insulated magnetite particles in a titanium sol–gel precursor mixture at 60–90°C. The coating
process was found to be influenced by pH, alkoxide precursor concentration, aging time and reaction
temperature. A mechanism for the formation of the titanium dioxide coating has been proposed. The
photocatalytic performance of the prepared particles was found to be related to the preparation conditions.

Introduction photocatalyst removal by the use of an external

magnet, simplifying the downstream recovery
Water purification by photocatalysis has gained stage. A stable magnetic photocatalyst is produced
significant attention over the past three decades by coating magnetite (Fe3O4) particles with an
(Hoffmann et al., 1995). The semiconductor, tita- insulating silicon dioxide (SiO2) layer to avoid
nium dioxide (TiO2), is the most commonly photodissolution of the iron oxide core (Beydoun
applied photocatalyst since it is inexpensive, et al., 2002) and then deposition of a photoactive
chemically stable and its photogenerated holes and titanium dioxide (TiO2) phase onto these insulated
electrons are highly oxidising and reducing, magnetite particles.
respectively (Tryk et al., 2000). In terms of reactor Methods available for particle coating include
design, slurry type reactors are more efficient than wet chemistry methods such as the sol–gel tech-
their immobilised counterparts. They are, how- nique (Ocana et al., 1994; Philipse et al., 1994),
ever, at an inherent disadvantage due to the diffi- dry methods such as the aerosol combustion
culties in removing the fine photocatalyst particles technique (Mayville et al., 1987; Wooldridge,
from the treated effluent. 1997) or chemical vapour deposition (Powell et al.,
Traditional methods for solid–liquid separation 1996; Choy, 2003). Various advantages and dis-
such as coagulation, flocculation and sedimenta- advantages are associated with each technique. In
tion are tedious and expensive to apply in a pho- the case of aerosol combustion, the produced
tocatalytic process due to the chemicals required materials are of high purity. However, because of
and the additional purification stage needed to the extremely high temperatures needed, it is dif-
wash the coagulant from the photocatalyst. One ficult to control the morphology of the coated
approach to overcome this has been to develop a particles. This results in mixed particle configura-
magnetic photocatalyst which allows for easy tions, ranging from uniform particles of each oxide
692

to one oxide being encapsulated by the other particles were prepared by the modified alkoxide
(Wooldridge, 1997). In the instance of chemical method as outlined by Gopal et al. (1997), with
vapour deposition, the powders produced are also modifications made for coating purposes. The
of good purity but the cost of production is high uniformity of the surface coating is considered to
due to the sophisticated reactor required for the depend on the rate of precipitation of the coating
process (Choy 2003). The sol–gel process is material as well as the surface charges of the two
advantageous in that it is capable of producing species undergoing heterocoagulation. Studies into
photocatalysts with excellent chemical homogene- the effects of preparation conditions including pH,
ity, high purity and more uniform phase distribu- precursor concentrations, reaction temperature
tion in multicomponent systems (Aruna et al., and aging time are presented. An investigation
1996). While a clear disadvantage of the sol–gel into the stability of the coated particles when
technique is the number of steps required to subject to attrition is also reported.
achieve high product purity, it remains the most
effective coating method as it allows close control
during early-stage processing (Hu et al., 2000), Experimental
therefore products (in the form of powders, films
or coatings) can be developed to fulfil specific Preparation of insulated magnetite (Fe3O4/SiO2)
demands.
For these reasons, a magnetic photocatalyst has Magnetite particles were prepared by ageing
been prepared using the conventional sol gel Fe(OH)2 gels at 90°C in the presence of KNO3 as
method, by the hydrolysation of alkoxide precur- described by Sugimoto and Matijevic (Sugimoto
sors to give separate silica and titania layers et al.,1980). Surface coating of the magnetite par-
(Beydoun et al., 2001). The original method ticles was carried out by the hydrolysis of an eth-
involved deposition of an amorphous titanium anolic-tetraethyl orthosilicate (TEOS) solution
dioxide coating onto the insulated magnetite under basic conditions. 0.2 g/l of magnetite parti-
through surface nucleation and growth. Whilst the cles were dispersed in a 0.51 M H2O, 0.79 M
amorphous coating has been found to be homo- NH4OH and 15 M ethanol solution. The required
geneous, it is a photocatalytically inactive form of amount of TEOS (0.0037 M) was added dropwise
titanium dioxide (Ohtani et al., 1997). Amorphous to the solution while being stirred by an overhead
titanium dioxide is converted to crystalline tita- mechanical stirrer (800 rpm) at room temperature.
nium dioxide by calcination at a high temperature. The mixture was aged for a minimum of 18 h. The
Heat treatment of TiO2 fine particles alters their particles were recovered by centrifugation and
characteristics including the number of surface Ti– washed three times with ethanol. The silica coated
OH groups (which are required for the photore- magnetite particles were dried in an oven at 60°C
action), the surface area and the crystalline phase for 24 h.
of TiO2 (Yasumori et al., 1994). Previous studies
identified heat treatment as a key step in deter- Preparation of crystalline titanium dioxide coated
mining the final properties of the prepared samples Fe3O4/SiO2 particles (FSTC) at low temperature
and to be detrimental to both their photoactivity
and magnetic properties (Beydoun et al., 2000). A Fe3O4/SiO2 suspension was prepared by dis-
An alternative preparation method was sought persing 0.25 g/l Fe3O4/SiO2 particles in 51.5 M
whereby the heat treatment step could be elimi- H2O. Solution pH was varied at the beginning of
nated. each experiment to be either 1.2, 3.2, 5.2 or 7.2.
Other than surface nucleation and growth, sur- The acidic conditions were obtained by adjustment
face coatings may also be formed by heterocoag- with 0.5 M HNO3. The mixture was immersed in
ulation of a preformed coating material with the either a 60°C or 90°C water bath with an overhead
core particles (Gherardi et al., 1986). In this study mechanical stirrer (500 rpm). A predetermined
an attempt was made to preform crystalline tita- amount of titanium isopropoxide (TISOP) dis-
nium dioxide and directly coat these particles onto persed in 2-propanol was added dropwise to the
insulated magnetite cores through heterocoagula- mixture. The TISOP concentration ranged from
tion. The preformed crystalline titanium dioxide 0.002 to 0.008 M with 2-propanol making up the
 693

final volume to 177 ml. The final mixture was Photoactivity testing
maintained at the required reaction temperature
for the duration of the aging period. Aging times A small, batch photocatalytic reaction system
ranged from 1 to 3 h. The particles (FSTC) were consisting of a spiral photoreactor illuminated by
recovered by centrifugation and washed twice with a near-UV illumination lamp (peak wavelength at
water and dried in an oven at 60°C for 48 h. 360 nm) and a conductivity meter for measuring
carbon dioxide generation was used to assess the
Preparation of amorphous titanium dioxide coated photocatalytic activity of the prepared particles. A
Fe3O4/SiO2 particles (FSTH450) followed by high detailed description of the procedure can be found
temperature heat treatment (conventional sol–gel in Matthews (Matthews, 1988). Catalyst loading
method) was 0.1 wt% and the system was equilibrated with
ambient conditions. The photocatalytic oxidation
Amorphous TiO2 was deposited onto Fe3O4/SiO2 rate of 100 lg of carbon, added as sucrose, in a
by the hydrolysis and condensation of titanium 30 ml suspension of the photocatalyst was used as
butoxide (TBOT). The final mixture consisted of the test reaction. By monitoring the rate of for-
0.25 g/l Fe3O4/SiO2, 0.4 M H2O and 0.005 M mation of carbon dioxide, the rate of sucrose
TBOT. A measured amount of silica coated mineralisation was determined.
magnetite particles (0.1 g) was dispersed in 25 ml
of ethanol in an ultrasonic bath for 30 min. This Attrition studies
suspension was transferred to a reaction vessel
containing the required amounts of water and Attrition tests were carried out by shearing a
ethanol. A predetermined amount of TBOT was photocatalyst suspension (0.2 wt%) in a beaker
dissolved in 40 ml of ethanol and then added to using an overhead mechanical stirrer. Shearing
the Fe3O4/SiO2/water/ethanol mixture. The reac- rates of 0, 200, 350 and 500 rpm were employed
tion was aged at room temperature for 3 h with for a duration of 1 h. The particles were allowed to
continuous stirring using an overhead stirrer. The settle over an external magnet and the supernatant
amorphous titania coated particles were recov- was analysed for turbidity using a light scattering
ered by centrifugation and washed three times instrument (TurbiScan MA 2000) (Mengual et al.,
with ethanol and twice with water. The particles 1999). Higher turbidity (lower transmittance)
were dried in an oven at 60°C for 2 days and then indicates the presence of TiO2 detached from the
calcined at 450°C for 3 h to transform the surface of the core particles and reflects a decrease
amorphous TiO2 coating to the photoactive in coating stability.
crystalline phase.

Particle characterisation Results and discussion

All samples were characterised using Transmission Aging solutions of titanium isopropoxide at 90°C
Electron Microscopy (TEM) (Phillips CM200 in the presence of seed particles (silica coated
electron microscope). X-ray Fluorescence analysis magnetite) under various conditions yielded dis-
(X-ray Spectrometer PW 2400) was used to persions with different properties. Depending on
determine the composition of the samples. X-ray the experimental conditions, three scenarios may
Diffraction analysis (Siemens D5000 diffractome- arise. Firstly, a dispersion of independent TiO2
ter) was used to determine the crystal phase. nanocrystals may form. When this occurs, a mixed
Crystallite size was calculated using the Scherrer system is obtained where the seed particles (Fe3O4/
equation (Moore et al., 1989). The effective SiO2) and the newly precipitated phase (TiO2)
diameter of the particles was obtained from Pho- coexist as separate phases (Gherardi et al., 1986).
ton Correlation Spectroscopy using the dynamic The second scenario is the crystalline TiO2 parti-
light scattering technique (Brookhaven Instru- cles may independently precipitate and interact to
ments). Zeta Potential values, as a function of pH, various degrees with the Fe3O4/SiO2, forming a
were obtained from electrophoretic mobility mea- coating around the insulated magnetite particles
surements (ZetaPlus Brookhaven Instrument). (heterogeneous coagulation) (Kawahashi et al.,
694

Table 1. Preparation conditions and nature of the titania coated insulated magnetite system. Fe3O4/SiO2 concentra-
tion = 250 mg/l

pH TISOP (M) Reaction temperature (°C) Aging time (h) Nature of system

1.2 0.004 90 1 NH1

3.2 0.004 90 1 H2
5.2 0.004 90 1 HI3
7.2 0.004 90 1 NH
3.2 0.002 90 1 H
3.2 0.006 90 1 HI
3.2 0.008 90 1 HI
3.2 0.004 60 1 HI
3.2 0.004 90 2 H
3.2 0.004 90 3 H
1
NH = No heterocoagulation between insulated magnetite and precipitated titania, presence of independent titania and
insulated magnetite particles.
2
H = Heterocoagulation between insulated magnetite and precipitated titania, no independent titania.
3
HI = Heterocoagulation between insulated magnetite and precipitated titania, presence of independent titania

1990). Lastly, the reactants may adsorb onto the particles can be observed from the TEM images
surface of the seed particles and the TiO2 crystals presented in Figure 2. The TEM images show
may directly precipitate on the seed particles magnetite platelets encapsulated by a silica dioxide
through surface nucleation and growth (Philipse layer for all samples. Crystalline TiO2 was also
et al., 1994). While the distinction may not always observed to deposit on the insulated magnetite
be clear, the results are presented in terms of these particles at pH 3.2 and 5.2.
three scenarios. A summary of the preparation At pH 1.2, both the Fe3O4/SiO2 and formed
conditions and the nature of the systems obtained titanium dioxide particles are positively charged
are provided in Table 1. The influence of different resulting in an electrostatic repulsion between these
experimental conditions on the mechanisms of two particle types. This repulsion effect forces
particle formation is discussed later in detail. precipitation of the crystalline titanium dioxide
away from the surface of the silica coated magnetite
Effect of pH particles, resulting in the formation of independent
titanium dioxide particles. This effect has also been
It is expected that pH influences both the precipi- reported by Gherardi and Matijevic (1986) when
tation rate of titanium dioxide and the surface studying the interactions of precipitated haematite
charge of the two species that undergo heteroge- with titania. Figure 2a showed no coating follow-
neous coagulation. The surface charges of the ing supernatant removal, whilst a mixed system of
Fe3O4/SiO2 and titanium dioxide particles at var-
ious pH values were determined by zeta potential
measurements and are presented in Figure 1. The 30
isoelectric points of the Fe3O4/SiO2 and the titania 20
particles were determined to be 2.25 and 5.25,
Zeta Potential (mV)

10 IEP of Fe3O4/SiO2 IEP of TiO2

respectively. These values are in agreement with
0
those reported in the literature for silica dioxide 1 2 3 4 5 6 7 8
(Iler, 1979) and titanium dioxide (Barringer et al., -10

1982; Roessler et al., 2002). In addition, since the -20

isoelectric point of the Fe3O4/SiO2 is close to that -30

of silica dioxide, it can be assumed the magnetite is -40
encapsulated by the silica dioxide coating. pH

Experiments examining the effect of pH on Insulated Magnetite Titanium Dioxide

particle coating were carried out at pH conditions Figure 1. Zeta Potential as a function of pH for the pre-
of 1.2, 3.2, 5.2 and 7.2. The morphology of the pared insulated magnetite (Fe3O4/SiO2) and TiO2 particles.
 695

Figure 2. TEM micrographs of TiO2 coated Fe3O4/SiO2 particles prepared by aging for 1 h at 90°C, using a TISOP con-
centration of 0.004 M and at a pH of (a) 1.2, (b) 1.2 before removal of supernatant, (c) 3.2, (d) 5.2, (e) 7.2 and (f) 7.2 before
removal of supernatant.
696

uncoated insulated magnetite aggregates and surface shell produced by the heterocoagulation
nanosized titanium dioxide particles was evident process is not as smooth as one formed by surface
before supernatant removal (Figure 2b) nucleation (Ocana et al., 1991). In this work, the
The optimum surface coverage of TiO2 crystals roughness of the coating suggests that the dominant
on core particles was achieved at pH 3.2. As can be coating mechanism is heterocoagulation. This is in
seen from Figure 1, at pH 3.2, the magnetic parti- agreement with the observations for yttrium coated
cles have a negative charge while the precipitated latex particles (Kawahashi et al. 1990), haematite
TiO2 particles have a positive charge. Hence, coat- coated titania (Gherardi et al., 1986); chromium
ing by heterocoagulation occurred. The morphol- coated haematite (Garg et al., 1988); yttrium
ogy of the particles (Figure 2c) also supports the coated haematite (Aiken et al., 1988); and manga-
postulation of interaction between the two particle nese coated haematite (Haq et al., 1997).
types. The titania nanocrystallites appear to have To further confirm heterocoagulation as the
adhered to the Fe3O4/SiO2 particles, producing a dominating mechanism for particle coating at
rough coating. It is known that the uniformity of a pH 3.2, the following experiments were performed.

Figure 3. TEM of TiO2 coated Fe3O4/SiO2 particles prepared by aging for 1 h at 90°C, pH 3.2 and TISOP concentrations of
(a) 0.002 M, (b) 0.004 M, (c) 0.006 M and (d) 0.008 M.
 697

Firstly, in the absence of seed particles, the concentration of the insulated magnetite particles
homogeneous precipitation of titanium dioxide and pH were maintained at 0.25 g/l and 3.2,
was found to occur at pH 3.2. Additionally, when respectively. TEM images of the resulting particles
the insulated magnetite dispersion was mixed with are shown in Figures 3 a–d.
preformed TiO2 particles (at a pH of 3.2), similar TEM images of the effect of TISOP concentra-
morphologies were obtained as seen in Figure 2c. tion on coating showed that at an initial concen-
The TEM images are not shown here. This reaf- tration of 0.002 M minimal TiO2 deposition was
firms the postulation that, in this system surface observed (Figure 3a). At a TISOP concentration
nucleation was not responsible for interactions of 0.004 M, good surface coverage of TiO2 was
between the TiO2 and Fe3O4/SiO2, since hetero- seen (Figure 3b). At both these concentrations, no
coagulation was established upon the mixing of independent TiO2 particles were present as suffi-
preformed dispersions of both particles. cient negatively charged Fe3O4/SiO2 sites were
At a pH of 5.2, deposition of titanium dioxide available for heterocoagulation to occur. Further
on the insulated magnetite was also observed but increases in TISOP concentration did not have a
the coating was not as thick as that obtained at significant effect on the coating thickness (Fig-
pH 3.2 (Figure 2d). At this pH, the Fe3O4/SiO2 ures 3c and d), but rather on the presence of
particles are negatively charged whilst the titanium independent titania particles. As the TISOP con-
dioxide particles have a close to neutral charge. centration was increased, the yield of TiO2 crystals
According to the zeta potential measurements also increased but the number of Fe3O4/SiO2 sites
(Figure 1), the difference in magnitude of the sur- remained the same.
face charges is reduced. It is known that if the The results indicate a certain ratio of TISOP
potential on two kinds of solids is of the same sign concentration to Fe3O4/SiO2 core particles must
(in this case, one of the solids is close to zero) but be maintained to produce coated particles in the
differs in magnitude, heterocoagulation can still absence of free TiO2 particles. These findings are
occur (Barouch et al., 1985). The result is a in agreement with others, where the nature of the
decrease in attraction between the two solids as final dispersion strongly depends on the extent of
shown by the thinner TiO2 coating and an increase the surface area available for coating relative to
in free titania in the system. the amount of material to be deposited on that
At a pH of 7.2, the precipitated titanium dioxide surface (Aiken et al., 1988). Examples are yttrium
particles have the same negative charge as the core coated haematite systems (Garg et al., 1988) and
insulated magnetite particles, thus repulsive elec- titania coated zinc oxide systems (Ocana et al.,
trostatic forces will exist between them. From 1991).
Figure 2e, Fe3O4/SiO2 particles with a minimal In addition, in systems where coating occurred,
titanium dioxide coating can be observed. On the the coating comprised of multilayered titanium
other hand, Figure 2f clearly shows the presence of dioxide crystals (refer to TEM images). This effect
titanium dioxide particles in the supernatant. is due to the initial formation of anatase aggre-
The pH may also influence the kinetics of tita- gates prior to the heterocoagulation process. Once
nium dioxide precipitation. However, as indicated the exposed surface of the seed particles has been
by the presence of independent titanium dioxide covered by titanium dioxide aggregates through
crystals, the pH does not have a critical influence heterocoagulation, the resulting overall charge on
on the rate of titanium dioxide formation. In this the surface of the titania coated particles is
system, surface charge was found to control the expected to change. At this point further interac-
heterocoagulation process with an optimum coat- tion is prevented and the increased presence of
ing achieved at pH of 3.2 where the difference in independent titanium dioxide particles does not
the magnitude of the surface charges between increase the surface coverage of the Fe3O4/SiO2
titanium dioxide and Fe3O4/SiO2 is the largest. particles.

Effect of Precursor Concentration Effect of reaction temperature

Experiments were conducted at TISOP concen- The effect of coating the insulated magnetite at
trations of 0.002, 0.004, 0.006 and 0.008 M. The temperatures of 60°C and 90°C was considered.
698

The experiments were conducted at pH 3.2, the obtained (Figure 4 (b)). At 60°C, needle-like
TISOP concentration used was 0.004 M and aging shaped particles were observed (Figure 4d). The
was for 1 h. Figure 4 (a) shows the resultant tita- difference in the morphologies is attributed to the
nia coated Fe3O4/SiO2 particles prepared at a different phases of the formed TiO2, where needle-
temperature of 90°C, while Figure 4 (c) represents like shaped particles are characteristic of rutile
the preparation temperature of 60°C. The accom- TiO2 and small spherical crystals are characteristic
panying TEM images (Figure 4 (b) and (d)) are of of anatase TiO2 (Yang et al., 2002; Yin et al.,
TiO2 particles obtained under the same experi- 2002). This was confirmed by XRD as shown in
mental conditions but in the absence of Fe3O4/ Figure 5. The formation mechanisms of the single
SiO2 seed particles. These are referred to as phase titania have been described previously by
‘‘blank’’ tests. Watson et al. (2003). Interestingly, only small
During the blank test conducted at a reaction nanosized anatase crystals were deposited when
temperature of 90°C, small crystals of TiO2 were seed particles were present in the system both at

Figure 4. (a) TEM image of TiO2 coated Fe3O4/SiO2 particles by aging for 1 h at 90°C, pH 3.2 and TISOP concentration of
0.004 M. (b) TEM of the system as described in (a) but in the absence of Fe3O4/SiO2 cores. (c) TEM of TiO2 coated Fe3O4/
SiO2 particles by aging for 1 h at 60°C, pH 3.2 and TISOP concentration of 0.004 M. (d) TEM of the system as described in (c)
but in the absence of Fe3O4/SiO2 cores.
 699

90°C and 60°C as observed in Figures 4 (a) and (Zhang et al., 2002) as well as coating thickness
(c), respectively. Additional experiments were (Plaza et al., 1997). Experiments were conducted
undertaken to explore the deposition of rutile onto to investigate whether improvements to the TiO2
the Fe3O4/SiO2 by varying reactant concentration, coating were achievable with aging time. TEM
pH and aging times, however, in all instances images of particles prepared at aging times of 1, 2
anatase was formed. and 3 hours are presented in Figures 4a, 6a and b,
The following postulation is proposed to respectively. The experiments were conducted at
explain the precipitation of titanium dioxide in its pH 3.2, TISOP concentration of 0.004 M and a
anatase form (as opposed to rutile) when seed reaction temperature of 90°C.
particles are present. The presence of seed particles No significant differences in surface coverage
affects the reaction kinetics and induces secondary were observed between the samples aged at dif-
nucleation. When seed particles (the insulated ferent times with no further evidence of precipi-
magnetic core particles) exist in solution, an tation of free titanium dioxide particles.
alternative scenario is created for the precipitation
and formation of the new phase. At 90°C, higher Effect of heat treatment
collision energies enable the titania octahedra to
overcome electrostatic repulsion forces and form Heating of the titania coated insulated magnetite
the anatase structure (Gopal et al., 1997; Watson particles (FSTC) at 150°C, 250°C and 450°C was
et al., 2003). However at 60°C, it is postulated that studied with the aim of improving coating
the presence of seed particles affects the equilib- robustness and minimising attrition in the pho-
rium of the system in turn influencing the precip- toreactor. TEM images (Figure 7a–c) show that
itation process. The formation of anatase particles the morphology of the coated particles does not
at 60°C is due to the presence of Fe3O4/SiO2 change significantly with further heat treatment,
particles lowering the activation energy for sec- however an increase in crystal size with increased
ondary nucleation of a new phase (Qian et al., heat treatment occurred as shown in Table 3.
1997). It is well known that anatase has a lower These results are consistent with the literature
activation energy than rutile and therefore, is the (Tanaka et al., 1993; Yanagisawa et al., 1997;
more favourable phase to form (Ovenstone et al., Jung et al., 1999; Yu et al., 2001; Song et al.
1999). 2002; Chen et al., 2003). Yanagisawa et al.
A further observation from Figure 4 is that (1999) found in the temperature region of 250°C
TiO2 surface coverage was greater when the reac- onwards the ionic product of the crystallisation
tion was carried out at 90°C compared to 60°C. medium increased rapidly for titania with maxi-
This is expected as at a higher reaction tempera- mum restructuring and growth of the amorphous
ture, nucleation is favoured and thus, higher phase to anatase existing at this temperature and
yields. The higher titanium dioxide presence in the higher. In this study, the deposited FSTC crys-
sample coated at 90°C was confirmed by XRF tals were initially anatase and only a slight
(Table 2). increase in crystal size was observed. This indi-
cates the anatase titanium dioxide structure on
Effect of aging time the Fe3O4/SiO2 particles was stable in terms of
phase and crystal size for the temperatures tes-
Aging time has been found to influence phase ted. By comparison, particles coated by the
transition and nucleation of the final products conventional alkoxide method (FSTH) possess a

Table 2. Compositions of FSTC particles prepared at different reaction temperatures

Sample code Sample description Composition (mass%)

FSTC90 TiO2 coated Fe3O4/SiO2 prepared by heterocoagulation at reaction Fe3O4: 37; SiO2: 6; TiO2 57
temperature of 90°C.
FSTC60 TiO2 coated Fe3O4/SiO2 prepared by heterocoagulation at reaction Fe3O4: 46; SiO2: 7; TiO2 47
temperature of 60°C.
700

M
treatment temperature. This is postulated to be
A M M M
due to pore collapse from sintering. A number of
A MA A MA
(a) studies have reported similar findings with
A
decreases in porosity and densification during heat
treatment with particle size remained unchanged
Intensity

A A
(b) A A A A A
(Yanagisawa et al., 1997; Song et al., 2002; Zhang
M
et al., 2002).
A M M M
(c) A MA A MA

Attrition studies
R
R R
(d) R R R
Attrition results at different shearing rates and
20 30 40 50 60 following sonification are presented in Figure 8.
2 theta These studies were designed to examine the
robustness of the coating as the particles experi-
Figure 5. XRD pattern of (a) TiO2 coated Fe3O4/SiO2
particles at 90°C, (b) TiO2 particles produced at 90°C in the ence shear forces in the photoreactor.
absence of Fe3O4/SiO2 cores, (c) TiO2 coated Fe3O4/SiO2 The blank sample, containing ultrapure water
particles at 60°C and (d) TiO2 particles produced at 60°C in alone, indicated that approximately 10% of the
the absence of Fe3O4/SiO2 cores. A = characteristic ana- transmittance was lost as background. The FSTC
tase peaks, R = characteristic rutile peaks, M = charac- particles exhibited a supernatant transmittance of
teristic magnetite peaks. 90% at all shearing rates considered indicating no
detachment of TiO2. Under sonified conditions the
more uniform coating (Figure 7 (d)) than the TiO2 coating of the FSTH particles maintained a
FSTC particles and overall agglomerate size is high stability, while a drop in transmittance was
larger. observed for the FSTC samples, indicating TiO2
Heat treatment of the FSTC samples did not detachment.
have an effect on particle size as seen in Table 3. Figure 8 indicates that increases in the heat
The effect of sintering on particle size is minimal treatment temperature of FSTC lead to increases
due to the impact of the sintering process occur- in coating stability. The improvement is postulated
ring on a micro-scale whilst overall particle size to be due to consolidation of the TiO2 on the silica
effects are on a macro-scale. The specific surface surface during sintering of the TiO2 crystals.
area however decreased with an increase in heat Similar observations were made by Plaza et al.

Figure 6. TEM images of TiO2 coated Fe3O4/SiO2 particles prepared at 90°C, pH of 3.2, TISOP concentration of 0.004 M and
aged for (a) 2 h and (b) 3 h.
 701

Figure 7. TEM images of TiO2 coated Fe3O4/SiO2 particles prepared by aging for 1 h at 90°C, pH 3.2, TISOP concentration
of 0.004 M and heat treated for 3 h at (a) 150°C, (b) 250°C, (c) 450°C. (d) TEM image of TiO2 coated Fe3O4/SiO2 particles
prepared by conventional sol–gel method (FSTH450).

Table 3. Particle and TiO2 coating characteristics of magnetic photocatalysts prepared by the heterocoagulation and
conventional sol–gel procedures

Sample code Sample description Particle size Surface area Crystal size
(nm) (m2/g) (nm)

FSTC90* Dried at 60°C for 48 h. 560 143 6

FSTC90HT150* Dried at 60°C for 48 h, heat treated at 250°C for 3 h. 559 133 6
FSTC90HT250* Dried at 60°C for 48 h, heat treated at 150°C for 3 h. 540 127 7
FSTC90HT450* Dried at 60°C for 48 h, heat treated at 450°C for 3 h. 562 118 8
FSTH450 Dried at 60°C for 48 h, calcined at 450°C for 3 h. 750 46 12
*
FSTC particles were prepared under the conditions identified as optimum for the deposition of TiO2 onto Fe3O4/SiO2 via
heterocoagulation. Conditions were pH 3.2, TISOP concentration of 0.004 M, reaction temperature 90°C and 3 h aging.
702

100
90
80
Transmission (%)
70
60
50
40
30
20
10
0
m

m
er

0
u
m

45
90

50

50

50
rp

rp

rp
at

TH
TC

T1

T4
T2
w

00

50

00
Q

H
H
-2

-3

-5

FS

FS
M

90

90
90
90

90

90

TC
TC

TC
TC

TC

TC

FS

FS

FS
FS

FS

FS

Figure 8. Transmittance of the supernatant of prepared particles after 1 h shearing or sonification. )200, )350 and )500 rpm
indicate shearing speeds and u indicates ultra-sonification.

(1997) after heat treatment of yttrium coated Photoactivity tests

haematite particles.
The high stability of the FSTH450 sample is The photocatalytic performance of the particles
believed to be a combination of the coating was compared on the basis of the rate of carbon
mechanism and the calcination stage associated dioxide generation during the oxidation of 100 lg
with the conventional sol–gel preparation method. of carbon, added as sucrose. A concentration of
Under the reaction conditions of this procedure a 100 lg of carbon was used as it represents condi-
TiO2 coating is formed on the surface of the insu- tions close to surface saturation of the catalyst by
lated magnetic core by heterogeneous precipitation the substrate. The results are shown in Figure 9.
(Ocana et al., 1991). This involves reactants Figure 9 shows the particles prepared by het-
adsorbing on the surface of the seed particles, fol- erocoagulation (FSTC90) exhibit a higher photo-
lowed by formation of the amorphous TiO2 phase. activity compared to the magnetic photocatalyst
This differs from the FSTC samples whereby TiO2 prepared conventionally (FSTH450). It can also be
coating occurs via heterocoagulation. The heat seen that heat treatment of the FSTC particles
treatment step also contributes to strengthening the resulted in a significant drop in photoactivity
stability of the coating as a result of sintering of the between heat treatment temperatures of 150°C and
TiO2 crystals at high temperatures. It is also pos- 250°C. Heat treatment may alter particle structure
sible that interactions between the silica and the
titania may arise at 450°C, leading to a chemical 120

bonding between the coating and the insulation 100

Carbon Oxidised ( gC)

(Castillo et al., 1994; Muralidharan et al., 1997;

FSTC90
Viswanath et al., 1998). All these factors contribute 80

to the improved stability of the coated particles 60

FSTC90HT150

prepared at higher temperatures. FSTC90HT250

40
Following each experimental run in the photo- FSTC90HT450
reactor, the FSTC particles were collected and the 20
FSTH450
TiO2 coating stability analysed. In each instance,
0
90% transmission was obtained for the superna- 0 5 10 15 20 25
tant of all the samples, indicating the particle Time (min)
coatings were unaffected under the shear condi- Figure 9. Photoactivity test of prepared particles. Particle
tions inside the photoreactor. details are provided in Table 3.
 703

(phase transformation) or cause the desorption of a result of adsorbed organics (terminal titanium
species from the particle surface (Yasumori et al., alkoxide (–OC3H7)) that are incorporated into the
1994). TEM images (Figure 7) and Table 3 indi- titanium structure. These were found in the sam-
cated minimal changes in particle size, surface area ples prepared at 90°C and the heat treated at
or crystal size occurred. Therefore, the photoac- 150°C. No similar peaks were evident for the
tivity results cannot be completely explained in samples heat treated at the higher temperatures.
terms of changing morphology. The peak at 1630 cm)1 and the broad band at
Thermogravimetric analysis (TGA) of single- 3000–3600 cm)1 are due to O–H bonds of adsor-
phase TiO2 particles (TC90) produced at the same bed water molecules (Park et al., 1997; Deng et al.,
conditions as the FSTC particles (Figure 10) dis- 1998). From Figure 11, the intensity of these peaks
played evidence of weight loss between 150°C and decreased with increasing heat treatment temper-
280°C. The major cause of weight loss in this ature, indicating the removal of adsorbed water
region is the partial oxidation of residual organics and is consistent with literature reports (Bickley
(Ying et al., 1992; Tang et al., 2002; Chen et al., et al., 1991; Jung et al., 2000; Znaidi et al., 2001).
2003; Goutailler et al., 2003). The removal of alkyl Clearly, these results confirm the removal of the
groups at heat treatment temperatures of 250°C alkyl groups at temperatures greater than 250°C.
and 450°C is confirmed by the FTIR spectra in Additionally, irreversible dehydroxylation of the
Figure 11. It is postulated that partial oxidation of titania surface can occur at high temperatures,
the alkyl groups on the surface of the TiO2 crystals having an adverse effect on the photoreaction
at temperatures between 150°C to 250°C influences process (Matthews, 1993). Figure 11 also shows
the photocatalytic efficiency of the particles. Jung the removal of the O-H peaks at temperatures
and Park have reported that the removal of these greater than 250°C which may contribute to the
alkyl groups produces defect sites which are in fact decreased activity of the samples.
advantageous for the photoreaction process (Jung
et al., 1999). However, in this instance it is antic-
ipated that, along with removal of the alkyl Conclusion
groups, heat treatment leads to the destruction of
any potentially active defect sites which may also In this study, it has been shown that nanosized
have formed (Yu et al., 2001). crystalline titanium dioxide can be directly
In Figure 11, the bands at 400–1250 cm)1 are deposited onto insulated magnetite particles. The
characteristic of the formation of a Ti–O–Ti net- deposition mechanism was ascribed to heteroco-
work (Sivakumar et al., 2002; Uekawa et al., agulation between titanium dioxide crystals and
2002). The peak at approximately 1400 cm)1 cor- the silica dioxide layer. Deposition of the crystals
responds to the C–O stretch bond present in was found to be strongly dependent on parame-
residual organics (Zheng et al., 2000). This peak is ters such as pH and precursor concentration,

C-O
O-H

O-Ti-O O-H
(a) TC90
Derivative Weight (% / oC)

Transmittance %

(b) TC90HT150
150 - 280oC

TC90 (c) TC90HT250

(d) TC90HT450

0 500 1000 1500 2000 2500 3000 3500 4000 4500

0 100 200 300 400 500 600 Wavenumbers (cm-1)

Temperature (oC)
Figure 11. FTIR spectra of blank TiO2 particles prepared
Figure 10. Thermogravimetric analysis of blank TiO2 pre- by the same method as the FSTC particles with (a) no
pared at the same conditions as the FSTC90 particles. further heat treatment and at heat treatment temperatures
Derivative Weight (change in weight) with temperature. of (b) 150°C, (c) 250°C and (d) 450°C.
704

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