Shape effects of goethite particles on their photocatalytic activity in the decomposition of acetaldehyde

Article abstract of DOI:10.1039/c3cy00768e

The relationship between the photocatalytic activity of goethite (α-FeOOH) and the shape of its particles was investigated. Goethite particles with various morphologies (rod-like, array-like, and pancake-like) were synthesized through the hydrolysis of a ferric nitrate solution or the oxidation of Fe(OH)₂. All the particles synthesized consisted of {110} and {021} facets. The photocatalytic activities of the different samples were examined for the decomposition of acetaldehyde under irradiation. It was found that the photocatalytic activity of goethite is affected by both the shape and the specific surface area of its particles. Further investigation of the effects of the particle shape indicated that the photocatalytic activity of the {021} facets is higher than that of the {110} facets of goethite. The goethite particles (short rod-like and pancake-like particles), therefore, exhibited higher photocatalytic activities than other particle types (long rod-like and array-like particles) owing to the greater {021} facets.

Full text of DOI:10.1039/c3cy00768e

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PAPER
Shape effects of goethite particles on their
photocatalytic activity in the decomposition
of acetaldehyde
Cite this: Catal. Sci. Technol., 2014,
4, 164
Seiji Kakuta,* Taiki Numata and Toru Okayama
The relationship between the photocatalytic activity of goethite (α-FeOOH) and the shape of its particles
was investigated. Goethite particles with various morphologies (rod-like, array-like, and pancake-like)
2
were synthesized through the hydrolysis of a ferric nitrate solution or the oxidation of Fe(OH) . All the
particles synthesized consisted of {110} and {021} facets. The photocatalytic activities of the different
samples were examined for the decomposition of acetaldehyde under irradiation. It was found that the
photocatalytic activity of goethite is affected by both the shape and the specific surface area of its
particles. Further investigation of the effects of the particle shape indicated that the photocatalytic activity
of the {021} facets is higher than that of the {110} facets of goethite. The goethite particles (short rod-like
and pancake-like particles), therefore, exhibited higher photocatalytic activities than other particle types
(long rod-like and array-like particles) owing to the greater {021} facets.
Received 4th October 2013,
Accepted 26th October 2013
DOI: 10.1039/c3cy00768e
www.rsc.org/catalysis
activity than cubic ones, in which {100} facets are domi-
nant. The higher photocatalytic activity of the {111} facets
is attributed to the presence of dangling bonds from the
Introduction
1
6
Iron oxides and iron oxyhydroxides have been studied exten-
sively as photocatalysts for various types of reactions (e.g.,
the splitting of water and the decomposition of organic
Cu atoms. In the case of anatase TiO
2
, it has been reported
that the {001} and {100} facets exhibit high activity with
1
–10
17–19
pollutants)
because of their visible-light responsiveness,
respect to photocatalytic reactions.
that the {100} facets are dominant in TiO
development of highly active TiO2 as a photocatalyst.
Having determined
low cost, nontoxic nature, and natural abundance. Of the
various such compounds investigated, goethite (α-FeOOH)
is known to be a visible-light-responsive photocatalyst (band
2
has led to the
2
0
6,13
Goethite particles with various shapes have been synthesized;
11
gap: 2.0–2.5 eV). In contrast to other iron oxides and
oxyhydroxides, goethite can be synthesized easily under ambi-
ent conditions and, thus, has been studied for use in various
environmental applications, such as the photocatalytic decom-
however, the effects of the shape of the goethite particles
on their photocatalytic activity from a crystallographic point
of view still remain unknown. Knowing the relationship
between the particle shape of goethite and its photocatalytic
activity is important in order to be able to synthesize a more
active and efficient photocatalyst.
In the present study, goethite particles with diverse mor-
phologies (i.e., rod-like, pancake-like, and array-like particles)
were synthesized. The photocatalytic activities of these parti-
cles were evaluated in the oxidation of acetaldehyde under
irradiation, with the focus being on the morphological factors
affecting the photocatalytic activities of the goethite particles.
5–7,12–15
position of dyes, organic acids, and aldehydes.
There-
fore, the preparation of highly active goethite as photocatalyst
is an important goal in modern materials chemistry and green
chemistry, and much effort has been devoted to the synthesis
of goethite nanoparticles with high specific surface areas such
6
,7
that they exhibit high photocatalyst activity.
In the last decade, it has been recognized that the activity
of a photocatalyst depends not only on the specific surface
area of the photocatalyst particles but also on their shape.
This is because the anisotropy of the photocatalyst crystals
results in the photocatalytic activity being different on differ-
Experimental
ent faces of the crystals. For instance, octahedral Cu
2
O parti-
Preparation of goethite particles
cles, which consist of {111} facets, show higher photocatalytic
The goethite particles investigated in this study were prepared
21,22
through two methods: the oxidation of Fe(OH) (method A)
2
7,23
and the hydrolysis of a ferric nitrate solution (method B).
Method A. Goethite particles were obtained by the
New Energy Technology Section, Aomori Prefectural Industrial Technology
Research Center, 221-10, Yamaguchi, Nogi, Aomori, 030-0142, Japan.
E-mail: seiji_kakuta@aomori-itc.or.jp
3
−3
oxidation of Fe(OH) . 22 cm of a 10 mol dm NaOH solution
2
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3
−3
was dropped into 200 cm of a 40 mmol dm FeSO solution,
a cut-off filter (Suruga Seiki, S76-L42) was mounted in the
irradiation window so as to irradiate the samples with light of
wavelength λ > 420 nm. The light intensity was measured
using a power meter (Gentec, PSV-3103V2). The gaseous prod-
ucts were analyzed using a gas chromatograph with a thermal
conductivity detector (Shimadzu, GC-14B) and a packed
column (Restek, ShinCarbon ST). Ar was used as the carrier gas.
4
and the mixture was stirred in an Ar atmosphere. The mixture
was then stirred continuously for 12 h while air was bubbled
into it at a rate of ~100 ml min . A resulting brownish-yellow
precipitate was separated by centrifugation and washed
−
1
3
repeatedly (more than 5 times) with 500 cm of deionized
water. The precipitate thus obtained was dried in air at 80 °C.
This yielded a sample of goethite labeled A-1. Goethite parti-
cles of a different type were also synthesized using the same
method, except that the synthesis was performed using
00 cm of a 40 mmol dm FeSO solution that contained
vol% ethylene glycol. The thus-obtained photocatalyst is
Results and discussion
3
−3
2
2
The structural and optical characteristics of each sample were
identified from the corresponding XRD patterns, SEM images,
specific surface areas, and diffuse reflectance spectra.
4
6
labeled as A-2.
Method B. Goethite particles were also prepared by
The XRD patterns of all the samples (Fig. 1) could be
indexed to pure goethite (JCPDS 29-713) since no peaks char-
acteristic of any other Fe species were noticed. The intensities
of the peaks in the spectrum for A-1 were similar to those of
the corresponding peaks in the spectrum for A-2. Therefore,
A-1 and A-2 can be said to possess essentially the same crys-
tallinity. The same was true for B-1 and B-2 as well.
3
−3
hydrolyzing a ferric nitrate solution. 100 cm of a 100 mmol dm
Fe(NO ) solution (pH of 1.8) was heated to and kept at 80 °C
3
3
for 48 h. The obtained precipitate was washed with deionized
water in a centrifuge and dried at 80 °C. This sample of
3
goethite particles is labeled as B-1. In addition, 100 cm of a
−
3
1
00 mmol dm Fe(NO
3 3
) solution (pH of 12.8) was heated to
and kept at 100 °C for 24 h. The pH of the ferric nitrate
solution was adjusted by adding drops of a 10 mol dm
Fig. 2 shows the shapes of the different types of goethite
particles observed using scanning electron microscopy. The
−
3
NaOH solution. The obtained precipitate was also washed
with deionized water and dried at 80 °C. This sample is
labeled as B-2.
Characterization of goethite particles
The shapes of the different goethite samples were observed
using scanning electron microscopy (SEM) (Hitachi, SU6600).
The specific surface area of the samples was determined by
the single point Brunauer–Emmett–Teller method using a
Micromeritics FlowSorb II 2300, AutoMate 23 instrument.
Approximately 0.1–0.2 g of the samples was taken for mea-
surements. The measurements were performed using a
mixture of nitrogen and helium gases at a molar volume ratio
of 3 : 7. Prior to analysis, the samples were degassed at
1
30 °C for 4 h. Powder X-ray diffraction (XRD) patterns of the
samples were obtained using a Shimadzu XD-610 diffractom-
eter with CuKα radiation (0.154 nm). The diffuse reflectance
spectra of the samples were measured using an ultraviolet–
visible spectrophotometer (Jasco, V-570) equipped with an
integrating sphere (Jasco, ISN-470). The diffuse reflectance
data were converted into the corresponding Kubelka–Munk
functions. The size distributions of the different types of
particles in solution were measured using a dynamic light-
scattering particle-size analyzer (HORIBA, LB-550).
Photocatalytic experiments
The photocatalytic activity of each sample was investigated in
terms of the oxidation of acetaldehyde. The photocatalytic
3
experiments were performed in air in a 20 cm airtight glass
3
−3
vessel containing 10 cm of 50 mmol dm acetaldehyde (pH
of 4.8) and 10 mg of the photocatalyst (A-1, A-2, B-1, or B-2). A
1
50 W xenon lamp (Ushio, UXL-151D-0) with a light intensity
Fig. 1 XRD patterns of the A-1, A-2, B-1, and B-2 samples and of
goethite (JCPDS card no. 29-713).
−
2
of ~100 mW cm was used as the light source. Additionally,
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Fig. 2 SEM images of the different photocatalyst samples. a: A-1, b: A-2, c: B-1, and d: B-2.
particles of samples A-1 (Fig. 2a) and A-2 (Fig. 2b) were rod-like
in shape and had a length of 100–200 nm. Commonly, goethite
crystals are often acicular and elongated along the crystallo-
form an asterisk-like sheet. This sheet is stacked on top of
another such sheet parallel to the crystallographic c-axis direc-
tion, resulting in the pancake-like particles. Therefore, the
pancake-like particles essentially consisted of {110} and {021}
facets; that is, the sides of the particles consisted of {021}
facets, while the top and bottom were {110} facets. On the
other hand, the B-2 particles had an array-like structure
(Fig. 2d). Rod-shaped goethite particles (similar to both A-1
and A-2 particles) that are longer than the A-2 particles com-
bine along the crystallographic b-axis (minor axis) to form
7,24
graphic c-axis direction,
terminating in {110} facets on the
elongated faces, and are capped by {021} surfaces (Fig. 3a). In
case of both samples A-1 and A-2 (Fig. 2a and b), the shape of
the edge of the rod was in good agreement with the acicular
elongated shape of common goethite crystals. This indicates
that the rod-like particles in A-1 and A-2 consist of {110} and
{021} facets. The length of the A-2 particles was greater than
6
that of the particles of A-1. The width of the A-1 particles was,
however, almost the same as that of the A-2 particles. The dif-
ferences in the shapes of the two types of particles are
discussed in detail later. Particles of B-1 had a pancake-like
array-like structures (Fig. 3c). Hence, the array-like particles
also consisted of {110} and {021} facets. These results indicate
that goethite particles of a different shape were successfully
prepared by the above-mentioned methods and that these
particles were made up of {110} and {021} facets.
(
oblate) shape (Fig. 2c). Goethite with a particle structure sim-
2
5,26
ilar to that of B-1 has been reported previously.
As shown
The specific surface areas of the different types of particles
2
−1
2
−1
2 −1
in Fig. 3b, this structure consists of six rod-shaped goethite
particles (similar to both A-1 and A-2 particles) that intersect
at 60° in the horizontal plane, coupling over one another to
were as follows: ~73 m g for A-1, ~70 m g for A-2, ~59 m g
2
−1
for B-1, and ~48 m g for B-2. The diffuse reflectance spectra
of A-1, A-2, B-1, and B-2 are shown in Fig. 4. The absorption
edges of all the samples had a wavelength of ~550 nm. The
1
1
band gap of goethite has been reported to be 2.0–2.5 eV; the
results obtained in this study were in good agreement with
this value. In addition, it has been shown that a decrease in
the size of photocatalyst particles can lead to a blue shift of
the absorption edge of the photocatalyst owing to a quantum
confinement effect. Hence, the band gap energies as deter-
mined from Fig. 4 may also suggest that the intrinsic oxidative/
reductive powers of all samples were similar.
2
7
Fig.
3 Schematic illustrations of the different shapes of the
38
The photocatalytic activities of the different samples were
photocatalyst particles made using the software VESTA. a: Rod-like
particles (A-1 and A-2), b: pancake-like particles (B-1), and c: array-like
particles (B-2).
investigated with respect to the decomposition of acetalde-
7
hyde (CH
3
CHO + 5/2O
2
→ 2CO
2
+ 2H
2
O). Fig. 5 shows the
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CO produced exhibited a linear dependence on the irradia-
2
tion time; this was true for all the samples, indicating that all
the samples acted as stable photocatalysts. The rates of CO
2
formation for the different samples are listed in Table 1. The
sample with the highest activity was A-1. The rate of CO pro-
2
duction in the case of A-2 was almost the same as that in the
case of B-1. The sample with the lowest activity was B-2. How-
ever, the order in which different samples ranked in terms
of their photocatalytic activities was different from the order
of their specific surface areas. Table 1 also shows the rates of
CO formation per unit surface area for the different samples.
2
A-1 had a specific surface area similar to that of A-2; however,
the photocatalytic activity per unit surface area of A-1 was
higher than that of A-2. Similarly, the photocatalytic activity
per unit surface area of B-1 was higher than that of B-2. These
results indicated that the photocatalytic activity of the sam-
ples was affected by their particle shape as well as by their
specific surface area.
To elucidate the effects of the particle shape of the sam-
ples on their photocatalytic activity, the particle shapes of
the different samples were investigated in detail. First, the
shapes of A-1 and A-2, both of which were rod-like, were com-
pared. Fig. 6 shows the results of a statistical analysis of the
distribution of the dimensions of the rod-like particles of A-1
and A-2 based on SEM observations. The length and width
were 101 ± 34 and 30 ± 8 nm, respectively, in the case of sample
A-1 and 187 ± 63 and 29 ± 7 nm, respectively, for sample
A-2. The width of the A-1 particles was almost the same as
that of the A-2 particles; however, the length of the A-2 parti-
cles was almost twice that of the A-1 particles. Considering
that the crystals of both sample A-1 and sample A-2 consisted
of {110} and {021} facets (Fig. 2 and 3a), the shortening of the
particle length in the case of sample A-1 increased the relative
surface area of the {021} facets. That is, the relative surface
area of the {021} facets in A-1 was greater than that in A-2.
Next, to investigate the dispersion states of the different sam-
ples in the reaction solution, the distributions of the sizes of
the particles in the reaction solution were determined using a
dynamic light-scattering particle size analyzer. The aggrega-
tion of the particles of a photocatalyst in solution often causes
a decrease in its photocatalytic activity, since the surface area
Fig. 4 Diffuse reflectance spectra of A-1, A-2, B-1, and B-2 converted
into the corresponding Kubelka–Munk functions.
2
8,29
decreases owing to the aggregation.
Fig. 7 shows the parti-
−3
cle sizes of samples A-1 and A-2 dispersed in a 50 mmol dm
acetaldehyde solution after 5 h of irradiation. The median
diameters of the A-1 and A-2 particles were ~115 nm and ~179 nm,
respectively; these values were in good agreement with the
Fig. 5 Time courses of the amounts of CO
B-1, and B-2 as photocatalyst (the amount of photocatalyst used:
2
produced using A-1, A-2,
−3
1
0 mg; acetaldehyde concentration: 50 mmol dm (pH of 4.8); light
−2
intensity: ~100 mW cm ).
2 1 s
Table 1 Rates of CO production (k ) and rate constants (k ) normalized
to the Brunauer–Emmett–Teller (BET) specific surface areas (SSAs)
time courses of the amounts of CO
2
produced using the differ-
−1
a
−1
−2
Sample
k
1
(μmol h
)
k
s
(μmol h
m )
ent samples under irradiation. During photocatalysis, all sam-
ples oxidatively induced the formation of CO . However, CO
was not produced in the dark or under irradiation without a
photocatalyst present. Hence, it was verified that acetaldehyde
degrades through a photocatalytic process. The amount of
A-1
A-2
B-1
B-2
0.46
0.20
0.22
0.10
0.63
0.28
0.37
0.20
2
2
a
s 1
k = k /(the amount of photocatalyst used (i.e., 10 mg) × SSA).
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results reveal that the photocatalytic activity of the {021} facets
is higher than that of the {110} facets. In a catalytic reaction
involving photocatalyst particles, the carriers photogenerated
in the particles need to react efficiently with the reactants
adsorbed on the surfaces of the photocatalyst particles; how-
ever, electron–hole recombination takes place readily in the
particles because of the short carrier diffusion distance (i.e.,
1
,30
owing to the low mobility of the carriers).
Therefore, the sur-
face properties of the photocatalyst particles often affect their
3
1–34
photocatalytic activity.
In general, it is considered that the
surface hydroxyl group (M–OH, M = metal) plays an important
role in photocatalytic reactions. The surface hydroxyl groups
accept holes generated under irradiation to form hydroxyl
3
1–33,35
radicals and thus prevent electron–hole recombination.
Therefore, it is expected that the presence of a greater number
of hydroxyl groups results in a higher reaction rate. It has been
reported that, in goethite, the density of Fe–OH sites on the
2
{
021} facets (3.4 sites per nm ) is higher than that on the {110}
2
24,36
facets (2.8 sites per nm ).
Thus, the high photocatalytic
Fig. 6 Distributions of the sizes of A-1 and A-2 particles, as deter-
mined from SEM images. The number of particles measured per sam-
ple type was 100.
activity of the {021} facets seen in this work can be correlated
to the presence of a greater number of hydroxyl species on the
surfaces of the facets.
In the photocatalytic decomposition of organic compounds
by iron oxides and iron oxyhydroxides, molecular oxygen
7
works as an electron acceptor for the reduction reaction. It is
considered that the lattice surface of iron (≡Fe(III), ≡ denotes
1
2
the lattice surface of iron oxides) acts as a reduction site.
−
Two types of hole scavengers, i.e. OH and the organic com-
pound being decomposed, are considered to participate in
the oxidation reactions, with the Fe–OH site acting as an oxi-
1
2,37
dation site for both scavengers.
Therefore, it can be
deduced that {021} facets with a high density of Fe–OH sites
may have high reactivity for oxidation reactions.
The correlation of photocatalytic activities between {110}
facets and {021} facets also explains the difference in the
photocatalytic activities of B-1 and B-2 (Fig. 5 and Table 1).
As mentioned above, the particles of samples B-1 and B-2
have pancake-like and array-like structures, respectively. Since
the relative surface area of the {021} facets is greater in B-1,
the photocatalytic activity per unit surface area in the case of
B-1 is superior to that in the case of B-2 (Table 1). The photo-
catalytic activity of B-1 was similar to that of A-2, even though
the specific surface area of the B-1 particles was lower than
that of the A-2 particles. These results suggest that shape-
selective synthesis is important for producing highly active
goethite photocatalyst particles as well as for obtaining parti-
cles with a high specific surface area.
Fig. 7 Distributions of the sizes of A-1 and A-2 particles in a 50 mM
aqueous solution of acetaldehyde after 5 h of irradiation, as measured
by the dynamic light-scattering method.
particle sizes estimated from the SEM images (Fig. 5). This
result indicated that the particles of A-1 and A-2 are
monodispersed in the reactive solution and that the effects of
particle aggregation could be ignored for both samples. It also
suggested that the photocatalytic activities shown in Fig. 4 were
essentially affected only by the shapes of the particles. Consid-
ering that the photocatalytic activity per unit surface area in
the case of A-1 was superior to that for A-2 (Table 1), these
Conclusions
In this study, the photocatalytic activities of goethite particles
with different morphologies (rod-like, pancake-like, and
array-like) were examined through the decomposition of acet-
aldehyde under irradiation. The activity of each particle type
depended on its shape as well as on its specific surface area.
A detailed investigation of the particle shapes revealed that
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the {021} facets of goethite have a higher photocatalytic activ-
ity than do the {110} facets. The short rod-like and pancake-
like particles of goethite, owing to the high relative surface
area of {021} facets, exhibited higher photocatalytic activities
than did the long rod-like and array-like particles. These
results indicated that the shape-selective synthesis of goethite
that takes into account the high-activity facets will lead to a
more active photocatalyst.
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