Characterization of nano-InVO4 powders synthesized by the hydrothermal process on various In/V molar ratio and soaking conditions

Article abstract of DOI:10.1016/j.jallcom.2009.03.093

Various hydrothermal parameters such as the In/V ratio of starting materials, soaking time and additives were conducted to examine the resulting phase formation and particle morphology of InVO₄. The possible reaction during InVO₄ for

Full text of DOI:10.1016/j.jallcom.2009.03.093

Journal of Alloys and Compounds 481 (2009) 740–745
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Characterization of nano-InVO powders synthesized by the hydrothermal
4
process on various In/V molar ratio and soaking conditions
a
b
a
b,∗
June-Min Yao , Chung-Kang Lee , Shien-Jen Yang , Chii-Shyang Hwang
a
Department of Materials Science and Engineering, Far East University, Hsin-Shih, Tainan, 74448 Taiwan
Department of Materials Science and Engineering, National Cheng Kung University, Tainan, 70101 Taiwan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Various hydrothermal parameters such as the In/V ratio of starting materials, soaking time and addi-
tives were conducted to examine the resulting phase formation and particle morphology of InVO4.
The possible reaction during InVO4 formation was proposed. The powders were characterized by
X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy and BET
Received 12 December 2008
Received in revised form 10 March 2009
Accepted 14 March 2009
Available online 25 March 2009
(Brunauer–Emmett–Teller) surface area measurement. Optical properties were investigated by UV–vis
and Fourier transform infrared spectroscopy. The XRD patterns indicated a single InVO4 phase formed at
the starting In/V ratio of 1/2 and In(OH)3, Na2V6O16 coexisted with InVO4 when the In/V ratio was 1/1
and 1/3, respectively. For the vanadium-rich samples (In/V = 1/3), the second phase of Na2V6O16 increased
with the increasing soaking time and demonstrated significant absorption in UV–vis spectra than pure
InVO4. The size and shape of InVO4 particles could be tailored from rod-like into rounded square particles
by PVP additives, acting as structure-directing-agent (SDA), up to 0.3 wt%.
Keywords:
Characterization
Hydrothermal process
InVO4
Na2V6O16
©
2009 Elsevier B.V. All rights reserved.
1. Introduction
InVO4 can be synthesized by solid-state reactions from In O3
2
and V O5, which need high temperature, long reaction and usually
2
Orthorhombic InVO has received considerable attention due to
result in difficulty in controlling the composition, homogeneity and
size of the product. As a result, the InVO4 activity of water splitting
reaction is not active enough to produce H2 to a commercialized
level. The sol-gel process [12,13] and hydrothermal method [14]
4
its special photocatalytic and electrochemical properties, such as
evolved H2 from hydrolysis of water molecules as a clean energy
source [1,2], anode material of Li-ion rechargeable battery due
to its high intercalation capacity [3,4], counter electrode of elec-
trochromic device [5] and gas sensor of ethanol [6]. The complex
phase transformation and crystalline structure of InVO4 have been
reported by Touboul and co-workers [7–9] and Roncaglia et al. [10].
have been proposed to obtain nano-scaled InVO particle or porous
4
films. Zhang et al. reported the sol–gel process using an amorphous
complex precursor of precipitated In(OH)3 and V O5 dissolved
2
in diethylenetriaminepentaacetic (DTPA) solution and calcined to
◦
A high-temperature stable orthorhombic (Cmcm) InVO -III phase
above 600 C to form orthorhombic InVO4 with an average grain
4
◦
is formed when annealed at 800 C for 6 h by solid-state reaction
size of 80 nm [12]. Xiao et al. synthesized 20–40 nm InVO –III
4
method in accordance with the scheme below [8]:
particles from NaVO3 and InCl3 solution under hydrothermal con-
ditions at 150 C for 4 h [15]. Huang and Yu studied the influences
◦
◦
amorphous
◦
monoclinic
◦
◦
680 C
100
C
520
C
610
C
InVO4 · 2.4H2O−→ InVO4 −→ InVO4 − I−→InVO4 − II −→ InVO4 − III
(1)
of various experimental conditions on the hydrothermal process,
including reaction temperature, time and molar ratio of starting
materials (InCl /NaVO ) for the InVO synthesis [14]. They obtained
orthorhombic
Ye et al. have investigated the photocatalytic efficiency of InMO
4
3
3
4
5
+
5+
5+
series (M = V , Nb , Ta ) under visible light irradiation [11]. The
InVO4 depicts a much narrower band gap (1.9 eV) than those of
InTaO4 (2.6 eV) and InNbO4 (2.5 eV) due to their different electron
orbits of transition metal Ta (5d-metal), Nb (4d-metal) and V (3d-
metal) [11]. This fact suggests InVO4 is possibly more efficient at
photocatalytic production of H2 from water splitting under visible
the pure InVO phase with average crystallite size around 20–40 nm
4
◦
at 150–210 C for 4 h when the In/V molar ratio equals to 1/2. Chen
et al. reported there is an optimal pH value range (pH ≤ 7.0) for the
formation of pure InVO4 phase during the hydrothermal reaction
[6]. They also examined the shape modification of InVO4
particles
using the organic additives as a structure-directing agent (SDA),
like cetyltrimethyl-ammonium bromide (CTAB), sodium dodecyl
sulfate (SDS), polyvinyl pyrrolidone (PVP) and ethylene diamine
tetraacetic acid (EDTA) in the solution during the hydrothermal
process. Various morphologies such as rod-like, irregular parti-
cle and brick-like samples prepared from different SDA additives
light irradiation than InTaO4 and InNbO4.
∗ Corresponding author. Tel.: +886 6 2757575 62940; fax: +886 6 2346290.
E-mail address: cshwang@mail.ncku.edu.tw (C.-S. Hwang).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.03.093

J.-M. Yao et al. / Journal of Alloys and Compounds 481 (2009) 740–745
741
were shown in TEM imagines with particle sizes ranging from
00–750 nm [6].
1
The particle size, morphology, homogeneity and surface mod-
ification, could play an important role in the physical/chemical
property of photocatalyst powders, leading to variation in their
catalytic activity. Wang et al. proposed possible chemical reactions
involved during hydrothermal synthesis of CeVO4 to explain their
particle size dependence on pH value [16]. It is suggested similar
chemical reactions can also be adopted in the hydrothermal syn-
thesis of InVO4 as shown below [6,16].
In3 + 3OH− → In(OH)3
+
(2)
(3)
(4)
VO (OH)] → VO43− + H+
2−
[
3
3−
→ InVO + 3OH−
In(OH) + VO
3
4
4
However, there were so far only limited studies discussing the
influences of hydrothermal parameters on InVO4 properties, it
should to be further investigated to improve their performance like
catalytic activity. In this study, InVO powders were synthesized by
Fig. 1. XRD patterns of InVO4 synthesized by solid-state reaction of (a) Vss and
hydrothermal process of (b) V1–24 h (c) V2–24 h (d)V3–24 h.
4
the hydrothermal process with split conditions like In/V molar ratio,
soaking time and PVP additives in some samples to modify parti-
cle shape. Solid-state reaction was also conducted in this study for
comparison.
spectra from 800–350 nm wavelength deducting the background spectrum of total-
reflection material of BaSO4.
2. Experiment
3. Results and discussion
2.1. Powder synthesis
3.1. Synthesis and phase analysis
The InVO4 powders were firstly prepared by solid-state reaction using starting
powders of In2O3 and V2O5 with purity of 99.99%. The stoichiometric amounts of
◦
well-mixing powders were reacted in a platinum crucible in air at 800 C for 12 h.
Fig. 1 shows the XRD patterns of powders synthesized by solid-
InVO4 was also synthesized by the hydrothermal process from NH4VO3 (ammonium
monovanadate, Merck, purity of 99%) and InCl3 solution. The InCl3 was obtained from
In2O3 dissolved in 60 C HCl solution. The composition of starting solution of InCl3
◦
state reaction at 800 C for 12 h and by hydrothermal processes at
◦
2
00 C for 12–48 h. The XRD pattern shown in Fig. 1a, prepared by
◦
solid-state reaction denoted as Vss, is in good agreement with the
orthorhombic InVO4-III phase (JCPDS 48-0898). For hydrothermal
samples, V2–24 h (Fig. 1c) shows good agreement with Vss of the
and NH4VO3 was blended and controlled at a specific In/V molar ratio of 1, 2 and 3
respectively, denoted as V1, V2 and V3 samples from now on. The controlled In/V
solutions were well mixed and the starting pH value of solution was adjusted to
be stable at 3.0, 6.0 and 10.0 respectively, by adding the proper amount of sodium
hydroxide under continuous stirring for 30 min. However, both samples synthesized
from solution pH 3.0 or 10.0 displayed complicated combinations of vanadate com-
pounds in contrast to the one with a pH 6.0. So, all the samples in this study were
spreaded at a solution pH value of 6.0. Various amounts of PVP (Merck, Polyvidone
InVO -III phase; however, V1–24 h and V3–24 h (Fig. 1b, d) revealed
4
second phases of In(OH)3 and Na V O respectively co-existed
2
6
16
with InVO4 phase.
During the InVO4 hydrothermal reaction, the In(OH)3 particles
will first precipitate as described in Eq. (2) previously and act as
2
5, MW = 72,000, purity of 99%) of 0.1, 0.3 and 1.0 wt.% were added in V2–24 h solu-
tion. The feedstock was then transferred into a Teflon-lined stainless steel autoclave.
nucleation and growth sites. When the NH VO3 powders were
◦
4
The soaking time of hydrothermal treatment was split into 12, 24 or 48 h at 200 C.
incorporated into the solution and brought about the transforma-
After completing the hydrothermal process and cooling to room temperature, the
resulting mixture of precipitate and solution were centrifuged, washed and filtered
tion of VO33 into VO4 under a controlled pH value of solution,
−
3−
+
−
3−
with distilled water and ethanol several times to remove remained ions (Na , OH
,
In(OH)3 began to react with VO4
and yielded InVO4 particles.
−
+
3−
Cl , NH4 , VO3 ) until the filtrate was clear. The samples were then dried in air at
Since the pH value of the precursor solution in this study was
controlled at 6.0 by adjusting of NaOH addition, the molar ratio of
In/V became the dominant factor deciding what kind of resulting
phases will be obtained after the hydrothermal reaction. The In/V
molar ratio of 1/2 should be aimed at the starting composition to
synthesize the single phase of InVO4 by the hydrothermal process,
despite being initially twice the amount of vanadium than indium.
The excess vanadate complex was possibly removed from the solu-
tion during the centrifugation and filtration processes. Moreover,
the diffraction peaks of In(OH)3 and Na V O noted in Fig. 1b and
◦
1
05 C for 12 h.
2.2. Characteristics
The crystal structure and phase identification were analyzed by X-ray diffrac-
tion (XRD) using Cu-K␣ radiation (ꢀ = 1.5406 Å) and a Siemens D5000 diffractometer
equipped with a rotating anode operated at 40 kV and 40 mA. Signal counts was accu-
◦
◦
◦
◦
mulated every 0.04 (2ꢁ) at a scan speed of 2 /min over the range from 10 to 80 . The
average crystallite size of InVO4 (D112 ) was estimated from Debye–Scherrer equa-
◦
◦
tion by performing an extra slow scan of 0.15 /min ranging from 2ꢁ = 32.8–33.6 for
the strongest peak of (1 1 2) crystal plane. The quartz powders with grain sizes large
than 4 ␮m are used to correct the diffraction angle. The particle morphology and
chemical composition were obtained by field emission microscope (FESEM, Philips
XL-40FEG) and scanning electron microscope (SEM, JEOL JSM-5610LV) assisted by X-
ray energy dispersive spectrometer (EDS, Oxford 6589). The high resolution image
of the particle was carried out with the transmission electron microscope (TEM,
JEOL JEM-2010). The Fourier transform infrared (FT-IR) spectra were recorded on
a PerkinElmer spectrum one spectrophotometer. The powders were loaded at a
2
6
16
d of V1–24 h and V3–24 h respectively, suggest these second phases
result form extra indium ions forming In(OH) in V1–24 h, and extra
3
vanadium ions forming Na V O in V3–24 h. These phenomena
2
6
16
can be reasonably explained in view of LeChatelier’s principle as
the hydrothermal reaction proceeded following the Eqs. (2) and
(4).
−1
reflective stage, scanning from 400 to 4000 cm . The specific surface area of InVO4
powders was determined by BET (Brunauer–Emmett–Teller) measurement on nitro-
The crystallite sizes estimated by the Scherrer formula are listed
in Table 1. It is clear the Vss sample prepared by solid-state reaction
possessed a much larger crystallite size of about 150 nm than all the
other hydrothermally processed samples with crystallite sizes of
about 30–38 nm. The samples of In/V = 1/2 (V2-series) favor form-
◦
gen adsorption at 77 K (Micrometrics ASAP2010, USA) after being degassed at 120
C
for 12 h. The external surface area and micro-pore surface area can be calculated
from t-Plot calculations based on Halsey equation or Harkins and Jura equation [17].
The ultraviolet–visible absorption spectroscopy of InVO4 powder was performed
using a PerkinElmer Lambda 35 spectrometer with an integrating sphere accessory.
The sample powder of 0.2 g was loaded inside a circular quartz cell, and collected
ing the InVO -III phase with larger crystallite sizes than the samples
4

742
J.-M. Yao et al. / Journal of Alloys and Compounds 481 (2009) 740–745
Table 1
Crystallite sizes of InVO4 prepared by solid-state reaction and hydrothermal process
based on the XRD results of (1 1 2) peak.
Samples
Vss
2ꢁ(112)
ꢂ2ꢁ(112)
0
FWHM
0.114
0.259
0.248
0.235
0.237
0.286
0.287
0.291
Crystallite size (nm)
33.325
33.239
33.278
33.218
33.178
33.228
33.183
33.169
149.8
34.3
36.1
38.6
38.2
30.6
30.5
30.0
V1–24h
V2–12h
V2–24h
V2–48h
V3–12h
V3–24h
V3–48h
−0.086
−0.047
−0.107
−0.147
−0.097
−0.142
−0.156
of the V1- and V3-series. Because the second phases of In(OH)3
and Na V O existed respectively in V1- and V3-sample except
2
6
16
3+
3−
InVO phase, it suggested that the excess ions of In or VO4 may
4
produce the second phases and consequently lower the remain-
ing ion concentration of In3 or VO4 inside the solution used for
InVO4 synthesis, resulting in a smaller crystallite size of V1- and
V3-samples than the V2-sample.
+
3−
As shown in Fig. 2, the second phase of Na V O becomes
2
6
16
more apparent with the increased soaking time from 12 to 48 h
during the hydrothermal reaction in V3- series. The percentile
ratio of Na V O to InVO4 has been calculated from the normal-
Fig. 3. FT-IR spectra of (a) Vss (b) V1–24 h (c) V2–24 h (d) V3–24 h.
2
6
16
ized ratio of relative intensity corresponding to the (0 0 1) peak of
Na V O and (1 1 2) peak of InVO , that is, the secondary phase
InVO4 in the XRD patterns of V2-samples were recorded, because
the content of the second phase were too few to resolve.
The FT-IR spectroscopy is expected to provide more informa-
tion of InVO4 structural analysis. The vibrational characteristics of
2
6
16
4
(
Na V O )/primary phase InVO = I(001)/I(112). The calculated ratio
2 6 16 4
is 0.06, 0.48 and 0.68 for the soaking time of 12, 24 and 48 h, respec-
tively. This means the lengthened soaking time would raise the
amount of Na V O phase by migration of vanadium ions from
orthorhombic InVO samples performed by FT-IR spectra in the
4
2
6
16
−
1
−1
6
00–2000 cm range showed the main bands at 750–1000 cm
InVO4 into Na V O16 . So, the V3–48 h sample would be relatively
2
6
(Fig. 3). The crystal of InVO4 is composed of two kinds of poly-
more vanadium-deficient than the V3–12 h sample with the same
hedra, octahedron of InO6 and tetrahedron of VO . They possess
unit structure of InVO . As shown in Table 1, the negative displace-
4
4
two groups of different V–O distances, two short ones of 1.662 Å
and two long ones of 1.791 Å [7]. The InO6 octahedra are con-
nected to each other by sharing edges that form chains along the
[0 0 1] direction and are linked by the VO4 tetrahedra based on
ment shift of ꢂ2␪(112) toward the lower Bragg angle increased as the
soaking time was increased from 12 to 48 h of V3-samples, hint-
ing the larger d-spacing of V3–48 h than V3–12 h. Since the radius
5
+
3+
of V ion (0.36 Å) is smaller than In ion (0.79 Å), this may be
−
the report of Oshikiri et al. [18]. The absorption band at 755 cm
1
5
+
attributed to more V ions crowded in the unit structure of InVO4
−
1
than In3+ ions. When the soaking time increased, more V5+ ions
will migrate from InVO4 into Na V O16 , enhancing the formation
and 898 cm
are attributed to vibration of V–O–In, and that at
−
1
9
52 cm is because of the expansion and contraction vibration of
2
6
−
1
V–O [15]. The additional peak at 1150 cm of V1–24 h was identi-
fied as In(OH) compared with the spectra of pure In(OH) powders,
and the absorption band at 1390 cm of V3–24 h was supposed to
be attributed to Na V O16 , which was also noted in the XRD pattern
of the Na V O phase. This fact leads to larger d-spacing in InVO4
2
6
16
of V3–48 h than V3–12 h. Note a similar ꢂ2␪(112) shift was also seen
in the V2-sample with a smaller amount. But no other peaks except
3
3
−
1
2
6
of Fig. 2c.
3.2. Morphologies of InVO4
The SEM images of InVO4 powders synthesized by solid-state
reaction and hydrothermal process at various In/V ratios are shown
in Fig. 4. There are significant differences in the morphologies and
particle sizes among these InVO samples. The Vss powders (Fig. 3a)
4
with a primary particle size around 2–5 ␮m, prepared by solid-state
◦
reaction at 800 C for 12 h, agglomerated into larger particle sizes of
about 15 ␮m. The powders sintered each other resulting in very low
2
surface areas of 0.45 m /g (Table 2) and appeared to be highly crys-
tallized compared to other hydrothermally treated samples (shown
in XRD patterns of Fig. 1). There are some soft agglomerates com-
posed of many uniform nanorod crystals of hydrothermal powders
observed in the SEM photographs of Fig. 4b, e. Both In(OH)3 and
InVO4 phases are present in the V1–24 h sample with a similar
rod-like shape around 200 nm in length and 50 nm in diameter.
Yang et al. reported the nanorod bundles of In(OH) obtained
3
◦
from the hydrothermal process at 140 C, consisting of individual
parallel nanorods of In(OH)3 with diameters ranging from 30 to
◦
4
5 nm, transformed into In O3 after dehydration at 600 C with
Fig. 2. XRD patterns of V3-series with various soaking time (a) V3–12 h (b) V3–24 h
2
(
c) V3–48 h.
the sustained morphology of In(OH)3 [19]. Since In(OH)3 is firstly

J.-M. Yao et al. / Journal of Alloys and Compounds 481 (2009) 740–745
743
Fig. 4. SEM images of as-formed powders from solid-state reaction of (a)Vss (b)V1–24 h (c)V2–24 h (d)V2–48 h (e)V3–24 h (f) typical EDS spectrum of V2–24 h sample.
obtained from the mixing solution of InCl and NaOH, and then
on their crystallite size with the study by Huang et al. [14]. The
3
reacted with VO43− to form InVO4 precipitates as described in Eqs.
V3–24 h precipitates observed in Fig. 3e were composed of two
kinds of morphologies of sword-blade precipitates and nanorods.
These are supposed to be the Na V O and InVO particles respec-
(
2)–(4). It is recommended the InVO particles in the V1–24 h sam-
4
ple after hydrothermal treatment retain the original morphology
2
6
16
4
and size of In(OH) , and some residual In(OH)3 particles present
tively, inferred from the XRD analysis shown in Fig. 2b.
3
with nanorod bundles are noted in Fig. 4b. The typical hydrother-
mal InVO4 agglomerates with an average diameter of 30–50 nm
and no clear morphological differences were found between the
V2–24 h and V2–48 h samples (Fig. 4c, d). This is consistent with
the soaking time of hydrothermal process having little influence
The chemical composition of the as-synthesized hydrother-
mal powders were investigated by performing EDS analysis and
provided further evidences to support the XRD results (Fig. 1).
The EDS result of V2–24 h pointed out the In/V/O atomic ratio
as 16.88/16.51/66.60, which was very close to the stoichiometric
Table 2
Specific surface area and particle size estimation from BET analysis.
2
2
2
Samples
External surface area (m /g)
Micro-pore surface area (m /g)
BET surface area (m /g)
Estimated particle size (nm)
Vss
V2–24h
V2–24h–PVP0.1
V2–24h–PVP0.3
V2–24h–PVP1.0
0.45
17.88
18.11
24.27
20.52
0.42
0.84
2.43
1.84
0.36
0.87
18.72
20.54
26.11
20.88
3009.5
67.8
61.8
48.6
60.8

744
J.-M. Yao et al. / Journal of Alloys and Compounds 481 (2009) 740–745
Fig. 5. TEM images of V2–24 h samples (a) without PVP (b) 0.1wt% PVP (c) 0.3 wt% PVP (d) 1.0 wt% PVP.
ratio (1/1/4) of the pure InVO4 phase. However, the amount of
indium ions was greater than the vanadium ions in V1–24 h with the
In/V/O atomic ratio of 19.49/14.65/65.86, and less in V3–24 h with
the In/V/O atomic ratio of 14.70/18.07/67.23. The results could be
referred to the coexisting phases of In(OH)3 and Na V O accom-
panying InVO4 respectively in the V1–24 h and V3–24 h samples.
Representative TEM micrographs shown in Fig. 5a–d revealed
different particle sizes and morphologies corresponding to V2–24 h
and some other PVP-added samples. The V2–24 h without PVP
additives were composed mostly of nanorods with lengths of
area and BET surface area of synthesized powders are shown in
Table 2. The external surface area emerges from the exterior sur-
faces of InVO powders, and the micro-pore surface area originates
4
from the porous structure of the samples. The BET surface area
equals the summation of the external and micro-pore surface area.
The Vss sample prepared by solid-state reaction possessed a surface
2
6
16
2
area of 0.87 m /g. The hydrothermally synthesized sample V2–24 h
demonstrated a drastic increment in the external surface compared
to Vss sample because of a large reduction in particle size from the
␮m-level (Vss sample) to the nm-level (V2–24 h sample). Note that
the PVP-added samples display higher BET surface area because
of the reduced particle size derived from the structure-directing
effect of PVP agent. When the PVP amount was 0.3 wt.%, the pow-
ders revealed a higher surface area value than others. The estimated
particle size from the BET results were calculated following the for-
mula: D = 6×1000/(r×A), where D is the particle size estimated from
140–180 nm and diameters of 40–55 nm (Fig. 5a). The other PVP-
added samples depicted more inhomogeneous and irregular size
distribution and shape as shown in Fig. 5b,d. For samples with
0
.1 wt.% PVP additive, some small particles with original InVO4
nanorods were observed. When PVP additives rose to 0.3 wt.%,
all the particle morphologies changed into a square shape with
rounded corners (Fig. 5c). Many more disparities of particle shape in
the sample with 1.0 wt.% PVP additive were found (Fig. 5d). The XRD
patterns (not shown here) of sample with 1.0 wt.% PVP revealed the
3
BET measurement; r is powder density (4.73 g/cm ); A is the specific
BET surface area; the shape factor is taken as 6 in the assumption
of spherical particle. The particle sizes of 48–68 nm are consistent
with the TEM micrographs (Fig. 5).
coexistence of InVO , V O5 and In(OH) phases, which may explain
4
3
3
the disparities of particle shape noted in Fig. 5d. The TEM and XRD
results suggested that PVP could act as a shape director to modify
the InVO4 particle morphology and the favorable amount of PVP
additive is about 0.3 wt.%.
The study of the specific surface area of InVO4 powders mea-
sured from BET analysis, external surface area, micropore surface
3.3. Photophysical properties of InVO4
Diffuse reflectance spectroscopy (DRS) is often used to char-
acterize the electronic state of semiconductor. Fig. 6 shows the
absorbance spectra converted from the DRS spectra of synthesized

J.-M. Yao et al. / Journal of Alloys and Compounds 481 (2009) 740–745
745
4. Conclusions
InVO4 powders were prepared by the hydrothermal and solid-
state reaction process. The characteristics of synthesized powders
highly depended on hydrothermal parameters, like modulation of
the starting composition (molar ratio of In/V), pH value, soaking
time and additive of polyvinyl pyrrolidone (PVP). It is preferred
to form a single phase of InVO4 at In/V ratio of 1/2 and solution
pH value of 6.0. The as-formed hydrothermal InVO4 particles pre-
sented a rod-like shape with diameters of 30–50 nm and lengths of
100–200 nm. The additive of PVP of 0.3 wt% can modify their shapes
into rounded square particles. The second phases of In(OH)3 and
Na V O existed in samples of In/V ratio as 1/1 and 1/3 respec-
2
6
16
tively, and demonstrated more absorption in the UV–vis spectra
than pure InVO . FT-IR spectroscopy confirms the vibrational band
4
behavior for InVO4.
Acknowledgments
The authors would like to thank the National Science Coun-
cil of the Republic of China, Taiwan for financially supporting this
research under Contract No. NSC94-2216-E-006-021 and NSC93-
2
622-E-269-028-CC3.
Fig. 6. UV–vis spectra of (a) anatase TiO2 and InVO4 samples of (b) Vss (c) V1–24 h
d) V2–24 h (e) V2–48 h (f) V3–24 h.
(
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InVO4 powders. The UV-light responsive photocatalyst of anatase
titanium oxide, (TiO , Alfa, purity of 99.9%, 325 mesh), is also
2
included as a reference. This displays a significant absorbance
drop above 420 nm belonging to the wavelength of visible light.
All samples of InVO4 showed absorption bands in the visible light
region of the spectrum, which was the major characterization
of the orthorhombic InVO4 photocatalyst [2]. The spectrum of
highly crystallized InVO4 of the Vss sample prepared by solid-state
reaction showed obvious absorption in the visible light region from
a wavelength of 420 nm and declined gradually above 600 nm. In
contrast to this, the hydrothermally synthesized InVO4 of V2–24 h
and V2-48 h revealed higher absorption around the wavelength
of 600–800 nm than the Vss sample. V1–24 h powders possessed
a much lower absorption than the V2-sample. This is because the
In(OH)3 phase is included which impedes the absorption of InVO4.
Interestingly, the V3–24 h sample showed the highest absorption
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of visible light despite the coexistence of Na V O and InVO . We
2
6
16
4
thought the Na V O compound should be responsible for higher
2
6
16
absorption of visible light.