Corundum type indium oxide nanostructures: Ambient pressure synthesis from InOOH, and optical and photocatalytic properties

Article abstract of DOI:10.1039/c6ra23693f

A simple, cost effective, surfactant free and scalable synthesis of rhombohedral In₂O₃ (rh-In₂O₃) nanostructures with controllable size and shape has been developed under ambient pressure by thermal dehydration of InOOH nanostructures. The InOOH nanostructures have been prepared by solvothermal reaction between indium nitrate hydrate with tetramethylammonium hydroxide (TMAH) in anhydrous methanol at 140 °C without any surfactant. The structure and morphology of the nanostructures have been characterized in detail by X-ray powder diffraction (XRD), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM). The studies reveal that highly crystalline nanostructures of InOOH and rh-In₂O₃ with rice-grain type morphology are formed. The optical properties of the InOOH and rh-In₂O₃ nanostructures have been explored by UV-visible diffuse reflectance spectroscopy (UV-DRS) and room-temperature photoluminescence (PL) studies. The direct band gap of as-synthesized InOOH and rh-In₂O₃ nanostructures was estimated to be 3.75 and 2.95 eV, respectively, from the diffuse reflectance absorbance spectra. Both InOOH and rh-In₂O₃ nanostructures show intense blue emission under UV excitation which is attributed to the presence of oxygen vacancies. The thermal stability of the rh-In₂O₃ phase has been studied by differential scanning calorimetry (DSC), differential thermal analysis (DTA) and dilatometry of the as prepared sample. The potential of InOOH and rh-In₂O₃ nanostructures as photocatalytic materials for hydrogen generation from water/methanol (2 : 1) mixtures under UV/vis irradiation has also been evaluated for the first time.

Full text of DOI:10.1039/c6ra23693f

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PAPER
Corundum type indium oxide nanostructures:
ambient pressure synthesis from InOOH, and
optical and photocatalytic properties†
a
Cite this: RSC Adv., 2016, 6, 108393
Anshu Singhal, D. Jain,a M. R. Pai,a S. Agouram,b D. Errandoneac and A. K. Tyagia
*
A simple, cost effective, surfactant free and scalable synthesis of rhombohedral In2O3 (rh-In2O3)
nanostructures with controllable size and shape has been developed under ambient pressure by thermal
dehydration of InOOH nanostructures. The InOOH nanostructures have been prepared by solvothermal
reaction between indium nitrate hydrate with tetramethylammonium hydroxide (TMAH) in anhydrous
methanol at 140 ꢀC without any surfactant. The structure and morphology of the nanostructures have
been characterized in detail by X-ray powder diffraction (XRD), Raman spectroscopy, and high-resolution
transmission electron microscopy (HRTEM). The studies reveal that highly crystalline nanostructures of
InOOH and rh-In2O3 with rice-grain type morphology are formed. The optical properties of the InOOH
and rh-In2O3 nanostructures have been explored by UV-visible diffuse reflectance spectroscopy (UV-
DRS) and room-temperature photoluminescence (PL) studies. The direct band gap of as-synthesized
InOOH and rh-In2O3 nanostructures was estimated to be 3.75 and 2.95 eV, respectively, from the diffuse
reflectance absorbance spectra. Both InOOH and rh-In2O3 nanostructures show intense blue emission
under UV excitation which is attributed to the presence of oxygen vacancies. The thermal stability of the
rh-In2O3 phase has been studied by differential scanning calorimetry (DSC), differential thermal analysis
(DTA) and dilatometry of the as prepared sample. The potential of InOOH and rh-In2O3 nanostructures
as photocatalytic materials for hydrogen generation from water/methanol (2 : 1) mixtures under UV/vis
irradiation has also been evaluated for the first time.
Received 23rd September 2016
Accepted 7th November 2016
DOI: 10.1039/c6ra23693f
www.rsc.org/advances
(Eg z 3.75 eV), electrical conductivity (native n-type semi-
conductor), and high charge-carrier mobility.7 Because of these
1. Introduction
characteristics, the nanostructures of In2O3 have been widely
investigated as optoelectronic devices, catalysts, sensors, and in
solar cells.7–13 In2O3 is a polymorphic material and most of the
literature studies are conducted on c-In2O3 which is the most
stable phase thermodynamically and can easily be obtained at
ambient pressure. It has a body-centered cubic bixbyite-type
crystal structure (bcc-In2O3, I3a, No. 206) and consists of two
types of In3+ ions (they are surrounded by oxygen in the octa-
hedral and trigonal prismatic coordination alternatively).14 The
other polymorph of In2O3 is metastable rh-In2O3 which is
usually described as a high temperature–high pressure phase
and transforms irreversibly into c-In2O3 upon heating.15 It has
Nanostructures of transparent conducting oxide (TCO) have
attracted extensive attention in fundamental as well as applied
research because of their transparency, range of crystal struc-
tures and technologically relevant properties.1–6 In2O3 is one of
the most widely studied TCO mainly because of its transparency
aChemistry Division, Bhabha Atomic Research Centre, HBNI, Trombay, Mumbai-400
085, India. E-mail: anshus@barc.gov.in; Fax: +91 22 2550 5151; Tel: +91 22 2559
2235
b
´
Departmento de Fısica Aplicada y Electromagnetism, Universidad de Valencia, Dr.
Moliner 50, Burjassot 46100, Spain
c
´
Departamento de Fısica Aplicada-ICMUV, Universidad de Valencia, Dr. Moliner 50,
ꢀ
Burjassot 46100, Spain
rhombohedral structure (rh-In2O3, R3c, No. 167) consisting of
hexagonal close packed oxygen ions with In3+ ions lling up
two-thirds of the six-coordinate C3v sites. The potential use of
In2O3 in the form of low dimensional nanostructures has trig-
gered interest in studying its metastable phases. Further,
metastable rh-In2O3 phase has several interesting physical
properties, such as more stable conductivity than its cubic
counterpart and structural similarity with metal doped oxides
like indium tin oxide (ITO) and it will be of great interest to
study it for fundamental as well as applied science.
† Electronic supplementary information (ESI) available: Diagram of reaction
assembly and the emission spectrum of lamp in UV-visible wavelengths used
for photocatalytic activity studies, XRD patterns of the reaction intermediates
isolated, characterization data for In(OH)3 and c-In2O3 nanocubes obtained by
the solvothermal reaction between In(NO3)3$2.55H2O and excess TMAH, XRD
patterns for the material obtained aer DSC experiment, DTA measurement
data for the rh-In2O3 sample, XRD patterns for the rh-In2O3 nanostructures
heated in the temperature range 650–800 ꢀC, characterization data for the
c-In2O3 nanoparticles obtained by thermal annealing of InOOH nanostructures,
XRD pattern of the used catalyst, pore size distribution data for InOOH and
rh-In2O3 nanostructures. See DOI: 10.1039/c6ra23693f
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The reversal of the phase stability in nanocrystalline mate- 2.55. Methanol (99.5%, S. D. Fine Chemicals, India) was dried
rials has been associated with large surface to volume ratios in and distilled over magnesium methoxide prior to use.
reduced dimensions. The high surface area of nanostructures
can lead to the spontaneous stabilization of metastable phases, 2.1 Synthesis of InOOH
owing to the surface energy and/or surface stress contributions,
In(NO3)3$2.55H2O (1.01 g, 2.91 mmol) was dissolved in 25 ml
both of which are dependent on size.16 In recent past there have
anhydrous methanol in beaker and 0.796 g (8.74 mmol) of
been several reports on the synthesis of rh-In2O3 nanostructures
under ambient pressure.17–27 The same rhombohedral structure
can also be synthesized by the application of combined high-
pressure and high-temperature, but this method is economi-
cally less efficient than ambient pressure chemical synthesis.28
TMAH solution was added drop wise to it over a period of een
minutes while sonicating the reaction mixture in an ultrasonic
bath. Aer the addition was complete, the contents of the
beaker were transferred to a Teon lined stainless steel auto-
ꢀ
clave. The autoclaveꢁw1as heated to a temperature of 140 C at
The stabilization of rh-In2O3 under atmospheric pressure has
been achieved by annealing of crystalline In(OH)3,18 or InOOH
nano structures17,19–25 which were mainly prepared by the
solution-based surfactant-assisted method. The literature
reports on synthesis of rh-In2O3 reveal that the solvothermal
method has been extensively used to prepare the intermediate
InOOH and rh-In2O3 under mild conditions. However, the sol-
vothermal methods employed use various organic/toxic solvents
such as glycerol,25a ethylene glycol,25b triethylene glycol,26
dimethylformamide,19a diethylenetriamine25c etc. and various
surfactants which might bring about great difficulties for the
large-scale production and environmental protection. The use
of surfactants to control the growth of nanoparticles may also
inuence the toxicity of the nanoparticles.29 Thus, the devel-
opment of simple, mild surfactant free and economical
synthetic pathways for the large-scale synthesis of well-
ꢀ
the rate of 3 C min and maintained at this temperature for
4 h. It was then cooled to room temperature naturally. The
resulting white product was retrieved by centrifugation and
washed with anhydrous methanol (3 ꢂ 5 ml) and then dried
under vacuum at room temperature for 1 hour to get InOOH
nanopowder.
2.2 Synthesis of rh-In2O3
InOOH nanopowder obtained by above mentioned procedure
was loaded in a quartz boat and heated in a tube furnace at
350 ꢀC (heating rate 3 ꢀC minꢁ1) for 4 h in air. Then it was
cooled to room temperature naturally. Thermal dehydration of
InOOH readily afforded rh-In2O3 nanostructures.
2.3 Characterization
controlled rh-In2O3 nanostructures under ambient conditions The crystal phases of the as-prepared products were character-
still remains a challenge.
ized using a Philips X'Pert pro powder X-ray diffractometer with
˚
In this study, we report a low temperature simple synthetic Cu-Ka radiation (l ¼ 1.5418 A) operating at 40 kV and 30 mA. A
approach for controlled synthesis of corundum type indium standard Si single crystal was used for instrumental correction
oxide (rh-In2O3) nanostructures by dehydration of InOOH and calibration. Morphological and structural analyses of the
nanostructures at ambient pressure at 350 ꢀC. Orthorhombic samples were carried out by TEM and HRTEM (FEI Field
InOOH nanostructures have been prepared through a simple Emission Gun (FEG) TECNAI G2 F20 S-TWIN high resolution
solvothermal reaction between indium nitrate hydrate and tet- transmission electron microscopes; accelerating voltage 200
ramethylammonium hydroxide at 140 ꢀC without using any kV). The TEM samples were prepared by dispersing the samples
surfactant. The prepared nanostructures have been character- in ethanol, and placing a drop of the dispersion on a Cu TEM
ized in detail for their structure and morphology and their grid covered with a holey carbon lm, which was then dried.
optical and photocatalytic properties for hydrogen generation The diffuse reectance UV-vis spectra were recorded with
via water splitting under UV-vis irradiation have been studied a JASCO spectrophotometer (model: V-530, Japan). BET surface
for the rst time. In recent years, semiconductor-based photo- areas were measured with a Thermoscientic Surfer instrument
catalytic solar-energy conversion by water splitting and CO2 with nitrogen as the adsorbing gas. The pore-size distribution
reduction has been considered very promising to address the and pore volume were calculated from desorption branch by
energy and environmental challenges.30 As wide band gap p- using the Barret–Joyner–Halenda (BJH) method. Raman spec-
block semiconductors, both InOOH31–34 and rh-In2O3 (ref. 27, tral studies were carried out using a micro-Raman spectrometer
35 and 36) nanostructures have been extensively used as pho- (model: STR-300, SEKI Technotron, Japan). The differential
tocatalysts for the degradation of organic pollutants, but to the thermal analysis was carried out using a Setaram Setsys ther-
best of our knowledge there is no report of these semi- mogravimetry analyzer under argon. DSC thermograms of
conductors being used as photocatalyst for hydrogen genera- nanopowders were recorded in a heat ux differential scanning
tion via water splitting.
calorimeter (model: DSC 823e, Mettler-Toledo GmBH, Switzer-
land). Samples (ꢃ20 mg) were taken in platinum pan with
covering lid and thermograms were recorded in the tempera-
ture range 0–675 C under owing argon atmosphere (ꢃ60 ml
2. Experimental details
ꢀ
Indium nitrate hydrate, In(NO3)3$xH2O (99.99%) and TMAH minꢁ1) with a heating rate of 10 ꢀC minꢁ1. Linear thermal
(25% solution in water) were procured from Aldrich. The exact dilation of as-prepared samples was measured in an integrated
water content in precursor indium nitrate was determined thermo mechanical analyzer (Model: Setsys TMA 1600, M/s.
using thermogravimetric studies and value of x was found to be Setaram Instrumentation, France). As-prepared powder
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samples were pressed in the form of green pellets (6 mm observed that if commercially available methanol (contains
diameter and 2–3 mm height) using a uniaxial hydraulic press 0.05% water) is used for the solvothermal reaction then
at a pressing load of 2.5 tonnes. These pellets were used for a mixture of InOOH and In(OH)3 is obtained which on
linear dilation measurement under owing argon atmosphere annealing gives a mixture of rh-In2O3 and bcc-In2O3. In some
(ꢃ60 ml minꢁ1) with near zero load condition (load ¼ 1 g) in cases direct preparation of rh-In2O3 nanopowders involve
temperature range 40–900 ꢀC. Uniform heating/cooling rate calcinations of In(OH)3 intermediates formed,18 but in our
(10 ꢀC minꢁ1) was used for all the experiments. Measurements study, we noted that direct thermal dehydration of In(OH)3 at
were made during both heating and cooling cycles. Sample ambient pressure produces only c-In2O3. Highly crystalline
dilation was evaluated from the measured dilation curves by In(OH)3 nanostructures were obtained by the solvothermal
subtracting the blank, which was run under identical experi- reaction between In(NO3)3$2.55H2O and excess TMAH (pH > 9)
mental conditions. All steady-state luminescence and lifetime at 140 ꢀC for 3 h and their thermal dehydration at 300 ꢀC for 2 h
measurements were carried out using an Edinburgh Instru- readily affords bcc-In2O3 nanostructures. The Bragg reections
ments' FLSP 920 system with a 450 W Xe lamp, nanosecond observed in the XRD pattern shown in Fig. S3a and S3b†
hydrogen ash lamp as the excitation sources. EPR spectra was conrm the formation of body-centred cubic In(OH)3 (JCPDS
recorded at 100 K using Bruker X-band spectrometer (EMX card no. 01-076-1463) and cubic bixbyite In2O3 (JCPDS no. 88-
series; EMM-1843) using
a standard rectangular cavity 2160, space group Ia3), respectively. No XRD peaks representing
ER4119HS operating at 9.60 GHz with a 100 kHz modulation other crystalline phases were detected, indicating that the nal
frequency. Diphenyl picrylhydrazyl (DPPH) radical was used as products exhibited excellent crystallinity and high purity. It is
reference for the calibration of g factor.
clear from the low resolution TEM micrographs shown in
Fig. S3c and S3d† that the materials presents cubic morphology
with edge of the cube varying between 50–80 nm for In(OH)3
nanocubes. It is clear from the above observations that the
water content in the reaction medium as well as pH of the
reaction medium inuences the phase and morphology of the
products obtained. Yan et al.37 have also observed similar
behavior while synthesizing InOOH via a solvothermal route
using indium nitrate and a binary solution of water and N,N-
dimethylformamide.
2.4 Photocatalytic activity under UV/vis irradiation
The photocatalytic activity of the InOOH and rh-In2O3 nano-
structures was evaluated at room temperature with water/
methanol (2 : 1 v/v, total volume 15 ml) mixtures in the pres-
ence of 0.1 g catalyst in a closed rectangular quartz cell (10 ꢂ 2.1
ꢂ 2.1 cm3) equipped with sampling and evacuation ports. The
reaction mixture was evacuated before irradiation to provide air
free conditions as oxygen acts as an electron scavenger and, on
photo-adsorption, blocks the active sites for the reaction. The
water/methanol mixtures with suspended photocatalyst were
then irradiated under a SAIC UV-visible lamp. The reaction
products were analyzed every 2 h by using a gas chromatograph
(Netel Michro-1100, India) equipped with a thermal conduc-
tivity detector (TCD); a molecular sieves column with argon as
the carrier was employed in the isothermal temperature mode
at 50 ꢀC oven temperature. The details of the reaction assembly
and emission spectrum of the UV-visible lamp are shown in
Fig. S1, ESI.†
3.2 Structure and morphology
3.2.1 InOOH. Fig. 1a shows the XRD pattern of the white
product prepared by the solvothermal reaction between
ꢀ
In(NO3)3$2.55H2O and stoichiometric TMAH at 140 C for 4 h.
The reection peaks can be indexed to orthorhombic InOOH
(JCPDS card 71-2283). The peaks are broad indicating the
nanocrystalline nature of the product. No characteristic peaks
due to In(OH)3 are seen in the XRD pattern indicating the high
purity of the product obtained. The major crystal planes are
assigned to the diffraction peaks. The measured lattice
˚
˚
parameters are found to be a ¼ 5.252 A, b ¼ 4.578 A and c ¼
3. Results and discussion
3.1 Synthesis
˚
3.254 A and compare well with the literature values (JCPDS 71-
˚
˚
˚
2283, a ¼ 5.260 A, b ¼ 4.560 A, c ¼ 3.270 A). The phase purity of
The solvothermal reaction between In(NO3)3$2.55H2O and InOOH samples prepared has also been examined with Raman
ꢀ
TMAH in anhydrous methanol (solution pH ꢃ 9) at 140 C for spectroscopy. InOOH has an orthorhombic crystal structure
4 h readily affords InOOH nanoparticles. To understand how with space group symmetry P21nm containing two formula units
the solvothermal reaction between In(NO3)3$2.55H2O and per unit cell. The Raman spectrum of InOOH nanostructures
TMAH unfolds, the reaction intermediates aer immediate (Fig. 1b) show bands at 178, 269, 324, 451, and 755 cmꢁ1
mixing of reactants, 1 and 2 h of the reaction were isolated. XRD consistent with literature reports.37,38
analyses of the solid intermediates obtained reveal that InOOH
Detailed information about the crystal structure and
is already formed aer 1 h of the solvothermal reaction. No morphology of InOOH nanostructures has been studied by
peaks due to In(OH)3 could be detected in the XRD patterns of TEM. Fig. 1c shows the low magnication image of the InOOH
the intermediates (Fig. S2, ESI†). Thermal dehydration of nanostructures. Due to lack of any stabilizing surfactants, the
ꢀ
InOOH nanoparticles at 350 C for 4 h readily affords rh-In2O3 particles are agglomerated but it is still possible to see some
nanostructures. It was noted that the use of fresh anhydrous elongated grains with length varying between 8–20 nm and
methanol is essential to obtain InOOH nanostructures. During width varying between 4–8 nm. The high resolution (HR) TEM
˚
the preparation of InOOH by solvothermal route, it was image of nanostructures (Fig. 1d) shows lattice spacing of 2.88 A
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Fig. 1 (a) XRD pattern, (b) room temperature Raman spectrum, (c) low resolution TEM image and (d) high resolution TEM image of InOOH
nanostructures. Inset in (d) shows fast Fourier transform of the HR image.
which corresponds to the (101) interplanar distance of ortho- where A1u, A2u, A2g, and Eu are infrared active or Raman inactive
rhombic InOOH. Fast Fourier transform of the HR image shown modes, and A1g and Eg are Raman active modes. The room
as inset in Fig. 1d clearly indicates the monocrystalline nature temperature Raman spectrum of rh-In2O3 exhibits peaks at 166,
of the selected grain.
224, 274, 312, 386, 501 and 592 cmꢁ1 in agreement with liter-
3.2.2 rh-In2O3. Fig. 2a shows the XRD patterns of the pale- ature reports17,23,28 The peaks at 166 and 501 cmꢁ1 are attributed
yellow product obtained by annealing the InOOH precursor at to A1g modes while the Raman peaks at 224, 274, 312, 386 and
350 C for 4 h. The reection peaks in the XRD pattern can be 592 cmꢁ1 are assigned to Eg modes. A weak and broad band
ꢀ
indexed to rh-In2O3 (JCPDS card 22-0336) and no other impu- observed at 450 cmꢁ1 may be an overtone of band at 224 cmꢁ1
.
rities peaks are observed, indicating that the precursor InOOH Interestingly, the weak and broad peak observed at 312 cmꢁ1 for
is completely transformed into rh-In2O3 under the annealing the rh-In2O3 nanostructures prepared by us and reported by
conditions. The miller indices are indicated on each diffraction other authors also17,23 is not observed for the bulk rh-In2O3
peak. The measured lattice parameters were found to be a ¼ b ¼ sample prepared by high pressure and high temperature
5.485 A and c ¼ 14.53 A, which are in good agreement with the route.28 The reasons for such discrepancy can be related with
˚
˚
˚
˚
reference (JCPDS card 22-0336, a ¼ b ¼ 5.487 A, c ¼ 14.510 A). the fact that nanoparticles prepared by chemical methods and
Room temperature Raman spectrum for the rh-In2O3 nano- samples prepared under extreme compression could be crys-
structures is shown in Fig. 2b. Rhombohedral In2O3 structure tallographically slightly different.
6
ꢀ
belongs to space group R3c, D3d. On the basis of the group
Fig. 3 shows TEM data for rh-In2O3 nanostructures. Fig. 3a
theory analysis, the optical modes of rh-In2O3 have the irre- shows low magnication image for the sample and it is clear
ducible representation, as shown below:39
from the image that the material presents rice grain type of
morphology. Both the length and width of the nanoparticles are
Gaussian distributed around 20 nm and 13 nm, respectively
Gopt ¼ 2A1g + 5Eg + 2A1u + 2A2u + 3A2g + 4Eu
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(expected atom%: In, 40; O, 60; observed: In, 44; O: 56% at)
suggesting that the nanostructures are composed of In2O3.
3.3 Thermal analysis and phase stability of rh-In2O3
nanostructures
As mentioned earlier, there are several literature reports avail-
able in recent times on ambient pressure synthesis of meta-
stable rh-In2O3, however, not many studies provide adequate
information on stability aspects of rh-In2O3. In this report, we
have investigated the stability of the rh-In2O3 phase using DSC,
DTA and dilatometry.
In the DSC thermogram one would expect an exothermic
peak if metastable rh-In2O3 transforms to more stable c-In2O3
phase. DSC thermogram of as-prepared rh-In2O3 sample along
with thermograms recorded on the same sample in repeated
thermal cycling is shown in Fig. 4. The DSC thermogram shows
two prominent endothermic peaks at 112.4 ꢀC and 383 ꢀC,
respectively, along with a broad exothermic peak at ꢃ570 ꢀC.
However, during repeated thermal cycling only an endotherm
around 90–100 ꢀC is observed. The sample weight measured
before and aer DSC run indicated ꢃ2.8 wt% loss. The phase
composition of the nal material obtained aer DSC experi-
ment was veried by XRPD studies which conrmed that rh-
In2O3 sample does not transform to cubic In2O3 (Fig. S4, ESI†).
In the light of above observations, the observed DSC results are
interpreted as follows: the endotherm at 112.4 ꢀC has been
assigned to the loss of adsorbed moisture on the surface of rh-
ꢀ
In2O3 nanostructures, the endotherm seen at 383 C is attrib-
uted to loss of residual surface functionalities (such as residual
–OH groups) present on the surface of rh-In2O3 nanostructures.
The presence of a broad exothermic peak at ꢃ570 ꢀC may be due
to heat of formation of water associated with the loss of surface
–OH groups. Thermal cycling on the same sample (without
exposure to ambient atmosphere) shows a weak endothermic
Fig. 2 (a) XRD pattern, and (b) room temperature Raman spectrum of
rh-In2O3 nanostructures.
ꢀ
peak between 100–110 C. While we are not able to assign the
(scale bar is 20 nm), with standard deviations 5 nm for the
length and 3 nm for the width. The minimum and maximum
values observed for the length (width) are 10 nm (6 nm) and
30 nm (20 nm). The selected area diffraction (SAD) pattern of
the nanopowder (Fig. 3b) features well-dened ring patterns
indicating the polycrystalline nature of the sample. The
measured interplanar distance (d-values) (from rst ring to
outer one for rst 8 rings) are 3.968, 2.895, 2.739, 2.392, 1.994,
exact reason for such an endotherm, one possibility is adsorp-
tion of moisture present in carrier gas (Ar) during DSC cooling
cycle. Similar low temperature endotherms are observed for
nanoparticles of different oxide materials studied in our lab
(unpublished results). DTA is another thermo analytic tech-
nique which is used to study phase transitions in materials.
Fig. S5† shows the data from DTA measurement on as prepared
rh-In2O3 sample. An exothermic peak is seen at ꢃ735 ꢀC clearly
indicating the phase transition from rhombohedral to cubic
phase of In2O3.
˚
1.826, 1.604 and 1.394 A, respectively, corresponding to the
planes (012), (104), (110), (113), (024), (116), (214) and (220),
consistent with the rhombohedral phase structure of In2O3. The
HRTEM image of the nanostructures (Fig. 3c) shows well-
dened lattice planes, indicating the single crystalline nature
3.3.1 Thermal expansion studies. The crystal structure of
two polymorphs of In2O3 suggests that the transition from rh-
In2O3 to c-In2O3 is likely to occur with volume expansion
˚
of the nanostructures. The lattice spacings of 2.74 and 2.86 A
(theoretical densities: rh-In2O3 ¼ 7.312 g cmꢁ3; c-In2O3
¼
observed in the HRTEM image correspond to (110) and (104)
interplanar distances of rh-In2O3, respectively. Fast Fourier
Transform (FFT) of the selected zone shown as inset in Fig. 3c
conrms the monocrystalline structure of the selected nano-
grain. Fig. 3d shows a localized EDX spectrum of an individual
In2O3 nanoparticle. The observed atom% values of In and O are
well in agreement with the expected values for rh-In2O3
7.123 g cmꢁ3). This behavior can be easily observed in linear/
volume expansion of the samples. Thus, linear thermal expan-
sion of as-prepared sample (powder compacted to give green
ꢀ
pellet) was measured in the temperature range 25–900 C and
Fig. 5 shows the dilation behavior recorded on the same sample
in two successive heating–cooling cycles. It is clear from the
Fig. 5a that the sample initially expands up to ꢃ540 ꢀC, and then
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Fig. 3 TEM data for rh-In2O3 nanostructures. (a) Low resolution TEM micrograph. (b) SAD pattern. (c) High resolution TEM micrograph showing
lattice fringes with FFT pattern shown as inset. (d) TEM-EDX spectrum.
Fig. 4 DSC thermograms recorded on as-prepared rh-In2O3 sample
and thermally cycled samples.
Fig. 5 (a) Linear thermal dilation profile of as prepared sample and (b)
subsequent thermal cycling on the same sample. Inset in 5a shows the
derivative plot of linear dilation profile recorded on as-prepared
sample. Onset of phase transformation around 675–700 ꢀC can be
ꢀ
shrinks up to 675 C. ꢀThere is an abrupt but nite expansion
clearly seen in the derivative plot.
between 675 and 700 C, and thereaerꢀthe material expands
nearly with the same rate as before 540 C. The nal material
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clearly reveal that rh-In2O3 phase is stable upto 700 ꢀC, at 750 ꢀC
both rh-In2O3 and c-In2O3 phases coexist and at 800 ꢀC, the
transformation from rh-In2O3 to c-In2O3 phase is complete.
Fig. S7, ESI† presents the detailed characterization data for the
In2O3 nanoparticles obtained by heating InOOH nanoparticles
ꢀ
at 800 C for 2 h. Nearly spherical and highly crystalline nano-
particles of c-In2O3 were formed.
3.4 Optical properties
The optical properties of InOOH and rh-In2O3 nanostructures
have been studied by UV-DRS and room temperature PL studies.
Fig. 6 shows the diffuse reectance UV-visible absorption
spectra and plots of [ahn]2 versus hn (as insets) for InOOH and
rh-In2O3 powder samples respectively. The Kubelka–Munk
method was used to convert the diffuse reectance to the
absorbance (Y-axis). The band gaps were determined by the
absorption edges extrapolated from the [ahn]2 and hn plots. The
estimated direct band gaps for the as-prepared InOOH and rh-
In2O3 nanostructures were 3.75, and 2.95 eV, respectively in
accord with literature data.31,32b
Photoluminescence (PL) is a very useful tool to determine the
structure, defect, and impurity in the nanocrystalline oxides.
Literature studies on PL of InOOH and rh-In2O3 nanostructures
reveal that PL of the nanostructures obtained by different
synthetic procedures varies greatly. The reported luminescence
peak positions range from 280 nm to 640 nm for InOOH21,40 and
from 350 to 650 nm for rh-In2O3 (ref. 17, 19b, 21, 23 and 41)
nanostructures depending upon the preparation method
employed. Xu et al.21 observed PL emission peaks in the range
280–580 nm in both InOOH quantum dots and nanowires. Tang
et al.40 studied the PL properties of InOOH nanowires synthe-
sized at different heating rates and observed PL emission peaks
at 456 and 496 nm for theꢁsa1mples synthesized at growth rate of
Fig. 6 UV-vis diffuse reflectance spectra for (a) InOOH and (b) rh-
In2O3 nanostructures. Insets show the plots of (ahn)2 vs. hn derived
from reflectance data showing band gap values.
ꢁ1
4 C min and 8 C min , respectively. Yu17 Dong,23 and Lee
et al.19b have observed PL peaks centered at 413 and 517 nm for
rh-In2O3 microspheres, 378, 398 and 420 nm from rh-In2O3
nanobres and 360, 400, 470 nm from rh-In2O3 nanocubes. The
rh-In2O3 hierarchical microcrystals reported by Jiang et al.41
show PL emission peaks at 413, 460, 494 and 519 nm. It is clear
from the above discussion that different preparation methods
used produce nanostructures with different dimensionality,
sizes, morphology, crystallinity and defect structure which
inuences the luminescence properties of the nanostructures
prepared. Near band edge emissions can be favored by the high
crystal quality and quantum connement effect21 and emissions
in the visible range are attributed to low crystallinity and
structural defects which are mainly oxygen vacancies.
ꢀ
ꢀ
obtained aer heating up to 900 ꢀC when subjected to a re-run
of TMA experiment, shows only expansion without any slope
variation (Fig. 5b). These results can be interpreted as follows:
ꢀ
initial expansion up to 540 C is indicative of normal physical
ꢀ
behavior of as-prepared material, aer 540 C, initial stage of
sintering commences along with the loss of residual surface
–OH groups and a net shrinkage is observed. The abrupt change
in the slope of the dilation curve at ꢃ700 ꢀC which indicates
a sudden expansion (clearly indicated in the derivative curve,
inset Fig. 5a) is quite interesting and suggests the onset of rh-
In2O3 / c-In2O3 phase transition. The phase composition of
the nal material obtained aer dilatometry experiments veri-
ed by PXRD studies reveals the presence of the cubic phase of
In2O3 in the nal product conrming the transformation of
metastable rhombohedral phase to stable cubic phase. To
corroborate this fact, X-ray powder diffraction patterns have
been recorded for the rh-In2O3 nanopowders aer annealing
them at different temperatures from 650–800 ꢀC in an electrical
furnace (1 h at each temperature with a heating rate of 3 ꢀC
minꢁ1). The results of the study are shown in Fig. S6, ESI,† and
The PL properties of InOOH, and rh-In2O3 nanostructures
prepared by us have been investigated at excitation wavelength
of 300 nm. The room temperature PL spectra of InOOH and rh-
In2O3 nanopowders are shown in Fig. 7. A very intense blue
emission peak is seen at 468 nm for InOOH nanostructures,
while two peaks at 451 and 469 nm are observed for rh-In2O3. As
discussed above, photoluminescence in nanostructures of
transparent conducting oxides is usually dominated by defect-
based emission42–44 with a small or negligible contribution
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EPR and optical spectroscopies hence allows one to get a more
complete insight into defect structures. The oxygen vacancies
can be present in three different charge states in the nano-
crystalline oxides: VO0 , VO+ , V+O+. The presence of singly ionized
oxygen vacancies can easily be observed by electron para-
magnetic resonance spectroscopy as they are paramagnetic.
Fig. 8 shows the results of EPR measurements carried out on
InOOH and rh-In2O3 samples. A distinct EPR signal with g ¼
2.002 appears for the InOOH nanostructures (Fig. 8a), while
a very broad and weak signal with g z 2.1 appears for rh-In2O3
(Fig. 8b). The data suggests that both the nanostructures
possess the same type of paramagnetic centers, i.e., singly
ionized oxygen vacancies. These are assumed to be the recom-
bination centers for the luminescence processes. The PL decay
curves for InOOH and rh-In2O3 nanostructures at emission
wavelength peak 468 nm (lexc ¼ 300 nm) are shown in Fig. 9.
Lifetime value is found to be 1.47 ns for InOOH and 1.14 ns for
rh-In2O3 nanostructures indicating faster electron–hole
recombination rate in the latter.
Fig.
7 Room temperature PL spectra for InOOH and rh-In2O3
nanostructures.
from band edge UV emission. InOOH and rh-In2O3 are both n-
type semiconductors because of presence of oxygen vacancies.
The oxygen vacancies act as the radiative center in the lumi-
nescence process by forming defect levels located inside the gap
and trapping electrons from the valence band.45 In this study,
we have prepared both InOOH and rh-In2O3 nanostructures
under mild conditions at low temperatures of 140 ꢀC and 350 ꢀC
respectively, thus, oxygen vacancies can easily be generated
during the synthetic process. In particular, solution based
synthetic methods lead to a high density of oxygen vacancies
because of incomplete crystallization. Thus, the PL bands of
InOOH and rh-In2O3 nanostructures observed in the blue
region of visible spectrum are mainly attributed to the effect of
oxygen vacancies or oxygen deciencies, which are inevitable
defects occurring during their synthetic procedure. Under the
excitation of 300 nm irradiation, the electrons are excited from
the valence band (VB) to the conduction band (CB). The elec-
trons move freely in the CB and nally relax to the oxygen
vacancies. The recombination of an electron occupying oxygen
vacancies with a photo-excited hole yields the blue emission at
468 nm and at 451 and 469 nm for InOOH and rh-In2O3
nanostructures respectively. Electron paramagnetic resonance
(EPR), is a powerful technique for characterizing the magnetic
properties of defect centers on an atomic scale. Correlation of
3.5 Photocatalytic activity
As p-block wide band gap semiconductors, both InOOH and rh-
In2O3 are attractive candidates for photocatalytic studies under
Fig. 9 Photoluminescence (PL) decay curves of InOOH and rh-In2O3
nanostructures at. emission wavelength of 468 nm.
Fig. 8 EPR spectra of InOOH and rh-In2O3 nanostructures at 100 K.
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generation via water splitting. The intrinsic stability of InOOH
nanostructures during the course of the photocatalytic experi-
ment was conrmed by XRD of the sample aer the photo-
catalytic reaction. The XRD pattern obtained for the spent
photocatalyst was the same as that of fresh sample (Fig. S8,
ESI†).
The photocatalytic activities of a semiconductor photo-
catalyst are governed by bulk (structure and electronic) and
surface properties. The surface properties, viz., N2 BET surface
areas, cumulative pore volumes and mean pore size diameter
for InOOH and rh-In2O3 nanostructures are listed in Table 1.
The pore size distribution and pore volumes are shown in
Fig. S9, ESI.†
It is clear from the data in Table 1 that, both surface area and
pore volume decrease when InOOH sample is thermally
annealed at 350 ꢀC to obtain rh-In2O3 nanopowders. Also, both
the materials show mesoporosity with mesoporous of varying
sizes but rh-In2O3 has larger pores in a wide range as compared
to InOOH nanopowders (Fig. S9, ESI†).
Fig. 10 Photocatalytic H2 yield (mmol) obtained from water/methanol
mixtures under UV visible light for InOOH and rh-In2O3
nanostructures.
The photoactivity of InOOH is attributed to its appropriate
electronic structure and its high surface area accompanied by
mesoporosity. In general, a higher surface area is benecial to
the reduced probability of electron–hole recombination and
promotes migration of photogenerated carriers. The more
dispersive conduction band in InOOH will also promote the
mobility of the photoexcited electrons and enhance the charge
separation contributing to its photocatalytic activity. We believe
that the difference in photoactivity of InOOH and rh-In2O3
nanostructures is most likely due to the difference in their bulk
and surface properties. InOOH has a larger band gap than rh-
In2O3. While this reduces the light that can be absorbed it may
raise the valence band maximum to higher energy levels relative
to redox potentials of adsorbed molecules. This increases the
oxidation power of electrons and facilitates electron transfer
from the InOOH to adsorbed molecules just like in case of
anatase TiO2.50 Surface properties of a semiconductor may play
a role in the adsorption of molecules and subsequent charge
transfer to the molecule. InOOH has larger surface area as
compared to rh-In2O3 nanostructures implying that it has more
surface defects as compared to the latter. Surface defects are
crucial for charge trapping and separation at the surface and
thus InOOH with more surface defects will trap the charge
carriers more efficiently as compared to rh-In2O3 leading to
better photocatalytic activity. The supporting evidence for this
comes from life time measurements carried out during PL
studies on InOOH and rh-In2O3 nanostructures. Lifetime value
is found to be 1.47 ns for InOOH and 1.14 ns for rh-In2O3
UV-vis irradiation. In case of InOOH the VB is composed of the
O 2p orbitals, while the CB is composed of the hybridizations of
the 5s and 5p orbitals of indium and is thus more dispersive
because of dispersive characteristics of s orbitals.46 Similarly, In
case of rh-In2O3 also the VB is composed mainly of oxygen 2p
like states and CB consists mainly of indium 5s-like states.47 The
more dispersive conduction band for both InOOH and rh-In2O3
will promote the mobility of the photoexcited electrons and
enhance the charge separation. Further, the edge of conduction
band (ECB) for InOOH is ꢁ0.4 V (vs. NHE), the edge of valence
band (EVB) is 3.3 V (vs. NHE),31,48 and the energy value of
calculated conduction band edge (ECB) for rh-In2O3 is ꢁ3.80 eV
(vs. absolute vacuum scale).49 Thus, photogenerated charge
carriers in both InOOH and rh-In2O3 nanostructures have
strong enough redox abilities. In view of these observations,
photocatalytic properties of InOOH and rh-In2O3 nano-
structures have been explored for hydrogen generation from
water/methanol (2 : 1) mixtures under UV/vis irradiation (16%
UV + 84% visible). Fig. 10 shows the results of H2 evolution with
InOOH and rh-In2O3 nanostructures without any cocatalyst. The
observed hydrogen yields are low. InOOH nanostructures show
H2 yield of 54 mmol gꢁ1 hꢁ1, while rh-In2O3 nanostructures
show negligible activity. The apparent quantum efficiency (AQE)
achieved using InOOH is only 0.1%. Thus, both InOOH and rh-
In2O3 nanostructures are not efficient photocatalysts for H2
Table 1 Particle size, N2-BET surface area and porosity of InOOH and rh-In2O3 nanostructures
Particle size
from TEM
Specic surface
BJH pore size
distribution
Pore volume
S. No.
1.
Sample
InOOH
rh-In2O3
area (m2 gꢁ1
)
(cm3 gꢁ1
)
Length: 8–20 nm
Width: 4–8 nm
Length: 20 nm
Width: 10–15 nm
53
43
Mesoporous in the
range 20–150 A
Mesoporous in the
range 40–300 A
0.175
˚
2.
0.133
˚
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nanostructures indicating faster electron–hole recombination
rate in the latter.
7 (a) O. Bierwagen, Semicond. Sci. Technol., 2015, 30, 024001;
(b) S. Y. Han, G. S. Herman and C. H. Chang, J. Am. Chem.
Soc., 2011, 133, 5166–5169.
8 G. Shen, B. Liang, X. Wang, H. Huang, D. Chen and
Z. L. Wang, ACS Nano, 2011, 5, 6148–6155.
4. Conclusions
In conclusion, we have successfully developed a low tempera-
ture simple solution synthetic route to synthesize nano-
9 V. P. Reddy, A. V. Kumar, K. Swapna and K. R. Rao, Org. Lett.,
2009, 11, 1697–1700.
structures of InOOH with controllable sizes and shape using 10 K. R. Reyes-Gil, E. A. Reyes-Garcia and D. Raery, J. Phys.
inexpensive starting materials. The synthetic method is scalable Chem. C, 2007, 111, 14579–14588.
and can be extended to get nanoparticles of In(OH)3 and c-In2O3 11 H. Yang, S. Wang and Y. Yang, CrystEngComm, 2012, 14,
with cube like morphology by just varying the pH of the reaction 1135–1142.
mixture. Elongated rh-In2O3 nanostructures can be easily ob- 12 E. Li, Z. Cheng, J. Xu, Q. Pan, W. Yu and Y. Chu, Cryst. Growth
tained by thermal annealing of InOOH nanostructures at Des., 2009, 9, 2146–2151.
350 ꢀC. XRD, HRTEM and Raman studies reveal that phase pure 13 R. Katoh, A. Furube, T. Yoshihara, K. Hara, G. Fujihashi,
materials are obtained. Thermal studies carried out on as
prepared rh-In2O3 samples reveal that on heating above 700 ꢀC,
S. Takano, S. Murata, H. Arakawa and M. Tachiya, J. Phys.
Chem. B, 2004, 108, 4818–4822.
metastable corundum type In2O3 transforms irreversibly into 14 S. Z. Karazhanov, P. Ravindran, P. Vajeeston, A. Ulyashin,
˚
cubic bixbyite type In2O3. The nanostructures exhibit intense
visible blue light emission which can be attributed to oxygen
T. G. Finstad and H. Fjellvag, Phys. Rev. B: Condens. Matter
Mater. Phys., 2007, 76, 075129.
vacancies present in the materials synthesized. As the structural 15 (a) C. T. Prewitt, R. D. Shannon, D. B. Rogers and
defects in TCOs are also responsible for electrical conductivity,
the ability to tune their optical properties by size controlled
synthesis of InOOH and rh-In2O3 nanostructures is promising
A. W. Sleight, Inorg. Chem., 1969, 8, 1985–1993; (b)
A. Gurlo, S. Lauterbach, G. Miehe, H. J. Kleebe and
R. Riedel, J. Phys. Chem. C, 2008, 112, 9209–9213.
for application in multifunctional optoelectronic materials and 16 H. Zhang and J. F. Baneld, J. Mater. Chem., 1998, 8, 2073–
devices. The potential of InOOH and rh-In2O3 nanostructures as 2076.
photocatalytic materials for hydrogen generation from water/ 17 D. Yu, S. H. Yu, S. Zhang, J. Zuo, D. Wang and Y. Qian, Adv.
methanol (2 : 1) mixtures under UV/vis irradiation has been Funct. Mater., 2003, 13, 497–501.
evaluated for the rst time. InOOH nanostructures show H2 18 (a) M. Epifani, P. Siciliano, A. Gurlo, N. Barsan and
yield of 54 mmol gꢁ1 hꢁ1, while rh-In2O3 nanostructures show
negligible activity. The difference in their photocatalytic
behavior is most likely due to their different bulk and surface
properties.
U. Weimar, J. Am. Chem. Soc., 2004, 126, 4078–4079; (b)
V. D. Ashok and S. K. De, J. Phys. Chem. C, 2011, 115,
9382–9392.
19 (a) C. Chen, D. Chen, X. Jiao and C. Wang, Chem. Commun.,
2006, 4632–4634; (b) C. H. Lee, M. T. Kim, A. Kim, J. Paek,
J. W. Lee, S.-Y. Choi, K. Kim, J.-B. Park and K. Lee, J. Am.
Chem. Soc., 2006, 128, 9326–9327.
Acknowledgements
The authors thank Dr A. Ballal, Molecular Biology Division, B. A. 20 (a) Z. Zhuang, Q. Peng, J. Liu, X. Wang and Y. Li, Inorg.
R. C. for carrying out low-resolution TEM studies on In(OH)3
and c-In2O3 samples. The authors also thank Dr R. M. Kadam
and Dr V. Rane, Radiochemistry Division, B. A. R. C for carrying
out EPR studies on InOOH and rh-In2O3 nanostructures. One of
Chem., 2007, 46, 5179–5187; (b) J. Q. Xu, Y. P. Chen,
Q. Y. Pan, Q. Xiang, Z. X. Cheng and X. W. Dong,
Nanotechnology, 2007, 18, 115615; (c) Y. Fan, Z. Li, L. Wang
and J. Zhan, Nanotechnology, 2009, 20, 285501.
the authors, D. E. thanks the nancial support to his research by 21 X. Xu and X. Wang, Inorg. Chem., 2009, 48, 3890–3895.
Spanish Ministerio de Economıa y Competitividad (MINECO) 22 L.-Y. Chen, Y. Liang and Z.-D. Zhang, Eur. J. Inorg. Chem.,
under Grants No. MAT2013-46649-C04-01/02/03 and No.
2009, 903–909.
MAT2015-71070-REDC (MALTA Consolider). The authors also 23 H. Dong, Z. Chen, L. Sun, L. Zhou, Y. Ling, C. Yu, H. H. Tan,
thank SC-SIE at Universitat de Valencia for support with the
transmission electron microscope measurements.
C. Jagadish and X. Shen, J. Phys. Chem. C, 2009, 113, 10511–
10516.
24 (a) H. Yang, L. Liu, H. Liang, J. Wei and Y. Yang,
CrystEngComm, 2011, 13, 5011–5016; (b) S. S. Farvid and
P. V. Radovanovic, J. Am. Chem. Soc., 2012, 134, 7015–7024.
25 (a) P. Li, H. Fan, Y. Cai, M. Xu, C. Long, M. Li, S. Lei and
X. Zou, RSC Adv., 2014, 4, 15161–15170; (b) L.-Y. Chen,
Z.-X. Wang and Z.-D. Zhang, New J. Chem., 2009, 33,
1109–1115; (c) Y. Tang and J. Ma, RSC Adv., 2014, 4,
25692–25697.
References
1 Special Issue of the MRS Bull.. 2000, 25, 15.
2 T. Minami, Semicond. Sci. Technol., 2005, 20, S35–S44.
3 P. D. C. King and T. D. Veal, J. Phys.: Condens. Matter, 2011,
23, 334214.
4 A. Stadler, Materials, 2012, 5, 661–683.
5 J. Song, S. A. Kulinich, J. Li, Y. Liu and H. Zeng, Angew. 26 W.-H. Zhang and W.-Z. Zhang, J. Phys. Chem. Solids, 2013, 74,
Chem., Int. Ed., 2015, 54, 462–466.
1271–1275.
6 I. Hamberg and C. G. Granqvist, J. Appl. Phys., 1986, 60, R123.
108402 | RSC Adv., 2016, 6, 108393–108403
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
27 M. Wu, C. Wang, Y. Zhao, L. Xiao, C. Zhang, X. Yu, B. Luo, 36 J. Yin and H. Cao, Inorg. Chem., 2012, 51, 6529–6536.
B. Hu, W. Fan and W. Shi, CrystEngComm, 2015, 17, 2336– 37 T. Yan, X. Wang, J. Long, H. Lin, R. Yuan, W. Dai, Z. Li and
2345.
28 B. Garcıa-Domene, J. A. Sans, F. J. Manjon, 38 X. Xue and M. Kanzaki, J. Phys. Chem. B, 2007, 111, 13156–
S. V. Ovsyannikov, L. S. Dubrovinsky, D. Martinez-Garcia, 13166.
O. Gomis, D. Errandonea, H. Moutaabbid, Y. Le Godec, 39 J. R. Ferraro and K. Nakamoto, Introductory Raman
X. Fu, New J. Chem., 2008, 32, 1843–1846.
´
´
~
´
´
H. M. Ortiz, A. Munoz, P. Rodrıguez-Hernandez and
Spectroscopy, Academic Press, Boston, 1994.
40 S. Tang, J. Zhang, S. Wu, C. Hu, Y. Li, L. Jiang and Q. Cui, J.
Phys. Chem. C, 2014, 118, 21170–21176.
C. Popescu, J. Phys. Chem. C, 2015, 119, 29076–29087.
29 A. Hoshino, K. Fujioka, T. Oku, M. Suga, Y. F. Sasaki,
T. Ohta, M. Yasuhara, K. Suzuki and K. Yamamoto, Nano 41 H. Jiang, L. Zhao, L. Gai, L. Ma, Y. Ma and M. Li,
Lett., 2004, 4, 2163–2169. CrystEngComm, 2013, 15, 7003–7009.
30 (a) W. Ong, L. Tan, S. Chai, S.-T. Yong and A. R. Mohamed, 42 L. Zhang, L. Yin, C. Wang, N. Iun, Y. Qi and D. Xiang, J. Phys.
Nano Res., 2014, 7, 1528–1547; (b) W. Ong, L. Tan, S. Chai Chem. C, 2010, 114, 9651–9658.
and S. Yong, Dalton Trans., 2015, 44, 1249–1257; (c) Y. Wei, 43 H. M. Xiong, D. G. Shchukin, H. Mohwald, Y. Xu and
J. Su, X. Wan, L. Guo and L. Vayssieres, Nano Res., 2016, 9, Y. Y. Xia, Angew. Chem., Int. Ed., 2009, 48, 2727–2731.
1561–1569; (d) M. Zeng, Z. Chai, X. Deng, Q. Li, S. Feng, 44 A. van Dijken, E. A. Meulenkamp, D. Vanmaekelbergh and
J. Wang and D. Xu, Nano Res., 2016, 9, 2729–2734; (e) A. Meijerink, J. Lumin., 2000, 90, 123–128.
T. Sun, J. Song, J. Jia, X. Li and X. Sun, Nano Energy, 2016, 45 H. Cao, X. Qiu, Y. Liang and Q. Zhu, Appl. Phys. Lett., 2003,
26, 83–89; (f) F. Li, L. Zhang, J. Tong, Y. Liu, S. Xu, Y. Cao
and S. Cao, Nano Energy, 2016, 27, 320–329.
31 Z. Li, Z. Xie, Y. Zhang, L. Wu, X. Wang and X. Fu, J. Phys.
Chem. C, 2007, 111, 18348–18352.
32 (a) L. Song, C. Chen, S. Zhang and Q. Wei, Catal. Commun.,
2011, 12, 1051–1054; (b) S. Ge, B. Wang, J. Lin and L. Zhang,
CrystEngComm, 2013, 15, 721–728.
33 Y. Song, L. Xu, W. Shi and J. Guan, J. Nanopart. Res., 2014, 16,
2295.
34 S. Yang, C.-Y. Xu, S.-P. Hu, W.-S. Wang, J. Yu and L. Zhen,
Bull. Korean Chem. Soc., 2016, 37, 522–528.
35 Y. S. Cho and Y. D. Huh, Bull. Korean Chem. Soc., 2010, 31,
1769–1772.
83, 761–763.
46 J. Sato, H. Kobayashi, K. Ikarashi, N. Saito, H. Nishiyama and
Y. Inoue, J. Phys. Chem. B, 2004, 108, 4369–4375.
47 P. D. C. King, T. D. Veal, F. Fuchs, C. Y. Wang, D. J. Payne,
A. Bourlange, H. Zhang, G. R. Bell, V. Cimalla,
O. Ambacher, R. G. Egdell, F. Bechstedt and
C. F. McConville, Phys. Rev. B: Condens. Matter Mater.
Phys., 2009, 79, 205211.
48 M. A. Butler and D. S. Ginley, J. Electrochem. Soc., 1978, 125,
228–232.
49 Y. Xu and M. A. A. Schoonen, Am. Mineral., 2000, 85, 543–
556.
50 M. Batzill, Energy Environ. Sci., 2011, 4, 3275–3286.
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