Shape Transition of TiO2 Nanocube to Nanospindle Embedded on Reduced Graphene Oxide with Enhanced Photocatalytic Activity

Article abstract of DOI:10.1021/acs.cgd.6b01096

In this study, we report a facile synthetic route for the transition of anatase TiO₂ nanocube to nanospindle with coexposed {001} and {101} facets on a reduced graphene oxide (rGO) platform. Initially, a TiO₂ nanocubes/graphene oxide suspension in an ethanol-water solvent mixture was hydrothermally treated at 180 °C for various reaction durations to perform the in situ growth of TiO₂ and simultaneous reduction of graphene oxide to rGO. All composites resulted in the formation of anatase TiO₂ with a surface heterojunction formed by coexposed {101} and {001} facets embedded on an rGO sheet. Interestingly, it was observed that 4 h of hydrothermal treatment resulted in the formation of a TiO₂ nanospindle with enhanced {001} high energy facet % embedded on an rGO sheet. The photocatalytic performance of the products was evaluated for the photodegradation of malachite green under simulated solar irradiation. All TiO₂@ rGO composites exerted enhanced activity as compared to the native TiO₂ cube owing to enhanced adsorption and fast electron migration by the rGO platform. Furthermore, the TiO₂ spindle @rGO exhibited the highest photocatalytic activity with a 6 fold increase in efficiency over the native TiO₂ cube. This superior performance of the nanospindle was ascribed to the synergistic effect of enhanced {001} facet percentage in the surface heterojunction and close interfacial contact with the rGO platform.

Full text of DOI:10.1021/acs.cgd.6b01096

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Article
Shape Transition of TiO2 Nanocube to Nanospindle Embedded on
reduced Graphene Oxide with Enhanced Photocatalytic Activity
Soumita Mukhopadhyay, Debabrata Maiti, Arindam Saha, and Parukuttyamma Sujatha Devi
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01096 • Publication Date (Web): 10 Oct 2016
Downloaded from http://pubs.acs.org on October 13, 2016
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Shape Transition of TiO
2
Nanocube to Nanospindle Embedded on Reduced Graphene
Oxide with Enhanced Photocatalytic Activity
Soumita Mukhopadhyay, Debabrata Maiti, Arindam Saha and Parukuttyamma Sujatha Devi*
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Sensor and Actuator Division, CSIR-Central Glass and Ceramic Research Institute, 196,
Raja S.C. Mullick Road, Jadavpur, Kolkata 700032, India
ABSTRACT: In this study, we report a facile synthetic route for the transition of anatase
TiO nanocube to nanospindle with coexposed {001} and {101} facets on reduced graphene
2
oxide (rGO) platform. Initially, TiO
2
nanocubes/graphene oxide suspension in ethanolꢀwater
o
solvent mixture was hydrothermally treated at 180 C for various reaction durations to
perform the in-situ growth of TiO and simultaneous reduction of graphene oxide to rGO.
2
2
All composites resulted in the formation of anatase TiO with surface heterojunction formed
by coexposed {101} and {001} facets embedded on rGO sheet. Interestingly, it was observed
that four hours of hydrothermal treatment resulted in the formation of TiO nanospindle with
2
enhanced {001} high energy facet % embedded on rGO sheet. The photocatalytic
performance of the products were evaluated for the photodegradation of malachite green
(
MG) under simulated solar irradiation. All TiO
activity as compared to the native TiO cube owing to enhanced adsorption and fast electron
migration by rGO platform. Furthermore, the TiO spindle @rGO exhibited highest
photocatalytic activity with ~6 fold increase in efficiency over native TiO cube. This
2
@ rGO composites exerted enhanced
2
2
2
superior performance of the nanospindle was ascribed to the synergistic effect of enhanced
{001} facet percentage in the surface heterojunction and close interfacial contact with rGO
platform.
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1
. INTRODUCTION
Titanium dioxide (TiO
2
) is one of the most extensively investigated semiconducting metal
oxides specially in energy induced environmental applications such as photoanode in DSSC,
Liꢀion storage battery, photocatalyst in solar water splitting and photocatalytic dye removal
owing to its strong oxidizing power, nontoxicity, chemical durability, low cost and long term
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stability. But all these applications extensively depend upon the morphology, surface
3,4
structures and crystalline phases with exposed low index facets. Among these anatase TiO
2
with exposed high reactive {001} facet has been researched greatly for environmental
applications owing to high surface energy and superior surface atomic structures as observed
4
ꢀ6
from both theoretical and experimental evidences. As observed in the literature, in most of
the cases the synthesis of this high energy facets {001} of anatase TiO involve highly
corrosive fluorination method or few with nitrogen, sulphur or carbonate ion incorporation.
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,8
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,10
In addition, very recently many researchers have devoted their attention towards the
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fabrication of coexposed facets.
heterojunction where one facet acts as oxidative site and the other as reductive site.
Formation of surface heterojunction can be significantly explained by surface atomic
Tailoring of coꢀfacets leads to the formation of surface
13ꢀ20
11,15,16
arrangements on the basis of density functional theory (DFT) calculations.
Existing
reports states that {001} surface exhibits larger Ti−O−Ti bond angles, where 2p states of the
surface O atoms are destabilized and very reactive. In contrast {101} surface exerts lower
Ti−O−Ti bond angles. According to the DFT electronic structure calculations, the Fermi
level of {001} facets penetrate through valence band whereas the Fermi level of {101} facets
15
remains at the top of the valence band. As reported, to attain the Fermi level equilibrium,
when {001} and {101} facets come in contact to each other, they form a surface
heterojunction where the photogenerated electrons migrate to {101} facet from {001} facet
15
and holes migrate in the reverse direction during the photocatalytic process. Consequently,
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{101} facet acts as reductive site and {001} facet acts as oxidative site in this surface
heterojunction and the photogenerated charge spontaneously get separated within a single
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component to exert superior performance as compared to the single faceted TiO2.
Furthermore, the coupling of the TiO with carbonaceous materials such as carbon nanotubes,
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graphite oxide, fullerenes or graphene could form an excellent heterojunction by extending
their optical response towards visible region, harvest more photons under solar spectrum and
hinder the excitons recombination, thereby significantly enhance the photocatalytic
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activity.
Compared to the other conventional carbonaceous materials, graphene with giant
2D morphology and highly conjugated planar surface exert significant adsorption properties
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towards the aromatic dye molecules owing to the пꢀ п stacking interaction. Excellent
physicochemical stability and flexible 2D plane with high electronic conductivity craft it as a
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promising candidate in energy induced environmental applications.
reported graphene coupled TiO
Liꢀion storage battery, hydrogen evolution and photocatalytic dye degradation.
Some groups have
2
with exposed {001} facets for various applications such as
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Jiang et
al. Sheng et al. and Gu et al. prepared grapheneꢀTiO with exposed {001} and studied
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2
on photocatalytic degradation of MB dye. Lu et al. synthesized TiO
with exposed {001} facets and investigated photocatalytic degradation of MO dye. Sun et
al. prepared nanoꢀsized TiO sheets with {001} facets on graphene nanosheets for
2
/graphene composites
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phocactalytic degradation of RhB dye. In contrast, till date, very few of them have reported
TiO /rGO composite with coexposed facet. As for example, very recently, Xiong et al
2
2
reported the synthesis of graphene supported TiO nanocrystals with coexposed {001} and
3
2
{
2
101} facets and its performance in CO reduction.
In this study, we have demonstrated the conversion of anatase nanocube to anatase
nanospindle with surface heterojunction formed by coexposed {001} and {101} facets of
TiO
2
embedded on rGO. The shape transition occurs as a function of reaction duration
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employing TiO
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cube/ graphene oxide (GO) suspension in ethanolꢀwater solvent mixture
o
under hydrothermal treatment at 180 C. 5% GO loading and 4 hours reaction time was
optimized for the formation of nanospindles from nanocubes under hydrothermal condition.
To investigate its photocatalytic performance, malachite green (MG) dye was used as probe
molecules for water contaminants as this dye is widely used in aquaculture in addition to
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textile, leather and paper industries. TiO spindle/rGO exhibited excellent photocatalytic
activity with ~6 fold increase in efficiency over native TiO
2
cube. The detailed mechanism
for its superior performance was thoroughly investigated.
2
. EXPERIMENTAL SECTION
.1. Materials. Titanium isopropoxide (TTIP,97%) and Graphite powder were purchased
SO , NaNO , KMnO and H were obtained
2
from Sigma Aldrich. Oleic acid, NaOH, H
2
4
3
4
2 2
O
from Merck India and used as received. Malachite green chloride (MG) was purchased from
Alfa Aesar.
2.2. Preparation sodium oleate. Sodium oleate was prepared following same method as
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reported in the earlier work. Briefly, 16 gm of sodium hydroxide was added in 200 ml of
o
ethanol and stirred at ~60 C to dissolve it completely. Then 20 ml of oleic acid was added to
the solution and stirring was continued further for 2 hours. As obtained yellowish white
precipitate was centrifuged at 5000 rpm followed by washing with hexane to remove excess
o
oleic acid from the precipitate. Finally the yellowish white product was dried at 70 C for 12
h.
2
2.3. Synthesis of TiO nanocube. The synthetic condition for the preparation of uniform
3
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anatase TiO
2
cubes was adopted from the work reported by Sugimoto et al. Initially, a stock
4+
solution of Ti was prepared by mixing titanium (IV) isopropoxide (TTIP) with
triethanolamine (TEOA) at a molar ratio of TTIP:TEOA = 1:2 to avoid the exceedingly fast
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hydrolysis of Ti , followed by addition of deionized water (DI) to prepare a solution of 0.5
ꢀ3
4+
ꢀ3
mol dm in Ti . Then, 0.02 mol dm of Naꢀoleate was mixed with the stock solution under
constant stirring. The pH of the mixture was adjusted to 10±0.5 and the total volume of the
stock solution was made upto 30 mL. The resulting mixture was placed into a closed flask
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o
and aged for 24 h at 100 C for gelation. Finally, the viscous gel was transferred to a Teflonꢀ
o
lined autoclave and aged at 140 C for 72h to nucleate and grow the titania particles. Then, the
product was centrifuged at 14,000 rpm followed by washing with water for several times and
o
dried under vacuum at 120 C.
2
.4. Synthesis of Graphene Oxide (GO). GO was synthesized following a modified
4
0
Hummer’s method. Briefly, 0.25 g of graphite and 0.4 g of NaNO
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were taken in 12.5 mL of
concentrated H SO and stirred in an ice bath for 15 min. Then 2.0 g of KMnO
2
4
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was added in
o
small portions to obtain a greenish purple suspension followed by stirring for 6h at 35 C. The
dark brown colour paste was diluted up to 100 mL with the slow addition of deionized water
(
DI) and sonicated for 20 mins. 2 mL of 30% H O was added dropwise to quench the
2 2
solution. Then goldenꢀbrown sol was centrifuged and washed with luke warm DI water for
several times to adjust the pH to ∼6. Finally the product was air dried at 40 °C for 24 h. The
product obtained was dispersed in water by 30 minutes sonication followed by centrifugation
at 3000 rpm for 15 minutes. The supernatant obtained was collected as graphene oxide (GO).
2 2
2.5. Synthesis of TiO @rGO Composites. Different TiO @rGO composites were prepared
by varying the reaction duration and concentration of GO under hydrothermal treatment
keeping other conditions constant. To synthesize TiO @rGOꢀ5%, initially, 2 ml of GO (0.5
2
mg/ml) was sonicated in 35 mL of deionized water for 2h to accomplish uniform dispersions
2
and exfoliation of GO. Then, 20 mg of TiO cubes dispersed in 5 ml ethanol were added
slowly to the GO suspension and vigorous stirred for 1h to get a homogeneous suspension.
Then the suspension was transferred to a 50 ml Teflonꢀlined autoclave and heated at 180°C
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for 2h, 4h and 6h to obtain TiO
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@rGO and the corresponding products were designated as
TiO @rGOꢀ2h, TiO @rGOꢀ4h and TiO @rGOꢀ6h, respectively. The asꢀobtained black
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coloured products were centrifuged and washed with DI water for several times and vacuum
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dried for 24h at 60 C. Controlled experiments were also performed varying the mass ratio of
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solution was also replaced with deionised water under identical reaction conditions for
comparison.
2.6. Characterization. The crystalline phase purity of the products were characterized by X
ray diffraction studies on an X’pert pro MPD XRD of PAN analytical system with CuKα
radiation (λ = 1.5406 Å). The bright field morphology and highꢀresolution transmission
electron microscopy (HRTEM) images were investigated using transmission electron
microscopy (TEM) on a Tecnai G2 30ST (FEG) Ultra High ResolutionTransmission Electron
Microscope operating at 200 kV. The UVꢀVisible absorbance spectra were obtained on a UVꢀ
VisꢀNIR Spectrophotometer (SHIMADZU UVꢀ3600). Nitrogen physisorption measurements
of all the samples were carried out by using a Quantachrome (iQ3) instrument after
o
evacuation at 150 C for 4 h. The specific surface area was calculated by BET method
whereas desorption cumulative pore volume and pore size distribution were calculated by
BJH method. The emission characteristics were recorded at room temperature on a Steady
State Spectrofluorometer (QMꢀ40, Photon Technology International, PTI) using a 150 watt
Xenon lamp as the excitation source at excitation wavelengths of 315 nm. Xꢀray
photoemission spectroscopy measurements were carried out in a PHI 5000 Versa probe II
scanning XPS microprobe (ULVACꢀPHI, U.S.). The measurements were monitored at room
ꢀ10
temperature and at a base pressure better than 6 x 10 mbar. All the spectra were acquired
with monochromatic Al Kα (hν = 1486.6 eV) radiation with a total resolution of about 0.7 eV
and a beam size of 100 ꢁm.
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2.7 Photocatalytic Measurements. Photocatalytic performance of native TiO cube and
TiO
Visible mixed light exposure equipped with an air cooled 250 W highꢀpressure mercury lamp
250W, HP) and under visible light irradiation using a tungsten halogen lamp (Phillips,
2
@rGO composites prepared at variable reaction conditions were investigated under UVꢀ
(
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250W) equipped with a cut filter of λ ≥420 nm. Illumination intensity of both irradiation
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sources were fixed at an average intensity of 2.5 mW/cm , measured using a Digital LUX
Meter, modelꢀ101A. Malachite green (MG) dye, which is widely used in aquaculture in
addition to many textile, leather and paper industries was used as probe molecule for organic
pollutants in waste water. To investigate the concentration dependent responding ability of
the composite a series of experiments were performed with variation in dye concentration
ꢀ5
ꢀ4
from 2.5 x 10 M to 10 M. Initially, 10 mg of each sample was dispersed in 25 mL of the
dye solution and allowed to stir for 30 mins in dark to equilibrate the adsorptionꢀdesorption
phenomenon. Immediately 3mL aliquots were withdrawn, conveniently separated by
centrifugation and the absorbance spectra of the supernatant was immediately checked by
Schimadtzu UVꢀvis spectrophotometer. Then the mixture was irradiated with air cooled
mercury lamp and tungsten halogen lamp to carry out the photodegradation under UVꢀVisible
and Visible light (λ ≥420 nm) exposure, respectively. At regular time intervals of irradiation,
aliquots were withdrawn, centrifuged and absorbance was monitored by spectrophotometer.
Rate of photodegradation reaction was also compared with controlled experiments using
Degussa P25 (commercial TiO ) and without using any photocatalyst (blank experiment).
2
Reproducibility of the efficient photocatalyst was studied for four consecutive runs. After
each cycle the catalyst was centrifuged and washed thoroughly with water/ethanol mixture
followed by drying under vacuum.
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. RESULTS AND DISCUSSION
2
Figure 1 shows the Xꢀray diffraction patterns and phase purity of bare GO, rGO, TiO and the
TiO @rGO composites prepared at different reaction durations under hydrothermal
2
o
treatment. XRD patterns in Figure 1a and b depict the characteristic peaks at 10.2 (002) and
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24.5 (002) with an interlayer spacings of 0.9 nm and 0.37 nm corresponded to GO and rGO,
respectively. As shown in Figure 1(cꢀf), all the reflections pattern agrees well with the
formation of pure anatase TiO
2
corresponding to JCPDS Card No. 21ꢀ1272. The average
crystal size of the asꢀprepared native TiO
2
was estimated using Scherer equation applied to
o
(101) reflection was 19.87 nm. Since, the peak at 24.5 corresponded to the rGO overlapped
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with the anatase TiO
(dꢀf) and Table 1. Lower crystalline pattern, peak broadening and shifting towards lower
angle indicate the grafting of TiO onto rGO in the TiO @rGO composites as compared to
2
(101) and give rise to a broad XRD pattern as observed from Figure
1
2
2
2
5,41
2
the native TiO .
Figure 1. XRD patterns of bare GO, rGO,TiO cube and TiO @rGO composite
prepared at different reaction durations.
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The morphology and size of the asꢀprepared products were investigated by bright field TEM
and HRTEM studies. Figure 2a exhibits the largeꢀscale TiO
2
nanocubes with average size of
nanocube with dꢀ
21 nm. HRTEM image shows the clear lattice fringes of a single TiO
2
spacings of ~0.351 and 0.237 nm corresponded to the (101) and (004) lattice planes, indicate
the presence of {101} and high energy {001} facets, respectively as observed in Figure 2b.
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Figure 2. (a) Bright field TEM (histogram inset) and (b) HRTEM images of TiO
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cube.
Figure 3(aꢀd) display the bright field images and Figure 3(e,f) show the HRTEM of
TiO @rGOꢀ4h composite prepared under hydrothermal condition for 4h. It is interesting to
2
2
observe that the TiO nanocubes were transformed to nanospindle in the composite with a
mean length and width of 40 and 18 nm, respectively. The strong hydrophilic nature of
oxygen functionalities on exfoliated GO sheets, makes it highly soluble in aqueous medium
leading to the golden brown homogeneous suspension. Then the vigorous stirring of water
dispersible TiO nanocubes into GO suspension results in their anchoring onto the GO
2
surfaces. After hydrothermal treatment this golden yellow TiO /GO dispersion turned into
2
dark black dispersion of TiO
2
/rGO. Digital photograph of the suspensions shown in Scheme 1
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6
clearly evidences this phenomenon. The gradual shape and size transition of TiO nanocubes
2
to nanospindles embedded on rGO as a function of reaction duration could be observed in
Figure S1.
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Figure 3. TEM (aꢀd) bright field and (e,f) HRTEM images of TiO @rGOꢀ4h (FFT inset).
2
The germination of nanospindles from the well dispersed TiO nanocubes embedded on rGO
sheets can be observed in Figure S1 (a,b) after 2h of hydrothermal treatment. But on
prolonging the time beyond 4h the breakage of the nanospindles occurs (Figure S1c,d). So 4h
reaction duration under hydrothermal treatment was optimized to obtain the well dispersed
nanospindles on rGO sheet.
The formation of nanospindles could be explained from HRTEM images and surface
42
chemistry of TiO on rGO under hydrothermal condition. As observed in HRTEM (Figure
2
3e) nanospindle formation resulted in enhanced exposure of {001} facet. This {001} facet
o
makes an angle of 68.3 with the {101} facet as shown in Figure 3f. The analogous FFT
18,43
pattern (inset images of Figure 3f) also agrees well with the theoretical value.
It has been
demonstrated that the crystallographic facets with high surface energies diminish quickly for
1
8
reducing the surface energy during crystal formation. Therefore, anatase TiO
2
crystals with
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exposed high energy {110} and {001} crystallographic facets are very rare. But in our study,
we observed the existence of {101} and {001} cofacet in all the cases. The appearance of
high energy {001} facet along with the regular {101} in the nanocube was attributed to the
usage of triethanolamine (TEOA) and sodium oleate, as complexing agent and shape
controller, respectively. While complex formation, TEOA restricts the growth parallel to the
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ꢀ
cꢀaxis whereas carboxylate groups (COO ) of oleate, simultaneously adsorb onto the (004)
polar plane of TiO
2
and reduce the surface energy of {001} facet to expose it in the
39
o
nanocube. Similarly, during composite formation under hydrothermal treatment at 180 C
for 4 hours, oxygen containing functional groups of GO, viz. carboxylate, carbonyl, epoxy
and/or hydroxyl groups further interact with this exposed (004) plane to reduce its surface
energy further which results in the formation of nanospindles with enhanced {001} facet.
Formation of TiO nanospindle from nanocube embedded on rGO sheet shown in Scheme 1.
2
2
Scheme 1. Schematic representation for the hydrothermal synthesis of TiO spindle
To confirm this phenomenon, a control experiment was performed by replacing GO solution
with the same amount of distilled water as solvent keeping the other reaction parameter
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constant for the in-situ growth of TiO
instead of nanospindle formation as evident from Figure S2 (a,b). This observation strongly
reveals the firm attachment of TiO nanocubes with the oxygen functionalities of GO during
2 2
cubes. In absence of GO, TiO cubes agglomerated
2
composite formation. The concentration (%) of GO was also varied keeping other reaction
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parameters constant. Figure S2(c,d) reveal that at a particular concentration of TiO , at 1%
2
2
and 10% GO concentration, the complete homogeneous shape transition of TiO from cube to
spindle embedded on rGO sheet was majorly affected. Hence 5% concentration of GO was
optimized for the homogeneous nanospindle formation on rGO platform.
Figure S3 shows the FTIR spectra of GO, native TiO cube and TiO @rGO composite
2
2
prepared at 4h. This spectroscopy was performed to investigate the functionality and
interaction present in the composite. Typical FTIR spectrum of GO shows the characteristic
ꢀ1
band of oxygen containing functional groups like C–O alkoxy (1068 cm ), C–O epoxy (1192
ꢀ1
ꢀ1
ꢀ1
cm ), CꢀO stretching (1390 cm ) and C=O carbonyl (1728 cm ) suggest the complete
2
6
oxidation of graphite to graphene oxide (GO). In the spectrum of TiO @rGO, the oxygen
2
containing vibrational bands drastically reduced, indicating a significant reduction of GO
ꢀ1
under hydrothermal treatment. The bands at 3450 and 1620 cm correspond to the stretching
and bending vibration of –OꢀH, respectively were observed in all the samples. The
ꢀ1
characteristics bands at 2850 and 2920 cm (Figure S3b,c) were endorsed to the methylene
group (ꢀCH ) of TEOA attached to the native TiO . The broad band in range of 500ꢀ900
2
2
ꢀ1
cm (Figure S3 b,c) can be attributed to the combination of TiꢀOꢀTi and TiꢀOꢀC stretching
25
2
vibrations, indicating the strong chemical coupling between TiO and rGO in the composite.
Xꢀray photoelectron spectroscopy (XPS) was also carried out to investigate the chemical
nature of the TiO nanospindle@rGOꢀ4h composite as shown in Figure 4. Figure 4(a) shows
2
the survey spectra of the composite. The presence of TiO and rGO can be ascertained by
2
observing the binding energy of Ti 2p, O 1s and C 1s core level XPS spectra as shown in
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Figure 4(bꢀd). The XPS peaks at 458.8 and 464.6 eV corresponded to the binding energies of
4
4
the Ti 2p3/2 and 2p1/2 electrons (Figure 4b). The spin orbit splitting energy of 5.8 eV is the
4+
2
characteristic of Ti in the anatase TiO spindle. Figure 4(c) represents the high resolution O
1
s spectrum. By deconvolution, the observed three peaks were attributed to TiꢀOꢀTi (530.1
eV), O=CꢀOꢀTi (530.6 eV) and TiꢀOꢀH (531.8 eV). On deconvolution of core level XPS of C
s, four peaks were observed corresponded to CꢀC/C=C (284.5 ± 0.2 eV), CꢀO (286.1 eV),
C=O (287.2 eV), overlapping of TiꢀOꢀC and OꢀC=O (288.8±0.1 eV) as demonstrated in
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Figure 4d.
Figure 4. (a) XPS survey and core level spectra of (b) Ti 2p (c) O 1s and (d) C 1s of
TiO @rGO composite prepared at 4h reaction duration.
2
2
Minute observation reveals the fact that defect containing sp carbon related peaks at ~284.5
eV were predominant whereas oxygen containing functional groups were faint in the
composite. These results strongly suggest that the oxygen functionalities of the graphene
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ꢀ
oxide , mainly carboxylate (COO ) group effectively interacted with TiO and hydrothermal
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treatment for 4h resulted in the formation of TiO
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/rGO composite with exposed cofacets
which is in good conformity with the HRTEM, Raman and FTIR results.
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UVꢀVisible absorbance spectra of TiO
different reaction duration are presented in Figure 5a. From the figure it is observed that
interaction of rGO to the TiO cubes extends its photoresponse towards visible region and
2 2
cubes and TiO @rGO composites prepared at
2
Figure 5. (a) UVꢀVis spectra and (b) band gap calculation of TiO
TiO @rGO composite prepared at different reaction duration.
2
cube and
2
consequently enhance its light harvesting capability. The optical band gap of the products
have been evaluated from the absorption spectrum using the Tauc’s relation as shown in
Figure 5b.The band gap of 3.08, 2.98, 2.91 and 2.93 eV were obtained for native TiO cubes
2
and TiO
2
@ rGO nanocomposites prepared at 2h, 4h and 6h, respectively. Narrowing in the
band gap occurs due to the formation of TiꢀOꢀC bond which is supported by IR and XPS
2
6,27
studies and also in good conformity with other results reported in the literature.
This band
gap narrowing indicates more efficient utilization of solar spectrum and consequently more
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photoinduced charge generation to exert superior phocatalytic performance over native TiO2
nanocubes.
2 2
Figure 6 depicts the photoluminescence spectra of TiO cubes and TiO @rGO composites
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prepared at 2h, 4h and 6h, respectively in the wavelength range of 350–550 nm.
Photoluminescence emission mainly arises from the recombination of photoꢀinduced charged
2 2
Figure 6. Emission spectra of TiO cube and TiO @rGO composite prepared at different
reaction durations.
carriers indicating the separation capability. As rGO sheet acts as an electron acceptor
materials, in the composite effective interaction of photoinduced charge carriers of TiO with
the close interfacial contact of rGO sheet reduces their radiative recombination and hence
2
3
5
quench the PL intensity.
Respective shifting in emission intensity may be due to the
change in surface properties as a function of size and shape of the TiO
2
.
Raman spectra as shown in Figure 7 and S3 confirm the phase purity and crystallinity of
native TiO
2
cubes and TiO
2
@rGO composites. It also provides a proper distinction between
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GO and rGO present in the composite. As evident from Figure 7, the characteristic E band at
g
ꢀ1
ꢀ1
ꢀ1
1
43.4 cm and 639 cm , the B1g band at 397cm and the (A1g + B1g) mode centred at 516
ꢀ1
cm confirm the presence of anatase TiO
2
. But minute observation reveals the fact that in the
ꢀ1
composite, the characteristic E
g
mode becomes broader and blue shifted from 143.4 cm to
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ꢀ1
148 cm . It may happen for the phonon confinement effect and developed strain owing to the
2
5
change in size and shape of TiO in the composite. It also confirms the strong interaction
2
between TiO
dynamics.
2
and rGO sheets which is a crucial factor for efficient electron transfer
Figure 7. Raman spectra of TiO
The percentage of anatase TiO with exposed {001} facet in the composite as mentioned in
the HRTEM can be quantified from the intensity ratio of E
observed that in the native TiO cube the {001} facet was only 14.7% which exposed upto
2 2
cube, GO and TiO @rGOꢀ4h.
2
46
g
and A1g Raman band. It was
2
18.3% to 36.8% for the composite prepared at 2h and 4h, respectively under hydrothermal
treatment. But after 6h reaction duration the exposure of {001} facet diminished to 26.4 % as
ꢀ1
ꢀ1
shown in Table 1. Further investigation of Raman spectra in the 1100 cm to 2000 cm
ꢀ1
reveals the information about the carbonaceous materials. The peaks at 1350 cm and
ꢀ1
26
1
597cm designated as the D and G bands, respectively. The intensity ratio of the D band to
the G band (I /I ) is used to measure the degree of defects present in the GO and rGO
D
G
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materials. Pristine GO shows the intensity ratio of 0.86 indicating the presence of lower
defects. As evident from Table 1, this value increases upto 0.95 to 1.07 indicating the
2
existence of higher defects with increased sp domains in the composite. It may be due to the
fact that preceding hydrothermal treatment TiO /GO mixture was exfoliated for long time
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which leads to the formation of fragmented smaller sp domains along the reactive sites
26
resulting in high intensity of D band. In all the composites, blue shifting in G band from
ꢀ1
1
597 to 1607 cm were observed as compared to the pristine GO. Both the change in band
intensity ratio (I
D G
/I ) and blue shift of the G band confirm the presence of reduced graphene
25
sheets in all the composites.
2 2
Table 1. Structural parameters of bare TiO cube, GO and TiO @ rGO composites
Catalyst
TiO cube
I
101/004
I
101/200 FWHM(101)
I
Eg (144 cmꢀ1)
I
A1g (514 cmꢀ1) facet(001) (%)
I
D
ꢀ
I
G
ꢀ
I
D
/I
G
2
3.272
2.61
0.489
0.62
0.812
0.87
ꢀ
1
0.147
0.183
0.368
0.264
ꢀ
14.7
18.3
36.8
26.4
ꢀ
ꢀ
TiO @rGOꢀ2h 3.154 1.44
1
1
1
ꢀ
0.169
0.236
0.559
0.19
0.178
0.22
0.95
1.07
1.06
0.86
2
TiO
2
2
@rGOꢀ4h 2.99
@rGOꢀ6h 3.06
1.47
1.58
ꢀ
TiO
0.528
0.221
Bare GO
ꢀ
In our study, the appearance of high energy {001} facet (14.7%) along with the regular {101}
in the nanocube was attributed to the usage of triethanolamine (TEOA) and sodium oleate, as
complexing agent and shape controller, respectively. During complex formation, TEOA
ꢀ
restricts the growth parallel to the cꢀaxis whereas carboxylate groups (COO ) of oleate,
2
simultaneously adsorb onto the (004) polar plane of TiO and reduce the surface energy of
1
{
001} facet to expose it in the nanocube. Similarly, while composite formation under
o
hydrothermal treatment at 180 C for 4 hours, oxygen containing functional groups of GO,
viz. carboxylate, carbonyl, epoxy and/or hydroxyl groups further interact with this exposed
(
004) plane to reduce its surface energy further which results in the formation of nanospindles
with enhanced {001} facet (36.8% as calculated from Raman spectra). Presence of all these
functional groups in graphene oxide (GO) can be observed from FTIR spectra as shown in
2
Figure S3. Reduction of all these bands was observed after TiO @rGO composite formation.
This phenomenon was further confirmed from the C1s core level XPS spectrum (Figure 4d).
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As evident from the figure that defect containing sp carbon related peaks at ~284.5 eV was
predominant whereas oxygen containing functional groups were faint in the composite. These
46
results strongly suggest that the oxygen functionalities of the graphene oxide carboxylate
ꢀ
groups (COO ) in particular effectively interacted with TiO
2
and hydrothermal treatment for
4h resulted in the formation of TiO
2
/rGO composite with coexposed facets. Moreover,
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presence of TiꢀOꢀC bond as observed in Xꢀray photoelectron spectroscopy (Figure 4d), FTIR
(Figure S3) and UV spectra (Figure 5) also approve this postulate.
Photocatalytic activity
Figure 8 (aꢀd) and Figure 9(aꢀb) depict the comparative photocatalytic performance of native
TiO
exposure, respectively. The histogram pattern as shown in Figure 8a illustrates the adsorption
efficiency of TiO @rGOꢀ4h as a function of dye concentration after 30 mins. stirring in dark.
2 2
cube and TiO @rGO composites under UVꢀvisible mixed light and visible light
2
Figure 8. (a) Adsorption (%) histogram after 30 min as a function of MG dye
ꢀ4
concentration using TiO
2
@rGOꢀ4h photocatalyst; (b) residual absorption spectra of 10
M MG dye, (c) photocatalytic degradation curve and (d) corresponding degradation
kinetics under UVꢀvisible mixed light exposure employing P25, TiO
2
cube and
TiO
2
@rGO composites prepared at different reaction conditions.
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It was observed from the figure that 98%, 60%, 42% and 24% of dye readily adsorbed onto
ꢀ5
ꢀ5
ꢀ5
ꢀ4
TiO
2
@rGOꢀ4h composite at 2.5 x 10 (M), 5 x 10 (M), 7.5 x 10 (M) and 10 (M) of dye
concentration, respectively. Earlier reports demonstrated that the carbonaceous materials
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possess amazing adsorption properties to be used in different environmental applications.
Most of the industrial dyes are typically aromatic in nature and cause πꢀπ stacking
25
interactions with the graphene aromatic domains to get adsorbed significantly. This
significant adsorption leads to the high concentration of the probe molecules onto the catalyst
surface to exert superior photocatalytic performance. Hence, in order to monitor the
ꢀ4
photocatalytic degradation kinetics 10 M dye was used as probe molecule. As evident from
Figure 8 and 9, under both types of irradiations blank experiment exerted only <10%
photodegradation of MG dye. In contrast, addition of photocatalyst remarkably enhances the
degradation rate and thereby confirms the efficacy of the photocatalyst. Interestingly, it was
observed that all the TiO composites embedded on rGO sheet exerted superior activities as
2
compared to native TiO cube and standard TiO Degussa P25 owing to the significant
2
2
adsorption and fast electron migration for rGO loading. It is noteworthy to mention that
among various composite photocatalysts, TiO @rGOꢀ4h exhibited excellent performance.
This phenomenon may be attributed to the growth kinetics of anatase cubes to anatase
2
ꢀ4
spindles on rGO sheet. Absorption spectra of remaining 10 M MG dye after 30 minutes of
mercury lamp irradiation shown in Figure 8b. From the HRTEM image it was observed that
all the synthesized photocatalysts possess coexposed {101} and high energy {001} facets
which form a surface heterojunction to facilitate the photoctatalytic activity. This surface
heterojunction facilitate the well separation of photogenerated electrons and holes to {101}
and {001} facets, respectively where {101} facet acts as reductive site and {001} acts as
15
oxidative site. From the Raman spectra it was observed that TiO cubes exerted ~14.7 %
2
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exposed reactive {001} facet which increased upto 36.8% for TiO spindles on rGO. This
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3ꢀ37
{
001} facet is highly reactive for photocatalytic activity.
percentage (%) in the surface heterojunction of nanospindle TiO
observed from HRTEM image (Figure 3e). The influence of rGO content ratio to TiO
The increased {001} facet
@rGOꢀ4h was also
in the
2
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composite and their photocatalytic performance was also investigated under identical
experimental conditions which demonstrated that the optimal ratio of rGO content to TiO2
also influence the photocatalytic activity. Adsorption capabilities of all the employed
photocatalysts in this study is shown in Figure S5. The order of apparent rate constant as
calculated for the photodegradation kinetics (shown in Table 2) are as follows: P25< TiO
cube < TiO @rGOꢀ2h < TiO @rGOꢀ6h < TiO @rGOꢀ10% <TiO @rGOꢀ1% < TiO @rGOꢀ
h. Similar trend was also noticed for exclusively visible light induced photodegradation as
2
2
2
2
2
2
4
observed in Figure 9 (aꢀb) but the extent of degradation was lower as compared to that under
UVꢀvisible light exposure. It may happen due to the restricted photoresponding ability of
47
2
TiO under exclusively visible light irradiation.
Figure 9. (a) Photocatalytic degradation curve and (b) corresponding degradation kinetics
for the degradation of MG dye under visible light exposure employing P25, TiO cube and
TiO @rGO composites prepared at different reaction conditions.
2
2
The photonic efficiencies (ξ) of all the photocatalysts were also calculated by using the
48
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ξ / % = (k C V/I A) × 100ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ(1)
o
o
−1
−2
−1
where I
o
is the photon flux (einsteins s cm ), k is the degradation rate constant (s ), A is
2
−1
the illuminated area (cm ), C
o
is the initial concentration of MG dye (mol L ), λ is the
average wavelength maxima of irradiation source (m), and V is the volume of aqueous
solution of MG dye (L). Photonic efficiency provides a relative evaluation for catalytic
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4
9,50
performances under identical reactor and reaction conditions.
best photonic efficiency of ~14.46 % was achieved for TiO
As shown in Table 2, the
@rGOꢀ4h under UVꢀvisible
2
light irradiation as compared to the other photocatalysts. In order to unravel the distinction in
photocatalytic performance, BET surface area of all the photocatalysts were investigated by
nitrogen isothermal sorption process (Figure 10). Corresponding pore size and pore volume
were calculated by BJH method as shown in inset of Figure 10(aꢀf). As evident from figure,
type IV isotherm with a wellꢀdefined hysteresis loop indicates the wellꢀdeveloped
2
mesoporous characteristics for all the photocatalysts. Native TiO nanocube exhibited a BET
2
specific surface area of 71.8 m /g with an average pore volume of 0.219 cc/g.
Figure 10. Nitrogen adsorptionꢀdesorption isotherm of (a) native TiO
2
cube, (b) TiO
2
@
@
rGOꢀ 2h, (b) TiO @ rGOꢀ 4h,(c) TiO @ rGOꢀ 6h, (d) TiO @ rGOꢀ 1% and (f) TiO
2
2
2
2
rGOꢀ 10%; Corresponding pore size distribution curves are shown inset.
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1
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2
2
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2
2
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2
But after coupling with rGO, the surface area raised upto 80.4 m /g for TiO spindle@ rGOꢀ
2
5% and becomes maximum for TiO
2
@ rGOꢀ10% of 89.34 m /g. Increase in surface area as
well as pore volume capacity on coupling with rGO is a beneficial factor for better
26
photocatalytic performance. In this experiment, results clearly indicate that though
adsorption capabilities (Figure S5) solely depend on the exposed surface area, photocatalytic
performance, besides governed by the charge transport phenomenon which lead to the
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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7
8
9
0
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0
superior performance for TiO
2
@ rGOꢀ4h. It is noteworthy to mention that higher rGO
loading in TiO @ rGOꢀ10% provides more exposed surface sites for the improved adsorption
2
of dye molecules onto the aromatic moieties of rGO via пꢀп stacking interactions but fails to
provide effective photocatalytic TiO sites for photoredox reactions resulting in the
2
diminished performance as compared to TiO
2
@ rGOꢀ4h.
Table 2. Photacatlytic performance of native TiO
2
and TiO @rGO composites
2
Catalyst
Surfac Adsorpti
UVꢀVisible light
Degradation Rate
%) in 30
mins
Visible light irradiation
e area
on (%)
ξ (%) Degradation
(%) in 60
Rate
ꢀ1
ξ (%)
2
(
m /g)
ꢀ1
(
(min )
(min )
mins
TiO cube
71.80
74.96
80.40
84.10
78.30
89.34
9
46
57
97
64
89
78
0.018
0.019
0.11
2.37
2.5
42
59
88
66
78
75
0.0061
0.0098
0.032
0.4
0.63
2.1
2
TiO
2
@rGOꢀ2hꢀ5%
21
24
20
16
30
TiO @rGOꢀ4hꢀ5%
14.46
3.95
9.35
5.93
2
TiO @rGOꢀ6h ꢀ5%
0.03
0.0149
0.0226
0.0152
0.95
1.44
0.97
2
TiO
2
2
@rGOꢀ4hꢀ1%
@rGOꢀ4hꢀ10%
0.071
0.045
TiO
On light exposure, native TiO
2
cubes exerted higher efficiency than commercial Degussa P25
2
(
anatase 80% and rutile 20%, SBET 52 m /g) owing to the presence of high surface active sites
with porosity as well as pure anatase phase with coexposed {101} and {001} facets
facilitating higher charge separation efficiency and photoelectrons mobility within single
phase. But superior efficiency of all the TiO @rGO composites attributed to the better dye
2
adsorption capability and higher contribution from high reactive {001} facet %. In case of
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TiO
2
@rGOꢀ4h, the surface heterojunction formed with enhanced {001} to {101} ratio
resulted in the remarkable enhancement in photocatalytic performance. In addition, loading
of rGO further extend the photoelectron delocalization from {101} facet due to the higher
25
work function of graphene. Consequently, the exciton recombination is further restricted
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resulting in significantly enhanced photocatalytic activity. But in TiO @rGOꢀ2h and
2
TiO
2
@rGOꢀ6h, the observed {001} facet was only 18.3 % and 26.4 % as estimated from the
on rGO (Figure S1)
Raman spectra (Figure S4) with the evolution of random shape TiO
2
resulting in the declination in photocatalytic performance. On the other hand, as revealed
from the degradation kinetics, higher rGO content though exhibited better adsorption
capability, it may block the exposed catalytic surface of TiO which in turn reduced the
2
2
5
degradation efficiency of TiO
may be ascribed to the coupled effect of effective surface heterojunction formation within
pseudo 1D anatase TiO and close interfacial contact with rGO platform. This phenomenon
2 2
@rGOꢀ10%. Hence the excellent reactivity of TiO @rGOꢀ4h
2
strongly suggests that the improved photocatalytic activity requires significant adsorption,
well separation of photogenerated charge carriers along the surface heterojunction and close
interfacial contact of surface active sites with rGO for faster electron migration to carry out
2
the photoredox reactions. Reproducibility of TiO @rGOꢀ4h catalyst was also monitored
under simulated solar irradiation (UVꢀVisible light). As shown in Figure S6 after four
consecutive cycles no significant decrease in efficiency indicates the excellent stability of the
catalyst. Considering the coexistence of both type of heterojunctions viz. surface
15
heterojunction within TiO
2
and its coupling with rGO, overall charge transfer mechanism
for the excellent photocatalytic performance of TiO
irradiation is shown in Scheme 2.
2
spindle@rGO under simulated solar
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9
0
Scheme 2. A schematic representation for the photocatalytic mechanism of TiO spindle
2
4
. CONCLUSIONS
Growth kinetics of anatase cube to spindle embedded on rGO sheet has been successfully
demonstrated here with series of experiments. The formation of surface heterojunction with
coexposed (101) and (001) facets were observed in all the photocatalysts. This surface
heterojunction leads to the spatial separation of photogenerated charge carriers to exert better
photocatalytic activity where (101) acts as reductive sites and (001) as oxidative sites. All the
samples exhibited mesoporosity with a very high surface area leading to enhanced
photocatalytic performance. Under UVꢀvisible light irradiation the excellent photocatalytic
2
performance of pseudo 1D nanospindle TiO on rGO platform was attributed to the coupled
effect of effective surface heterojunction formation with enhanced {001}high energy facet
percentage and optimal loading of rGO (5%). The rGO sheet further extend the
delocalization of spatially separated photoelectrons from the {101} reductive site and
restrains their recombination. This phenomenon strongly suggests that the improved
photocatalytic activity requires significant adsorption, well separation of photogenerated
charge carriers along the surface heterojunction and close interfacial contact with rGO
platform for faster electron migration. It is expected that this work will offer a facile route for
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the in-situ growth of TiO
2
@rGO composite with coexposed reactive facets which could find
applications in diverse areas.
ASSOCIATED CONTENT
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Supporting Information
Electronic supplementary information (ESI) available: TEM, FTꢀIR, Raman, adsorption (%)
histogram and recycling performance.
AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: psujathadevi@cgcri.res.in, psujathadevi@gmail.com
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
SM is indebted to the University Grant Commission (UGC), Govt. of India, for the award of
Senior Research Fellowship. PSD acknowledges Ministry of New and Renewable Energy
(MNRE) for financial support under CSIRꢀTAPSUN programme.
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