Rutile TiO2/CoSe nanocomposite: An efficient photocatalyst for photodegradation of model organic dyes under visible light irradiation

Article abstract of DOI:10.1016/j.molliq.2018.12.119

Here we propose new type II semiconducting nanocomposite TiO₂/CoSe for effective separation of the photo-induced charges, thus enhancing the visible light harvesting capability of TiO₂. The formation of TiO₂/CoSe nanocompo

Full text of DOI:10.1016/j.molliq.2018.12.119

Accepted Manuscript
Rutile TiO2/CoSe nanocomposite: An efficient photocatalyst for
photodegradation of model organic dyes under visible light
irradiation
Swapnil K. Warkhade, Sangesh P. Zodape, Umesh R. Pratap, Atul
V. Wankhade
PII:
S0167-7322(18)34309-5
DOI:
Reference:
https://doi.org/10.1016/j.molliq.2018.12.119
MOLLIQ 10195
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
20 August 2018
30 November 2018
22 December 2018
Please cite this article as: Swapnil K. Warkhade, Sangesh P. Zodape, Umesh R. Pratap,
Atul V. Wankhade , Rutile TiO2/CoSe nanocomposite: An efficient photocatalyst for
photodegradation of model organic dyes under visible light irradiation. Molliq (2018),
https://doi.org/10.1016/j.molliq.2018.12.119
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ACCEPTED MANUSCRIPT
Rutile TiO2/CoSe nanocomposite: An efficient photocatalyst for photodegradation of model
organic dyes under visible light irradiation
Swapnil K. Warkhade, Sangesh P. Zodape, Umesh R. Pratap, Atul V. Wankhade*
Department of Chemistry, Visvesvaraya National Institute of Technology, Nagpur, India
Email: - atulwa2006@yahoo.co.in
Abstract:
Here we propose new type II semiconducting nanocomposite TiO2/CoSe for effective separation
of the photo-induced charges, thus enhancing the visible light harvesting capability of TiO2. The
formation of TiO2/CoSe nanocomposite is confirmed by XRD, FE-SEM, HR-TEM, XPS and
BET characterization techniques. Remarkable results were obtained during photodegradation
study of both Rhodamine B (99.86 %) and Methylene Blue (97.85 %) within just 90 min of
.-
visible light irradiation. The mechanistic investigation demonstrates that the Superoxide (O2 )
radicals is most reactive species participates in photocatalytic dye degradation. The synthesized
photocatalyst has a good reusability, it can be successfully reuse for six batches of photocatalytic
reactions.
Keywords; TiO2/CoSe nanocomposite, Type II Semiconductor, Photocatalysis, Dye degradation,
1. INTRODUCTION:
Since the discovery of photocatalytic degradation of virulent organic pollutants using
semiconductor materials, photocatalysis has been globally acknowledged technique for control
environmental remediation [1-3]. TiO2 has attracted unusual attentions among the most of the
semiconductor materials owning to its high catalytic efficiency, stability, and inertness [4].
However, the wide band gap of TiO2 (Eg = 3.2 anatase and 3.4eV rutile) and the faster rate of
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recombination of photo-generated hole and electrons sternly preclude the utilization of visible
light and photocatalytic efficiency of TiO2 [5-6]. In order to alleviate these perpetual problems,
various methods have been developed to emphasize the photocatalytic efficiency of TiO2 under
visible light [7-10]. The various techniques like, fixation of dopant, fabrication of composites
with lower energy band gap semiconductor materials and tuning morphology of nanostructured
materials have been employed to enhance the visible light absorption potential of TiO2 [11-13].
Furthermore, report reveals that the photocatalytic efficiency can also be enhanced by designing
nanocomposite hetero-structures, these nanocomposites provides the hetero-junction for the
separation of photo-induced holes and electrons which reduces the possibility of their
recombination and thereby enhancing the photocatalytic activity [14-17]. To tune the energy
band gap for increasing the visible light harvesting ability, the consolidation of TiO2 with
semiconductor chalcogenides eg. CuS, CdS, CdSe, PbS, and CdTe has emerged as decent
approach to sensitize the TiO2 nanomaterials in the visible region [18-23]. However these Cd and
Pb based chalcogenides are hazardous and suffers from stability issue, which in terms is
detrimental for environment [24]. Indeed, depending upon the electron affinity and alignment of
band gap energy levels, heterojunction nanocomposites are classified in to three types such as
type-I, type-II, and type-III [25]. In case of type-II semiconductor, the conduction band (CB) of
semiconductor 1 (dopant) should be higher than the CB of semiconductor 2 also the valence
band (VB) of dopant should be higher than the VB of semiconductor 2.
In seek of greener and clean approach, for the first time here we intend to fabricate
TiO2/CoSe nanocomposite with type-II band structure, where CoSe will serves as an alternative
to hazardous Cd and Pb based photosensitizers. The narrow band gap of CoSe (1.8 eV), high
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stability and ability of CoSe to tailor the wide band gap of TiO2 has pampered us to use the CoSe
nanomaterials to sensitize TiO2.
This synthesized nanocomposite catalyst has potential to harvest visible light and helps in
photodegradation of Rhodamine B dye and Methylene Blue dye as the model organic pollutant.
The synthesized TiO2/CoSe nano-photocatalyst shows propitious photocatalytic performance
over the TiO2 and CoSe by harvesting more visible light.
2 Experimental:
2.1 Chemicals
Selenous acid (10 mmol aqueous solution of SeO2), Cobalt Chloride (CoCl2.6H2O), Cetyl
trimethyl-Ammonium Bromide (CTAB) and all other chemicals were purchased from Sigma-
Aldrich and used without further purification. The stock solution of 0.5 g/L of Rhodamine B
(RhoB) and methylene blue (MB) was prepared by addition of appropriate amount of dye in
deionized water. This stock solution was further diluted for the study of batches of kinetics
experiments.
2.2 Synthesis of TiO2 Nanomaterials;
For the synthesis of TiO2 nanomaterials, the hydrothermal method was adopted. In the
typical synthesis, 5 mmol of TiCl3 was slowly dissolved in 100 ml of deionized water kept in ice
water bath, to this 1 mol % of CTAB was drop wise added and stirred for 60 minutes. The milky
white colored reaction mixture was then transferred to Teflon-lined stainless-steal autoclave and
thermally treated at 140 ºC for 24 h. The white precipitate formed was washed several times with
deionize water to ensure the complete removal of chloride ions. Finally the white powder of TiO2
was dried overnight at 70 ºC under vacuum.
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2.3 Synthesis of TiO2/CoSe nanocomposite;
For the synthesis of layered TiO2/CoSe nanocomposite, 2 mmol of TiO2 powder was
disseminating in 50 ml of deionized water and stirred for 10 min to form homogeneous
dispersion and sonicated for 30 min. To this solution, 1mmol each of CoCl2.6H2O and selenous
acid were added with constant stirring. The pH of the reaction mixture was maintained in range
of 8-10 with help of NH4OH, and was further sonicated for another 30 min to immobilized CoSe
on the charged surface of TiO2. In order to improve the crystallinity of CoSe, reaction mixture
was again subjected to hydrothermal treatment in Teflon-lined stainless-steal autoclave at 140 ºC
for 24 h. The black colored precipitate obtained was separated out by centrifugation and washed
several times with deionized water followed by ethanol. Finally, the precipitate was dried in
vacuum oven at 70 ºC for overnight.
2.4 Characterizations;
X-Ray Powder Diffraction (model AXS D8 Advance Bruker, Germany with Cu-K α
radiation [λ=1.54056Å].) technique was used to illustrate the phase purity of synthesized
TiO2/CoSe nanocomposite. To study the morphological pieces of evidence, Field-Emission
Scanning Electron Microscope (JEOL Model JSM-7600F) and High-Resolution Transmission
Electron Microscopy (instrument Tecnai G2, F40 with 300kv operation voltage) are used to
elucidate shape and size. The Diffusion Reflectance Spectroscopy (DRS) of synthesized
materials was studied by using UV-2401 Shimadzu spectrophotometer. The Brunauere–
Emmette–Teller (BET) analysis of N2 adsorption study was performed by using Micromeritics
ASAP 2020 V3.04H instrument at –196°C. XPS study of synthesized material was studied by
using a Perkin-Elmer PHI 5300 ESCA instrument. The photocatalytic degradation of dye was
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studied by using UV-Visible spectrophotometer (Shimadzu UV 1800 at wavelength λmax 554
nm).
2.5 Photocatalytic activity of TiO2/CoSe nanocomposite:
The photocatalytic efficiency of synthesized TiO2/CoSe nanocomposite was investigated by
using RhoB and MB as model organic pollutant under visible light irradiation. The
photocatalytic performance of catalyst was studied using homemade photo-reactor (30cm H x 25
cm W x 25 cm L) with centralize magnetic stirrer and 250 ml round bottom flask. Just one 100W
tungsten filament lamp (Intensity approx. 28000lux) was used as a source visible light, which
was placed at the distance of 10cm from the bottom of round bottom flask to minimize the heat
transfer.
2.6 Photo-degradation of the dye by using photocatalyst;
For photocatalytic degradation, 0.5 g/L (optimized) of synthesized photocatalyst was
disseminating in 100 ml of 0.06 g/L aqueous solution of RhoB dye. Before irradiation, this
solution was kept in dark with constant stirring for 30 min to achieve the maximum adsorption-
desorption equilibrium, followed by irradiation with visible light. After the regular interval of 15
min. of visible light irradiation, 5 ml aliquots of the dye solution was centrifuged, and the
concentration of residual dye solution was checked at 554 nm and 664 nm for RhoB and MB
solution respectively. Additionally, in order to explore the effectiveness of TiO2/CoSe
nanocomposite nanostructure comparative photocatalytic study was carried out with bare
counterparts of synthesized material (TiO2 and CoSe). The photocatalytic efficiencies (PE) of the
photocatalysts for the degradation of RhoB were calculated by using equation (1).
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푪풐 − 푪
푪풐
( )
푷푬 % =
× ퟏퟎퟎ
(ퟏ)
Where, 푪풐 is the initial concentration of dye and 푪 is the concentration of residual dye solution
after t minuets of irradiation time.
2.7 Optimization of dose of TiO2/CoSe nanocomposite ;
To study the effect of dose of TiO2/CoSe photocatalyst on photo-degradation of RhoB
dye, batches of experiment were carried out by varying the amount of photocatalyst in the range
of 0.10 to 0.70 g/L, whereas the concentration of RhoB dye solution (0.06 g/L) and temperature
of reaction were kept constant.
2.8 Optimization of concentration of Dye;
To optimize the concentration of aqueous solution of RhoB dye, the sets of photocatalytic
experiments were carried out by varying the dye concentration in the range of 0.01-0.08 g/L,
where the optimized amount of photocatalyst (0.5 g/L) was used under ambient conditions.
2.9 Photocatalytic mechanistic investigation;
To explore the exact mechanistic pathway of photocatalytic degradation of RhoB dye
using the TiO2/CoSe nanocomposite under visible light irradiation, 0.1mmol of quenchers like
ammonium oxalate (AO), i-propyl alcohol (IPA), silver nitrate (SN) and benzoquinone (BQ)
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were added for trapping the hole (h+VB), hydroxyl radical (OH˙), electron (e-) and superoxide
radicals (O2˙-) respectively.
2.10
Reusability of TiO2/CoSe nanocomposite;
To scrutinize the reusability of synthesized TiO2/CoSe nanocomposite, it was reused for 6
successive photodegradation cycles for the degradation of RhoB dye. Before reusing, the catalyst
was washed several times with distilled water followed by absolute ethanol and was dried in hot
air oven at 70 ºC.
2 RESULTS AND DISCUSSION;
2.1 Structural and morphological study;
2.1.1 Powder X-Ray Diffraction Technique;
In order to explore the structure and phase purity of synthesized TiO2/CoSe
nanocomposite powder X-ray diffraction technique was used; the results are given in figure 1.
The figure displays the comparative diffraction patterns of TiO2/CoSe nanocomposite and its
bare counterparts (Rutile TiO2 and CoSe) synthesized by same protocol. The diffraction patterns
of synthesized bare rutile TiO2 and CoSe are well matched with the standard JCPDS (894202
and 892004 respectively) data with no other phase impurity. In case of synthesized TiO2/CoSe
nanocomposite, the diffraction pattern exhibits peaks at 2θ 26.79º, 35.56º, 40.80º, 43.20º, 53.56º,
55.68º and 64.82 which corresponds to (110), (101), (111), (210), (211), (220) and (002) planes
of rutile TiO2 respectively, while the peaks at 2θ 29.26º, 31.52º, 44.93º, 51.29º, 58.81º and 62.40º
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corresponds to (100), (101), (102), (110), (200) and (112) planes of CoSe respectively. After the
closer investigation of PXRD, intensity of peaks of TiO2 was found to be weaker in case of
TiO2/CoSe nanocomposite, Which can be attributed to the assorted growth of CoSe nanomaterial
over synthesized TiO2 nanomaterials, which results in the diminished intensity of the diffraction
peaks of TiO2 and lowering of lattice constant of TiO2 in TiO2/CoSe nanocomposite as compare
to bare TiO2.While the dramatically enhancement in intensity of characteristic peak for (100)
plane of CoSe was observed which suggest the growth of CoSe along 100 plane. Additionally,
figure 1 shows the slight shifting of diffraction peaks in case of synthesized TiO2/CoSe
nanocomposite as compared to bare TiO2 and CoSe, which suggest the formation of hetero-
junction among CoSe and TiO2 nanomaterials. From the above discussion here we conclude
successful formation of TiO2/CoSe nanocomposite, which is further supported by SEM and TEM
analysis.
The particle size of synthesized TiO2/CoSe nanocomposite, bare TiO2 and CoSe was
calculated by using the Debye-Scherrer equation [D=0.9λ/β Cos θ], whereas, the λ is wavelength
of X-Ray radiation (1.5406Å), β is full-width half maxima (FWHM), whereas θ is for Braggs
angle of X-Ray diffraction peak. The particle size of bare counterparts TiO2 and CoSe was found
to be 25.31 nm and 21.14 nm respectively, whereas the particle size of TiO2/CoSe
nanocomposite was found to be some escalate (59.52 nm) due to the formation of hetero-junction
in between CoSe and TiO2 nanomaterials (Table 1).
2.1.2 Morphological study;
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To investigate the intact morphology of synthesized TiO2/CoSe nanocomposite field
emission scanning electron microscopy (FESEM) technique was used. Figure 2A and 2B reveals
the formation TiO2/CoSe nanocomposite, with average diameter between 100-150 nm and length
ranging in between 50-100 µm. As can be seen in figure, there is formation of mix asymmetric
nano rods of TiO2 consolidated with haphazard agglomerated spherical CoSe nanomaterials in
synthesized TiO2/CoSe nanocomposite. The elemental composition of the synthesized
TiO2/CoSe nanocomposite was analysed by Energy Dispersive Spectroscopy (EDAX) as given
in figure 2C. The figure 2C (inset) reveals the presence of Ti, O, Co, and Se in the synthesized
TiO2/CoSe nanocomposite materials and their respective atomic percentile. The formation for
TiO2/CoSe nanocomposite and their evidences were examined by using TEM and HR-TEM
analysis as portrayed in figure 3. TEM (figure 3A) analysis of synthesized TiO2/CoSe
nanocomposite reviles that the average 65 nm sized CoSe nanoparticles were randomly
distributed over surface of TiO2 nanorods (average size 100 nm). The highly contrast TEM
image exhibits consolidation among TiO2 and CoSe, which endorses the successfully formation
hetero-junction in TiO2/CoSe nanocomposite. Further, in order to search the strong pieces of
evidence of formation of TiO2/CoSe nanocomposite, here we studied synthesized nanocomposite
by using HRTEM analysis. The HRTEM image of synthesized TiO2/CoSe nanocomposite
decisive the random non-uniform distribution of two different consecutive lattice fringes as
shown in figure 3B. The inter-planer distance between two lattice fringes was found to be 0.33
nm which resembles to the (110) plane of rutile TiO2 whereas 0.30 nm inter-planer distance was
corresponds to the (100) plane of CoSe.
2.2 DRS studies;
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Diffusion reflectance spectroscopy (DRS) was used to execute the energy band gap of
synthesized TiO2/CoSe nanocomposite and their bare counterparts. Figure 4 unveils the distinct
absorption edge of synthesized bare rutile TiO2 and CoSe was found to be 3.4 eV in UV region
and 1.95 eV in visible region respectively. Due to the wide energy band gap (3.4 eV) in between
conduction and valence band of rutile TiO2, it shows the lesser photocatalytic activity in visible
light region. Additionally, The CoSe has lower energy band gap (1.9 eV) which is liable for
faster recombination of photogenerated holes and electrons resulting in decreased photocatalytic
efficiency. The absorption edge of synthesized TiO2/CoSe nanocomposite was found to be 2.7
eV which enables it to absorb light in visible region. This shifting of absorption edge for
TiO2/CoSe nanocomposite (2.7 eV) is the indication of the formation of type-II nanocomposite,
which increases the interfacial contact between TiO2 and CoSe nanoparticles, which facilities the
trapping of one of the charge carriers in TiO2 while other is transferred to the CoSe, resulting in
diminished rate of recombination. Thus empowers the visible light harvesting capacity of
synthesized TiO2/CoSe nanocomposite and anticipates as a preferable photocatalyst under visible
light irradiation as compared to other.
2.3 X-Ray photoelectron spectroscopy (XPS);
The XPS analysis was studied to explicate the chemical state and surface composition of
synthesized TiO2/CoSe nanocomposite. The full-scale XPS spectra of TiO2/CoSe nanocomposite
show the presence of Ti, O, Co, and Se. The binding energies of Ti 2p, O 2p, Co 3p and Se 3d
respectively are portrayed in figure S1. After the deconvolution of Ti 2p XPS signal, two
stronger peaks were observed at binding energy of 465.01 eV and 459.1 eV which corresponds
to the spin-orbital splitting of photoelectrons Ti 2p1/2 and Ti 2p3/2 respectively [figure 5A]. The
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peak severance of 5.91 eV in between the Ti 2p1/2 and Ti 2p3/2 gives the excellent agreement of
presence of Ti in Ti [IV] state [26]. Two main reported spin-orbital splitting of photoelectrons
for bare O1s were found at 530.90 eV and 529.60 eV respectively. Whereas, in synthesized
TiO2/CoSe nanocomposite, the binding energy of O1s was found at 532.4 eV and 531.5 eV
respectively [figure 5B] and the single peak of O1s strongly supports the presence of one type of
oxygen [27]. The deconvolution of Co 2p and Se 3d XPS signal shown in the figure 5C and
figure 5D respectively, the spin-orbital splitting of Co 2p1/2, comes to the binding energy 795.82
eV and 793.45 while 781.10 eV, 778.20 eV for Co 2p3/2 and 55.40 eV binding energy
correspond to the Se 3d5/2 orbital. However the binding energy observed in the XPS analysis
corresponds to the Ti, O, Co and Se elements has slightly shifted as compared to the previously
reported binding energy of bare counterparts (TiO2 and CoSe) [28]. This might be due to the
migration of electron from CoSe nanomaterials to TiO2 nanomaterials, which strongly endorses
the formation of heterojunction among TiO2 and CoSe nanomaterials.
2.4 Fourier transformed Infra-red spectroscopy;
The FT-IR of synthesized TiO2/CoSe nanocomposite is depicted in figure 6A, the
transmission peaks at 3750 cm-1 and 1680 cm-1 is for stretching and bending of hydroxyl group.
The presence of broad peak in the region of 3400 to 2750 cm-1 can be attributed to the uniform
distribution of hydroxyl group and illustrations the stronger interaction of hydroxyl group on
TiO2 with CoSe. The sharp and intense peak was observed in the region 462 cm-1 which
indicates the bonding between the Co-Se [29], along with these two sharp peaks were observed
at 1180 cm-1 and 1010 cm-1 which correspond to the Co-O interaction between TiO2/CoSe
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nanocomposites. While in case of the bare TiO2 these significant peaks were missing in finger
print region as given in supplementary information (Figure S 2)
These FE-SEM, EDAX, TEM, HR-TEM, DRS, XPS and FTIR results demonstrated the
successfully formation of hetero-structured TiO2/ CoSe nanocomposite.
2.5 Brunauere–Emmette–Teller (BET);
The Brunauere–Emmette–Teller (BET) analysis of synthesized TiO2/CoSe nanocomposite
catalyst was analyzed by measuring the amount of adsorption-desorption of N2 over the surface
of catalyst as given in figure S3. The specific surface area of as-synthesized TiO2/CoSe
nanocomposite catalyst was found to be 5.0913 m²/g. BJH model was used for pore size and
pore volume analysis. Where, the micropours nature of synthesized photocatalyst was confirmed,
the pore volume and pore size were found to be 0.017783 cm³/g and 1063.446 Å respectively.
2.6 Photocatalytic Activity;
2.6.1 Photocatalytic study using RhoB dye;
By considering the pervasive hazardous effect of dyes in industries effluents, here in we
appraise the worth of synthesis of TiO2/CoSe nanocomposite nanostructure for the photocatalytic
degradation of the aqueous solution of RhoB under visible light irradiation. The gradual decrease
in the absorbance of dye solution was observed with irradiation time as given in figure S4. This
can be attributing to decrease in the concentration of RhoB dye without formation of any other
side products. The figure 6B shows the comparatively photocatalytic activity of synthesized
TiO2/CoSe nanocomposite and their bare counterparts respectively. The photocatalytic efficiency
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of bare TiO2 and CoSe were found to be just ~ 15% and 18 % respectively, whereas the
photocatalytic degradation efficiency for RhoB using TiO2/CoSe nanocomposite was found to be
significantly higher (99.86%) within 90 min of visible light irradiation. The synthesized
TiO2/CoSe nanocomposite alleviates all perpetual problems and certifies the fact that
photocatalytic efficiency of TiO2 can be boost by integration of TiO2 nanomaterials with lower
energy band gap semiconductor materials. To ascertain the photodegradation, we have
performed the batch experiments in dark; it takes about 30 min for the attainment of adsorption-
desorption equilibrium. After 30 min, there is no significant decrease in dye concentration due to
adsorption. These results attribute that the synthesized nanocomposite has capacity to adsorbed
dye solution accompanied by wide potential of photodegradation. Additionally, in order to check
the possibility of degradation of dye due to photosensitization, the experiment in absence of
photocatalyst (TiO2/CoSe nanocomposite) was carried out with an irradiation of visible light.
The results reveals that there is no significant drop in the concentration of dye even on prolong
irradiate with visible light, which endorses the degradation of dye was carried out by
photocatalysis rather than visible light photosensitization of RhoB. Comparative investigation of
photocatalytic degradation of MB under similar reaction condition and optimized dose of
photocatalyst results in 97.85% photocatalytic degradation of MB in 90 min, as shown if figure
6C. The kinetics of photodegradation of RhoB and MB solution was studied by using integral
form of pseudo first order kinetic equation to examine the rate constant.
풍풏 (푪푪풐) = −풌풕
(ퟐ)
Where, Co is initial concentration of dye after 30 min adsorption, C is the concentration of dye at
t time interval and k is the rate constant. The logarithmic plot in between ln(C/Co) and time t was
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found to be linear with higher regression factor (R2 = 0.995) which endorses the pseudo first-
order kinetics for the photodegradation of RhoB solution as given in figure 6D. The rate constant
for photodegradation of RhoB was found to be 5.4E-2 min-1, whereas the rate constant for
photodegradation of methylene blue was also comparable (4.8E-2 min-1). Additionally, there is
no change in the diffraction pattern and morphology of synthesized TiO2/CoSe nanocomposite
even after 6 cycle of photodegradation (Figure S5 and Figure S6 respectively), which endorses
the higher chemical and physical stability of the synthesized TiO2/CoSe nanocomposite.
2.7 Type II nanocomposite;
The effectual passivation of CoSe on the TiO2 (TiO2/CoSe nanocomposite) has been carried
out in present work which not only increases the interfacial contact between TiO2 and CoSe, but
also helps to suppress the recombination of photogenerated holes and electrons and thereby
enhancing spectral response towards longer wavelength. The band gap position of (valence band
and conduction band) of synthesized TiO2/CoSe nanocomposite was calculated by using the
following equations [30].
푬푪푩 = 푿 − 푬ퟎ − ퟏퟐ 푬품
(ퟑ)
(ퟒ)
푬푽푩 = 퐄푪푩 + 푬품
Where 푬푪푩 is conduction band energy, 푿 is the electronegativity of material (geometric mean of
absolute electronegativity of each atom), 푬품 band gap, 푬ퟎ is the scaling factor of reference
hydrogen electrode (푬ퟎ = −ퟒ. ퟓ풆푽) and 푬푽푩 is the valence band energy. By using these
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equations, the band gap positions of conduction band and valence band was calculated, to
calculate these positions of synthesized TiO2/CoSe nanocomposite we used the band gap of bare
TiO2 and CoSe. From these calculated results (Table 2), we conclude that the energy of valence
band of CoSe lies in between the valence band and conduction band of TiO2, while the
conduction band of CoSe lies above the conduction band of TiO2, which results in the formation
of type II semiconductor. The CB and VB positions of synthesized TiO2/CoSe nanocomposite
and the mechanistic migration of electrons and hole are explained in figure 7.
The photo-induced electrons plausibly move from CB of CoSe to CB of TiO2 on irradiation of
visible light which cases the accumulation of electrons in CB of TiO2 which results in the
effectively separation of photoinduced charges through hetero-junction. Thus, the enhancement
in the photocatalytic activity can be attributed to the synergetic effect of both formation of
hetero-junction and presence of type II energy band gap in synthesized nanostructure this
ultimately leads to enhancing in photocatalytic efficiency.
2.8 Optimization of dose of Photocatalyst and Dye concentrations;
3.8.1 Optimization of Photocatalyst
To investigate the influence of amount of photocatalyst on the photocatalytic efficiency
herein we used various amount of photocatalyst ranging from the 0.1 to 0.7 g/L under ambient
conditions. The result obtained during the batches of experiments are manifest in figure S7, the
figure reveals that initially there is increase in the photocatalytic efficiency with increases in the
dose of photocatalyst which is due to increase in active sites of catalyst. While above the
optimized dose of 0.5g/L of photocatalyst the photocatalytic efficiency decreases, this might be
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due to agglomeration and sedimentation of catalyst which inhibits the photon absorption. The
screening effect by photocatalyst and opacity of dye solution possibly shields the penetration of
visible light which results in the decrease in the photocatalytic efficiency.
3.8.2 Optimization of Dye Concentration;
To optimization of RhoB concentration and its effect on the photocatalytic efficiency,
batches of experiments were carried out by varying dye concentration (0.01-0.08g/L) with
optimized amount of photocatalyst (0.5g/L) as shown in figure S8. Initially, there is enhancement
in the photocatalytic degradation up to 0.06g/L of RhoB solution, further the degradation
efficiency of photocatalyst gradually decreases with increase in the concentration of dye
solution, and this can be attributed to increase in opacity of dye solution which possibly absorbs
more visible light than the photocatalyst.
3.9 Mechanistic investigation of dye degradation;
For investigating mechanistic pathway, the degradation of RhoB dye under visible light
irradiation using the TiO2/CoSe nanocomposite as a photocatalyst was studied. The CoSe
nanoparticles of composite could possibly allocate the photon energy to the TiO2 nanorods by
promoting the photo-induced electrons from CB of CoSe to the CB of TiO2 resulting into the
better charge separation. Thus, the synthesized nanocomposite possesses the capability of
retarding the rate of recombination of photoexcitons. As evident from quencher study, the
migration of photoelectrons on the surface of composite leads to formation of highly active
superoxide (O2˙-) radicals. These superoxide radicals have potential to decompose the RhoB dye
and convert into CO2 and water as final degradation product. The identification of contributing
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reactive species involved in degradation is most important to accomplish the detail mechanistic
route of degradation. Thus, in order to understand degradation pathway, a series of quenchers
were used to identify reactive species involved in photodegradation, here we used ammonium
oxalate (AO) to trap the holes (h+, VB) in valence band, silver nitrate (SN) for trapping electrons
(e+, CB) in conduction band, benzoquinone (BQ) for trapping the superoxide (O2˙-) radicals and
i-PrOH (IPA) for hydroxyl (OH˙) radicals. Figure 8 reveals there is decrease in photocatalytic
efficiency as we add quenchers in dye solution. There is slight decrease in photocatalytic
efficiency on the addition of SN quenchers as shown in figure, this assigns that electrons are
hardly contributing (5%) in photocatalytic degradation. While in case of addition of IPA and AO
there is about 11% and 14% decrease in photocatalytic efficiency respectively, which reveals that
holes (h+, VB) and hydroxyl (OH˙) radicals actively participates in photocatalytic activity but are
acting as secondary reactive species. However, in case of BQ, there is drastic decreased in of
photocatalytic efficiency (70%) which confirming that the superoxide (O2˙-) radicals are the
primary reactive species for degradation of dye.
Thus, from above results here we conclude that the superoxide (O2˙-) radicals are the main
reactive species involved in the photocatalysis while holes (h+, VB) and hydroxyl (OH˙) slightly
contribute to photocatalysis and electrons have the least contribution in photodegradation of
RhoB dye.
3.10 Reusability of photocatalyst;
To study the reusability of synthesized photocatalyst we performed the batches of
experiment by using the same photocatalyst and photocatalytic degradation results were
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portrayed in figure 9. We have reusing photocatalyst which actively degrade dye up to 6 cycles
with small drop in of degradation efficiency (~6%). From this result, we conclude that synthesis
TiO2/CoSe nanocomposite is one of the promising photocatalysts with good reusability and
lower operation cost.
4
Conclusion;
Present work provides the ideal strategy to retard the recombination of charge carriers and
band gap engineering of TiO2 in order to harvest visible light by constructing the CoSe/TiO2
nanocomposite. The enhancement in the photocatalytic activity of TiO2/CoSe nanocomposite as
compare to the bare counterpart on irradiation of visible light strongly supports possibility of
lowers in the recombination of photoinduced charge carriers and enhancing the capability to
harvest visible light. The formation of superoxide (O2˙-) reactive species was most prominent
during the photocatalytic experiment which helps in the degradation of dye in quenching
experiments. These factors have possibly boosted the photocatalytic efficiency of synthesized
TiO2/CoSe nanocomposite photocatalyst and offers upper hand over bare TiO2 as a photocatalyst
in waste water treatment. Constructing such nanocomposite will open new arena for developing
band gap engineered semiconductor nanostructures for visible light harvesting and other
environmental applications.
5
Acknowledgement;
We would like to thank Director, Visvesvaraya National Institute of Technology, Nagpur, India,
for providing laboratory facility. We are also thankful to Sophisticated Test and Instrument
Facility, Cochin University, Kerala, and Sophisticated Test and Instrument Facility, IIT Bombay
for providing instrument facility.
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Figure caption;
Figure 1: Comparative X-Ray diffraction (XRD) study of synthesized TiO2/CoSe
nanocomposite and bare counterpart (CoSe and TiO2).
Figure 2: Study of Scanning Electron Microscope (SEM) of synthesized TiO2/CoSe
nanocomposite and their Energy Dispersive Spectroscopy (EDAX).
Figure 3: A] Study of Tunneling Electron Microscope (TEM), B] High-resolution Transmission
Electron Microscope (HR-TEM) and inset SAED pattern of synthesized TiO2/CoSe
nanocomposite.
Figure 4: Comparative study of UV-Absorbance of synthesized TiO2/CoSe nanocomposite and
bare counterpart (CoSe and TiO2).
Figure 5: XPS analysis of synthesized TiO2/CoSe nanocomposite and their deconvolution of
respective elements.
Figure 6: A] Infrared spectra of TiO2/CoSe nanocomposite, B] Comparative photocatalytic
degradation of Rhodamine B dye solution by using TiO2/CoSe nanocomposite and there
counterpart (CoSe and TiO2), C] Comparative photocatalytic degradation of Rhodamine B and
Methylene blue dye solution and D] Comparative degradation rate of Rhodamine B and
Methylene blue dye solution.
Figure 7: Diagrammatic representation of synthesized type II TiO2/CoSe nanocomposite with
the mechanistic rout of dye degradation.
Figure 8: Graphically representation of effect of quenchers on the photocatalytic efficiency of
synthesized TiO2/CoSe nanocomposite.
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Figure 9: The reusability and rejuvenation of synthesized TiO2/CoSe nanocomposite during the
cycles of photocatalysis.
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Table No. 1: Comparative particle size calculations for synthesized TiO2/CoSe nanocomposite
and their bare counterparts by using Debye-Scherrer equation [D = 0.9λ/βCos θ].
Table No. 2: Calculated CB and VB Positions of TiO2 and TiO2/CoSe nanocomposite
respectively.
Sr. No.
Nanomaterials
Particle size was calculated from Debye-Scherrer
equation
[D = 0.9λ/βCos θ]. In (nm)
21.14
1
2
3
CoSe
TiO2
25.31
TiO2/CoSe
59.52
Table No. 1
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Table No. 2
Sr. no
Sample
TiO2
VB (eV)
3.33
CB (eV)
-0.063
1
2
TiO2/CoSe
3.33,0.41
-0.063,-1.29
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Highlights:



Rutile TiO2 nanorods decorated with CoSe nanoparticles.
TiO2/CoSe nanocomposite: A visible light harvesting type II semiconductor.
Enhanced photocatalytic efficiency is attributed to the retarded rate of recombination of
photoexcitons.

Superoxide radical is most reactive species responsible for the photodegradation.
28

Figure 1