Convenient architectures of Cu2O/SnO2 type II p-n heterojunctions and their application in visible light catalytic degradation of rhodamine B

Article abstract of DOI:10.1039/c6ra07150c

3D hierarchical heterostructures of p-type Cu₂O/n-type SnO₂ were synthesized by impregnating porous SnO₂ nanospheres with different weight percentages of Cu₂O by a simple hydrothermal route. The heterostructures were then evaluated for their photocatalytic performance under visible light irradiation for degradation of rhodamine B (RhB), an organic pollutant. In addition, the effect of varying Cu₂O content, and thereby the composition, morphology and photocatalytic activity of the Cu₂O/SnO₂ nanocomposite were investigated. XRD revealed that the Cu₂O/SnO₂ nanocomposite comprised of cassiterite tetragonal structured SnO₂ and cubic structured Cu₂O. HR-TEM and FE-SEM of the as synthesized nanocomposite indicated nearly monodispersed nanospheres with an average size around 60 nm. A UV-vis absorption edge of the Cu₂O/SnO₂ nanocomposite synthesized in a 1:1 ratio was observed at 484 nm (E_g ～ 2.56 eV) that indicated considerable absorption in the visible region of the solar spectrum. An enhancement in photocatalytic activity was observed with an increase in Cu₂O impregnation and the complete photo-oxidation of RhB was possible with the 1:1 mole ratio Cu₂O/SnO₂ nanocomposite in 90 min under optimized conditions. Photocatalytic activity of the Cu₂O/SnO₂ nanocomposite for the degradation of RhB was much higher than that of individual nanoparticles, primarily attributed to the anti-recombination effect of photogenerated electrons and holes at the junction due to formation of p-n heterojunctions. Identification of intermediates and a detailed mechanism of the photo-oxidation process was accomplished by using analytical techniques such as high-performance liquid chromatography [HPLC] and UV-vis diffuse reflectance [DR] spectroscopy.

Full text of DOI:10.1039/c6ra07150c

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Convenient Architectures of Cu O/SnO Type II p-n
2
2
Heterojunctions and their Application in Visible
Light Catalytic Degradation of Rhodamine B
†
a, b, d
a, b, c
b, d
Shravanti Joshi,
Samuel J. Ippolito,
and Sunkara V. Manorama
*
a
Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Science, College
of Science, Engineering & Health, RMIT University, Melbourne, VIC 3001, Australia.
b
RMITꢀIICT Research Centre, CSIRꢀIndian Institute of Chemical Technology, Hyderabad ꢀ 500
007, India.
c
School of Engineering, RMIT University, Melbourne, VIC 3001, Australia.
d
Nanomaterials Laboratory, Inorganic & Physical Chemistry Division, CSIRꢀIndian Institute of
Chemical Technology, Hyderabad ꢀ 500 007, India. Eꢀmail address: manorama@iict.res.in, Tel.:
91 40 27193225; Fax: +91 40 27160921.
+
*
To whom all Correspondences should be addressed
1

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Abstract
3
D hierarchical heterostructures of pꢀtype Cu O/nꢀtype SnO were synthesized by
2 2
impregnating porous SnO nanospheres with different weight percentages of Cu O by a simple
2
2
hydrothermal route. The heterostructures were then evaluated for their photocatalytic
performance under visible light irradiation for degradation of Rhodamine B (RhB), an organic
pollutant. In addition, the effect of varying Cu O content, and thereby the composition,
2
morphology and photocatalytic activity of Cu O/SnO nanocomposite were investigated. XRD
2
2
revealed that the Cu O/SnO nanocomposite comprised of cassiterite tetragonal structured SnO
2
2
2
and cubic structured Cu O. HRꢀTEM and FEꢀSEM of the as synthesized nanocomposite
2
indicated nearly monodispersed nanospheres with an average size around 60 nm. A UVꢀvis
absorption edge of Cu O/SnO nanocomposite synthesized in 1:1 ratio was observed at 484 nm
2
2
(Eg ~2.56 eV) that indicated considerable absorption in the visible region of solar spectrum. An
enhancement in photocatalytic activity was observed with increase in Cu O impregnation and the
2
complete photoꢀoxidation of RhB was possible with the 1:1 mole ratio Cu O/SnO2
2
nanocomposite in 90 min under optimized conditions. Photocatalytic activity of the Cu O/SnO2
2
nanocomposite for the degradation of RhB was much higher than that of individual
nanoparticles, primarily attributed to the antiꢀrecombination effect of photogenerated electrons
and holes at the junction due to formation of pꢀn heterojunctions. Identification of intermediates
and detailed mechanism of photoꢀoxidation process was accomplished by using analytical
techniques such as highꢀperformance liquid chromatography [HPLC] and UVꢀvis diffuse
reflectance [DR] spectroscopy.
2

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1. Introduction
Semiconductorꢀmediated photocatalysis, a well known potential green route for degradation of
toxic chemical pollutants is currently a topic of widespread research especially in environmental
remediation applications. In this context, over the last few years, our group has been involved in
the development of several materials by tailoring the morphologies, controlling crystallinity and
1
ꢀ5
noble metal doping to explore its efficacy for photocatalytic applications. One such material
being tin dioxide (SnO ), which as literature suggests is one of the most extensively studied
2
6
ꢀ9
oxide semiconductor due its wide range applications in the field of energy and environment.
Naturally occurring as nꢀtype semiconducting oxide, SnO has a strong absorption in UV region
2
1
0ꢀ11
with a large bandgap (E ~3.8eV) which limits it application in photocatalysis.
In order to
g
extend its light absorption into the visible region, various strategies have been considered, such
as doping with metals (Au, Ag, Pt, Zn etc) or forming heterojunctions by coupling with other pꢀ
1
2ꢀ17
type or nꢀtype oxide semiconductors.
The former strategy of doping of metals in oxide
semiconductors has been very ineffective and consequently exhibits low photocatalytic
1
8
activity. The latter strategy of forming heterojunctions is anticipated to be advantageous
because the resulting internal field that is builtꢀup when the junction is formed facilitates
5
,19ꢀ21
efficient migration of charge carriers leading to improved photocatalytic performance.
Photocatalysts composed of pꢀn heterojunctions would render optimum performance based on
some primary considerations such as; (i) to achieve highest output from the nꢀtype/pꢀtype oxide
semiconductors, a minimum or ideally zero conduction band discontinuity between the two
materials, (ii) work function of the nꢀtype oxide less than or equal to the electron affinity of the
2
2
pꢀtype oxide, to aid easy transfer of charge carriers across the barrier. Based on these criteria, pꢀ
2
3
type transition metal oxide Cu O (E ~1.9ꢀ2.2 eV) was selected among other materials, to be
2
g
3

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introduced into the SnO matrix to form the pꢀn heterojunction. Further, the valence and
2
conduction bands of Cu O being higher than those for SnO , the coupling of Cu O with SnO
2
2
2
2
would thermoꢀdynamically facilitate the transfer of charge carriers, which would be an added
advantage in enhancing the photocatalytic activity by impeding recombination and extending the
2
4ꢀ26
lifetime of charge carriers.
Recently, Tian and coꢀworkers reported tubeꢀlike ternary αꢀ
Fe O @SnO @Cu O sandwich coreꢀshellꢀshell heterostructures showing enhanced
2
3
2
2
photocatalytic properties on the photodegradation of Rhodamine B (RhB) under simulated
2
6
sunlight irradiation. Albeit such reports, current global situation justifies the search for newer
materials with improved photocatalytic properties. The correlation between photocatalytic
operational parameters, electronic properties (namely band gap (E ), electron affinity (χ) and
g
work function (ϕ)) would all lead to a better understanding of the phenomena, the materials
properties and the structural properties, consequently, aiding the discovery of suitable and better
performing photocatalysts.
In the present study, we report 3D hierarchical heterostructures of pꢀtype Cu O/nꢀtype SnO
2
2
synthesized by means of a simple hydrothermal route where Cu O was impregnated into porous
2
SnO nanoarchitectures. A series of nanocomposites with different weight percentages of Cu O
2
2
were synthesized with the aim of optimizing the composition for best photocatalytic activity.
Rhodamine B (RhB), an organic dye, was used to evaluate the photocatalytic performance of
nanocomposites under visible light irradiation. The fundamental photocatalytic mechanism was
probed by critically evaluating the electronic properties along with the crystallinity and
morphology of the metal oxides. Furthermore, correlating them with the optical properties and
finally the photocatalytic performances, a probable chemical reaction pathway has been proposed
4

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based on the spectroscopic and chromatographic evaluation of the products that were retrieved at
different stages of the reaction.
2. Experimental Section
2.1. Chemicals
All chemicals were of analytical reagent (AR) grade and used without further purification.
Sodium stannate trihydrate (Na SnO .3H O), cupric (II) acetate monohydrate (Cu
2
3
2
(
CO CH ) .H O) and Dꢀ(+)ꢀglucose monohydrate (C H O .H O) were procured from Sigmaꢀ
2 3 2 2 6 12 6 2
Aldrich. Rhodamine B (RhB), an organic dye, was purchased from Molychem. Millipore
deionised water (18.2 MΩ at RT) was used throughout the experimentation.
2.2. Synthesis of SnO
2
Nanospheres
4
Synthesis of SnO nanospheres was carried out by the method reported in our earlier work.
2
Briefly, 1 g of sodium stannate trihydrate was dissolved in 30 mL of deionised water to get 0.125
M stannate solution that was taken in a roundꢀbottom flask followed by addition of 2 g of Dꢀ(+)ꢀ
glucose monohydrate and then kept for stirring at 50 °C. After 12 h of reaction, as formed white
precipitate of SnO nanospheres about 50nm in diameter were collected by centrifugation,
2
washed with deionised water and dried at 60 °C.
2.3. Synthesis of Cu
2
O/SnO Nanocomposite
2
Stoichiometric amount of the above described preꢀsynthesized SnO and cupric (II) acetate
2
monohydrate as starting materials were added to 40 ml of deionised water. This solution was
5

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stirred magnetically for 20 min and ultrasonicated for 60 min to obtain homogeneity, then
transferred to Teflonꢀlined stainless steel autoclave and placed in a programmable oven. The
temperature was raised to 180 °C at a heating rate of 10 °C/min and held at constant temperature
for 3 h. After completion of reaction, the resultant mixture was centrifuged and washed several
times with deionised water to obtain the product that was finally dried in an oven at 60 °C.
2.4 Characterizations
The Xꢀray diffractograms (XRD) of the asꢀsynthesized materials were recorded on
PANalytical Empyrean XRD equipment in the reflection mode using a zero background sample
holder. Incident radiation was generated using a CuKα source, λ = 1.5406 Å and data was
collected in the 2ϴ range, 10 to 80° in continuous scanning mode with a step size of 0.01° using
a Pixel 3D detector. Additionally, transmission electron microscope (TEM) (Philips Technai FEI
F12, operating at 120kV) was used to investigate the morphology of synthesized nanostructures.
The samples for TEM were typically prepared by dispersing the material in water by ultraꢀ
sonication and drop drying onto a formvar coated copper grid. Fieldꢀemission scanning electron
micrographs (FEꢀSEM) were recorded on a JEOL JSMꢀ7610F (JEOL Inc. Japan) and Energy
dispersive Xꢀray analysis (EDAX) performed at 15 kV using OXFORD detector interfaced with
the FEꢀSEM. High resolution TEM image was taken at 200kV on a JEOL 2100F TEM/STEM
with Oxford XꢀMaxN 80T EDXS Detector, Gatan Imaging Filter (EELS Spectrometer) and
Gatan Orius SC1000 CCD Camera. A UVꢀvisꢀNIR spectrophotometer (Varian Cary 5000
U.S.A) was used to record diffused reflectance spectra of solid nanocomposites and absorbance
spectra of dye solutions in the wavelength range of 200ꢀ800 nm. The Xꢀray Photo Electron
Spectroscopy (XPS) spectra were obtained using M/S.SPECS XPS system (M/s.SPECS Surface
6

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Nano Analysis GmbH, Germany) equipped with twin anode Xꢀray source and a hemispherical
analyzer with a singleꢀchannel detector. The spectra were obtained with 150W nonꢀ
monochromatic Al Kα radiation (1486.6 eV). The HPLC data was recorded on a HPꢀVectra
(Hewlett Packard, Waldron, Germany) computer system using ꢀM A diode array detector
20
(PDA) and LCꢀSolution data acquiring software (Shimadzu, Kyoto, Japan). Waters Symmetry
C18 Column (250 × 4.6 mm id; 5ꢁm, Waters, Ireland) was used for separation. Chromatographic
separation was achieved using a mobile phase mixture of ACN: KH PO in water in the ratio of
2
4
4
5: 55 (v/v) and degassed using a vacuum degasser before use. The flow rate was set at 1.0
mL/min and the column was maintained at ambient temperature. The injection volume was 20
L and the detector wavelength was tuned to 535 nm. The specific area was determined by the
ꢁ
surface area and pore size distribution measurements using a Nova 4000e surface area and pore
size analyzer from Quantachrome instruments at liquid nitrogen temperature.
2.5 Photocatalytic Evaluations
In a typical photocatalytic reaction, 30 mg of as synthesized Cu O/SnO nanocomposite was
2
2
ꢀ5
ultrasonically dispersed in 30 mL of aqueous Rhodamine B (RhB) (1 x 10 M) in a quartz
reactor. The solution was magnetically stirred for 30 minutes in dark to equilibrate the
adsorption/desorption of RhB dye on the catalyst surface. After 30 minutes of stirring, the
solution was irradiated with visible light produced by 400 W high pressure sodium vapour lamp
(SAIC, India) whilst continually stirring to ensure uniform exposure of the suspension
o
throughout the process. The reactor temperature was maintained below 4 C by continuously
circulating cold water. At periodic time interval, aliquots (3 mL) of aqueous RhB solution was
withdrawn from the photo reactor and analyzed after centrifugation at 13,000 rpm for 15 min to
7

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separate out any inorganic particles. The absorption spectra of the supernatant of the centrifuged
solutions were recorded in the range 400ꢀ800 nm. The degradation of RhB was monitored by
observing the characteristic absorption peak of RhB at λmax = 554 nm. Controlled photocatalytic
experiment was also performed following identical protocol without the nanocomposite, whose
outcome of negligible degradation of RhB, ascertained that the nanocomposite is solely
responsible for the photodegradation process of the dye. All experiments were conducted at room
temperature and in air. Recyclability was verified by repeating the photodegradation of RhB for
five consecutive cylces under similar conditions. After each cycle, the catalyst was washed with
o
ethanol first then with deionized water followed by overnight drying in an oven at 60 C. To this,
calculated volume of fresh solution of RhB was added before each run of photocatalytic
evaluation.
3. Results and Discussion
3.1. XRD, TG-DTA and FTIR Analysis
The Xꢀray diffractograms of the as synthesized pure SnO and Cu O/SnO composites with
2
2
2
different Cu O mole ratios are presented in Fig. 1. Peaks observed (Fig. 1a) confirm that the
2
synthesized material is SnO with tetragonal cassiterite crystal structure (space group: P4 /mmm,
2
2
JCPDS card no. 21ꢀ1250). The XRD patterns (Fig. 1bꢀe) corresponding to different weight
percentages of Cu O in SnO matrix to form composites in various mole ratios confirmed that
2
2
Cu O impregnation did not significantly alter the structure of the individual oxides. As seen in
2
Figure 1b, the 0.125:1 mole ratio Cu O/SnO does not show any copper oxide peaks. This could
2
2
be either due to very low content or the amorphous nature of copper oxide species that may have
8

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2
7
been dispersed uniformly inside the SnO nanospheres. Beyond mole ratio of 0.125:1, presence
2
of Cu O peaks becomes discernable. The XRD pattern of Cu O/SnO nanocomposite in 1:1 mole
2
2
2
ratio (Fig. 1e), showed peaks corresponding to cassiterite SnO and cubic structured Cu O (space
2
2
group: P4 /mmm, JCPDS card no.21ꢀ1250 and space group: Pn3m (224), JCPDS card no.77ꢀ
2
0
199 respectively). This suggests that Cu O and SnO coexist in the nanocomposite as
2 2
independent phases. The broad nature of the SnO peaks observed here can be ascribed to the
2
4
nanocrystallite nature of the constituent particles. The crystallite size was calculated by taking
the average after applying the Scherrer equation to the major diffraction peaks in the XRD
2
8
pattern. The average crystallite size was 4.5 nm for pure SnO and that for the Cu O/SnO was
2
2
2
o
about 5.7 nm. Slight shift (~ 0.8 ) in (101) peak position towards higher 2ϴ value is observed in
the pattern of Cu O/SnO nanocomposites compared to pure SnO that can be attributed to
2
2
2
2
9
increase in oxygen vacancies, residual stresses or lattice contraction induced by defects or the
2
8
presence of secondary species in the material. Existence of such defects in nanocomposite is
encouraging since, it has been reported extensively that lightꢀmediated activity such as
photocatalysis is predominantly dependent on defects like oxygen vacancies and oxygen
interstitials, in addition to other factors such as phase purity, nature of dopant, method of
3
0
synthesis, shape, particle size, morphology and surface area. Xꢀray diffractograms evinced
absence of peaks due to any other Cu or Sn phase indicating that nanocomposites synthesized
here are composed of SnO and Cu O only. It is worth mentioning here that the copper oxide
2
2
phases became dominant in the nanocomposite with increase in cupric (II) acetate monohydrate
content (Fig. S1†).
Interestingly, we observed that when, only cupric (II) acetate monohydrate was independently
o
treated hydrothermally at 180 C for 3 h, mixed phase of copper oxides was obtained (Fig.
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S2.1†). As synthesized material was confirmed to be CuO with monoclinic tenorite crystal
structure (space group: C2/c (15), JCPDS card no. 80ꢀ0076) with Cu O in minor quantity. This
2
observation suggests that the cupric (II) acetate monohydrate decomposition follows the
26,31
conversion to Cu O and subsequently transforms to CuO.
Here, we anticipated presence of
2
copper oxides in mixed phase with amorphous Cu O in minor percentage. This was further
2
confirmed from TGꢀDTA and FTꢀIR analysis. TG and DTA trace for as synthesized 1:1 mole
o
ratio Cu O/SnO nanocomposite from ambient temperature to 800 C revealed all the above
2
2
stages (Fig. S3†). FTꢀIR analysis also confirmed the formation of phase pure SnO and showed
2
ꢀ1
the presence of Cu O as a broad peak in the range 605ꢀ670 cm (Fig. S4.a†).
2
3.2. FE-SEM, HRTEM and EDS Analysis
Particle size and specific morphology of asꢀsynthesized 1:1 mole ratio Cu O/SnO2
2
nanocomposite were examined by TEM (Fig. 2a), FEꢀSEM (Fig. 2b and Fig. 2c) and HRꢀTEM
Fig. 2dꢀf). The representative TEM and FEꢀSEM images of the nanocomposite shows nearly
(
uniform highly dispersed nanospheres with size around 60 nm (Fig. 2aꢀc). The high
magnification TEM images clearly revealed that each nanosphere is composed of primary
nanoparticles around 2ꢀ6 nm in size (Fig. 2d and Fig. 2e). Morphology of Cu O/SnO2
2
nanocomposite is identical to pure SnO thereby implying that the Cu O inclusion does not alter
2
2
the morphology of the composite. From HRTEM image (Fig. 2f), lattice fringes belonging to two
different interplanar distances of 0.24 and 0.355 nm were observed, which could be assigned to
(
111) plane of Cu O and (110) plane of SnO respectively, in agreement with Xꢀray
2
2
2
6,31,42
diffractograms (Fig. 1).
This image further confirmed the formation of pꢀn heterojunction
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at the interface between Cu O and SnO in addition to establishing efficient assimilation of Cu O
2
2,
2
particles into the SnO nanospheres.
2
Fig. 3a is the typical FEꢀSEM micrograph showcasing the area over which elemental mapping
was effectuated. Elemental mapping of 1:1 mole ratio Cu O/SnO nanocomposite showed
2
2
uniform distribution of the three elements (Sn, Cu and O) throughout the sample (Fig. 3bꢀe).
Quantitative elemental analysis of the nanocomposite displayed nonꢀstoichiometric ratio, and
showed Cu/Sn atomic% as 7.85:12.33 (Fig. 3f). Similarly, the EDAX data of the 0.5:1 mole ratio
composite showed Cu/Sn atomic% as 4.95:13.14 though lower but much closer to calculated
stoichiometry (Fig. S5†). Further, these observations are reflected in Xꢀray diffractograms (Fig.
1
) where the diffraction peaks due to Cu O are of much lesser intensity than the SnO peaks even
2 2
for the nanocomposite synthesized in 1:1 mole ratio. This discrepancy in theoretically estimated
stoichiometry and the analytical data of as synthesized samples can be justified by a simple
explanation. When starting with phase pure SnO nanospheres and followed by incorporation of
2
increasing weight percentages of Cu O, initially at low concentrations as expected all Cu O
2
2
particles get assimilated into the nanospheres and hence are not reflected in the XRD pattern, but
at high concentrations the assimilating ability decreases, primarily attributed to spatial
considerations. The excess Cu O which does not get incorporated into the SnO matrix would be
2
2
expunged during the synthesis process. This leads to heterogeneous distribution and hence
incongruence of Cu O concentration estimated from EDAX compared to theoretical estimates.
2
But for the sake of convenience, throughout the study, the nomenclature used for the material
identification in theoretical estimates would be maintained for all the as synthesized samples.
The N adsorptionꢀdesorption isotherm of the Cu O/SnO nanocomposite in 1:1 mole ratio is of
2
2
2
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type IV with type H4 hysteresis loop (Fig. S6†). The BET surface area of the nanocomposite
2
ꢀ1
calculated from the linear part of multipoint plot was found to be 81.61m .g
3.3. Optical Absorption Properties
UVꢀvis diffuse reflectance spectra of samples were recorded to obtain information pertaining
to photophysical properties of the asꢀsynthesized samples. Reflectance (%R) as a function of
wavelength of the pure SnO nanospheres that translated into an indirect band gap of about Eg
2
~3.75 eV corresponding to an absorption edge at around 331 nm is in agreement with reported
4
value. Also, reflectance (%R) as a function of wavelength of the mixed phase copper oxide,
o
hydrothermally synthesized at 180 C for 3 h, revealed a direct band gap of E ~1.65 eV
g
corresponding to an absorption edge at about 752 nm (Fig. S7†). As expected, the mixed phase
copper oxide and SnO absorb in the visible and UV region, and these results are consistent with
2
10,11,23,26
their standard bandꢀtoꢀband transitions.
Fig. 4 displays the typical plots of absorbance and F(R ) as a function of wavelength for the
∞
Cu O/SnO nanocomposites synthesized in various mole ratios. The pure SnO nanospheres
2
2
2
showed negligible absorption in visible region contrary to the high red shifted absorption
exhibited by Cu O/SnO nanocomposites (Fig. 4a), thus substantiating the impregnation of pꢀ
2
2
type metal oxide, that is, Cu O in SnO matrix. The visible light absorption shown by Cu O/SnO
2
2
2
2
nanocomposite can be attributed to the presence of mixed phase copper oxide with a band gap of
about 1.65 eV, as well as the presence of increased oxygen vacancies and defects in the
2
8ꢀ29
material.
The absorption edge of 1:1 mole ratio Cu O/SnO nanocomposite was observed at
2 2
4
84 nm, corresponding to a calculated band gap of E ~2.56 eV (Fig. 4b). Hence it can be
g
inferred that enhanced absorption in the visible region is maintained upon composite formation
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throughout all mole ratios and more evident in the case of nanocomposite synthesized in 1:1
mole ratio and above. Similar red shift in case of composites prepared by impregnation of pꢀtype
1
9ꢀ21
oxide into nꢀtype oxide or viceꢀversa had been observed by previous researchers.
As the
succeeding results indicate, these light absorption properties of the various composites influence
the photocatalytic performance significantly.
3.4. XPS Analysis
To attest phase purity and confirm the oxidation states of the constituent oxides, the 1:1 mole
ratio Cu O/SnO nanocomposite was further assessed by Xꢀray photoemission spectroscopy
2
2
(XPS) (Fig. 5). The C 1s peak centered at 284.60 eV (characteristic of CꢀC, C=C and CꢀH bonds)
and assigned to aliphatic carbon was used as an internal standard to calibrate all binding energies
for residual surface charge due to photoemission. The survey spectrum of Cu O/SnO
2
2
nanocomposite showed peaks only from Sn, Cu and O (Fig. 5a). Fig. 5b and 5c illustrates the
core level XPS spectra of the Sn and Cu. The Sn 3d level shows peaks at 486.30 eV and 494.90
eV which can be assigned to 3d5/2 and 3d3/2 respectively (Fig. 5b) confirming the +4 oxidation
32ꢀ33
state of Sn in the tetragonal rutile structure of SnO2.
The spin orbital splitting energy of 8.6
3
4
eV is in agreement with reported values. Deconvolution of Cu 2p core level spectrum of copper
at 932.80 eV (Fig. 5c) resulted in two peaks, where peak at 932.5 and 934.07 eV confirmed
2
6,31
presence of Cu in +1 and +2 state respectively.
Cu 2p XPS spectrum represented by the
characteristic 2p3/2 and 2p1/2 peaks centered at 934.07 eV and 953.86 eV respectively indicated
3
5
presence of Cu in +2 oxidation state. Further, the existence of the two shakeꢀup satellite peaks,
one in the range 938ꢀ947 eV (which could be deconvoluted into two Gaussian peaks centered at
941.18 and 943.58 eV) and the other at 961.88 eV in the spectra also reiterates the chemical state
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2
+
36
of copper in nanocomposite as Cu (CuO). As suggested by Liu and coꢀworkers, XPS being a
surface characterization technique and hence capable of detecting species within shallow depths.
The reason why the CuO could not be detected in XRD pattern could be due to low
concentration and/or amorphous nature that too in the interior of the heterostructure. In addition,
oxidation of Cu O to CuO during processing and experimental conditions has been widely
2
3
1,37ꢀ38
reported.
Nevertheless, we anticipate that presence of CuO if any, does not in any way
hinder the photocatalytic performance of the nanocomposite. On the contrary, presence of CuO
3
9ꢀ42
would actually improve the performance as proposed by other researchers.
The
deconvolution of O 1s core level XPS spectrum of oxygen (Fig. 5d), represented by a broad
asymmetric peak resulted into four peaks centered at 529.88 eV, 530.78 eV, 531.98eV and
43
5
32.78 eV. The peak at 529.88 eV can be assigned to lattice oxygen in the Cu O. and the one at
2
5
30.78 eV is due to lattice oxygen in SnO (i.e. SnꢀOꢀSn) which is in good agreement with XRD
2
4
4ꢀ45
data as discussed in preceding section.
The higher energy peak at 531.98 eV could be
2
ꢀ
ꢀ
attributed to the oxygen species such as O or O ions present within the matrix of Cu O/SnO
2
2
4
6ꢀ47
44ꢀ49
nanocomposite,
and the peak at 532.78 eV can be assigned to adsorbed hydroxyl species.
Overall, from the XRD, FEꢀSEM, HRꢀTEM, elemental mapping, XPS, FTIR, microꢀRaman (Fig.
S4.b†) and PL analyses (Fig. S4.c†), we can conclude that the asꢀsynthesized Cu O/SnO2
2
nanocomposite was confirmed to be hierarchical heterostructure consisting of Cu O and SnO2
2
with CuO in small percentages forming well defined intimate pꢀn interfaces.
4. Visible Light Induced Photocatalytic Performance
4.1. Evaluation of Photocatalytic Performance
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The photodegradation of RhB, a facile process was chosen as the model reaction to evaluate
the performance efficacy of the synthesized Cu O/SnO2 nanocomposite as a potential
2
photocatalyst. The characteristic absorption of RhB at 554 nm was used as the indicator to
monitor the degradation activity as a function of irradiation time. Fig. 6a and 6b shows the
temporal evolution of absorption spectra of RhB solution over asꢀsynthesized nanocomposite
under visible light irradiation. Under visible light illumination, limited (< 15%) degradation of
RhB solution in the absence of photocatalyst (Fig. 6a) represents the blank reaction. Fig. 6b
shows the degradation profile of RhB over 1:1 mole ratio Cu O/SnO nanocomposite in 90 min.
2
2
The UVꢀvis absorption spectra of the aliquots drawn from the reaction mix at regular time
intervals shows that the intensity of the absorption peak at 554 nm gradually decreases with
increase in irradiation time. The decrease in absorbance is also accompanied with a shift of the
main absorption peak to lower wavelength. This hypsochromic shift of the major absorption peak
here can be attributed to dye degradation through stepꢀbyꢀstep Nꢀdeethylation mechanism,
similar phenomena have been well documented in earlier photocatalytic dye degradation
49ꢀ50
studies.
In comparison with Cu O/SnO nanocomposite (Fig. 6b) the wavelength shift is not
2 2
so pronounced for the case of pure SnO nanospheres and mixed phase copper oxide (Fig. S8†),
2
which could be due to low efficiency of the photocatalysts in degrading the dye.
The blue shift in the absorption peak of RhB with increase in irradiation time (Fig. 6b) is an
indication of the evolution of intermediates in the reaction mix, due to the destruction of RhB
5
1ꢀ52
structure including both, chromophore and aromatic ring
(Fig. S9†). This has been
adequately supported by HPLC analysis in succeeding section. In addition, temporal monitoring
via UVꢀvis spectroscopy of RhB photodegradation mediated by nanocomposites synthesized in
various mole ratios under visible light were examined along with wavelength shift of absorption
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maxima at 554nm as a function of irradiation time (Fig. S10ꢀS11†). The most active
photocatalyst, that is, 1:1 mole ratio Cu O/SnO nanocomposite showed complete photoꢀ
2
2
oxidation of RhB in 90 min on exposure to visible light (Fig. 6c). This clearly reveals that RhB
undergoes degradation in presence of Cu O/SnO nanocomposite, thus substantiating our claim
2
2
of this composition as an efficient photocatalyst. Our results also indicate that degradation time
decreased significantly with increase in mole ratio of Cu O and gave the best photocatalytic
2
performance for 1:1 mole ratio Cu O/SnO nanocomposite. Further we observed saturation in
2
2
terms of photocatalytic efficiency when Cu O content in composite is > 1 that is, for 1.5:1 and
2
2
:1 mole ratio Cu O/SnO nanocomposite (Fig. S12†).
2 2
The following section describes our attempts to throw light on the kinetics of the
photocatalytic degradation. The linear simulation of degradation of dye concentration can be
accounted for, by a pseudo firstꢀorder LangmuirꢀHinshelwood (LꢀH) model, well established for
5
3
heterogeneous photocatalysis. Fig. 6d depicts the plot of ln (C /C) of RhB degradation versus
o
the irradiation time (t) in presence of asꢀsynthesized photocatalyst. The approximate linearity of
5
3
this graphical representation can be well expressed by equation 1:
ꢄꢂ
ꢀ
ꢀꢁꢂꢃ ꢄꢅ ꢀ= ꢀꢆꢅ
(1)
(2)
54
The photocatalytic degradation rate constant (DC) was calculated by equation 2:
ꢄꢂꢉꢀꢄꢅ
ꢇꢄ =ꢀꢈ
ꢊ x 100%
ꢄꢂ
ꢀ1
Where κ is the apparent firstꢀorder rate constant (min ), C is the initial concentration of RhB
o
solution which reached absorbency balance and C is the concentration of the dye solution at the
t
irradiation time (t). The firstꢀorder rate constant was calculated from the gradient of the graph of
ln (C /C) versus the irradiation time (t) (Fig. 6d) and briefly summarized in Table. 1. Other
o
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photocatalytic parameters calculated using analytical equations are also tabulated in Table. 1.
Here it is observed that percentage degradation constant of 93.11 is highest for equimole
Cu O/SnO nanocomposite in 90 min exposure to visible irradiation. The firstꢀorder rate constant
2
2
ꢀ1
(
κ) value for pure SnO and mixed phase copper oxides are estimated to be 0.34 and 0.63 min
2
with standard error of ± 0.01 and ± 0.03, and those for Cu O/SnO nanocomposites lie within the
2
2
ꢀ1
range of 0.90ꢀ2.05 min . The firstꢀorder rate constant (κ) for 1:1 mole ratio Cu O/SnO
2
2
heterojunction sample is 2.05, a nearly 2ꢀfold higher than the value for individual SnO and
2
mixed phase copper oxide.
4.2. Recyclability of the Cu
2
O/SnO
2
Nanocomposite
To evaluate the recyclability of the Cu O/SnO nanocomposite as an efficient
2
2
photocatalyst, additional four cycles of RhB photodegradation were performed over the used
nanocomposite (Fig. 7). XRD, FEꢀSEM and EDAX results revealed that even after five repeated
cycles of the photocatalyst for the photodegradation reaction, the crystal structure, primary
morphology and the elemental composition of the nanocomposite was not affected, thereby
confirming its stability and reusability character (Fig. S13ꢀS16†). The average photocatalytic
efficiency of Cu O/SnO nanocomposite for the five cycles of photodegradation for 90 min of
2
2
irradiation time was found to be 93.65% with standard deviation of 2.3. Moreover, simplistic
approach used in synthesis of hierarchical nanocomposite also allowed easy retrieving of the
catalyst from the reaction mixture by simple low speed centrifugation or filtration.
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4.3. HPLC Analysis
In order to confirm that the RhB degradation is not just a visible decolorization effect but is
complete photooxidation, as intended by this study, and to understand the temporal variations
during the photocatalysis of RhB, the above photodegradation study was supplemented and
monitored by HPLC studies accomplished on a HPꢀVectra system equipped with a photodiode
array (PDA) detector with wavelength tuned to 535 nm. Fig. 8 is the stacked plot of HPLC
chromatograms, wherein Fig. 8a represents the chromatogram of aqueous RhB solution used as
reference. Fig. 8b is the chromatogram corresponding to the study with equimole Cu O/SnO2
2
nanocomposite dispersed in RhB dye solution which was kept in the dark for 30 minutes prior to
start of irradiation. Here the characteristic peaks of RhB dye were observed at retention time of
~19 min and ~10 min exactly matching with reference spectra (Fig. 8a).The spectra after 90
minutes of irradiation time (Fig. 8c), showed a peak due to Nꢀdeethylated intermediate at 5.687
min retention time, due to N, NꢀdiethylꢀN'ꢀethylrhodamine (DER) in addition to characteristic
5
0
peaks of RhB dye. With increase in irradiation time, decrease in RhB concentration was
observed and steady increase in the concentration of other intermediates associated with the
formation and transformation of the reaction intermediates.
Our limitations with respect to the HPLC technique hindered complete identification of
intermediates formed during photooxidation process, because of which peaks at retention time
2.8 and 3.8 min could not be identified from the available literature. Fig. 8d depicts the dye
decolorization sequence during photocatalysis process; here it was observed that the intense pink
colour of RhB decreases as irradiation time increases and at the end of 90 min becomes nearly
colourless. The degree of mineralization reached maximum of 80.6% in 90 min of visible light
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irradiation for RhB over 1:1 mole ratio Cu O/SnO nanocomposite (Fig. 8e). These findings
2
2
demonstrate the excellent photocatalytic performance of the nanocomposite.
4.4. Proposed Mechanism of the Photocatalytic Activity
Visible light induced photodegradation of organic pollutants over pꢀn heterojunction
1
9ꢀ21
composites is widely discussed in the literature.
On the basis of such documented literature
and supplemented by novel findings from the experimental results, a plausible mechanism for
dye degradation is proposed and delineated in Scheme. 2. Scheme 2a represents the synthesis
methodology adopted to obtain Cu O/SnO2 nanocomposite. Principle of photocatalytic
2
degradation of RhB over nanocomposite is illustrated in Scheme. 2b. The enhanced
photodegradation of RhB over Cu O/SnO nanocomposite under visible light irradiation can be
2
2
appreciated considering the energy band model of Cu O/SnO nanocomposite. Cu O is a pꢀtype
2
2
2
2
3
narrow band semiconductor with work function of 4.8ꢀ4.9 eV, while SnO is nꢀtype wide band
2
5
5
semiconductor with work function of 4.52 eV. According to the band model (Scheme. 2c)
under the influence of visible light only Cu O would be excited because of its interband
2
transitions, while SnO remains largely inactive due to its wide band gap. As a result, it can be
2
reasonably assumed that this high visible light absorbance of Cu O is solely responsible for
2
electrons and holes generation.
At the Cu O/SnO heterojunction, majority of photogenerated electrons are injected into SnO
2
2
2
nanospheres across the pꢀn interface because of the position of the conduction band of Cu O that
2
is more negative than that of SnO resulting in the electric field at the interface directing the
2
injection of electrons into SnO and the photogenerated holes remain accumulated in the VB of
2
CuO. Thus the Cu O impregnation in SnO promotes the transfer of electrons from Cu O CB to
2
2
2
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SnO CB and these free electrons combine with available oxygen dissolved in water to form
2
superoxide radicals which facilitates dye degradation. Simultaneously, the photogenerated holes
that have accumulated in Cu O VB combine with water molecules to form hydroxyl radicals
2
which react with dye and degrade it. Overall summary of reactions taking place during
16, 56ꢀ61
photodegradation of RhB can be represented by the following chemical equations:
ꢉ
ꢗ
ꢋꢄꢌ ꢎ/ꢏꢐꢎ ꢑ + ꢀꢒꢓꢀ → ꢀꢏꢐꢎ . ꢔ
+ꢀꢄꢌ ꢎ. ꢒ ꢀ
(3)
(4)
(5)
(6)
(7)
(8)
(9)
ꢍ
ꢍ
ꢍ
ꢄꢕꢀꢀ
ꢍ
ꢖꢕ
ꢏꢐꢎ . ꢔꢉ +ꢀꢎ ꢀ→ ꢀꢏꢐꢎ +ꢀꢎ ꢀ
•ꢀꢉ
ꢍ
ꢄꢕꢀ
ꢍ
ꢍ
ꢍ
ꢄꢌ ꢎ. ꢒꢗ +ꢀꢎꢘ ꢀ→ꢀꢎꢘ +ꢀꢄꢌ ꢎ
ꢉ
•
ꢍ
ꢖꢕ
ꢍ
•
ꢀꢉ
•
ꢉ
ꢎ
+ꢀꢘ ꢎꢀ → ꢎꢘ +ꢀꢎꢘ
ꢍ
ꢍ
ꢍ
•
•
ꢎꢘ +ꢀꢘ ꢎꢀꢀ →ꢀꢎꢘ +ꢀꢘ ꢎ
ꢍ
ꢍ
ꢍ
ꢍ
•
ꢘ ꢎ ꢀ→ꢀ2ꢎꢘ
ꢍ
ꢍꢀ
•
ꢙ ꢕꢋꢄ ꢘ ꢄꢁꢝ ꢎ ꢑ +ꢀꢎꢘ ꢀ→ ꢄꢎ +ꢀꢘ ꢎ + ꢞꢔꢃꢟꢠꢞꢠꢅꢡꢂꢐꢀꢀꢢꢟꢂꢞꢌꢣꢅꢤꢀ
ꢒ
ꢍꢚ ꢛꢜ
ꢍ
ꢛ
ꢍ
ꢍ
Documented results manifest faithful agreement here that the pꢀn heterojunction plays a crucial
role and this can be explained by simple hypothesis as below: Heterojunction formation between
Cu O and SnO at the interface and as a result of Fermi level alignment facilitates favorable
2
2
5
5
charge transfer. At thermal equilibrium state, band bending takes place (Scheme. 2c), and once
the equilibrium is attained at the Cu O/SnO interface; such that Cu O and SnO attain negative
2
2
2
2
and positive charge respectively, thus leading to creation of electric field. This electric field is
responsible for the movement of photogenerated electrons to the conduction band of SnO and
2
retention of holes in the valence band of Cu O during the photocatalysis process, thereby
2
preventing charge carrier recombination. This retarded recombination rate is directly manifested
in the form of improved photocatalytic performance exhibited by the nanocomposite. It can
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hence be inferred that the enhanced photocatalytic activity in the present study is directly
attributed to combined effect of hierarchical nature of SnO nanoarchitectures, the improved
2
visible light adsorption due to Cu O impregnation into SnO nanospheres and finally to the
2
2
formation of pꢀn heterojunction at interface of Cu O and SnO . In addition, desirable features
2
2
such as high hole concentration, high hole mobility and low lattice mismatch between SnO and
2
2
6
Cu O. Furthermore, Cu O played a crucial role in improving the photocatalytic activity. The
2
2
proposed mechanism on enhanced visible light induced photocatalysis is in line with the reports
5
6ꢀ61
on other pꢀn heterojunctions.
5. Conclusions
This study exemplifies the simplistic nature of pꢀn nanocomposite synthesized by a facile
chemical modification, where the nanoarchitectures not only retained the original hierarchical
structure but also exhibited remarkable photoresponse, thus presenting a convenient method to
achieve functional materials. In summary, Cu O/SnO nanocomposites were synthesized in
2
2
various mole ratios by adopting a simple hydrothermal route by appropriate variation of the
starting precursors. Surface morphology and elemental mapping of the nanocomposite by HRꢀ
TEM, FEꢀSEM and EDAX revealed that the morphology of the SnO remained intact in the
2
nanocomposites even after impregnation with Cu O. XPS studies confirmed the oxidation states
2
4
+
+
of tin and copper as Sn (SnO ) and Cu (Cu O). The investigation of photocatalytic
2
2
performance of the composite established that the 1:1 mole ratio Cu O/SnO nanocomposite
2
2
demonstrated highest photocatalytic performance under visible light radiation, compared to
mixed phase copper oxide, pure SnO and other compositions. The visible light response
2
observed here could be attributed to the major role played by Cu O in extending the UV response
2
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of SnO into the visible region. The phenomenon of separation of photogenerated electrons and
2
holes, facilitated by the potential barrier at the pꢀtype Cu O/nꢀtype SnO heterojunction enhanced
2
2
the photoresponse. In addition to this, the hierarchical structure of SnO and other features such
2
as high surface to volume ratio, large surface area and porosity of nanocomposite also
contributed to the improved performance. Photocatalytic activity of such heterostructures could
be further improved by suitably varying the material impregnated, tailoring the morphologies
and controlling the heterojunction fabrication at the micro and nanoscale, which would
significantly promote its application not only in waste water remediation but also in water
splitting and sensing applications.
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RSC Advances
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DOI: 10.1039/C6RA07150C
†Electronic supplementary information (ESI) available:
XRD patterns of phase pure CuO and Cu O/SnO nanocomposite in various mole ratios, TGꢀ
2
2
DTA trace, FTꢀIR spectra, microꢀRaman and Photoluminescence spectrum, EDAX analysis, N2
adsorptionꢀdesorption isotherm, Photocatalytic kinetic study of Cu O/SnO nanocomposite in
2
2
various mole ratios, its corresponding UVꢀDRS data and evaluation of Cu O/SnO2
2
nanocomposite in equimole ratio for its reusability.
Acknowledgments
SJ acknowledges RMIT University, Australia for financial assistance in the form of junior
research fellowship (CLPꢀ0092), as well as sincerely appreciates the encouragement and help
received from Dr. Pratyay Basak, Dr. Rohit K. Rana, Dr. Suvarna R. Bathe and A. G. Mahesh.
Heartfelt gratitude towards Ms Y. Swarnalatha and B. Arun for their help with FEꢀSEM/EDS. SJ
and SVM duly acknowledge the strong support of the CSIR XIIꢀFYP Projects M2D (CSCꢀ0134)
and Intel COAT (CSCꢀ0114) for the grants received. The authors are very much thankful to Ms
Sara Khalid, Dr. C Anandan (CSIRꢀNAL, India) and Dr. Selvakannan Periasamy (RMIT
University, Australia) for their help with HPLC, XPS and HRꢀTEM respectively.
27

RSC Advances
Page 28 of 38
View Article Online
DOI: 10.1039/C6RA07150C
Fig. 1 Representative stacked plot of Xꢀray diffraction patterns of as synthesized (a) Porous
SnO nanospheres, Cu O impregnated in SnO matrix to form nanocomposites in various mole
2
2
2
ratios (b) 0.125:1, (c) 0.25:1, (d) 0.5:1 and (e) 1:1.
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Page 29 of 38
RSC Advances
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DOI: 10.1039/C6RA07150C
Fig. 2 Structural evaluation was carried out using TEM, FEꢀSEM and HRꢀTEM of Cu O/SnO2
2
nanocomposite in 1:1 mole ratio, (aꢀb) Representative TEM and FEꢀSEM micrographs
demonstrates nearly monodispersed nanospheres, (c) Panoramic image, (dꢀe) High magnification
TEM micrographs revealing hierarchical heterostructures and (f) HRꢀTEM image.
29