Studies on structural defects in bare, PVP capped and TPPO capped copper oxide nanoparticles by positron annihilation lifetime spectroscopy and their impact on photocatalytic degradation of rhodamine B

Article abstract of DOI:10.1039/c6ra12795a

Surface defects are potential sites for modifying the activities of photoexcited electrons and holes, and may consequently affect photocatalytic activity. Here we have demonstrated how surface defects in copper oxide nanoparticles can be engineered by ele

Full text of DOI:10.1039/c6ra12795a

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Studies on structural defects in bare, PVP capped and TPPO capped copper oxide nanoparticles
by positron annihilation lifetime spectroscopy and its impact on
photocatalytic degradation of Rhodamine B
Aarti Sharma1, Raj Kumar Dutta1,2*, Anirban Roychowdhury3, Dipankar Das3
1Centre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee 247667, India
2Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India
3UGCꢀDAE Consortium for Scientific Research, Kolkata Centre, III/LBꢀ8, Bidhannagar, Kolkataꢀ
700098, India
*Corresponding author: Fax: +91ꢀ1332ꢀ286202, Tel. +91ꢀ1332ꢀ285280, Email: duttafcy@iitr.ernet.in
Abstract
The surface defects are potential sites for modifying the activities of photoexcited electrons and holes,
and may consequently affect photocatalytic activity. Here we have demonstrated how surface defects in
copper oxide nanoparticles can be engineered by electron rich capping agent (like TPPO) to drastically
improve the rate of photocatalytic degradation of Rhodamine B dye. The steady state
photoluminescence spectroscopy indicated metal vacancies and cluster vacancy defects in the as
synthesized batches of bare, PVP (polyvinylpyrolidone) capped and TPPO (triphenylphosphine oxide)
capped copper oxide nanoparticles (NPs). The positron annihilation lifetime spectroscopy studies
confirmed that these electron deficient defects were mainly located at the grain boundaries. As
compared to the bare and the PVP capped CuO NPs, the positron lifetime parameter ‘τ1’ for TPPO
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capped CuO NPs was significantly small, which is due to faster annihilation of positrons trapped in free
volume defect sites by the local electrons. Unlike PVP, the electron rich TPPO is capable of transporting
more electrons to the electron deficient defect sites, due to which the rate of photocatalytic dye
degradation was three to four folds higher for the TPPO capped CuO NPs (k = 0.556 hꢀ1) than
polyvinylpyrolidone (PVP) capped CuO NPs (k = 0.168 hꢀ1) and bare CuO NPs (k = 0.143 hꢀ1). The
reduced rate of photocatalytic dye degradation for bare and PVP capped CuO NPs were attributable to
trapping of photoexcited electrons at the defect sites. The photocatalytic dye degradation was associated
with generation of hydroxyl radicals and H2O2 species. The maximum photocatalytic dye degradation by
TPPO capped CuO NPs was corroborated with highest concentration of H2O2 detection (125 ꢁM),
followed by 79 ꢁΜ for PVP capped CuO NPs and 65 ꢁΜ for bare CuO NPs. A detailed mechanism of
the role of defects in photocatalytic process has been outlined.
Keywords: Copper oxide nanoparticles, Photocatalytic dye degradation, Positron annihilation lifetime
spectroscopy, Defects, Reactive oxygen species.
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1. Introduction
The discovery of TiO2 as a photocatalyst by Fujishima and Honda in 1972 led to its profound
applications in wastewater treatment and solar energy.1ꢀ8 This class of materials has unique electronic
band structure comprising of filled valence band (VB) and empty conduction band (CB).9 The
photocatalytic application depends on absorption of light energy equal to or greater than the band gap of
the material. The wastewater treatment by such semiconductor type materials is attributed to generation
•
of reactive oxygen species (ROS), e.g., O2 − (superoxide radicals), H2O2 (hydrogen peroxide) and OH•
(hydroxyl radicals). These are formed due to interaction of photoexcited electron in conduction band (eꢀ
CB) and the holes in valence band (h+VB) with with O2 and H2O, respectively.10ꢀ15 The major drawback of
bulk TiO2 as a photocatalyst is attributed to its wide band gap corresponding to the UV region.
Consequently, photocatalysts of smaller band gaps are getting more scientific attention. In this regard,
CuO is a promising material as a photocatalyst for dye degradation owing to good chemical stability,
high resistance to photocorrosion and relative low cost.16ꢀ22 The band gap of bulk CuO is 1.2 eV which
correspond to near infrared region. Such a small band gap may pose a limitation for ROS generation
owing to enhanced recombination of the electrons and holes. However, the band gap can be tuned to
visible region by reducing the particle size of CuO particles to the order of excitonic radius of CuO and
satisfying the quantum confinement effect.23 In addition, decrease in particles size would lead to
increase in the surface area of the nanoparticles and facilitate enhanced surface reactivity.
Furthermore, defects play important role in photocatalytic activity.24,25 In this regard, method employed
for synthesizing nanoparticles can be considered relevant for incorporating defects in nanoparticles.
Chemical precipitation method is reasonably simple and reproducible, where the nanoparticles are inꢀ
situ capped with surfactants or other suitable agents to inhibit agglomeration.26 The choice of capping
agents might play an important role for passivating the defect states and dangling bond density at the
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surface of the nanoparticles, which might be beneficial for improving efficiency of the photocatalytic
dye degradation.27 Such studies on role of surface defects of nanoparticles on photocatalytic dye
degradation are not systematically reported.
In the present study, we have employed photoluminescence (PL) spectroscopy and positron annihilation
lifetime spectroscopy (PALS) as a probe for studying structural defects in bare and capped CuO NPs
synthesized by precipitation method. Two different capping agents, e.g., triphenylphosphine oxide
(TPPO) and polyvinylpyrrolidone (PVP), were chosen for stabilizing nanoparticles.28, 29 Both are polar
capping agents that can bind with nanoparticles. The difference is their molecular structure, i.e., TPPO
has three bulky phenyl groups while PVP comprises of long CꢀH chain and pyrrolidone ring, tend to
inhibit agglomeration of the nanoparticles. In general, the metal vacancies and oxygen vacancies are
well characterized by photoluminescence spectroscopy.30 The PALS study are suitable to characterize
void oriented defects in inorganic nanomaterials.31,32 Positron interacts with local electrons and it may
either be annihilated to produce two photons of 511 keV energy or it may interact with electron to form
a hydrogenꢀlike atom called positronium atom (Ps) which may be trapped in a free volume defect site
and are eventually annihilated. Since defect sites are characterized with lower electron density than the
defectꢀfree material, so the positron lifetime in a defect rich material is longer and is a measure of the
defect size, especially at the grain boundary.33 In the present work, positron lifetimes of the bare and
capped CuO NPs were compared for assessing effect of capping on surface defects in CuO NPs. All the
batches of CuO NPs were subjected to photocatalytic degradation studies using rhodamine B as a model
dye to understand the role of defects in CuO NPs on photocatalytic activity. The photocatalytic
degradation of rhodamine B was studied with respect to specific assays related to generation of reactive
oxygen species and was correlated with the nature of defects probed by PALS.
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2.
Experimental
Chemicals
2.1.
Copper acetate (Cu(CH3COO)2, > 98 %), acetic acid (CH₃COOH, 99.5 %), sodium hydroxide (NaOH, >
98 %), polyvinylpyrolidone (PVP, average molecular weight 40,000), tertiary butyl alcohol, ammonium
oxalate (AO), terepthalic acid and ethanol (> 99 %) were procured from HiMedia Pvt. Ltd., India.
Potassium iodide, ammonium molybdenum, potassium biphthalate, triphenylphosphine oxide (TPPO >
98 %) and Rhodamine B (Rh B) dye were procured from Sigma Aldrich, GmbH, Germany. All
precursors used in this study were of analytical grade and were used without further purification.
2.2.
Synthesis of bare and capped CuO nanoparticles
The bare and capped CuO nanoparticles were prepared by the coꢀprecipitation method. Briefly, 150 mL
of 2 mM copper acetate aqueous solution was added with 2 mL acetic acid and 0.1 g capping agent
o
(TPPO and PVP) in a roundꢀbottomed flask and refluxed at 100 C for 1 h. Then 1.0 M NaOH was
added to the solution until a black colored precipitate was formed at pH 7ꢀ8. The precipitate was
centrifuged at room temperature, washed with deionized water (Millipore), followed by washing with
ethanol and eventually dried at 60 oC overnight in a temperature controlled oven.
2.3.
Characterization of nanoparticles
The Xꢀray powder diffraction (XRD) patterns were recorded on a Bruker AXS D8 Advance
diffractometer using the graphite monochromatized Cu Kα radiation (λ =1.5406 Å) in a 2θ range from
20o to 80o at a scanning rate of 2o/min. Transmission electron microscopy (TEM) images were recorded
with a FEI TechnaiꢀG2 microscope, operated at an acceleration voltage of 200 kV. The sample for TEM
measurement was prepared by dropping a highly diluted dispersion of the batches of CuO NPs
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nanoparticles on respective copper grid of mesh 100. The samples were air dried overnight before
measuring by TEM. The morphology and elemental composition was measured by Field Emissionꢀ
scanning electron microscope coupled with energy dispersive xꢀray analysis (FEꢀSEM coupled with
EDX, Zeiss ultraplus) operated at 15 kV. The samples for FEꢀSEM analysis were prepared by dispersing
powdered nanoparticles on a carbon tape and then sputter coated a very thin layer of gold. The UVꢀ
visible absorption spectroscopy studies were performed in the wavelength range of 200–800 nm
(Shimadzu, UVꢀ1800). The photoluminescence spectra were recorded by Hitachi Fꢀ4600 fluorescence
spectrophotometer at an excitation energy 300 nm. The positron annihilation lifetime measurements
were carried out using a fast–fast coincidence system consisting of two 1 inch tapered off BaF2
scintillators coupled to XP 2020Q photomultiplier tubes. The prompt time resolution of the system using
22
a 60Co source with Na gate was 290 ps and measurements were done by using 22Na source embedded
in thin Niꢀfoil. The lifetime spectra were deꢀconvoluted using the code PATFIT88 after incorporating
necessary source corrections due to thin Ni foil.34 The Fourier transformed infrared (FTꢀIR) spectra were
recorded on Thermo NICOLET 6700 FTIR in KBr pellets. The Brunauer Emmett and Teller (BET)
specific surface area of the nanoparticles were measured by Quantachrome Nova 2200 analyser at liquid
nitrogen temperature using nitrogen adsorption–desorption method.
2.4.
Photocatalytic degradation of Rhodamine B dye
The photocatalytic degradation studies of Rhodamine B (RhB) dye were performed at room temperature
in 50 mL of dye solution (1×10ꢀ5 M) by treating with batches of 1 mg/mL of bare and capped CuO NPs,
respectively. The respective batches of dyeꢀphotocatalyst suspensions were exposed to long UV light (λ
= 360 nm) of 30 W at a distance of 30 cm. Prior to the UV light exposure, the dyeꢀphotocatalyst
suspensions were magnetically stirred for 60 min in the dark to ensure adsorption ꢀ desorption
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equilibrium between the photocatalyst and RhB dye. To differentiate between the photocatalysis and the
photolysis of RhB, control experiment under UV light exposure was carried in the absence of NPs as
catalyst. In all the cases, the kinetics of decolorization of the dye was studied by withdrawing 1 mL
aliquot dye solution at specific time interval. The dye concentration in the supernatant fraction was
measured after centrifuging at 15,000 rpm at 25oC for 5 min using Beckman Coulter AllegraTM Xꢀ22R.
2.5. ROS generation and ROS scavenging studies
2.5.1. H2O2 determination
The concentration of hydrogen peroxide generated in the reaction medium was determined by the iodide
method as given in literature.35 Briefly, the batches of bare and capped CuO photocatalysts were
dispersed in water (1 mg/mL) and exposed to UV light. The aliquots (2–3 ml) of dispersion were drawn
after specific time interval and centrifuged. Three millilitre of iodide reagent (0.4 M potassium iodide,
0.06 M NaOH, approximately 10ꢀ4 M ammonium molybdate) and 3 ml of 0.1 M potassium biphthalate
were added the supernatant. Then the absorbance of the solution was measured by UVꢀvisible
spectrophotometer at λ = 351 nm.
2.5.2. Hydroxyl radical (OH•) assay
The batches of bare and capped CuO photocatalyst of 50 mg were suspended in aqueous solution (50
mL) containing 0.01 M NaOH and 3.0 mM terephthalic acid. The suspensions were magnetically stirred
in the dark for 60 min to achieve adsorption–desorption equilibrium between the photocatalyst and
terephthalic acid. The solution was exposed to UV light (8 W), kept at a distance of 30 cm for 2 h.
Aliquot of 5 mL of the solution was withdrawn and centrifuged. The fluorescence spectrum of the
supernatant liquid was measured at λex = 315 nm.
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2.5.3 Scavenging of reactive oxygen species (ROS)
The effect of ROS generated during photocatalytic reaction was indirectly studied by monitoring dye
degradation in the presence of photocatalyst and specific ROS scavenger. Batches of RhB dye solutions
were treated with 1 mg/mL each of the bare and capped CuO as photocatalysts. The solutions were
stirred in dark for 1 h. Then 1 mM tertiary butyl alcohol as OH• radical scavenger)36, and 1 mM
ammonium oxalate as hole scavenger)37 were added to the respective dye solutions and exposed to UV
light as discussed above for photocatalytic dye degradation process. At regular interval, aliquot of the
dye solution from respective batches of dye degradation media, were take out, centrifuged and the
supernatant was measured for dye concentration.
2.6.
Stability and reusability of CuO nanoparticles
The stability of catalysts against photocorrosion and reusability of the catalyst was investigated by
conducting five degradation cycles with same CuO photocatalyst (bare and capped). For this, 1 mg/mL
bare and capped CuO NPs were taken in RhB dye solution and the photocatalytic degradation studies
were performed. After each cycle, the photocatalyst was separated by centrifuging, washed with distilled
water and further used for next cycle. For each cycle studies, the dyeꢀphotocatalyst suspension was
subjected to 60 min dark stirring followed by photocatalytic degradation in UV light.
3.
Results and discussion
Structural studies
3.1.
The XRD patterns of bare and capped CuO (by TPPO and PVP, respectively) in Figure 1, confirmed the
synthesis of pure phase monoclinic structure of CuO with Bragg reflections at (110), (002), (111), (20 ),
(020), (202), (11 ), (002), (31 ) and (220) planes which matched with the standard JCPDS file (Card
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No. 48ꢀ1548). The XRD pattern revealed peak broadening, attributable to smaller crystallite size and
lattice strain in the structure.38 The average crystallite size of the bare and capped CuO NPs calculated
from Debye–Scherrer equation are given in Tableꢀ1. The TEM analysis of bare CuO revealed formation
of spherical shaped nanoparticles of about 17 nm diameter (Figure 2a), while the particle sizes for PVP
capped and TPPO capped CuO NPs were less than 10 nm diameter (Figure 2b, 2c). The decrease in the
particle size was due to the effect of capping on CuO NPs. The polycrystalline nature of the as
synthesized TPPO capped CuO nanoparticle is reflected from the selected area electron diffraction
(SAED) pattern (Figure 3). The FEꢀSEM images of all the batches of bare and capped CuO NPs
revealed agglomeration of the nanoparticles and their chemical composition comprising of is reflected
from the characteristic Xꢀrays peaks of Cu and O in the EDS spectrum (given as Supporting
Information, Figure S1aꢀc) The BET surface area of bare, PVP capped and TPPO capped CuO were
measured to be 73.27 m2/g, 91. 02 m2/g and 105.82 m2/g.
3.2. FTꢀIR spectra analysis
Figure S2 represents FTꢀIR spectra of the bare and the capped CuO nanoparticles recorded in the range
of 4000ꢀ400 cmꢀ1 (given as Supporting Information). The hygroscopic nature of the CuO nanoparticles
is evident for observation of broad peak in the region 3000 ꢀ 3500 cmꢀ1. In the TPPO capped CuO
nanoparticles, the band at 1422 cmꢀ1 corresponds to C=C stretch corresponding to the aromatic structure
of TPPO molecule. A broad and weak peak at 1076 cmꢀ1 is attributed to coꢀordinate bond between the
oxygen atom of P=O bond of TPPO ligand and the Cu2+ ions. A similar FTꢀIR signature is reported for
TOPOꢀcapped TiO2 system.39 The FTꢀIR spectrum of PVP capped CuO nanoparticles revealed a peak at
1637 cmꢀ1 corresponding to C=O stretching of carbonyl group on pyrrolidone ring of PVP. The
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observed peak position is at lower energy with respect to carbonyl bond which is atributable to C=O
bond due interaction of carbonyl bond with Cu2+ ion. The peaks observed at 1400 cmꢀ1 and 1340 cmꢀ1
correspond to CꢀN stretching, while the peak at 1026 cmꢀ1 is attributable to carbon ring in cyclic
compounds. Moreover, the peaks at 589 cmꢀ1, 507 cmꢀ1, 440 cmꢀ1 are vibrational frequencies of Cu – O
bond.
3.3.
Optical spectroscopy studies
The band gaps of the CuO nanoparticles were determined from diffused reflectance spectroscopic study.
The KubelkaꢀMunk (KꢀM) theory was used to convert reflectance data to absorption mode. The band
gap of the samples was estimated from the Tauc’s plot, using the Tauc expression:40
(αhν)1/n = A(hνꢀ Eg)
(1)
where α, hν, A and Eg are the absorption coefficient, incident light frequency, proportionality constant
and band gap, respectively. The value of exponent ‘n’ determines the nature of electronic transition; e.g.,
n= ½ refers to direct transition, and n = 2 refers to indirect transition. Assuming direct transition for
CuO nanoparticles, the linear region of the Tauc plot (αhν)2 against hν was extrapolated to (αhν)2 = 0,
which corresponded to the band gap energy of the material (Figure 4). The band gap for bare, PVP
capped and TPPO capped CuO were determined to be 2.59 eV, 2.71 eV and 2.86 eV, which were larger
than that of the bulk CuO (1.2 eV). The increase in the band gap is attributable to the decrease in the
particle size of the asꢀsynthesized CuO nanoparticles and well supported by TEM results discussed
above.
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The photoluminescence (PL) spectroscopy was used to investigate the electronic and optical properties
of nanoparticles as well as to elucidate the energy levels within the band gap region corresponding to the
defect sites.41,42 The emission spectra of bare and capped CuO revealed intense sharp peak at 452 nm,
468 nm and 488 nm (Figure 5), and our results are consistent with those reported in the literature.43,44
The emission at 468 nm and 488 nm are assigned to energy levels of defect sites, e.g. VCu (copper
vacancies), Cui (copper interstitial) and Vo (oxygen vacancy) in CuO nanoparticles.45 The weak
emission in bare and PVP capped CuO nanoparticles may be attributed to the nonꢀradiative transition
because of the dangling bonds and defects present at the surface of these nanoparticles. In the case of
TPPO capping, the increase in the PL intensity increased due to could be attributed to passivation of
surface defects and favor radiative recombination.46
3.4.
Probing defects by positron annihilation lifetime spectroscopy
The asꢀsynthesized bare and capped CuO nanoparticles might possess one or more types of the defects,
e.g., copper vacancy (VCu), oxygen vacancy (Vo), Cu interstitial (ICu), voids, cluster defects, triple
junction defects, etc. The electron densities at these defect sites are less as compared to the defect free
materials or in the bulk of the materials. Since positrons are annihilated with local electrons so positron
annihilation lifetime spectroscopy (PALS) is expected to probe these defects except for oxygen
vacancies, as the oxygen vacancies exists in neutral charge state. It is important to note two features,
e.g., (a) most of the defects probed by positrons are at the grain boundaries and (b) the lifetime of the
positrons are more in the defect rich materials.47,48
The recorded positron annihilation lifetime spectra of the batches of bare and capped CuO NPs are given
as Supporting Information, Figure S3 and the different defect sizes were estimated by deꢀconvulating
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the respective spectrum with three lifetime components, τ1, τ2 and τ3 with their relative intensities I1, I2
and I3, respectively (given in Table 1). The defect sizes due to cation vacancies are expectedly smaller
than that of the voids, so the shortest lifetime τ1 is assigned to positron annihilation at grain boundaries
due to the presence of copper vacancies (VCu) or VCu clusters. The τ1 values for bare CuO NPs and the
one capped with PVP were found to be 160±4 ps and 163±5 ps, respectively. On the other hand, the τ1
for TPPO capped CuO was very small (148 ±5 ps), and was relatively similar to the bulk CuO reported
in literature.49 The longer lifetime (τ1) for PVP capped CuO and bare CuO indicate lesser electron
densities at the defect sites, which are primarily attributed as cationic monovacancies, e.g., VCu or
smaller sized structural defects e.g., VCu cluster vacancies at the grain boundaries.
It may be remarked that the lone pairs of electrons at the oxygen atoms of TPPO might be sharing with
the electron deficient defects sites at the grain boundaries of CuO NPs and thereby neutralized the
defects. Notably, the PALS result is consistent with the literature report on pꢀtype CuO comprising Cu
vacancies that act as shallow and efficient hole producers.50 The lifetime component τ2 was found to be
of similar order (ranging between 325±3 ps and 333±4 ps) for all the studied batches. The lifetime τ2 is
attributable to larger sized free volume at the grain boundaries, namely diꢀvacancies, larger copper
vacancy clusters or nanovoids at the intersection of multiple grain boundaries (e.g., triple junctions). The
longest lifetime component τ3 is attributable to formation of orthopositronium atoms in free volume
defects.33 However, the relative intensities of the τ3 component were negligible as compared to τ1 and τ2
(less than 0.14 %) and therefore it is not relevant for interpreting defects in the asꢀsynthesized bare or
capped CuO NPs. The mean positron annihilation lifetime (τm) is statistically better reliable parameter
for assessing overall defects and it is given as:
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Notably, the mean positron lifetime (τm) was found minimum for the batch of TPPO capped CuO NPs
and the maximum mean positron lifetime was found for PVP capped CuO NPs. From these results it
may be surmised that the defects owing to Cu monovacancies or divacancies and nanovoids at the grain
boundaries were significantly less for TPPO capped CuO NPs as compared to bare CuO NPs and PVP
capped CuO NPs. As these defects are electron deficient so the electron density of capping agent would
be important for neutralizing the defects. In the case of TPPO capping, the electron deficient defects at
the grain boundaries of CuO NPs were likely to be neutralized by the transfer of lone pair of electron on
the oxygen of P=O. Similar electron transfer process can occur from the carbonyl group of the PVP. It
may be remarked that the transfer of electron is more efficient for TPPO than PVP owing to the greater
polarization of electrons in the P=O bond than C=O bond.
3.5.
Photocatalytic degradation of rhodamine B dye (RhB dye)
The kinetics of photocatalytic degradation of RhB dye treated with bare and capped CuO NPs under UV
light was studied by using LangmuirꢀHinshelwood model, given as:51
ln(C/Co) = kt,
(3)
where ‘Co’ is the initial concentration of the dye solution, ‘C’ is the dye concentration at time ‘t’ and ‘k’
is the pseudoꢀfirst order rate constant. The linear plot between ln(Co/C) and time (t) of UV light
exposure for all the batches of bare and capped CuO NPs are shown in Figure 6, which indicated the
photocatalytic RhB degradation followed a pseudo firstꢀorder kinetics. The degradation rate constant (k),
calculated from the slope of ln(Co/C) verses ‘t’ plot, was three folds higher for TPPO capped CuO NPs
(k = 0.556 hꢀ1) than PVP capped CuO NPs (k = 0.168 hꢀ1) and bare CuO NPs (k = 0.143 hꢀ1). An
interesting correlation could be observed between the rate of dye degradation and its positron lifetime τ1
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values in bare and capped CuO NPs. The efficiency of dye degradation was inversely related to the
positron lifetime of τ1 component and hence the dye degradation efficiency is related to the structural
defects in the photocatalysts.
3.6. Mechanism of photocatalytic dye degradation by CuO NPs
The photocatalytic action of CuO NPs is attributable to sequence of events triggered by absorption of
light of suitable energy that is greater than or equal to the band gap of CuO NPs as photocatalyst.
Consequently, photoexcited electrons are transferred from valence band to conduction band, leaving the
holes at the valence band. Primarily there are two process for the photoexcited electrons and holes, e.g.,
(a) these charge carriers migrate from bulk to surface, where electrons reduce adsorbed electron
acceptor (e.g., O2) and holes oxidize adsorbed donor species (e.g., H2O); (b) the electrons (eꢀ) and holes
(h+) recombine. Amongst these two processes, the former one may lead to generation of a plethora of
reactive oxygen species (ROS) which subsequently cause dye degradation by cleaving the molecular
structure of dye.52,53 The feasibility of ROS generation would depend on the favourable combinations of
potentials of photogenerated electron and holes with those of following relevant redox reactions:
CuO + hν → CuO (
(4)
(5)
(6)
(7)
(8)
+ H2O → H+ + OH•
O2 +
→
Oꢀ2 + 2H+ +
2H2O + 2
→ H2O2
→ 2H2O2 + 2H+
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H2O2 → 2 OH•
(9)
The potentials of the photogenerated electrons and holes correspond to the potential energies of the
bottom of conduction band (ECB) and the top of valance band (EVB), respectively. These potential
energies in a metal oxide nanoparticles can be determined as:54
ECB
=
χ
(AaBb) + 1/2 Eg + Eo ; and EVB = ECB + Eg ;
(10)
where, Eg is the band gap of the semiconductor, Eo is the scale factor taken as ꢀ4.50 eV, i.e., the energy
of free electrons on the normal hydrogen electrode (NHE scale). The parameter χ(AaBb) corresponds to
the absolute electronegativity of a semiconductor material type AaBb, which is calculated as the
geometric mean of the electronegativity of the constituent atoms. In our calculation, the
electronegativity values for Cu and O were taken as 4.48 eV, and 7.54 eV respectively.55 The ECB values
for bare CuO NPs, PVP capped CuO NPs and TPPO capped CuO NPs were calculated as +0.05, ꢀ0.04
and ꢀ0.12 eV, respectively. The corresponding EVB values were +2.56, +2.64, +2.74 eV, respectively.
It is noted that the potential energy of ECB for both the batches of capped CuO NPs and bare CuO NPs
• −
are lesser than the EH (redox potential) of O2/O2 (ꢀ0.2 eV vs. NHE).56 Since electrons transit from
•−
higher potential to lower potential, so reduction of molecular oxygen to superoxide radicals (O2 ) by the
photoexcited electrons is not thermodynamically a favourable process. However, the potentials of
photogenerated electrons in CuO NPs are higher than the redox potentials of O2/H2O2 (0.68 V vs NHE)
and H2O/H2O2 (1.80 V vs NHE).57 It rendered favourable condition for inꢀsitu produce H2O2 species
which is a storage tank for generating various reactive oxygen species in the photocatalytic
processes.58,59 In addition, the EH of H2O/OH• (+2.2 eV vs. NHE scale)55 is at a higher potential than that
of EVB of all the batches of CuO NPs. This is a favourable condition for holes mediated oxidation of
H2O into OH• radicals.
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The energy levels of the redox couples leading to generation of respective reactive oxygen species e.g.,
H2O2 and OH• for CuO nanoparticles as photocatalyst and their role in subsequent photocatalytic dye
degradation of Rhodamine B dye is schematically presented in schemeꢀ1.
3.6.1 Estimation of H2O2 and OH•
Firstly, concentration of H2O2 generated in the photochemical process by different batches of bare and
capped CuO NPs was determined by iodometry and results are given in Figure 7. Notably, 125 ꢁM
H2O2 was produced by TPPO capped CuO, while 79 ꢁΜ and 65 ꢁΜ concentrations of H2O2 were
formed in the batches of PVP capped CuO NPs and bare CuO NPs. Higher concentrations of OH•
radicals were produced in the batch of TPPO capped CuO NPs as compared to the PVP capped CuO
NPs and bare CuO NPs, which was confirmed from the hydroxyl radical specific terepthalic acid (TA)
assay (Figure 8). In this assay the OH• radicals react with (TA) and generate 2ꢀhydroxyterephthalic acid
(TAOH), which emits a unique fluorescence signal with a peak at 426 nm when excited at 315 nm.60
3.6.2 Inhibition of photocatalytic dye degradation by ROS scavenging strategies
The role of reactive oxygen species towards dye degradation was further studied from inhibition of
degradation of RhB dye by CuO NPs in the presence of tertiary butyl alcohol and ammonium oxalate as
scavengers of OH• and photogenerated holes, respectively and the results are given given as Supporting
Information, Figure S4. The RhB dye degradation is remarkably suppressed with the addition of
ammonium oxalate, which is reflected from the decrease in the rate of dye degradation from k = 0.565 hꢀ
1 to k = 0.088 hꢀ1. This is attributable to scavenging of photogenerated holes and consequently the ROS
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generation was limited. Further, the rate of dye degradation decreased from k = 0.565 hꢀ1 to k = 0.101 hꢀ1
in presence of tertiary butyl alcohol, which is a hydroxyl radical scavenger (given as Supporting
Information, Figure S4). The above results confirmed hole mediated hydroxyl radical generation as the
major event responsible for degradation of RhB dye.
3.7.
Effect of surface defects on photocatalytic degradation
Surface defects in metal oxide NPs are known to influence photocatalytic activity.61,62 The defects can
play two different roles, i.e., (a) it can improve the separation of photogenerated electrons and holes so
that they are available for ROS generation and cause dye degradation;63, (b) it can also act as a trap
centre to neutralize these charge carriers and inhibit photocatalytic dye degradation.64ꢀ67
In the case of CuO NPs, the specific rate constant for TPPO capped CuO NPs was marginally higher
(i.e. 0.005 g mꢀ2 hꢀ1) than PVP capped CuO NPs (0.002 g mꢀ2 hꢀ1) and bare CuO NPs (0.002 g mꢀ2 hꢀ1). It
implied that the effect of large surface area of TPPO capped CuP NPs could not be ruled out. In
addition, the nature of defects in the batches of bare and capped CuO nanoparticles would impact the
rate of photocatalytic degradation. Comparing the parameters obtained from PALS studies and
photocatalytic dye degradation, it is found that positron lifetime (τ1) is inversely correlated with the rates
of photocatalytic dye degradation. The enhanced photocatalytic dye degradation by TPPO capped CuO
NPs was associated with smaller freeꢀvolume defects reflected from the τ1 component of PALS, which
is characteristic of Cu monovacancies or smaller clusters of Cu vacancies at the grain boundaries.
Further, the intensities (I1) of τ1 are of similar order for all batches of capped and bare CuO NPs, so the
population of defects in these batches of CuO are not relevant. In other words, it may be remarked that
larger τ1 values (for PVP capped and bare CuO) indicated larger freeꢀvolumes corresponding to copper
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vacancy defects at grain boundaries as compared to smaller τ1 value (for TPPO capped CuO). It may be
remarked that the larger copper vacancy defects at the grain boundaries of PVP capped CuO NPs and
bare CuO NPs were better trapping sites for neutralizing the photogenerated charge carriers. This would
explain the reason for less ROS generation and consequently less efficient photocatalytic dye
degradation by PVP capped CuO and bare CuO NPs. Whereas, smaller τ1 component in TPPO capped
CuO NPs corresponds to reduced Cu vacancies defects, which is not favourable for trapping the
photogenerated charge carriers and hence facilitate the generation of hydroxyl radical for enhanced
photocatalytic reaction. It may be commented here that the defects owing to oxygen vacancies (if any)
were not probed by positron lifetime spectroscopy and hence its effect on the rate of photocatalytic
activity of CuO NPs could not be assessed. However, the oxygen vacancies in CuO might not be very
important as its energy level below the conduction band is not convenient for electron transfer process
for bringing out reduction of oxygen to superoxide radical species.
3.8.
Stability of CuO nanoparticles
In order to investigate the photostability and reꢀussability of CuO NPs, the five cycles of
photodegradation process (i.e., equilibration in dark followed by UV light exposure) were repeated. In
all the five cycles, same percentage of degradation of RhB dye was achieved in 5 h of UV light exposure
and their respective rate constants (k) found to be of similar order (given as Supporting Information,
Figure S5). A stable photocatalytic performance has been observed and our result complimented the
literature report on the stability of CuO against photocorrosion.19 These results are encouraging to design
supported photocatalyst made of CuO NPs for undertaking large volume textile dye degradation.
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4. Conclusion
Highly crystalline batches of bare, PVP capped and TPPO capped CuO NPs of diameter less than 10 nm
were synthesized by chemical precipitation method. The band gaps of these nanoparticles were in the
range of 2.59 and 2.86 eV, and which are suitable as photocatalyst for Rhodamine blue dye degradation
under the exposure of light with λ = 360 nm. Overall, the photocatalytic dye degradation was correlated
with inꢀsitu generation of H2O2 and OH• in the reaction medium via interaction with photogenerated
holes. The systematic variation in the photocatalytic dye degradation was correlated with structural
defects like metal vacancies and cluster vacancies at the grain boundaries of the nanoparticles, as
revealed from positron annihilation lifetime spectroscopy. The surface defects were considered as
trapping sites of photogenerated electrons and holes, that lead to less ROS generation and subsequent
slowing down of photocatalytic dye degradation by PVP capped CuO and bare CuO NPS. In the case of
TPPO capped CuO NPs, the capping agent is electron rich, which could passivate the surface defects,
and consequently exhibited enhanced photocatalytic degradation of Rhodamine B as a model dye.
5.
Acknowledgment
A.S. is thankful to Ministry of Human Resource Development (MHRD), Govt of India for
awarding Senior Research Fellowship. R.K.D is thankful to Board of Research in Nuclear
Sciences (BRNS), Government of India for financial support (2011/37C/37/BRNS).
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FIGURES
bare CuO
PVP capped CuO
TPPO capped CuO
30
40
50
60
70
80
2θ
Fig. 1
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60
40
20
0
30
20
10
0
5
6
7
8
6
7
8
9
10
Particle size (nm)
Particle size (nm)
(b)
(a)
30
20
10
0
12 14 16 18 20
Particle size (nm)
(c)
Fig. 2
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Fig. 3
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bare CuO
PVP capped CuO
TPPO capped CuO
1.6
1.2
0.8
0.4
0.0
2
3
4
5
Eg (eV)
Fig. 4
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468 nm
452 nm
488 nm
TPPO capped CuO
PVP capped CuO
bare CuO
350
400
450
500
550
Wavelength (nm)
Fig. 5
29