XANES, EXAFS and photocatalytic investigations on copper oxide nanoparticles and nanocomposites

Article abstract of DOI:10.1039/c4ra16217j

CuO nanoparticles (NPs) and Cu₂O/CuO and CuO/TiO₂ nanocomposites (NCs) have been synthesized by using a modified co-precipitation method with three different schemes of synthesis. The crystal structures and morphologies of the samples have been investigated using synchrotron X-ray diffraction and transmission electron microscopy, respectively. The detailed local electronic structures of the NPs and NCs have been determined using X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy. O K-, Cu K- and Cu L-edge XANES spectra revealed a dominant +2 valence state of Cu in the case of the CuO NPs and CuO/TiO₂ NCs, although Cu⁺¹ was dominant in the Cu₂O/CuO NCs. A comparison of the local atomic structure around the Cu sites revealed shorter Cu-O bond distances in the as-synthesized samples with respect to the bulk CuO or Cu₂O. The Ti K-edge EXAFS fittings for the CuO/TiO₂ NCs revealed that the local anatase TiO₂ phase was formed, with a Ti-O bond distance of 1.98 ?. We further demonstrated that the CuO NPs, and Cu₂O/CuO and CuO/TiO₂ NCs can serve as effective photocatalysts towards the degradation of two novel water pollutants, (i) methyl orange (MO) and (ii) potassium dichromate (PD), under visible light irradiation. It was found that the Cu₂O/CuO NCs exhibit a higher photocatalytic activity towards the degradation of MO and PD than the CuO NPs or CuO/TiO₂ NCs. The mechanism of the photodegradation of MO and PD is also discussed in terms of possible chemical reactions, along with the electronic structure and surface properties of the samples.

Full text of DOI:10.1039/c4ra16217j

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XANES, EXAFS and Photocatalytic investigations on copper oxide Nanoparticles and
Nanocomposites
1
*
1
1
1
2
Aditya Sharma , Mayora Varshney , JaeꢀHun Park , TaeꢀKyun Ha , KeunꢀHwa Chae and
1
ψ
HyunꢀJoon Shin
1. Pohang Accelerator Laboratory, POSTECH, Pohang 790ꢀ784, South Korea
2
. Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 136ꢀ791,
South Korea
Abstract
CuO nanoparticles (NPs), Cu O/CuO and CuO/TiO nanocomposites (NCs) have been
2
2
synthesized by using modified coꢀprecipitation method with three different schemes of synthesis.
Crystal structure and morphology of the samples have been investigated by the synchrotron Xꢀ
ray diffraction and transmission electron microscopy, respectively. The detailed local electronic
structure of NPs and NCs has been determined by the Xꢀray absorption near edge structure
(XANES) and the extended Xꢀray absorption fine structure (EXAFS) spectroscopy. O Kꢀedge,
Cu Kꢀ and Cu Lꢀedge XANES spectra revealed the dominating +2 valence state of Cu in case of
+
1
CuO NPs and CuO/TiO NCs, although Cu was dominated in the Cu O/CuO NCs. A
2
2
comparison of local atomic structure around the Cu sites revealed the shorter Cu – O bond
distances in the asꢀsynthesized samples with respect to the bulk CuO or Cu O. The Ti Kꢀedge
2
EXAFS fittings for CuO/TiO NCs revealed that the local anatase TiO phase has been formed,
2
2
with Ti – O bond distance of 1.98 Å. We further demonstrated that the CuO NPs, Cu O/CuO and
2
CuO/TiO NCs can serve as effective photocatalyst towards the degradation of two novel water
2
1

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pollutants, (i) methyl orange (MO) and (ii) potassium dichromate (PD), under the visible light
irradiation. It was found that the Cu O/CuO NCs exhibit higher photocatalytic activity towards
2
the degradation of MO and PD than the CuO NPs or CuO/TiO NCs. The mechanism of the
2
photodegradation of MO and PD is also discussed in terms of possible chemical reactions, along
with the electronic structure and surface properties of the samples.
ψ
Corresponding authors; shj001@postech.ac.kr , *adityaiuac@gmail.com
2

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1. Introduction
Recently, nanostructures of TiO , SnO and ZnO have been reported for their photocatalytic
2
2
property towards the degradation of water pollutants by generating electronꢀhole pairs under the
light irradiation [1ꢀ2]. These metal oxides are known as nꢀtype transparent conducting oxides
(TCOs) and have been utilized in the gas sensing [3], electrodes for the Li ion batteries [4], dyeꢀ
sensitized solar cells [5] and medicine applications [6]. Most of these TCOs have large energy
band gap (> 3 eV), which lies in the ultra violet (UV) range of the solar spectrum. Therefore, it is
hard to generate electronꢀhole pairs, from these materials, under the visible light irradiation. This
limits the photocatalytic efficiency of such TCOs in the solar light. Hence, there is thirst to
develop new photocatalyst materials with low energy band gap and high photocatalytic
efficiency under the visible light irradiation.
Copper oxides (CuO and Cu O) are known as important pꢀtype semiconductor materials having
2
direct energy band gap and unique optical and magnetic properties [7ꢀ9]. In the CuO, the lattice
has a monoclinic symmetry (mꢀCuO; space groupꢀC2/c) and each atom of this compound has
four nearest neighbors of their kind. Cu atoms are at the centre of O rectangle and the O atoms
are at the centre of the distorted tetrahedron of Cu [10]. The crystallographic structure of Cu O is
2
highly symmetric (space groupꢀ Pn 3m ) with six atoms per unit cell. The O atoms form a bodyꢀ
centered cubic lattice, and the Cu atoms are situated on the vertices of a tetrahedron around the
9
each O atom [10]. CuO has an open Cu 3d configuration (3d ) and known to an
antiferromagnetic semiconductor with energy band gap of ~ 1.7 eV. However, Cu O has
2
1
0
essentially full Cu 3d shell (3d ) and present larger energy band gap ~ 2.1 eV [11].
3

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By considering the low band gap energy and easy accessibility of copper oxides, there are efforts
to degrade the water pollutants (methyl orange (MO), potassium dichromate (PD) and
phenoxazinꢀ3ꢀone etc.) by using the copper oxide as sensitizer in a composite photocatalyst such
as, CuO/ZnO [12], CuO/zeolite [13], CuO/BiVO [14] and CuO/Cu coreꢀshell [15]. In these
4
studies, the photocatalytic activity was not satisfactory when CuO or Cu O NPs alone were used
2
as the photocatalyst. Few recent reports are available on the photocatalytic properties of CuO
nanostructures with unusual morphology, under the UV light irradiation [16ꢀ20]. The preparation
of unusual morphology requires sophisticated synthesis procedure and high operational
temperature/pressure. Besides this, Cu metallic phases also formed in such synthesis methods
which affect the photocatalyst performance of the products [17]. Therefore, it is desirable to
develop stable catalyst materials with low cost synthesis and high efficiency. It is expected that
the catalytic properties of copper oxide may, further, be tailored by maneuvering its electronic
structure (either by adding of foreign elements or by making mixed polymorphs).
To understand the role of copper oxide in the catalytic reactions, it is advantageous to establish a
relation between the electronic structure and catalytic activity of compounds. In most of the
previous reports, Xꢀray photoelectron spectroscopy (XPS) has been employed to determine the
oxidation state of copper [18ꢀ22]. Although, this technique often provides information regarding
the oxidation state of copper in the certain thin samples but; (i) it is mainly surface probing
technique and (ii) generally involves cumbersome fittings procedures because the Cu 2p XPS
+
+2
peaks of Cu , Cu and metallic Cu all appears within a very narrow energy range. Therefore,
other element specific techniques like Xꢀray absorption spectroscopy (XAS), within low and
high energy range, may be favorable to investigate the local atomic structure, coordination
number, valence state and hybridization of the probed atoms. In this study, we employed
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modified coꢀprecipitation synthesis for the preparation of stable CuO NPs, Cu O/CuO and
2
CuO/TiO NCs. Comprehensive electronic structure study by XAS and photocatalytic
2
investigations suggest that the as prepared materials can serve as an effective photocatalyst for
the degradation of MO and PD under the visible light irradiation.
2. Experimental
2
.1 Synthesis of CuO NPs, CuO/Cu O and CuO/TiO NCs
2
2
For the synthesis of copper oxide photocatalyst materials, we applied previously established coꢀ
precipitation method [23ꢀ24] with modified strategy. There were three major synthesis schemes
used in the present work: (a) In the first scheme, 5 g of Cu(CH3COO) .5H O (Aldrich, 99.9
3
2
purity) was mixed into 200 ml of double distilled (DI) water with constant stirring for 45 minutes
at room temperature. (b) In the second scheme, 1.5 g of PVA was dissolved into 25 ml DI water
and then added, with stirring, into similar set of previously prepared Cu(CH3COO) .5H O
3
2
solution. (c) In the third scheme, the clear solutions of Cu(CH3COO) .5H O and TiCl .5H O
3
2
4
2
were prepared according to their molar ratio (10 mol% of Ti was taken with respect to Cu) and
then mixed together with stirring. In the each scheme of synthesis, when the chemicals had been
fairly dissolved, the diluted NH OH solution was dropꢀwise added under the constant stirring up
4
to the pH ~ 9. The brownꢀblue colored precipitates, so formed, were washed several times with
DI water and then dried at 100 °C in air. The dark brown/blue colored samples were fairly
crushed in the mortarꢀpestle and the fine powdered samples were collected carefully. The
samples, synthesized by the all there different schemes, are named as; S1 (CuO NPs), S2
(
Cu O/CuO NCs) and S3 (CuO/TiO NCs), respectively. Synthesis schemes, used chemicals,
2
2
sample names and composition of samples are briefed in the Table 1.
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2.2 Photocatalytic experiments
The evaluation of the photocatalytic activities of the asꢀsynthesized samples, by monitoring the
degradation of MO and PD aqueous solutions, were performed at room temperature. The
procedure was as follow: (i) 0.2 g/L aqueous solutions of MO and PD were prepared in the dark
and stirred for 45 minutes. (ii) 0.1 g of the asꢀsynthesized samples were dispersed into 50 ml of
previously prepared MO and PD aqueous solution and stirred for 30 minutes in dark to ensure
the adsorption of MO and PD onto the catalyst surface. (iii) A commercial 150 W tungstenꢀ
filament bulb was used as a light irradiation source. To avoid the thermal effects; the bulb was
kept ~ 20 inches above from the beaker, the beaker was placed in the cold water plate and the
water was changed after each 15 minutes. (iv) At regular intervals (30 min), 10 ml of the
suspensions were sampled and the NPs and NCs were separated by centrifugation for 10 min. (v)
Changes of MO and PD concentrations with irradiation time were monitored by measuring the
UVꢀvisible absorption spectra.
2.3 Characterization
The crystal structure and phases of asꢀprepared samples were characterized using the synchrotron
Xꢀray diffraction (λ = 1.240 Å) performed at Xꢀray scattering beam line (3D beam line of
Pohang Accelerator laboratory (PAL), South Korea). The morphology and crystallite sizes were
studied with JOELꢀJEMꢀ2200FS high resolution transmission electron microscope (HRꢀTEM)
accompanied with selected area electron diffraction (SAED). To prepare the samples for the HRꢀ
TEM measurements, the powder samples were fairly dispersed in the ethanol solution using the
sonicator and then suspended powders were dropꢀwise loaded on the carbon coated Cu grids. Xꢀ
ray absorption near edge structure (XANES) spectra at the O Kꢀedge, Cu Lꢀedge and Ti Lꢀedge
were collected in the total electron yield (TEY) mode at 10D (PALꢀKIST) beam line. The photon
6

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energy resolution of this beam line was better than 0.6 eV (at O Kꢀedge). The extended Xꢀray
absorption fine structure (EXAFS) measurements at Cu Kedge and Ti Kꢀedge were performed at
1D (PALꢀKIST) beam line. This beam line utilizes a Si (311) double crystal monochromator and
the higher harmonics were effectively removed by detuning the crystals to 70% of the maximum
intensity. Ionization chambers, filled with Ar gas, were used to record the intensity of the
incident and the transmitted Xꢀrays (the sample is placed between the first and second ionization
chamber). The UVꢀvisible absorption spectra of catalyst samples were collected by using Varian
Caryꢀ100 UVꢀvisible spectrophotometer.
3. Results
3.1 XRD results
Fig. 1 illustrates the XRD patterns of asꢀsynthesized samples. All of the XRD spectra were
analyzed by Powder X software. The diffraction patterns of sample S1 fairly resemble to the
monoclinic phase of CuO (space groupꢀC2/c, lattice parameters; a =4.689 Å, b =3.426 Å, c
=5.132 Å, α= γ=90°, β=99.653°). The high intensity peak at 2θ =29.2° and other low intense
peak at 2θ=33.7° in the sample S2 fairly resemble to the cubic Cu O (space groupꢀ Pn 3 m , lattice
2
parameters; a=b=c=4.2696 Å, α=β=γ=90°). However, some other low intensity diffraction
peaks at 28.3°, 39.8° and 48.6° match with the monoclinic CuO. Thus the sample S2 reveals the
formation of composite Cu O/CuO phases. The diffraction peaks of sample S3 fairly match with
2
the CuO phase, while the peaks at 25.5° and 31.6° were roughly fitted with either Ti O
2
3
rhombohedral phase or TiO anatase phase. Since the rhombohedral Ti O is known to an
2
2
3
unstable phase of titanium oxide, the XRD patterns of sample S3 suggest the formation of copper
oxide and titanium oxide composite phases. The average grain size was calculated using the
Scherrer relation; D = 0.9λ/βCosθ (D is the grain size, λ is the wavelength of used Xꢀrays and β
7

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is the full width at half maximum of the diffraction peak). The estimated grain sizes of all three
samples are tabulated in the Table 1.
3.2 TEM-SAED results
Fig. 2 (a), (b) and (c) show the TEM images of samples S1, S2 and S3 respectively. Left and
right insets in each image show the size distribution histograms and SAED patterns, respectively.
The size distribution of nanoparticles is tabulated in the Table 1. The encircled area in the TEM
images shows the closer view of the particles and crystallographic planes. The ring patterns in
the SAED images are convincing the crystalline nature of the samples. However, aggregation of
NPs is visible in all of the TEM images. Such aggregation is expected in the wet chemically
synthesized samples due to the, marginal, presence of hydroxyl groups with the NPs surface [23ꢀ
24].
3.3 O K edge XANES
XANES measurements probe the density of states of empty/partially filled electronic states by
considering the excitation of an inner shell electron to those states that are allowed by dipole
selection rule [25ꢀ26]. The major feature in the O Kꢀedge spectra of various transition metal
oxide compounds can be rationalized using the molecular orbital theory [26]. According to this,
O Kꢀedge XANES features are the result of a transition of O 1s electron to the various partially
occupied and unoccupied molecular orbitals of the oxide, taking into account the crystalꢀfield
splitting effects. In the case of cubic or monoclinic type compounds (i.e., CeO , ZrO2, CuO and
2
2
2
Cu O, etc.) the metal d , d and d orbitals point towards the oxygen atoms, while the d and
x ꢀ y
2
xy
xz
yz
2
z
2
2
2
d orbitals point between the ligands. As a result, the e (group of d
and d ) orbital lowered
z
g
x
ꢀ
y
in energy and the t2g orbital (group of the d , d , and d ) rose in energy [26]. The energy
xy
xz
yz
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difference between e and t is called crystalꢀfield splitting parameter and represented by 10 Dq.
g
2g
Fig.3 presents the feature rich O Kꢀedge XANES spectra of asꢀsynthesized samples. There are
four major peaks present in the O Kꢀedge spectra of sample S1 at 531.8 eV, 536.5 eV, 540.6 eV
and 545.3 eV, respectively. The first peak in the spectrum of sample S1 can be assigned to a
9
1
s → 3e transition of CuO [27]. The 3e orbital is partially filled because of d configuration of
g
g
+
2
Cu in CuO and is formed by the hybridization of O 2p and Cu 3d states [27]. Rest of the peaks
at higher energy are due to the transitions to the hybridize orbitals of O (2p and 3p) and Cu (4s
and 4p) [27]. The energy separation of the first two peaks (~4.6 eV) is consistent with the
previous reports [25, 27] and strengthened the formation of dominating CuO phase in the first
synthesis scheme. Interesting variations has been observed in the spectrum of sample S2 with the
presence of peak like feature ~ 534.4 eV (marked by asterisk). The previous reports have shown
that Cu O exhibited 1s → e transition at higher energy (~2.7 eV) than that of CuO [27]. In the
2
g
present study, too, this peak is ~ 2.6 eV higher in the energy over the 1s → 3e transition peak (at
g
5
31.8 eV) of CuO polymorphs, suggesting the formation of, mixed, Cu O and CuO phases in the
2
sample S2. The intensity of first peak (at 531.8 eV) is quite lower in the sample S2, indicating
low concentration of CuO phase in this sample. The sample S3, also, exhibits remarkable
changes in the O Kꢀedge features (marked by down arrows). Except the four sharp features of
CuO phase, there are two more features present at 533.2 eV and 537.9 eV. These spectral
features did not match with the spectral features of CuO and Cu O phases and may arise due to
2
the hybridization of O 2p orbitals with titanium d obitals. Appearance of such features is
intriguing and requires detailed investigations. In this regard, we have collected Ti Lꢀedge and Ti
Kꢀedge XANES and Ti Kꢀedge EXAFS spectra from this sample and shall be discussed in the
following sections of the paper.
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3.4 Cu K-edge XANES
To further investigate the structural and electronic structure differences in the as synthesized
samples, Cu Kꢀedge XANES spectra were collected and have been presented in the Fig. 4. It is
clear from the Fig.4 that the sample S1 shows intense white line peak at 8995.6 eV, shakedown
like feature at 8986.1 eV and weak absorption preꢀedge feature at 8972 eV which are attributed
to the 1s→4p (continuum), 1s→4p features of CuO (Cu in +2 valence state) and 1s→3d
z
transition, respectively [28ꢀ29]. The 1s→3d is forbidden transition by the dipole selection rules,
and, thus, the 1s→3d preꢀedge feature is attributed to the 3d+4p orbital mixing as well as
vibronic coupling and direct quadrupole coupling [29]. In the previous reports, this dipoleꢀ
forbidden transition is considered as the identification of +2 valence state of Cu, because this
+
1
+1
preꢀedge feature is not observed in the compounds of Cu [30]. In the Cu compounds, the
1
0
1
s→3d transition does not exist because of the d closed shell configuration. The closer
examination of preꢀedge feature (see the inset of Fig. 4) revealed that this feature is present in the
S1 and S3 samples and absent in case of S2 sample, indicating dominating +2 valence state of
Cu in the samples S1 and S3. Interesting variations in the spectral features and peak position
have been observed in the XANES spectrum of sample S2. The white line position shifts to the
lower photon energy (8993.2 eV) and an apparent shoulder ~ 8979.3 eV is also present in the
spectrum. The Cu Kꢀedge position has been reported to shifts towards the lower energy as the
oxidation state of Cu decreases and the Cu O presents remarkable shoulder at ~9 eV beneath the
2
white line peak [28]. Here, the intense white line peak and the dominant shoulder, in the sample
S2, can be assigned to the 1s→4p (continuum) and 1s→4p p transition of Cu O (Cu in +1
x,y
z
2
valence state), respectively [28]. Therefore, the Cu Kꢀedge structural features of sample S2
validate the formation of dominating Cu O phase in the second scheme of synthesis, as
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evidenced by XRD results. The preꢀedge, shakedown feature and white line peak positions of S3
sample fairly match with the spectral features of sample S1. Furthermore, we could not observe
significant variations in the Cu Kꢀedge spectral features of this sample due to the presence of
titanium oxide phases, as they were observed in the XRD spectra. This signifies the dominating
local CuO structure and minor titanium oxide phases in the sample S3. To further probe the
valence state of Cu in the asꢀsynthesized samples, systematic, Cu L ꢀedge XANES
3
,2
measurements were performed and shall be discussed in the following section.
3.5 Cu L-edge, Ti L-edge, and Ti K-edge XANES
Since the Cu Lꢀedge XANES spectrum represents the dipole transitions of Cu 2p3/2 and 2p1/2
electrons into the empty dꢀstates [27, 31ꢀ32], therefore, in order to probe the 3d character of Cu
in asꢀsynthesized samples the Cu L ꢀedge XANES spectra were collected from all three
3
,2
samples and presented in the Fig. 5. It is clear from the Fig. 5 that sample S1 shows two distinct
peaks at 935.4 eV and 955.3 eV. These two features are assigned to the transitions from Cu 2p3/2
(
L3) and 2p1/2 (L ) electrons into the empty dꢀstates, respectively [27, 31ꢀ32]. Presence of a bump
2
like feature at ~943.1 eV and the energy separation between L and L edge (19.9 eV) are
3
2
consistent with the reported values [27] and, thus, signify the formation of CuO phase (Cu is +2
valence state) in this sample. Noticeable changes in the spectral features can be seen in the
sample S2. There are three major differences in the L edge spectra between sample S1 and S2: (i)
the peak position of Cu Lꢀedge features of sample S2 appears at higher energy than those of
sample S1, (ii) the energy separation between L and L features, which is determined by the
3
2
spinꢀorbit coupling and known to be dependent on the oxidation state of Cu [27], is lesser in the
case of sample S1 and (iii) post edge peak ~938.8 eV appears in sample S2 (marked by asterisk).
The spectral features and energy separation between L and L edges of sample S2 are quite
3
2
1
1

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similar to those of reported Cu O thin films [27] and thus suggest the formation of Cu O phases
2
2
(Cu is +1 valence state) in this sample. The spectral features and energy separation between L3
and L edges of sample S3 resemble to those of sample S1, indicating that valence state of Cu
2
remains +2 in this sample. Our XRD results have shown titanium oxide phases in the sample S3
and O Kꢀedge spectra show some intriguing features. Therefore, in order to study the valence
state of Ti and local electronic structure surrounding the Ti atom, in the sample S3, the Ti Lꢀedge
and Ti Kꢀedge XANES spectra were also collected and presented in the Fig. 6. Fig 6 (a) shows
the Ti Lꢀedge XANES spectrum of sample S3 along with the spectra of rutile phase TiO (Rꢀ
2
TiO ) and anatase phase TiO (AꢀTiO ) reference samples. The Ti Lꢀedge spectrum of sample S3
2
2
2
demonstrates four sharp features at 459.4 eV, 461.2 eV, 464.8 eV and 467.1 eV, namely i, j, k,
and l, respectively. The first two features constitute L edge and last two features contribute to
3
the L edge [33]. Interestingly, preꢀedge peak (i’) and L second white line splitted peak (j’) have
2
3
also been appeared in the spectrum of RꢀTiO and AꢀTiO . However, the feature j’ appears at low
2
2
energy in case of RꢀTiO and at higher energy for AꢀTiO [33]. The 10 Dq value and the spectral
2
2
features of sample S3 fairly resemble to that of AꢀTiO (Ti in +4 valence state) [33] and
2
suggesting the formation of local AꢀTiO phase in the sample S3. The peak l of sample S3 shows,
2
unusual, strong intensity. This is due to the overlapping of Ti L edge features with the second
2
diffraction order of Cu L edge. Fig. 6 (b) shows the Ti Kꢀedge XANES spectrum of sample S3.
3
The spectrum is mainly divided into two regions; (i) preꢀedge region (4940 to 4970 eV) and (ii)
above threshold region (4970 to 5050 eV). Within the preꢀedge region there are two low intense
features (m and n) and a sharp preꢀedge (o). Above the threshold there are three multiple
scattering (MS) resonance features (p, q, and r) of the excited photoelectrons, scattered by
neighbor atoms. According to recent investigations, the peak m is purely quadrupolar t ; n is
2
g
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dipolar in nature but includes also a minor e quadrupolar component while o is a pure dipolar
g
feature [34ꢀ35]. From the MS calculations [35], features n and o may be associated with the
transition to unoccupied states made up by mixing of Tiꢀ4p orbitals and higherꢀshell Tiꢀ3d
orbitals. The spectral features p, q, and r are mainly due to MS contribution of the ejected
photoelectron from higherꢀshell neighboring atoms [34ꢀ35]. Ti Kꢀedge XANES spectrum, above
the threshold, of AꢀTiO exhibits sharp q peak and broad/splitting r peak while the RꢀTiO shows
2
2
splitted q peak and intense r peak [36ꢀ37]. Thus the Ti Kꢀedge spectral features of the sample S3
convince spectral features of the AꢀTiO and strengthened the formation of AꢀTiO in this sample,
2
2
as demonstrated by XRD and Ti Lꢀedge XANES spectra. Further, the local atomic investigations
may be advantageous to probe the different phases, coordination number and bond distances
between the atoms by using EXAFS analysis. The EXAFS analysis along with the Fourier
transform (FT) data shall be discussed in the following section.
3.6 Cu K-edge and Ti K -edge EXAFS
To study the coordination number of metal atoms and bond distances between metalꢀmetal and
metalꢀoxygen atoms, we performed the EXAFS analysis at the Cu Kꢀedge and Ti Kꢀedge.
Normalization of the raw data and background noise correction were performed using Athenaꢀ
Artemis program package [38]. To determine the relative bond length and local atomic structure
with respect to the absorbing atom, the EXAFS data were FT to the rꢀspace. To simulate the FT
data (in rꢀspace), systematic theoretical structure of CuO was generated using the ATOM and
ꢀ1
FEFF codes [39]. The data range taken for the transformation was 2ꢀ13.5 Å in kꢀspace.
Structural parameters were obtained, without the phase corrections, by fitting the data in rꢀspace,
3
within the interval of 1ꢀ4 Å. The FT of k ꢀweighted χ(k), along with the fit, for the Cu Kꢀedge
EXAFS of the asꢀsynthesized samples are shown in the Fig. 7 (aꢀd). The shell parameters [bondꢀ
1
3

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2
distance (R), coordination number (N) and DebyeꢀWaller factor (σ )] are presented in Table 2.
The first and second shells in the FTs arise from the single scattering paths of cationꢀoxygen and
cationꢀcation, respectively [28]. Third and other higher shells originate from the single and MS
contributions from a variety of paths and thus larger uncertainties in the third shell fitted
parameters have been reported [40]. Here, we first focus on the Cu – O and Cu – Cu shells of all
three samples. It is clear from the Fourier transformed EXAFS oscillations of sample S1 that a
sharp feature at 1.5 Å is appeared followed by the other intense feature at 2.4 Å. The first feature
corresponds to Cu – O bonds and the second feature represents the Cu – Cu bonds of CuO [28,
41]. The Fourier transformed EXAFS oscillation features of sample S2 are quite different to
those of the sample S1. There are mainly three spectral features, in the sample S2, at 1.2 Å, 1.5 Å
and 2.4 Å, respectively. To simulate these spectral features, we generated the theoretical
structures of CuO and Cu O for the same sample (S2). It is clear from the Fig. 7(b) that the
2
spectral feature at 1.5 Å was better simulated for the theoretical CuO structure; however, the
peaks ~ 1.2 Å and 2.4Å were fairly resembled with the theoretical Cu O structure (see Fig. 7 (c)).
2
Previous EXAFS studies have shown that the Cu – O and Cu – Cu bond distances were lower in
the Cu O compound because of the highly symmetric cubic structure of Cu O and octahedral
2
2
environment of Cu in Cu O [41]. Therefore, the first peak in the FT oscillations of sample S2 is
2
the signature of Cu – O bond from Cu O phase and the second peak represents the Cu – O bond
2
from the CuO phase. To further probe the local atomic structure around Cu, in the sample S3, the
Cu Kꢀedge EXAFS data were simulated with the theoretical CuO structure and presented in Fig.
7
(d). It is clear from the Fig. 7 (d) that Cu – O shell is well fitted, but the Cu – Cu and other
higher scattering paths are not fitted with the theoretical CuO structure, which may be due the
coꢀexistence of TiO phases in this sample. Similar scarce fitted EXAFS oscillations have also
2
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4

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been reported in the CuO/TiO nanorod systems [28]. To examine the TiO phase and Ti
2
2
coordination with O in the sample S3, Ti Kꢀedge EXAFS data were collected and the theoretical
AꢀTiO structure was generated using the Athenaꢀ Artemis program package with the same
2
parameters (k and rꢀranges) as applied to simulate the Cu Kꢀedge. It is clear from the Fig. 7 (e)
that AꢀTiO structure is fairly simulated for the Ti Kꢀedge EXAFS, for the sample S3, and
2
strengthened the formation of AꢀTiO phase in this sample. The Ti – O and Ti – Ti bond
2
distances and coordination number (without phase correction) obtained from this fitting are listed
in the Table 2. It can be noticed from the Table 2 that the DebyeꢀWaller factor is higher and Cu –
O and Cu – Cu bond lengths are shorten in the case of sample S2. The DebyeꢀWaller factor
represents the structural perturbations in the materials and has been reported with higher values
for the small sized NPs [40]. Therefore, a large DebyeꢀWaller factor, in the sample S2, may
indicate the structural constraints because of the coexistence of small sized CuO and Cu O
2
polymorphs. Further, small sized NPs possess rather high surface free energy (or large surface
area) which results in a contraction of the cell parameters of the NPs [40]. Thus, in the present
case of sample S2, lower Cu – O or Cu – Cu bond lengths in comparison to the reported bulk
CuO or Cu O may be due to the size effect.
2
3.7 Photocatalytic activity
As a demonstration of the application of the asꢀsynthesized samples, MO and PD were chosen as
model organic dyestuffs to evaluate the photocatalytic performance. Fig. 8 (aꢀd) and Fig.9 (aꢀd)
show the photocatalytic performance of all three samples towards the degradation of MO and PD,
respectively. Control experiments, without any catalyst materials, were also performed for MO
and PD under the same light irradiation condition. It is clear from the Fig. 8a that UVꢀvis
absorption spectra of MO exhibit a strong absorption peak at ~465 nm and less intense peak at
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~
270 nm. These two absorption peaks are the typical absorptions of azobenzene of MO dye [43].
Similarly, the two major absorption peaks in PD aqueous solution (see Fig.9 (a)) appeared at
360 nm and ~260nm and were attributed to the absorption of Cr O ligands [44]. Interestingly,
~
2
7
there is no measureable change in the absorption spectra of MO and PD aqueous solutions under
the light irradiation when no catalyst was added. This is in accordance with the previous reports
where no degradation of MO and PD could detect under the light radiation [16ꢀ19]. Fig. 8 (b)ꢀ(d)
and Fig. 9 (b)ꢀ(d) show that photodegradation of MO and PD has been established under the
presence of all three photocatalyst samples.
To understand the photocatalytic response of the samples, we first enlighten the working
mechanism of photocatalysis which involves several key processes [15, 18ꢀ19]. In the first step
of photocatalysis process, significant adsorption of pollutant dyes (MO and PD in the present
case) is expected to take place on the surface of used catalyst materials. In the second step, the
excitation of electrons takes place from the valance band (VB) to the conduction band (CB) of
the photocatalyst material under the light irradiation (see eq. (1)). The photoelectrons are
¯
expected to scavenge by the molecular oxygen (O ) to create the superoxide radical anion (O )
2
2
(
eq. (2)) and hydrogenꢀperoxide (H O ) (eq. (3)):
2 2
ꢀ
+
hv (light irradiation)+ Photocatalyst sample
→
e + h
(1)
(2)
(3)
ꢀ
¯
ꢀ
O + e
→ O2
2
+
2
H + O +2 e
→
H O
2 2
2
Either the holes or the intermediate products, produced in reactions 1ꢀ3, or both participate to
ꢀ
generate the hydroxyl radicals [(OH ) and (
̇
OH)] in the solution as follows (eq. 4ꢀ5);
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+
+
H O + Photocatalyst sample (h )
→
̇
(4)
(5)
2
¯
ꢀ
O +H O
→ O + OH + ̇O H
2
2
2
2
The ̇OH radicals work as a powerful oxidizing agent and degrade the most of the water pollutant
dyes [18ꢀ19, 45]. The reactive oxygen species and holes contribute to the oxidative pathways in
the degradation of organic pollutants [45] as following;
¯
Organic Pollutants + O2
→
CO + H O
(6)
2
2
+
Organic Pollutant + h
→
CO + H O
(7)
2
2
On the other hand the electrons, in the conduction band, may reduce the Cr 2 O 72 − ligands via the
following reaction:
ꢀ
ꢀ
+3
2
−
n(Cr 2 O 7
)
+ n(OH )+ ne
n Cr + n(H O)
(8)
2
In the reaction 8, the n is non zero and arbitrary number. We further calculated the degradation
rate of MO and PD by considering the intensity of absorbance peaks, as a function of light
irradiation time by using the following relation;
A − A
0
t
(9)
D =
× 100 %
t
A0
where the D (%) is the degradation rate of MO and PD aqueous dyes at an irradiation time of t
t
minutes, A is the initial absorbance and A is the absorbance at an irradiation time of t minutes.
0
t
Fig. 10 (a) and (b) represents the photodegradation rates of MO and PD dyes, respectively, for all
three catalyst samples as a function of light irradiation time. It is clear from the Fig. 10 that the
photo degradation rates of sample S1 and sample S3 are quite comparable and the sample S2
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exhibits superior photodegradation rates for the MO and PD dyes under the same light irradiation
condition. The higher degradation rates of sample S2 could be understand as follows;
The smaller sized NPs posse higher surface area and offer extensive adsorption reactions of the
pollutants [18, 45]. Furthermore, higher migration rates of photoꢀgenerated charge carriers,
towards the surface, have also been observed in case of smaller sized NPs [46]. Our XRD and
TEM results have confirmed the formation of smaller sized (4 nm – 6 nm) Cu O/CuO NCs in
2
sample S2, resulting the high DebyeꢀWaller factor and shorter Cu – O bond distances in the
EXAFS analysis. Such smaller sized Cu O/CuO NCs may offer adequate adsorption and
2
oxidation/reduction reactions on their surface and resulting high photocatalyst performance. The
schemes of the photocatalysis for all three samples are presented in the Fig. 11 (a – c). In the Fig.
1
1 (b), we demonstrated that the CuO and Cu O both work as the electronꢀhole generation
2
centers and thus effectively degrade the pollutants via oxidation and reduction processes. On the
other hand the sample S3, however, encloses TiO crystallites along with CuO NPs but the
2
energy of visible light photons is not sufficient to generate the electronꢀhole pairs from TiO NPs
2
because of the large energy band gap (~ 3.2 eV) of TiO . Therefore, only CuO serves as the
2
electronꢀhole production center in the sample S1 and S3 (see Fig. 11 (a) and (c)) and hence lower
degradation rates of MO and PD could achieve for the samples S1 and S3.
Conclusions
We have reported a modified coꢀprecipitation method for the synthesis of CuO NPs, Cu O/CuO
2
and CuO/TiO NCs. XRD and TEM results illustrate the formation of crystalline NPs of
2
disparate sizes. We adequately applied the XANES and EXAFS techniques to probe the local
electronic/atomic structure of the asꢀsynthesized samples. Spectral features and 10 Dq values in
the XANES indicate the formation of typical mꢀCuO phase in the sample S1, while the XANES
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spectral features at O Kꢀedge, Cu Lꢀedge and Cu Kꢀedge of sample S2 authenticate the cubic
Cu O phases in the sample S2. The Cu Kꢀedge EXAFS spectra of S1 sample were better fitted
2
with the monoclinic CuO structure (with Cu – O bond distance of 1.91 Å and coordination
number 3.8), however partial CuO and Cu O theoretical structures could be produced for the
2
sample S2 (with short bond distance 1.84 Å and coordination number 1.72 for Cu O phase),
2
indicating single phase CuO formation in the sample S1 and mixed CuO and Cu O phases in the
2
sample S2. Fourier transformed spectra of Cu Kꢀedge EXAFS and Ti Kꢀedge EXAFS validate
the formation of mꢀCuO and AꢀTiO phases in sample S3 and support our XANES investigations.
2
All of the samples exhibited excellent photocatalytic properties and significantly degraded the
MO and PD dye pollutants under the visible light irradiation. It is expected that higher adsorption
reactions of MO and PD take place on the surface of smaller sized NPs (sample S2) and
generation of the plenty of electronꢀhole pairs from the CuO and Cu O centers enhances the
2
photocatalysis reaction under the light irradiation. In the mechanism of photocatalyst reaction,
we believe that the destruction of organic dyes under the visible light irradiation by the
photocatalyst material is initiated mainly through hydroxyl radicals, and the resultant water
molecules.
Acknowledgment
Aditya Sharma, Mayora Varshney and HyunꢀJoon Shin would like to acknowledge the financial
support by the Basic Science Research Program (No. 2008ꢀ0062606, CELAꢀNCRC) through the
National Research Foundation of Korea (NRF) and by the Converging Research Center Program
(NRFꢀ2014M3C1A8048817) through the Ministry of Science, ICT and Future Planning, Korea.
1
9

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Authors are also thankfull to Dr. NamꢀSuk Lee (NINTꢀPOSTECH) and Dr. Taeyeol Jeon (3D
beam line PAL) for their help in the TEM and XRD measurements.
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Figure captions
Fig.1 Xꢀray diffraction patterns of sample S1 (CuO NPs), S2 (Cu O/CuO NCs) and S3
2
(
CuO/TiO NCs).
2
Fig. 2. TEM images of (a) sample S1 (CuO NPs), (b) sample S2 (Cu O/CuO NCs) and (c)
2
sample S3 (CuO/TiO NCs). (Left and right insets show the histograms of size distribution and
2
SAED patterns, respectively).
Fig.3 O Kꢀedge XANES spectra of samples S1 (CuO NPs), S2 (Cu O/CuO NCs) and S3
2
(
CuO/TiO NCs). The spectra were vertically displaced for clarity.
2
Fig.4 Cu Kꢀedge XANES spectra of samples S1 (CuO NPs), S2 (Cu O/CuO NCs) and S3
2
(
CuO/TiO ). The inset shows that the preꢀedge features are present in S1 and S3 samples and
2
absent in the S2 sample.
Fig. 5. Cu L ꢀedge XANES spectra of samples S1 (CuO NPs), S2 (Cu O/CuO NCs) and S3
3
,2
2
(CuO/TiO2).
Fig. 6. (a) Ti L ꢀedge XANES spectra of sample S3, RꢀTiO and AꢀTiO . (b) Ti Kꢀedge XANES
3
,2
2
2
spectrum of sample S3.
Fig. 7. Magnitude of FT along with fittings of; (a) sample S1 witted with the theoretical structure
of CuO, (b) sample S2 fitted with theoretical structure of CuO, (c) sample S2 fitted with the
theoretical structure of Cu O, (d) sample S3 fitted with the theoretical structure of CuO, (e)
2
sample S3 fitted with the theoretical structure of anatase TiO2.
2
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Fig. 8. UVꢀvisible absorption spectra taken at different time of; (a) MO solution without catalyst,
b) MO solution with sample S1 (CuO NPs), (c) MO solution with sample S2 (Cu O/CuO NCs)
(
2
and (d) MO solution with sample S3 (CuO/TiO NCs).
2
Fig. 9. UVꢀvisible absorption spectra taken at different time of; (a) PD solution without catalyst,
(
b) PD solution with sample S1 (CuO NPs), (c) PD solution with sample S2 (Cu O/CuO NCs)
2
and (d) PD solution with sample S3 (CuO/TiO NCs).
2
Fig. 10. Degradation rates (%) of MO and PD for the samples S1 (CuO NPs), S2 (Cu O/CuO
2
NCs) and S3 (CuO/TiO NCs) as a function of light irradiation time.
2
Fig. 11. Schematic schemes of the generation of electronꢀhole pairs and degradation of MO and
PD pollutants, via oxidation and reduction reactions, of; (a) sample S1, (b) sample S2 and (c)
sample S3
2
5

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Table captions
Table 1: Summary of synthesis schemes of samples, sample manes, particle sizes calculated by
XRD and TEM and details of compounds.
Table 2. Structural parameters (obtained from the fittings of Cu Kꢀedge and Ti Kꢀedge EXAFS
data) of first two coordination shells around the Cu and Ti atoms.
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S3
S2
S1
2
0
25
30
35
40
45
2
theta (Degree)

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3
03x104mm (150 x 150 DPI)