CuO/CuSCN valence state heterojunctions with visible light enhanced and ultraviolet light restrained photocatalytic activity

Article abstract of DOI:10.1039/c3cc49492f

CuSCN is applied, for the first time, in a photocatalytic system to form CuO/CuSCN valence state heterojunctions, which exhibited enhanced visible light driven photocatalytic activity and, surprisingly, ultraviolet light restrained activity. Proper migration of photo-generated carriers is proposed to explain the photocatalytic process. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3cc49492f

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CuO/CuSCN valence state heterojunctions with
visible light enhanced and ultraviolet light
restrained photocatalytic activity†
Cite this: Chem. Commun., 2014,
0, 3814
5
Received 14th December 2013,
Accepted 17th January 2014
a
a
a
a
a
Gang Wang, Baibiao Huang,* Lu Wang, Zeyang Wang, Zaizhu Lou,
a
a
b
Xiaoyan Qin, Xiaoyang Zhang and Ying Dai
DOI: 10.1039/c3cc49492f
www.rsc.org/chemcomm
CuSCN is applied, for the first time, in a photocatalytic system to most promising p-type window material for nanocrystalline solar cells
form CuO/CuSCN valence state heterojunctions, which exhibited due to its transparency in the visible light spectrum range, reasonable
À4
À1
6
enhanced visible light driven photocatalytic activity and, surprisingly, hole conductivity (Z5 Â 10 S cm ) and chemical stability. The
ultraviolet light restrained activity. Proper migration of photo- band gap of CuSCN is about 3.6 eV, which positions the conduction
generated carriers is proposed to explain the photocatalytic process. band and valence band at about À3.1 eV and +0.5 eV (NHE),
7
respectively. Although, CuSCN has a wide forbidden band, there is
hardly any photocatalytic activity under UV light irradiation due to the
practical applications in environmental treatment and new energy
high recombination of the photo-generated carriers. CuO is also an
Due to the enormous potential of photocatalytic technology for
1
development, photocatalytic systems have become a hot research
topic worldwide. However, the rapid recombination of photo-
generated charge carriers for single semiconductors has seriously
impeded improvements in photocatalytic efficiency. Nanostructured
heterojunctions offer the opportunity to design new high efficiency
composite photocatalytic systems. Some examples, including AgI/
important p-type transition metal oxide with a narrow band gap of
1
.7 eV, which is a potential field material, excellent catalyst and also has
8
photoelectrochemical properties. However, pure CuO has limited
photocatalytic activity under visible light irradiation. Most reports
9
about CuO in photocatalytic systems discuss composite structures,
10,11
such as CuO–TiO
2
and CuO–ZnO photocatalysts.
Therefore, CuO is
2,3
BiOI and AgCl/H
2
WO
4
ÁH
2
O
have been reported, which confirm
an excellent candidate material to composite with other appropriate
semiconductors to explore new photocatalytic systems.
that heterojunction photocatalysts are an effective way to expand the
photo-absorption range and to facilitate charge separation. Generally,
efforts have been focused on modifying traditional wide band gap
photocatalysts via compositing suitable narrow band gap semicon-
In this paper, the valence state heterojunction photocatalyst CuO/
CuSCN has been successfully synthesized via a facile one-step method
for the first time. The obtained CuO/CuSCN contains two copper
valence states in which the photocatalytic activity is attributed to the
suitable band structures of CuO and CuSCN and the valence change of
4
ductors based on semiconductor theory. It is known that valence
changes usually occur in photocatalytic reactions when the electrons
5
and holes are generated by suitable light excitation. Nevertheless,
+
2+
Cu and Cu . A photocatalytic mechanism of visible light enhanced
and ultraviolet light restrained photocatalytic activity is also proposed.
Visible light could excite the CuO semiconductor to generate electrons
and holes, and the electrons would partially transfer to the valence
band (VB) of the CuSCN semiconductor which is slightly more positive
than the conduction band (CB) of CuO. The main construction in the
plenty of semiconductors cannot utilize these excited electrons and
holes to achieve valence changes due to their high rate of recombina-
tion. Therefore, it is possible to obtain a series of photocatalysts by
compositing two materials with different metallic element valence
states to form valence state heterojunction photocatalysts, which
could efficiently separate the photo-induced electrons and holes.
Copper based compounds possess significant advantages in photo-
conversion efficiency and the promotion of charge separation via an
0
VB of CuSCN is Cu 3d, which could become Cu after accepting
+
electrons and then revert back to Cu via reaction with dissolved
oxygen. Thereby, the electron and hole have been separated efficiently,
and the hole in the VB of CuO could oxidize methylene blue (MB)
molecules. When the CuSCN semiconductor is also excited by UV light,
the electrons in the CuSCN CB would transfer to the CuO CB, which
could enhance the recombination of electron–hole pairs in the CuO
semiconductor. Therefore, the photocatalytic activity has been limited.
Fig. 1A shows the X-ray powder diffraction patterns of the
as-prepared CuO/CuSCN, pure alpha-CuSCN, and CuO. It clearly
displays that the peaks of the CuO/CuSCN sample (Fig. 1B) coincide
0
+
2+
interfacial charge transfer between Cu , Cu , Cu under light irradia-
tion. CuSCN is a traditional wide band gap inorganic semiconductor,
which has a p-type hole-conducting material, and is thought to be the
a
State Key Lab of Crystal Materials, Shandong University, Jinan 250100,
People’s Republic of China. E-mail: bbhuang@sdu.edu.cn
b
School of Physics, Shandong University, People’s Republic of China
†
Electronic supplementary information (ESI) available: Experimental section
and Fig. S1–S8. See DOI: 10.1039/c3cc49492f
3
814 | Chem. Commun., 2014, 50, 3814--3816
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Fig. 1 (A) XRD patterns of pure CuSCN, CuO/CuSCN, and pure CuO
crystals and (B) UV-vis diffuse reflectance spectra of the samples.
Fig. 2 XPS results of the Cu 2p (A) and O 1s (B) in the CuO/CuSCN composite.
observed in the SEM images (Fig. S3D, ESI†). The morphologies of
with those of the standard orthorhombic alpha-CuSCN ( JCPDS No. pure CuSCN and CuO are microrods and nanosheets, respectively,
9-582) and CuO phases (JCPDS No. 89-2531). The peaks in Fig. 1A a which is confirmed by the SEM images as shown (Fig. S3E and F,
2
and c match those of pure CuSCN and CuO. The as-prepared CuO/ ESI†). The microstructure of the CuO/CuSCN composite can also be
CuSCN (Fig. 1A b) displays both the peaks of CuO, and pure alpha- confirmed by the TEM image (Fig. S6, ESI†).
CuSCN crystals, and no other peaks were found, which indicates that
the as-prepared samples are the CuO/CuSCN composite of high prepared samples (Cu(NO
purity. X-ray photoelectron spectroscopy (XPS). XPS spectra of Cu 2p and O
The elemental composition and chemical status of the as-
3 2
)
:NaSCN = 1:2) were also examined by
XRD patterns of CuO/CuSCN synthesized via different molar 1s in CuO/CuSCN are given in Fig. 2. The Cu 2p spectra of CuO/
ratios of raw materials are given (Fig. S1, ESI†). We observed CuO CuSCN consists of two individual peaks at about 931.5 eV and
3 2
peaks when the molar ratio of Cu(NO ) and NaSCN were about 1 : 1 951.5 eV which can be attributed to Cu 2p3/2 and Cu 2p1/2 and also
and 1 : 2 (Fig. S1A and B, ESI†) and no CuO peaks were observed with a peak separation of 20 eV, indicating that the valence state of
12
when the molar ratio of Cu(NO
Fig. S1C and D, ESI†). Although CuO could not be detected by XRD different peaks at Cu that feature a Cu 2p3/2 binding energy at
the colours of the samples are fuscous, which could reflect the DRS 933.6 eV and Cu 2p1/2 binding energy at 953.4 eV. There is also a
3 2
) and NaSCN increased to 1 : 3 or 1 : 4 the Cu ion is +1. The Cu 2p peaks could be further divided into two
2+
(
1
3
(
UV-vis diffuse reflectance spectra) results (Fig. 1B). When the molar satellite pattern of 943 eV in the Cu 2p3/2 XPS spectra. The molar
+
2+
ratio of Cu(NO ) and NaSCN was 1 : 1, the as-prepared sample had ratio of Cu /Cu is about 5.38 in the sample based on the XPS results
3
2
the strongest absorption in the visible light region among the four of Cu 2p. The spectrum of O 1s consists of peaks situated at binding
different molar ratio samples. The absorbency of the four samples in energies of 529.0 eV, 530.8 eV and 533.0 eV. It is well recognized that
14
the visible light region decreased when the molar ratio of NaSCN 529.0 eV is assigned to the CuO lattice oxygen. The peak at 530.8 eV
increased, which resulted from the different amounts of CuO is the binding energy of O atoms on adsorption, and the binding
nanocrystals formed on the surface of the CuSCN microcrystals.
energy 533.0 eV has been assigned in the literature to the presence of
15
2+
The formation of the CuO/CuSCN composite is described in the water. Therefore, Cu ions exist in the form of CuO, and the CuO/
2
+
schematic diagram (Fig. S2, ESI†). [Cu(NH
3
)
4
]
ions could be gener- CuSCN composite is obtained.
The photocatalytic activity of CuO/CuSCN was estimated by the
CuSCN and CuO were gradually formed during the evaporation degradation of methylene blue (MB) under visible light irradiation.
process at 80 1C. The mixed solution could release NH slowly during Fig. 3 shows the photo-degradation activity of MB dye as a function of
the heating process, which could restrict the release speed of Cu
ated after excess NH ÁH O was added into the Cu(NO ) solution, and
3
2
3 2
3
2+
irradiation time over different photocatalysts or light sources. MB dye
ions in solution. Due to the vastly different solubility between CuSCN could be decomposed completely over the CuO/CuSCN composite
and CuO, an abundance of CuO nanocrystals could form at the start photocatalyst under visible light irradiation in 60 minutes (Fig. 3a). As
À
of the heating reaction, and then the SCN ions can consume the shown in Fig. 3b, the photocatalytic activities of CuO, CuSCN and
2+
Cu ions in the CuO nanocrystals to generate CuSCN microcrystals. mechanically mixed CuO/CuSCN (molar ratio 1 : 5) are also measured
2+
À
During oxidation–reduction reactions between Cu and SCN ions, for comparison and do not exhibit photocatalytic activity under visible
there was also some (SCN) gas that escaped into the air with heating light irradiation. The photocatalytic activities of the same sample
2
at 80 1C, which offered an opportunity for the residual CuO nano- (raw material molar ratio 1 : 2) are evaluated under different light
crystals to form on the surface of CuSCN microcrystals. As the amount source irradiation as shown in Fig. 3d. CuO/CuSCN has higher
À
of SCN ions increased fewer CuO nanocrystals formed, which caused photocatalytic efficiency under irradiation by visible light than under
the colours of the obtained samples that to become lighter.
The morphologies of the samples prepared by different molar presented in the following section.
ratios of Cu(NO and NaSCN were investigated by SEM as shown Photocatalytic activities of the samples that were synthesized with
Fig. S3A–D, ESI†). In the obtained composites, the morphological different raw material molar ratios are shown in Fig. S4 (ESI†). When
structures of the CuSCN crystals are microrods, which are surrounded the molar ratio of Cu(NO ) and NaSCN is 1 : 2, the obtained sample
irradiation by UV + Vis light and the possible reasons will be
3 2
)
(
3
2
by CuO nanosheets. With increasing molar ratio of NaSCN, the possesses the greatest photocatalytic activity. Based on the experi-
amount of CuO gradually decreased, which can be clearly observed mental results above, the molar ratio of raw materials could control
(
Fig. S3A–D, ESI†). When the molar ratio between Cu(NO
3 2
) and the amount of CuO nanocrystals on the surface of the CuSCN
NaSCN is 1 : 4, the CuO in the obtained samples could not be microcrystals. Therefore, CuO nanocrystals not only enhance
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Fig. 4 Schematic diagram of the band structures of CuO/CuSCN valence
state heterojunctions and the possible photocatalytic mechanism. (A) Band
structure of the CuSCN and CuO semiconductor. (B) and (C) show the
electron migration mechanism of photoinduced carriers under UV + Vis or
visible light irradiation, respectively.
the CuO CB migrate to the CuSCN VB and combine with holes in the
CuSCN VB. The stimulated electrons in the CuSCN CB can also transfer
to the CB of CuO, and then recombine with holes in the CuO VB.
Fig. 3 Photodegradation of MB dye as a function of irradiation time. (a)
Photocatalytic activity of the CuO/CuSCN (1 : 2) composite under visible
light irradiation. (b) Comparative photocatalytic results of different sam-
ples. (c) Photocatalytic activity of the CuO/CuSCN (1 : 2) composite under Therefore, restricting the photocatalytic activity.
UV-vis light irradiation. (d) Photocatalytic activities (1 : 2) comparison of the
results of UV + Vis and Vis light irradiation.
A CuO/CuSCN valence state heterojunction photocatalyst was
synthesized by a facile one-step low temperature chemical bath
method. While the pure CuSCN and CuO do not have photocatalytic
activity, the as-prepared CuO/CuSCN heterojunction photocatalyst
exhibited strong visible light absorption and photocatalytic activity
against MB. The photocatalytic activity could also be controlled by
adjusting the molar ratio of raw materials, and the sample with the
most powerful photocatalytic activity is obtained when the molar ratio
absorption in the visible light region, but also tune the photocatalytic
activities of the CuO/CuSCN composites.
It is known that photocatalytic activity is greatly affected by the
recombination of the photo-generated electrons and holes which could
decrease the quantum yield. Photoluminescence emission spectroscopy
is an effective approach to understanding the separation capacity of the
photo-generated carriers because of the direct result of the recombina-
tion of the free carriers. Herein, 320 nm is chosen as the excitation
wavelength, the PL spectra of CuO/CuSCN, pure CuSCN and CuO are
displayed in Fig. S5 (ESI†). It can be observed that pure CuSCN and
CuO (Fig. S5B, ESI†) have strong emission peaks, deriving from the
direct electron–hole recombination of the band transition while the
CuO/CuSCN composites have lower peaks, suggesting that the photo-
induced carriers can migrate easily between the CuO and CuSCN
crystals and that recombination of the charges can be greatly limited.
On the basis of the experimental and theoretical results, a schematic
diagram of the band levels of CuO/CuSCN and the possible photo-
catalytic reaction mechanism are given. Fig. 4A shows the band-edge of
CuSCN and CuO. Neither CuSCN nor CuO possess photocatalytic activity
under suitable light irradiation which might be due to the fast
recombination of electrons and holes according to the PL results above.
The photocatalytic activity results reveals that the CuO/CuSCN valence
state heterojunction photocatalyst has much greater activity under
visible light irradiation than under irradiation by UV + Vis light. We
know that visible light only excites the CuO semiconductor to generate
electrons and holes (Fig. 4C), and that the electrons could migrate to the
VB of CuSCN, which is mainly a hybrid of Cu 3d. The electron can be
3 2
of Cu(NO ) and NaSCN is 1 : 2. Meanwhile, visible light enhanced and,
surprisingly, ultraviolet light restrained photocatalytic activity is dis-
played by the CuO/CuSCN valence state heterojunction photocatalyst.
Based on our systematic analysis, proper migration of photo-generated
carriers is proposed to explain the visible light enhanced and ultraviolet
light restrained photocatalytic processes. At the same time, this valence
state heterojunction photocatalyst can extend the scope of photocata-
lytic development which will lead to more potential applications.
This work was financially supported by a research Grant from the
National Basic Research Program of China (973 Program; No.
2
(
013CB632401), the National Natural Science Foundation of China
No. 21333006, 11374190, 21007031 and 51002091).
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+
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9
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16
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816 | Chem. Commun., 2014, 50, 3814--3816
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