Fabrication of Au-CuO hybrid plasmonic nanostructured thin films with

Materials Research Bulletin 123 (2020) 110707
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Materials Research Bulletin
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Fabrication of Au-CuO hybrid plasmonic nanostructured thin films with
enhanced photocatalytic activity
T
Kavita Sahu, Shipra Choudhary, Satyabrata Mohapatra*
University School of Basic and Applied Sciences, Guru Gobind Singh Indraprastha University, Dwarka, New Delhi 110078, India
A R T I C LE I N FO
A B S T R A C T
Keywords:
Au-CuO
Nanohybrid
MB
MG
Thin films of Au-CuO plasmonic nanohybrids were fabricated by simple thermal evaporation. SEM and XRD
investigations unveiled the impact of annealing on the structural properties and morphology of the synthesized
Au-CuO hybrid thin film. Au nanoparticle decoration onto CuO thin film significantly altered the photocatalytic
activity, structural and optical properties of CuO thin film. The as-prepared Au-CuO nanostructured hybrid
plasmonic thin film showed improved photocatalytic efficiency for decomposition of MG and MB than pristine
CuO and annealed Au-CuO thin film samples. The improved photocatalytic efficiency of Au-CuO hybrid nanostructured thin film is attributed to reduction in the recombination of charge carriers in CuO nanostructures
and improvement in light utilization capability due to attachment of Au nanoparticles on nanostructured CuO
thin film. Cracks in Au-CuO nanohybrid thin film facilitated enhanced adsorption of dye molecules and played
an essential role in improving the photocatalytic efficiency.
1. Introduction
CuO nanostructures have attracted worldwide attention because of
their fascinating physical and chemical properties as well as their potential applications in various fields [1–3]. CuO nanostructures are
extensively used in solar cells [4], gas sensors [5], photodetectors [6],
supercapacitors [7], magnetic storage media [8] and removal of organic
pollutants from water [9,10]. Water contamination is increasing vastly
due to fast industrial development and this has led to the hazardous
effects on environment and mankind. Therefore, removal of organic
pollutants from water is essential and for that photocatalytic treatment
is extremely useful [11–20].
CuO nanostructures are widely utilized as catalysts in photocatalytic
treatment because of their remarkable properties such as narrow band
gap, low cost and less-toxic nature [21–23]. Synthesis of CuO nanostructures can be done by various techniques such as microwave assisted
method [24], hydrothermal method [25], reactive sputtering [26],
electrodeposition [27], sol-gel method [28] and thermal evaporation
[29]. Among these methods, thermal evaporation is a promising route
for the fabrication of CuO nanostructures because of its cost effectiveness and ease of fabrication. Efficiency of CuO as a photocatalyst mainly
depends on the optical and morphological properties of prepared nanostructures [30–32]. Wang et al. [33] explored the size effect of CuO
sheet-like nanostructures on their photocatalytic activity towards the
⁎
degradation of RhB dye. They observed that CuO sheet-like nanostructures degrade 96.7 % RhB dye, while commercial CuO powder
degrades only 39.6 % in 9 h. Zaman et al. [34] reported synthesis of
CuO petals and structures with flower-like morphology and examined
their photocatalytic behavior for the degradation RhB and MB dyes.
They observed higher rate constant of CuO petal-like nanostructures
(0.0019 min−1) as compared to flower-like nanostructures
(0.0008 min−1) for the degradation of MB.
CuO nanostructures used as photocatalysts have some limitations
such as high rate of recombination of charge carriers and less utilization
of sun light, which limits their photocatalytic efficiency [35,36]. Coupling of plasmonic metal (Au, Ag, Pd) nanostructures with semiconductor enhances the photocatalytic activity by promoting the separation between the charge carriers and enhancing the light utilization
capability [37–40]. Zhang et al. [41] synthesized CuO nanoflakes and
Au-CuO nanohybrids by wet chemical technique and examined their
photocatalytic performance towards the degradation of RhB dye. AuCuO hybrids having rate constant of 0.64 h−1 showed 3.56 times enhanced photocatalytic activity than pristine CuO nanoflakes (0.18 h−1)
due to enhanced charge separation and improved visible light absorption capability. Yu et al. [42] reported enhanced photocatalytic activity
of Au decorated CuO nanowires as compared to CuO nanowires. They
observed that Au decorated CuO nanowires degrade 92.7 % of RhB dye
in 8 h while CuO nanowires arrays degrade only 65% of RhB in the
Corresponding author.
E-mail address: [email protected] (S. Mohapatra).
https://doi.org/10.1016/j.materresbull.2019.110707
Received 24 August 2019; Received in revised form 8 November 2019; Accepted 25 November 2019
Available online 28 November 2019
0025-5408/ © 2019 Elsevier Ltd. All rights reserved.
Materials Research Bulletin 123 (2020) 110707
K. Sahu, et al.
Fig. 1. (a–d) SEM images of Au-CuO thin film samples CP, CAuP, CAu400 and CAu500.
properties and thereby significantly improving its photocatalytic behavior for removal of organic pollutants from water. Photocatalytic
action of the fabricated Au-CuO nanohybrid thin films under illumination with natural sun light and visible light from He-Ne laser was
investigated. Au-CuO hybrid plasmonic nanostructures without annealing exhibited superior sun light and visible light active photocatalytic activities than pristine CuO and annealed Au-CuO nanostructured thin film samples and led to decomposition of MG and MB.
Au-CuO nanostructured thin film exhibited extremely enhanced photocatalytic activities than CuO thin films reported in the literature. The
mechanism underlying the improved activity of Au-CuO nanohybrid
photocatalyst film was tentatively proposed. The present work focuses
on the development of nanostructured Au-CuO hybrid thin films as
emerging highly efficient and reusable photocatalysts for wastewater
treatment.
2. Experimental details
Fig. 2. XRD patterns of prepared thin film of CuO and Au-CuO hybrid thin
films.
Au-CuO hybrid plasmonic thin films were deposited onto silica glass
substrates by thermal evaporation. In the first step, thermal evaporation
of Cu on silica glass substrates followed by ageing in oxidizing ambient
for 30 days led to the formation of CuO thin film,while in the second
step Au thin film (5 nm) was deposited onto prepared CuO thin films
using thermal evaporation of Au. Au deposited CuO thin film was further annealed at two different temperatures (400⁰C and 500⁰C) for 2 h
in Ar atmosphere. Pristine thin film of CuO is hereafter referred as CP
and Au-CuO hybrid plasmonic nanostructures without annealing and
annealed at 400⁰C and 500⁰C are mentioned as CAuP, CAu400 and
CAu500, respectively. X-ray diffraction (XRD) analysis employing
Panalytical X’pert Pro diffractometer with Cu Kα radiation (λ
=0.154 nm) was employed to analyze the crystal structure of the prepared thin films. Scanning electron microscopy (SEM, ZEISS) was carried out to examine the morphology of the Au-CuO hybrid plasmonic
thin films. Chemical state of the Au-CuO thin film was analyzed using
XPS (Omicron nanotechnology, Oxford instruments) with Al Kα of
same exposure time.
However, in practical scenario these reported nanomaterials in
powder form suffer from some limitations as photocatalysts due to
difficulty in separation, recovery and recyclability. These issues can be
conquered by using photocatalysts in the form of thin films instead of
powder since thin film photocatalysts can be used repeatedly without
any complex recycling process.
In view of this, we have fabricated a highly efficient reusable AuCuO nanohybrid thin film photocatalyst for the removal of organic dyes
by a very simple and cost-effective technique. Au-CuO plasmonic hybrid nanostructured thin films consisting of Au nanoparticles and CuO
nanostructures were fabricated by simple thermal evaporation method
combined with annealing. Decoration of Au nanoparticles onto nanostructured CuO thin film was found to appreciably alter its optical
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K. Sahu, et al.
Fig. 3. XPS spectra of (a) Cu 2p, (b) O 1s and (c) Au 4f from Au-CuO hybrid thin film.
Fig. 4. Schematic possible growth mechanism of Au-CuO hybrid nanostructures.
In this study, Au-CuO hybrid plasmonic thin films were dipped in 5 mL
aqueous solutions of 5 μM of organic dyes (MB and MG) in glass vials
separately. Decomposition of MG and MB dyes in the presence of thin
film of Au-CuO hybrid photocatalysts was monitored for 30, 60 and
90 min of sunlight exposure. After sunlight exposure, UV–vis spectroscopy was used to study the photocatalytic activity of plasmonic AuCuO hybrid thin films for the removal of dyes. The efficiency of Au-CuO
hybrid plasmonic thin films for photocatalytic degradation of dyes was
estimated using the following relation.
energy 1486.7 eV. Optical studies and photocatalytic behavior of AuCuO hybrid plasmonic thin films were explored with the help of UV–vis
spectroscopy (HITACHI U-3300).
2.1. Photocatalytic studies
In order to investigate the photocatalytic behavior of the prepared
Au-CuO hybrid plasmonic thin films towards degradation of organic
dyes under sunlight exposure, we used Au-CuO thin films as photocatalysts and MB and MG dyes as representative organic contaminants.
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K. Sahu, et al.
face centered cubic phase of Au. It was observed that the intensity of
(111) peak of Au increased along with its narrowing with the increase
in annealing temperature. This signifies the growth of Au nanostructures due to annealing, which affects the morphology of prepared
Au-CuO hybrid plasmonic nanostructures. The average crystallite sizes
of Au nanoparticles in samples CAuP, CAu400 and CAu500 were found
to be 11, 15 and 17 nm, respectively. XPS analysis was employed to
examine the surface chemical state of Au-CuO hybrid thin film and the
results are shown in Fig. 3. XPS signals of Cu 2p region in Au-CuO
hybrid nanostructures are shown in Fig. 3(a). Two main peaks found at
932.8 and 952.7 eV are allocated to Cu 2p3/2 and Cu 2p1/2 of CuO,
respectively. Fig. 3(b) shows the O 1s spectra of the prepared Au-CuO
hybrid nanostructured thin film. Observed peaks at 530.5 and 532.4 eV
are allocated to the O2− and oxygen adsorbed on CuO surface. Fig. 3(c)
depicts the Au 4f XPS spectrum of Au-CuO hybrid thin film. Observed
peaks at 84.2 and 87.9 eV are allocated to the core levels of Au 4f7/2 and
Au 4f5/2, respectively and are assigned to Au(0) in the prepared Au-CuO
hybrid nanostructures [43–46]. Mechanism underlying growth of AuCuO hybrid thin film is revealed in Fig. 4. Thermal evaporation of Au
onto nanostructured CuO thin film with cracks leads to deposition of Au
atoms on the surface of CuO nanostructures. Adsorption of impinging
Au atoms on the CuO surface and their surface diffusion leads to the
nucleation of Au nanoclusters and their growth into Au nanoparticles.
Annealing results in growth of Au nanoparticles by conventional Ostwald ripening.
Fig. 5. Optical absorption spectra of thin film of CuO and Au-CuO hybrid nanostructured thin films.
η=
C0 - C
C0
(1)
where C0 is the initial absorbance of dye (without catalyst) before
sunlight illumination and C represents the absorbance of dye in the
solution (with catalyst) after illumination.
3.2. Optical studies
2.1.1. In-situ capture experiment
In this process, four different scavengers viz. TBA (tertiary-butyl
alcohol), BQ (benzoquinone), FA (formic acid) and CN (cupric nitrate)
were used as the representative scavenging chemicals for hydroxyl radicals (•OH ), superoxide radicals (•O2−), holes (h+ ) and electrons (e− ),
respectively. 5 mM solutions of CN, FA and TBA and 0.5 mM of BQ were
added into the photocatalytic systems to investigate the mechanism
behind the photocatalytic activity of prepared Au-CuO hybrid thin
films. Au-CuO hybrid thin film samples were immersed in different
scavenger containing dye solutions and then illuminated to sunlight.
Following thin film photocatalyst removal the solutions were analyzed
using UV–vis absorption spectroscopy.
3. Results and discussion
Fig. 5 presents the UV–vis absorbance spectra of samples CP, CAuP,
CAu400 and CAu500. All the prepared samples show broad absorption
band of CuO in the wavelength range from 300 to 500 nm. In addition
to this peak, two broad absorption bands near 610 nm and 671 nm were
observed in the CuO hybrid thin film. For Au-CuO thin film sample
CAuP, one broad absorption band around 608 nm was observed. This
absorption band changes into a very broad band blue shifted to 560 nm
for the thin film sample annealed at 400 °C (sample CAu400). Further
increase in annealing temperature to 500 °C (sample CAu500) resulted
in a slight increase in the intensity of the absorption band red shifted to
606 nm. It shows the presence of Au nanoparticles and their aggregates
in the prepared samples CAuP, CAu400 and CAu500. As seen from the
absorbance data, Au decorated pristine CuO thin film without annealing (CAuP) shows higher absorbance than other samples.
3.1. Morphology and structural properties
3.3. Evaluation of photocatalytic activity
Fig. 1(a–d) show the SEM images of prepared Au-CuO hybrid
plasmonic thin film samples CP, CAuP, CAu400 and CAu500, respectively. Cracks in CuO and Au-CuO hybrid thin film were observed in
samples CP and CAuP as shown in Fig. 1(a–b). As clearly seen from the
SEM images, thermal annealing of Au-CuO hybrid plasmonic thin film
led to increase in size of Au nanoparticles in Au-CuO hybrid thin film.
The average size of Au nanoparticles in Au-CuO hybrid plasmonic thin
film was estimated to be 46, 51 and 76 nm for the samples CAuP,
CAu400 and CAu500, respectively. XRD was done to study the crystal
structure of prepared CuO and Au-CuO thin films. Fig. 2 displays the
XRD patterns of the prepared samples CP, CAuP, CAu400 and CAu500.
XRD data clearly show the presence of peaks corresponding to CuO and
Au in the samples CAuP, CAu400 and CAu500. XRD patterns of the
prepared samples show monoclinic CuO and face centered cubic Au.
XRD pattern of sample CP shows three peaks marked (1̄11), (1̄12) and
(1̄13) which were well matched with the standard data (JCPDS
89–5895) of monoclinic phase of CuO. The XRD patterns of samples
CAuP, CAu400 and CAu500 show peaks marked (1̄11), (1̄12) and (1̄13) ,
which were well matched with standard data (JCPDS 89–5895) of
monoclinic phase of CuO whereas the peaks marked (111), (200), (220)
and (311) matched well with the standard data (JCPDS 00-001-1172) of
Fig. 6(a–d) presents the temporal variations in absorption spectra of
MB dye with the synthesized thin film samples CP, CAuP, CAu400 and
CAu500. It can be distinctly observed that Au-CuO hybrid plasmonic
thin film without annealing (CAuP) shows enhanced photocatalytic
performance over other samples. Au decorated CuO thin film without
annealing degraded 84% of MB dye in 90 min. Kinetics of MB decomposition in the presence of Au-CuO hybrid plasmonic nanostructures are
shown in Fig. 6(e). The efficiency of Au-CuO photocatalysts for decomposition of MB is shown as a function of exposure time in Fig. 6(f).
Fig. 7(a–d) shows the temporal variations in the absorption spectra of
MG with prepared thin film photocatalysts CP, CAuP, CAu400 and
CAu500. Au-CuO hybrid plasmonic thin film without annealing (CAuP)
shows enhanced photocatalytic activity as compared to other samples.
Au decorated CuO sample (CAuP) degraded 85% of MG dye in 90 min.
The degradation kinetics of MG in the presence of the synthesized
photocatalysts are displayed in Fig. 7(e). The efficiency of Au-CuO
photocatalysts for decomposition of MG is shown as a function of exposure time in Fig. 7(f). First order kinetics equation was used to determine the rate constants for decomposition of dyes
C = C0 exp(−kt )
4
(2)
Materials Research Bulletin 123 (2020) 110707
K. Sahu, et al.
Fig. 6. (a–d) Temporal variations in the absorption spectra of MB with photocatalysts. (e) Degradation kinetics of MB by thin film photocatalysts. (f) Efficiency of the
synthesized thin film photocatalysts for the decomposition of MB.
hybrid plasmonic thin film sample was found to degrade 72.3 % of MB
dye in 2.5 h. Kinetics of MB decomposition in the presence of thin film
photocatalysts are presented in Fig. 8(c). Efficiency of the prepared
nanostructured CuO and Au-CuO hybrid plasmonic thin film photocatalysts for removal of MB are shown in Fig. 8(d). The enriched photocatalytic activity of the as-deposited Au-CuO plasmonic thin film can
be easily understood as follows. When photon of energy hυ strikes nanostructured CuO surface, excitation of electrons takes place from the
valence band to its conduction band leaving holes in the valence band
(Eq. 3). Contact of the photogenerated holes with water and photogenerated electrons with oxygen leads to the formation of hydroxyl
radicals and superoxide radicals, respectively (Eq. 4–6). Superoxide
radicals and hydroxyl radicals react with organic dyes and degrade
them into non-toxic compounds (equation 7 & 8). Photocatalytic
where C0 denotes the starting concentration of dye (without catalyst), C
denotes the concentration of dye with photocatalyst after sunlight exposure. The determined rate constants for MB degradation in the presence of thin film photocatalysts CP, CAuP, CAu400 and CAu500 are
0.008, 0.021, 0.014 and 0.016 min−1, respectively. The rate constant
values for MG degradation in the presence of photocatalysts CP, CAuP,
CAu400 and CAu500 are found to be 0.007, 0.160, 0.0159 and
0.0163 min−1, respectively. Photocatalytic activities of CuO and AuCuO hybrid thin film samples were also evaluated by degradation of MB
under 632.8 nm visible light irradiation from He-Ne laser. Fig. 8(a–b)
displays the temporal variations in the absorption spectra of MB with
the synthesized thin film photocatalysts under 632.8 nm visible light
exposure. Au-CuO hybrid plasmonic thin film showed highly enhanced
photocatalytic activity as compared to pristine CuO thin film. Au-CuO
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Fig. 7. (a–c) Temporal variations in the absorption spectra of MG with photocatalysts. (e) Degradation kinetics of MG by the synthesized thin film photocatalysts. (f)
Efficiency of the thin film photocatalysts for the decomposition of MG.
of CuO to Au nanoparticles because of the difference in their Fermi
energy levels. Fermi level of the Au-CuO nanohybrids is lower than the
conduction band of CuO. Thus electron transfer from CuO nanostructures to Au nanoparticles enhances the separation between charge
carriers by reducing their recombination rate. Fig. 9(a) shows the
general schematic representation that describes the charge transportation in Au-CuO hybrid plasmonic nanostructures resulting in the decomposition of dye.
decomposition of dye by CuO nanostructures is defined by the equations as shown below
CuO + hυ → e− (C . B.) + h+ (V . B.)
(3)
h+
→ •OH
(4)
•O2−
(5)
e−
h+
+
OH−
+ O2 →
+ H2 O →
H+
+ •OH
(6)
•OH + organic dye → degradation products
(7)
•O2−
(8)
+ organic dye → degradation products
3.4. Role of active radicals
Reactive species significantly contribute to the photocatalytic removal of organic pollutants. TBA, CN, BQ and FA act as representative
In case of Au nanoparticle decorated CuO hybrid nanostructures,
flow of photogenerated electrons takes place from the conduction band
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Fig. 8. (a–b) Temporal variations in the absorption spectra of MB with thin film photocatalysts under He-Ne laser exposure. (c) Degradation kinetics of MB by thin
film photocatalysts. (d) Efficiency of the synthesized thin film photocatalysts for the decomposition of MB under He-Ne laser exposure.
Fig. 9. (a) General schematic representation of
possible mechanism of photocatalytic decomposition of dye by Au-CuO hybrid plasmonic
nanostructure. (b) Efficiency of Au-CuO plasmonic thin film for photocatalytic removal of
MB in the absence and presence of scavengers.
(c) C/C0 versus illumination time results of MB
decomposition for three runs in the presence of
Au-CuO nanohybrid thin film.
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K. Sahu, et al.
performance, higher surface area of the film because of cracks formed
in CuO thin film leading to efficient adsorption of dye molecules and
the smaller crystallite size of Au nanoparticles which helps in improving
charge separation.
Table 1
Comparative list showing the rate constants of catalysts towards the degradation of organic dye.
Photocatalyst
Rate constant
(min−1)
Reference
CuO petal-like nanostructures
CuO flower-like nanostructures
Ag-CuO nanostructures
CuO nanoflakes
Au-CuO nanohybrid nanostructured thin
film
CuO thin film
Au-CuO hybrid nanostructured thin film
CuO thin film
Au-CuO hybrid nanostructured thin film
0.0019 (MB)
0.0008 (MB)
0.0102 (MB)
0.003 (RhB)
0.0106 (RhB)
Ref
Ref
Ref
Ref
Ref
0.008
0.021
0.007
0.160
This
This
This
This
(MB)
(MB)
(MG)
(MG)
4. Conclusion
29
29
35
36
36
In summary, we have prepared Au-CuO plasmonic nanohybrid thin
films by simple thermal evaporation. Visible light and sunlight driven
photocatalytic activities of the prepared Au-CuO nanohybrid thin films
were evaluated. Au-CuO hybrid plasmonic thin film without annealing
showed superior photocatalytic activities over pristine CuO and annealed Au-CuO thin film samples and led to complete degradation of
MG and MB in 90 min. UV–vis absorption studies showed that Au-CuO
hybrid thin film without annealing has higher absorbance of light in the
UV–vis region. Effective suppression of charge carriers in nanostructured CuO thin film and improvement in its light utilization capability upon Au nanoparticle decoration are the main factors leading to
the enhanced photocatalytic behavior of Au-CuO plasmonic thin film.
Superior photocatalytic performance and simple synthesis of hybrid
nanostructured thin film of Au-CuO can enrich efficient material systems for the sunlight and visible light driven reusable photocatalysis
towards efficient removal of organic pollutants.
work
work
work
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scavengers for hydroxyl radicals (%OH), photogenerated electrons (e−),
superoxide radicals (•O2−) and photogenerated holes (h+), respectively.
The efficiency of Au-CuO plasmonic thin film for photocatalytic removal of MB in the absence and presence of these scavengers for the
same sunlight exposure conditions is shown in Fig. 9(b). Addition of BQ
resulted in decrease in photocatalytic efficiency from 84.2%–79.1%,
while the efficiency reduced to 68.2% with the addition of FA. This
clearly indicates that •O2− and photogenerated electrons (e–) radicals
play minor roles in the photocatalytic degradation process. The photocatalytic degradation efficiency of Au-CuO plasmonic thin film was
reduced from 84.2%–59.3% when CN was added into the system. When
TBA was added into the system, the photocatalytic degradation efficiency of Au-CuO plasmonic thin film was significantly inhibited (reduced from 84.2%–12.6%). This shows that %OH significantly affects
the photocatalytic efficiency of the prepared Au-CuO plasmonic thin
film. Hydroxyl radicals were observed to be the main active species
accountable for the photocatalytic removal of MB in water by Au-CuO
hybrid plasmonic thin film.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is financially supported by Department of Science and
Techonology (DST), New Delhi under WOS-A scheme (SR/WOS-A/ PM10/2017 (G&C)). The authors thank Dr. Rahul Singhal for the XPS
studies.
3.5. Reusability of photocatalysts
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The reusability of Au-CuO nanohybrids was examined for the decomposition of MB under the same reaction conditions. Fig. 9(c) shows
the C/C0 versus illumination time results of MB decomposition for three
runs in the presence of Au-CuO nanohybrids thin film. No significant
change in the photocatalytic activity was observed even after three
cycles which shows the reusability of the prepared Au-CuO hybrid
plasmonic thin film photocatalysts towards the degradation of organic
dyes in water.
Decoration of Au nanoparticles onto nanostructured thin film of
CuO significantly improves the absorption of visible light due to SPR
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