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One-pot facile synthesis of branched Ag-ZnO heterojunction nanostructure as highly efficient photocatalytic catalyst
Accepted Manuscript Title: One-pot facile synthesis of branched Ag-ZnO heterojunction nanostructure as highly efficient photocatalytic catalyst Author: Qingli Huang Qitao Zhang Saisai Yuan Yongcai Zhang Ming Zhang PII: DOI: Reference:
S0169-4332(15)01545-7 http://dx.doi.org/doi:10.1016/j.apsusc.2015.06.197 APSUSC 30718
To appear in:
APSUSC
Received date: Revised date: Accepted date:
5-5-2015 16-6-2015 29-6-2015
Please cite this article as: Q. Huang, Q. Zhang, S. Yuan, Y. Zhang, M. Zhang, One-pot facile synthesis of branched Ag-ZnO heterojunction nanostructure as highly efficient photocatalytic catalyst, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.06.197 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphic abstract
The branched Ag-ZnO heterojunction nanostructures with enhanced photocatalytic properties were prepared by a facile, green and one-pot hydrothermal method, which showed high photodegradation activities whether in RhB aqueous solution or in CH3CHO gaseous phase.
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Highlights
●A branched Ag-ZnO nanostructure was synthesized by a one-pot method. ●RhB aqueous solution and CH3CHO gas can be fully degraded by Ag-ZnO. ●The branched Ag-ZnO possessed the enhanced photocatalytic properties.
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One-pot facile synthesis of branched Ag-ZnO heterojunction nanostructure as highly efficient photocatalytic catalyst Qingli Huang a,b∗, Qitao Zhangb,c, Saisai Yuanb,c , Yongcai Zhangb, Ming Zhanga,b a
b
Testing center, Yangzhou University, Yangzhou city, Jiangsu 225009, China Key Laboratory of Environmental Material and Environmental Engineering of
Jiangsu Province, College of Chemistry and Chemical Engineering, Yangzhou University, Jiangsu 225002, China c
Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of
Technology, Kitakyushu 804-8550, Japan ∗
To whom correspondence should be addressed. Fax: ++86-514-87979244 E-mail: [email protected] 3
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ABSTRACT： In this paper, the branched Ag-ZnO heterojunction nanostructure and the branched ZnO were synthesized successfully by a facile, green and one-pot hydrothermal
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method. Such branched heterojunction and the comparing branched pure ZnO were characterized by X-ray diffraction, field emission scanning electron microscopy high
resolution
transmission
electron
microscopy
(HRTEM),
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(FESEM),
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energy-dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy (PL) and UV-vis diffuse reflectance spectra (DRS).
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The photocatalytic degradation of RhB aqueous solution and acetaldehyde (CH3CHO)
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gas results both showed that the branched Ag-ZnO heterojunction possessed the enhanced photocatalytic properties in comparison to the branched ZnO and Ag-ZnO
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counterparts due to its special interface structures and fast separation of its
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photogenerated charge carriers. This method is simple, feasible and can provide an important clue for synthesis and application of other branched metal/semiconductor heterojunction nanostructures.
KEYWORDS: branched Ag-ZnO; electron microscopy; photocatalytic; acetaldehyde
1. Introduction
Branched metal/semiconductor heterojunction nanostructures have received broad attention due to their distinguished performance in electronics, optics and photonics [1-7].
It has been found that the desirable performances of heterojunction
nanostructures could be obtained by controlling the compositions, sizes, morphologies, and organization patterns of primary blocks. Many efforts have been made on the 4
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synthesis of branched metal/semiconductor heterojunction nanostructures employing various methods [1-7]. However, most of these methods usually involve at least two steps: metal nanocrystal preparation and the subsequent growth of semiconductor onto
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the as prepared nanomaterials. Tremendous challenges still exist to design a simple and feasible route to synthesize aforementioned heterojunctions at a high quality and
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in large quantities. One step or one-pot hydrothermal synthetic method has showed an
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extraordinary ability in the controllable fabrication of these heterojunction nanomaterials not only because it is simple, but also because it can produce
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high-quality heterojunction nanomaterials.
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With the rapid development of industry, an enormous amount of industrial dyes and volatile organic compounds (VOCs) have brought serious threats to our health.
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Increasing attention has been paid to the degradation of industrial dyes as well as
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VOCs. ZnO is one of the most promising candidates for the degradation of organic pollutants because of its low cost, environmental friendliness and wide band gap (3.37 eV) [8-10]. A copious amount of studies have been conducted on the synthesis and photocatalytic testing of ZnO nanocrystals for the degradation of organic pollutants both in aqueous solution and gaseous phase [11-17]. However, the quick recombination of the photo-excited electrons and holes in ZnO always leads to reduced photocatalytic efficiency. One of the methods to increase the efficiency of the photocatalytic
activity
is
to
develop
heterogeneous
metal/semiconductor
nanocomposite materials [18-20]. Ag-ZnO nanocomposites boost great research interest due to their high photocatalytic performance [21-35]. For example, Zhang et
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al. have reported a facile and template-free method for the controllable synthesis of Ag/ZnO composites with hollow coupled structures, which have enhanced photodegradation efficiency and electrocatalytic activities [23]. Deng et al. have
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prepared Ag–ZnO nanocomposites by loading Ag nanoparticles on flower-like ZnO microspheres by a two-step method. In comparison with the pure flower-like ZnO the
Ag–ZnO
nanocomposites
show
enhanced
photocatalytic
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microspheres,
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performances [27]. Sun et al. have obtained Ag/ZnO heterostructure nanocrystals with enhanced photocatalytic performance by a novel method using glucose as a reducing
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agent [31]. Liang et al. have synthesized porous 3D flower-like Ag/ZnO
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heterostructural composites with tunable silver contents by hydrothermal and photo-chemical deposition methods, without using any pore-directing reagents and
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surfactants. The photocatalytic performance of these heterostructures can be
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controlled by adjusting the reactant concentration such as silver nitrate [34]. Though various methods have been used to successfully synthesize the Ag/ZnO nanocrystal, the design and synthesis of Ag–ZnO hybrid nanocrystals with specific morphologies and well-defined interface still needs extra exploration. Unlike the previous cases, uniform Ag-ZnO heterojunction nanostructure was prepared by a one-pot hydrothermal method in this work. It has turned out that the typical Ag-ZnO heterojunction nanostructure displayed high photodegradation activities either in RhB aqueous solution or in CH3CHO gaseous phase. This Ag-ZnO heterojunction photocatalysts also possessed excellent stability and high efficiency. Moreover, photocatalytic mechanisms were also discussed briefly. The synthesis
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strategy revealed in this study offers important clues for the preparation of metal–semiconductor
hybrid
nanocrystals
with
tunable
morphologies
and
performances.
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2. Experimetal section 2.1 Materials
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All the chemical reagents used in this work include silver nitrate (AgNO3),
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hydrogen peroxide (H2O2, Wt 30%), zinc nitrate hexahydrate (Zn(NO3)2﹒6H2O), diethanolamine (C4H11NO2), rhodamine B (RhB), commercial ZnO, aceteldehyde
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(CH3CHO) and ethanol (C2H5OH). All chemicals were analytically pure and were
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2.2 Methods
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the experiment.
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used as received without further purification. Deionized water was used throughout
A typical synthesis of branched ZnO: 6.0 mmol of zinc nitrate hexahydrate
(Zn(NO3)2﹒6H2O) was dissolved in 50 mL of distilled water with magnetic stirring for 10 min. And 6 mL diethanolamine (C4H11NO2) was added into the above solution with stirring and a white suspension were obtained. Then 6 mL H2O2 (Wt 5%) was added stirred for 10 min. Then the mixed solution was added the distilled water to 85 mL and transferred into a Teflon-lined autoclave of 100 mL capacity and. After being sealed and heated at 180 ℃ for 12 h, the autoclave was cooled to room temperature naturally. The resulting products were collected by centrifugation, washed with distilled water and ethanol for several times, and finally dried in vacuum at 60 ℃ for 6 h. 7
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A typical synthesis of branched Ag–ZnO heterojunctions: 6.0 mmol zinc nitrate hexahydrate (Zn(NO3)2﹒6H2O) was dissolved in 50 mL distilled water with magnetic stirring for 10 min. And 6 mL diethanolamine (C4H11NO2) was added into the above
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solution with stirring, followed with 6 mL H2O2 (Wt 30%) added. Finally, 9 mL silver nitrate aqueous solution (AgNO3, 0.1 M) was poured into the solution. And the
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mixed solution was transferred into a Teflon-lined autoclave of 100 mL capacity,
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filled up to 85% of the total volume with deionized water. After being sealed and
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heated at 180 ℃ for 12 h, the autoclave was cooled to room temperature naturally. The resulting products were collected by centrifugation, washed with distilled water
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2.3 Characterization
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and ethanol for several times, and finally dried in vacuum at 60 ℃ for 6 h.
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The phase purity of the products was characterized by X-ray diffraction (XRD, German Bruker AXSD8 ADVANCE X-ray diffractometer) using a X-ray diffractometer with Cu Kα radiation (λ=1.5418 Å). Morphology information of the as-prepared particles was obtained on a Japan Hitachi S-4800 field emission scanning electron microscope (SEM). Transmission electron microscope (TEM) images, high resolution transmission electron microscopy (HRTEM) images, high angle annular dark field (HAADF) images and elemental mapping images were obtained on an American FEI Tecnai G2 F30 S-TWIN field-emission transmission electron microscopy (operated at 300 kV). Diffuse reflectance spectra (DRS) were measured using a UV–vis spectrophotometer (Shimadzu,UV-2600) equipped with an integrating
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sphere unit (Shimadzu,ISR-2600 Plus). Fluorescence spectra were obtained using a photoluminescence spectrometer (JASCO, FP-8500). X-ray photoelectron spectra (XPS) were recorded on a ESCALAB 250Xi system (Thermo Scientific). The specific
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surface area was carried out on a 3flex surface characterization analyzer (micromeritics, America).
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2.4 Photocatalytic testing
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RhB aqueous solution photodegradation evaluation test: the photocatalytic
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activities of as-prepared products were tested by degrading an organic dye, rhodamine B (RhB) in aqueous solution. In the photocatalytic experiments, 20 mg as-synthesized
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nanocrystals were added into a bottle which contains 100 mL RhB aqueous solution (1×10-5 M), and the solution was stirred in the dark for 30 min to reach
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adsorption–desorption equilibrium between the catalyst and RhB. Then, the mixture was irradiated using two 125 W UV lamp. After the mixture was irradiated for a given time and about 3 mL was taken out and immediately centrifuged. The degradation of RhB was monitored by measuring the absorbance of the solution using a Cary 5000 UV-vis spectrophotometer.
CH3CHO gaseous phase photodegradation evaluation test: 0.1 g samples were
spread on the bottom of a watch-glass. The watch-glass was put and 125 mL acetaldehyde/ pure air mixture gases (550 ppm) were injected into the Tedlar bag (Polyvinyl fluoride, As ONE Co.Ltd). The bag was located in dark room for 2 h to ensure the adsorption equilibrium. Then the samples were irradiated under LED lamp (central wavelength of 365 nm for UV light and 435 nm for visible light) with 1 9
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mW/cm2 light intensity. The concentration variation of acetaldehyde and CO2 evolution were monitored by on-line gas chromatography (Agilent Technologies, 3000A Micro-GC, TCD detector) equipped with OV1 and PLOT-Q columns.
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3. Results and discussion The morphology of the as-prepared branched Ag-ZnO heterostructure was
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characterized by SEM. A low-magnification image of Ag-ZnO is shown in Fig. 1a. It
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clearly reveals that the well dispersed samples are composed of urchin-like flowers with the diameters in the range of 2–3 µm, and the flowers are uniform in shape with
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the center as common growth points for all of the constituent nanorods. From the
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high-magnification image in Fig. 1b, the nanorod is about 100 nm in diameter and 1 µm in length. The composition of the Ag–ZnO hybrids is examined by EDS. The
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EDS spectrum of the Ag–ZnO is shown in Fig. 1c. The existence of silver, zinc, and
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oxygen confirms the silver and ZnO components and the mass content of Ag was about 10.38 wt%, according to the semi-quantitative analysis of EDS. The XRD patterns of the as-synthesized samples are shown in Fig. 1d. Different from one set of diffraction patterns in the branched ZnO structure (ESI, Fig.S1 and S2), two sets of diffraction patterns are found for the as-synthesized Ag-ZnO sample: those marked with “#” are indexed to hexagonal wurtzite ZnO (JCPDS 36-1451), while the one marked with “*” is ascribed to face-centered cubic (fcc) metallic Ag (JCPDS 04-0783). No other crystalline impurities can be observed in the diffraction patterns. The transmission electron microscopy (TEM) images further confirm that the hybrid flowerlike structures are constructed from orderly radial nanorod arrays with
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the rod oriented lengthwise from the centre the Ag core to the surface of the flower (Fig. 2a and 2b). The size of the Ag core is about 500 nm. High-resolution TEM (HRTEM) image (Fig. 2c) displays clear lattice fringes of the ZnO nanorod.
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Combining with selected area electron diffraction (SAED) in Fig. 2d, it confirms the crystal structure of ZnO with d002 = 0.26 nm and d010=0.28 nm and the growth
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direction is determined to be along the c axis. Interestingly, though the growth
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direction of single rod in the branched ZnO structure (ESI, Fig. S3a) is also along the c axis, the other growth direction is [110], which is different from that of Ag-ZnO
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heterojunction (ESI, Fig. S3b and S3c). Fig. 2e is a HRTEM image showing the
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hetero-interface between Ag and ZnO. The interplanar distance of 0.26 nm corresponds to (002) crystal plane of ZnO, and that of 0.23 nm in the attached
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nanoparticle corresponds to Ag (111) crystal plane. ZnO nanorods are attached to Ag
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core tightly without any voids, which well agree with the XRD results as shown in Fig. 1d. And the corresponding fast Fourier transform (FFT) patterns of the hetero-interface between Ag and ZnO are shown in Fig. 2g. It reveals that the basal plane of ZnO nanorods grows on the basis of Ag (111) facets. The high angle annular dark field (HAADF) images and energy-dispersive spectroscopy elemental mapping images are shown in Fig. 3. The different contrast means different element information (Fig. 3a and 3b). It further confirms that the branched Ag-ZnO heterojunction nanostructures were designed. The enlarged image in Fig. 3c shows many holes are scattered on the surface of ZnO nanorods, which can enhance the specific surface area and defects of ZnO rods (the specific surface area of
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Ag-ZnO is 4.23 m2/g; the specific surface area of ZnO is 2.01 m2/g). The elemental map of the Ag-ZnO nanostructures was obtained using energy-dispersive spectroscopy as shown in Fig. 3d, e and f. Different colors indicate the presence of
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different elements, where yellow, red and green refer to the presence of Zn, O, and Ag respectively. It can be clearly seen that the center core is Ag and ZnO nanorods are in
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the epitaxial growth direction.
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To clarify the elemental composition and chemical state of the as-synthesized branched Ag-ZnO heterojunction nanostructures, XPS spectra were carried out with
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the binding energies calibrated using C 1s (284.8 eV), as shown in Fig. 4. The peaks
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can be ascribed to Zn, O, C, and Ag (Fig. 4a,) confirming the heterostructure is prepared comparing to the only Zn, O, C elements present in the branched ZnO (ESI,
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Fig. S4). High-resolution spectra of Zn, O and Ag species obtained from branched
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Ag-ZnO heterojunction and branched ZnO are shown in Fig. 4(b)–(d) and Fig. S4 (b)–(d) respectively. Two strong peaks in Fig. 4b center on 1020.9 and 1043.9 eV, which are in agreement with the binding energies of Zn 2p3/2 and Zn 2p1/2 respectively. There is a little shift comparing to pure branched ZnO, which may be attribute to the electron transfer between ZnO and Ag. In Fig. 4c, O 1s profile is asymmetric and can be fitted into two symmetrical peaks at 529.9 eV (red line) and 531.3 eV (green line), which agree well with the reported value for ZnO [36, 37]. The two peaks originate from the lattice oxygen of ZnO and adsorbed oxygen respectively. The two peaks centered at 367.3 and 373.3 eV in Fig. 4d can be attributed to Ag 3d5/2 and Ag 3d3/2 respectively. It should note that the peaks of Ag
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shift obviously to lower binding energy compared with the standard value (Ag 3d5/2, 368.2 eV; Ag 3d3/2, 374.2 eV). The binding energy of Ag for the Ag-ZnO sample is much too lower than zerovalent Ag, indicating the transfer of electrons from Ag
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nanoparticles to ZnO nanorods, which is in good agreement with the fabrication of Ag-ZnO heterojunctions in the previous literatures [38-40]. Therefore, the XPS result
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further confirms the sample is composed of ZnO and Ag.
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The structure and morphologies of the branched Ag-ZnO heterojunction nanostructures closely depend on different AgNO3 volume, while other conditions
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remain unchanged. When the volume of AgNO3 was decreased to 6 mL, the branched
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heterojunction structures disappeared. A lot of nanoparticles with ca. 100 nm in size and hexagonal prism structures with the diameter of ca.1 µm and length of ca. 3 µm
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were found in Fig 5a. When the volume of AgNO3 was decreased to 3 mL, the
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products remain hexagonal prism structures with nanoparticles scattering around it. In spite of the length of hexagonal prism structure unchanged, the diameter increase to ca. 2 µm and the size of Ag nanoparticles decreased to 70 nm. Further decreasing the volume of AgNO3 to 0.6 mL, the diameter of the hexagonal prism further increase to ca. 3 µm keeping the length of hexagonal prism structure unchanged. And the size and the amount of Ag nanoparticles are all decreased. A completely different morphology and phase is formed when no AgNO3 was added in Fig 5d. XRD pattern shows that the product is Zn5(CO3)2(OH)6 (JCPDS 72-1100, Fig.S5). Overall, it can be inferred from the SEM studies that the volume of silver nitrate does play a very crucial role in the growth of Ag-ZnO crystals. According to early report [4], ZnO nanorods can grow
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directly on the facets of Ag without any other ancillary seed-assisted process. There is an equilibrium between the nucleation of Ag and the growth of ZnO, which is determined by the amount of AgNO3. When a small amount of AgNO3 was added, no
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heterojunction nanostructure was obtained due to the fast rate of the growth of ZnO. However, when excessive AgNO3 was added, ZnO-Ag heterojunction nanostructure
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was found according to the way of early report on the formation of ZnO-Ag [4]. Other
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parameters were also investigated. In the absence of diethanolamine, none of products were obtained. In the absence of H2O2, none of the morphologies of ZnO described
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previously can be formed (ESI, Fig.S6a). And large micro-rods of ZnO were found by
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decreasing the concentration of H2O2 to 15% and 5% (ESI, Fig.S6b and S6c). On the basis of the results discussed above and further investigations of related
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literatures [41, 42], a possible formation mechanism is proposed, as schematically
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shown in Scheme 2. Firstly, diethanolamine hydrolyzed and produced a lot of hydroxyl radicals in reaction system and caused Zn2+ and Ag+ forming precursors (Zn(OH)42-) and Ag2O···Ag+. Then, the open frameworks were formed between Ag2O···Ag+ and Zn(OH)42- due to the weak interparticle forces, such as van der Waals and electrostatic interactions. Subsequently, diethanolamine has a certain reducing ability due to its two hydroxyl groups which can reduce Ag2O species to tiny Ag nanoparticles when the temperature was increased. At the same time, H2O2 acts as an oxidant, which can dissolve unstable Ag nanoparticles to Ag+ due to the potentials higher than that of Ag+/Ag (E0 = 0.7996 V). The new Ag+ can be absorbed on the surface of remained Ag2O, which further sustains the frameworks between
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Ag2O···Ag+ and Zn(OH)42-. There is a dynamic equilibrium between the reduction and oxidation, which provides the buffer and creates in situ stable nucleation sites for epitaxial growth of ZnO. As the reaction proceeded, Ag2O was reduced into stable Ag
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particles by diethanolamine, which in turn catalyzed the assembling of ZnO on the surface of Ag particles. Finally, ZnO can grow around the Ag core and eventually
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formed a branched structure. Here, the Ag nanoparticles act as a catalyzer to induce
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the formation of branched Ag-ZnO heterojunctions. The overall possible chemical reaction equations during the above experiment were listed as followed. Compared to
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the branched structure reported by Fan et al. [4], the maximum of more than eight
H2O + NH(CH2CH2OH)2
→
2Ag+ + H2O2 → Ag2O+H2O
NH2+(CH2CH2OH)2 +OH-
(1)
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H2O2.
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nanorods could be observed on a single Ag nanocrystal due to the etchant effect of
(2)
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Ag++Ag2O +Zn2++ 4OH-→Zn(OH)42-+ Ag2O···Ag+
(3)
Zn(OH)42-+ Ag2O···Ag+ + NH2+(CH2CH2OH)2→ZnO-Ag+other products in water (4) The optical absorption property of the samples, which is relevant to the electronic structure feature, is tested to explain the change in photocatalytic activity. The optical absorption of as-prepared samples were measured by diffuse reflectance spectra (DRS), and shown in Fig. 6. Relative to the branched ZnO and normal Ag-ZnO counterpart, the branched Ag-ZnO heterojunction possesses higher reflection peaks in ultraviolet region (365 nm). This may increase the UV catalytic activity to some extent. Fig. 7 shows the representative PL spectra of the as-prepared samples with the
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excitation wavelength of 325 nm at room temperature. Three samples reveal similar emission spectrum. In the ultraviolet region, the emission at around 380 nm was observed, which is due to radiative recombination of free excitions. Though the exact
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origins are currently still highly controversial and ambiguous, the visible emission may be due to the recombination of photo-generated holes with the electrons in
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intrinsic or extrinsic defects, such as oxygen vacancies (VO˙) and interstitial oxygen
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(Oi00) [43-47]. When a suitable scavenger or surface defect state is available to trap the electrons or holes, undesirable recombination can be prevented effectively. As the
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redox reactions might occur on the surface of the VO˙ and Oi00 defects, the oxygen
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defects can be considered to be the active sites of the ZnO photocatalyst. Compared to the branched ZnO and other Ag-ZnO composites, the formation of the branched
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Ag-ZnO heterojunction reduces the intensity of the UV emission with a clear red-shift.
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The intensity decrease and the red-shift of the UV emission for the Ag-ZnO heterojunction can be ascribed to the electron trapping effect of Ag nanoparticles, which act as electron acceptor, hindering the recombination of photogenerated carriers on ZnO. According to earlier reports, the electron trapping effect of Ag NPs is importantly favorable for the improvement of the photocatalytic activity of ZnO due to the enhancement of the separation.
From Fig.8 (a-b), we can conclude that the branched Ag-ZnO heterojunction photocatalyst shows the highest CO2 generation concentration (1100ppm) under UV light (365nm) , which is nearly 1.2 times, 7.6 times and 2.9 times higher activity than that of branched ZnO (900ppm), normal Ag-ZnO counterpart (145ppm) and
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commercial ZnO (384ppm). Namely, 550 ppm CH3CHO gas is fully degraded to CO2 by this branched Ag-ZnO photocatalyst within 16 hours at room temperature under 365 nm UV light irradiation (Fig.8b). In the same time, the photocatalytic
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performance of the branched Ag-ZnO heterojunction under visible light (435nm) was also investigated in Fig.8a. It can be seen that the CO2 generation concentration
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(1100ppm) under UV light (365nm) is 8.9 times higher activity than that under visible
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light (124nm). The photocatalytic performance improvement of the branched Ag-ZnO heterojunction photocatalyst is mainly attributed to its intimate interfacial contact
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between Ag core and ZnO nanorods. This unique Ag-ZnO structure is beneficial for
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photogenerated charge carriers (holes and electrons) separation to promote the photocatalytic enhancement, which is also well accordance with the PL analysis
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results in Fig.7. Therefore, many more photogenerated holes and electrons prefer to
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be involved in the photocatalytic reaction rather than be annihilated by the form of radiation. Meanwhile, the branched Ag-ZnO heterojunction is with good stability (ESI, see the XRD and DRS sprectra after photocatalytic reaction in Fig.S7 and S8) and high efficiency because this photocatalyst still retains 99% photocatalytic activity after three CH3CHO degradation recycles (Fig. 8d). Besides, the photodegradation experiments of RhB aqueous solution were further carried out to investigate the universality of this heterojunction photocatalyst. Obviously, the RhB dyes photodegradation results in solution in Fig.8c present a similar tendency with CH3CHO ones in gaseous phase. In summary, this in-situ prepared branched Ag-ZnO heterojunction is with excellent photodegradation performance in RhB dye and
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CH3CHO gas under UV light irradiation. The mechanisms of the enhanced photocatalytic activity of Ag-ZnO have been proposed by others [27, 34]. The enhanced photocatalytic performance in UV region
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was attributed to the formation of the Schottky barriers at Ag and ZnO interface when they are in direct contact, which improved the segregation of charges and prevented
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the charge recombination. For the branched heterostructures, it is stressed here that
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the center Ag particles and ZnO nanorods with high carrier transport property, serve as spatially extended catalyzing centers to provide direct and fast electron/hole
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transfer to their acceptors (H2O, O2, RhB and CH3CHO), which increases the chance
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of organic pollutants to be degraded, compared with commonly used Ag–ZnO and ZnO particles (Scheme 1). In fact, photocatalysis is a complicated process, which is
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associated with not only the structure-related physical properties, but also
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structure-related catalytic processes.
4. Conclusions
In summary, a facile and effective hydrothermal method was introduced to the one-pot synthesis of branched Ag-ZnO heterojunction nanostructures. The products were characterized by XRD, FESEM, HRTEM, XPS, PL and UV–vis. Morphological control of the metal–ZnO interface was achieved by adjusting the reaction parameters. The branched Ag-ZnO heterojunction nanostructures show its superior photocatalytic activity over the branched ZnO and its Ag-ZnO counterparts, which demonstrates its potential application as nanocatalysts both in dyes and VOCs degradation. The simplicity and generality of our method for different metals together with the growth 18
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selectivity offer great advantages and motivate further research on its application to other metal–metal oxide systems. References:
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an
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Huang, T.S. Chan, D. Huang, R. Liu, D.P. Tsai, Hydrogen generation: plasmonic ZnO/Ag embedded structures as collecting layers for photogenerating electrons in
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[26] C.Q. Chen, Y.H. Zheng, Y.Y. Zhan, X.Y. Lin, Q. Zheng, K.M. Wei, Enhanced Raman scattering and photocatalytic activity of Ag/ZnO heterojunction nanocrystals, Dalton Trans. 40 (2011) 9566-9570.
[27] Q. Deng, H.B. Tang, G. Liu, X.P. Song, G.P. Xu, Q. Li, D. H.L. Ng, G.Z. Wang, The fabrication and photocatalytic performances of flower-like Ag nanoparticles/ZnO nanosheets-assembled microspheres, Appl. Surf. Sci. 331 (2015) 50–57. [28] Z.Z. Han, L.L. Ren, Z.H. Cui, C.Q. Chen, H.B. Pan, J.Z. Chen, Ag/ZnO flower
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heterostructures as a visible-light driven photocatalyst via surface plasmon resonance, Appl. Catal. B 126 (2012) 298-305. [29] Y.S. Liu, S.H. Wei, W. Gao, Ag/ZnO heterostructures and their photocatalytic
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[34] Y.M. Liang, N. Guo, L.L. Li, R.Q. Li, G.J. Ji, S.C. Gan, Fabrication of porous 3D
flower-like
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performance, Mater. Res. Bull. 48 (2013) 4287–4293. [36] P. Gao, Z.Y. Liu, D.D. Sun, The synergetic effect of sulfonated graphene and silver as co-catalysts for highly efficient photocatalytic hydrogen production of ZnO
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d
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[41] J.J. Feng, Q.C. Liao, A.J. Wang, J.R. Chen, Mannite supported hydrothermal synthesis of hollow flower-like ZnO structres for photocatalytic application, CrystEngComm 13 (2011) 4202-4210.
[42] J.S. Xie, Q.S. Wu, One-pot synthesis of ZnO/Ag nanospheres with enhanced photocatalytic activity, Mater. Lett. 64 (2010) 389-392. [43] F. Xu, Y.T. Shen, L.T. Sun, H.B. Zeng, Y.N. Lu, Enhanced photocatalytic activity of hierarchical ZnO nanoplate-nanowire architecture as environmentally safe and
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facilely recyclable photocatalyst, Nanoscale 3 (2011) 5020-5025. [44] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, Mechanisms behind green photoluminescence in ZnO phosphor powders, J. Appl. Phys. 79 (1996)
ip t
7983-7990. [45] Y.H. Zheng, C.Q. Chen, Y.Y. Zhan, X.Y. Lin, Q. Zheng, K.M. Wei, J.F. Zhu, Y.J.
cr
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between structure and property, Inorg. Chem. 46 (2007) 6675-6682.
[46] S.B. Zhang, S.H. Wei, A. Zunger, “Intrinsic n-type versus p-type doping
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075205-1-7.
[47] A.V. Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, The kinetics
d
of the radiative and nonradiative processes in nanocrystalline ZnO particles upon
Ac ce pt e
photoexcitation, J. Phys. Chem. B 104 (2000) 1715-1723. Fig. Caption:
Fig. 1. (a) low-magnification, (b) enlarged-magnification of SEM images, (c) EDS spectrum (d) XRD pattern of branched Ag-ZnO heterostructure. Fig. 2. (a) low-magnification (b) single branched Ag-ZnO heterostructure of the branched Ag/ZnO heterostructure, (c) HRTEM and (d) SAED image of single ZnO nanorod (e) HRTEM (f) the corresponding FFT image of Ag core. Fig. 3. (a-c) HAADF STEM and elemental mapping images of the branched Ag/ZnO heterostructure (d) Zn elemental mapping (e) O elemental mapping (f) Ag elemental mapping. 25
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Fig. 4. XPS spectra of the branched Ag-ZnO heterostructure (a) the scan (b) Zn2p (c) O1s (d) Ag3d Fig. 5. SEM images of Ag-ZnO hybrids prepared with different volume of AgNO3
ip t
solution: (a) 6mL (b) 3 mL and (c) 0.6 mL (d) 0 mL (keeping the other parameter unchaged).
cr
Fig. 6. DRS spectra of the as-prepared samples.
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Fig. 7. PL spectra of as-prepared samples.
Fig. 8. The photocatalytic performance evaluation curves of the as-prepared
an
photocatalysts (a) CO2 generation ( b); CH3CHO concentration; (c) RhB degradation
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and (d) three recycles of CH3CHO photodegradation evaluation by the branched Ag-ZnO.
Ac ce pt e
heterostructures.
d
Scheme 1. A proposed photocatalytic mechanism of the branched Ag-ZnO
Scheme 2.
The mechanism of the formation of ZnO-Ag heterojunction
nanostructure.
Fig.
26
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5
#(101)
30
#(110)
*(111)
40
50
60
70
*(311) #(004)
#(100)
20
10
#(103) *(220) #(220) #(112) #(201)
Zn
# ZnO * Ag
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Zn Ag
#(002)
Ag 4.18
cr
ip t 0
(d) Ag 10.38
*(200) #(102)
O
Weight % O Zn 17.71 71.91 Atom % O Zn 48.07 47.75
an
Counts(a.u)
Zn
Intensity (a.u.)
(c)
80
2θ (deg.)
Energy(Kev)
Ac ce pt e
d
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Fig. 1
27
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Ac ce pt e
d
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Fig. 2
28
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Ac ce pt e
d
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Fig. 3
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400
600
800
1000
1200
1020
1025
Binding Energy (ev)
1035
(d)
530
532
534
367.33
1040
1045
1050
us
373.33
an
XPS Intensity (a.u)
XPS Intensity (a.u)
(c)
528
1030
Binding Energy (ev)
cr
200
1043.93
ip t
XPS Intensity (a.u) 0
1020.93 (b)
XPS Intensity (a.u)
(a)
536
365
Binding Energy (ev)
370
375
380
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Binding Energy (ev)
Ac ce pt e
d
Fig. 4
30
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d
Fig. 5
31
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Reflectance ( %)
the branched ZnO the branched Ag-ZnO Ag-ZnO 0.6mL AgNO3
300
400
500
600
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200
700
800
Wavelength ( nm )
Ac ce pt e
d
Fig. 6
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450
500
Wavelength (nm)
600
Ac ce pt e
d
M
an
Fig. 7
550
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400
cr
ip t
Intensity (a.u.)
the branched Ag-ZnO the branched ZnO Ag-ZnO (0.6mLAgNO3)
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600
the branched ZnO the branched Ag-ZnO Ag-ZnO 0.6mL AgNO3 the branched Ag-ZnO under visible light commercial ZnO
(a)
1400 1200
500
CH3CHO concentration (ppm)
(b)
the branched ZnO the branched Ag-ZnO Ag-ZnO 0.6mL AgNO3
400
1000
300
800 600
100
200 0 0
5
10
15
20
0
25
0
Irradiation Time (h)
1.0
(c)
(d)
0.9
CO2 generation (ppm)
0.8 0.7
15
an
C/C0
0.6 0.5 0.4 0.3 0.1 0
20
40
60
20
25
80
Recycle 1 Recycle 2 Recycle 3
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75
Irradiation Time ( h )
d
Time (minutes)
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no any catalysts) Ag-ZnO 0.6mLAgNO3 the branched ZnO the branched Ag-ZnO
0.2 0.0
10
Irradiation Time (h)
cr
1.1
5
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200
400
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CO2 generation (ppm)
1600
Ac ce pt e
Fig. 8
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Ac ce pt e
d
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Scheme 1.
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d
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an
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Scheme 2.
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S0169-4332(15)01545-7 http://dx.doi.org/doi:10.1016/j.apsusc.2015.06.197 APSUSC 30718
To appear in:
APSUSC
Received date: Revised date: Accepted date:
5-5-2015 16-6-2015 29-6-2015
Please cite this article as: Q. Huang, Q. Zhang, S. Yuan, Y. Zhang, M. Zhang, One-pot facile synthesis of branched Ag-ZnO heterojunction nanostructure as highly efficient photocatalytic catalyst, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.06.197 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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an
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Graphic abstract
The branched Ag-ZnO heterojunction nanostructures with enhanced photocatalytic properties were prepared by a facile, green and one-pot hydrothermal method, which showed high photodegradation activities whether in RhB aqueous solution or in CH3CHO gaseous phase.
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Highlights
●A branched Ag-ZnO nanostructure was synthesized by a one-pot method. ●RhB aqueous solution and CH3CHO gas can be fully degraded by Ag-ZnO. ●The branched Ag-ZnO possessed the enhanced photocatalytic properties.
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One-pot facile synthesis of branched Ag-ZnO heterojunction nanostructure as highly efficient photocatalytic catalyst Qingli Huang a,b∗, Qitao Zhangb,c, Saisai Yuanb,c , Yongcai Zhangb, Ming Zhanga,b a
b
Testing center, Yangzhou University, Yangzhou city, Jiangsu 225009, China Key Laboratory of Environmental Material and Environmental Engineering of
Jiangsu Province, College of Chemistry and Chemical Engineering, Yangzhou University, Jiangsu 225002, China c
Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of
Technology, Kitakyushu 804-8550, Japan ∗
To whom correspondence should be addressed. Fax: ++86-514-87979244 E-mail: [email protected] 3
Page 3 of 36
ABSTRACT： In this paper, the branched Ag-ZnO heterojunction nanostructure and the branched ZnO were synthesized successfully by a facile, green and one-pot hydrothermal
ip t
method. Such branched heterojunction and the comparing branched pure ZnO were characterized by X-ray diffraction, field emission scanning electron microscopy high
resolution
transmission
electron
microscopy
(HRTEM),
cr
(FESEM),
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energy-dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy (PL) and UV-vis diffuse reflectance spectra (DRS).
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The photocatalytic degradation of RhB aqueous solution and acetaldehyde (CH3CHO)
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gas results both showed that the branched Ag-ZnO heterojunction possessed the enhanced photocatalytic properties in comparison to the branched ZnO and Ag-ZnO
d
counterparts due to its special interface structures and fast separation of its
Ac ce pt e
photogenerated charge carriers. This method is simple, feasible and can provide an important clue for synthesis and application of other branched metal/semiconductor heterojunction nanostructures.
KEYWORDS: branched Ag-ZnO; electron microscopy; photocatalytic; acetaldehyde
1. Introduction
Branched metal/semiconductor heterojunction nanostructures have received broad attention due to their distinguished performance in electronics, optics and photonics [1-7].
It has been found that the desirable performances of heterojunction
nanostructures could be obtained by controlling the compositions, sizes, morphologies, and organization patterns of primary blocks. Many efforts have been made on the 4
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synthesis of branched metal/semiconductor heterojunction nanostructures employing various methods [1-7]. However, most of these methods usually involve at least two steps: metal nanocrystal preparation and the subsequent growth of semiconductor onto
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the as prepared nanomaterials. Tremendous challenges still exist to design a simple and feasible route to synthesize aforementioned heterojunctions at a high quality and
cr
in large quantities. One step or one-pot hydrothermal synthetic method has showed an
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extraordinary ability in the controllable fabrication of these heterojunction nanomaterials not only because it is simple, but also because it can produce
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high-quality heterojunction nanomaterials.
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With the rapid development of industry, an enormous amount of industrial dyes and volatile organic compounds (VOCs) have brought serious threats to our health.
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Increasing attention has been paid to the degradation of industrial dyes as well as
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VOCs. ZnO is one of the most promising candidates for the degradation of organic pollutants because of its low cost, environmental friendliness and wide band gap (3.37 eV) [8-10]. A copious amount of studies have been conducted on the synthesis and photocatalytic testing of ZnO nanocrystals for the degradation of organic pollutants both in aqueous solution and gaseous phase [11-17]. However, the quick recombination of the photo-excited electrons and holes in ZnO always leads to reduced photocatalytic efficiency. One of the methods to increase the efficiency of the photocatalytic
activity
is
to
develop
heterogeneous
metal/semiconductor
nanocomposite materials [18-20]. Ag-ZnO nanocomposites boost great research interest due to their high photocatalytic performance [21-35]. For example, Zhang et
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al. have reported a facile and template-free method for the controllable synthesis of Ag/ZnO composites with hollow coupled structures, which have enhanced photodegradation efficiency and electrocatalytic activities [23]. Deng et al. have
ip t
prepared Ag–ZnO nanocomposites by loading Ag nanoparticles on flower-like ZnO microspheres by a two-step method. In comparison with the pure flower-like ZnO the
Ag–ZnO
nanocomposites
show
enhanced
photocatalytic
cr
microspheres,
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performances [27]. Sun et al. have obtained Ag/ZnO heterostructure nanocrystals with enhanced photocatalytic performance by a novel method using glucose as a reducing
an
agent [31]. Liang et al. have synthesized porous 3D flower-like Ag/ZnO
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heterostructural composites with tunable silver contents by hydrothermal and photo-chemical deposition methods, without using any pore-directing reagents and
d
surfactants. The photocatalytic performance of these heterostructures can be
Ac ce pt e
controlled by adjusting the reactant concentration such as silver nitrate [34]. Though various methods have been used to successfully synthesize the Ag/ZnO nanocrystal, the design and synthesis of Ag–ZnO hybrid nanocrystals with specific morphologies and well-defined interface still needs extra exploration. Unlike the previous cases, uniform Ag-ZnO heterojunction nanostructure was prepared by a one-pot hydrothermal method in this work. It has turned out that the typical Ag-ZnO heterojunction nanostructure displayed high photodegradation activities either in RhB aqueous solution or in CH3CHO gaseous phase. This Ag-ZnO heterojunction photocatalysts also possessed excellent stability and high efficiency. Moreover, photocatalytic mechanisms were also discussed briefly. The synthesis
6
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strategy revealed in this study offers important clues for the preparation of metal–semiconductor
hybrid
nanocrystals
with
tunable
morphologies
and
performances.
ip t
2. Experimetal section 2.1 Materials
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All the chemical reagents used in this work include silver nitrate (AgNO3),
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hydrogen peroxide (H2O2, Wt 30%), zinc nitrate hexahydrate (Zn(NO3)2﹒6H2O), diethanolamine (C4H11NO2), rhodamine B (RhB), commercial ZnO, aceteldehyde
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(CH3CHO) and ethanol (C2H5OH). All chemicals were analytically pure and were
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2.2 Methods
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the experiment.
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used as received without further purification. Deionized water was used throughout
A typical synthesis of branched ZnO: 6.0 mmol of zinc nitrate hexahydrate
(Zn(NO3)2﹒6H2O) was dissolved in 50 mL of distilled water with magnetic stirring for 10 min. And 6 mL diethanolamine (C4H11NO2) was added into the above solution with stirring and a white suspension were obtained. Then 6 mL H2O2 (Wt 5%) was added stirred for 10 min. Then the mixed solution was added the distilled water to 85 mL and transferred into a Teflon-lined autoclave of 100 mL capacity and. After being sealed and heated at 180 ℃ for 12 h, the autoclave was cooled to room temperature naturally. The resulting products were collected by centrifugation, washed with distilled water and ethanol for several times, and finally dried in vacuum at 60 ℃ for 6 h. 7
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A typical synthesis of branched Ag–ZnO heterojunctions: 6.0 mmol zinc nitrate hexahydrate (Zn(NO3)2﹒6H2O) was dissolved in 50 mL distilled water with magnetic stirring for 10 min. And 6 mL diethanolamine (C4H11NO2) was added into the above
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solution with stirring, followed with 6 mL H2O2 (Wt 30%) added. Finally, 9 mL silver nitrate aqueous solution (AgNO3, 0.1 M) was poured into the solution. And the
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mixed solution was transferred into a Teflon-lined autoclave of 100 mL capacity,
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filled up to 85% of the total volume with deionized water. After being sealed and
an
heated at 180 ℃ for 12 h, the autoclave was cooled to room temperature naturally. The resulting products were collected by centrifugation, washed with distilled water
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2.3 Characterization
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and ethanol for several times, and finally dried in vacuum at 60 ℃ for 6 h.
Ac ce pt e
The phase purity of the products was characterized by X-ray diffraction (XRD, German Bruker AXSD8 ADVANCE X-ray diffractometer) using a X-ray diffractometer with Cu Kα radiation (λ=1.5418 Å). Morphology information of the as-prepared particles was obtained on a Japan Hitachi S-4800 field emission scanning electron microscope (SEM). Transmission electron microscope (TEM) images, high resolution transmission electron microscopy (HRTEM) images, high angle annular dark field (HAADF) images and elemental mapping images were obtained on an American FEI Tecnai G2 F30 S-TWIN field-emission transmission electron microscopy (operated at 300 kV). Diffuse reflectance spectra (DRS) were measured using a UV–vis spectrophotometer (Shimadzu,UV-2600) equipped with an integrating
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sphere unit (Shimadzu,ISR-2600 Plus). Fluorescence spectra were obtained using a photoluminescence spectrometer (JASCO, FP-8500). X-ray photoelectron spectra (XPS) were recorded on a ESCALAB 250Xi system (Thermo Scientific). The specific
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surface area was carried out on a 3flex surface characterization analyzer (micromeritics, America).
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2.4 Photocatalytic testing
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RhB aqueous solution photodegradation evaluation test: the photocatalytic
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activities of as-prepared products were tested by degrading an organic dye, rhodamine B (RhB) in aqueous solution. In the photocatalytic experiments, 20 mg as-synthesized
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nanocrystals were added into a bottle which contains 100 mL RhB aqueous solution (1×10-5 M), and the solution was stirred in the dark for 30 min to reach
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adsorption–desorption equilibrium between the catalyst and RhB. Then, the mixture was irradiated using two 125 W UV lamp. After the mixture was irradiated for a given time and about 3 mL was taken out and immediately centrifuged. The degradation of RhB was monitored by measuring the absorbance of the solution using a Cary 5000 UV-vis spectrophotometer.
CH3CHO gaseous phase photodegradation evaluation test: 0.1 g samples were
spread on the bottom of a watch-glass. The watch-glass was put and 125 mL acetaldehyde/ pure air mixture gases (550 ppm) were injected into the Tedlar bag (Polyvinyl fluoride, As ONE Co.Ltd). The bag was located in dark room for 2 h to ensure the adsorption equilibrium. Then the samples were irradiated under LED lamp (central wavelength of 365 nm for UV light and 435 nm for visible light) with 1 9
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mW/cm2 light intensity. The concentration variation of acetaldehyde and CO2 evolution were monitored by on-line gas chromatography (Agilent Technologies, 3000A Micro-GC, TCD detector) equipped with OV1 and PLOT-Q columns.
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3. Results and discussion The morphology of the as-prepared branched Ag-ZnO heterostructure was
cr
characterized by SEM. A low-magnification image of Ag-ZnO is shown in Fig. 1a. It
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clearly reveals that the well dispersed samples are composed of urchin-like flowers with the diameters in the range of 2–3 µm, and the flowers are uniform in shape with
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the center as common growth points for all of the constituent nanorods. From the
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high-magnification image in Fig. 1b, the nanorod is about 100 nm in diameter and 1 µm in length. The composition of the Ag–ZnO hybrids is examined by EDS. The
d
EDS spectrum of the Ag–ZnO is shown in Fig. 1c. The existence of silver, zinc, and
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oxygen confirms the silver and ZnO components and the mass content of Ag was about 10.38 wt%, according to the semi-quantitative analysis of EDS. The XRD patterns of the as-synthesized samples are shown in Fig. 1d. Different from one set of diffraction patterns in the branched ZnO structure (ESI, Fig.S1 and S2), two sets of diffraction patterns are found for the as-synthesized Ag-ZnO sample: those marked with “#” are indexed to hexagonal wurtzite ZnO (JCPDS 36-1451), while the one marked with “*” is ascribed to face-centered cubic (fcc) metallic Ag (JCPDS 04-0783). No other crystalline impurities can be observed in the diffraction patterns. The transmission electron microscopy (TEM) images further confirm that the hybrid flowerlike structures are constructed from orderly radial nanorod arrays with
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the rod oriented lengthwise from the centre the Ag core to the surface of the flower (Fig. 2a and 2b). The size of the Ag core is about 500 nm. High-resolution TEM (HRTEM) image (Fig. 2c) displays clear lattice fringes of the ZnO nanorod.
ip t
Combining with selected area electron diffraction (SAED) in Fig. 2d, it confirms the crystal structure of ZnO with d002 = 0.26 nm and d010=0.28 nm and the growth
cr
direction is determined to be along the c axis. Interestingly, though the growth
us
direction of single rod in the branched ZnO structure (ESI, Fig. S3a) is also along the c axis, the other growth direction is [110], which is different from that of Ag-ZnO
an
heterojunction (ESI, Fig. S3b and S3c). Fig. 2e is a HRTEM image showing the
M
hetero-interface between Ag and ZnO. The interplanar distance of 0.26 nm corresponds to (002) crystal plane of ZnO, and that of 0.23 nm in the attached
d
nanoparticle corresponds to Ag (111) crystal plane. ZnO nanorods are attached to Ag
Ac ce pt e
core tightly without any voids, which well agree with the XRD results as shown in Fig. 1d. And the corresponding fast Fourier transform (FFT) patterns of the hetero-interface between Ag and ZnO are shown in Fig. 2g. It reveals that the basal plane of ZnO nanorods grows on the basis of Ag (111) facets. The high angle annular dark field (HAADF) images and energy-dispersive spectroscopy elemental mapping images are shown in Fig. 3. The different contrast means different element information (Fig. 3a and 3b). It further confirms that the branched Ag-ZnO heterojunction nanostructures were designed. The enlarged image in Fig. 3c shows many holes are scattered on the surface of ZnO nanorods, which can enhance the specific surface area and defects of ZnO rods (the specific surface area of
11
Page 11 of 36
Ag-ZnO is 4.23 m2/g; the specific surface area of ZnO is 2.01 m2/g). The elemental map of the Ag-ZnO nanostructures was obtained using energy-dispersive spectroscopy as shown in Fig. 3d, e and f. Different colors indicate the presence of
ip t
different elements, where yellow, red and green refer to the presence of Zn, O, and Ag respectively. It can be clearly seen that the center core is Ag and ZnO nanorods are in
cr
the epitaxial growth direction.
us
To clarify the elemental composition and chemical state of the as-synthesized branched Ag-ZnO heterojunction nanostructures, XPS spectra were carried out with
an
the binding energies calibrated using C 1s (284.8 eV), as shown in Fig. 4. The peaks
M
can be ascribed to Zn, O, C, and Ag (Fig. 4a,) confirming the heterostructure is prepared comparing to the only Zn, O, C elements present in the branched ZnO (ESI,
d
Fig. S4). High-resolution spectra of Zn, O and Ag species obtained from branched
Ac ce pt e
Ag-ZnO heterojunction and branched ZnO are shown in Fig. 4(b)–(d) and Fig. S4 (b)–(d) respectively. Two strong peaks in Fig. 4b center on 1020.9 and 1043.9 eV, which are in agreement with the binding energies of Zn 2p3/2 and Zn 2p1/2 respectively. There is a little shift comparing to pure branched ZnO, which may be attribute to the electron transfer between ZnO and Ag. In Fig. 4c, O 1s profile is asymmetric and can be fitted into two symmetrical peaks at 529.9 eV (red line) and 531.3 eV (green line), which agree well with the reported value for ZnO [36, 37]. The two peaks originate from the lattice oxygen of ZnO and adsorbed oxygen respectively. The two peaks centered at 367.3 and 373.3 eV in Fig. 4d can be attributed to Ag 3d5/2 and Ag 3d3/2 respectively. It should note that the peaks of Ag
12
Page 12 of 36
shift obviously to lower binding energy compared with the standard value (Ag 3d5/2, 368.2 eV; Ag 3d3/2, 374.2 eV). The binding energy of Ag for the Ag-ZnO sample is much too lower than zerovalent Ag, indicating the transfer of electrons from Ag
ip t
nanoparticles to ZnO nanorods, which is in good agreement with the fabrication of Ag-ZnO heterojunctions in the previous literatures [38-40]. Therefore, the XPS result
cr
further confirms the sample is composed of ZnO and Ag.
us
The structure and morphologies of the branched Ag-ZnO heterojunction nanostructures closely depend on different AgNO3 volume, while other conditions
an
remain unchanged. When the volume of AgNO3 was decreased to 6 mL, the branched
M
heterojunction structures disappeared. A lot of nanoparticles with ca. 100 nm in size and hexagonal prism structures with the diameter of ca.1 µm and length of ca. 3 µm
d
were found in Fig 5a. When the volume of AgNO3 was decreased to 3 mL, the
Ac ce pt e
products remain hexagonal prism structures with nanoparticles scattering around it. In spite of the length of hexagonal prism structure unchanged, the diameter increase to ca. 2 µm and the size of Ag nanoparticles decreased to 70 nm. Further decreasing the volume of AgNO3 to 0.6 mL, the diameter of the hexagonal prism further increase to ca. 3 µm keeping the length of hexagonal prism structure unchanged. And the size and the amount of Ag nanoparticles are all decreased. A completely different morphology and phase is formed when no AgNO3 was added in Fig 5d. XRD pattern shows that the product is Zn5(CO3)2(OH)6 (JCPDS 72-1100, Fig.S5). Overall, it can be inferred from the SEM studies that the volume of silver nitrate does play a very crucial role in the growth of Ag-ZnO crystals. According to early report [4], ZnO nanorods can grow
13
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directly on the facets of Ag without any other ancillary seed-assisted process. There is an equilibrium between the nucleation of Ag and the growth of ZnO, which is determined by the amount of AgNO3. When a small amount of AgNO3 was added, no
ip t
heterojunction nanostructure was obtained due to the fast rate of the growth of ZnO. However, when excessive AgNO3 was added, ZnO-Ag heterojunction nanostructure
cr
was found according to the way of early report on the formation of ZnO-Ag [4]. Other
us
parameters were also investigated. In the absence of diethanolamine, none of products were obtained. In the absence of H2O2, none of the morphologies of ZnO described
an
previously can be formed (ESI, Fig.S6a). And large micro-rods of ZnO were found by
M
decreasing the concentration of H2O2 to 15% and 5% (ESI, Fig.S6b and S6c). On the basis of the results discussed above and further investigations of related
d
literatures [41, 42], a possible formation mechanism is proposed, as schematically
Ac ce pt e
shown in Scheme 2. Firstly, diethanolamine hydrolyzed and produced a lot of hydroxyl radicals in reaction system and caused Zn2+ and Ag+ forming precursors (Zn(OH)42-) and Ag2O···Ag+. Then, the open frameworks were formed between Ag2O···Ag+ and Zn(OH)42- due to the weak interparticle forces, such as van der Waals and electrostatic interactions. Subsequently, diethanolamine has a certain reducing ability due to its two hydroxyl groups which can reduce Ag2O species to tiny Ag nanoparticles when the temperature was increased. At the same time, H2O2 acts as an oxidant, which can dissolve unstable Ag nanoparticles to Ag+ due to the potentials higher than that of Ag+/Ag (E0 = 0.7996 V). The new Ag+ can be absorbed on the surface of remained Ag2O, which further sustains the frameworks between
14
Page 14 of 36
Ag2O···Ag+ and Zn(OH)42-. There is a dynamic equilibrium between the reduction and oxidation, which provides the buffer and creates in situ stable nucleation sites for epitaxial growth of ZnO. As the reaction proceeded, Ag2O was reduced into stable Ag
ip t
particles by diethanolamine, which in turn catalyzed the assembling of ZnO on the surface of Ag particles. Finally, ZnO can grow around the Ag core and eventually
cr
formed a branched structure. Here, the Ag nanoparticles act as a catalyzer to induce
us
the formation of branched Ag-ZnO heterojunctions. The overall possible chemical reaction equations during the above experiment were listed as followed. Compared to
an
the branched structure reported by Fan et al. [4], the maximum of more than eight
H2O + NH(CH2CH2OH)2
→
2Ag+ + H2O2 → Ag2O+H2O
NH2+(CH2CH2OH)2 +OH-
(1)
d
H2O2.
M
nanorods could be observed on a single Ag nanocrystal due to the etchant effect of
(2)
Ac ce pt e
Ag++Ag2O +Zn2++ 4OH-→Zn(OH)42-+ Ag2O···Ag+
(3)
Zn(OH)42-+ Ag2O···Ag+ + NH2+(CH2CH2OH)2→ZnO-Ag+other products in water (4) The optical absorption property of the samples, which is relevant to the electronic structure feature, is tested to explain the change in photocatalytic activity. The optical absorption of as-prepared samples were measured by diffuse reflectance spectra (DRS), and shown in Fig. 6. Relative to the branched ZnO and normal Ag-ZnO counterpart, the branched Ag-ZnO heterojunction possesses higher reflection peaks in ultraviolet region (365 nm). This may increase the UV catalytic activity to some extent. Fig. 7 shows the representative PL spectra of the as-prepared samples with the
15
Page 15 of 36
excitation wavelength of 325 nm at room temperature. Three samples reveal similar emission spectrum. In the ultraviolet region, the emission at around 380 nm was observed, which is due to radiative recombination of free excitions. Though the exact
ip t
origins are currently still highly controversial and ambiguous, the visible emission may be due to the recombination of photo-generated holes with the electrons in
cr
intrinsic or extrinsic defects, such as oxygen vacancies (VO˙) and interstitial oxygen
us
(Oi00) [43-47]. When a suitable scavenger or surface defect state is available to trap the electrons or holes, undesirable recombination can be prevented effectively. As the
an
redox reactions might occur on the surface of the VO˙ and Oi00 defects, the oxygen
M
defects can be considered to be the active sites of the ZnO photocatalyst. Compared to the branched ZnO and other Ag-ZnO composites, the formation of the branched
d
Ag-ZnO heterojunction reduces the intensity of the UV emission with a clear red-shift.
Ac ce pt e
The intensity decrease and the red-shift of the UV emission for the Ag-ZnO heterojunction can be ascribed to the electron trapping effect of Ag nanoparticles, which act as electron acceptor, hindering the recombination of photogenerated carriers on ZnO. According to earlier reports, the electron trapping effect of Ag NPs is importantly favorable for the improvement of the photocatalytic activity of ZnO due to the enhancement of the separation.
From Fig.8 (a-b), we can conclude that the branched Ag-ZnO heterojunction photocatalyst shows the highest CO2 generation concentration (1100ppm) under UV light (365nm) , which is nearly 1.2 times, 7.6 times and 2.9 times higher activity than that of branched ZnO (900ppm), normal Ag-ZnO counterpart (145ppm) and
16
Page 16 of 36
commercial ZnO (384ppm). Namely, 550 ppm CH3CHO gas is fully degraded to CO2 by this branched Ag-ZnO photocatalyst within 16 hours at room temperature under 365 nm UV light irradiation (Fig.8b). In the same time, the photocatalytic
ip t
performance of the branched Ag-ZnO heterojunction under visible light (435nm) was also investigated in Fig.8a. It can be seen that the CO2 generation concentration
cr
(1100ppm) under UV light (365nm) is 8.9 times higher activity than that under visible
us
light (124nm). The photocatalytic performance improvement of the branched Ag-ZnO heterojunction photocatalyst is mainly attributed to its intimate interfacial contact
an
between Ag core and ZnO nanorods. This unique Ag-ZnO structure is beneficial for
M
photogenerated charge carriers (holes and electrons) separation to promote the photocatalytic enhancement, which is also well accordance with the PL analysis
d
results in Fig.7. Therefore, many more photogenerated holes and electrons prefer to
Ac ce pt e
be involved in the photocatalytic reaction rather than be annihilated by the form of radiation. Meanwhile, the branched Ag-ZnO heterojunction is with good stability (ESI, see the XRD and DRS sprectra after photocatalytic reaction in Fig.S7 and S8) and high efficiency because this photocatalyst still retains 99% photocatalytic activity after three CH3CHO degradation recycles (Fig. 8d). Besides, the photodegradation experiments of RhB aqueous solution were further carried out to investigate the universality of this heterojunction photocatalyst. Obviously, the RhB dyes photodegradation results in solution in Fig.8c present a similar tendency with CH3CHO ones in gaseous phase. In summary, this in-situ prepared branched Ag-ZnO heterojunction is with excellent photodegradation performance in RhB dye and
17
Page 17 of 36
CH3CHO gas under UV light irradiation. The mechanisms of the enhanced photocatalytic activity of Ag-ZnO have been proposed by others [27, 34]. The enhanced photocatalytic performance in UV region
ip t
was attributed to the formation of the Schottky barriers at Ag and ZnO interface when they are in direct contact, which improved the segregation of charges and prevented
cr
the charge recombination. For the branched heterostructures, it is stressed here that
us
the center Ag particles and ZnO nanorods with high carrier transport property, serve as spatially extended catalyzing centers to provide direct and fast electron/hole
an
transfer to their acceptors (H2O, O2, RhB and CH3CHO), which increases the chance
M
of organic pollutants to be degraded, compared with commonly used Ag–ZnO and ZnO particles (Scheme 1). In fact, photocatalysis is a complicated process, which is
d
associated with not only the structure-related physical properties, but also
Ac ce pt e
structure-related catalytic processes.
4. Conclusions
In summary, a facile and effective hydrothermal method was introduced to the one-pot synthesis of branched Ag-ZnO heterojunction nanostructures. The products were characterized by XRD, FESEM, HRTEM, XPS, PL and UV–vis. Morphological control of the metal–ZnO interface was achieved by adjusting the reaction parameters. The branched Ag-ZnO heterojunction nanostructures show its superior photocatalytic activity over the branched ZnO and its Ag-ZnO counterparts, which demonstrates its potential application as nanocatalysts both in dyes and VOCs degradation. The simplicity and generality of our method for different metals together with the growth 18
Page 18 of 36
selectivity offer great advantages and motivate further research on its application to other metal–metal oxide systems. References:
ip t
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Fig. 1. (a) low-magnification, (b) enlarged-magnification of SEM images, (c) EDS spectrum (d) XRD pattern of branched Ag-ZnO heterostructure. Fig. 2. (a) low-magnification (b) single branched Ag-ZnO heterostructure of the branched Ag/ZnO heterostructure, (c) HRTEM and (d) SAED image of single ZnO nanorod (e) HRTEM (f) the corresponding FFT image of Ag core. Fig. 3. (a-c) HAADF STEM and elemental mapping images of the branched Ag/ZnO heterostructure (d) Zn elemental mapping (e) O elemental mapping (f) Ag elemental mapping. 25
Page 25 of 36
Fig. 4. XPS spectra of the branched Ag-ZnO heterostructure (a) the scan (b) Zn2p (c) O1s (d) Ag3d Fig. 5. SEM images of Ag-ZnO hybrids prepared with different volume of AgNO3
ip t
solution: (a) 6mL (b) 3 mL and (c) 0.6 mL (d) 0 mL (keeping the other parameter unchaged).
cr
Fig. 6. DRS spectra of the as-prepared samples.
us
Fig. 7. PL spectra of as-prepared samples.
Fig. 8. The photocatalytic performance evaluation curves of the as-prepared
an
photocatalysts (a) CO2 generation ( b); CH3CHO concentration; (c) RhB degradation
M
and (d) three recycles of CH3CHO photodegradation evaluation by the branched Ag-ZnO.
Ac ce pt e
heterostructures.
d
Scheme 1. A proposed photocatalytic mechanism of the branched Ag-ZnO
Scheme 2.
The mechanism of the formation of ZnO-Ag heterojunction
nanostructure.
Fig.
26
Page 26 of 36
5
#(101)
30
#(110)
*(111)
40
50
60
70
*(311) #(004)
#(100)
20
10
#(103) *(220) #(220) #(112) #(201)
Zn
# ZnO * Ag
us
Zn Ag
#(002)
Ag 4.18
cr
ip t 0
(d) Ag 10.38
*(200) #(102)
O
Weight % O Zn 17.71 71.91 Atom % O Zn 48.07 47.75
an
Counts(a.u)
Zn
Intensity (a.u.)
(c)
80
2θ (deg.)
Energy(Kev)
Ac ce pt e
d
M
Fig. 1
27
Page 27 of 36
ip t cr us an
Ac ce pt e
d
M
Fig. 2
28
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ip t cr us an
Ac ce pt e
d
M
Fig. 3
29
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400
600
800
1000
1200
1020
1025
Binding Energy (ev)
1035
(d)
530
532
534
367.33
1040
1045
1050
us
373.33
an
XPS Intensity (a.u)
XPS Intensity (a.u)
(c)
528
1030
Binding Energy (ev)
cr
200
1043.93
ip t
XPS Intensity (a.u) 0
1020.93 (b)
XPS Intensity (a.u)
(a)
536
365
Binding Energy (ev)
370
375
380
M
Binding Energy (ev)
Ac ce pt e
d
Fig. 4
30
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ip t cr us an M Ac ce pt e
d
Fig. 5
31
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ip t cr an
us
Reflectance ( %)
the branched ZnO the branched Ag-ZnO Ag-ZnO 0.6mL AgNO3
300
400
500
600
M
200
700
800
Wavelength ( nm )
Ac ce pt e
d
Fig. 6
32
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450
500
Wavelength (nm)
600
Ac ce pt e
d
M
an
Fig. 7
550
us
400
cr
ip t
Intensity (a.u.)
the branched Ag-ZnO the branched ZnO Ag-ZnO (0.6mLAgNO3)
33
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600
the branched ZnO the branched Ag-ZnO Ag-ZnO 0.6mL AgNO3 the branched Ag-ZnO under visible light commercial ZnO
(a)
1400 1200
500
CH3CHO concentration (ppm)
(b)
the branched ZnO the branched Ag-ZnO Ag-ZnO 0.6mL AgNO3
400
1000
300
800 600
100
200 0 0
5
10
15
20
0
25
0
Irradiation Time (h)
1.0
(c)
(d)
0.9
CO2 generation (ppm)
0.8 0.7
15
an
C/C0
0.6 0.5 0.4 0.3 0.1 0
20
40
60
20
25
80
Recycle 1 Recycle 2 Recycle 3
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75
Irradiation Time ( h )
d
Time (minutes)
M
no any catalysts) Ag-ZnO 0.6mLAgNO3 the branched ZnO the branched Ag-ZnO
0.2 0.0
10
Irradiation Time (h)
cr
1.1
5
ip t
200
400
us
CO2 generation (ppm)
1600
Ac ce pt e
Fig. 8
34
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ip t cr us
Ac ce pt e
d
M
an
Scheme 1.
35
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ip t cr
Ac ce pt e
d
M
an
us
Scheme 2.
36
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