CuO Nanofibers: Toward

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Article

Tuning Composition of Electrospun ZnO/CuO Nanofibers: Towards Controllable and Efficient Solar Photocatalytic Degradation of Organic Pollutants Amene Naseri, Morasae Samadi, Niyaz Mohammad Mahmoodi, Ali Pourjavadi, Hamid Mehdipour, and Alireza Zaker Moshfegh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10414 • Publication Date (Web): 22 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Tuning Composition of Electrospun ZnO/CuO Nanofibers: Towards Controllable and Efficient Solar Photocatalytic Degradation of Organic Pollutants Amene Naseri†, Morasae Samadi‡, Niyaz Mohammad Mahmoodi§, Ali Pourjavadiǁ, Hamid Mehdipour‡, and Alireza Z. Moshfegh†,‡,* †

Institute for Nanoscience and Nanotechnology, Sharif University of Technology, P.O. Box

14588-8969, Tehran, Iran ‡

Department of Physics, Sharif University of Technology, P.O. Box 11555-9161, Tehran, Iran

§

Department of Environmental Research, Institute for Color Science and Technology, P.O. Box

16688-14811, Tehran, Iran ǁ

Department of Chemistry, Sharif University of Technology, P.O. Box 11555-9516, Tehran, Iran

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 41

ABSTRACT. ZnO/CuO nanofibers, with different CuO concentrations, were fabricated by onestep electrospinning of polymer precursor and annealing in the air. Scanning electron microscopy (SEM) showed smooth and beadless morphology for the synthesized nanofibers, while X-ray diffraction (XRD) analysis revealed formation of hexagonal and monoclinic crystalline structure phases for ZnO and CuO nanofibers, respectively. X-ray photoelectron spectroscopy (XPS) analysis confirmed presence of CuO on the surface of ZnO nanofibers. For further confirming the formation of chemical bonds, Fourier transform inferared (FTIR) spectroscopy was employed. The effect of Cu contents in the overall electronic band structure of ZnO was explained by the density functional theory (DFT) calculations. Diffuse reflectance spectroscopy (DRS) showed that ZnO band gap energy reduced with increasing the amount of CuO contents due to the presence of the Cu(3d) energy states above the valence band. Comparing photocatalytic activity of ZnO/CuO nanofibers samples with different CuO concentrations under similar sunlight irradiation conditions revealed that the ZnO/(0.5wt%) CuO sample exhibited the highest performance among all samples. This was explained by an effective suppression of electron-hole recombination as verified by both photoluminescence (PL) and photocurrent density measurements. By means of charge carrier scavengers, it was found that holes and hydroxyl radicals are the main surface species for photocatalytic degradation of methylene blue (MB) over the ZnO/(0.5wt%) CuO nanofiber. Furthermore, the optimized sample demonstrated a great activity for degradation of bisphenol A (BPA) with a rate constant of 3.4×10-2 min-1. Finally, a photocatalytic degradation mechanism based on the main reactive oxygen species (ROS) and calculated band positions was proposed.


2

Page 3 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION In recent decades, growing population and industrial development have led to releasing huge amounts of highly toxic chemical pollutants into the environment globally.1 Several approaches including biological, chemical, physical as well as advanced oxidation processes (AOPs) have been employed to handle the contamination removal from wastewaters for environmental remediation.2 Among them, AOP based on oxide semiconductors and related compounds (as photocatalysts) is highly promising, especially for degradation of organic pollutants.3,4,5 This method is a "green" technology for complete elimination of contaminants under sunlight irradiation as well as ambient conditions.6 Applying ZnO as a semiconductor photocatalyst with approximately 3.3 eV band gap energy has received much attention due to its high chemical and mechanical stability, inexpensiveness, non-toxicity, high quantum efficiency, as well as high redox potential. In addition, the optical band gap of ZnO is suitable for producing hydrogen and oxygen molecules from decomposition of water. It is also used to produce reactive oxygen species (ROS), including hydroxyl radical (.OH) and superoxide anion radical (.O2-) as well as hydrogen peroxide (H2O2). This is because the redox potentials of these species fall exactly within the band gap of the ZnO semiconductor photocatalyst.7 However, generation of charge carriers in ZnO is limited only under UV light irradiation (that accounts for about 5% of solar light) and their fast recombination rate are two limiting factors that reduce the photocatalytic efficiency of this semiconductor.8 Therefore, a variety of methods such as doping,9 sensitization10,11 and coupling with other semiconductors,12,13 have been used recently in our group to enhance its photocatalytic activity under the visible light.


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 41

CuO, a p-type semiconductor with reported direct band gap in the range of 1.2-1.79 eV, 14,15

is a non-toxic, chemically stable, naturally abundant material with high thermal and

electrical conductivities.14 Because of its narrow band gap, CuO is usually used as a co-catalyst in combination with large band gap catalysts such as ZnO16 and TiO217 to enhance their photocatalytic activity under solar light irradiation. In the ZnO/CuO nanocomposites, chargecarriers separation is more effective and recombination rate is greatly reduced, which improve the photodegradation rate.18 Also, Irie et al. have proposed a mechanism for interfacial charge transfer between TiO2 and Cu(II) . In the proposed mechanism, the photo-excited electrons of TiO2 transferred to the Cu(II) ions, producing Cu(I) ions on the TiO2 surface. The produced Cu(I) then can reduce O2 in a multi-electron process, while the remaining holes in the TiO2 valence band (VB) can decompose gas pollutant exposed to the photocatalyst.19 Malwal et al. have reported that the formation of p-n heterojunction at the interface of ZnO@CuO can result in a better charge separation and thus enhanced photocatalytic activity.20 In this work, we have used the electrospinning technique as a cost-effective and broadly applicable method for fabrication of nanofibers with large aspect ratio, great interconnectivity, and high porosity which make them perfect candidates for photocatalytic applications.21,22,23 To the best of our knowledge, tuning the photocatalytic activity of electrospun ZnO/CuO nanofibers with variation of CuO concentrations has not been investigated so far. In the present work, we have synthesized both pure ZnO and CuO, as well as ZnO/(x wt%) CuO nanofibers, where x = 0.1, 0.5, 1, 2.5, 5, 10, and 50 to compare their photocatalytic activity. Various characterization methods, including TGA/DTA, FT-IR, FESEM, XRD, XPS and DRS have been used to determine the structural, electrical, and optical properties of the prepared nanofibers. Also, Density Functional Theory-based model of Cu-doped ZnO (Zn1-xCuxO composite) has been


4

Page 5 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


developed to show the effect of Cu atoms in the electronic band structure of ZnO with different Cu concentrations. Kinetics of photocatalytic activity of the nanofibers has been also studied by measuring the degradation rate of two kinds of organic pollutants, namely methylene blue (MB) and bisphenol A (BPA). To better understand the exact mechanism of the photocatalytic activity, various scavengers have been utilized. Finally, the role of different ROS is discussed and a charge transfer mechanism is proposed based on analysis of ROS action, PL intensities as well as photocurrent density measurements. 2. EXPERIMENTAL AND COMPUTAIONAL METHODS 2.1. Preparation of electrospun nanofibers of ZnO/CuO. A solution containing PVA was prepared in DI water by dissolving of 1 g PVA (Mw = 88000-97000, 87-89% hydrolysed, Alfa Aesar) in 10 mL DI water using magnetic stirring for 3 h at 90 °C. Then an appropriate amount of cupric acetate monohydrate (Merck, > 99.0%), 1 mL ethanol, and 2 mL acetic acid were added to the polymer solution under constant stirring for 1 h at 80 °C. A suitable amount of zinc acetate dihydrate (Merck, 99.5 - 101.0 %) was then added and further stirring was performed for 3 h at 80 °C to obtain a homogeneous solution.24 The Cu to Zn (Cu/Zn) weight ratios in the precursor solutions were considered as: 50/50, 10/90, 5/95, 2.5/97.5, 1/99, 0.5/99.5, 0.1/99.9 and 0/100, for synthesized ZnO/(x wt%) CuO nanofibers, which x values were 50, 10, 5, 2.5, 1, 0.5, 0.1 and 0, respectively. A summarized sample names is listed in Table 1.

Table 1. Sample Names of the ZnO/(x wt%) CuO Nanofibers With Their Corresponding CuO Concentrations. Sample name

ZnO

ZC0.1

ZC0.5

ZC1

ZC2.5

ZC5

Amount of x

0

0.1

0.5

1

2.5

5

ZC10

10

ZC50

CuO

50

100


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 41

The precursor solutions containing different Cu concentrations were loaded into two plastic syringes, with 0.7 mm diameter stainless steel needles, which were placed at opposite sides of a collector of homemade electrospinning setup. During electrospinning, the fibers deposited on the collector covered with aluminium foil. For all experiments, applied potential between the needle and collector, needle-to-collector distance, spinning rate and flow rate were kept at 12 kV, 12 cm, 200 rpm and 0.4 mL/h, respectively. The “as spun” nanofibers with different Cu concentrations were then annealed at 460 °C for 1 hour. 2.2. Characterization. The morphology of the synthesized ZnO/CuO nanofibers was investigated by field-emission scanning electron microscopy (FESEM, S4160 Hitachi). Exact annealing temperature of the “as spun” nanofibers was determined by thermal gravimetric and differential thermal analysis (TGA/DTA) (Rheometric Scientific STA1500). For phase identification and crystallinity determination, XRD patterns of nanofibers were collected on a PW 3710 Philips diffractometer fitted with a Cu Kα radiation source (λ=1.54056Å). X-ray photoelectron spectroscopy (XPS) equipped with an AlKα (1486.6 eV) X-ray source was employed at a pressure lower than 10−7 Pa to determine surface chemical composition of the prepared nanofibers. Calibration of all binding energy values was performed by fixing the C(1s) core level binding energy at the 284.6 eV as a reference. All XPS peaks were deconvoluted by means of SDP software (version 4.1) with 90% Gaussian-10% Lorentzian peak fitting. The diffuse reflectance UV–Vis spectra (DRS) of the nanofiber samples were obtained by using an Ava Spec-2048TEC spectrophotometer. Photoluminescence spectroscopy (PL) with Varian Cary Eclipse Fluorescence Spectrophotometer was used to study photo-induced charge recombination rate for different samples.


6

Page 7 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.3. Specific surface area (SSA) determination. One of the imporatant factors to study catalytic and photocatalytic activity is SSA.25 For several decades, MB adsorption has been used for determination of SSA of different materials.26,27 To calculate the SSA, 10 mg of each sample was added to 25 cc of a standard solution of MB. The obtained suspension was kept under constant stirring in darkness for 24 h. The suspension was then centrifuged and the absorption spectrum of supernatant was recorded. The SSA could be calculated by the following equation;26

SSA =

N A A MB (C0 - Ce )V M MB ms

where, NA is the Avogadro’s constant (6.02×1023/mol), AMB is the area covered by one MB molecule (usually supposed to be 1,35 nm2), MMB is the molecular mass of MB, C0 and Ce are the initial and equilibrium concentrations of MB, respectively, V is the MB solution volume and mS is the mass of the photocatalyst sample. 2.4. Photocatalytic activity tests. For assessment of photocatalytic activity of all prepared ZnO/CuO nanofibers, MB and BPA were used as model compounds. Photocatalytic degradation of the MB in aqueous solution over the nanofiber samples was performed under similar solarlight irradiation conditions. To examine photocatalytic activity under sunlight irradiation, 6 mg of each sample suspended in 15 mL of 10-5 M MB dye. Before irradiation, a mixture suspension was stirred in the dark for 60 min to obtain adsorption-desorption equilibrium of the dye. Then, the reaction mixtures were illuminated via a 300W Xe lamp with a power of 800±50 Lux, while the distance from the lamp to top surface of the solution was kept fixed (18 cm). After each 30 min, 1.5 mL of solution was drawn to analyze optical absorption with JASCO V-530 UV-Vis. spectrophotometer. The variation of the MB absorbance at 664 nm was then monitored for MB


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 41

photodegradation under similar conditions. This wavelength is assigned to the absorption by the π-conjugated system in the MB.

To further verify the photocatalytic activity of nanofibers, the photodegradation of BPA was also carried out by adding 6 mg of the optimized (ZC0.5) and pure ZnO samples in 15 mL of 20 ppm BPA at room temperature. After stirring for 1h in the dark to obtain adsorptiondesorption equilibrium, the suspension was illuminated by the same source as the one used for the degradation of MB dye. Then a small volume of suspension (about 0.2 mL) was taken out every 5 min and filtered through a 0.22 µm microporous membrane. The remaining amount of BPA was analyzed by high-performance liquid chromatography (HPLC) (SYKAM) in a specific time period. The HPLC system was equipped with a C18 reverse phase column and a UV-Vis. S3210 detector at 230 nm. The mobile phase used was a mixture of acetonitrile and DI water (50/50, v/v) with a flow rate of 0.8 mL/min and for each run 20 µL of sample was injected in this study. 2.5. Photoelectrode preparation and photocurrent density measurement. Photocurrent densities were recorded for both ZC0.5 and ZnO samples to investigate the charge separation efficiency under similar conditions. For electrode preparation, 15 mg of photocatalyst (ZC0.5 and ZnO) and 10 µL of Nafion solution (~5% in a mixture of lower aliphatic alcohols and water) were dispersed in a mixed solvent of water/isopropanol (3:1 v/v, 1 mL). Then 50 µL of suspension was coated on FTO (8 Ω.□-1 , about 0.5×1 cm2 surface area) by the spin coating method at 500 rpm for 2 minutes. The chronoamperometry experiments were done on a threeelectrode galvano-stat/potentiostat (Autolab PGSTAT302) in a three-electrode system in which the sample coated FTO acted as working electrode, Pt wire as auxiliary electrode and Ag/AgCl


8

Page 9 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electrode as the reference electrode. An aqueous solution of sodium sulfate (0.5 M) was used as electrolyte. The irradiation source was a Xenon lamp with a natural sunlight spectrum. 2.6. Density Functional Theory of Band Structure of Zn1-xCuxO Composites. To understand the role of Cu content in the overall electronic band structure of ZnO and clarify the origin of change in the photocatalytic activity of the ZnO nanofiber, we have developed a first-principle model of the Zn1-xCuxO composite with different amount of Cu contents. Structure optimization and band structure calculations of Zn1-xCuxO compounds are performed within the framework of Density Functional Theory as implemented in Quantum Espresso Package.28 Interactions between ion cores and valence electrons is modelled using ultrasoft pseudopotential in the Vanderbilt form. Based on our XRD measurements, which indicate that wurtzite phase is formed for ZnO nanofibers, a 2 2 2 hexagonal unit cell containing 32 atoms (16 Zn and 16 O atoms) is used to model wurtzite Zn1-xCuxO composite with different Cu concentrations (see Scheme 1). The energy cutoff for all plane wave basis is set at 30 Ry, while the convergence for selfconsistent field iterations is set at 10 eV/atom. Each periodic structure optimization is

performed until the force on each atom falls below 10 eV/Ǻ. The electronic configurations Zn3d104s2 and O2s22p4 have been set for valence shells of the Zn and O, respectively, and the remaining near core electrons are kept frozen as core states.


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 41

Scheme 1. Schematic illustration of (side view (a) and top view (b)) of the optimized crystal structures of 2 × 2 × 2 orthorombic unit cells for pure Cu-doped ZnO. The blue, red, and yellow balls represent zinc, oxygen and copper atoms, respectively.

To obtain more accurate electronic band structures of optimized structures, DFT+Ud+Up method has been adopted with Hubbard Ud,Zn and Up,O parameters set at 10 and 7 eV, respectively.29 In order to optimize a Cu doped ZnO structure with a specific Cu concentration, a number of Zn atoms (rather than O atoms) are replaced substitutionally with Cu dopants which electronically nearly resemble the Zn atoms. Using this atomic substitution, we can avoid distortion in the lattice structure and thus get convergence for lattice relaxation. Monkhorst-Pack k-points grid sampling of 4 × 4 × 2 is used for integration in first Brillouin zone. For each optimized Cu-doped ZnO structure, partial and total density of states are computed to get clear insight into the contributions of atomic orbitals of each atomic element (Zn, O, and Cu atoms) in overall electronic band structure and also quantify the change in the energy band gap with Cu concentration.

3. RESULTS AND DISCUSSION In this section, the results of the experimental techniques stated above are presented, analyzed and discussed. Moreover, first principal theoretical modelling is also implemented in order to determine the electronic band structure of the samples. 3.1. TGA/DTA. The thermal behavior of as-spun ZnO and CuO and their mixtures at extreme CuO concentrations, namely ZC50 (highest CuO concentration) and ZC0.5 (lowest CuO concentration), are shown in Figure 1. All organic and volatile compounds such as acetate groups


10

Page 11 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and water are removed under 460 °C thermal annealing. As depicted in the figures, there is a slight weight loss after the annealing. Figure 1(a) shows two exothermic DTA peaks at 248 and 418 °C, which correspond to decomposition of zinc acetate and pyrolysis of PVA respectively.30,31 These two peaks in DTA data correspond to 31 and 9.5 % weight loss in the TGA curve, respectively. Comparison of the pure samples and nanocomposites shows that the thermal behaviors of mixed precursors are different from the pure samples. It is clear that the metal acetate decomposition and formation of crystalline metal oxide structures occur in the range of 200-300 °C in the composite nanofibers.32 As shown in Figure 1(b), for zinc acetate/(0.5wt%) cupric acetate/PVA nanofibers, dehydration of cupric acetate and degradation of PVA occur at 169 and 435 °C, respectively. In the case of zinc acetate/(50wt%) cupric acetate/PVA, the dehydration and pyrolysis occur at 88 and 425 °C, respectively (Figure 1(c)). Also, the process of cupric acetate/PVA annealing starts initially by dehydration of cupric acetate (88 °C), and subsequent decomposition of cupric acetate and formation of crystalline CuO occur at 233 and 246 °C, respectively (see Figure 1(d)). Finally, the pyrolysis of PVA takes place at 405 °C and no weight loss was observed thereafter. Therefore, these measurements determine the stages of the one hour-long thermal annealing of crystalline metal oxide at 460 °C.


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 41

Figure 1. Thermal behaviors of the ZnO/CuO as-spun nanofiber samples with different CuO concentrations: The TGA and DTA thermograms curves of the pure ZnO (a), ZC0.5 (b), ZC50 (c), and pure CuO (d) nanofiber samples.

3.2. FT-IR. The FT-IR spectra of the pure ZnO and CuO nanofibers and composite samples with low and high percentages of CuO are shown in Figure 2. As expected, the characteristic peaks of inorganic metal oxides emerge in the fingerprint region of FT-IR spectrum (400-700 cm-1).33 The peaks at 583 and 527 cm-1 are attributed to the Cu-O stretching vibrations.34 The emerged characteristic peak of Zn-O stretching vibration at 439 cm-1 confirms the formation of pure inorganic metal oxide.30 FT-IR spectra of the composite nanofibers of the ZnO/CuO with two different CuO concentrations are very similar to the pure ZnO. The only difference that appears is a slight shift of the metal oxide peak towards higher wavenumbers for higher CuO concentration, which changes from 445 to 452 cm-1.34


12

Page 13 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. FT-IR spectra of the ZnO/CuO nanofiber samples with different CuO concentrations: curves (a) and (d) are FT-IR spectra for pure CuO and pure ZnO nanofiber samples, respectively, and curves (b) and (c) are FT-IR spectra for ZC50 and ZC0.5 nanofiber samples, respectively.

Bands with low intensity at 3443 cm-1 (CuO), 3410 cm-1 (ZnO), 3443 cm-1 (ZC50) and 3447 cm-1 (ZC0.5) can be attributed to the stretching vibration of O–H as a result of adsorption of water molecules on the surface of samples.10 Other weak bands at 1620 cm-1 (CuO), 1639 cm-1 (ZnO), 1635 cm-1 (ZC50) and 1635 cm-1 (ZC0.5) are due to the stretching of C=C bond. Also, the bands emerged at 1426 cm-1 (CuO), 1434 cm-1 (ZnO), 1450 cm-1 (ZC50), 1442 cm-1 (ZC0.5) correspond to the CH2 bending modes.34 3.3. FESEM. The typical FESEM images of “as-spun” and annealed nanofibers are shown in Figure 3. The prepared nanofibers are randomly oriented and have beadless structure and smooth surface. By comparing Figures 3(a) and (b) it is found that the PVA/ zinc acetate nanofibers become thinner as a result of the annealing. The distribution of nanofiber diameter is quantified in the histogram according to their corresponding SEM images. It is seen that average diameter


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 41

decrease from 208 to 100 nm upon the annealing. In the case of zinc acetate/cupric acetate (0.5wt%)/PVA, the measured mean diameters before and after annealing at 460 °C for 1 h are approximately 322 and 120 nm, respectively (see Figs 3(c) and (d)). The reduction of average diameter of nanofibers is believed to be due to the PVA removal and ZnO crystallization after sample calcination at 460 °C. Similar trends for change of nanofiber diameter with annealing have been reported by others.35 3.4. XRD. To investigate the crystal structure of the nanofibers, XRD analysis has been used. Figure 4 displays XRD patterns of ZnO, CuO and composite ZnO/CuO nanofibers with different CuO concentrations. In the XRD pattern for pure ZnO nanofibers, all the emerged diffraction peaks indicate the formation of the hexagonal structure, thus, ZnO is crystalized in wurtzite phase (JCPDS: 36-451). It should be noted that no impurity peaks are observed for the samples. The calculated value of c/a ratio (a and c are the hexagonal lattice parameters) for pure ZnO nanofibers is about 1.596, which is in the range of 1.5930-1.6038, as reported by previous works.36 The XRD pattern of the pure CuO nanofibers (Fig. 4) shows emergence of nine characteristic peaks which can be attributed to the formation of (110), (002), (111), 2 02, (020),

(202), (113), 3 11, (220) planes of a tenorite (monoclinic) crystal structure (JCPDS: 481548).34 Also, no peaks were observed related to Cu2O compound in XRD patterns of pure CuO and composite nanofibers at any CuO concentration.


14

Page 15 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. FESEM images (left) and histogram of nanofiber diameter (right): (a) and (b) ((e) and (f)) are SEM images (diameter histograms) of as spun pure ZnO nanofibers before and after annealing, respectively. (c) and (d) ((g) and (h)) are SEM images (diameter histograms) of as spun pure ZC0.5 nanofiber before and after annealing, respectively.

By reducing CuO concentration, no peak related to CuO appears in the XRD pattern,37 whereas, along with wurtzite ZnO peaks, clear peaks for tenorite CuO emerged when the amount of CuO is increased (see red XRD pattern for ZC50 in Figure 4). It is worth noted that no peak for any impurity phase is seen in all XRD patterns.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 41

Figure 4. XRD patterns of the pure ZnO, pure CuO, as well as ZnO/CuO composite nanofibers with different CuO concentrations. The CuO concentration increases from the top-most curve to the bottom-most curve.

3.5. XPS. This analysis is carried out to determine the exact ionic states of the copper and zinc species on the surface in the prepared samples. An XPS of elemental compositions of ZnO/CuO nanofiber samples with a certain amount of CuO (here, ZC10) has been presented in Figure 5. In the full survey spectrum shown in Figure 5(a), all the observed peaks can be attributed to presence of C, O, Zn, and Cu and no peak for impurities can be seen. This is a very clear indication that only ZnO/CuO nanofibers have formed and no impurity exists in the sample. The only detected carbon peaks are due to the adventitious elemental carbon atoms and/or adsorbed carbon adatoms on the sample surface during its exposure to the ambient atmosphere.13 The deconvoluted Cu(2p) core-level in Figure 5(b), fits well with a set of single peaks located at 933.2 and 952.3 eV, which correspond to Cu(2p3/2) and Cu(2p1/2) core levels, respectively, and both indicate the presence of Cu2+ in the formed samples.38 Also, the shake-up satellite peaks located at 940, 942.8 and 961.3 eV are attributed to the Cu(3d) hole states.39 The spin-orbit-


16

Page 17 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. A full XPS survey scan of the ZnO/CuO nanofiber sample (ZC10) (a). High resolution XPS spectra of the Cu(2p) core level (b) and the Zn(2p) core level (c) of the nanofiber sample.

coupled in the Cu(2p) region (Fig. 5(b)) shows two peaks which are centered at 930.3 and 949.3 eV for the Cu(2p3/2) and Cu(2p1/2), respectively.40 The appearance of these peaks can be attributed to the presence of Cu+ species only on the surface of the nanofiber sample. It should be noted that the sum of the area below the peaks of Cu+/Cu2+ is very small, see Table 2. The small amount of reduction from Cu2+ to Cu+ could be due to the following effects: 1) X-ray exposure during XPS measurements, 2) carbon formation during the annealing of the nanofibers in the air, and 3) acetic acid formation during the zinc acetate and copper acetate hydrolysis.34


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 41

The presence of Zn2+ in Zn wurtzite structure is characterized by two peaks at 1021.6 and 1044.8 eV for the Zn(2p) core level, see Fig. 5(c). These peaks are related to the conveyed binding energies of the Zn(2p3/2) and Zn(2p1/2) states, respectively.10

Table 2. XPS Binding Energy and Corresponding Normal Area for High Resolution Spectrum of the Cu(2p) Core Level of the ZC10 Nanofiber Sample.

Cu+

Cu2+

BE (eV)

Normal Area

BE (eV)

Normal Area

Cu(2p3/2)

930.3

37.95

933.2

184.43

Strong Cu2+ satellite peaks

-

-

940.0

15.49

-

-

942.8

49.19

Cu(2p1/2)

949.3

25.42

952.3

101.13

Strong Cu2+ satellite peak

-

-

961. 3

48.09

3.6 DRS. Optical diffuse reflectance spectra of the synthesized nanofibers are obtained at the room temperature. Band gap energies ( ) of pure ZnO and ZnO/CuO nanofibers are estimated by Tauc's plot using the following relation;41 ℎ = ℎ −

(1)

where h, ν, α and C are Plank's constant, photon frequency, absorption coefficient and a constant, respectively. By extrapolating the linear region of (αhν)2 versus the photon energy axis the optical band gaps can be estimated. For the pure ZnO nanofibers, the band gap energy was


18

Page 19 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


estimated about 3.15 eV and for various ZnO/CuO composite nanofibers, it varies from 2.93 to 1.94 eV depending on the amount of Cu content in the samples (see Figure 6). Thus, one can conclude that the presence of more CuO in the nanofiber sample narrows the band gap energy.

Figure 6. Plot of (αhν)2 versus photon energy for the ZnO/CuO nanofiber samples with different CuO concentrations.

3.7. DFT Theoretical results. Density functional theory (DFT) is used to calculate the electronic band structure of the Zn1-xCuxO samples. The results reveal the fact that the addition of Cu atoms introduces a number of defect states above the ZnO valence band and thus narrowing the band gap (Fig. 7(b)). Based on our calculations demonstrated in the Fig. 7(a) and (b), band gap narrowing, up to 2.2 eV (in comparison to the 3.6 eV in pure ZnO) is observed for the lowest Cu concentration ( = 0.06) considered. It should be noted that the band gap calculated for the pure ZnO ( = 3.6 eV) is close to the measured band gap = 3.15 eV, using DRS method.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 41

Figure 7. Electronic band structures and density of states for ZnCuO composite: (a) and (b) panels show band structures of Zn1-xCuxO structure with = 0 and 0.06 Cu concentrations, respectively, while (c) and (d) show corresponding partial density of states of atomic orbitals for each atom. The Fermi energy is set at zero for all the panels.

Partial density of states (PDOS) of each atom for pure ZnO and Cu-doped ZnO structures are calculated and shown in the Figs. 7(c) and (d), respectively. Considering the band structure of pure ZnO, one can see that the valence band is mainly consisted of O(2p) and Zn(3d) orbitals as well as small contribution from O(2s) orbitals (see Fig. 7(c)). Moreover, the conduction band is largely made of Zn(4s) orbitals with smaller contributions from O(2s) and O(2p) orbitals. For the Cu-doped ZnO structure, the valence band contributions come mainly from not only the O(2p) and Zn(3d) orbitals, but also Cu(3d) atomic orbitals are involved (see Figure 7(d)). Moreover, the orbital contributions at the bottom of conduction band are almost unchanged. It is clearly


20

Page 21 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


seen that the Cu(3d) orbitals just close to the Fermi level, act the main role in narrowing the band. Some O(2p) orbital states also can be seen populating the energy levels below the Fermi energy and above the valence band (see Figure 7(d)). The appearance of these states can be attributed to hybridization of O(2p) and Cu(3d) orbitals. Total densities of states (TDOS) for three different Cu concentrations have been displayed in Figure 8. The decrease in the energy band gap (as a result of the Cu(3d) defect states) is quite visible, which strongly confirms the measured trend of band gap narrowing in Table 3.

Figure 8. Total density of states of the Zn1-xCuxO composite with Cu concentrations = 0 (black solid curve), x= 6% (blue dotted curve) and 18 % (red dashed curve). The Fermi energy is set at zero. 3.8 Photocatalytic activity measurements. Photocatalytic activity of the nanofibers is evaluated by measuring the rate of decolorization of the MB, as a model compound, under sunlight irradiation. It is well known that the kinetics of light-induced degradation of organic pollutants follows a first-order reaction model, obeying Langmuir-Hinshelwood model.42,43 The kinetics of the photodegradation is described by the following equation;44 Ln $ %& ' = ()

(2)


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 41

where % and are MB concentrations at ) = 0 and later time ), respectively. ( is the apparent first order rate constant, which represents the rate of photocatalytic activity of a sample. It should be emphasized that the first-order kinetics used here has been verified by results of almost all experiments reported by others.44 Figure 9(a) shows Ln(C0/C) as a function of sunlight irradiation time (up to 120 min) over the pure ZnO, pure CuO, and their different composite nanofibers. As shown in the Figure 9(a), photocatalytic activities of all samples are higher than photolysis. The optimized ZC0.5 sample exhibits the highest photocatalytic activity (( = 3.11 10 min, ) compared to other samples and the color of the solution changes completely from blue to colorless in the specific time period chosen. For better comparison among the samples, variety of k for different amounts of CuO concentrations in the nanofibers has been plotted in Figure 9 (b). The order of photodecomposition rates for different nanofiber samples is obtained as: ZC0.5 > ZC1 > ZC0.1 > ZC2.5 > ZC5 > pure ZnO > ZC10 > ZC50 > pure CuO. BPA is a monomer commonly used for the production of the various plastics and epoxy resins and also is found in compact discs, food can linings, adhesives and etc. BPA continuous exposure, even at low levels can result in many diseases like prostate and breast cancer.45 To further verify photocatalytic activity of the optimized sample (ZC0.5), we have also studied photodegradation of BPA, under similar light irradiation conditions. The k values measured for BPA photodegradation over the ZC0.5 and pure ZnO samples were 3.4 10 and 1.9

10 min,, respectively. It is clearly seen in Figure 9(c) that the ZC0.5 nanofiber sample is about 1.8 times more active than the ZnO nanofiber sample for BPA photodegradation under the


22

Page 23 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


same sunlight conditions. A similar enhancement in photodegradation has also been reported for Cu-deposited titanate nanotubes (or CuxO-deposited TiO2) under UV light irradiation.46

Figure 9. Plots of Ln (C0/C) as function of sunlight exposure time for MB photodegradation over the ZnO/CuO nanofibers with different CuO concentrations (a). Variation of apparent rate constant with CuO concentration for the ZnO/CuO nanofiber exposed to the MB under similar irradiation conditions (b). Ln (C0/C) as a function of sunlight exposure time for BPA photodegradation over the pure ZnO (circle) and the ZC0.5 (triangle) nanofiber photocatalysts samples as compared to photolysis (c).

Table 3. The Band Gap Energy and Reaction Rate Constants Under Sunlight Irradiation for Various ZnO/CuO Nanofibers with Different Compositions.

Sample

Band gap energy (eV)

k (10-2min−1)

Pure ZnO

3.15

1.03

ZC0.1

2.93

1.91

ZC0.5

2.90

3.11

ZC1

2.87

2.24

ZC2.5

2.76

1.50

ZC5

2.65

1.29

ZC10

2.60

0.84

ZC50

1.94

0.79

Pure CuO

1.32

0.77


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 41

The amounts of SSA obtained are 17.9 m2/g for ZnO nanofibers and 6.3 m2/g for ZC0.5 nanofibers. As it was mentioned in FESEM results of ZnO and ZC0.5 nanofibers, their mean diameters were 100 and 120 nm, respectively. Therefore, higher SSA of ZnO nanfibers can be correlated to the thinner nanofibers size. As the ZC0.5 nanofibers with lower SSA showed better photocatalytic activity, this implies that maybe the presence of CuO in ZnO nanofibers is a more effective factor than SSA. Therefore, the addition of optimized content of CuO (0.5 wt%) causes an enhancement in the photocatalytic activity. 3.9. Photodegradation mechanism. 3.9.1. Role of reactive oxygen species. In order to determinate the types of surface species responsible for degradation of MB over the ZC0.5 sample under sunlight irradiation and also understand the underlying mechanism, we have used several charge carrier scavengers. Photocatalytic activity is evaluated in the presence of various scavengers with concentration of 10 mM. Isopropyl alcohol (IPA) is introduced into the system as an .OH scavenger,37 and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) is used as hole scavenger.47 Also, AgNO3 has been employed to quench electron,48 and KI is used as both .OH and hole scavengers.49 As shown in Figure 10 (a), by using AgNO3 rate constant of photocatalytic activity increases to 4.58 10 min,. This is good confirmation that the electrons do not have any role in photocatalytic activity and by effectively eliminating their presence the rate of electronhole pair recombination can be largely decreased. Thus, one can conclude that the photogenerated holes have more active role than electrons and maybe the hydroxyl radicals generated through hole-induced water oxidation exhibit important role.50 The apparent constant rate k is reduced to 0.95 10 and 1.58 10 min, by addition of IPA for hydroxyl radical quenching and EDTA for hole scavenging, respectively. Therefore, all these results can well


24

Page 25 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


verify the roles of holes and hydroxyl radicals (via hole-induced water oxidation) in the photocatalytic degradation reaction. Moreover, in these measurements KI has been employed as both holes and .OH scavengers and the photoactivity of the ZC0.5 nanofibers effectively prohibited. In this case, the rate constant (k) value is reduced to 0.69 10 min, which is similar to the photolysis rate constant. Therefore, these data can indicate that the two species (holes and hydroxyl radicals) are both the main species for photocatalytic degradation of MB by the ZC0.5 nanofibers. In order to further elucidate the primary mechanism for photodegradation, photoluminescence spectra and photocurrent densities of pure-ZnO and optimized-photocatalyst (ZC0.5) samples have been examined. A comparison between the PL spectra has been made and the results will be discussed in the next subsection and then photocurrent densities will be compared. 3.9.2. Photoluminescence spectra of nanofiber samples. Photoluminescence (PL) spectra are useful data for revealing the trapping, migration, and transfer of charge carriers because PL emission is highly dependent on the recombination of free carriers.51 The PL spectra of the pure ZnO and ZCO.5 nanofibers shown in Figure 10(b) contain three emission peaks. The PL spectrum of pure ZnO sample has one UV emission and two visible peaks. The near band-edge narrow UV peak located at 390 nm can be attributed to direct radiative recombination of excitons,51 and the visible peaks at ca. 424 and 526 nm appeared as a result of charge carrier relaxation due to the presence of surface related trap states. These trap states originate from the zinc interstitials and oxygen vacancies in the ZnO crystal.52 The spectrum of the ZC0.5 nanofibers shows similar positions for the peaks, but the peak intensities reduced significantly (see Fig. 10(b)). In the case of excitonic PL signals, the lower peak intensity shows the reduction


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 41

of charge carrier recombination. Diminishing of peak intensities could be due to the presence of very small CuO content in ZnO nanofiber sample which can largely suppress photo-induced electron-hole recombination via effective charge carrier separation. A similar observation has been also reported for the (ZnO)1-x(CdO)x nanofibers recently.13 3.9.3 Photocurrent densities of nanofiber samples. In order to more clarify the photodegradation mechanism, we have measured the photocurrent response of the ZC0.5 samples and pure ZnO. Figure 10(c) shows the photocurrent responses of ZnO and ZC0.5 samples versus time by turning on and off the illumination. The irradiation process was repeated with an interval of 25 seconds over 400 seconds. The photocurrent density of ZC0.5 is larger than that of ZnO. This enhancement in the photocurrent points out the more effective photogenerated charge separation in ZC0.5 samples and is the other reason for an enhancement in its photocatalytic activity.

Figure 10. The measured apparent rate constants in the presence of different scavengers used in this work (a). Photoluminescence (PL) spectra of the pure ZnO (solid) and the optimized ZC0.5 (dashed) nanofibers (b) measured at excitation wavelength of λ=325 nm. Photocurrent density vs. time for the ZnO and ZC0.5 samples at constant potential (0.5 V vs. Ag/AgCl) (c).

3.9.4. Proposed mechanism. Since CuO is a p-type semiconductor with a narrow band gap energy of 1.35-1.79 eV,15 it is expected to be a visible light active photocatalyst. However, it has


26

Page 27 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


rather low photocatalytic activity,53 which can be due to inappropriate potentials of the valence and conduction bands, that are more negative and positive for reduction of O2 to .O2- and oxidation of H2O to .OH, respectively.54 In our experimental work, when small amounts of CuO are loaded into ZnO nanofibers, the photocatalytic activity of ZnO nanofibers is improved. For the ZC0.5 nanofiber sample, the measured rate constant is 3.11 10 min,, which is about 3 times higher than that of pure ZnO nanofibers. But, when CuO concentration is increased to 10wt% and 50wt% the value of k reduces to 0.84 10 and 0.79 10 min,, respectively (see Table 3). This nearly one order decrease in k value can be explained by the fact that when higher amounts of CuO (Cu content) exist in ZnO nanofiber sample, the energy band gap is remarkably narrowed by the presence of defect states in the gap (see Figures 7(b) and 7 (d)), and as a result the electron-hole recombination becomes more effective. Thus, effective irradiative electron-hole recombination becomes the main channel for the loss of holes in the system, which in turn could dramatically diminish the photocatalytic activity of the sample.16 Previous works have shown that combined band structure of ZnO and CuO can work as overall electronic band structures of type I semiconductor heterojunction,55,56 in which CB and VB of CuO structure fall between those of wurtzite ZnO structure.16,57 Using the following equation, one can calculate energy at the top of the valence band (012 ) of a semiconductor with band gap ;58 345 = 6 − 7 + 0.5

(3)

where χ is the absolute electronegativity of the semiconductor, which is the geometric mean of the absolute electronegativity of the constituent atoms, and 7 is the energy of free electrons in the hydrogen scale (∼4.5 eV). Also, in order to calculate the energy at the bottom of the


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 41

conduction band (ECB) vs. normal hydrogen electrode (NHE), the following equation can be used;58 95 = 345 −

(4)

The reported absolute electronegativities (χ) for CuO and ZnO are 5.81 and 5.79 eV, respectively.59 From our DRS data (Table 3), band gap energies of 3.15 and 1.32 eV have been obtained for the pure ZnO and CuO samples, respectively, in a good agreement with results of our band structure calculations. Using these measured band gaps in above two equations, the positions of the valence and conduction edges for ZnO and CuO have been calculated and shown in Scheme 2. The band edge positions obtained are in good agreement with previously reported data.59 Having known the roles of reactive oxygen species and using the sketched band edge alignments for ZnO and CuO structures, a mechanism for charge transfer between ZnO and CuO can be proposed in Scheme 2. Under the Xe lamp irradiation, the electrons and holes are generated in the ZnO and CuO structures. Because of the special band edge alignment for the ZnO/CuO compound, the photo-excited electrons in the ZnO conduction band can be transferred to the CuO conduction band (eq. 1).57 ZnO / CuO+ hν → e- (CuO) + h+ (ZnO)

(1)

This electron-hole separation and electron transfer in ZnO/CuO nanofiber (due to the band alignment) can be responsible for the observed low PL intensity measured for the optimized ZnO/CuO nanofiber sample (ZC0.5) (see Figure 10(b)). Also, the CuO has a conduction band edge position of 0.65 eV vs. NHE, thus it cannot reduce

oxygen

molecule

to

superoxide

anion

radical,

neither

kinetically

nor


28

Page 29 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


thermodynamically.37 Based on the interface charge transfer (IFCT) model, transferred photoexcited electrons from ZnO CB can react with Cu(II) to produce Cu(I):19

Cu(II) + e- → Cu(I)

(2)

Scheme 2. Schematic diagram of the band edge alignment of the ZnO/CuO system along with the proposed mechanism of charge transfer as well as the intermediate reactions for degradation of pollutant.

Subsequently, produced Cu(I) can reduce the dissolved O2 molecule to H2O2 through a reaction involving two electrons:. 2Cu(I) + O 2 + 2H + → 2Cu(II) + H 2 O 2

(3)

This is a reaction channel in which free electrons are removed from the system.60 Then, from hydrogen peroxide and the reaction of water molecules with generated holes, the hydroxyl radicals are produced:

H 2O 2 → 2 • OH

(4)


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

H 2 O + h + → H + + • OH

Page 30 of 41

(5)

and then these .OH radicals can decompose the organic pollutants as following: • OH + pollutant → intermediates → CO + H O 2 2

(6)

The holes generated as a result of electron excitation (to the conduction band) in the ZnO electronic structure can play an important role when they migrate to the surface of ZnO nanofibers. They not only (as stated above) can participate in water oxidation, which gives rise to the production of hydroxyl radicals as well, but also can solely react with pollutants and turn it to the same byproduct as the hydroxyl radical-MB reaction yields:57 h + + pollutant → intermediates → CO 2 + H 2 O

(7)

Therefore, presence of enough holes from optimized ZC0.5 nanofibers and as-produced hydroxyl radicals can lead to effective degradation of MB. Similarly, other organic pollutants (such as BPA) can be degraded (and thus removed) by reaction with the holes and hydroxyl radicals produced from ZnO/CuO nanofibers under sunlight irradiation.

4. CONCLUSIONS Smooth and beadless ZnO/CuO composite nanofibers with different CuO concentrations were fabricated by electrospinning of the solution of zinc acetate/cupric acetate/PVA precursor followed by annealing in air at 460 °C. DRS results showed that by increasing CuO concentration, ZnO band gap energy decreases. This experimental fact was verified by DFT calculations of the electronic band structure for the Cu-doped wurtzite ZnO structure. Using this


30

Page 31 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


model, it was found that Cu(3d) defect states above the valence band of ZnO were responsible for the band gap narrowing with an increase in Cu concentrations. Photodegradation measurements showed that the ZnO/(0.5wt%) CuO exhibited 3 times improvement in degradation of MB and 1.8 times enhancement in decomposition of BPA as compared to pure ZnO nanofibers under similar sunlight irradiation conditions. Photoluminescence emission spectra and photocurrent density measurements revealed that the observed lower PL intensity and enhanced photocurrent could be due to the presence of the optimized CuO content (0.5wt% reported here), which could highly suppress photo-induced electron-hole recombination via effective charge separation. At higher CuO concentrations, more effective electron-hole recombination occurred due to the band gap narrowing resulted in lower photocatalytic activity. A number of charge carrier scavengers were used in order to determine the main species involved in the photocatalytic degradation of the methylene blue. The holes and hydroxyl radicals were found to be the main species responsible for photocatalytic degradation over the ZnO/(0.5wt%) CuO. Finally, it was proposed that the optimized ZnO/CuO nanofiber (ZnC0.5) can be a good candidate for effective removal of pollutants from environment by tuning and controlling the amount of CuO contents in the ZnO/CuO nanofibers.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 41

AKNOWLEDGEMENT Authors would like to thank Research Council of Sharif University of Technology and Institute for Color Science and Technology for supporting this research project. Support of Iran National Science Foundation (INSF) for funding the project through a grant No. 92026525 is highly acknowledged. We would also like to thank Dr. S. Yousefzadeh, M. Doroudian, M. Soltani and N. Sarikhani for fruitful discussions.


32

Page 33 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1)

Jing, L.; Zhou, W.; Tian, G.; Fu, H. Surface Tuning for Oxide-Based Nanomaterials as

Efficient Photocatalysts. Chem. Soc. Rev. 2013, 42, 9509-9549. (2)

Advanced Oxidation Processes for Water and Wastewater Treatment. Parsons, S., Ed.;

Water Intelligence Online, 2004. (3)

Méndez-Medrano, M. G.; Kowalska, E.; Lehoux, A.; Herissan, A.; Ohtani, B.; Bahena,

D.; Briois, V.; Colbeau-Justin, C.; Rodríguez-López, J. L.; Remita, H. Surface Modification of TiO2 with Ag Nanoparticles and CuO Nanoclusters for Application in Photocatalysis. J. Phys. Chem. C 2016, 120, 5143-5154. (4)

Bayati, M. R.; Golestani-Fard, F.; Moshfegh, A. Z.; Molaei, R. A Photocatalytic

Approach in Micro Arc Oxidation of WO3–TiO2 Nano Porous Semiconductors Under Pulse Current. Mater. Chem. Phys. 2011, 128, 427-432. (5)

Liu, H. R.; Shao, G. X.; Zhao, J. F.; Zhang, Z. X.; Zhang, Y.; Liang, J.; Liu, X. G.; Jia, H.

S. ;Xu, B. S. Worm-Like Ag/ZnO Core–Shell Heterostructural Composites: Fabrication, Characterization, and Photocatalysis. J. Phys. Chem. C 2012, 116, 16182-16190. (6)

Wang, H.; Zhang, L.; Chen, Z.; Hu, J.; Li, S.; Wang, Z.; Liu, J.; Wang, X.

Semiconductor Heterojunction Photocatalysts: Design, Construction, and Photocatalytic Performances. Chem. Soc. Rev. 2014, 43, 5234-5244. (7)

Rehman, S.; Ullah, R.; Butt, A. M.; Gohar, N. D. Strategies of Making TiO2 and ZnO

Visible Light Active. J. Hazard. Mater. 2009, 170, 560-569. (8)

Subash, B.; Krishnakumar, B.; Velmurugan, R.; Swaminathan, M.; Shanthi, M. Synthesis

of Ce Co-doped Ag-ZnO Photocatalyst with Excellent Performance for NBB Dye Degradation under Natural Sunlight Illumination. Catal. Sci. Tech. 2012, 2, 2319-2326.


33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(9)

Page 34 of 41

Samadi, M.; Zirak, M.; Naseri, A.; Khorashadizade, E.; Moshfegh, A. Z. Recent Progress

on Doped ZnO Nanostructures for Visible-Light Photocatalysis. Thin Solid Films 2016, 605, 219.

(10)

Samadi, M.; Shivaee, H. A.; Zanetti, M.; Pourjavadi, A.; Moshfegh, A. Visible Light

Photocatalytic Activity of Novel MWCNT-Doped ZnO Electrospun Nanofibers. J. Mol. Catal. A: Chem. 2012, 359, 42-48. (11)

Samadi, M.; Shivaee, H. A.; Pourjavadi, A.; Moshfegh, A. Z. Synergism of Oxygen

Vacancy and Carbonaceous Species on Enhanced Photocatalytic Activity of Electrospun ZnOCarbon Nanofibers: Charge Carrier Scavengers Mechanism. Appl. Catal. A: Gen 2013, 466, 153160. (12)

Zirak, M.; Akhavan, O.; Moradlou, O.; Nien, Y.; Moshfegh, A. Vertically Aligned

ZnO@ CdS Nanorod Heterostructures for Visible Light Photoinactivation of Bacteria. J. Alloys Compd. 2014, 590, 507-513. (13)

Samadi, M.; Pourjavadi, A.; Moshfegh, A. Z. Role of CdO Addition on the Growth and

Photocatalytic Activity of Electrospun ZnO Nanofibers: UV vs. Visible Light. Appl. Surf. Sci. 2014, 298, 147-154. (14)

Mageshwari, K.; Nataraj, D.; Pal, T.; Sathyamoorthy, R.; Park, J. Improved

Photocatalytic Activity of ZnO Coupled CuO Nanocomposites Synthesized by Reflux Condensation Method. J. Alloys Compd. 2015, 625, 362-370. (15)

Chen, H.; Leng, W.; Xu, Y. Enhanced Visible-Light Photoactivity of CuWO4 through a

Surface-Deposited CuO. J. Phys. Chem. C 2014, 118, 9982-9989.


34

Page 35 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(16)

Qamar, M. T.; Aslam, M.; Ismail, I. M. I.; Salah, N.; Hameed, A. Synthesis,

Characterization, and Sunlight Mediated Photocatalytic Activity of CuO Coated ZnO for the Removal of Nitrophenols. ACS Appl. Mater. Interf. 2015, 7, 8757-8769. (17)

Praveen Kumar, D.; Shankar, M. V.; Mamatha Kumari, M.; Sadanandam, G.; Srinivas,

B.; Durgakumari, V. Nano-Size Effects on CuO/TiO2 Catalysts for Highly Efficient H2 Production under Solar Light Irradiation. Chem. Commun. 2013, 49, 9443-9445. (18)

Chabri, S.; Dhara, A.; Show, B.; Adak, D.; Sinha, A.; Mukherjee, N. Mesoporous CuO-

ZnO p-n Heterojunction Based Nanocomposites with High Specific Surface Area for Enhanced Photocatalysis and Electrochemical Sensing. Catal. Sci. Tech. 2016, 6, 3238-3252. (19)

Irie, H.; Kamiya, K.; Shibanuma, T.; Miura, S.; Tryk, D. A.; Yokoyama, T.; Hashimoto,

K. Visible Light-Sensitive Cu(II)-Grafted TiO2 Photocatalysts: Activities and X-ray Absorption Fine Structure Analyses. J. Phys. Chem. C, 2009, 113, 10761-10766. (20) Malwal, D.; Gopinath, P. Enhanced Photocatalytic Activity of Hierarchical Three Dimensional Metal Oxide@CuO Nanostructures Towards the Degradation of Congo Red Dye under Solar Radiation. Catal. Sci. Tech. 2016, 6, 4458-4472. (21)

Reneker, D. H.; Yarin, A. L. Electrospinning Jets and Polymer Nanofibers. Polymer

2008, 49, 2387-2425. (22)

Peng, S.; Jin, G.; Li, L.; Li, K.; Srinivasan, M.; Ramakrishna, S.; Chen, J. Multi-

Functional Electrospun Nanofibres for Advances in Tissue Regeneration, Energy Conversion & Storage, and Water Treatment. Chem. Soc. Rev. 2016, 45, 1225-1241. (23)

Malwal, D.; Gopinath, P. Fabrication and Characterization of Poly(ethylene oxide)

Templated Nickel Oxide Nanofibers for Dye Degradation. Environ. Sci.: Nano 2015, 2, 78-85.


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(24)

Page 36 of 41

Vijayakumar, G. N. S.; Devashankar, S.; Rathnakumari, M.; Sureshkumar, P. Synthesis

of Electrospun ZnO/CuO Nanocomposite Fibers and Their Dielectric and Non-Linear Optic Studies. J. Alloys Compd. 2010, 507, 225-229. (25) Moshfegh, A. Z. Nanoparticle Catalysts. J. Phys. D: Appl. Phys. 2009, 42, 233001 (30pp). (26)

Yang, C.; Shen, J.; Wang, C.; Fei, H.; Bao, H.; Wang, G. All-Solid-State Asymmetric

Supercapacitor Based on Reduced Graphene Oxide/Carbon Nanotube and Carbon Fiber Paper/Polypyrrole Electrodes. J. Mater. Chem. A 2014, 2, 1458-1464. (27)

Yukselen, Y.; Kaya, A. Suitability of the Methylene Blue Test for Surface Area, Cation

Exchange Capacity and Swell Potential Determination of Clayey Soils. Eng. Geol. 2008, 102, 38-45. (28)

Giannozzi, P. et al. QUANTUM ESPRESSO: A Modular and Open-Aource Software

Project for Quantumsimulations of Materials. J. Phys.: Condens. Matter. 2009, 21, 395502. (29)

Ma, X.; Lu, B.; Li, D.; Shi, R.; Pan, C.; Zhu, Y. Origin of Photocatalytic Activation of

Silver Orthophosphate from First-Principles. J. Phys. Chem. C 2011, 115, 4680-4687. (30)

Yang, X.; Shao, C.; Guan, H.; Li, X.; Gong, J. Preparation and Characterization of ZnO

Nanofibers by Using Electrospun PVA/Zinc Acetate Composite Fiber as Precursor. Inorg. Chem. Commun. 2004, 7, 176-178. (31)

Wang, Y.; Zhang, J.; Chen, X.; Li, X.; Sun, Z.; Zhang, K.; Wang, D.; Yang, B.

Morphology-Controlled Fabrication of Polygonal ZnO Nanobowls Templated from Spherical Polymeric Nanowell Arrays. J. Coll. Interf. Sci. 2008, 322, 327-332.


36

Page 37 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(32)

Saravanan, R.; Karthikeyan, S.; Gupta, V. K.; Sekaran, G.; Narayanan, V.; Stephen, A.

Enhanced Photocatalytic Activity of ZnO/CuO Nanocomposite for the Degradation of Textile Dye on Visible Light Illumination. Mater. Sci. Eng. C 2013, 33, 91-98. (33)

Yang, C.; Cao, X.; Wang, S.; Zhang, L.; Xiao, F.; Su, X.; Wang, J. Complex-Directed

Hybridization of CuO/ZnO Nanostructures and Their Gas Sensing and Photocatalytic Properties. Ceram Int. 2015, 41, 1749-1756. (34)

Sahay, R.; Sundaramurthy, J.; Kumar, P. S.; Thavasi, V.; Mhaisalkar, S. G.;

Ramakrishna, S. Synthesis and Characterization of CuO Nanofibers, and Investigation for its Suitability as Blocking Layer in ZnO NPs Based Dye Sensitized Solar Cell and as Photocatalyst in Organic Dye Degradation. J. Solid State Chem. 2012, 186, 261-267. (35)

Li, D.; Xia, Y. Fabrication of Titania Nanofibers by Electrospinning. Nano Letters 2003,

3, 555-560. (36)

Niskanen, M.; Kuisma, M.; Cramariuc, O.; Golovanov, V.; Hukka, T. I.; Tkachenko, N.;

Rantala, T. T. Porphyrin Adsorbed on the 101 0 Surface of the Wurtzite Structure of ZnOConformation Induced Effects on the Electron Transfer Characteristics. Phys. Chem. Chem. Phys. 2013, 15, 17408-17418. (37)

Moniz, S .J. A.; Tang, J. Charge Transfer and Photocatalytic Activity in CuO/TiO2

Nanoparticle Heterojunctions Synthesised through a Rapid, One-pot, Microwave Solvothermal Route. Chem. Cat. Chem. 2015, 7, 1659-1667. (38)

Poulston, S.; Parlett, P.; Stone, P.; Bowker, M. Surface Oxidation and Reduction of CuO

and Cu2O Studied Using XPS and XAES. Surf. Interf. Anal. 1996, 24, 811-820.


37


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(39)

Page 38 of 41

Senthilkumar, V.; Kim, Y. S.; Chandrasekaran, S.; Rajagopalan, B.; Kim, E. J.; Chung, J.

S. Comparative Supercapacitance Performance of CuO Nanostructures for Energy Storage Device Applications. RSC Adv. 2015, 5, 20545-20553. (40)

Lee, J. -B.; Lee, H. -J.; Seo, S.-H.; Park, J.-S. Characterization of Undoped and Cu-

Doped ZnO Films for Surface Acoustic Wave Applications. Thin Solid Films 2001, 398, 641646. (41)

Sathishkumar, P.; Sweena, R.; Wu, J. J.; Anandan, S. Synthesis of CuO-ZnO

Nanophotocatalyst for Visible Light Assisted Degradation of a Textile Dye in Aqueous Solution. Chem. Eng. J. 2011, 171, 136-140. (42)

Bayati, M. R.; Golestani-Fard, F.; Moshfegh, A. Z. How Photocatalytic Activity of the

MAO-Grown TiO2 Nano/Micro-Porous Films Is Influenced by Growth Parameters? Appl. Surf. Sci. 2010, 256, 4253-4259. (43)

Bechambi, O.; Jlaiel, L.; Najjar, W.; Sayadi, S. Photocatalytic Degradation of Bisphenol

A in the Presence of Ce–ZnO: Evolution of Kinetics, Toxicity and Photodegradation Mechanism. Mater. Chem. Phys. 2016, 173, 95-105. (44)

Du, P.; Bueno-López, A.; Verbaas, M.; Almeida, A. R.; Makkee, M.; Moulijn, J. A.; Mul,

G. The Effect of Surface OH-Population on the Photocatalytic Activity of Rare Earth-Doped P25-TiO2 in Methylene Blue Degradation. J. Catal. 2008, 260, 75-80. (45)

Chiang, K.; Lim, T. M.; Tsen, L.; Lee, C. C. Photocatalytic Degradation and

Mineralization of Bisphenol A by TiO2 and Platinized TiO2. Appl. Catal. A: Gen., 2004, 261, 225-237.


38

Page 39 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(46)

Doong, R. -A.; Chang, S. -M.; Tsai, C. -W. Enhanced Photoactivity of Cu-Deposited

Titanate Nanotubes for Removal of Bisphenol A. Appl. Catal. B: Environmental, 2013, 129, 4855. (47)

Wang, Y.; Shi, R.; Lin, J.; Zhu, Y. Enhancement of Photocurrent and Photocatalytic

Activity of ZnO Hybridized with Graphite-Like C3N4. Ene. Environ. Sci. 2011, 4, 2922-2929. (48)

Chen, Z.; Zhang, N.; Xu, Y.-J. Synthesis of Graphene–ZnO Nanorod Nanocomposites

with Improved Photoactivity and Anti-Photocorrosion. Crystengcomm 2013, 15, 3022-3030. (49)

Li, Y.; Wang, J.; Yao, H.; Dang, L.; Li, Z. Efficient Decomposition of Organic

Compounds and Reaction Mechanism with BiOI Photocatalyst under Visible Light Irradiation. J. Mol. Catal. A: Chem. 2011, 334, 116-122. (50)

Zheng, P.; Pan, Z.; Li, H. Y.; Bai, B.; Guan, W. S. Effect of Different Type of

Scavengers on the Photocatalytic Removal of Copper and Cyanide in the Presence of TiO2@yeast Hybrids. J. Mater. Sci-Mater.: El. 2015, 26, 6399-6410. (51)

Djurisic, A. B.; Leung, Y. H. Optical Properties of ZnO Nanostructures. Small, 2006, 2,

944-961. (52)

Pal, M.; Bera, S.; Sarkar, S.; Jana, S. Influence of Al Doping on Microstructural, Optical

and Photocatalytic Properties of Sol–Gel Based Nanostructured Zinc Oxide Films on Glass. RSC Adv. 2014, 4, 11552-11563. (53)

Arai, T.; Yanagida, M.; Konishi, Y.; Iwasaki, Y.; Sugihara, H.; Sayama, K. Promotion

Effect of CuO Co-catalyst on WO3-Catalyzed Photodegradation of Organic Substances. Catal. Commun. 2008, 9, 1254-1258. (54)

Chen, H.; Xu, Y. Photocatalytic Organic Degradation over W-rich and Cu-rich CuWO4

under UV and Visible Light. RSC Adv. 2015, 5, 8108-8113.


39


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(55)

Page 40 of 41

Wang, Z.; Liu, Y.; Huang, B.; Dai, Y.; Lou, Z.; Wang, G.; Zhang, X.; Qin, X. Progress

on Extending the Light Absorption Spectra of Photocatalysts. Phys. Chem. Chem. Phys. 2014, 16, 2758-2774. (56)

Himmetoglu, B.; Wentzcovitch, R. M.; Cococcioni, M. First-Principles Study of

Electronic and Structural Properties of CuO. Phys. Rev. B, 2011, 84, 115108. (57)

Liu, Z.; Bai, H.; Xu, S.; Sun, D. D. Hierarchical CuO/ZnO "Corn-Like" Architecture for

Photocatalytic Hydrogen Generation. Int. J. Hyd. Ene. 2011, 36, 13473-13480. (58)

Zhou, P.; Yu, J.; Jaroniec, M. All‐Solid‐State Z‐scheme Photocatalytic Systems. Adv.

Mater. 2014, 26, 4920-4935. (59)

Xu, Y.; Schoonen, M. A. The Absolute Energy Positions of Conduction and Valence

Bands of Selected Semiconducting Minerals. Am. Mineral. 2000, 85, 543-556. (60)

Shan, W.; Hu, Y.; Zheng, M.; Wei, C. The Enhanced Photocatalytic Activity and Self-

Cleaning Properties of Mesoporous SiO2 Coated Cu-Bi2O3 Thin Films. Dalton Trans. 2015, 44, 7428-7436.


40

Page 41 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


"TOC Graphic"


41

CuO Nanofibers: Toward

Recommend Documents