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Facile synthesis of cobalt-doped zinc oxide thin films for highly efficient visible light photocatalysts
Accepted Manuscript Title: Facile Synthesis of Cobalt-Doped Zinc Oxide Thin Films for Highly Efficient Visible Light Photocatalysts Author: Ozlem ALTINTAS YILDIRIM Hanife ARSLAN ¨ ˘ Savas¸ SONMEZO GLU PII: DOI: Reference:
S0169-4332(16)31712-3 http://dx.doi.org/doi:10.1016/j.apsusc.2016.08.069 APSUSC 33818
To appear in:
APSUSC
Received date: Revised date: Accepted date:
28-6-2016 11-8-2016 12-8-2016
Please cite this article as: Ozlem ALTINTAS YILDIRIM, Hanife ARSLAN, ¨ ˘ Savas¸ SONMEZO GLU, Facile Synthesis of Cobalt-Doped Zinc Oxide Thin Films for Highly Efficient Visible Light Photocatalysts, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.08.069 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.
Facile Synthesis of Cobalt-Doped Zinc Oxide Thin Films for Highly Efficient Visible Light Photocatalysts
Ozlem ALTINTAS YILDIRIM1,*, Hanife ARSLAN2,3, and Savaş SÖNMEZOĞLU,3
1
Department of Metallurgical and Materials Engineering, Selcuk University, Konya, Turkey.
2
Department of Metallurgical and Materials Engineering, Karamanoglu Mehmetbey University,
Karaman, Turkey. 3
Nanotechnology R&D Laboratory, Karamanoglu Mehmetbey University, Karaman, Turkey.
*To whom all correspondence should be addressed: e-mail: [email protected] phone: +90 (332) 223-1997 fax: +90 (332) 241-0635
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Graphical Abstract
2
Highlights
Photocatalytically active Co-ZnO thin film was obtained by sol-gel method.
Co2+ doping narrowed the band gap of pure ZnO to an extent of 3.18 eV.
Co-ZnO was effective in MB degradation under visible light.
Optimum dopant content to show high performance was 3 at. %.
3
Abstract Cobalt-doped zinc oxide (Co:ZnO) thin films with dopant contents ranging from 0 to 5 at.% were prepared using the sol–gel method, and their structural, morphological, optical, and photocatalytic properties were characterized. The effect of the dopant content on the photocatalytic properties of the films was investigated by examining the degradation behavior of methylene blue (MB) under visible light irradiation, and a detailed investigation of their photocatalytic activities was performed by determining the apparent quantum yields (AQYs). Co2+ ions were observed to be substitutionally incorporated into Zn2+ sites in the ZnO crystal, leading to lattice parameter constriction and band gap narrowing due to the photoinduced carriers produced under the visible light irradiation. Thus, the light absorption range of the Co:ZnO films was improved compared with that of the undoped ZnO film, and the Co:ZnO films exhibited highly efficient photocatalytic activity (~92% decomposition of MB after 60-min visible light irradiation for the 3 at.% Co:ZnO film). The AQYs of the Co:ZnO films were greatly enhanced under visible light irradiation compared with that of the undoped ZnO thin film, demonstrating the effect of the Co doping level on the photocatalytic activity of the films.
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1.
INTRODUCTION
In recent decades, the presence of organic dye compounds used in textile and paint industries as well as household chemicals has caused serious detrimental effects for living ecosystems as a result of water pollution [1]. Traditional biological treatment methods can be unsuccessful in decolorizing and mineralizing the stable dye molecules. Today, semiconductor-assisted photocatalysts have attracted considerable attention in wastewater treatment because of their high photosensitivity, environmentally friendly non-toxic nature, and low cost [2]. These type semiconductor-based photocatalysts have been also used as gas sensors and antibacterial applications [3, 4]. Although titanium dioxide (TiO2) is widely used as a photocatalyst among the various oxide-type semiconductor photocatalysts (i.e., TiO2, iron trioxide, and tungsten trioxide), zinc oxide (ZnO) is one of the most preferred materials because of its outstanding properties such as its similar band gap to TiO2, low cost, chemical stability, and non-toxicity [5-7]. ZnO is an important semiconductor material with a wide band gap (3.3 eV) and large exciton binding energy (60 meV) [8]. However, semiconductor-based photocatalysts containing ZnO can only absorb UV light with a wavelength of less than 385 nm, which accounts for less than 10% of solar irradiation. Thus, expanding the absorption spectral range is critical in improving photocatalytic applications of ZnO. To absorb interior lighting or visible light in the solar spectrum, the band gap energy of ZnO should be reduced or split into several sub-gaps. Techniques such as semiconductor combination [9], noble metal deposition [10, 11], nonmetal doping [12, 13] and
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transition metal (TM) doping [14, 15] have been used to improve the photocatalytic efficiency of semiconductor catalysts. TM doping provides more photons to be absorbed and decomposes organic pollutants by altering the coordination environment of Zn with TM ion substitution into the ZnO lattice, thus modifying the electronic band structure of ZnO [16]. Therefore, doping ZnO with TM ions appears to be effective in expanding its photoresponse from the UV light region into the visible range. TM ions act as trapping sites by receiving the photogenerated e- and h° carriers, hindering recombination of the carriers and thus enhancing the photocatalytic activity of the ZnO [17]. Among the various types of TM-doped ZnO materials, cobalt-doped ZnO (Co:ZnO) nanostructures with controllable band structure are particularly attractive because of the abundant electronic states of Co and minor effect of Co on the ZnO structure [18]. To date, several researchers have studied the photocatalytic activities of Co:ZnO nanostructures under UV light illumination [19, 20]. However, detailed studies on the visible light photocatalytic efficiency of Co:ZnO systems against stable dye molecules have rarely been reported. In addition, a survey of the literature shows that only powder form of Co-ZnO is explicitly investigated and least attention is made towards the thin films [21-24]. Consequently, the main purpose of this study was to explore the effects of Co doping on the photocatalytic activity of ZnO thin films and determine the relevant mechanism. In this work, we prepared undoped ZnO and Co:ZnO films with Co contents of 1, 2, 3, 4, and 5 at.% via the sol–gel method. Methylene blue (MB) degradation under visible light irradiation was used to determine the effect of the Co addition on the photocatalytic activities of the Co:ZnO films. To determine the photocatalytic mechanism, detailed structural, morphological, and optical analyses of the Co:ZnO films were performed, and the kinetic parameters of the 6
photocatalytic reaction were investigated. The Co:ZnO films exhibited improved visible light photoresponses for the degradation of MB solution compared with that of the undoped ZnO film, and 3 at.% Co was determined to be the optimum content to achieve the best photocatalytic response among the studied conditions.
2. EXPERIMENTAL DETAILS 2.1.
Synthesis of undoped ZnO and Co:ZnO nanoparticles
Pure ZnO and Co:ZnO nanoparticles were successfully synthesized using the sol–gel method. In a typical process, 0.87 g zinc acetate dihydrate was first dissolved in a mixture of 10 mL 2methoxyethanol and 1 mL ethanolamine at 70 °C and then subjected to magnetic stirring for 2 h. Cobalt acetate, used as source of the Co dopant, was added into the precursor solution at various concentrations (1, 2, 3, 4, and 5 at.%) under vigorous stirring. The solutions were stirred for 1 h using a magnetic stirrer at 70 °C to obtain a clear homogeneous and transparent sol, which served as the coating solution after being stored for 24 h at room temperature. 2.2.
Preparation of undoped ZnO and Co:ZnO thin films
After the above synthesis procedure, a spin-coating process was used to deposit the ZnO solution on 2x2 cm glass substrates. The spinning process was performed using 30 μL of solution spun in air at 2000 rpm for 30 s. This process was repeated 25 times to achieve a uniform surface. After each spinning process, the samples were subjected to repeated pre-annealing processes at 400 °C for 5 min. Finally, the post-annealing process was performed in an oven at 550 °C for 1 h.
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2.3.
Characterization of undoped ZnO and Co:ZnO films
The crystal structures of the pure ZnO and Co:ZnO films were identified using X–ray diffraction (XRD, Bruker D8 Advanced) in the 2θ range of 20°–70°. The surface morphology and roughness of the samples were analyzed using scanning electron microscopy (SEM, Zeiss LS–10) and atomic force microscopy (AFM, Nanomagnetics Instruments, Ambient AFM). The composition of the films was determined using X-ray photoelectron spectroscopy (XPS, SPECS EA 300) and energy-dispersive
X-ray
spectroscopy
(EDS).
UV–Vis
absorption
and
transmission
measurements were also obtained using a SPECORD S600 spectrophotometer. The photoluminescence (PL) spectra of the samples were recorded at room temperature using a PTI Quanta Master 30 PC spectrofluorophotometer with a Xe lamp as the excitation source. 2.4.
Photocatalytic characterization
The photocatalytic activities of the thin films were evaluated based on the visible light photocatalytic degradation of MB dye in a cylindrical custom-built photoreactor. Three 23-W xenon lamps with a cut-off filter of 420 nm were used as the visible light source, and the reaction was maintained at room temperature by circulating with cool water. These thin films were placed in a 12 ppm MB suspension in aqueous solution. The distance between the top of the reactor and lamp was adjusted to be 13 cm. During the photoreaction process, 3 mL of the solution was removed in a constant time, the photocatalyst was separated from the solution by centrifugation, and the concentration of the remaining clear liquid was determined using the UV-Vis spectrophotometer. The photocatalytic degradation of MB was estimated from the reduction in the absorption intensity of MB at a fixed wavelength λmax = 664 nm. In addition to the photocatalytic degradation, the apparent quantum yield (AQY) of the photocatalytic process was
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estimated using the equation given in Ref. [25]. This estimated AQY is lower than the real quantum yield because the number of absorbed photons is usually smaller than the number of incident photons unless a dispersed system where light scattering comes into play is not used. Furthermore, the degradation percentage of MB was calculated using the relation % degradation = [(A0-At)/A0] × 100,
(1)
where A0 is the initial absorbance of dye and At is the absorbance of dye at time t.
3. RESULTS AND DISCUSSION 3.1 Crystal structure and orientation of the undoped ZnO and Co:ZnO films The crystal structure and orientation of the undoped ZnO and Co:ZnO films prepared via the sol– gel method on the glass substrate at 550 °C were characterized using XRD, and the results are presented in Fig. 1. Fig. 1(a) shows that all the diffraction peaks of both the undoped and Co:ZnO films match those of the hexagonal wurtzite ZnO crystal structure (JCPDS card no: 361451). No evidence of impurity phases related to any type of cobalt or cobalt oxide was observed within the detection limit of the XRD, which suggests that the addition of up to 5 at.% Co to ZnO does not change the crystal structure of ZnO within the studied deposition temperature range. The intensity order of the XRD peaks of all the samples deviates from the standard JCPDS card order. The presence of relatively higher intensity [0002] diffraction lines indicate that the ZnO and Co:ZnO films grew with the c-axis preferred orientation perpendicular to the substrate plane.
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Similar findings have also been reported by several groups in the literature [26, 27]. This result can be attributed to the different chemical activity and surface energy of the Zn- and Oterminated polar surfaces of the ZnO crystal. ZnO usually prefers to grow along the [0002] direction to reduce its highest surface energy compared with the other planes. After formation of the first ZnO clusters on the glass substrate, the Zn-terminated chemically active positive polar surface supplies an appropriate environment for local enrichment of Zn on the ZnO cluster [28]. According to this theory, non-homogenously distributed regions of the (0002) surface of the cluster act as a catalyst for film growth. The incorporation of Co ions into the ZnO lattice may produce lattice distortion of the host lattice. To investigate the effect of the Co doping on the crystal structure, the peak positions of the ZnO diffractions were examined in detail. Fig. 1(b) shows the dependence of the XRD peak positions on the Co content. The 2θ values as a function of the Co content are listed in Table 1. Co incorporation results in a peak shift to higher 2θ values, which indicates substitutional incorporation of Co atoms into the ZnO crystal. As observed in Table 1, the c-axis lattice constant calculated from the (0002) plane decreased from 5.134 to 5.075 Å as the Co content increased from 0 to 5 at.%. The smaller value of the c-axis lattice parameter in the Co:ZnO films than in the undoped film indicates a slight narrowing of the lattice due to substitution of the Co ions with slightly smaller ionic size (Co2+ = 0.58 Å and Zn2+ = 0.60 Å). According to these results, the Co addition obeys Vegard’s law and indicates the substitutional incorporation of Co ions into the ZnO crystal [29, 30]. It was also observed that [0002] diffraction intensity of 3 at.% Co containing Co:ZnO film decreased compared with other films. This result shows that for 3 at.% Co containing Co:ZnO film, substitution of Co ions and reducing of the surface energy hinder the crystal growth through [0002] direction more effectively. It was worth noting that 10
ionic substitution takes place up to a limited extent. Therefore, for 3 at.% Co containing Co:ZnO films, the majority of Co ions are probably incorporated into substitutional sites, while for Co:ZnO films with 4 and 5 at.% Co content, some of the Co ions may be located into the interstitial sites. A similar observation was also reported for Mg2+ doped ZnO [31]. Co atoms sitting in the Zn2+ sites may introduce strains in the host lattice because of the ionic size differences between Co2+ and Zn2+ ions for the same coordination number. The XRD peak position shift and lattice contraction indicate that the effect of the Co addition on the ZnO film is a uniform state of compressive stress with tensile components parallel to the c-axis [32]. The stress (σ) in the plane of the film was estimated using the following formula from the c-axis lattice parameter value of the film [33] (the results are listed in Table 1):
.
(2)
Here, CFilm is the c-axis lattice constant of the films, and CBulk is the standard c-axis lattice constant of crystalline ZnO (0.052 nm) in the JCPDS card. A positive σ indicates the presence of a compressive stress [33]. σ of the 5 at.% Co:ZnO film was almost twice that of the undoped film. Several features can result in an enhancement of σ, including defect formation, substitution of ions with different ionic radii, lattice parameter mismatch, and thermal coefficient differences between the substrate and film. In our case, compressive stress can appear as a result of substitution of Co2+ ions into the ZnO crystal [34]. Similar observations have also been reported when doping ZnO with different materials [35, 36]. The reasons for the compressive stress will be further clarified in Section 3.3.
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The oxidation state of Co was investigated using XPS measurements. Fig. 2 presents the XPS spectra of the undoped ZnO and Co:ZnO film with the highest Co content (5 at.% Co), which was selected to examine the metallic Co formation. The survey spectrum of the undoped ZnO in Fig. 2(a) shows the major signals originating from Zn, O, and C elements and the minor signals originating from the glass substrate (Si, Na, and Ca). The survey spectrum of the 5 at.% Co:ZnO film show the same major and minor signals and one additional signal originating from Co, which confirms the presence of Co in the film structure. High-resolution spectral regions for Zn(2p), O(1s), and Co(2p) are presented in Figs. 2(b–d), respectively. Table 2 summarizes the corresponding binding energies (BEs) in eV for the Zn(2p), O(1s), and Co(2p) signals. In Fig. 2(b), because of the strong spin orbit coupling, the Zn(2p) signals appear in two separate peaks as Zn(2p3/2) and Zn(2p1/2) centered at 1021.2 and 1044.3 eV, respectively, for the undoped ZnO film, which are in agreement with the previously reported BE values for ZnO [37]. The position of these signals slightly shifted toward high BE values for the Co:ZnO films. In Fig. 2(c), the O(1s) signal located at 530.9 eV for the undoped ZnO film is associated with the lattice oxygen ions neighboring with Zn2+ ions in the ZnO lattice. This BE value is in agreement with that of stoichiometric ZnO [38]. For the Co:ZnO films, there is also a slight shift in the peak location of the O(1s) signal to higher BE values with Co addition. These positional shifts for the Zn(2p) and O(1s) signals can be explained by the change in ionic character and thus the electronegativity differences for the bonds between O and cations. For the undoped ZnO film, the cation is Zn2+, whereas for the Co:ZnO film, the cation can be Zn2+ or Co2+. According to the Pauling principle, the Co–O bond has a smaller electronegativity difference than the Zn–O bond. Thus, the presence of Co–O bonds in the Co:ZnO film causes a higher valence electron density, 12
resulting in a higher possibility of electron transfer from the conduction band of ZnO to the 3d band of Co. A similar observation was also reported for Cu-doped ZnO nanostructures [16]. Fig. 2(d) presents the high-resolution XPS spectra of Co(2p). According to a previous report, the peak position of Co changes with the ionic character of the bonds; the Co(2p3/2) peak is attributed to the Co–Co bonding at 778.1 eV and Co–O bonding at 780 eV [39]. Therefore, in our experiments, the Co(2p3/2) peak at 780.2 eV in the 5 at.% Co:ZnO sample corresponds to the Co–O bond. This result indicates the absence of precipitates with Co–Co bonds because the highest amount of Co addition remains below the solubility limit of Co in ZnO in the studied temperature range. Furthermore, the Co(2p1/2) peak is observed at 796.1 eV. Thus, the 15-eV BE differences between the Co(2p1/2) and Co(2p3/2) peaks confirm that Co is present as Co2+ [39]. In addition, Co2+ has a weak ion scattering ability; therefore, the regional spectra of Co exhibits high noise [40]. 3.2 Film morphology of the undoped ZnO and Co:ZnO films SEM micrographs of the ZnO films coated on glass substrates are presented in Fig. 3. The general view of the film structure shown as an inset in the SEM image of the undoped ZnO film indicates homogeneous and continuous film formation over the large area of the glass substrate. As observed in the SEM images, rod-like ZnO structures with typical hexagonal morphology that are approximately 200 nm in diameter were grown perpendicular to the substrate surface. Thus, the SEM micrographs agree well with the XRD results. For photocatalytic applications, the surface morphology and roughness are important parameters because they affect both the quality and light-scattering phenomena of the surface [41]. The surface morphology and roughness of the undoped and 1, 3, and 5 at.% Co:ZnO films were 13
characterized using AFM in contact mode with a 10 μm × 10 μm scan area. Fig. 4 shows the 2D and 3D surface morphology of selected film compositions. All the films have uniformly distributed grains over the substrate surface. According to the 2D and 3D images, the undoped ZnO films are comprised of an elongated grain structure (Fig. 4(a) and 4(a1)), whereas the 1 and 3 at.% Co:ZnO films have relatively rough grain structures (Figs. 4(b), 4(b1) and 4(c), 4(c1), respectively). In addition, the 5 at.% Co:ZnO film exhibits an increased tendency to agglomerate into larger grains and thus has a larger grain size (Figs. 4(d) and 4(d1)). The root-mean-square roughness of the undoped ZnO film was 12.5 nm, whereas, for the 1, 3, and 5 at.% Co:ZnO films, it was estimated to be 4.4, 27.1, and 30.9 nm, respectively. The AFM analyses indicate that the roughness of the film surface decreased with the 1 at.% Co addition compared with that of the undoped ZnO film and then increased with further Co addition. This phenomenon may have originated from damage from Co ions on the surface of the initially formed nuclei/cluster resulting in new nucleation sites during the deposition of the film structure [42]. Therefore, for the 1 at.% Co:ZnO film, nucleation is the most dominant step in the crystallization process involving nucleation, growth, and coalescence. However, for the 3 and 5 at. % Co:ZnO films, because of the increase in the surface energy of the nuclei/cluster, growth and coalescence are the more dominant steps in the crystallization process. Therefore, a greater Co addition results in the formation of larger grains with higher roughness. Similar observations have also been reported for La-doped ZnO thin films [43]. 3.3 Optical properties of the undoped ZnO and Co:ZnO films The effect of the Co addition on the optical properties of the ZnO films was investigated using UV-Vis spectroscopy analyses (Fig. 5). The transmittance spectra of the undoped ZnO and Co:ZnO films coated on glass substrates are presented in Fig. 5(a). All the films exhibited high 14
transparency of nearly 75% in the visible range; the transparency decreased upon increasing the Co addition because increasing the doping element content results in a light loss on the films. The inset in Fig. 5(a) shows the appearance of the ZnO films as a function of the Co content. The undoped ZnO film was colorless, whereas the Co:ZnO films were greenish, with their color gradually changing to dark green upon increasing the Co content. Therefore, a change in the transmission spectrum of the 5 at.% Co:ZnO film occurred. The transmittance of all the films exhibited a sharp decrease at approximately 380 nm, which is related to the absorption band edge of ZnO. For all the films, the high transmittance in the visible range of the spectrum and sharp reduction in the absorption band edges in the UV region of the spectrum indicate the good optical and crystal quality of the films in agreement with the XRD analyses. The observed oscillations in the transmission spectra can be attributed to the light interference originating from the different refractive indices and therefore multiple reflections between the ZnO film and glass substrate. Fig. 5(b) presents an enlarged regional plot of the transmittance of the ZnO films in the 480–700 nm range. The transitions peaks observed at 565, 615, and 660 nm are attributed to tetrahedrally coordinated Co2+ ions entering the wurtzite structure and are in good agreement with the XRD analyses [26]. These transitions in the visible range are attributed to the charge transfer between donor and acceptor levels placed within the band gap of ZnO [27]. UV-Vis absorption spectra and band gap determination curves of the undoped ZnO and Co:ZnO films coated on the glass substrate are presented in Fig. 6. The absorption spectra of all the films (Fig. 6(a)) indicate that the UV visible absorption edge of the undoped ZnO film appears at 354 nm and is related to the wurtzite type crystal structure of ZnO [44]. The band edge for the Co:ZnO samples appears at 356, 358, 359, 361, and 367 nm for the 1, 2, 3, 4, and 5 at.% Co 15
additions, respectively. The absorption band edge for the undoped ZnO film undergoes a red shift to higher wavelength compared with that of the Co:ZnO films, as observed in Fig. 6(a). This absorption band edge shift indicates a change in the electronic band structure of the ZnO resulting from substitutional incorporation of the dopant Co atoms [45]. Based on the red shift in the absorption onset with increasing Co addition, a shrinkage in the band gap energy is expected. Fig. 6(b) shows the band gap energy values of ZnO as a function of the Co content, which were determined by plotting the absorption coefficient (α) vs. photon energy (hυ) and extrapolating the straight-line portion of this plot to the hυ axis. As clearly observed in Fig. 6(b), a red shift occurs in the direct band gap energy with doping from 3.34 eV (0 at.% Co) to 3.30, 3.23, 3.20, 3.10 and 3.06 eV corresponding to the 1, 2, 3, 4 and 5 at.% Co:ZnO films, respectively. The red shift of the band gap energy of ZnO with doping has previously been observed and explained by several factors including the size, morphology, annealing temperature, film thickness, defect state, and dopant content [46-48]. In our case, the effects of quantum confinement and morphology can be disregarded because of the large sizes and similar morphologies of the samples. Thus, this contraction may be explained by the combined effects of (i) the presence of defect states such as Zn interstitials and O vacancies and (ii) the dopant content [49]. At this point, the function of defect states is considered to be the most important contributor to the band gap contraction because of the distinct exchange interaction between the defect state and valence band electrons. According to the literature, s–d and p–d exchange interactions between the localized d electrons of the Co2+ ions and band electrons of ZnO results in negative and positive corrections of the conduction and valence band edges, respectively, thus leading to band gap narrowing [50, 51].
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Room-temperature PL spectra of all the ZnO films obtained using a 526-nm excitation wavelength are presented in Fig. 7. The PL spectrum of the undoped ZnO film is a combination of two peaks: (i) a sharp green emission peak (at 528 nm) originating from O vacancies [52] and (ii) a broad red emission peak with lower intensity (at ~680 nm) originating from Zn interstitials [53]. However, for the Co:ZnO films, the intensities of both the green and red emissions decreased with increasing Co content. In addition, the red emission nearly disappeared for the Co:ZnO films with higher Co content, which indicates that the amount of defect states decreased with increasing Co content. Therefore, the residual compressive stress with Co addition determined by the XRD analyses does not depend on the defect formation; it should instead originate from the increased amount of substitutionally incorporated Co ions in ZnO resulting from the ionic radii differences between Co2+ and Zn2+ ions. 3.4 Photocatalytic properties of the undoped ZnO and Co:ZnO films The photocatalytic properties of the undoped ZnO and Co:ZnO films were examined under visible light based on the decomposition of MB. Fig. 8 shows the photodegradation reactions of all the ZnO films. Fig. 8(a) presents the real-time UV-Vis absorption spectra of the photodegradation of MB solutions containing the undoped and 1, 3, and 5 at.% Co:ZnO films. To calculate the degradation of MB, the maximum absorption band observed at 664 nm was used. For comparison, a dye solution without catalyst was also exposed to the visible light under the same experimental conditions and showed insensible change with respect to time. According to Fig. 8(a), the degradation of MB for all the undoped ZnO and Co:ZnO samples increased with irradiation time. Fig. 8(b) shows the appearance of the dye solution under visible light with respect to time for the 3 at.% Co:ZnO film. The color of the solution changed from dark blue to light blue as time progressed. At the end of the studied irradiation time range (120 min), the 17
color was bluish-white, indicating the presence of a small amount of non-degraded MB in the solution. Fig. 8(c) shows that the change of the visible light degradation of MB as a function of Co content undergoes an exponential decrease of the dye absorbance under the visible light irradiation. After 20-min irradiation, the degradation percentage of MB with undoped ZnO was approximately 20%; this value was much better for all the Co:ZnO films. Therefore, it can be concluded that the addition of Co has a positive effect on ZnO for the degradation of MB. This result can be attributed to the enhanced absorption in the visible light range resulting from the substitutionally incorporated Co ions. The best MB photodegradation was approximately 88% for the 3 at.% Co:ZnO film after 40 min of visible light irradiation. Qiu Xiaoqing et al. reported that for Co:ZnO samples, visible light irradiation is a more time-consuming process than UVlight irradiation for the degradation of RhB dye solution [20]. Therefore, the degradation time for our visible light degradation of Co:ZnO samples is better than that of previously reported Co:ZnO samples. To better distinguish the photocatalytic efficiency of the undoped ZnO and Co:ZnO photocatalysis, the kinetic parameters were determined from the MB degradation results and are presented in Fig. 9. Fig. 9(a) presents fitted data of the pseudo first-order kinetics [54]. The Co: ZnO films clearly exhibited much higher photocatalytic activity than the undoped ZnO film. Furthermore, the degradation rate of the Co:ZnO films slightly increased up to the 3 at.% Co addition and then decreased. The rate constant values determined from the slope of the fitted curves are presented as a histogram in Fig. 9(b). According to the histogram, the rate constant k values of the Co:ZnO films are almost three times higher than that of the undoped ZnO film. Up to 3 at.% Co doping, the photocatalytic abilities first increased and then slightly decreased with further doping. These results indicate both that the Co addition considerably affected the 18
photocatalytic ability of ZnO and that the 3 at.% Co addition is the optimal dopant content for Co:ZnO films. To explore the effects of Co ions on the visible light photocatalysis, we also studied the AQY of the photocatalytic process, as shown in Fig. 9(c). In the absence of any photocatalyst, the MB solution exhibited a very low AQY (4.88%) in the visible region (664 nm) for 60 min. In the visible region, when Co+2 ions were incorporated into the ZnO wurtzite crystal structure, the films exhibited high AQY values, which are attributed to the red shift in the absorption onset, suppressing the e-–h° pair recombination and enhanced photogenerated charge separation across the ZnO inside [25, 55, 56]. Among the Co:ZnO thin films, the 3% dopant content resulted in the highest AQY (85.96%), and the 1% dopant content resulted in the lowest AQY (82.93%). This result is due to the increase in crystallinity, large active surface area, and increased catalytic active sites. Thus, the AQY values of the Co:ZnO thin films in the visible region were always higher than that of the bare ZnO (41.12%) thin film, indicating improved photocatalytic activity of the ZnO thin film in the presence of Co ions. The solution pH is an important parameter in photocatalytic degradation reactions for industrial applications. To find out the effect of the pH on the photocatalytic degradation efficiency, degradation of 3 at. % Co-doped ZnO photocatalyst were investigated by varying the initial MB solution pH values of 3.0 (acidic medium), 7.0 (neutral medium) and 11.0 (basic or alkaline medium), respectively, by adding either hydrochloric acid or sodium hydroxide (0.1 M) and the results are represented in Fig. 10 (a). The photodegradation efficiency of 3 at. % Co-doped ZnO photocatalyst is slightly higher in alkaline (98.65 %) and neutral (97.40 %) solutions than that in acidic (78.02 %) solutions after 120 minutes. It is interesting to note that photodegradation at alkaline pH of 11 is close to the neutral pH of 7.0 suggesting that the effect of pH at these 19
medium did not significantly influence the photocatalytic performance of Co-doped ZnO thin films under visible-light irradiation. The nearly same MB degradation at these pH values may be due to the higher interaction between cationic MB and negatively charge catalyst surface [57]. Furthermore, this result is similar to that observed by other studies [58, 59]. Therefore, we performed all studies at neutral pH medium. We further explored the recycling ability, which is essential to find the photocatalytic stability and the reusability of the catalyst, under visible-light irradiation, and the results of recycling process are shown in Fig. 10 (b). After recycling five times over 120 min reaction, there were no remarkable changes in the degradation even if the degradation rate decreased from 97% to 93% during first and last cycle, respectively.
This decrement can be attributed to the some
photocatalyst washout during the recovery steps. This clearly indicates that the synthesized Codoped ZnO thin films are reusable and stable enough for industrial and environmental applications. According to the literature, several possible factors affect the photocatalytic activity of TMdoped oxide materials: (i) particle sizes and morphologies, (ii) surface properties, and (iii) dopant content [10, 60, 61]. In our case, the dependence of the photocatalytic activity could not explained by factors (i) and (ii) because of the similar particle and surface properties of the samples. Thus, the dopant content of the electronic structure of the Co:ZnO films is reasonably the dominant factor affecting the photocatalytic activity. With Co addition, to change the band structure, there should be a strong electronic interaction between the ZnO and Co species or d–d transition of Co ions [40].
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In general, the photocatalytic activity of a semiconductor oxide is closely related to two combined steps: the formation of photogenerated e-–h° pairs resulting from excitation of electrons from the valence band in the oxide material and the retention of e-–h° pair recombination. Up to 3 at.% Co doping, the band gap narrowing of Co:ZnO resulting from substitutional incorporation of Cu ions into the ZnO lattice is responsible for the enhancement of the photocatalytic activity. With reduction in the band gap energy up to 3 at.% Co content, the higher number of carriers results in a better photocatalyst. However, a slight decrease in the photocatalytic efficiency was observed upon further increasing the Co content (4 and 5 at.% Co:ZnO). This result may originate from the substitutional doping combined with interstitial doping of Co ions. Similar finding has been also reported for Mn-doped ZnO [31]. In a sample containing a high amount of Co, the majority of Co ions are substituted into Zn sites, whereas excess Co ions may be located in interstitial sites, which would act as trapping or recombination centers for photo-excited electrons and holes. Therefore, a slight decrease was observed in the photocatalytic ability of the 4 and 5 at.% Co:ZnO films.
4. CONCLUSIONS This work describes highly photocatalytically active Co:ZnO films prepared by the sol–gel method on glass substrates. The effects of the Co addition on the structural, morphological, optical, and photocatalytic properties of undoped ZnO and Co:ZnO films were investigated and discussed in detail. The results indicate that the Co addition leads to narrowing of the c-axis lattice parameter, as demonstrated by the shift in the XRD peak positions of wurtzite ZnO to lower diffraction angle values. In addition, narrowing of the band gap of ZnO was demonstrated
21
by the change in the absorption edge to longer wavelengths, as determined by the absorption band edge values. These results indicated that Co2+ ions were substitutionally incorporated into Zn2+ sites within the ZnO lattice. The photocatalytic activities of the films were investigated by examining MB dye degradation under visible light irradiation. The 3 at.% Co content was determined to be optimal for the photocatalytic activity of the Co:ZnO films. The Co:ZnO films presented here are highly photocatalytically active under visible light irradiation and promise a breakthrough in the large-scale utilization of photocatalysts using visible light to overcome organic and/or water pollution. Furthermore, the recycling results imply that the photocatalyst is stable enough and could be easily reused which is favorable for potential environmental and industrial applications.
Acknowledgments The authors would like to thank the European Cooperation in Science and Technology through COST Action MP1302 Nanospectroscopy, Scientific and Technological Research Council of Turkey (TUBITAK Grant number 112T981), Scientific Research Commission of Karamanoğlu Mehmetbey University under Project No. 30–M–12 and the Scientific Research Foundation of Selcuk University under Project No. 15401123 for the financial support of this research. Finally, the author (H. Arslan) would like to thank TUBITAK under Project No. 114Z956 for their financial supports during her studies.
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28
FIGURE CAPTIONS
Fig. 1. (a) Overview and (b) enlarged region of the experimental XRD patterns of undoped ZnO and Co:ZnO thin films with different Co contents.
Fig. 2. XPS spectra for undoped ZnO and Co:ZnO thin films with different Co contents: (a) survey analyses and high-resolution regional spectra of (b) Zn(2p), (c) O(1s), and (b) Co(2p) signals.
Fig. 3. SEM micrographs of the undoped ZnO and Co:ZnO thin films with different Co contents. The inset in the SEM micrograph of the undoped ZnO shows a magnified SEM image as a visual aid demonstrating the coating homogeneity.
Fig. 4. (a)–(d) 2D and (a1)–(d1) 3D AFM images of undoped ZnO and Co:ZnO thin films with different Co contents.
Fig. 5. (a) Overview and (b) enlarged region of the transmittance spectra of the undoped ZnO and Co:ZnO thin films with different Co contents. The inset in (a) shows the appearance of all the undoped ZnO and Co:ZnO thin films.
Fig. 6. UV–Vis spectra and (b) plots of (αhυ)2 as a function of photon energy with values of the band gap for undoped ZnO and Co:ZnO thin films with different Co contents. The band edge locations are indicated in (a).
Fig. 7. PL spectra for undoped ZnO and Co:ZnO thin films with different Co contents. 29
Fig. 8. (a) Real-time UV-Vis absorption spectra of the photodegradation of MB solutions containing the undoped and 1, 3, and 5 at.% Co:ZnO films. (b) Appearance of dye solution under visible light as a function of time for the 3 at.% Co:ZnO film. (c) Temporal profile of MB degradation of Co:Zn thin films with different Co contents under visible light illumination.
Fig. 9. (a) Fitted data of the pseudo first-order kinetics. (b) Histogram of the rate constant values determined from the slope of the fitted curves in (a). (c) AQY values of the photocatalytic process.
Fig. 10. (a) Effect of pH on the photocatalytic degradation of MB with 3 at.% Co-doped ZnO thin film and (b) the recycling ability of 3 at.% Co-doped ZnO photocatalyst for MB degradation under visible-light irradiation.
30
TABLE HEADINGS
Table 1. (002) peak position, lattice parameters (c), and stress (σ) value for the undoped ZnO and Co:ZnO thin films with different Co contents.
Table 2. Binding energy values (in eV) of Zn(2p3/2), Zn(2p1/2), O(1s), Co(2p1/2), and Co(2p3/2), for the undoped ZnO and Co:ZnO thin films with different Co contents.
31
Table 1. Peak Co:Zn Lattice Position (degree ratio Parameter of 2) (at.%) (c, Å)
α (GPa)
0
34.55
5.134
6.32
1
34.58
5.128
6.84
2
34.59
5.110
8.41
3
34.61
5.084
10.68
4
34.63
5.071
11.81
5
36.64
5.075
11.46
32
Table 2. Binding energy (eV) Co:Zn ratio (at.%)
Zn (2p3/2)
Zn (2p1/2)
O(1s)
Co(2p3/2)
Co(2p1/2)
0
1021.2
1044.3
530.9
-
-
5
1021.4
1044.5
531.1
780.2
796.1
33
S0169-4332(16)31712-3 http://dx.doi.org/doi:10.1016/j.apsusc.2016.08.069 APSUSC 33818
To appear in:
APSUSC
Received date: Revised date: Accepted date:
28-6-2016 11-8-2016 12-8-2016
Please cite this article as: Ozlem ALTINTAS YILDIRIM, Hanife ARSLAN, ¨ ˘ Savas¸ SONMEZO GLU, Facile Synthesis of Cobalt-Doped Zinc Oxide Thin Films for Highly Efficient Visible Light Photocatalysts, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.08.069 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.
Facile Synthesis of Cobalt-Doped Zinc Oxide Thin Films for Highly Efficient Visible Light Photocatalysts
Ozlem ALTINTAS YILDIRIM1,*, Hanife ARSLAN2,3, and Savaş SÖNMEZOĞLU,3
1
Department of Metallurgical and Materials Engineering, Selcuk University, Konya, Turkey.
2
Department of Metallurgical and Materials Engineering, Karamanoglu Mehmetbey University,
Karaman, Turkey. 3
Nanotechnology R&D Laboratory, Karamanoglu Mehmetbey University, Karaman, Turkey.
*To whom all correspondence should be addressed: e-mail: [email protected] phone: +90 (332) 223-1997 fax: +90 (332) 241-0635
1
Graphical Abstract
2
Highlights
Photocatalytically active Co-ZnO thin film was obtained by sol-gel method.
Co2+ doping narrowed the band gap of pure ZnO to an extent of 3.18 eV.
Co-ZnO was effective in MB degradation under visible light.
Optimum dopant content to show high performance was 3 at. %.
3
Abstract Cobalt-doped zinc oxide (Co:ZnO) thin films with dopant contents ranging from 0 to 5 at.% were prepared using the sol–gel method, and their structural, morphological, optical, and photocatalytic properties were characterized. The effect of the dopant content on the photocatalytic properties of the films was investigated by examining the degradation behavior of methylene blue (MB) under visible light irradiation, and a detailed investigation of their photocatalytic activities was performed by determining the apparent quantum yields (AQYs). Co2+ ions were observed to be substitutionally incorporated into Zn2+ sites in the ZnO crystal, leading to lattice parameter constriction and band gap narrowing due to the photoinduced carriers produced under the visible light irradiation. Thus, the light absorption range of the Co:ZnO films was improved compared with that of the undoped ZnO film, and the Co:ZnO films exhibited highly efficient photocatalytic activity (~92% decomposition of MB after 60-min visible light irradiation for the 3 at.% Co:ZnO film). The AQYs of the Co:ZnO films were greatly enhanced under visible light irradiation compared with that of the undoped ZnO thin film, demonstrating the effect of the Co doping level on the photocatalytic activity of the films.
4
1.
INTRODUCTION
In recent decades, the presence of organic dye compounds used in textile and paint industries as well as household chemicals has caused serious detrimental effects for living ecosystems as a result of water pollution [1]. Traditional biological treatment methods can be unsuccessful in decolorizing and mineralizing the stable dye molecules. Today, semiconductor-assisted photocatalysts have attracted considerable attention in wastewater treatment because of their high photosensitivity, environmentally friendly non-toxic nature, and low cost [2]. These type semiconductor-based photocatalysts have been also used as gas sensors and antibacterial applications [3, 4]. Although titanium dioxide (TiO2) is widely used as a photocatalyst among the various oxide-type semiconductor photocatalysts (i.e., TiO2, iron trioxide, and tungsten trioxide), zinc oxide (ZnO) is one of the most preferred materials because of its outstanding properties such as its similar band gap to TiO2, low cost, chemical stability, and non-toxicity [5-7]. ZnO is an important semiconductor material with a wide band gap (3.3 eV) and large exciton binding energy (60 meV) [8]. However, semiconductor-based photocatalysts containing ZnO can only absorb UV light with a wavelength of less than 385 nm, which accounts for less than 10% of solar irradiation. Thus, expanding the absorption spectral range is critical in improving photocatalytic applications of ZnO. To absorb interior lighting or visible light in the solar spectrum, the band gap energy of ZnO should be reduced or split into several sub-gaps. Techniques such as semiconductor combination [9], noble metal deposition [10, 11], nonmetal doping [12, 13] and
5
transition metal (TM) doping [14, 15] have been used to improve the photocatalytic efficiency of semiconductor catalysts. TM doping provides more photons to be absorbed and decomposes organic pollutants by altering the coordination environment of Zn with TM ion substitution into the ZnO lattice, thus modifying the electronic band structure of ZnO [16]. Therefore, doping ZnO with TM ions appears to be effective in expanding its photoresponse from the UV light region into the visible range. TM ions act as trapping sites by receiving the photogenerated e- and h° carriers, hindering recombination of the carriers and thus enhancing the photocatalytic activity of the ZnO [17]. Among the various types of TM-doped ZnO materials, cobalt-doped ZnO (Co:ZnO) nanostructures with controllable band structure are particularly attractive because of the abundant electronic states of Co and minor effect of Co on the ZnO structure [18]. To date, several researchers have studied the photocatalytic activities of Co:ZnO nanostructures under UV light illumination [19, 20]. However, detailed studies on the visible light photocatalytic efficiency of Co:ZnO systems against stable dye molecules have rarely been reported. In addition, a survey of the literature shows that only powder form of Co-ZnO is explicitly investigated and least attention is made towards the thin films [21-24]. Consequently, the main purpose of this study was to explore the effects of Co doping on the photocatalytic activity of ZnO thin films and determine the relevant mechanism. In this work, we prepared undoped ZnO and Co:ZnO films with Co contents of 1, 2, 3, 4, and 5 at.% via the sol–gel method. Methylene blue (MB) degradation under visible light irradiation was used to determine the effect of the Co addition on the photocatalytic activities of the Co:ZnO films. To determine the photocatalytic mechanism, detailed structural, morphological, and optical analyses of the Co:ZnO films were performed, and the kinetic parameters of the 6
photocatalytic reaction were investigated. The Co:ZnO films exhibited improved visible light photoresponses for the degradation of MB solution compared with that of the undoped ZnO film, and 3 at.% Co was determined to be the optimum content to achieve the best photocatalytic response among the studied conditions.
2. EXPERIMENTAL DETAILS 2.1.
Synthesis of undoped ZnO and Co:ZnO nanoparticles
Pure ZnO and Co:ZnO nanoparticles were successfully synthesized using the sol–gel method. In a typical process, 0.87 g zinc acetate dihydrate was first dissolved in a mixture of 10 mL 2methoxyethanol and 1 mL ethanolamine at 70 °C and then subjected to magnetic stirring for 2 h. Cobalt acetate, used as source of the Co dopant, was added into the precursor solution at various concentrations (1, 2, 3, 4, and 5 at.%) under vigorous stirring. The solutions were stirred for 1 h using a magnetic stirrer at 70 °C to obtain a clear homogeneous and transparent sol, which served as the coating solution after being stored for 24 h at room temperature. 2.2.
Preparation of undoped ZnO and Co:ZnO thin films
After the above synthesis procedure, a spin-coating process was used to deposit the ZnO solution on 2x2 cm glass substrates. The spinning process was performed using 30 μL of solution spun in air at 2000 rpm for 30 s. This process was repeated 25 times to achieve a uniform surface. After each spinning process, the samples were subjected to repeated pre-annealing processes at 400 °C for 5 min. Finally, the post-annealing process was performed in an oven at 550 °C for 1 h.
7
2.3.
Characterization of undoped ZnO and Co:ZnO films
The crystal structures of the pure ZnO and Co:ZnO films were identified using X–ray diffraction (XRD, Bruker D8 Advanced) in the 2θ range of 20°–70°. The surface morphology and roughness of the samples were analyzed using scanning electron microscopy (SEM, Zeiss LS–10) and atomic force microscopy (AFM, Nanomagnetics Instruments, Ambient AFM). The composition of the films was determined using X-ray photoelectron spectroscopy (XPS, SPECS EA 300) and energy-dispersive
X-ray
spectroscopy
(EDS).
UV–Vis
absorption
and
transmission
measurements were also obtained using a SPECORD S600 spectrophotometer. The photoluminescence (PL) spectra of the samples were recorded at room temperature using a PTI Quanta Master 30 PC spectrofluorophotometer with a Xe lamp as the excitation source. 2.4.
Photocatalytic characterization
The photocatalytic activities of the thin films were evaluated based on the visible light photocatalytic degradation of MB dye in a cylindrical custom-built photoreactor. Three 23-W xenon lamps with a cut-off filter of 420 nm were used as the visible light source, and the reaction was maintained at room temperature by circulating with cool water. These thin films were placed in a 12 ppm MB suspension in aqueous solution. The distance between the top of the reactor and lamp was adjusted to be 13 cm. During the photoreaction process, 3 mL of the solution was removed in a constant time, the photocatalyst was separated from the solution by centrifugation, and the concentration of the remaining clear liquid was determined using the UV-Vis spectrophotometer. The photocatalytic degradation of MB was estimated from the reduction in the absorption intensity of MB at a fixed wavelength λmax = 664 nm. In addition to the photocatalytic degradation, the apparent quantum yield (AQY) of the photocatalytic process was
8
estimated using the equation given in Ref. [25]. This estimated AQY is lower than the real quantum yield because the number of absorbed photons is usually smaller than the number of incident photons unless a dispersed system where light scattering comes into play is not used. Furthermore, the degradation percentage of MB was calculated using the relation % degradation = [(A0-At)/A0] × 100,
(1)
where A0 is the initial absorbance of dye and At is the absorbance of dye at time t.
3. RESULTS AND DISCUSSION 3.1 Crystal structure and orientation of the undoped ZnO and Co:ZnO films The crystal structure and orientation of the undoped ZnO and Co:ZnO films prepared via the sol– gel method on the glass substrate at 550 °C were characterized using XRD, and the results are presented in Fig. 1. Fig. 1(a) shows that all the diffraction peaks of both the undoped and Co:ZnO films match those of the hexagonal wurtzite ZnO crystal structure (JCPDS card no: 361451). No evidence of impurity phases related to any type of cobalt or cobalt oxide was observed within the detection limit of the XRD, which suggests that the addition of up to 5 at.% Co to ZnO does not change the crystal structure of ZnO within the studied deposition temperature range. The intensity order of the XRD peaks of all the samples deviates from the standard JCPDS card order. The presence of relatively higher intensity [0002] diffraction lines indicate that the ZnO and Co:ZnO films grew with the c-axis preferred orientation perpendicular to the substrate plane.
9
Similar findings have also been reported by several groups in the literature [26, 27]. This result can be attributed to the different chemical activity and surface energy of the Zn- and Oterminated polar surfaces of the ZnO crystal. ZnO usually prefers to grow along the [0002] direction to reduce its highest surface energy compared with the other planes. After formation of the first ZnO clusters on the glass substrate, the Zn-terminated chemically active positive polar surface supplies an appropriate environment for local enrichment of Zn on the ZnO cluster [28]. According to this theory, non-homogenously distributed regions of the (0002) surface of the cluster act as a catalyst for film growth. The incorporation of Co ions into the ZnO lattice may produce lattice distortion of the host lattice. To investigate the effect of the Co doping on the crystal structure, the peak positions of the ZnO diffractions were examined in detail. Fig. 1(b) shows the dependence of the XRD peak positions on the Co content. The 2θ values as a function of the Co content are listed in Table 1. Co incorporation results in a peak shift to higher 2θ values, which indicates substitutional incorporation of Co atoms into the ZnO crystal. As observed in Table 1, the c-axis lattice constant calculated from the (0002) plane decreased from 5.134 to 5.075 Å as the Co content increased from 0 to 5 at.%. The smaller value of the c-axis lattice parameter in the Co:ZnO films than in the undoped film indicates a slight narrowing of the lattice due to substitution of the Co ions with slightly smaller ionic size (Co2+ = 0.58 Å and Zn2+ = 0.60 Å). According to these results, the Co addition obeys Vegard’s law and indicates the substitutional incorporation of Co ions into the ZnO crystal [29, 30]. It was also observed that [0002] diffraction intensity of 3 at.% Co containing Co:ZnO film decreased compared with other films. This result shows that for 3 at.% Co containing Co:ZnO film, substitution of Co ions and reducing of the surface energy hinder the crystal growth through [0002] direction more effectively. It was worth noting that 10
ionic substitution takes place up to a limited extent. Therefore, for 3 at.% Co containing Co:ZnO films, the majority of Co ions are probably incorporated into substitutional sites, while for Co:ZnO films with 4 and 5 at.% Co content, some of the Co ions may be located into the interstitial sites. A similar observation was also reported for Mg2+ doped ZnO [31]. Co atoms sitting in the Zn2+ sites may introduce strains in the host lattice because of the ionic size differences between Co2+ and Zn2+ ions for the same coordination number. The XRD peak position shift and lattice contraction indicate that the effect of the Co addition on the ZnO film is a uniform state of compressive stress with tensile components parallel to the c-axis [32]. The stress (σ) in the plane of the film was estimated using the following formula from the c-axis lattice parameter value of the film [33] (the results are listed in Table 1):
.
(2)
Here, CFilm is the c-axis lattice constant of the films, and CBulk is the standard c-axis lattice constant of crystalline ZnO (0.052 nm) in the JCPDS card. A positive σ indicates the presence of a compressive stress [33]. σ of the 5 at.% Co:ZnO film was almost twice that of the undoped film. Several features can result in an enhancement of σ, including defect formation, substitution of ions with different ionic radii, lattice parameter mismatch, and thermal coefficient differences between the substrate and film. In our case, compressive stress can appear as a result of substitution of Co2+ ions into the ZnO crystal [34]. Similar observations have also been reported when doping ZnO with different materials [35, 36]. The reasons for the compressive stress will be further clarified in Section 3.3.
11
The oxidation state of Co was investigated using XPS measurements. Fig. 2 presents the XPS spectra of the undoped ZnO and Co:ZnO film with the highest Co content (5 at.% Co), which was selected to examine the metallic Co formation. The survey spectrum of the undoped ZnO in Fig. 2(a) shows the major signals originating from Zn, O, and C elements and the minor signals originating from the glass substrate (Si, Na, and Ca). The survey spectrum of the 5 at.% Co:ZnO film show the same major and minor signals and one additional signal originating from Co, which confirms the presence of Co in the film structure. High-resolution spectral regions for Zn(2p), O(1s), and Co(2p) are presented in Figs. 2(b–d), respectively. Table 2 summarizes the corresponding binding energies (BEs) in eV for the Zn(2p), O(1s), and Co(2p) signals. In Fig. 2(b), because of the strong spin orbit coupling, the Zn(2p) signals appear in two separate peaks as Zn(2p3/2) and Zn(2p1/2) centered at 1021.2 and 1044.3 eV, respectively, for the undoped ZnO film, which are in agreement with the previously reported BE values for ZnO [37]. The position of these signals slightly shifted toward high BE values for the Co:ZnO films. In Fig. 2(c), the O(1s) signal located at 530.9 eV for the undoped ZnO film is associated with the lattice oxygen ions neighboring with Zn2+ ions in the ZnO lattice. This BE value is in agreement with that of stoichiometric ZnO [38]. For the Co:ZnO films, there is also a slight shift in the peak location of the O(1s) signal to higher BE values with Co addition. These positional shifts for the Zn(2p) and O(1s) signals can be explained by the change in ionic character and thus the electronegativity differences for the bonds between O and cations. For the undoped ZnO film, the cation is Zn2+, whereas for the Co:ZnO film, the cation can be Zn2+ or Co2+. According to the Pauling principle, the Co–O bond has a smaller electronegativity difference than the Zn–O bond. Thus, the presence of Co–O bonds in the Co:ZnO film causes a higher valence electron density, 12
resulting in a higher possibility of electron transfer from the conduction band of ZnO to the 3d band of Co. A similar observation was also reported for Cu-doped ZnO nanostructures [16]. Fig. 2(d) presents the high-resolution XPS spectra of Co(2p). According to a previous report, the peak position of Co changes with the ionic character of the bonds; the Co(2p3/2) peak is attributed to the Co–Co bonding at 778.1 eV and Co–O bonding at 780 eV [39]. Therefore, in our experiments, the Co(2p3/2) peak at 780.2 eV in the 5 at.% Co:ZnO sample corresponds to the Co–O bond. This result indicates the absence of precipitates with Co–Co bonds because the highest amount of Co addition remains below the solubility limit of Co in ZnO in the studied temperature range. Furthermore, the Co(2p1/2) peak is observed at 796.1 eV. Thus, the 15-eV BE differences between the Co(2p1/2) and Co(2p3/2) peaks confirm that Co is present as Co2+ [39]. In addition, Co2+ has a weak ion scattering ability; therefore, the regional spectra of Co exhibits high noise [40]. 3.2 Film morphology of the undoped ZnO and Co:ZnO films SEM micrographs of the ZnO films coated on glass substrates are presented in Fig. 3. The general view of the film structure shown as an inset in the SEM image of the undoped ZnO film indicates homogeneous and continuous film formation over the large area of the glass substrate. As observed in the SEM images, rod-like ZnO structures with typical hexagonal morphology that are approximately 200 nm in diameter were grown perpendicular to the substrate surface. Thus, the SEM micrographs agree well with the XRD results. For photocatalytic applications, the surface morphology and roughness are important parameters because they affect both the quality and light-scattering phenomena of the surface [41]. The surface morphology and roughness of the undoped and 1, 3, and 5 at.% Co:ZnO films were 13
characterized using AFM in contact mode with a 10 μm × 10 μm scan area. Fig. 4 shows the 2D and 3D surface morphology of selected film compositions. All the films have uniformly distributed grains over the substrate surface. According to the 2D and 3D images, the undoped ZnO films are comprised of an elongated grain structure (Fig. 4(a) and 4(a1)), whereas the 1 and 3 at.% Co:ZnO films have relatively rough grain structures (Figs. 4(b), 4(b1) and 4(c), 4(c1), respectively). In addition, the 5 at.% Co:ZnO film exhibits an increased tendency to agglomerate into larger grains and thus has a larger grain size (Figs. 4(d) and 4(d1)). The root-mean-square roughness of the undoped ZnO film was 12.5 nm, whereas, for the 1, 3, and 5 at.% Co:ZnO films, it was estimated to be 4.4, 27.1, and 30.9 nm, respectively. The AFM analyses indicate that the roughness of the film surface decreased with the 1 at.% Co addition compared with that of the undoped ZnO film and then increased with further Co addition. This phenomenon may have originated from damage from Co ions on the surface of the initially formed nuclei/cluster resulting in new nucleation sites during the deposition of the film structure [42]. Therefore, for the 1 at.% Co:ZnO film, nucleation is the most dominant step in the crystallization process involving nucleation, growth, and coalescence. However, for the 3 and 5 at. % Co:ZnO films, because of the increase in the surface energy of the nuclei/cluster, growth and coalescence are the more dominant steps in the crystallization process. Therefore, a greater Co addition results in the formation of larger grains with higher roughness. Similar observations have also been reported for La-doped ZnO thin films [43]. 3.3 Optical properties of the undoped ZnO and Co:ZnO films The effect of the Co addition on the optical properties of the ZnO films was investigated using UV-Vis spectroscopy analyses (Fig. 5). The transmittance spectra of the undoped ZnO and Co:ZnO films coated on glass substrates are presented in Fig. 5(a). All the films exhibited high 14
transparency of nearly 75% in the visible range; the transparency decreased upon increasing the Co addition because increasing the doping element content results in a light loss on the films. The inset in Fig. 5(a) shows the appearance of the ZnO films as a function of the Co content. The undoped ZnO film was colorless, whereas the Co:ZnO films were greenish, with their color gradually changing to dark green upon increasing the Co content. Therefore, a change in the transmission spectrum of the 5 at.% Co:ZnO film occurred. The transmittance of all the films exhibited a sharp decrease at approximately 380 nm, which is related to the absorption band edge of ZnO. For all the films, the high transmittance in the visible range of the spectrum and sharp reduction in the absorption band edges in the UV region of the spectrum indicate the good optical and crystal quality of the films in agreement with the XRD analyses. The observed oscillations in the transmission spectra can be attributed to the light interference originating from the different refractive indices and therefore multiple reflections between the ZnO film and glass substrate. Fig. 5(b) presents an enlarged regional plot of the transmittance of the ZnO films in the 480–700 nm range. The transitions peaks observed at 565, 615, and 660 nm are attributed to tetrahedrally coordinated Co2+ ions entering the wurtzite structure and are in good agreement with the XRD analyses [26]. These transitions in the visible range are attributed to the charge transfer between donor and acceptor levels placed within the band gap of ZnO [27]. UV-Vis absorption spectra and band gap determination curves of the undoped ZnO and Co:ZnO films coated on the glass substrate are presented in Fig. 6. The absorption spectra of all the films (Fig. 6(a)) indicate that the UV visible absorption edge of the undoped ZnO film appears at 354 nm and is related to the wurtzite type crystal structure of ZnO [44]. The band edge for the Co:ZnO samples appears at 356, 358, 359, 361, and 367 nm for the 1, 2, 3, 4, and 5 at.% Co 15
additions, respectively. The absorption band edge for the undoped ZnO film undergoes a red shift to higher wavelength compared with that of the Co:ZnO films, as observed in Fig. 6(a). This absorption band edge shift indicates a change in the electronic band structure of the ZnO resulting from substitutional incorporation of the dopant Co atoms [45]. Based on the red shift in the absorption onset with increasing Co addition, a shrinkage in the band gap energy is expected. Fig. 6(b) shows the band gap energy values of ZnO as a function of the Co content, which were determined by plotting the absorption coefficient (α) vs. photon energy (hυ) and extrapolating the straight-line portion of this plot to the hυ axis. As clearly observed in Fig. 6(b), a red shift occurs in the direct band gap energy with doping from 3.34 eV (0 at.% Co) to 3.30, 3.23, 3.20, 3.10 and 3.06 eV corresponding to the 1, 2, 3, 4 and 5 at.% Co:ZnO films, respectively. The red shift of the band gap energy of ZnO with doping has previously been observed and explained by several factors including the size, morphology, annealing temperature, film thickness, defect state, and dopant content [46-48]. In our case, the effects of quantum confinement and morphology can be disregarded because of the large sizes and similar morphologies of the samples. Thus, this contraction may be explained by the combined effects of (i) the presence of defect states such as Zn interstitials and O vacancies and (ii) the dopant content [49]. At this point, the function of defect states is considered to be the most important contributor to the band gap contraction because of the distinct exchange interaction between the defect state and valence band electrons. According to the literature, s–d and p–d exchange interactions between the localized d electrons of the Co2+ ions and band electrons of ZnO results in negative and positive corrections of the conduction and valence band edges, respectively, thus leading to band gap narrowing [50, 51].
16
Room-temperature PL spectra of all the ZnO films obtained using a 526-nm excitation wavelength are presented in Fig. 7. The PL spectrum of the undoped ZnO film is a combination of two peaks: (i) a sharp green emission peak (at 528 nm) originating from O vacancies [52] and (ii) a broad red emission peak with lower intensity (at ~680 nm) originating from Zn interstitials [53]. However, for the Co:ZnO films, the intensities of both the green and red emissions decreased with increasing Co content. In addition, the red emission nearly disappeared for the Co:ZnO films with higher Co content, which indicates that the amount of defect states decreased with increasing Co content. Therefore, the residual compressive stress with Co addition determined by the XRD analyses does not depend on the defect formation; it should instead originate from the increased amount of substitutionally incorporated Co ions in ZnO resulting from the ionic radii differences between Co2+ and Zn2+ ions. 3.4 Photocatalytic properties of the undoped ZnO and Co:ZnO films The photocatalytic properties of the undoped ZnO and Co:ZnO films were examined under visible light based on the decomposition of MB. Fig. 8 shows the photodegradation reactions of all the ZnO films. Fig. 8(a) presents the real-time UV-Vis absorption spectra of the photodegradation of MB solutions containing the undoped and 1, 3, and 5 at.% Co:ZnO films. To calculate the degradation of MB, the maximum absorption band observed at 664 nm was used. For comparison, a dye solution without catalyst was also exposed to the visible light under the same experimental conditions and showed insensible change with respect to time. According to Fig. 8(a), the degradation of MB for all the undoped ZnO and Co:ZnO samples increased with irradiation time. Fig. 8(b) shows the appearance of the dye solution under visible light with respect to time for the 3 at.% Co:ZnO film. The color of the solution changed from dark blue to light blue as time progressed. At the end of the studied irradiation time range (120 min), the 17
color was bluish-white, indicating the presence of a small amount of non-degraded MB in the solution. Fig. 8(c) shows that the change of the visible light degradation of MB as a function of Co content undergoes an exponential decrease of the dye absorbance under the visible light irradiation. After 20-min irradiation, the degradation percentage of MB with undoped ZnO was approximately 20%; this value was much better for all the Co:ZnO films. Therefore, it can be concluded that the addition of Co has a positive effect on ZnO for the degradation of MB. This result can be attributed to the enhanced absorption in the visible light range resulting from the substitutionally incorporated Co ions. The best MB photodegradation was approximately 88% for the 3 at.% Co:ZnO film after 40 min of visible light irradiation. Qiu Xiaoqing et al. reported that for Co:ZnO samples, visible light irradiation is a more time-consuming process than UVlight irradiation for the degradation of RhB dye solution [20]. Therefore, the degradation time for our visible light degradation of Co:ZnO samples is better than that of previously reported Co:ZnO samples. To better distinguish the photocatalytic efficiency of the undoped ZnO and Co:ZnO photocatalysis, the kinetic parameters were determined from the MB degradation results and are presented in Fig. 9. Fig. 9(a) presents fitted data of the pseudo first-order kinetics [54]. The Co: ZnO films clearly exhibited much higher photocatalytic activity than the undoped ZnO film. Furthermore, the degradation rate of the Co:ZnO films slightly increased up to the 3 at.% Co addition and then decreased. The rate constant values determined from the slope of the fitted curves are presented as a histogram in Fig. 9(b). According to the histogram, the rate constant k values of the Co:ZnO films are almost three times higher than that of the undoped ZnO film. Up to 3 at.% Co doping, the photocatalytic abilities first increased and then slightly decreased with further doping. These results indicate both that the Co addition considerably affected the 18
photocatalytic ability of ZnO and that the 3 at.% Co addition is the optimal dopant content for Co:ZnO films. To explore the effects of Co ions on the visible light photocatalysis, we also studied the AQY of the photocatalytic process, as shown in Fig. 9(c). In the absence of any photocatalyst, the MB solution exhibited a very low AQY (4.88%) in the visible region (664 nm) for 60 min. In the visible region, when Co+2 ions were incorporated into the ZnO wurtzite crystal structure, the films exhibited high AQY values, which are attributed to the red shift in the absorption onset, suppressing the e-–h° pair recombination and enhanced photogenerated charge separation across the ZnO inside [25, 55, 56]. Among the Co:ZnO thin films, the 3% dopant content resulted in the highest AQY (85.96%), and the 1% dopant content resulted in the lowest AQY (82.93%). This result is due to the increase in crystallinity, large active surface area, and increased catalytic active sites. Thus, the AQY values of the Co:ZnO thin films in the visible region were always higher than that of the bare ZnO (41.12%) thin film, indicating improved photocatalytic activity of the ZnO thin film in the presence of Co ions. The solution pH is an important parameter in photocatalytic degradation reactions for industrial applications. To find out the effect of the pH on the photocatalytic degradation efficiency, degradation of 3 at. % Co-doped ZnO photocatalyst were investigated by varying the initial MB solution pH values of 3.0 (acidic medium), 7.0 (neutral medium) and 11.0 (basic or alkaline medium), respectively, by adding either hydrochloric acid or sodium hydroxide (0.1 M) and the results are represented in Fig. 10 (a). The photodegradation efficiency of 3 at. % Co-doped ZnO photocatalyst is slightly higher in alkaline (98.65 %) and neutral (97.40 %) solutions than that in acidic (78.02 %) solutions after 120 minutes. It is interesting to note that photodegradation at alkaline pH of 11 is close to the neutral pH of 7.0 suggesting that the effect of pH at these 19
medium did not significantly influence the photocatalytic performance of Co-doped ZnO thin films under visible-light irradiation. The nearly same MB degradation at these pH values may be due to the higher interaction between cationic MB and negatively charge catalyst surface [57]. Furthermore, this result is similar to that observed by other studies [58, 59]. Therefore, we performed all studies at neutral pH medium. We further explored the recycling ability, which is essential to find the photocatalytic stability and the reusability of the catalyst, under visible-light irradiation, and the results of recycling process are shown in Fig. 10 (b). After recycling five times over 120 min reaction, there were no remarkable changes in the degradation even if the degradation rate decreased from 97% to 93% during first and last cycle, respectively.
This decrement can be attributed to the some
photocatalyst washout during the recovery steps. This clearly indicates that the synthesized Codoped ZnO thin films are reusable and stable enough for industrial and environmental applications. According to the literature, several possible factors affect the photocatalytic activity of TMdoped oxide materials: (i) particle sizes and morphologies, (ii) surface properties, and (iii) dopant content [10, 60, 61]. In our case, the dependence of the photocatalytic activity could not explained by factors (i) and (ii) because of the similar particle and surface properties of the samples. Thus, the dopant content of the electronic structure of the Co:ZnO films is reasonably the dominant factor affecting the photocatalytic activity. With Co addition, to change the band structure, there should be a strong electronic interaction between the ZnO and Co species or d–d transition of Co ions [40].
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In general, the photocatalytic activity of a semiconductor oxide is closely related to two combined steps: the formation of photogenerated e-–h° pairs resulting from excitation of electrons from the valence band in the oxide material and the retention of e-–h° pair recombination. Up to 3 at.% Co doping, the band gap narrowing of Co:ZnO resulting from substitutional incorporation of Cu ions into the ZnO lattice is responsible for the enhancement of the photocatalytic activity. With reduction in the band gap energy up to 3 at.% Co content, the higher number of carriers results in a better photocatalyst. However, a slight decrease in the photocatalytic efficiency was observed upon further increasing the Co content (4 and 5 at.% Co:ZnO). This result may originate from the substitutional doping combined with interstitial doping of Co ions. Similar finding has been also reported for Mn-doped ZnO [31]. In a sample containing a high amount of Co, the majority of Co ions are substituted into Zn sites, whereas excess Co ions may be located in interstitial sites, which would act as trapping or recombination centers for photo-excited electrons and holes. Therefore, a slight decrease was observed in the photocatalytic ability of the 4 and 5 at.% Co:ZnO films.
4. CONCLUSIONS This work describes highly photocatalytically active Co:ZnO films prepared by the sol–gel method on glass substrates. The effects of the Co addition on the structural, morphological, optical, and photocatalytic properties of undoped ZnO and Co:ZnO films were investigated and discussed in detail. The results indicate that the Co addition leads to narrowing of the c-axis lattice parameter, as demonstrated by the shift in the XRD peak positions of wurtzite ZnO to lower diffraction angle values. In addition, narrowing of the band gap of ZnO was demonstrated
21
by the change in the absorption edge to longer wavelengths, as determined by the absorption band edge values. These results indicated that Co2+ ions were substitutionally incorporated into Zn2+ sites within the ZnO lattice. The photocatalytic activities of the films were investigated by examining MB dye degradation under visible light irradiation. The 3 at.% Co content was determined to be optimal for the photocatalytic activity of the Co:ZnO films. The Co:ZnO films presented here are highly photocatalytically active under visible light irradiation and promise a breakthrough in the large-scale utilization of photocatalysts using visible light to overcome organic and/or water pollution. Furthermore, the recycling results imply that the photocatalyst is stable enough and could be easily reused which is favorable for potential environmental and industrial applications.
Acknowledgments The authors would like to thank the European Cooperation in Science and Technology through COST Action MP1302 Nanospectroscopy, Scientific and Technological Research Council of Turkey (TUBITAK Grant number 112T981), Scientific Research Commission of Karamanoğlu Mehmetbey University under Project No. 30–M–12 and the Scientific Research Foundation of Selcuk University under Project No. 15401123 for the financial support of this research. Finally, the author (H. Arslan) would like to thank TUBITAK under Project No. 114Z956 for their financial supports during her studies.
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28
FIGURE CAPTIONS
Fig. 1. (a) Overview and (b) enlarged region of the experimental XRD patterns of undoped ZnO and Co:ZnO thin films with different Co contents.
Fig. 2. XPS spectra for undoped ZnO and Co:ZnO thin films with different Co contents: (a) survey analyses and high-resolution regional spectra of (b) Zn(2p), (c) O(1s), and (b) Co(2p) signals.
Fig. 3. SEM micrographs of the undoped ZnO and Co:ZnO thin films with different Co contents. The inset in the SEM micrograph of the undoped ZnO shows a magnified SEM image as a visual aid demonstrating the coating homogeneity.
Fig. 4. (a)–(d) 2D and (a1)–(d1) 3D AFM images of undoped ZnO and Co:ZnO thin films with different Co contents.
Fig. 5. (a) Overview and (b) enlarged region of the transmittance spectra of the undoped ZnO and Co:ZnO thin films with different Co contents. The inset in (a) shows the appearance of all the undoped ZnO and Co:ZnO thin films.
Fig. 6. UV–Vis spectra and (b) plots of (αhυ)2 as a function of photon energy with values of the band gap for undoped ZnO and Co:ZnO thin films with different Co contents. The band edge locations are indicated in (a).
Fig. 7. PL spectra for undoped ZnO and Co:ZnO thin films with different Co contents. 29
Fig. 8. (a) Real-time UV-Vis absorption spectra of the photodegradation of MB solutions containing the undoped and 1, 3, and 5 at.% Co:ZnO films. (b) Appearance of dye solution under visible light as a function of time for the 3 at.% Co:ZnO film. (c) Temporal profile of MB degradation of Co:Zn thin films with different Co contents under visible light illumination.
Fig. 9. (a) Fitted data of the pseudo first-order kinetics. (b) Histogram of the rate constant values determined from the slope of the fitted curves in (a). (c) AQY values of the photocatalytic process.
Fig. 10. (a) Effect of pH on the photocatalytic degradation of MB with 3 at.% Co-doped ZnO thin film and (b) the recycling ability of 3 at.% Co-doped ZnO photocatalyst for MB degradation under visible-light irradiation.
30
TABLE HEADINGS
Table 1. (002) peak position, lattice parameters (c), and stress (σ) value for the undoped ZnO and Co:ZnO thin films with different Co contents.
Table 2. Binding energy values (in eV) of Zn(2p3/2), Zn(2p1/2), O(1s), Co(2p1/2), and Co(2p3/2), for the undoped ZnO and Co:ZnO thin films with different Co contents.
31
Table 1. Peak Co:Zn Lattice Position (degree ratio Parameter of 2) (at.%) (c, Å)
α (GPa)
0
34.55
5.134
6.32
1
34.58
5.128
6.84
2
34.59
5.110
8.41
3
34.61
5.084
10.68
4
34.63
5.071
11.81
5
36.64
5.075
11.46
32
Table 2. Binding energy (eV) Co:Zn ratio (at.%)
Zn (2p3/2)
Zn (2p1/2)
O(1s)
Co(2p3/2)
Co(2p1/2)
0
1021.2
1044.3
530.9
-
-
5
1021.4
1044.5
531.1
780.2
796.1
33