- Email: [email protected]
Synthesis, structural, optical and photocatalytic behavior of Sn doped ZnO nanoparticles
Materials Science & Engineering B 253 (2020) 114497
Contents lists available at ScienceDirect
Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb
Synthesis, structural, optical and photocatalytic behavior of Sn doped ZnO nanoparticles N. Sivaa,b, D. Sakthia, S. Ragupathya, V. Aruna,c, N. Kannadasana,
T
⁎
a
Department of Physics, E.R.K. Arts and Science College, Erumiyampatti, Dharmapuri, Tamil Nadu 636905, India Department of Physics, Periyar University, Periyar Palkalai Nagar, Salem, Tamil Nadu 636011, India c Department of Energy Studies, Periyar University, Periyar Palkalai Nagar, Salem, Tamil Nadu 636011, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Photosensitivity Transition metal FESEM Methylene Blue
ZnO is widely used as a photocatalytic process due to high photosensitivity, strongly oxidizing nature, non-toxic, favorable band gap energy and the excellent chemical and mechanical stabilities of the material. The present work deals with the synthesis of ZnO doped with transition metal (Sn) by using a simple chemical precipitation method. XRD analysis confirms the substitution of Sn4+ ions into the ZnO matrix by increased lattice spacing and a reduced 2θ angle. Optical properties like reflectance and band gap were studied using UV–Vis–DRS spectroscopy. The morphology of the samples was observed using FESEM and TEM which explains the growth rate and shape of the prepared samples. Methylene Blue (MB) dye was used to evaluate the photocatalytic activity of synthesized ZnO nanoparticles and their performance with respect time were analyzed and presented.
1. Introduction Discharge of wastewater from textile industries into water bodies is causing a huge environmental hazard. The wastewater not only depletes the esthetic quality of water, but also renders it detrimental for aquatic flora and fauna. Of various dyes discharged into water bodies from textile industries, azo dyes constitute the largest portion. These dyes are known to be toxic, carcinogenic and can induce mutations [1]. So elimination of these recalcitrant dyes from the water bodies is an urgent issue. Due to the advancement of science and technology, many methods are available for the eradication of the dye molecules [2]. Recently, advanced oxidation processes (AOPs) have engendered great scientific interest worldwide. AOPs are based upon the in situ generation of hydroxyl radicals, which are responsible for the oxidation of dyes. Some of the AOPs are Fenton reaction [3], ozonation [4], sonophotocatalysis [5], electrochemical oxidation [6] and heterogeneous photocatalysis [7]. Out of these methods heterogeneous photocatalysis has emerged as a very efficient tool for the eradication of dyes. This method involves complete degradation of dye molecules to smaller molecular weight compounds and is carried out under ambient reaction conditions without the requirement of any additional oxidizing agent. Moreover, this is a cost effective technique and unlike other methods, this does not result in the formation of secondary pollutants. Photocatalytic treatment of wastewater by irradiated semiconductor has proven to be an effective process, which can lead to complete ⁎
mineralization of pollutants [8]. Moreover, semiconductor photocatalysts have been widely employed in the production of hydrogen by water splitting [9,10], purification of air [11], and other applications [12]. However, the fast recombination of the photogenerated electrons and holes hinders the commercialization of this technology. Therefore, it is important to promote photocatalytic efficiency by suppressing the recombination [13]. Previous studies indicated that photocatalytic activity largely depends on two factors: reactant adsorption behavior and the separation efficiency of electron–hole pairs [14]. The adsorption capacity can be improved by increasing the specific surface area of the catalysts, while semiconductor modifications by doping with a transitional metal ion [15], coupling with another semiconductor [16], and loading with noble metals [17,18] have been successfully carried out, in order to reduce the recombination of the electrons and holes. Among the various semiconductors, ZnO is widely used as an excellent material for the photocatalytic process due to the photosensitivity, the strongly oxidizing, non-toxic nature, the favorable band gap energy and the excellent chemical and mechanical stabilities of the material [19,20]. Moreover, it absorbs a large fraction of the solar spectrum, making it a perfect candidate for photocatalysis. Photocatalytic activity occurs when the ZnO absorbs a photon with an energy equal or greater than the material’s band gap energy that results in the formation of electron and hole pairs, which can subsequently migrate to the ZnO surface and react with adsorbed molecules to generate such reactive species as H2O2, superoxide anion radicals (•O2−) hydroxyl
Corresponding author E-mail address: [email protected] (N. Kannadasan).
https://doi.org/10.1016/j.mseb.2020.114497 Received 4 April 2018; Received in revised form 23 November 2019; Accepted 20 January 2020 0921-5107/ © 2020 Elsevier B.V. All rights reserved.
Materials Science & Engineering B 253 (2020) 114497
N. Siva, et al.
radicals (•OH) [21,22]. These species are very strong oxidizing and highly reactive agents that can degrade an organic pollutant into harmless compounds. However, ZnO has several weaknesses such as the fast recombination rate of the photogenerated electron and hole pairs and a low quantum yield in photocatalytic reactions in aqueous solutions, which can obstruct the photocatalytic degradation process. Significant effort has been focused to enhance the photocatalytic activity such as the rate of electron and hole pairs induced redox reaction and the rate of electron and hole recombination. It is known that the surface charge transfer processes and strong electron and hole recombination are strongly related to the structure and optical properties of the photocatalysis. Therefore, to enhance the photocatalytic activity the surface charge transfer processes should be increased and the recombination rate of electron and hole should be decreased. Various methods have been developed to reduce electron-hole recombination and increased the surface charge transfer. One of the interesting approaches to doped ZnO photocatalysis with transition metal ions, which have been shown to reduce band gap energy and improve charge separation between electron and hole by forming electron traps [23,24]. The hole will be able to migrate towards the surface of the photocatalysis and adsorbed organic compound. The doping of Sn in ZnO is expected to modify the absorption, luminescent, photocatalytic and other physical or chemical properties of ZnO because of the different structure of the electron shell and the similar size of Sn4+ (0.071 nm) and Zn2+ (0.074 nm) [25,26]. Manjula G. Nair et al. [27] have confirmed that the MB decomposition rate obtained using pure ZnO was much higher than that by doped ZnO. Japinder Kaur, Sonal Singhal [28] has investigated the transition metal doped ZnO prepared by thermally undoped ZnO was found to be better photocatalyst than doped ZnO. Changle Wu et al. [29] reported the Sn-doped ZnO nanorods are a kind of promising photocatalyst in remediation of water polluted by some chemically stable azo dyes. This work deals with the synthesis of ZnO doped with transition metal (Sn) by using a simple chemical precipitation method. Such method is a non-expensive and versatile approach that allows us to synthesize a great variety of crystalline metal oxides at low temperature compared to other synthesis methods [30]. The doping of Sn was chosen due to their comparatively ionic radii to the ionic radius of Zn2+ and ease to synthesize without change the ZnO crystalline structure over a wide range of dopant concentrations. Moreover, it is known than Co and Mn incorporated into ZnO lattices can form an impurity energy level and can facilitate mobility of charge carrier. Here we reported a systematic study on the influence of Sn ion doping degree on the structure, morphology and optical properties of ZnO nanoparticles. The photocatalytic oxidation of Methylene Blue (MB) in water was observed to study the influence of transition metal ions doping degree on the performance of ZnO.
drop to the above mixture. The entire solution was stirred magnetically at room temperature until a white precipitate was formed. The obtained dispersions were purified by dialysis against de-ionized water and ethanol several times to remove impurities. The yield products were dried in hot air oven at 100 °C for 6 h to evaporate water and organic material to the maximum extent. Finally, the obtained product was annealed in a muffle furnace at 300 °C for 3 h. The annealed powders were pulverized to fine powders using agate mortar for further characterizations. A similar method of preparation without the addition of dopant was used to synthesize undoped ZnO nanocrystals. 2.3. Characterization XRD patterns of as-synthesized samples were recorded with a X’PERT PRO diffractometer system with a Cu Kα irradiation (l = 1.5418 Å). Diffuse reflectance spectra were obtained using a Shimadzu UV-2500 spectrophotometer in the wavelength range 100–800 nm. The morphologies of samples were observed using scanning electron microscopes (FE-SEM, JEOL, JSM-6701F) and a high-resolution transmission electron microscopy (HR-TEM, JEOL, JEM-2100). Multipoint N2 adsorption-desorption experiments were carried out on a Micromeritix (ASAP 2020) analyzer using the BET gas adsorption method, at 77 K. 2.4. Photocatalytic experiment For the purpose of studying the photocatalytic activity of Sn doped ZnO nanoparticles, under Sun light irradiation, MB dye was degraded from the catalyst of Pure and Sn (0.075, 0.1 and 0.125 M) doped ZnO nanoparticles at room temperature. A 950 ± 20 W min−2 solar light was used in this dye degradation experiment. Where, 0.05 gm of Pure and Sn (0.075, 0.1 and 0.125 M) doped ZnO nanoparticles was added to a quartz photoreactor containing 100 ml of a 30 mg/l MB aqueous solution. After that, the prepared solution was kept at dark condition for accomplishing the adsorption and desorption equilibrium between dye and photocatalyst. Then, the sample was irradiated under sun light source at a regular time interval followed by filtered. After the filtration process, the obtained solution was utilized to analyze UV–Visible absorption behavior of the organic dyes. In the UV–Visible absorption spectra, the maximum absorption of MB dye was observed at λmax = 664 nm. 3. Result and discussion 3.1. X-ray diffraction The XRD patterns of both the undoped and the Sn-doped ZnO microspheres are shown in Fig. 1a. All the diffraction patterns can be indexed to a hexagonal phase ZnO according to the JCPDS 36–1451 [31] data files before and after Sn doping ((1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), and (2 0 1)). In this figure, the strong, sharp diffraction peaks indicate a high degree of crystallization. As shown in Fig. 1b, the (1 0 1) peak of the Sn-doped ZnO clearly exhibits a shift to the left compared to that of the undoped ZnO peaks. In addition, the other characteristic peaks of the Sn-doped ZnO samples also exhibit a slight shift compared to those of ZnO. It was observed that peak position has slightly shifted to lower diffraction angle in the doped samples when compared to the pure ZnO. This explains that Sn4+ ions have been completely dissolved into the lattice of ZnO. Since the ionic radius of Sn4+ (74 pm) is slightly larger than ionic radius of Zn2+ (60 pm) [32,33], Sn ion is expected to be soluble in ZnO and hence this had resulted in the peak shift to lower diffraction angle. A shift in the peak to lower theta value results the increment in the d-spacing and hence it is expected to increase the cell volume., the lattice parameters of ZnO were expected to increase upon the Sn4+ substitution on the Zn2+ position, thus resulting in an
2. Materials and methods 2.1. Chemicals All the chemicals used in this study are of AR grade with 99% purity (E. Merck chemicals) and used without further purification. Sample preparation and dilutions were made of ultrapure water. Zinc acetate dihydrate [Zn(CH3COO)2*2H2O], Tin Chloride dihyrate [SnCl2.2H2O] and potassium hydroxide (KOH) were used as precursors. 2.2. Synthesis of bare and Sn doped ZnO nanoparticles For the preparation of Sn-doped ZnO nanocrystals, 0.5 M of Zinc acetate dihydrate dissolved in 50 ml of deionized water was stirred vigorously by magnetic stirrer. Then, tin chloride dihyrate of preferred mole (0.05, 0.075, 0.1 and 0.125 M) prepared in 20 ml aqueous were mixed drop by drop to the above and stirred for 30 min. Finally, 2 M of potassium hydroxide in 50 ml of deionized water was added drop by 2
Materials Science & Engineering B 253 (2020) 114497
N. Siva, et al.
reported in the JCPDS data [34]. It can be also noted that the value of the FWHM of the 0.075, 0.10 and 0.125 M Sn-doped ZnO nanoparticles sample is higher than 0.05 M Sn-doped ZnO and pure ZnO crystalline planes, suggesting a reduction of crystallite size with increasing the Sn content. The crystallite size (D) of these samples is estimated using following Scherrer’s formula:
D=
0.89λ βcosθ
where λ is the wavelength of X-rays used; β is the broadening of the diffraction line measured at half its maximum intensity; and θ is the diffraction angle. The cell volume was calculated by using the formula V = 0.866a2c and the Zn-O bond length was deduced by using the equation 1
2
a rZn − O = ⎡ ⎛ ⎞ + (0.5 − u)2c 2⎤ ⎢⎝ 3 ⎠ ⎥ ⎣ ⎦ ⎜
where u =
2
⎟
( ) + 0.25. All the obtained lattice parameters, cell volume a2 3c 2
and Zn-O bond lengths are summarized in Table 1. The calculated average crystallite size of pure ZnO is 26 nm, and it decreases up to 21 nm for the Sn-doped ZnO samples. The decrease in the crystallite size with respect to increase in Sn concentration. The crystallite size decrease with Sn content is likely due to the substitution of tin into the zinc oxide lattice. It is discovered that on doping Sn ion there is a slight increase in the cell parameters (a = b) and bond length (rZn-O) which results in the expansion of the cell volume of the doped samples when compared to pure ZnO. In addition to this, the synthesized ZnO has cell volume exact value than that of the standard value for the bulk material i.e. 47.62 Å3 (JCPDS). Also, As a result of doping the c/a value reduces and the average bond length of the Zn-O has slightly increased from 1.9778 Å to 1.9779 Å, 1.9789 Å, 1.9795 Å and 1.9787 Å in Sn (0.05, 0.075, 0.1 and 0.125 M) doped ZnO samples. Hence, it can be understood that difference in the valency and ionic radii of the dopant (Sn4+) and the host (Zn2+) atoms has resulted in such incremental in the cell parameters, bond length and cell volume. 3.2. UV–Vis reflectance spectra For semiconductor materials, diffuse reflectance spectroscopy is a useful tool for characterizing the optical property, which is considered to be a key component in photocatalytic performance [35]. Fig. 2a shows the UV–Vis DRS of both the pure ZnO and the Sn-doped ZnO. For the undoped ZnO an absorption edge steeply rises to about 422 nm whereas, the Sn-doped ZnO exhibits red shifts of the absorption edge as well as a significant enhancement of light absorption in the 400–600 nm range (Fig. 2a). In general, there are a great number of oxygen vacancies on nanosized ZnO surfaces, and oxygen vacancies can give rise to surface states. The surface states are located between the valence and the conduction band. Therefore, the optical absorption between 400 and 600 nm is closely related to the surface states. The band gap of the samples were calculated using the Kubelka–Munk relation to convert the reflectance into a Kubelka–Munk function F(Rα), using the relation
Fig. 1. (a) X-ray diffraction patterns of pure and Sn doped ZnO nanoparticles. (b) X-ray diffraction patterns of pure and Sn doped ZnO nanoparticles-enlarged view of (1 0 0), (0 0 2) and (1 0 1) peaks.
increased lattice spacing and a reduced 2θ angle. This result revealed that the Sn4+ ions have been incorporated into the crystal lattice of ZnO and substituted for the fractional Zn ions. Moreover, the characteristic peaks of the Sn hybrid are not detected due to the high dispersion and low content of the Sn compound. The lattice parameters, ‘a’ and ‘c’, were calculated using equations:
sin2 θ =
F (R α ) =
(1 − R α )2 2R α
where, Rα is the reflectance of an infinitely thick sample with respect to a reference at each wavelength. The Kubelka–Munk function is a function equivalent to the absorption coefficient. The bandgap energy of the nanoparticles were calculated from the slope of the graph where [F(Rα)hν]2 were plotted against photon energy hν are 3.43 eV, 3.31 eV, 3.19 eV, 3.07 eV and 3.13 eV for pure ZnO and Sn (0.05, 0.075, 0.1 and 0.125 M) doped samples, respectively (Fig. 2b). This indicates that the Sn-doped ZnO possessed a narrower band gap compared to that of the undoped ZnO. Furthermore, this lower energy
λ2 2 λ2 (h + k 2 + hk ) + 2 (l 2) 3a2 4c
These values are presented in Table 1. It is observed that the lattice constant value of ZnO nano crystalline is almost the same as the one 3
Materials Science & Engineering B 253 (2020) 114497
N. Siva, et al.
Table 1 XRD derived parameters, Bond length and Band gap energy of ZnO and Sn doped ZnO nanoparticles. Samples
ZnO ZnO: ZnO: ZnO: ZnO:
Sn Sn Sn Sn
Crystallite size (nm)
(0.05 M) (0.075 M) (0.1 M) (0.125 M)
26 26 25 21 22
Lattice parameters (Å) a=b
c
c/a
3.248 3.249 3.252 3.254 3.251
5.213 5.211 5.209 5.207 5.210
1.6199 1.6038 1.6017 1.6001 1.6025
Cell Volume (Å3)
Bond length (Å)
Band gap Eg (eV)
47.6253 47.6363 47.7060 47.7464 47.6858
1.9778 1.9779 1.9789 1.9795 1.9787
3.43 3.31 3.19 3.07 3.13
lattice, the band gap of the resulting solid solution increases as SnO has a higher band gap (3.0 eV) [32] than ZnO. Thus, Sn (0.125 M) is incorporated into the ZnO lattice, its band gap further increases to 3.13 eV. Since localized 4 s orbital’s of Zn constitute the conduction band of ZnO, the addition of Sn to the ZnO lattice influences the conduction band by admixing extended Sn 5 s orbitals [34]. Thus, the conduction band is shifted toward the higher energy and a systematic increase in the valence band conduction band separation occurs with increasing amount of Sn in the ZnO lattice. This will lead to an increase in its Fermi level, and consequently the oxidation–reduction potential, impacting the photo catalytic property of ZnO. 3.3. Field emission scanning electron microscopy The synthesized pure ZnO and Sn-doped ZnO samples have been characterized by FE-SEM (see Fig. 3a–d). The images show the clear morphological changes due to the 0.1 M of Sn doped ZnO nanoparticles. The agglomerated spherical structures in the pure ZnO sample are shown in Fig. 3a. The size of these crystalline structures is in the range of ~15–45 nm. The ZnO nanostructure seen above change dramatically on 0.1 M Sn doped to ZnO nanoparticles. The morphological change recorded by FESEM for the less agglomerated spherical crystallites are observed in Sndoped ZnO samples. The size of these crystalline structures is in the range of ~10–35 nm. 3.4. Energy-dispersive spectrum Energy-dispersive spectrum analysis is performed to investigate the elemental composition of Sn-doped ZnO nanostructures. EDS analysis confirmed the presence of Zn, O, and Sn elements in the nanostructures. Fig. 3e exhibits a pure ZnO spectrum with two high intense peaks and single small peak, which are associated with O and Zn atoms, respectively. 3.5. Transmission electron microscopy
Fig. 2. (a) UV–Vis–diffuse reflectance spectra of pure and Sn doped ZnO nanoparticles. (b) Plot of band gap energy for pure and Sn-doped ZnO nanoparticles.
Fig. 4 (a, b) shows the TEM micrographs of pure ZnO and 0.1 M Sndoped ZnO nanoparticles. TEM micrographs in Fig. 4a shows spherical shaped ZnO nanoparticles, having size about 40–60 nm. Fig. 4b shows that the spherical nano assemblies present on the 0.1 M of Sn-doped ZnO sample, with size in the range of 10–25 nm, which is in good agreement with the crystallite size obtained from XRD line broadening. The morphological change we observed can be then explained on the basis of role of Sn4+ in governing the growth rate of planes of ZnO nanocrystals originated by different surface charges on ZnO planes. In the presence of a high concentration of tin, the adsorption of Sn4+ filling the ZnO growth sites suppresses the growth rate along c-axis. This balance of the crystal facets surface energy leads, consequently, to formation of particles with a rounded shape form.
value has been attributed to the strong absorption in the visible region due to localized levels existing in the forbidden gap because of the Sn doping. Upon irradiation with visible light, an electron–hole pair is generated within the effective band gap: an electron transition takes place for the new additional valence state to a conduction state. This transition requires smaller excitation energy compared with a ZnO inhabitant band gap (3.43 eV) depending on the particular dopant level within the band gap [36]. Therefore, it can be used as an efficient photo catalyst under visible-light irradiation. In addition, it can be seen that the band gap decreases with the increasing Sn content at low doping levels and reaches a minimum for Sn-doped ZnO (0.1 M). These reductions in the band gap attribute to a change in the position of the valence band because of the introduction of Sn 5 s orbitals [37]. With the increasing Sn concentration in the ZnO
3.6. BET studies The 4
nitrogen
adsorption-desorption
isotherms
and
the
Materials Science & Engineering B 253 (2020) 114497
N. Siva, et al.
Fig. 3. FE-SEM images of pure (a) and Sn (0.1 M) doped (c), corresponding size distribution curve (b, d) and EDS spectra of Sn (0.1 M) doped ZnO nanopartcles (e).
whereas, for Sn-doped ZnO, they are 54.19 m2g−1 and 0.511 cm3g−1 respectively. The high specific surface area of Sn-doped ZnO material can yield high photocatalytic activity. The inset in Fig. 5(a, b) shows the pore size distribution of bare and Sn-doped ZnO nanocrystals. The undoped and Sn-doped ZnO exhibit
corresponding pore size distribution curves of the pure and 0.1 M of Sndoped ZnO samples are shown in Fig. 5 (a, b). Both the samples exhibit a type IV isotherm, indicating the capillary condensation of the mesopore structure [38]. The specific surface area and the pore volume are calculated to be 29.86 m2 g−1 and 0.259 cm2 g−1, for pure ZnO,
Fig. 4. (a, b) TEM micrographs of undoped and Sn (0.1 M) doped ZnO nanoparticles. 5
Materials Science & Engineering B 253 (2020) 114497
N. Siva, et al.
3.9. Turnover number The effectiveness of the catalyst is expressed in this reaction system a “turnover number” (TON) [39].
TON =
(% Conversion )(Number of moles of substrate ) Number of moles of catalyst
3.10. Turnover frequency (TOF) The number of reaction cycles place in a given time period and is usually employed to study the reaction rate [40].
TOF =
TON Time
3.11. Estimation of the degradation percentage and rate constant The absorption spectra of irradiated samples were recorded at various time intervals and the rate of decolorization of MB was observed in terms of the change in the intensity at 664.5 nm. To study the enhancement of the photoactivity, comparative experiments were deployed. The experimental results demonstrated that the concentrations of MB revealed no degradation or reduction after 120 min in the absence of doped nanoparticles or light irradiation. It is suggested that the hydrolysis and photochemical processes can be neglected and that the photocatalytic experiment occurred in a pure photocatalytic system [41,42]. To understand the response of the immersed Sn-doped ZnO nanoparticles on the percentage of degradation of MB, have been followed under solar irradiation light at various time intervals with a UV–Vis spectrophotometer. The MB degradation results with immersed Sn-doped ZnO nanoparticles in the dark. Fig. 6 (a, b) and Fig. 7 shows the degradation percentage of the MB efficiency as a function of irradiation time in the presence of Sn-doped ZnO nanoparticles with different doping concentrations. Several researchers have reported that the kinetic behavior of a photocatalytic reaction can be described by a pseudo-first order model. The photocatalytic activity of all doped ZnO nanoparticles in this study obeys the pseudo-first-order reaction kinetics (Fig. 8 a, b). The apparent reaction rate constants k is 0.00517, 0.00667, 0.01263 and 0.00982 min−1 of the MB degradation using pure and (0.075, 0.1, 0.125 M) Sn doped ZnO nanoparticles, respectively. The rate constant values are found to be higher for 0.1 M Sn doped ZnO (0.01263 min−1) it is clear and the reason why 0.1 M Sn doped ZnO performs well in the photodegradation of MB.
Fig. 5. (a) The N2 adsorption – desorption isotherm and the BJH pore size distribution curves (Insect) of Pure ZnO nanoparticles. (b) The N2 adsorption – desorption isotherm and the BJH pore size distribution curves (Insect) of Sn (0.1 M) doped ZnO nanoparticles.
peaks at 22.24 and 47.12 nm, respectively. The Sn-doped ZnO has higher pore size and higher pore volume than the bare ZnO. The higher pore size and higher pore volume contribute to the effective electron transport at the dye degradation process.
3.12. Reason for the enhanced photocatalytic activity
3.7. Photocatalytic activity
Photocatalytic activity of ZnO has increased manifold upon doping with Sn ion. From the photocatalytic results it is observed that the highest photocatalytic activity was obtained for the 0.1 M Sn doped ZnO and the activity decrease gradually with further doping. To investigate the possible factors responsible for the highest activity we look into the analysis of structural, optical morphological, surface area, number of active sites, turnover number and turnover frequency properties. Looking into the structural variation, when Sn4+ larger ionic radii than Zn2+ with slightly is doped into ZnO lattice, it results in the expansion of the lattice i.e. increase in the bond length (rZn-O) and volume expansion. With an increase in the volume there is a small decrease in the band gap (see Table 1) for lower doping content (up to 0.1 M), after which the band gap again increases due to the starting formation of the secondary phase Zn2SnO4. This decrease in the bandgap has two major advantages, i.e. firstly; lower bandgap can facilitate electron excitation with comparatively low energy and hence can meet the requirements for visible light photocatalysis, and thereby the numbers of electrons are available in the CB for the photocatalytic reaction which leads to the enhanced photodegradation of the dye. In
To evaluate photocatalytic activity of ZnO and doped ZnO, degradation of MB was studied. The photocatalytic efficiency of catalysts was evaluated in terms of percentage of dye degraded using the following equation [35].
Percentage of degradation =
Co − Ct × 100 Co
where Co is the initial concentration of MB dye solutions (mg/L), Ct is the concentration of dye after irradiation after selected time interval (mg/L). 3.8. Number of active sites for Sn doped ZnO nanoparticles For adsorptions that are the effective sites for a particular heterogeneous catalytic reaction is called the active sites of the catalytic. The number of active sites depends on the concentration of the metal ions in the specified amount of the catalyst taken for a particular reaction. 6
Materials Science & Engineering B 253 (2020) 114497
N. Siva, et al.
Fig. 6. (a–d) Time dependent UV–Vis absorption spectra of the photocatalytic degradation of MB in the presence of pure and (0.075, 0.1 and 0.125 M) Sn doped ZnO nanoparticles.
generation of a hole (h+) in the VB. The photoelectron can easily be trapped by electronic acceptors like adsorbed O2, to further produce a superoxide radical anion (•O2¯) whereas, the photoinduced holes can be easily trapped by electron donors, such as OH¯ or organic pollutants, to further oxidize organic pollutants [35,36]. In addition to reacting with electron donors or acceptors adsorbed on the semiconductor surface mentioned above, the photogenerated electrons and holes can also recombine and dissipate the input energy as heat or get trapped in metastable states [43,44]. If a suitable scavenger or defect state is available to trap the electron or hole, recombination is prevented and subsequent redox reactions can occur [45].
addition to this, doping introduces intermediate states within the conduction band and valence band. These intermediate states help in trapping the excited electron that causes enhanced charge separation, thus inhibiting recombination. This is because after a certain critical value of doping, these impurity energy levels instead of acting as a charge separation center behave like a recombination center, thus facilitating recombination process. Further, these results are concordant with the calculated TON and TOF values given in Table 2. The observed the table, the degradation rate enhanced can be attributed to the Sn (0.1 M) doped ZnO nanoparticles, which can efficiently absorb photons under solar light compared to pure and other Sn doped ZnO nanoparticles.
Photocatalytic + hν → hVB++ eCB− Dye + hVB+→ Dye+→ Final species
3.13. Degradation mechanism
Dye + OH·→ Dye·→ Final species
When semiconductor nanocrystals are irradiated by light with energy higher or equal to the band gap, an electron (e¯) in the valence band (VB) can be excited to the conduction band (CB) with the
Fig. 9 shows that illustrate the band gap modification and 7
Materials Science & Engineering B 253 (2020) 114497
N. Siva, et al.
Table 2 Determination of the active sites, TON and TOF in the solar system for all the photocatalysts. Photocatalyst
Number of active sites 5 mg (×10−4 mol)
TON (×10−3 min−1)
TOF (×10−5 min−1)
ZnO ZnO: Sn (0.075 M) ZnO: Sn (0.1 M) ZnO: Sn (0.125 M)
6.144 5.853 6.606 5.659
7.938 9.775 11.926 10.080
6.615 8.145 9.938 8.400
Fig. 7. Plot of C/C0 versus irradiation time for the degradation of MB under solar light.
introduction of intermediate states with respect to Sn ion doping in the ZnO lattice. These defect states resulted in the delayed recombination thus making more electrons available for photocatalysis and thus increases the degradation rate.
Fig. 9. Schematic diagram representing the degradation mechanism in pure and Sn doped ZnO.
ZnO + hν → hVB++ eCB− nanoparticles were synthesized using simple and cost effective chemical precipitation method. The crystalline properties of the samples were analyzed using XRD and it reveals that the crystalline size decreases with the increase in doping concentrations due to the substitution of Sn4+ ions into ZnO lattices. Optical properties of ZnO nanoparticles were studied using UV–Vis–DRS spectroscopy and the reflectance and the band gap value decreases with increase in Sn doping concentrations. FESEM images reveal the formation of nanoparticles and their sizes were in nanometer region. EDS spectrum confirms the presence of Sn, Zn and O in the samples. TEM result shows the spherical nano assemblies for 0.1 M of Sn-doped ZnO sample, in the size ranges from 10 to 25 nm. Methylene Blue dye was used to evaluate the photocatalytic activity of synthesized ZnO nanoparticles. The effectiveness of the catalyst at very low concentrations is suggested by the calculation of the number of active sites, TON and TOF for all the systems under solar light illumination. The apparent reaction rate constants k is 0.00517 and 0.01263 min−1 of the MB degradation using 0.1 M Sn doped and
Sn4++ e−→ Sn3+ Sn3++ O2 → O2·−+ Sn4+ 3.14. Recyclability of Sn (0.1 M) doped ZnO nanoparticles The stability and reusability of the prepared Sn (0.1 M) doped ZnO nanoparticle photocatalyst was checked by using 5 mg of the catalyst for three cycles and in the third cycle, the percentage of degradation found 76.25% due to the loss of some amount of catalyst while washing and filtrating. The results unambiguously demonstrate that the Sn (0.1 M) doped ZnO photocatalyst is stable, efficient and can be reused for up to three cycles. 4. Conclusion As prepared and different concentrations of Sn doped ZnO
Fig. 8. (a, b) Effect of MB degradation and rate constant values of pure and different levels of Sn doped ZnO nanoparticles. 8
Materials Science & Engineering B 253 (2020) 114497
N. Siva, et al.
ZnO nanoparticles respectively. Sn (0.1 M) doped ZnO nanoparticles shows the higher photocatalytic activity of visible light illumination.
[22]
Declaration of Competing Interest [23]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[24]
[25]
References
[26]
[1] F.M.D. Chequer, T.M. Lizier, R. de Felício, M.V.B. Zanoni, H.M. Debonsi, N.P. Lopes, R. Marcos, D.P. de Oliveira, Analyses of the genotoxic and mutagenic potential of the products formed after the biotransformation of the azo dye Disperse Red 1, Toxic. Vitro 25 (2011) 2054–2063. [2] T. Robinson, G. McMullan, R. Marchant, P. Nigam, Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative, Bioresour. Technol. 77 (2001) 247–255. [3] M.S. Lucas, J.A. Peres, Decolorization of the azo dye reactive black 5 by fenton and photo-fenton oxidation, Dyes Pigm. 71 (2006) 236–244. [4] L. Zhao, J. Ma, Z. Sun, H. Liu, Mechanism of heterogeneous catalytic ozonation of nitrobenzene in aqueous solution with modified ceramic honeycomb, Appl. Catal. B 89 (2009) 326–334. [5] Y. He, F. Grieser, M. Ashokkumar, The mechanism of sonophotocatalytic degradation of methyl orange and its products in aqueous solutions, Ultrason. Sonochem. 18 (2011) 974–980. [6] A. Bedoui, M.F. Ahmadi, N. Bensalah, A. Gadri, Comparative study of Eriochrome black T treatment by BDD-anodic oxidation and Fenton process, Chem. Eng. J. 146 (2009) 98–104. [7] A.R. Khataee, M. Zarei, R. Ordikhani-Seyedlar, Heterogenous photocatalysis of a dye solution using supported TiO2 nanoparticles combined with homogenous photoelectrochemical process: molecular degradation products, J. Mol. Catal. A: Chem. 338 (2011) 84–91. [8] L. Zhang, H. Cheng, R. Zong, Y. Zhu, Photocorrosion Suppression of ZnO Nanoparticles via Hybridization with Graphite-like Carbon and Enhanced Photocatalytic Activity, J. Phys. Chem. C 113 (2009) 2368–2374. [9] M. Gratzel, Photoelectrochemical cells, Nature 414 (2001) 338–344. [10] J. Zhu, D. Chakarov, M. Zäch, in: L. Zang (Ed.), Energy Efficiency and Renewable Energy Through Nanotechnology, Springer, London, 2011, 441–486. [11] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental Applications of Semiconductor Photocatalysis, Chem. Rev. 95 (1995) 69–96. [12] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Photogeneration of Highly Amphiphilic TiO2 Surfaces, Nature 388 (1997) 431–432. [13] H. Fu, T. Xu, S. Zhu, Y. Zhu, Photocorrosion Inhibition and Enhancement of Photocatalytic Activity for ZnO via Hybridization with C, Environ. Sci. Technol. 42 (2008) 8064–8069. [14] Q. Xiao, Z. Si, J. Zhang, C. Xiao, X. Tan, Photoinduced hydroxyl radical and photocatalytic activity of samarium-doped TiO2 nanocrystalline, J. Hazard. Mater. 150 (2008) 62–67. [15] W.Y. Choi, A. Termin, M.R. Hoffmann, The role of metal ion dopants in quantumsized TiO2: correlation between photoreactivity and charge carrier recombination dynamics, J. Phys. Chem. 98 (1994) 13669–13679. [16] C. Li, T. Ahmed, M. Ma, T. Edvinsson, J. Zhu, A facile approach to ZnO/CdS nanoarrays and their photocatalytic and photoelectrochemical properties, Appl. Catal. B 138–139 (2013) 175–183. [17] C.G. Silva, R. Juarez, T. Marino, R. Molinari, H. Garcia, Influence of Excitation Wavelength (UV or Visible Light) on the Photocatalytic Activity of Titania Containing Gold Nanoparticles for the Generation of Hydrogen or Oxygen from Water, J. Am. Chem. Soc. 133 (2011) 595–602. [18] A. Primo, T. Marino, A. Corma, R. Molinari, H. Garcia, Efficient Visible-Light Photocatalytic Water Splitting by Minute Amounts of Gold Supported on Nanoparticulate CeO2 Obtained by a Biopolymer Templating Method, J. Am. Chem. Soc. 133 (2011) 6930–6933. [19] M. Shamshi Hassan, Touseef Amna, O-Bong Yang, Hyun-Chel Kim, MyungSeob Khil, TiO2 nanofibers doped with rare earth elements and their photocatalytic activity, Ceram. Int. 38 (2012) 5925–5930. [20] S. Anandan, A. Vinu, T. Mori, N. Gokulakrishnan, P. Srinivasu, V. Murugesan, K. Ariga, Photocatalytic degradation of 2,4,6-trichlorophenol using lanthanum doped ZnO in aqueous suspension, Catal. Commun. 8 (2007) 1377–1382. [21] K. Thongsuriwong, P. Amornpitoksuk, S. Suwanboon, Photocatalytic and
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38] [39] [40]
[41] [42]
[43]
[44]
[45]
9
antibacterial activities of Ag-doped ZnO thin films prepared by a sol–gel dip-coating method, J. Sol-Gel Sci. Technol 62 (2012) 304–312. Umar Ibrahim Gaya, Abdul Halim Abdullah, Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: a review of fundamentals, progress and problems, J. Photochem. Photobiol. C: Photochem. Rev. 9 (2008) 1–12. Ruby Chauhan, Ashavani Kumar, Ram pal chaudhary, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 98 (2012) 256–264. Ruby Chauhan, Ashavani Kumar, Ram Pal Chaudhary, Photocatalytic studies of silver doped ZnO nanoparticles synthesized by chemical precipitation method, J. Sol-Gel Sci. Technol. 63 (2012) 546–553. F.J. Sheini, M.A. More, S.R. Jadkar, K.R. Patil, V.K. Pillai, D.S. Joag, Observation of Photoconductivity in Sn-Doped ZnO Nanowires and Their Photoenhanced Field Emission Behavior, J. Phys. Chem. C 114 (2010) 3843–3849. J.-H. Sun, S.-Y. Dong, J.-L. Feng, X.-J. Yin, X.-C. Zhao, Enhanced sunlight photocatalytic performance of Sn-doped ZnO for Methylene Blue degradation, J. Mol. Catal. A: Chem 33 (2011) 145–150. Manjula G. Nair, M. Nirmala, K. Rekha, A. Anukaliani, Structural, optical, photo catalytic and antibacterial activity of ZnO and Co doped ZnO nanoparticles, Mater. Lett. 65 (2011) 1797–1800. Japinder Kaur, Sonal Singhal, Facile synthesis of ZnO and transition metal doped ZnO nanoparticles for the photocatalytic degradation of Methyl Orange, Ceram. Int. 40 (2014) 7417–7424. Wu. Changle, Li Shen, Yu. Huaguang, Qingli Huang, Yong Cai Zhang, Synthesis of Sn-doped ZnO nanorods and their photocatalytic properties, Mater. Res. Bull. 46 (2011) 1107–1112. N. Kannadasan, N. Shanmugam, K. Sathishkumar, S. Cholan, R. Ponnguzhali, G. Viruthagiri, The effect of Ce4+ incorporation on structural, morphological and photocatalytic characters of ZnO nanoparticles, Mater. Charact. 97 (2014) 37–46. N. Kannadasan, N. Shanmugam, K. Sathishkumar, S. Cholan, R. Ponnguzhali, G. Viruthagiri, Optical behavior and sensor activity of Pb ions incorporated ZnO nanocrystals, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 143 (2015) 179–186. C.B. Fitzgerald, M. Venkatesan, L.S. Dorneles, R. Gunning, P. Stamenov, J.M.D. Coey, Magnetism in dilute magnetic oxide thin films based on SnO 2, Phys. Rev. B. 74 (2006) 115307. R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. Sect. A. 32 (1976) 751–767. Zhanchang Pan, Pengwei Zhang, Xinlong Tian, Guo Cheng, Yinghao Xie, Huangchu Zhang, Xiangfu Zeng, Chumin Xiao, Hu. Guanghui, Zhigang Wei, Properties of fluorine and tin co doped ZnO thin films deposited by sol–gel method, J. Alloy. Compd. 576 (2013) 31–37. Haohao Wang, Ripon Bhattacharjee, I-Ming Hung, Langkai Li, Renjie Zeng, Material characteristics and electro chemical performance of Sn-doped ZnO spherical-particle photoanode for dye-sensitized solar cells, Electrochim. Acta 111 (2013) 797–801. M. Oshikiri, M. Boero, J.H. Ye, Z.G. Zou, G. Kido, Electronic structures of promising photocatalysts InMO4 (M = V, Nb, Ta) and BiVO4 for water decomposition in the visible wavelength region, J. Chem. Phys. 11 (2002) 77313–77318. Y.Q. Lei, G.H. Wang, S.Y. Song, W.Q. Fan, M. Pang, J.K. Tang, H.J. Zhang, Room temperature, template-free synthesis of BiO2I hierarchical structures: visible-light photocatalytic and electrochemical hydrogen storage properties, Dalton Trans. 39 (2010) 3273–3278. S.K. Mehar, G. Ranga Rao, Ultralayered Co3O4 for high performance supercapacitor applications, J. Phys. Chem. C 115 (31) (2011) 15646–15654. M. Boudart, Turnover rates in heterogeneous catalysis, Chem. Rev 95 (1995) 661–666. L. Gomathi Devi, R. Shyamala, Photocatalytic activity of SnO2–a-Fe2O3 composite mixtures: exploration of number of active sites, turnover number and turnover frequency, Mater. Chem. Front 2 (2018) 796. R. Ullah, J. Dutta, Photocatalytic degradation of organic dyes with manganesedoped ZnO nanoparticles, J. Hazard Mater 156 (2008) 194–200. Y. Hosogi, H. Kato, A. Kudo, Photocatalytic activities of layered titanates and niobates ion-exchanged with Sn2+ under visible light irradiation, J. Phys. Chem. C 112 (2008) 17678–17682. Y. Hosogi, Y. Shimodaira, H. Kobayashi, A. Kato, Role of Sn2+ in the Band Structure of SnM2O6 and Sn2M2O7 (M = Nb and Ta) and Their Photocatalytic Properties, Chem Mater 20 (2008) 1299–1307. Y. Peltzer, E.L. Blanca, A. Svane, N. Christensen, C. Rodri Guez, O. Cappannini, M. Moreno, Calculated static and dynamic properties of beta-Sn and SnO2 compounds, Phys. Rev. B 48 (1993) 15712–15718. M. Radeeka, P. Pasierb, K. Zakrzewska, M. Rekas, Transport properties of (Sn, Ti)O2 polycrystalline ceramics and thin films, Solid State Ionics 119 (1999) 43–48.
Contents lists available at ScienceDirect
Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb
Synthesis, structural, optical and photocatalytic behavior of Sn doped ZnO nanoparticles N. Sivaa,b, D. Sakthia, S. Ragupathya, V. Aruna,c, N. Kannadasana,
T
⁎
a
Department of Physics, E.R.K. Arts and Science College, Erumiyampatti, Dharmapuri, Tamil Nadu 636905, India Department of Physics, Periyar University, Periyar Palkalai Nagar, Salem, Tamil Nadu 636011, India c Department of Energy Studies, Periyar University, Periyar Palkalai Nagar, Salem, Tamil Nadu 636011, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Photosensitivity Transition metal FESEM Methylene Blue
ZnO is widely used as a photocatalytic process due to high photosensitivity, strongly oxidizing nature, non-toxic, favorable band gap energy and the excellent chemical and mechanical stabilities of the material. The present work deals with the synthesis of ZnO doped with transition metal (Sn) by using a simple chemical precipitation method. XRD analysis confirms the substitution of Sn4+ ions into the ZnO matrix by increased lattice spacing and a reduced 2θ angle. Optical properties like reflectance and band gap were studied using UV–Vis–DRS spectroscopy. The morphology of the samples was observed using FESEM and TEM which explains the growth rate and shape of the prepared samples. Methylene Blue (MB) dye was used to evaluate the photocatalytic activity of synthesized ZnO nanoparticles and their performance with respect time were analyzed and presented.
1. Introduction Discharge of wastewater from textile industries into water bodies is causing a huge environmental hazard. The wastewater not only depletes the esthetic quality of water, but also renders it detrimental for aquatic flora and fauna. Of various dyes discharged into water bodies from textile industries, azo dyes constitute the largest portion. These dyes are known to be toxic, carcinogenic and can induce mutations [1]. So elimination of these recalcitrant dyes from the water bodies is an urgent issue. Due to the advancement of science and technology, many methods are available for the eradication of the dye molecules [2]. Recently, advanced oxidation processes (AOPs) have engendered great scientific interest worldwide. AOPs are based upon the in situ generation of hydroxyl radicals, which are responsible for the oxidation of dyes. Some of the AOPs are Fenton reaction [3], ozonation [4], sonophotocatalysis [5], electrochemical oxidation [6] and heterogeneous photocatalysis [7]. Out of these methods heterogeneous photocatalysis has emerged as a very efficient tool for the eradication of dyes. This method involves complete degradation of dye molecules to smaller molecular weight compounds and is carried out under ambient reaction conditions without the requirement of any additional oxidizing agent. Moreover, this is a cost effective technique and unlike other methods, this does not result in the formation of secondary pollutants. Photocatalytic treatment of wastewater by irradiated semiconductor has proven to be an effective process, which can lead to complete ⁎
mineralization of pollutants [8]. Moreover, semiconductor photocatalysts have been widely employed in the production of hydrogen by water splitting [9,10], purification of air [11], and other applications [12]. However, the fast recombination of the photogenerated electrons and holes hinders the commercialization of this technology. Therefore, it is important to promote photocatalytic efficiency by suppressing the recombination [13]. Previous studies indicated that photocatalytic activity largely depends on two factors: reactant adsorption behavior and the separation efficiency of electron–hole pairs [14]. The adsorption capacity can be improved by increasing the specific surface area of the catalysts, while semiconductor modifications by doping with a transitional metal ion [15], coupling with another semiconductor [16], and loading with noble metals [17,18] have been successfully carried out, in order to reduce the recombination of the electrons and holes. Among the various semiconductors, ZnO is widely used as an excellent material for the photocatalytic process due to the photosensitivity, the strongly oxidizing, non-toxic nature, the favorable band gap energy and the excellent chemical and mechanical stabilities of the material [19,20]. Moreover, it absorbs a large fraction of the solar spectrum, making it a perfect candidate for photocatalysis. Photocatalytic activity occurs when the ZnO absorbs a photon with an energy equal or greater than the material’s band gap energy that results in the formation of electron and hole pairs, which can subsequently migrate to the ZnO surface and react with adsorbed molecules to generate such reactive species as H2O2, superoxide anion radicals (•O2−) hydroxyl
Corresponding author E-mail address: [email protected] (N. Kannadasan).
https://doi.org/10.1016/j.mseb.2020.114497 Received 4 April 2018; Received in revised form 23 November 2019; Accepted 20 January 2020 0921-5107/ © 2020 Elsevier B.V. All rights reserved.
Materials Science & Engineering B 253 (2020) 114497
N. Siva, et al.
radicals (•OH) [21,22]. These species are very strong oxidizing and highly reactive agents that can degrade an organic pollutant into harmless compounds. However, ZnO has several weaknesses such as the fast recombination rate of the photogenerated electron and hole pairs and a low quantum yield in photocatalytic reactions in aqueous solutions, which can obstruct the photocatalytic degradation process. Significant effort has been focused to enhance the photocatalytic activity such as the rate of electron and hole pairs induced redox reaction and the rate of electron and hole recombination. It is known that the surface charge transfer processes and strong electron and hole recombination are strongly related to the structure and optical properties of the photocatalysis. Therefore, to enhance the photocatalytic activity the surface charge transfer processes should be increased and the recombination rate of electron and hole should be decreased. Various methods have been developed to reduce electron-hole recombination and increased the surface charge transfer. One of the interesting approaches to doped ZnO photocatalysis with transition metal ions, which have been shown to reduce band gap energy and improve charge separation between electron and hole by forming electron traps [23,24]. The hole will be able to migrate towards the surface of the photocatalysis and adsorbed organic compound. The doping of Sn in ZnO is expected to modify the absorption, luminescent, photocatalytic and other physical or chemical properties of ZnO because of the different structure of the electron shell and the similar size of Sn4+ (0.071 nm) and Zn2+ (0.074 nm) [25,26]. Manjula G. Nair et al. [27] have confirmed that the MB decomposition rate obtained using pure ZnO was much higher than that by doped ZnO. Japinder Kaur, Sonal Singhal [28] has investigated the transition metal doped ZnO prepared by thermally undoped ZnO was found to be better photocatalyst than doped ZnO. Changle Wu et al. [29] reported the Sn-doped ZnO nanorods are a kind of promising photocatalyst in remediation of water polluted by some chemically stable azo dyes. This work deals with the synthesis of ZnO doped with transition metal (Sn) by using a simple chemical precipitation method. Such method is a non-expensive and versatile approach that allows us to synthesize a great variety of crystalline metal oxides at low temperature compared to other synthesis methods [30]. The doping of Sn was chosen due to their comparatively ionic radii to the ionic radius of Zn2+ and ease to synthesize without change the ZnO crystalline structure over a wide range of dopant concentrations. Moreover, it is known than Co and Mn incorporated into ZnO lattices can form an impurity energy level and can facilitate mobility of charge carrier. Here we reported a systematic study on the influence of Sn ion doping degree on the structure, morphology and optical properties of ZnO nanoparticles. The photocatalytic oxidation of Methylene Blue (MB) in water was observed to study the influence of transition metal ions doping degree on the performance of ZnO.
drop to the above mixture. The entire solution was stirred magnetically at room temperature until a white precipitate was formed. The obtained dispersions were purified by dialysis against de-ionized water and ethanol several times to remove impurities. The yield products were dried in hot air oven at 100 °C for 6 h to evaporate water and organic material to the maximum extent. Finally, the obtained product was annealed in a muffle furnace at 300 °C for 3 h. The annealed powders were pulverized to fine powders using agate mortar for further characterizations. A similar method of preparation without the addition of dopant was used to synthesize undoped ZnO nanocrystals. 2.3. Characterization XRD patterns of as-synthesized samples were recorded with a X’PERT PRO diffractometer system with a Cu Kα irradiation (l = 1.5418 Å). Diffuse reflectance spectra were obtained using a Shimadzu UV-2500 spectrophotometer in the wavelength range 100–800 nm. The morphologies of samples were observed using scanning electron microscopes (FE-SEM, JEOL, JSM-6701F) and a high-resolution transmission electron microscopy (HR-TEM, JEOL, JEM-2100). Multipoint N2 adsorption-desorption experiments were carried out on a Micromeritix (ASAP 2020) analyzer using the BET gas adsorption method, at 77 K. 2.4. Photocatalytic experiment For the purpose of studying the photocatalytic activity of Sn doped ZnO nanoparticles, under Sun light irradiation, MB dye was degraded from the catalyst of Pure and Sn (0.075, 0.1 and 0.125 M) doped ZnO nanoparticles at room temperature. A 950 ± 20 W min−2 solar light was used in this dye degradation experiment. Where, 0.05 gm of Pure and Sn (0.075, 0.1 and 0.125 M) doped ZnO nanoparticles was added to a quartz photoreactor containing 100 ml of a 30 mg/l MB aqueous solution. After that, the prepared solution was kept at dark condition for accomplishing the adsorption and desorption equilibrium between dye and photocatalyst. Then, the sample was irradiated under sun light source at a regular time interval followed by filtered. After the filtration process, the obtained solution was utilized to analyze UV–Visible absorption behavior of the organic dyes. In the UV–Visible absorption spectra, the maximum absorption of MB dye was observed at λmax = 664 nm. 3. Result and discussion 3.1. X-ray diffraction The XRD patterns of both the undoped and the Sn-doped ZnO microspheres are shown in Fig. 1a. All the diffraction patterns can be indexed to a hexagonal phase ZnO according to the JCPDS 36–1451 [31] data files before and after Sn doping ((1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), and (2 0 1)). In this figure, the strong, sharp diffraction peaks indicate a high degree of crystallization. As shown in Fig. 1b, the (1 0 1) peak of the Sn-doped ZnO clearly exhibits a shift to the left compared to that of the undoped ZnO peaks. In addition, the other characteristic peaks of the Sn-doped ZnO samples also exhibit a slight shift compared to those of ZnO. It was observed that peak position has slightly shifted to lower diffraction angle in the doped samples when compared to the pure ZnO. This explains that Sn4+ ions have been completely dissolved into the lattice of ZnO. Since the ionic radius of Sn4+ (74 pm) is slightly larger than ionic radius of Zn2+ (60 pm) [32,33], Sn ion is expected to be soluble in ZnO and hence this had resulted in the peak shift to lower diffraction angle. A shift in the peak to lower theta value results the increment in the d-spacing and hence it is expected to increase the cell volume., the lattice parameters of ZnO were expected to increase upon the Sn4+ substitution on the Zn2+ position, thus resulting in an
2. Materials and methods 2.1. Chemicals All the chemicals used in this study are of AR grade with 99% purity (E. Merck chemicals) and used without further purification. Sample preparation and dilutions were made of ultrapure water. Zinc acetate dihydrate [Zn(CH3COO)2*2H2O], Tin Chloride dihyrate [SnCl2.2H2O] and potassium hydroxide (KOH) were used as precursors. 2.2. Synthesis of bare and Sn doped ZnO nanoparticles For the preparation of Sn-doped ZnO nanocrystals, 0.5 M of Zinc acetate dihydrate dissolved in 50 ml of deionized water was stirred vigorously by magnetic stirrer. Then, tin chloride dihyrate of preferred mole (0.05, 0.075, 0.1 and 0.125 M) prepared in 20 ml aqueous were mixed drop by drop to the above and stirred for 30 min. Finally, 2 M of potassium hydroxide in 50 ml of deionized water was added drop by 2
Materials Science & Engineering B 253 (2020) 114497
N. Siva, et al.
reported in the JCPDS data [34]. It can be also noted that the value of the FWHM of the 0.075, 0.10 and 0.125 M Sn-doped ZnO nanoparticles sample is higher than 0.05 M Sn-doped ZnO and pure ZnO crystalline planes, suggesting a reduction of crystallite size with increasing the Sn content. The crystallite size (D) of these samples is estimated using following Scherrer’s formula:
D=
0.89λ βcosθ
where λ is the wavelength of X-rays used; β is the broadening of the diffraction line measured at half its maximum intensity; and θ is the diffraction angle. The cell volume was calculated by using the formula V = 0.866a2c and the Zn-O bond length was deduced by using the equation 1
2
a rZn − O = ⎡ ⎛ ⎞ + (0.5 − u)2c 2⎤ ⎢⎝ 3 ⎠ ⎥ ⎣ ⎦ ⎜
where u =
2
⎟
( ) + 0.25. All the obtained lattice parameters, cell volume a2 3c 2
and Zn-O bond lengths are summarized in Table 1. The calculated average crystallite size of pure ZnO is 26 nm, and it decreases up to 21 nm for the Sn-doped ZnO samples. The decrease in the crystallite size with respect to increase in Sn concentration. The crystallite size decrease with Sn content is likely due to the substitution of tin into the zinc oxide lattice. It is discovered that on doping Sn ion there is a slight increase in the cell parameters (a = b) and bond length (rZn-O) which results in the expansion of the cell volume of the doped samples when compared to pure ZnO. In addition to this, the synthesized ZnO has cell volume exact value than that of the standard value for the bulk material i.e. 47.62 Å3 (JCPDS). Also, As a result of doping the c/a value reduces and the average bond length of the Zn-O has slightly increased from 1.9778 Å to 1.9779 Å, 1.9789 Å, 1.9795 Å and 1.9787 Å in Sn (0.05, 0.075, 0.1 and 0.125 M) doped ZnO samples. Hence, it can be understood that difference in the valency and ionic radii of the dopant (Sn4+) and the host (Zn2+) atoms has resulted in such incremental in the cell parameters, bond length and cell volume. 3.2. UV–Vis reflectance spectra For semiconductor materials, diffuse reflectance spectroscopy is a useful tool for characterizing the optical property, which is considered to be a key component in photocatalytic performance [35]. Fig. 2a shows the UV–Vis DRS of both the pure ZnO and the Sn-doped ZnO. For the undoped ZnO an absorption edge steeply rises to about 422 nm whereas, the Sn-doped ZnO exhibits red shifts of the absorption edge as well as a significant enhancement of light absorption in the 400–600 nm range (Fig. 2a). In general, there are a great number of oxygen vacancies on nanosized ZnO surfaces, and oxygen vacancies can give rise to surface states. The surface states are located between the valence and the conduction band. Therefore, the optical absorption between 400 and 600 nm is closely related to the surface states. The band gap of the samples were calculated using the Kubelka–Munk relation to convert the reflectance into a Kubelka–Munk function F(Rα), using the relation
Fig. 1. (a) X-ray diffraction patterns of pure and Sn doped ZnO nanoparticles. (b) X-ray diffraction patterns of pure and Sn doped ZnO nanoparticles-enlarged view of (1 0 0), (0 0 2) and (1 0 1) peaks.
increased lattice spacing and a reduced 2θ angle. This result revealed that the Sn4+ ions have been incorporated into the crystal lattice of ZnO and substituted for the fractional Zn ions. Moreover, the characteristic peaks of the Sn hybrid are not detected due to the high dispersion and low content of the Sn compound. The lattice parameters, ‘a’ and ‘c’, were calculated using equations:
sin2 θ =
F (R α ) =
(1 − R α )2 2R α
where, Rα is the reflectance of an infinitely thick sample with respect to a reference at each wavelength. The Kubelka–Munk function is a function equivalent to the absorption coefficient. The bandgap energy of the nanoparticles were calculated from the slope of the graph where [F(Rα)hν]2 were plotted against photon energy hν are 3.43 eV, 3.31 eV, 3.19 eV, 3.07 eV and 3.13 eV for pure ZnO and Sn (0.05, 0.075, 0.1 and 0.125 M) doped samples, respectively (Fig. 2b). This indicates that the Sn-doped ZnO possessed a narrower band gap compared to that of the undoped ZnO. Furthermore, this lower energy
λ2 2 λ2 (h + k 2 + hk ) + 2 (l 2) 3a2 4c
These values are presented in Table 1. It is observed that the lattice constant value of ZnO nano crystalline is almost the same as the one 3
Materials Science & Engineering B 253 (2020) 114497
N. Siva, et al.
Table 1 XRD derived parameters, Bond length and Band gap energy of ZnO and Sn doped ZnO nanoparticles. Samples
ZnO ZnO: ZnO: ZnO: ZnO:
Sn Sn Sn Sn
Crystallite size (nm)
(0.05 M) (0.075 M) (0.1 M) (0.125 M)
26 26 25 21 22
Lattice parameters (Å) a=b
c
c/a
3.248 3.249 3.252 3.254 3.251
5.213 5.211 5.209 5.207 5.210
1.6199 1.6038 1.6017 1.6001 1.6025
Cell Volume (Å3)
Bond length (Å)
Band gap Eg (eV)
47.6253 47.6363 47.7060 47.7464 47.6858
1.9778 1.9779 1.9789 1.9795 1.9787
3.43 3.31 3.19 3.07 3.13
lattice, the band gap of the resulting solid solution increases as SnO has a higher band gap (3.0 eV) [32] than ZnO. Thus, Sn (0.125 M) is incorporated into the ZnO lattice, its band gap further increases to 3.13 eV. Since localized 4 s orbital’s of Zn constitute the conduction band of ZnO, the addition of Sn to the ZnO lattice influences the conduction band by admixing extended Sn 5 s orbitals [34]. Thus, the conduction band is shifted toward the higher energy and a systematic increase in the valence band conduction band separation occurs with increasing amount of Sn in the ZnO lattice. This will lead to an increase in its Fermi level, and consequently the oxidation–reduction potential, impacting the photo catalytic property of ZnO. 3.3. Field emission scanning electron microscopy The synthesized pure ZnO and Sn-doped ZnO samples have been characterized by FE-SEM (see Fig. 3a–d). The images show the clear morphological changes due to the 0.1 M of Sn doped ZnO nanoparticles. The agglomerated spherical structures in the pure ZnO sample are shown in Fig. 3a. The size of these crystalline structures is in the range of ~15–45 nm. The ZnO nanostructure seen above change dramatically on 0.1 M Sn doped to ZnO nanoparticles. The morphological change recorded by FESEM for the less agglomerated spherical crystallites are observed in Sndoped ZnO samples. The size of these crystalline structures is in the range of ~10–35 nm. 3.4. Energy-dispersive spectrum Energy-dispersive spectrum analysis is performed to investigate the elemental composition of Sn-doped ZnO nanostructures. EDS analysis confirmed the presence of Zn, O, and Sn elements in the nanostructures. Fig. 3e exhibits a pure ZnO spectrum with two high intense peaks and single small peak, which are associated with O and Zn atoms, respectively. 3.5. Transmission electron microscopy
Fig. 2. (a) UV–Vis–diffuse reflectance spectra of pure and Sn doped ZnO nanoparticles. (b) Plot of band gap energy for pure and Sn-doped ZnO nanoparticles.
Fig. 4 (a, b) shows the TEM micrographs of pure ZnO and 0.1 M Sndoped ZnO nanoparticles. TEM micrographs in Fig. 4a shows spherical shaped ZnO nanoparticles, having size about 40–60 nm. Fig. 4b shows that the spherical nano assemblies present on the 0.1 M of Sn-doped ZnO sample, with size in the range of 10–25 nm, which is in good agreement with the crystallite size obtained from XRD line broadening. The morphological change we observed can be then explained on the basis of role of Sn4+ in governing the growth rate of planes of ZnO nanocrystals originated by different surface charges on ZnO planes. In the presence of a high concentration of tin, the adsorption of Sn4+ filling the ZnO growth sites suppresses the growth rate along c-axis. This balance of the crystal facets surface energy leads, consequently, to formation of particles with a rounded shape form.
value has been attributed to the strong absorption in the visible region due to localized levels existing in the forbidden gap because of the Sn doping. Upon irradiation with visible light, an electron–hole pair is generated within the effective band gap: an electron transition takes place for the new additional valence state to a conduction state. This transition requires smaller excitation energy compared with a ZnO inhabitant band gap (3.43 eV) depending on the particular dopant level within the band gap [36]. Therefore, it can be used as an efficient photo catalyst under visible-light irradiation. In addition, it can be seen that the band gap decreases with the increasing Sn content at low doping levels and reaches a minimum for Sn-doped ZnO (0.1 M). These reductions in the band gap attribute to a change in the position of the valence band because of the introduction of Sn 5 s orbitals [37]. With the increasing Sn concentration in the ZnO
3.6. BET studies The 4
nitrogen
adsorption-desorption
isotherms
and
the
Materials Science & Engineering B 253 (2020) 114497
N. Siva, et al.
Fig. 3. FE-SEM images of pure (a) and Sn (0.1 M) doped (c), corresponding size distribution curve (b, d) and EDS spectra of Sn (0.1 M) doped ZnO nanopartcles (e).
whereas, for Sn-doped ZnO, they are 54.19 m2g−1 and 0.511 cm3g−1 respectively. The high specific surface area of Sn-doped ZnO material can yield high photocatalytic activity. The inset in Fig. 5(a, b) shows the pore size distribution of bare and Sn-doped ZnO nanocrystals. The undoped and Sn-doped ZnO exhibit
corresponding pore size distribution curves of the pure and 0.1 M of Sndoped ZnO samples are shown in Fig. 5 (a, b). Both the samples exhibit a type IV isotherm, indicating the capillary condensation of the mesopore structure [38]. The specific surface area and the pore volume are calculated to be 29.86 m2 g−1 and 0.259 cm2 g−1, for pure ZnO,
Fig. 4. (a, b) TEM micrographs of undoped and Sn (0.1 M) doped ZnO nanoparticles. 5
Materials Science & Engineering B 253 (2020) 114497
N. Siva, et al.
3.9. Turnover number The effectiveness of the catalyst is expressed in this reaction system a “turnover number” (TON) [39].
TON =
(% Conversion )(Number of moles of substrate ) Number of moles of catalyst
3.10. Turnover frequency (TOF) The number of reaction cycles place in a given time period and is usually employed to study the reaction rate [40].
TOF =
TON Time
3.11. Estimation of the degradation percentage and rate constant The absorption spectra of irradiated samples were recorded at various time intervals and the rate of decolorization of MB was observed in terms of the change in the intensity at 664.5 nm. To study the enhancement of the photoactivity, comparative experiments were deployed. The experimental results demonstrated that the concentrations of MB revealed no degradation or reduction after 120 min in the absence of doped nanoparticles or light irradiation. It is suggested that the hydrolysis and photochemical processes can be neglected and that the photocatalytic experiment occurred in a pure photocatalytic system [41,42]. To understand the response of the immersed Sn-doped ZnO nanoparticles on the percentage of degradation of MB, have been followed under solar irradiation light at various time intervals with a UV–Vis spectrophotometer. The MB degradation results with immersed Sn-doped ZnO nanoparticles in the dark. Fig. 6 (a, b) and Fig. 7 shows the degradation percentage of the MB efficiency as a function of irradiation time in the presence of Sn-doped ZnO nanoparticles with different doping concentrations. Several researchers have reported that the kinetic behavior of a photocatalytic reaction can be described by a pseudo-first order model. The photocatalytic activity of all doped ZnO nanoparticles in this study obeys the pseudo-first-order reaction kinetics (Fig. 8 a, b). The apparent reaction rate constants k is 0.00517, 0.00667, 0.01263 and 0.00982 min−1 of the MB degradation using pure and (0.075, 0.1, 0.125 M) Sn doped ZnO nanoparticles, respectively. The rate constant values are found to be higher for 0.1 M Sn doped ZnO (0.01263 min−1) it is clear and the reason why 0.1 M Sn doped ZnO performs well in the photodegradation of MB.
Fig. 5. (a) The N2 adsorption – desorption isotherm and the BJH pore size distribution curves (Insect) of Pure ZnO nanoparticles. (b) The N2 adsorption – desorption isotherm and the BJH pore size distribution curves (Insect) of Sn (0.1 M) doped ZnO nanoparticles.
peaks at 22.24 and 47.12 nm, respectively. The Sn-doped ZnO has higher pore size and higher pore volume than the bare ZnO. The higher pore size and higher pore volume contribute to the effective electron transport at the dye degradation process.
3.12. Reason for the enhanced photocatalytic activity
3.7. Photocatalytic activity
Photocatalytic activity of ZnO has increased manifold upon doping with Sn ion. From the photocatalytic results it is observed that the highest photocatalytic activity was obtained for the 0.1 M Sn doped ZnO and the activity decrease gradually with further doping. To investigate the possible factors responsible for the highest activity we look into the analysis of structural, optical morphological, surface area, number of active sites, turnover number and turnover frequency properties. Looking into the structural variation, when Sn4+ larger ionic radii than Zn2+ with slightly is doped into ZnO lattice, it results in the expansion of the lattice i.e. increase in the bond length (rZn-O) and volume expansion. With an increase in the volume there is a small decrease in the band gap (see Table 1) for lower doping content (up to 0.1 M), after which the band gap again increases due to the starting formation of the secondary phase Zn2SnO4. This decrease in the bandgap has two major advantages, i.e. firstly; lower bandgap can facilitate electron excitation with comparatively low energy and hence can meet the requirements for visible light photocatalysis, and thereby the numbers of electrons are available in the CB for the photocatalytic reaction which leads to the enhanced photodegradation of the dye. In
To evaluate photocatalytic activity of ZnO and doped ZnO, degradation of MB was studied. The photocatalytic efficiency of catalysts was evaluated in terms of percentage of dye degraded using the following equation [35].
Percentage of degradation =
Co − Ct × 100 Co
where Co is the initial concentration of MB dye solutions (mg/L), Ct is the concentration of dye after irradiation after selected time interval (mg/L). 3.8. Number of active sites for Sn doped ZnO nanoparticles For adsorptions that are the effective sites for a particular heterogeneous catalytic reaction is called the active sites of the catalytic. The number of active sites depends on the concentration of the metal ions in the specified amount of the catalyst taken for a particular reaction. 6
Materials Science & Engineering B 253 (2020) 114497
N. Siva, et al.
Fig. 6. (a–d) Time dependent UV–Vis absorption spectra of the photocatalytic degradation of MB in the presence of pure and (0.075, 0.1 and 0.125 M) Sn doped ZnO nanoparticles.
generation of a hole (h+) in the VB. The photoelectron can easily be trapped by electronic acceptors like adsorbed O2, to further produce a superoxide radical anion (•O2¯) whereas, the photoinduced holes can be easily trapped by electron donors, such as OH¯ or organic pollutants, to further oxidize organic pollutants [35,36]. In addition to reacting with electron donors or acceptors adsorbed on the semiconductor surface mentioned above, the photogenerated electrons and holes can also recombine and dissipate the input energy as heat or get trapped in metastable states [43,44]. If a suitable scavenger or defect state is available to trap the electron or hole, recombination is prevented and subsequent redox reactions can occur [45].
addition to this, doping introduces intermediate states within the conduction band and valence band. These intermediate states help in trapping the excited electron that causes enhanced charge separation, thus inhibiting recombination. This is because after a certain critical value of doping, these impurity energy levels instead of acting as a charge separation center behave like a recombination center, thus facilitating recombination process. Further, these results are concordant with the calculated TON and TOF values given in Table 2. The observed the table, the degradation rate enhanced can be attributed to the Sn (0.1 M) doped ZnO nanoparticles, which can efficiently absorb photons under solar light compared to pure and other Sn doped ZnO nanoparticles.
Photocatalytic + hν → hVB++ eCB− Dye + hVB+→ Dye+→ Final species
3.13. Degradation mechanism
Dye + OH·→ Dye·→ Final species
When semiconductor nanocrystals are irradiated by light with energy higher or equal to the band gap, an electron (e¯) in the valence band (VB) can be excited to the conduction band (CB) with the
Fig. 9 shows that illustrate the band gap modification and 7
Materials Science & Engineering B 253 (2020) 114497
N. Siva, et al.
Table 2 Determination of the active sites, TON and TOF in the solar system for all the photocatalysts. Photocatalyst
Number of active sites 5 mg (×10−4 mol)
TON (×10−3 min−1)
TOF (×10−5 min−1)
ZnO ZnO: Sn (0.075 M) ZnO: Sn (0.1 M) ZnO: Sn (0.125 M)
6.144 5.853 6.606 5.659
7.938 9.775 11.926 10.080
6.615 8.145 9.938 8.400
Fig. 7. Plot of C/C0 versus irradiation time for the degradation of MB under solar light.
introduction of intermediate states with respect to Sn ion doping in the ZnO lattice. These defect states resulted in the delayed recombination thus making more electrons available for photocatalysis and thus increases the degradation rate.
Fig. 9. Schematic diagram representing the degradation mechanism in pure and Sn doped ZnO.
ZnO + hν → hVB++ eCB− nanoparticles were synthesized using simple and cost effective chemical precipitation method. The crystalline properties of the samples were analyzed using XRD and it reveals that the crystalline size decreases with the increase in doping concentrations due to the substitution of Sn4+ ions into ZnO lattices. Optical properties of ZnO nanoparticles were studied using UV–Vis–DRS spectroscopy and the reflectance and the band gap value decreases with increase in Sn doping concentrations. FESEM images reveal the formation of nanoparticles and their sizes were in nanometer region. EDS spectrum confirms the presence of Sn, Zn and O in the samples. TEM result shows the spherical nano assemblies for 0.1 M of Sn-doped ZnO sample, in the size ranges from 10 to 25 nm. Methylene Blue dye was used to evaluate the photocatalytic activity of synthesized ZnO nanoparticles. The effectiveness of the catalyst at very low concentrations is suggested by the calculation of the number of active sites, TON and TOF for all the systems under solar light illumination. The apparent reaction rate constants k is 0.00517 and 0.01263 min−1 of the MB degradation using 0.1 M Sn doped and
Sn4++ e−→ Sn3+ Sn3++ O2 → O2·−+ Sn4+ 3.14. Recyclability of Sn (0.1 M) doped ZnO nanoparticles The stability and reusability of the prepared Sn (0.1 M) doped ZnO nanoparticle photocatalyst was checked by using 5 mg of the catalyst for three cycles and in the third cycle, the percentage of degradation found 76.25% due to the loss of some amount of catalyst while washing and filtrating. The results unambiguously demonstrate that the Sn (0.1 M) doped ZnO photocatalyst is stable, efficient and can be reused for up to three cycles. 4. Conclusion As prepared and different concentrations of Sn doped ZnO
Fig. 8. (a, b) Effect of MB degradation and rate constant values of pure and different levels of Sn doped ZnO nanoparticles. 8
Materials Science & Engineering B 253 (2020) 114497
N. Siva, et al.
ZnO nanoparticles respectively. Sn (0.1 M) doped ZnO nanoparticles shows the higher photocatalytic activity of visible light illumination.
[22]
Declaration of Competing Interest [23]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[24]
[25]
References
[26]
[1] F.M.D. Chequer, T.M. Lizier, R. de Felício, M.V.B. Zanoni, H.M. Debonsi, N.P. Lopes, R. Marcos, D.P. de Oliveira, Analyses of the genotoxic and mutagenic potential of the products formed after the biotransformation of the azo dye Disperse Red 1, Toxic. Vitro 25 (2011) 2054–2063. [2] T. Robinson, G. McMullan, R. Marchant, P. Nigam, Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative, Bioresour. Technol. 77 (2001) 247–255. [3] M.S. Lucas, J.A. Peres, Decolorization of the azo dye reactive black 5 by fenton and photo-fenton oxidation, Dyes Pigm. 71 (2006) 236–244. [4] L. Zhao, J. Ma, Z. Sun, H. Liu, Mechanism of heterogeneous catalytic ozonation of nitrobenzene in aqueous solution with modified ceramic honeycomb, Appl. Catal. B 89 (2009) 326–334. [5] Y. He, F. Grieser, M. Ashokkumar, The mechanism of sonophotocatalytic degradation of methyl orange and its products in aqueous solutions, Ultrason. Sonochem. 18 (2011) 974–980. [6] A. Bedoui, M.F. Ahmadi, N. Bensalah, A. Gadri, Comparative study of Eriochrome black T treatment by BDD-anodic oxidation and Fenton process, Chem. Eng. J. 146 (2009) 98–104. [7] A.R. Khataee, M. Zarei, R. Ordikhani-Seyedlar, Heterogenous photocatalysis of a dye solution using supported TiO2 nanoparticles combined with homogenous photoelectrochemical process: molecular degradation products, J. Mol. Catal. A: Chem. 338 (2011) 84–91. [8] L. Zhang, H. Cheng, R. Zong, Y. Zhu, Photocorrosion Suppression of ZnO Nanoparticles via Hybridization with Graphite-like Carbon and Enhanced Photocatalytic Activity, J. Phys. Chem. C 113 (2009) 2368–2374. [9] M. Gratzel, Photoelectrochemical cells, Nature 414 (2001) 338–344. [10] J. Zhu, D. Chakarov, M. Zäch, in: L. Zang (Ed.), Energy Efficiency and Renewable Energy Through Nanotechnology, Springer, London, 2011, 441–486. [11] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental Applications of Semiconductor Photocatalysis, Chem. Rev. 95 (1995) 69–96. [12] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Photogeneration of Highly Amphiphilic TiO2 Surfaces, Nature 388 (1997) 431–432. [13] H. Fu, T. Xu, S. Zhu, Y. Zhu, Photocorrosion Inhibition and Enhancement of Photocatalytic Activity for ZnO via Hybridization with C, Environ. Sci. Technol. 42 (2008) 8064–8069. [14] Q. Xiao, Z. Si, J. Zhang, C. Xiao, X. Tan, Photoinduced hydroxyl radical and photocatalytic activity of samarium-doped TiO2 nanocrystalline, J. Hazard. Mater. 150 (2008) 62–67. [15] W.Y. Choi, A. Termin, M.R. Hoffmann, The role of metal ion dopants in quantumsized TiO2: correlation between photoreactivity and charge carrier recombination dynamics, J. Phys. Chem. 98 (1994) 13669–13679. [16] C. Li, T. Ahmed, M. Ma, T. Edvinsson, J. Zhu, A facile approach to ZnO/CdS nanoarrays and their photocatalytic and photoelectrochemical properties, Appl. Catal. B 138–139 (2013) 175–183. [17] C.G. Silva, R. Juarez, T. Marino, R. Molinari, H. Garcia, Influence of Excitation Wavelength (UV or Visible Light) on the Photocatalytic Activity of Titania Containing Gold Nanoparticles for the Generation of Hydrogen or Oxygen from Water, J. Am. Chem. Soc. 133 (2011) 595–602. [18] A. Primo, T. Marino, A. Corma, R. Molinari, H. Garcia, Efficient Visible-Light Photocatalytic Water Splitting by Minute Amounts of Gold Supported on Nanoparticulate CeO2 Obtained by a Biopolymer Templating Method, J. Am. Chem. Soc. 133 (2011) 6930–6933. [19] M. Shamshi Hassan, Touseef Amna, O-Bong Yang, Hyun-Chel Kim, MyungSeob Khil, TiO2 nanofibers doped with rare earth elements and their photocatalytic activity, Ceram. Int. 38 (2012) 5925–5930. [20] S. Anandan, A. Vinu, T. Mori, N. Gokulakrishnan, P. Srinivasu, V. Murugesan, K. Ariga, Photocatalytic degradation of 2,4,6-trichlorophenol using lanthanum doped ZnO in aqueous suspension, Catal. Commun. 8 (2007) 1377–1382. [21] K. Thongsuriwong, P. Amornpitoksuk, S. Suwanboon, Photocatalytic and
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38] [39] [40]
[41] [42]
[43]
[44]
[45]
9
antibacterial activities of Ag-doped ZnO thin films prepared by a sol–gel dip-coating method, J. Sol-Gel Sci. Technol 62 (2012) 304–312. Umar Ibrahim Gaya, Abdul Halim Abdullah, Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: a review of fundamentals, progress and problems, J. Photochem. Photobiol. C: Photochem. Rev. 9 (2008) 1–12. Ruby Chauhan, Ashavani Kumar, Ram pal chaudhary, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 98 (2012) 256–264. Ruby Chauhan, Ashavani Kumar, Ram Pal Chaudhary, Photocatalytic studies of silver doped ZnO nanoparticles synthesized by chemical precipitation method, J. Sol-Gel Sci. Technol. 63 (2012) 546–553. F.J. Sheini, M.A. More, S.R. Jadkar, K.R. Patil, V.K. Pillai, D.S. Joag, Observation of Photoconductivity in Sn-Doped ZnO Nanowires and Their Photoenhanced Field Emission Behavior, J. Phys. Chem. C 114 (2010) 3843–3849. J.-H. Sun, S.-Y. Dong, J.-L. Feng, X.-J. Yin, X.-C. Zhao, Enhanced sunlight photocatalytic performance of Sn-doped ZnO for Methylene Blue degradation, J. Mol. Catal. A: Chem 33 (2011) 145–150. Manjula G. Nair, M. Nirmala, K. Rekha, A. Anukaliani, Structural, optical, photo catalytic and antibacterial activity of ZnO and Co doped ZnO nanoparticles, Mater. Lett. 65 (2011) 1797–1800. Japinder Kaur, Sonal Singhal, Facile synthesis of ZnO and transition metal doped ZnO nanoparticles for the photocatalytic degradation of Methyl Orange, Ceram. Int. 40 (2014) 7417–7424. Wu. Changle, Li Shen, Yu. Huaguang, Qingli Huang, Yong Cai Zhang, Synthesis of Sn-doped ZnO nanorods and their photocatalytic properties, Mater. Res. Bull. 46 (2011) 1107–1112. N. Kannadasan, N. Shanmugam, K. Sathishkumar, S. Cholan, R. Ponnguzhali, G. Viruthagiri, The effect of Ce4+ incorporation on structural, morphological and photocatalytic characters of ZnO nanoparticles, Mater. Charact. 97 (2014) 37–46. N. Kannadasan, N. Shanmugam, K. Sathishkumar, S. Cholan, R. Ponnguzhali, G. Viruthagiri, Optical behavior and sensor activity of Pb ions incorporated ZnO nanocrystals, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 143 (2015) 179–186. C.B. Fitzgerald, M. Venkatesan, L.S. Dorneles, R. Gunning, P. Stamenov, J.M.D. Coey, Magnetism in dilute magnetic oxide thin films based on SnO 2, Phys. Rev. B. 74 (2006) 115307. R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. Sect. A. 32 (1976) 751–767. Zhanchang Pan, Pengwei Zhang, Xinlong Tian, Guo Cheng, Yinghao Xie, Huangchu Zhang, Xiangfu Zeng, Chumin Xiao, Hu. Guanghui, Zhigang Wei, Properties of fluorine and tin co doped ZnO thin films deposited by sol–gel method, J. Alloy. Compd. 576 (2013) 31–37. Haohao Wang, Ripon Bhattacharjee, I-Ming Hung, Langkai Li, Renjie Zeng, Material characteristics and electro chemical performance of Sn-doped ZnO spherical-particle photoanode for dye-sensitized solar cells, Electrochim. Acta 111 (2013) 797–801. M. Oshikiri, M. Boero, J.H. Ye, Z.G. Zou, G. Kido, Electronic structures of promising photocatalysts InMO4 (M = V, Nb, Ta) and BiVO4 for water decomposition in the visible wavelength region, J. Chem. Phys. 11 (2002) 77313–77318. Y.Q. Lei, G.H. Wang, S.Y. Song, W.Q. Fan, M. Pang, J.K. Tang, H.J. Zhang, Room temperature, template-free synthesis of BiO2I hierarchical structures: visible-light photocatalytic and electrochemical hydrogen storage properties, Dalton Trans. 39 (2010) 3273–3278. S.K. Mehar, G. Ranga Rao, Ultralayered Co3O4 for high performance supercapacitor applications, J. Phys. Chem. C 115 (31) (2011) 15646–15654. M. Boudart, Turnover rates in heterogeneous catalysis, Chem. Rev 95 (1995) 661–666. L. Gomathi Devi, R. Shyamala, Photocatalytic activity of SnO2–a-Fe2O3 composite mixtures: exploration of number of active sites, turnover number and turnover frequency, Mater. Chem. Front 2 (2018) 796. R. Ullah, J. Dutta, Photocatalytic degradation of organic dyes with manganesedoped ZnO nanoparticles, J. Hazard Mater 156 (2008) 194–200. Y. Hosogi, H. Kato, A. Kudo, Photocatalytic activities of layered titanates and niobates ion-exchanged with Sn2+ under visible light irradiation, J. Phys. Chem. C 112 (2008) 17678–17682. Y. Hosogi, Y. Shimodaira, H. Kobayashi, A. Kato, Role of Sn2+ in the Band Structure of SnM2O6 and Sn2M2O7 (M = Nb and Ta) and Their Photocatalytic Properties, Chem Mater 20 (2008) 1299–1307. Y. Peltzer, E.L. Blanca, A. Svane, N. Christensen, C. Rodri Guez, O. Cappannini, M. Moreno, Calculated static and dynamic properties of beta-Sn and SnO2 compounds, Phys. Rev. B 48 (1993) 15712–15718. M. Radeeka, P. Pasierb, K. Zakrzewska, M. Rekas, Transport properties of (Sn, Ti)O2 polycrystalline ceramics and thin films, Solid State Ionics 119 (1999) 43–48.