- Email: [email protected]
Tuning the photocatalytic activity of ZnO by TM (TM = Fe, Co, Ni) doping
Materials Science in Semiconductor Processing 91 (2019) 333–340
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Tuning the photocatalytic activity of ZnO by TM (TM = Fe, Co, Ni) doping A. Mondal, N. Giri, S. Sarkar, S. Majumdar, R. Ray
⁎
T
Department of Physics, Jadavpur University, Kolkata 700032, India
A R T I C LE I N FO Keywords: Photocatalysis XPS Zn1-xTMxO (TM = Fe, Co, Ni) Methylene Blue
A B S T R A C T
The present study exhibits the enhancement of photocatalytic activity of transition metal (TM) doped ZnO (Zn1with TM = Fe, Co, Ni) towards aqueous solution of organic dyes such as methylene blue (MB) under ultraviolet and visible light irradiation. Wurtzite Zn1-xTMxO nanoparticles are synthesized by co-precipitation method with different x ranging from zero to almost maximum attainable dopant (x) by this method and characterized by X-ray diffraction, scanning electron microscopy, UV–vis diffuse reflectance spectroscopy and Xray photo electron spectroscopy. UV–vis diffuse reflectance spectroscopy illustrates the gradual red shift of ZnO band gap due to Fe and Co doping. The photocatalytic activity is found to increase gradually with Co doping (x) in Zn1-xCoxO whereas Zn1-xNixO causes a maximum degradation of (~83%) of MB for x = 0.07 which is much larger compared to that (~50%) due to pure ZnO for 150 min irradiation. However, enormous degradation (~94.5%) of MB by Zn1-xFexO under the same condition for x = 0.15 makes this member the best photocatalyst among Zn1-xTMxO (TM= Fe, Co. Ni) series.
xTMxO,
1. Introduction Photocatalysis under solar irradiation has attracted much attention as a promising way to address the environmental problems, especially removal of the non-biodegradable dyes from waste water stream. Some of the most adequate and extensively studied semiconductors that offer the potential for elimination of pollutants are TiO2, ZnO, CdS, V2O5, WO3 and α-Fe2O3 [1–5]. Among these, ZnO has emerged as an efficient and promising candidate in the environmental management system due to its photosensitivity, the strongly oxidizing power [6], environmental friendly nature and the excellent chemical and mechanical stabilities. Photocatalytic activity occurs when ZnO absorbs UV light with energy equal to or greater than its band gap and forms electron-hole pairs. These electron-hole pairs subsequently migrate to the ZnO surface and react with adsorbed molecules to generate reactive species as H2O2, superoxide anion radicals (∙O2 − ) and hydroxyl radicals (∙ OH) [7,8]. These strongly oxidizing and highly reactive agents can degrade an organic pollutant into nontoxic compound. However, ZnO has several shortcomings as photocatalyst such as wide band gap~ 3.37 eV [9] and large exciton binding energy ~ 60 meV [10] which leads to fast recombination of the photogenerated electron and hole pairs. As solar spectrum contains 46% visible, 47% IR and 5–7% of UV light, practical application of ZnO as photocatalyst demands absorption of light in visible range or IR. This requires narrowing of the band gap which can be achieved by doping with transition metal ions [11,12]. This doping
⁎
was also found to improve the separation between electron hole by forming electron traps. The hole will be able to migrate towards the surface of the photocatalyst and disintegrate the adsorbed pollutant. Thus surface area and surface defects of the nanoparticles play an important role in photocatalytic activity. Xu et al. have found that the photocatalytic properties of ZnO can be improved by doping Co2+ (0.5–5 mol%) by hydrothermal method [13]. The degradation rate of the MB dye has been studied by Kant et al. [14]. He reported the improvement of photocatalytic properties of Zn1-xNixO prepared by sol – gel technique with Ni doping in the range 0 ≤x ≤ 0.5. Yu et al. [15] reported a striking enhancement of photocatalytic activity of Zn1-xFexO for x = 0.01 and revealed that large surface defect is responsible for the enhanced activity. A comparative study of photocatalytic activity of ZnO nanoparticles doped with transition metals (Mn and Co) upto 12 at% prepared by a co-precipitation method has been reported by Saleh et al. [11]. As both surface area and surface defects of the nanoparticles are very much sensitive of synthesis procedure it greatly influences the photocatalytic activity. Here we have performed a comparative study of the photocatalytic activity of Zn1xTMxO (TM = Fe, Co, Ni) prepared by co-precipitation method. It is noteworthy that doping is not only confined within the diluted magnetic system (DMS) regime where x is in the range 0 ≤x <0.1, rather attempt was made for higher possible doping of TM which is attainable by the following synthesis procedure. We have used methylene blue (MB) as a pollutant, which is a hazardous dye as it causes hyper tension,
Corresponding author. E-mail address: [email protected] (R. Ray).
https://doi.org/10.1016/j.mssp.2018.12.003 Received 25 April 2018; Received in revised form 27 November 2018; Accepted 3 December 2018 1369-8001/ © 2018 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 91 (2019) 333–340
A. Mondal et al.
cyanosis, jaundice, shock. 2. Experimental details 2.1. Synthesis Zn1-xTMxO (TM = Fe, Co, Ni) nanoparticles with varying TM content (x), were synthesized in de-ionized water ambiance [16]. For Fe, Co, and Ni doping anhydrous FeCl3, (CH3COO)2Co·4H2O and (CH3COO)2Ni·4H2O were selected as precursor. Zinc acetate and precursor were taken in stoichiometric ratio as required by molecular formula. Aqueous (25 ml) precursor of required concentration was added drop by drop to 50 ml aqueous (CH3COO)2Zn·2H2O having required amount of Zn2+ as calculated from formula unit. A homogeneous solution was obtained by adding 1 g of PVP in 50 ml de-ionized water before adding the precursor. Then 0.4 g of 50 ml sodium hydroxide was added drop by drop to the above mixture which results a yellow voluminous precipitate. The precipitate was then isolated by centrifuging it against de-ionized water and ethanol several times. The product was dried at 120 °C for 2 h in air. Thus Zn1-xFexO (x = 0.0, 0.03, 0.06, 0.09, 0.13, 0.15, 0.20), Zn1-xNixO (x = 0.01, 0.02, 0.03, 0.07, 0.09, 0.11) and Zn1-xCoxO (x = 0.05, 0.10, 0.15, 0.25) were prepared successfully and selected for further study. Here, attempt was made to prepare each Zn1-xTMxO (TM= Fe, Co, Ni) series with different dopant (x) starting from low value to almost maximum value attainable by this co-precipitation method. We could prepare Zn0.80Fe0.20O, Zn0.75Co0.25O and Zn0.89Ni0.11O with maximum dopant content with Fe, Co and Ni respectively. An attempt for higher dopant of TM yielded phase segregation of transition metal oxide. Sincere care was taken to select only those doped samples for further study which were of single phase wurzite structure and any other impurity phase was absent. 2.2. Characterization: structure and morphology Crystal structures were studied at room temperature using a powder X-ray diffractometer (XRD) (Bruker D8 Advance) with the Cu Kαradiation. The morphology of the nanoparticles was studied in field emission scanning electron microscopy (FE-SEM) images using the microscope, JEOL (model: JSM-7610F). UV–VIS diffuse reflectance spectrophotometer (Shimadzu, UV 2401PC) was used to study optical absorption spectra in the wavelength region 200– 800 nm. 2.3. Photocatalysis study Photocatalytic activities of the Zn1-xTMxOnanoparticles were studied by degradation of MB dye aqueous solution under irradiation of Ultraviolet (UV) and visible (Vis) light. Photocatalysis studies were performed by using a home-made photo reactor [dimension of 1.5 ft × 1.5 ft × 1 ft (length ×breadth× height)] fitted with a mercury lamp (300 W, 280 < λ < 400 nm) and a xenon lamp (500 W, 400 < λ < 700 nm). Similar UV–Vis source was also used by Yu et al. [15]. Aqueous dispersion of Zn1-xTMxO nanoparticles were prepared by adding 0.04 g of Zn1-xTMxO into 100.0 ml of 5 ppm MB dye solution. The pH of this solution was 7.1–7.2. The dispersion was stirred in dark for 1 h so that MB dye molecules got adsorbed on the surface of nanocomposite. The homogeneous mixture was than irradiated by ultraviolet (UV) and visible light. As a preventive measure of loss of light, the beaker was covered by Al foil from all sides. After irradiation for a desired time, about 10 ml of the aqueous dispersion was sampled out and centrifuged to separate the Zn1-xTMxO photocatalysts. The concentration of residual MB in the filtrate was determined by Shimadzu UV-3101PC UV–VIS spectro-photometer. The mineralization degree was detected by a Shimadzu TOC-VCBH Total Organic Carbon (TOC) analyzer. To estimate the percentage of mineralization of MB dye by TOC measurement its concentration, mass of photocatalyst and irradiation conditions were kept same as were done in photocatalytic
Fig. 1. a. X-ray diffraction patterns of Zn1-xFexO with 0.0 ≤ x ≤ 0.20 at 300 K. b. X-ray diffraction patterns of Zn1-xCoxO with 0.0 ≤ x ≤ 0.25 at 300 K. c. Xray diffraction patterns of Zn1-xNixO with 0.0 ≤ x ≤ 0.11 at 300 K.
experiment.
3. Results and discussions 3.1. Crystal structure and morphology: XRD, EDX and FESEM studies Fig. 1a, b, and c illustrate the X-ray diffraction (XRD) peaks of each Zn1-xTMxO (TM = Fe, Co, Ni) series. All diffraction peaks exhibit the characteristics of hexagonal wurtzite structure (space group p63mc, JCPDF #36-1451). Absence of any impurity phase like excess TM or its oxide infers the successful substitution of Zn2+ by TM (Fe, CO, Ni) in the lattice. The lattice parameters (a = b ≠ c) of this hexagonal wurtzite structure are calculated from the interplanar spacing dhkl where 2 1/ dhkl = 4/3(h2+hk+k 2)/ a2 + l 2/ c 2 334
Materials Science in Semiconductor Processing 91 (2019) 333–340
A. Mondal et al.
Fig. 2 illustrates the variation of the hexagonal cell parameters a and c, and the unit cell volume of the Zn1-xTMxO with x for each series. For Fe doped ZnO, the unit cell volume does not show any systematic variation with x as shown in Fig. 2a inset. Whereas, a gradual decrease of the unit cell volume of Zn1-xCoxO with x has been illustrated in inset of Fig. 2b for 0 ≤ x ≤ 0.15 which increases beyond x = 0.15 exhibiting a minima around x = 0.15. This may be due to the substitution of tetrahedrally coordinated Zn2+ by Co2+as the ionic radius of Co2+ (0.58 Å) in tetrahedral coordination is smaller than that of Zn2+ (0.60 Å). Beyond x = 0.15 the observed increase of unit cell volume suggests additional interstitial incorporation of Co2+ or Co3+ as reported by Hays et al. [17]. Co3+ does not readily enter into tetrahedral coordination [18]. In the octahedral site of the wurtzite structure, Co2+ has a radius between 0.65 Å (low spin) and 0.74 Å (high spin) and that of Co3+are 0.54 Å (low spin) and 0.61 Å (high spin) [17]. So presence of Co2+ in the interstitial site may increase the unit cell volume. To identify the oxidation state of Co ion, X-ray photo electron spectroscopy (XPS) was studied. Presence of Co3+may be ruled out as revealed from XPS data of Zn0.75Co0.25O which is discussed later. In Fe doped ZnO, divalent Zn may be substituted either by Fe2+ or Fe3+ or by both. The ionic radii of Fe2+ and Fe3+ in tetrahedral position are 0.63 and 0.49 Å respectively. The substitution of Zn2+in ZnO lattice by Fe2+or Fe3+, leads to lattice distortion in reverse direction as the ionic size of Zn2+ is in between Fe2+ and Fe3+. Literature survey reveals that in Fe doped ZnO, Fe ions exist as Fe2+ [19], or Fe3+ [20] or both Fe2+ and Fe3+ may coexist [21,22]. It appears that the valence state of Fe ion in Zn1xFexO is very much sensitive of synthesis route. To identify the valence state of Fe, study of X-ray photo electron spectroscopy (XPS) was performed on Zn0.85Fe0.15O, a representative member of Zn1-xFexO series. It indicates the presence of Fe2+ only. Fig. 2c shows gradual decrease of unit cell volume of Zn1-xNixO with increasing Ni content. This infers the substitution of Zn2+ by Ni2+which has smaller ionic radius 0.55 Å compared to that of Zn2+ [23]. Oxidation state of Ni is also verified from XPS measurement. To probe the dopant content of Zn1-xTMxO (TM = Fe, Co, Ni), one representative member of each series for example, Zn0.85Fe0.15O, Zn0.75Co0.25O and Zn0.93Ni0.07O were selected and EDX measurement was performed. Fig. 3 depicts the energy-dispersive spectra (EDX) of Zn0.85Fe0.15O, Zn0.75Co0.25O and Zn0.93Ni0.07O which correspond to the best photocatalyst in their respective series. The atomic percentages of Fe, Co and Ni estimated from EDX analysis are 14.6%, 24.6% and 7.28%, respectively. It indicates that the amount of Fe, Co and Ni incorporated into the ZnO matrix are close to the claimed amount of dopants. Morphology and size of Zn1-xTMxO were investigated from FESEM images. The FESEM images of Zn1-xTMxO (Fe, Co, Ni) demonstrate rod like structure. Three of the representative (Zn0.80Fe0.20O, Zn0.90Co0.10O, Zn0.93Ni0.07O) images of each series are shown in Fig. 4. Inset shows the size distribution of the rods. Fig. 4d depicts the variation of particle size of Zn1-xTMxO with x. Size of Co and Ni doped particles do not vary in a
Fig. 2. a.Unit cell parameter of Zn1-xFexO (a and c) as a function of Fe doping (x). Inset shows the variation of unit cell volume with x. b.Unit cell parameter of Zn1-xCoxO (a and c) as a function of Co doping (x). Inset shows the variation of unit cell volume with x. c.Unit cell parameter of Zn1-xNixO (a and c) as a function of Ni doping (x). Inset shows the variation of unit cell volume with x.
Fig. 3. EDX spectrum acquired from (a) Zn0.85Fe0.15O, (b) Zn0.75Co0.25O and (c) Zn0.93Ni0.07O. 335
Materials Science in Semiconductor Processing 91 (2019) 333–340
A. Mondal et al.
Fig. 4. FESEM images of rod structured (a) Zn1-xFexO (x = 0.20), (b) Zn1-xCoxO (x = 0.10) and (c) Zn1-xNixO (x = 0.07). Insets indicate size distribution following Lorentz distribution shown by continuous line. (d)Variation of particle size of Zn1-xTMxO with x for different TM (=Fe, Co, Ni) doping.
respectively, whereas the corresponding satellite structures are observed at 860.0 and 878.7 eV which are similar to the reported results [26]. The Ni 2p3/2peak position is quite different from that of Ni metal (852.7 eV) and Ni2O3 (856.7 eV), while close to the value of NiO (853.8 eV) [26]. This may indicate the absence of Ni metal as well as Ni2O3 but indicates 2+oxidation state of Ni cation same as in NiO. On the contrary, XRD result did not exhibit any peak corresponding to NiO. Moreover, the absence of trace amount of antiferromagnetic NiO (Neel temperature TN = 525 K) can be ruled out from the detail magnetic results indicating the paramagnetic behavior of Zn1-xNixO in the temperature range 122–300 K as reported by Mondal et al. [27]. Thus XPS results show a clear evidence of Ni2+ oxidation state in Zn0.93Ni0.07O. The optical band-gap energy (Eg) of the Zn1-xTMxO was estimated from UV–Vis absorption spectra. Absorption corresponding to the transition from valence band to the conduction band can be used to determine the optical band gap of a material. For direct band gap system absorption coefficient is related with incident photon energy as (αhν )2 = A (hν − Eg ) , where Eg is the band gap of the material. Using this relation, the band gap of ZnO is evaluated in the standard Tauc plot as shown in Fig. 6. Fig. 7aillustrates red shift of band gap with gradual Fe doping (0 ≤ x ≤ 0.15) in ZnO which is favorable for the photocatalytic enhancement under solar radiation as discussed earlier. Similar red shift had also been reported in Fe doped ZnO [28]. On the other hand, blue shift of Fe doped ZnO films and nanocrystals were also reported by Xu et al. [29], Murugadoss et al. [30] and Parra-Palomino et al. [31], respectively. In fact, both increase and decrease of band gap, with respect to an undoped ZnO are reported which may be sensitive to the oxidation state of Fe. Fig. 7b also illustrates red shift of optical band gap with gradual Co doping in ZnO. Similar red shift had been reported in Co doped ZnO by Fitzgerald et al. [32]. As solar spectrum contains 46% visible, 47% IR and 5–7% of UV light, narrowing of the optical band gap increases the efficiency of photon absorption and hence enhances the photocatalytic activity under solar irradiation. Fig. 7c does not exhibit any regular variation of Eg with Ni doping.
regular manner with x. On contrary, size of Fe doped particles significantly increases beyond x = 0.13. 3.2. Spectroscopic characterization: XPS, UV–vis spectroscopy The transition metals may exist in a number of different oxidation states in oxides like Fe may exist as Fe2+or Fe3+ oxidation states, whereas, Co may exist as Co2+ or Co3+. X-ray photoelectron spectroscopy may reveal the chemical composition as well as the formal oxidation state of Fe and Co. A typical survey spectra of Zn0.85Fe0.15O, Zn0.75Co0.25O and Zn0.93Ni0.07O as shown in Fig. 5a, b and c,respectively reveal the spectroscopic signature of zinc, oxygen and Iron/cobalt/Nickel. The inset of Fig. 5a, b and c show high resolution XPS spectra of Fe 2p, Co 2p and Ni 2p, respectively. Fig. 5a depicts that Fe 2p1/2 and Fe 2p3/2 peaks are located at722.7and 709.4 eV along with their corresponding satellites at 728.3 and 714.5 eV, respectively. Literature shows that the peak position of Fe 2p1/2 and Fe 2p3/2 for metallic Fe are 719.9 eV and 706.5 eV, respectively [24], whereas, that of Fe2+are 722.3 eV and 709.0 eV [24]. In case of Fe3+ as found in Fe2O3, these peak positions are found to be at 724.6 eV and 711 eV, respectively [24]. Thus XPS peak positions along with ‘shoulder’ satellite show a clear evidence of Fe2+ oxidation state in Zn0.85Fe0.15O. Fig. 5b demonstrates two main peaks of Co 2p3/2 and Co 2p1/2 located at 780.4 and 796.0 eV, along with their satellites in Zn0.75Co0.25O.Reported XPS data [17,25] shows that there are clear differences between Co(0) in Co metal, Co2+in CoO and Co3+ in Co2O3 with respect to the primary and satellite peak of Co 2p3/2 and 2p1/ 2+ are also quite different from that of 2energies. The line shapes of Co 3+ Co . Comparison with these reference spectra provides useful fingerprints for determining the oxidation state of Co and reveals that Co, in Zn0.75Co0.25O, is in the 2+ oxidation state. The oxidation state of Ni doped in ZnO was also investigated by XPS experiment. The results are shown in Fig. 5c. Two main peaks of 2p3/2 and 2p1/2are found to be centered ~ 854.4 and ~ 872.4 eV, 336
Materials Science in Semiconductor Processing 91 (2019) 333–340
A. Mondal et al.
Fig. 5. High-resolution XPS survey scan of (a) Zn0.85Fe0.15O, (b) Zn0.75Co0.25O and (c) Zn0.93Ni0.07O. The colored solid lines in the insets correspond to Fe/Co/Ni 2p1/ 2, 2p3/2 and its satellite along with the resultant Gaussian fitted curve superimposed on the experimental data.
degrade MB by ε ~ 50% after 150 min of irradiation whereas, the degradation efficiency was significantly enhanced (~94.5%) by Zn0.85Fe0.15O under same condition as depicted in Fig. 8a. Figure 8dillustrates the comparative study of photocatalytic activity of Zn1xTMxO (TM = Fe, Co, Ni) with different doping (x) for a particular time (150 min) of irradiation. It illustrates a significantly sharp peak for Zn1xFexO with x = 0.15. This significant enhancement of this activity for x = 0.15 does not corroborate with the surface area as the particle size is much larger (Fig. 4d) compared to that of x = 0.13. This colossal increment of photocatalytic activity may be attributed to enhanced defect sites. Doping of ZnO with Fe adds defect sites in the vicinity of valence band and thus reduces the effective band gap. When UV–vis light is passed through sample the electron–hole pair is generated within the effective band gap. This transition corresponds to lower energy than the band gap of ZnO. Thus surface defect, reduction in band gap all act in harmony with the enhancement of photocatalytic
3.3. Photocatalytic properties The photocatalytic activity of Zn1-xTMxO (Fe, Co, Ni) was evaluated by the degradation of MB dye under UV–vis irradiation. Degradation of MB dye by the photocatalyst Zn1-xTMxO was monitored through the reduction of characteristic absorption peak height. The percentage of C −C degradation of MB dye defined as ε = t C 0 × 100 is the measure of 0 degradation efficiency of the photocatalyst [33]. Here c0 is the initial concentration of the dye MB and ct is the concentration of residual MB after t s irradiation. The variation of degradation efficiency (ε) of Zn1xFexO, Zn1-xCoxO and Zn1-xNixO with the time (t) of irradiation are shown in Fig. 8a, b, c, respectively. Photolysis experiment was also carried out by irradiating the MB solution with UV radiation. The degradation of MB, due to photolysis without TM doped ZnO photocatalyst was found to be 34% after 150 min of irradiation. Then as a photocatalyst ZnO nanoparticles were added and it was found to
Fig. 6. Taucs plot corresponding to UV–vis absorbance spectra of (a) Zn1-xFexO, (b) Zn1-xCoxO and (c) Zn1-xNixO are shown. 337
Materials Science in Semiconductor Processing 91 (2019) 333–340
A. Mondal et al.
Fig. 7. Variation of optical band gap of (a) Zn1-xFexO, (b) Zn1-xCoxO and (c) Zn1-xNixO with x.
activity and make Zn0.85Fe0.15O the best photocatalytic among all TM doped samples as reported here. Fig. 8 dreveals gradual rise of photocatalytic activity of Zn1-xCoxO with increase of Co doping (x) in the range 0 ≤ x ≤ 0.25. Degradation efficiency of Zn0.75Co0.25O for 150 min irradiation is 81%. On the other hand, Zn0.93Ni0.07O exhibits an optimum doping of Ni for which degradation (83%) is maximum. Beyond this dopant content, photocatalytic activity of Zn1-xNixO decreases. The excessive dopant ions induce intrinsic point defects which may serve as recombination centers and lead to quenching of the photocatalytic activity as explained by Chauhan et al. [34,35]. It is worthwhile to mention that the defects introduced by different TM having dissimilar ionic radius are different. So the photocatalytic activity can be tailored desirably by doping different TM in ZnO in appropriate amount; however, excessive dopant ions are detrimental [34]. Total organic carbons (TOC) content which is an important indicator of mineralization was also monitored during the photodegradation process. TOC conversion is calculated [36] using the following relation, TOCConversion(%) = TOC0 − TOCt , where, TOC0 and TOC0 TOCt represent the TOC at initial and after t s of photodegradation process, respectively. The kinetic linear simulation curves based on the Langmuir–Hinshelwood first-order kinetic model of TOC removal is shown in Fig. 9. The values of TOC rate constants κTOC are obtained from
TOC
Fig. 9. The ln TOC0 vs irradiation time plots of mineralization of MB by ZnO, t
Zn0.85Fe0.15O, Zn0.75Co0.25O and Zn0.93Ni0.07O as photocatalysts.
the slope of the linear curves and corresponding values of percentage of TOC conversion are tabulated in Table 1. Decrease of TOC content with the increase of irradiation time indicates that MB degrades into nontoxic compounds. The mineralization of MB dye is found to be higher in presence of TM doped ZnO photocatalysts compared to that of pure
Fig. 8. Photodegradation efficiency (ε) of (a) Zn1-xFexO (0 ≤ x ≤ 0.20), (b) Zn1-xCoxO (0 ≤ x ≤ 0.25) and (c) Zn1-xNixO (0 ≤ x ≤ 0.11) under different irradiation time.(d) Comparison of photocatalytical performance of Zn1-xFexO, Zn1-xCoxO and Zn1-xNixO for different dopant concentration under 150 min irradiation. 338
Materials Science in Semiconductor Processing 91 (2019) 333–340
A. Mondal et al.
degradation was investigated by performing three cycles of the photocatalytic experiment of Zn0.85Fe0.15O. In the first cycle, MB concentration decreases to ct / c0 = 5.5% in 150 min. After separating out the used Zn0.85Fe0.15O from the mixture, it was added to an identical aqueous solution of MB and stirred under irradiation for another 150 min. The third cycle was followed in a similar way. Fig. 11 exhibits almost negligible loss of activity during three cycles of photocatalytic experiments [40]. Thus, both the enhancement of photocatalytic activity and reusability of Zn0.85Fe0.15O make it advantageous for its application as a photocatalyst.
Table 1 Rate constant (κ ), photo degradation efficiency (ε), TOC rate constant (κTOC ) and percentage of TOC Conversion of a representative member of Zn1-xTMxO (TM = Fe, Co, Ni) photocatalyst series. Samples
κ (min−1)
ε
κTOC (min−1)
Percentage of TOC Conversion (%)
ZnO Zn0.85Fe0.15O Zn0.75Co0.25O Zn0.93Ni0.07O
0.00466 0.02049 0.01010 0.01216
50% 94.5% 81% 83%
0.00465 0.01774 0.01130 0.01120
49% 93.2% 80% 81.9%
4. Summary and conclusions Tailoring of photocatalytic activity of ZnO had been investigated by doping TM (=Fe, Co, Ni) with different doping concentration x. For each series of Zn1-xTMxO, attempt was made for maximum possible doping of TM which is achievable by the co-precipitation route as discussed earlier. Only single phase wurzite structured samples without any impurity phase were picked up for photocatalysis study. The photocatalytic activity of Zn1-xTMxO was investigated by studying the degradation of MB dye (5 ppm MB in 100 ml aqueous solution) under UV and visible light irradiation. The activity of Zn1-xTMxO was found to be significantly higher than that of undoped ZnO. This enhancement in activity may be due to increase in surface area, decrease in optical band gap, defect, increase in conductivity etc. A decrease in the particle size will increase the surface area and thus increase the active site on the surface, which consequently leads to a higher for photocatalytic activity. However, in the present study, the particle size corresponding to the best photocatalyst among the members Zn1-xTMxO is not the minimum one. Hence, the average crystallite size may play an important role but is not a decisive factor for better photocatalysis. Gradual decrease of band gap of ZnO due to TM doping make it efficient photocatalyst under sun light irradiation as solar spectrum contains 46% visible, 47% IR and 5–7% of UV light. Semiconductor having narrower band gap requires lesser energy for transition of electron from valance to conduction band and thus it can harvest more photons to excite the electron from the valence band. Moreover, dopant introduced in a ZnO matrix can serve as a trap for an electron or hole if the energy of that electron or hole is just below the conduction band or just above the valence band, respectively. This trapped electron or hole will be migrated to the surface of photocatalyst and undergoes redox reaction with MB molecule. Thus the defects caused by TM (= Fe, Co, Ni) doping in ZnO can suppress recombination of the electron–hole pairs and TM doped ZnO displays better photocatalytic properties than pure ZnO. It is observed here that the photocatalytic activity of Zn1xCoxO increases gradually with Co doping (x) whereas Zn1-xNixO exhibits a maximum activity at x = 0.07. However, Zn1-xFexO reveals significantly enhanced photocatalytic activity for x = 0.15. On further doping of Fe, activity decreases. The colossal activity of Zn0.85Fe0.15O which is measured by the percentage of degradation (94.5%) of MB makes this member the best photocatalyst among Zn1-xTMxO (TM= Fe, Co. Ni) series.
Fig. 10. Pseudo-first order kinetic plot for the degradation of MB using (a) ZnO, (b) Zn0.85Fe0.15O, (c) Zn0.75Co0.25O and (d) Zn0.93Ni0.07O as photocatalysts.
Fig. 11. Cyclic photodegradation of MB by Zn0.85Fe0.15O for three cycles.
ZnO. Moreover, Zn0.85Fe0.15O photocatalyst exhibits the maximum TOC conversion of MB as expected. The photocatalytic processes generally follows pseudo-first-order kinetic with respect to Langmuir–Hinshelwood reaction mechanism,
( ) = −κt , where,κ is the pseudo-first-order rate constant. This can be used to compare the photocatalytic activity of a series of photocatalysts [37–39]. The plots of ln ( ) as a function of irradiation times
ln
Ct C0
Acknowledgment
C0 Ct
(t) are shown in Fig. 10a–d for undoped ZnO and Zn0.85Fe0.15O, Zn0.75Co0.25O, Zn0.93Ni0.07O, respectively which correspond to the highest photodegradation efficiency in their respective series. The values of rate constants (κ ) obtained from the slope of the linear curves and corresponding degradation efficiency (ε) are tabulated in Table 1.
Authors would like to thank DST, Govt. of India for developing instrumental facilities like Advance X-ray powder diffractometer (Bruker D8) and FE-SEM (JEOL, JSM-7610F) under FIST program at Jadavpur University. S. Sarkar, A. Mondal and N. Giri wish to thank DST for INSPIRE fellowship scheme.
3.4. Reuse of the catalyst system
References
The stability and reusability of photocatalyst during photocatalytic reaction is a crucial factor for the practical applications and economical point of view. The reusability of the photocatalyst for MB photo-
[1] M.A. Lazar, W.A. Daoud, Achieving selectivity in TiO2-based photocatalysis, RSC Adv. 3 (2013) 4130–4140. [2] S. Cho, J.W. Jang, J.S. Lee, K.H. Lee, Porous ZnO–ZnSe nanocomposites for visible
339
Materials Science in Semiconductor Processing 91 (2019) 333–340
A. Mondal et al.
Al2O3(0001) by radio-frequency sputtering, Appl. Phys. 96 (2004) 4150–4153. [23] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. Sect. A 32 (5) (1976) 751–767. [24] T. Yamashita, P. Hayes, Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials, Appl. Surf. Sci. 254 (2008) 2441–2449. [25] R. He, B. Tang, C. Ton-That, M. Phillips, T. Tsuzuki, Physical structure and optical properties of Co-doped ZnO nanoparticles prepared by co-precipitation, J. Nanopart. Res. 15 (11) (2013) 1–8. [26] B. Pal, D. Sarkar, P.K. Giri, Structural, optical, and magnetic properties of Ni doped ZnO nanoparticles: correlation of magnetic moment with defect density, Appl. Surf. Sci. 356 (2015) 804–811. [27] A. Mondal, N. Giri, S. Sarkar, R. Ray, Magnetic properties of Zn1-xNixO, AIP Conf. Proc. 1953 (2018) 120017 (1–4). [28] Z.C. Chen, L.J. Zhuge, X.M. Wu, Y.D. Meng, Initial study on the structure and optical properties of Zn1−xFexO films, Thin Solid Films 515 (2007) 5462–5465. [29] L. Xu, X. Li, Growth and characterization of Mg-doped AlGaN–AlN short-period superlattices for deep-UV optoelectronic devices, J. Cryst. Growth 312 (2010) 756–761. [30] G. Murugadoss, Synthesis and characterization of transition metals doped ZnO nanorods, J. Mater. Sci. Technol. 28 (7) (2012) 587–593. [31] A. Parra-Palomino, O. Perales-Perez, R. Singhal, M. Tomar, J. Hwang, P.M. Voyles, Evidence of ferromagnetism in Zn1−xMxO (M = Ni,Cu) nanocrystals for spintronics, J. Appl. Phys. 18 (2007) 315606 (5 pp). [32] C.B. Fitzgerald, M. Venkatesan, J.G. Lunney, L.S. Dornelse, J.M.D. Coey, Anisotropic ferromagnetism in substituted zinc oxide, Appl. Surf. Sci. 247 (2005) 493–496. [33] A. Hamza, I.J. John, B. Mukhtar, Enhanced photocatalytic activity of calcined natural sphalerite under visible light irradiation, J. Mater. Res. Technol. 6 (1) (2017) 1–6. [34] R. Chauhan, A. Kumar, R.P. Chaudhary, Photocatalytic degradation of methylene blue with Fe doped ZnS nanoparticles, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 113 (2013) 250–256. [35] L. Wang, Z. Ji, J. Lin, P. Li, Preparation and optical and photocatalytic properties of Ce-doped ZnO microstructures by simple solution method, Mater. Sci. Semicond. Process. 71 (2017) 401–408. [36] O. Bechambi, A. Touati, S. Sayadi, W. Najjar, Effect of cerium doping on the textural, structural and optical properties of zinc oxide: role of cerium and hydrogen peroxide to enhance the photocatalytic degradation of endocrine disrupting compounds, Mater. Sci. Semicond. Process. 39 (2015) 807–816. [37] A. Ashkarrana, M. Fakharia, H. Hamidinezhadb, H. Haddadic, M.R. Nouranid, TiO2 nanoparticles immobilized on carbon nanotubes for enhanced visible-light photoinduced activity, J. Mater. Res. Technol. 4 (2) (2015) 126–132. [38] A. Peticaa, A. Floreab, C. Gaidaua, D. Balan, L. Anicai, Synthesis and characterization of silver-titania nanocomposites prepared by electrochemical method with enhanced photocatalytic characteristics, antifungal and antimicrobial activity, J. Mater. Res. Technol. 324 (2017) 1–13. [39] A. Ulyankina, I. Leontyev, M. Avramenko, D. Zhigunov, N. Smirnova, Large-scale synthesis of ZnO nanostructures by pulse electrochemical method and their photocatalytic properties, Mater. Sci. Semicond. Process. 76 (2018) 7–13. [40] Y. Yu, C. Yu, T. Xu, H. Sun, Optical and photocatalytic properties of indium phosphide nanoneedles and Nanotubes, Mater. Sci. Semicond. Process. 68 (2017) 270–274.
light photocatalysis, Nanoscale 4 (2012) 2066–2071. [3] S.G. Kumar, L.G. Devi, J. Review on modified TiO2 photocatalysis under UV/Visible light: selected results and related mechanisms on interfacial charge carrier transfer dynamics, Phys. Chem. A115 (2011) 13211–13241. [4] C. Karunakaran, V. Rajeswari, P. Gomathisankar, Optical, electrical, photocatalytic, and bactericidal properties of microwave synthesized nanocrystalline Ag–ZnO and ZnO, Solid State Sci. 13 (2011) 923–928. [5] R. Vinu, G. Madras, Environmental remediation by photocatalysis, J. Indian Inst. Sci. 90 (2010) 189–229. [6] E.R. Caraway, A.J. Hoffman, M. Hoffmann, Photocatalytic oxidation of organic acids onquantum-sized semiconductor colloids, Environ. Sci. Technol. 28 (1994) 786–793. [7] K. Thongsuriwong, P. Amornpitoksuk, S. Suwanboon, Photocatalytic and 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. [8] U.I. Gaya, A.H. Abdullah, Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide, J. Photochem. Photobiol. C: Photochem. Rev. 9 (2008) 1–12. [9] A. Moezi, A.M. McDonagh, M.B. Cortie, Chemical etching preparation of BiOI/ BiOBr heterostructures with enhanced photocatalytic properties for organic dye removal, J. Chem. Eng. 185/186 (2012) 1–22. [10] R. Srivastava, B.C. Yadav, Nanaostructured ZnO, ZnO-TiO2 and ZnO-Nb2O5 as solid state humidity sensor, Adv. Mater. Lett. 3. 3 (2012) 197–203. [11] R. Saleh, N.F. Djaja, Transition-metal-doped ZnO nanoparticles: synthesis, characterization and photocatalytic activity under UV light, Spectrochim. Part A: Mol. Biomol. Spectrosc. 130 (2014) 581–590. [12] Z.C. Chen, L.J. Zhuge, X.M. Wu, Y.D. Meng, Initial study on the structure and optical properties of Zn1−xFexO films, Thin Solid Films 515 (2017) 5462–5465. [13] C. Xu, L. Cao, G. Su, W. Liu, X. Qu, Y. Yu, Preparation, characterization and photocatalytic activity of Co-doped ZnO powders, J. Alloy. Compd. 497 (2010) 373–376. [14] S. Kant, A. Kumar, A comparative analysis of structural, optical and photocatalytic properties of ZnO and Ni doped ZnO nanospheres prepared by sol-gel method, Adv. Mater. Lett. 3 (4) (2012) 350–354. [15] X. Yu, D. Meng, C. Liu, K. Xu, J. Chen, C. Lu, Y. Wang, Enhanced photocatalytic activity of Fe-doped ZnO nanoparticles synthesized via a two-step sol–gel method, J. Mater. Sci: Mater. Electron 25 (2014) 3920–3923. [16] A. Mondal, S. Sarkar, N. Giri, S. Chatterjee, R. Ray, Magnetic phase separation in diluted magnetic system: Zn1-xFexO, Acta Metall. Sin. (Engl. Lett.) 30 (6) (2017) 521–527. [17] J. Hays, K.M. Reddy, N.Y. Graces, M.H. Engelhard, V. Shutthanandan, M. Luo, C. Xu, N.C. Giles, C. Wang, S. Thevuthasan, A. Punnoose, Fluorescent particles comprising nanoscale ZnO layer and exhibiting cell-specific toxicity, J. Phys.: Condens. Matter 19 (2007) 266203 (24pp). [18] A.S. Risbud, N.A. Spaldin, Z.Q. Chen, S. Stemmer, R. Seshadri, Magnetism in polycrystalline cobalt-substituted zinc oxide, Phys. Rev. B 68 (2003) 1–7 (205202). [19] Z.C. Chen, L.J. Zhuge, X.M. Wu, Y.D. Meng, Initial study on the structure and optical properties of Zn1-xFexO films, Thin Solid Films 515 (2007) 5462–5465. [20] L. Xu, X. Li, Influence of Fe-doping on the structural and optical properties of ZnO thin films prepared by sol–gel method, J. Cryst. Growth 312 (2010) 851–855. [21] D. Karmakar, S.K. Mandal, R.M. Kadam, P.L. Paulose, A.K. Rajarajan, T.K. Nath, A.K. Das, I. Dasgupta, G.P. Das, Ferromagnetism in Fe-doped ZnO nanocrystals: experiment and theory, Phys. Rev. B 75 (2007) 1–14 (144404). [22] K.J. Kim, Y.R. Park, J. Optical investigation of Zn1−xFexO films grown on
340
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Tuning the photocatalytic activity of ZnO by TM (TM = Fe, Co, Ni) doping A. Mondal, N. Giri, S. Sarkar, S. Majumdar, R. Ray
⁎
T
Department of Physics, Jadavpur University, Kolkata 700032, India
A R T I C LE I N FO Keywords: Photocatalysis XPS Zn1-xTMxO (TM = Fe, Co, Ni) Methylene Blue
A B S T R A C T
The present study exhibits the enhancement of photocatalytic activity of transition metal (TM) doped ZnO (Zn1with TM = Fe, Co, Ni) towards aqueous solution of organic dyes such as methylene blue (MB) under ultraviolet and visible light irradiation. Wurtzite Zn1-xTMxO nanoparticles are synthesized by co-precipitation method with different x ranging from zero to almost maximum attainable dopant (x) by this method and characterized by X-ray diffraction, scanning electron microscopy, UV–vis diffuse reflectance spectroscopy and Xray photo electron spectroscopy. UV–vis diffuse reflectance spectroscopy illustrates the gradual red shift of ZnO band gap due to Fe and Co doping. The photocatalytic activity is found to increase gradually with Co doping (x) in Zn1-xCoxO whereas Zn1-xNixO causes a maximum degradation of (~83%) of MB for x = 0.07 which is much larger compared to that (~50%) due to pure ZnO for 150 min irradiation. However, enormous degradation (~94.5%) of MB by Zn1-xFexO under the same condition for x = 0.15 makes this member the best photocatalyst among Zn1-xTMxO (TM= Fe, Co. Ni) series.
xTMxO,
1. Introduction Photocatalysis under solar irradiation has attracted much attention as a promising way to address the environmental problems, especially removal of the non-biodegradable dyes from waste water stream. Some of the most adequate and extensively studied semiconductors that offer the potential for elimination of pollutants are TiO2, ZnO, CdS, V2O5, WO3 and α-Fe2O3 [1–5]. Among these, ZnO has emerged as an efficient and promising candidate in the environmental management system due to its photosensitivity, the strongly oxidizing power [6], environmental friendly nature and the excellent chemical and mechanical stabilities. Photocatalytic activity occurs when ZnO absorbs UV light with energy equal to or greater than its band gap and forms electron-hole pairs. These electron-hole pairs subsequently migrate to the ZnO surface and react with adsorbed molecules to generate reactive species as H2O2, superoxide anion radicals (∙O2 − ) and hydroxyl radicals (∙ OH) [7,8]. These strongly oxidizing and highly reactive agents can degrade an organic pollutant into nontoxic compound. However, ZnO has several shortcomings as photocatalyst such as wide band gap~ 3.37 eV [9] and large exciton binding energy ~ 60 meV [10] which leads to fast recombination of the photogenerated electron and hole pairs. As solar spectrum contains 46% visible, 47% IR and 5–7% of UV light, practical application of ZnO as photocatalyst demands absorption of light in visible range or IR. This requires narrowing of the band gap which can be achieved by doping with transition metal ions [11,12]. This doping
⁎
was also found to improve the separation between electron hole by forming electron traps. The hole will be able to migrate towards the surface of the photocatalyst and disintegrate the adsorbed pollutant. Thus surface area and surface defects of the nanoparticles play an important role in photocatalytic activity. Xu et al. have found that the photocatalytic properties of ZnO can be improved by doping Co2+ (0.5–5 mol%) by hydrothermal method [13]. The degradation rate of the MB dye has been studied by Kant et al. [14]. He reported the improvement of photocatalytic properties of Zn1-xNixO prepared by sol – gel technique with Ni doping in the range 0 ≤x ≤ 0.5. Yu et al. [15] reported a striking enhancement of photocatalytic activity of Zn1-xFexO for x = 0.01 and revealed that large surface defect is responsible for the enhanced activity. A comparative study of photocatalytic activity of ZnO nanoparticles doped with transition metals (Mn and Co) upto 12 at% prepared by a co-precipitation method has been reported by Saleh et al. [11]. As both surface area and surface defects of the nanoparticles are very much sensitive of synthesis procedure it greatly influences the photocatalytic activity. Here we have performed a comparative study of the photocatalytic activity of Zn1xTMxO (TM = Fe, Co, Ni) prepared by co-precipitation method. It is noteworthy that doping is not only confined within the diluted magnetic system (DMS) regime where x is in the range 0 ≤x <0.1, rather attempt was made for higher possible doping of TM which is attainable by the following synthesis procedure. We have used methylene blue (MB) as a pollutant, which is a hazardous dye as it causes hyper tension,
Corresponding author. E-mail address: [email protected] (R. Ray).
https://doi.org/10.1016/j.mssp.2018.12.003 Received 25 April 2018; Received in revised form 27 November 2018; Accepted 3 December 2018 1369-8001/ © 2018 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 91 (2019) 333–340
A. Mondal et al.
cyanosis, jaundice, shock. 2. Experimental details 2.1. Synthesis Zn1-xTMxO (TM = Fe, Co, Ni) nanoparticles with varying TM content (x), were synthesized in de-ionized water ambiance [16]. For Fe, Co, and Ni doping anhydrous FeCl3, (CH3COO)2Co·4H2O and (CH3COO)2Ni·4H2O were selected as precursor. Zinc acetate and precursor were taken in stoichiometric ratio as required by molecular formula. Aqueous (25 ml) precursor of required concentration was added drop by drop to 50 ml aqueous (CH3COO)2Zn·2H2O having required amount of Zn2+ as calculated from formula unit. A homogeneous solution was obtained by adding 1 g of PVP in 50 ml de-ionized water before adding the precursor. Then 0.4 g of 50 ml sodium hydroxide was added drop by drop to the above mixture which results a yellow voluminous precipitate. The precipitate was then isolated by centrifuging it against de-ionized water and ethanol several times. The product was dried at 120 °C for 2 h in air. Thus Zn1-xFexO (x = 0.0, 0.03, 0.06, 0.09, 0.13, 0.15, 0.20), Zn1-xNixO (x = 0.01, 0.02, 0.03, 0.07, 0.09, 0.11) and Zn1-xCoxO (x = 0.05, 0.10, 0.15, 0.25) were prepared successfully and selected for further study. Here, attempt was made to prepare each Zn1-xTMxO (TM= Fe, Co, Ni) series with different dopant (x) starting from low value to almost maximum value attainable by this co-precipitation method. We could prepare Zn0.80Fe0.20O, Zn0.75Co0.25O and Zn0.89Ni0.11O with maximum dopant content with Fe, Co and Ni respectively. An attempt for higher dopant of TM yielded phase segregation of transition metal oxide. Sincere care was taken to select only those doped samples for further study which were of single phase wurzite structure and any other impurity phase was absent. 2.2. Characterization: structure and morphology Crystal structures were studied at room temperature using a powder X-ray diffractometer (XRD) (Bruker D8 Advance) with the Cu Kαradiation. The morphology of the nanoparticles was studied in field emission scanning electron microscopy (FE-SEM) images using the microscope, JEOL (model: JSM-7610F). UV–VIS diffuse reflectance spectrophotometer (Shimadzu, UV 2401PC) was used to study optical absorption spectra in the wavelength region 200– 800 nm. 2.3. Photocatalysis study Photocatalytic activities of the Zn1-xTMxOnanoparticles were studied by degradation of MB dye aqueous solution under irradiation of Ultraviolet (UV) and visible (Vis) light. Photocatalysis studies were performed by using a home-made photo reactor [dimension of 1.5 ft × 1.5 ft × 1 ft (length ×breadth× height)] fitted with a mercury lamp (300 W, 280 < λ < 400 nm) and a xenon lamp (500 W, 400 < λ < 700 nm). Similar UV–Vis source was also used by Yu et al. [15]. Aqueous dispersion of Zn1-xTMxO nanoparticles were prepared by adding 0.04 g of Zn1-xTMxO into 100.0 ml of 5 ppm MB dye solution. The pH of this solution was 7.1–7.2. The dispersion was stirred in dark for 1 h so that MB dye molecules got adsorbed on the surface of nanocomposite. The homogeneous mixture was than irradiated by ultraviolet (UV) and visible light. As a preventive measure of loss of light, the beaker was covered by Al foil from all sides. After irradiation for a desired time, about 10 ml of the aqueous dispersion was sampled out and centrifuged to separate the Zn1-xTMxO photocatalysts. The concentration of residual MB in the filtrate was determined by Shimadzu UV-3101PC UV–VIS spectro-photometer. The mineralization degree was detected by a Shimadzu TOC-VCBH Total Organic Carbon (TOC) analyzer. To estimate the percentage of mineralization of MB dye by TOC measurement its concentration, mass of photocatalyst and irradiation conditions were kept same as were done in photocatalytic
Fig. 1. a. X-ray diffraction patterns of Zn1-xFexO with 0.0 ≤ x ≤ 0.20 at 300 K. b. X-ray diffraction patterns of Zn1-xCoxO with 0.0 ≤ x ≤ 0.25 at 300 K. c. Xray diffraction patterns of Zn1-xNixO with 0.0 ≤ x ≤ 0.11 at 300 K.
experiment.
3. Results and discussions 3.1. Crystal structure and morphology: XRD, EDX and FESEM studies Fig. 1a, b, and c illustrate the X-ray diffraction (XRD) peaks of each Zn1-xTMxO (TM = Fe, Co, Ni) series. All diffraction peaks exhibit the characteristics of hexagonal wurtzite structure (space group p63mc, JCPDF #36-1451). Absence of any impurity phase like excess TM or its oxide infers the successful substitution of Zn2+ by TM (Fe, CO, Ni) in the lattice. The lattice parameters (a = b ≠ c) of this hexagonal wurtzite structure are calculated from the interplanar spacing dhkl where 2 1/ dhkl = 4/3(h2+hk+k 2)/ a2 + l 2/ c 2 334
Materials Science in Semiconductor Processing 91 (2019) 333–340
A. Mondal et al.
Fig. 2 illustrates the variation of the hexagonal cell parameters a and c, and the unit cell volume of the Zn1-xTMxO with x for each series. For Fe doped ZnO, the unit cell volume does not show any systematic variation with x as shown in Fig. 2a inset. Whereas, a gradual decrease of the unit cell volume of Zn1-xCoxO with x has been illustrated in inset of Fig. 2b for 0 ≤ x ≤ 0.15 which increases beyond x = 0.15 exhibiting a minima around x = 0.15. This may be due to the substitution of tetrahedrally coordinated Zn2+ by Co2+as the ionic radius of Co2+ (0.58 Å) in tetrahedral coordination is smaller than that of Zn2+ (0.60 Å). Beyond x = 0.15 the observed increase of unit cell volume suggests additional interstitial incorporation of Co2+ or Co3+ as reported by Hays et al. [17]. Co3+ does not readily enter into tetrahedral coordination [18]. In the octahedral site of the wurtzite structure, Co2+ has a radius between 0.65 Å (low spin) and 0.74 Å (high spin) and that of Co3+are 0.54 Å (low spin) and 0.61 Å (high spin) [17]. So presence of Co2+ in the interstitial site may increase the unit cell volume. To identify the oxidation state of Co ion, X-ray photo electron spectroscopy (XPS) was studied. Presence of Co3+may be ruled out as revealed from XPS data of Zn0.75Co0.25O which is discussed later. In Fe doped ZnO, divalent Zn may be substituted either by Fe2+ or Fe3+ or by both. The ionic radii of Fe2+ and Fe3+ in tetrahedral position are 0.63 and 0.49 Å respectively. The substitution of Zn2+in ZnO lattice by Fe2+or Fe3+, leads to lattice distortion in reverse direction as the ionic size of Zn2+ is in between Fe2+ and Fe3+. Literature survey reveals that in Fe doped ZnO, Fe ions exist as Fe2+ [19], or Fe3+ [20] or both Fe2+ and Fe3+ may coexist [21,22]. It appears that the valence state of Fe ion in Zn1xFexO is very much sensitive of synthesis route. To identify the valence state of Fe, study of X-ray photo electron spectroscopy (XPS) was performed on Zn0.85Fe0.15O, a representative member of Zn1-xFexO series. It indicates the presence of Fe2+ only. Fig. 2c shows gradual decrease of unit cell volume of Zn1-xNixO with increasing Ni content. This infers the substitution of Zn2+ by Ni2+which has smaller ionic radius 0.55 Å compared to that of Zn2+ [23]. Oxidation state of Ni is also verified from XPS measurement. To probe the dopant content of Zn1-xTMxO (TM = Fe, Co, Ni), one representative member of each series for example, Zn0.85Fe0.15O, Zn0.75Co0.25O and Zn0.93Ni0.07O were selected and EDX measurement was performed. Fig. 3 depicts the energy-dispersive spectra (EDX) of Zn0.85Fe0.15O, Zn0.75Co0.25O and Zn0.93Ni0.07O which correspond to the best photocatalyst in their respective series. The atomic percentages of Fe, Co and Ni estimated from EDX analysis are 14.6%, 24.6% and 7.28%, respectively. It indicates that the amount of Fe, Co and Ni incorporated into the ZnO matrix are close to the claimed amount of dopants. Morphology and size of Zn1-xTMxO were investigated from FESEM images. The FESEM images of Zn1-xTMxO (Fe, Co, Ni) demonstrate rod like structure. Three of the representative (Zn0.80Fe0.20O, Zn0.90Co0.10O, Zn0.93Ni0.07O) images of each series are shown in Fig. 4. Inset shows the size distribution of the rods. Fig. 4d depicts the variation of particle size of Zn1-xTMxO with x. Size of Co and Ni doped particles do not vary in a
Fig. 2. a.Unit cell parameter of Zn1-xFexO (a and c) as a function of Fe doping (x). Inset shows the variation of unit cell volume with x. b.Unit cell parameter of Zn1-xCoxO (a and c) as a function of Co doping (x). Inset shows the variation of unit cell volume with x. c.Unit cell parameter of Zn1-xNixO (a and c) as a function of Ni doping (x). Inset shows the variation of unit cell volume with x.
Fig. 3. EDX spectrum acquired from (a) Zn0.85Fe0.15O, (b) Zn0.75Co0.25O and (c) Zn0.93Ni0.07O. 335
Materials Science in Semiconductor Processing 91 (2019) 333–340
A. Mondal et al.
Fig. 4. FESEM images of rod structured (a) Zn1-xFexO (x = 0.20), (b) Zn1-xCoxO (x = 0.10) and (c) Zn1-xNixO (x = 0.07). Insets indicate size distribution following Lorentz distribution shown by continuous line. (d)Variation of particle size of Zn1-xTMxO with x for different TM (=Fe, Co, Ni) doping.
respectively, whereas the corresponding satellite structures are observed at 860.0 and 878.7 eV which are similar to the reported results [26]. The Ni 2p3/2peak position is quite different from that of Ni metal (852.7 eV) and Ni2O3 (856.7 eV), while close to the value of NiO (853.8 eV) [26]. This may indicate the absence of Ni metal as well as Ni2O3 but indicates 2+oxidation state of Ni cation same as in NiO. On the contrary, XRD result did not exhibit any peak corresponding to NiO. Moreover, the absence of trace amount of antiferromagnetic NiO (Neel temperature TN = 525 K) can be ruled out from the detail magnetic results indicating the paramagnetic behavior of Zn1-xNixO in the temperature range 122–300 K as reported by Mondal et al. [27]. Thus XPS results show a clear evidence of Ni2+ oxidation state in Zn0.93Ni0.07O. The optical band-gap energy (Eg) of the Zn1-xTMxO was estimated from UV–Vis absorption spectra. Absorption corresponding to the transition from valence band to the conduction band can be used to determine the optical band gap of a material. For direct band gap system absorption coefficient is related with incident photon energy as (αhν )2 = A (hν − Eg ) , where Eg is the band gap of the material. Using this relation, the band gap of ZnO is evaluated in the standard Tauc plot as shown in Fig. 6. Fig. 7aillustrates red shift of band gap with gradual Fe doping (0 ≤ x ≤ 0.15) in ZnO which is favorable for the photocatalytic enhancement under solar radiation as discussed earlier. Similar red shift had also been reported in Fe doped ZnO [28]. On the other hand, blue shift of Fe doped ZnO films and nanocrystals were also reported by Xu et al. [29], Murugadoss et al. [30] and Parra-Palomino et al. [31], respectively. In fact, both increase and decrease of band gap, with respect to an undoped ZnO are reported which may be sensitive to the oxidation state of Fe. Fig. 7b also illustrates red shift of optical band gap with gradual Co doping in ZnO. Similar red shift had been reported in Co doped ZnO by Fitzgerald et al. [32]. As solar spectrum contains 46% visible, 47% IR and 5–7% of UV light, narrowing of the optical band gap increases the efficiency of photon absorption and hence enhances the photocatalytic activity under solar irradiation. Fig. 7c does not exhibit any regular variation of Eg with Ni doping.
regular manner with x. On contrary, size of Fe doped particles significantly increases beyond x = 0.13. 3.2. Spectroscopic characterization: XPS, UV–vis spectroscopy The transition metals may exist in a number of different oxidation states in oxides like Fe may exist as Fe2+or Fe3+ oxidation states, whereas, Co may exist as Co2+ or Co3+. X-ray photoelectron spectroscopy may reveal the chemical composition as well as the formal oxidation state of Fe and Co. A typical survey spectra of Zn0.85Fe0.15O, Zn0.75Co0.25O and Zn0.93Ni0.07O as shown in Fig. 5a, b and c,respectively reveal the spectroscopic signature of zinc, oxygen and Iron/cobalt/Nickel. The inset of Fig. 5a, b and c show high resolution XPS spectra of Fe 2p, Co 2p and Ni 2p, respectively. Fig. 5a depicts that Fe 2p1/2 and Fe 2p3/2 peaks are located at722.7and 709.4 eV along with their corresponding satellites at 728.3 and 714.5 eV, respectively. Literature shows that the peak position of Fe 2p1/2 and Fe 2p3/2 for metallic Fe are 719.9 eV and 706.5 eV, respectively [24], whereas, that of Fe2+are 722.3 eV and 709.0 eV [24]. In case of Fe3+ as found in Fe2O3, these peak positions are found to be at 724.6 eV and 711 eV, respectively [24]. Thus XPS peak positions along with ‘shoulder’ satellite show a clear evidence of Fe2+ oxidation state in Zn0.85Fe0.15O. Fig. 5b demonstrates two main peaks of Co 2p3/2 and Co 2p1/2 located at 780.4 and 796.0 eV, along with their satellites in Zn0.75Co0.25O.Reported XPS data [17,25] shows that there are clear differences between Co(0) in Co metal, Co2+in CoO and Co3+ in Co2O3 with respect to the primary and satellite peak of Co 2p3/2 and 2p1/ 2+ are also quite different from that of 2energies. The line shapes of Co 3+ Co . Comparison with these reference spectra provides useful fingerprints for determining the oxidation state of Co and reveals that Co, in Zn0.75Co0.25O, is in the 2+ oxidation state. The oxidation state of Ni doped in ZnO was also investigated by XPS experiment. The results are shown in Fig. 5c. Two main peaks of 2p3/2 and 2p1/2are found to be centered ~ 854.4 and ~ 872.4 eV, 336
Materials Science in Semiconductor Processing 91 (2019) 333–340
A. Mondal et al.
Fig. 5. High-resolution XPS survey scan of (a) Zn0.85Fe0.15O, (b) Zn0.75Co0.25O and (c) Zn0.93Ni0.07O. The colored solid lines in the insets correspond to Fe/Co/Ni 2p1/ 2, 2p3/2 and its satellite along with the resultant Gaussian fitted curve superimposed on the experimental data.
degrade MB by ε ~ 50% after 150 min of irradiation whereas, the degradation efficiency was significantly enhanced (~94.5%) by Zn0.85Fe0.15O under same condition as depicted in Fig. 8a. Figure 8dillustrates the comparative study of photocatalytic activity of Zn1xTMxO (TM = Fe, Co, Ni) with different doping (x) for a particular time (150 min) of irradiation. It illustrates a significantly sharp peak for Zn1xFexO with x = 0.15. This significant enhancement of this activity for x = 0.15 does not corroborate with the surface area as the particle size is much larger (Fig. 4d) compared to that of x = 0.13. This colossal increment of photocatalytic activity may be attributed to enhanced defect sites. Doping of ZnO with Fe adds defect sites in the vicinity of valence band and thus reduces the effective band gap. When UV–vis light is passed through sample the electron–hole pair is generated within the effective band gap. This transition corresponds to lower energy than the band gap of ZnO. Thus surface defect, reduction in band gap all act in harmony with the enhancement of photocatalytic
3.3. Photocatalytic properties The photocatalytic activity of Zn1-xTMxO (Fe, Co, Ni) was evaluated by the degradation of MB dye under UV–vis irradiation. Degradation of MB dye by the photocatalyst Zn1-xTMxO was monitored through the reduction of characteristic absorption peak height. The percentage of C −C degradation of MB dye defined as ε = t C 0 × 100 is the measure of 0 degradation efficiency of the photocatalyst [33]. Here c0 is the initial concentration of the dye MB and ct is the concentration of residual MB after t s irradiation. The variation of degradation efficiency (ε) of Zn1xFexO, Zn1-xCoxO and Zn1-xNixO with the time (t) of irradiation are shown in Fig. 8a, b, c, respectively. Photolysis experiment was also carried out by irradiating the MB solution with UV radiation. The degradation of MB, due to photolysis without TM doped ZnO photocatalyst was found to be 34% after 150 min of irradiation. Then as a photocatalyst ZnO nanoparticles were added and it was found to
Fig. 6. Taucs plot corresponding to UV–vis absorbance spectra of (a) Zn1-xFexO, (b) Zn1-xCoxO and (c) Zn1-xNixO are shown. 337
Materials Science in Semiconductor Processing 91 (2019) 333–340
A. Mondal et al.
Fig. 7. Variation of optical band gap of (a) Zn1-xFexO, (b) Zn1-xCoxO and (c) Zn1-xNixO with x.
activity and make Zn0.85Fe0.15O the best photocatalytic among all TM doped samples as reported here. Fig. 8 dreveals gradual rise of photocatalytic activity of Zn1-xCoxO with increase of Co doping (x) in the range 0 ≤ x ≤ 0.25. Degradation efficiency of Zn0.75Co0.25O for 150 min irradiation is 81%. On the other hand, Zn0.93Ni0.07O exhibits an optimum doping of Ni for which degradation (83%) is maximum. Beyond this dopant content, photocatalytic activity of Zn1-xNixO decreases. The excessive dopant ions induce intrinsic point defects which may serve as recombination centers and lead to quenching of the photocatalytic activity as explained by Chauhan et al. [34,35]. It is worthwhile to mention that the defects introduced by different TM having dissimilar ionic radius are different. So the photocatalytic activity can be tailored desirably by doping different TM in ZnO in appropriate amount; however, excessive dopant ions are detrimental [34]. Total organic carbons (TOC) content which is an important indicator of mineralization was also monitored during the photodegradation process. TOC conversion is calculated [36] using the following relation, TOCConversion(%) = TOC0 − TOCt , where, TOC0 and TOC0 TOCt represent the TOC at initial and after t s of photodegradation process, respectively. The kinetic linear simulation curves based on the Langmuir–Hinshelwood first-order kinetic model of TOC removal is shown in Fig. 9. The values of TOC rate constants κTOC are obtained from
TOC
Fig. 9. The ln TOC0 vs irradiation time plots of mineralization of MB by ZnO, t
Zn0.85Fe0.15O, Zn0.75Co0.25O and Zn0.93Ni0.07O as photocatalysts.
the slope of the linear curves and corresponding values of percentage of TOC conversion are tabulated in Table 1. Decrease of TOC content with the increase of irradiation time indicates that MB degrades into nontoxic compounds. The mineralization of MB dye is found to be higher in presence of TM doped ZnO photocatalysts compared to that of pure
Fig. 8. Photodegradation efficiency (ε) of (a) Zn1-xFexO (0 ≤ x ≤ 0.20), (b) Zn1-xCoxO (0 ≤ x ≤ 0.25) and (c) Zn1-xNixO (0 ≤ x ≤ 0.11) under different irradiation time.(d) Comparison of photocatalytical performance of Zn1-xFexO, Zn1-xCoxO and Zn1-xNixO for different dopant concentration under 150 min irradiation. 338
Materials Science in Semiconductor Processing 91 (2019) 333–340
A. Mondal et al.
degradation was investigated by performing three cycles of the photocatalytic experiment of Zn0.85Fe0.15O. In the first cycle, MB concentration decreases to ct / c0 = 5.5% in 150 min. After separating out the used Zn0.85Fe0.15O from the mixture, it was added to an identical aqueous solution of MB and stirred under irradiation for another 150 min. The third cycle was followed in a similar way. Fig. 11 exhibits almost negligible loss of activity during three cycles of photocatalytic experiments [40]. Thus, both the enhancement of photocatalytic activity and reusability of Zn0.85Fe0.15O make it advantageous for its application as a photocatalyst.
Table 1 Rate constant (κ ), photo degradation efficiency (ε), TOC rate constant (κTOC ) and percentage of TOC Conversion of a representative member of Zn1-xTMxO (TM = Fe, Co, Ni) photocatalyst series. Samples
κ (min−1)
ε
κTOC (min−1)
Percentage of TOC Conversion (%)
ZnO Zn0.85Fe0.15O Zn0.75Co0.25O Zn0.93Ni0.07O
0.00466 0.02049 0.01010 0.01216
50% 94.5% 81% 83%
0.00465 0.01774 0.01130 0.01120
49% 93.2% 80% 81.9%
4. Summary and conclusions Tailoring of photocatalytic activity of ZnO had been investigated by doping TM (=Fe, Co, Ni) with different doping concentration x. For each series of Zn1-xTMxO, attempt was made for maximum possible doping of TM which is achievable by the co-precipitation route as discussed earlier. Only single phase wurzite structured samples without any impurity phase were picked up for photocatalysis study. The photocatalytic activity of Zn1-xTMxO was investigated by studying the degradation of MB dye (5 ppm MB in 100 ml aqueous solution) under UV and visible light irradiation. The activity of Zn1-xTMxO was found to be significantly higher than that of undoped ZnO. This enhancement in activity may be due to increase in surface area, decrease in optical band gap, defect, increase in conductivity etc. A decrease in the particle size will increase the surface area and thus increase the active site on the surface, which consequently leads to a higher for photocatalytic activity. However, in the present study, the particle size corresponding to the best photocatalyst among the members Zn1-xTMxO is not the minimum one. Hence, the average crystallite size may play an important role but is not a decisive factor for better photocatalysis. Gradual decrease of band gap of ZnO due to TM doping make it efficient photocatalyst under sun light irradiation as solar spectrum contains 46% visible, 47% IR and 5–7% of UV light. Semiconductor having narrower band gap requires lesser energy for transition of electron from valance to conduction band and thus it can harvest more photons to excite the electron from the valence band. Moreover, dopant introduced in a ZnO matrix can serve as a trap for an electron or hole if the energy of that electron or hole is just below the conduction band or just above the valence band, respectively. This trapped electron or hole will be migrated to the surface of photocatalyst and undergoes redox reaction with MB molecule. Thus the defects caused by TM (= Fe, Co, Ni) doping in ZnO can suppress recombination of the electron–hole pairs and TM doped ZnO displays better photocatalytic properties than pure ZnO. It is observed here that the photocatalytic activity of Zn1xCoxO increases gradually with Co doping (x) whereas Zn1-xNixO exhibits a maximum activity at x = 0.07. However, Zn1-xFexO reveals significantly enhanced photocatalytic activity for x = 0.15. On further doping of Fe, activity decreases. The colossal activity of Zn0.85Fe0.15O which is measured by the percentage of degradation (94.5%) of MB makes this member the best photocatalyst among Zn1-xTMxO (TM= Fe, Co. Ni) series.
Fig. 10. Pseudo-first order kinetic plot for the degradation of MB using (a) ZnO, (b) Zn0.85Fe0.15O, (c) Zn0.75Co0.25O and (d) Zn0.93Ni0.07O as photocatalysts.
Fig. 11. Cyclic photodegradation of MB by Zn0.85Fe0.15O for three cycles.
ZnO. Moreover, Zn0.85Fe0.15O photocatalyst exhibits the maximum TOC conversion of MB as expected. The photocatalytic processes generally follows pseudo-first-order kinetic with respect to Langmuir–Hinshelwood reaction mechanism,
( ) = −κt , where,κ is the pseudo-first-order rate constant. This can be used to compare the photocatalytic activity of a series of photocatalysts [37–39]. The plots of ln ( ) as a function of irradiation times
ln
Ct C0
Acknowledgment
C0 Ct
(t) are shown in Fig. 10a–d for undoped ZnO and Zn0.85Fe0.15O, Zn0.75Co0.25O, Zn0.93Ni0.07O, respectively which correspond to the highest photodegradation efficiency in their respective series. The values of rate constants (κ ) obtained from the slope of the linear curves and corresponding degradation efficiency (ε) are tabulated in Table 1.
Authors would like to thank DST, Govt. of India for developing instrumental facilities like Advance X-ray powder diffractometer (Bruker D8) and FE-SEM (JEOL, JSM-7610F) under FIST program at Jadavpur University. S. Sarkar, A. Mondal and N. Giri wish to thank DST for INSPIRE fellowship scheme.
3.4. Reuse of the catalyst system
References
The stability and reusability of photocatalyst during photocatalytic reaction is a crucial factor for the practical applications and economical point of view. The reusability of the photocatalyst for MB photo-
[1] M.A. Lazar, W.A. Daoud, Achieving selectivity in TiO2-based photocatalysis, RSC Adv. 3 (2013) 4130–4140. [2] S. Cho, J.W. Jang, J.S. Lee, K.H. Lee, Porous ZnO–ZnSe nanocomposites for visible
339
Materials Science in Semiconductor Processing 91 (2019) 333–340
A. Mondal et al.
Al2O3(0001) by radio-frequency sputtering, Appl. Phys. 96 (2004) 4150–4153. [23] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. Sect. A 32 (5) (1976) 751–767. [24] T. Yamashita, P. Hayes, Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials, Appl. Surf. Sci. 254 (2008) 2441–2449. [25] R. He, B. Tang, C. Ton-That, M. Phillips, T. Tsuzuki, Physical structure and optical properties of Co-doped ZnO nanoparticles prepared by co-precipitation, J. Nanopart. Res. 15 (11) (2013) 1–8. [26] B. Pal, D. Sarkar, P.K. Giri, Structural, optical, and magnetic properties of Ni doped ZnO nanoparticles: correlation of magnetic moment with defect density, Appl. Surf. Sci. 356 (2015) 804–811. [27] A. Mondal, N. Giri, S. Sarkar, R. Ray, Magnetic properties of Zn1-xNixO, AIP Conf. Proc. 1953 (2018) 120017 (1–4). [28] Z.C. Chen, L.J. Zhuge, X.M. Wu, Y.D. Meng, Initial study on the structure and optical properties of Zn1−xFexO films, Thin Solid Films 515 (2007) 5462–5465. [29] L. Xu, X. Li, Growth and characterization of Mg-doped AlGaN–AlN short-period superlattices for deep-UV optoelectronic devices, J. Cryst. Growth 312 (2010) 756–761. [30] G. Murugadoss, Synthesis and characterization of transition metals doped ZnO nanorods, J. Mater. Sci. Technol. 28 (7) (2012) 587–593. [31] A. Parra-Palomino, O. Perales-Perez, R. Singhal, M. Tomar, J. Hwang, P.M. Voyles, Evidence of ferromagnetism in Zn1−xMxO (M = Ni,Cu) nanocrystals for spintronics, J. Appl. Phys. 18 (2007) 315606 (5 pp). [32] C.B. Fitzgerald, M. Venkatesan, J.G. Lunney, L.S. Dornelse, J.M.D. Coey, Anisotropic ferromagnetism in substituted zinc oxide, Appl. Surf. Sci. 247 (2005) 493–496. [33] A. Hamza, I.J. John, B. Mukhtar, Enhanced photocatalytic activity of calcined natural sphalerite under visible light irradiation, J. Mater. Res. Technol. 6 (1) (2017) 1–6. [34] R. Chauhan, A. Kumar, R.P. Chaudhary, Photocatalytic degradation of methylene blue with Fe doped ZnS nanoparticles, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 113 (2013) 250–256. [35] L. Wang, Z. Ji, J. Lin, P. Li, Preparation and optical and photocatalytic properties of Ce-doped ZnO microstructures by simple solution method, Mater. Sci. Semicond. Process. 71 (2017) 401–408. [36] O. Bechambi, A. Touati, S. Sayadi, W. Najjar, Effect of cerium doping on the textural, structural and optical properties of zinc oxide: role of cerium and hydrogen peroxide to enhance the photocatalytic degradation of endocrine disrupting compounds, Mater. Sci. Semicond. Process. 39 (2015) 807–816. [37] A. Ashkarrana, M. Fakharia, H. Hamidinezhadb, H. Haddadic, M.R. Nouranid, TiO2 nanoparticles immobilized on carbon nanotubes for enhanced visible-light photoinduced activity, J. Mater. Res. Technol. 4 (2) (2015) 126–132. [38] A. Peticaa, A. Floreab, C. Gaidaua, D. Balan, L. Anicai, Synthesis and characterization of silver-titania nanocomposites prepared by electrochemical method with enhanced photocatalytic characteristics, antifungal and antimicrobial activity, J. Mater. Res. Technol. 324 (2017) 1–13. [39] A. Ulyankina, I. Leontyev, M. Avramenko, D. Zhigunov, N. Smirnova, Large-scale synthesis of ZnO nanostructures by pulse electrochemical method and their photocatalytic properties, Mater. Sci. Semicond. Process. 76 (2018) 7–13. [40] Y. Yu, C. Yu, T. Xu, H. Sun, Optical and photocatalytic properties of indium phosphide nanoneedles and Nanotubes, Mater. Sci. Semicond. Process. 68 (2017) 270–274.
light photocatalysis, Nanoscale 4 (2012) 2066–2071. [3] S.G. Kumar, L.G. Devi, J. Review on modified TiO2 photocatalysis under UV/Visible light: selected results and related mechanisms on interfacial charge carrier transfer dynamics, Phys. Chem. A115 (2011) 13211–13241. [4] C. Karunakaran, V. Rajeswari, P. Gomathisankar, Optical, electrical, photocatalytic, and bactericidal properties of microwave synthesized nanocrystalline Ag–ZnO and ZnO, Solid State Sci. 13 (2011) 923–928. [5] R. Vinu, G. Madras, Environmental remediation by photocatalysis, J. Indian Inst. Sci. 90 (2010) 189–229. [6] E.R. Caraway, A.J. Hoffman, M. Hoffmann, Photocatalytic oxidation of organic acids onquantum-sized semiconductor colloids, Environ. Sci. Technol. 28 (1994) 786–793. [7] K. Thongsuriwong, P. Amornpitoksuk, S. Suwanboon, Photocatalytic and 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. [8] U.I. Gaya, A.H. Abdullah, Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide, J. Photochem. Photobiol. C: Photochem. Rev. 9 (2008) 1–12. [9] A. Moezi, A.M. McDonagh, M.B. Cortie, Chemical etching preparation of BiOI/ BiOBr heterostructures with enhanced photocatalytic properties for organic dye removal, J. Chem. Eng. 185/186 (2012) 1–22. [10] R. Srivastava, B.C. Yadav, Nanaostructured ZnO, ZnO-TiO2 and ZnO-Nb2O5 as solid state humidity sensor, Adv. Mater. Lett. 3. 3 (2012) 197–203. [11] R. Saleh, N.F. Djaja, Transition-metal-doped ZnO nanoparticles: synthesis, characterization and photocatalytic activity under UV light, Spectrochim. Part A: Mol. Biomol. Spectrosc. 130 (2014) 581–590. [12] Z.C. Chen, L.J. Zhuge, X.M. Wu, Y.D. Meng, Initial study on the structure and optical properties of Zn1−xFexO films, Thin Solid Films 515 (2017) 5462–5465. [13] C. Xu, L. Cao, G. Su, W. Liu, X. Qu, Y. Yu, Preparation, characterization and photocatalytic activity of Co-doped ZnO powders, J. Alloy. Compd. 497 (2010) 373–376. [14] S. Kant, A. Kumar, A comparative analysis of structural, optical and photocatalytic properties of ZnO and Ni doped ZnO nanospheres prepared by sol-gel method, Adv. Mater. Lett. 3 (4) (2012) 350–354. [15] X. Yu, D. Meng, C. Liu, K. Xu, J. Chen, C. Lu, Y. Wang, Enhanced photocatalytic activity of Fe-doped ZnO nanoparticles synthesized via a two-step sol–gel method, J. Mater. Sci: Mater. Electron 25 (2014) 3920–3923. [16] A. Mondal, S. Sarkar, N. Giri, S. Chatterjee, R. Ray, Magnetic phase separation in diluted magnetic system: Zn1-xFexO, Acta Metall. Sin. (Engl. Lett.) 30 (6) (2017) 521–527. [17] J. Hays, K.M. Reddy, N.Y. Graces, M.H. Engelhard, V. Shutthanandan, M. Luo, C. Xu, N.C. Giles, C. Wang, S. Thevuthasan, A. Punnoose, Fluorescent particles comprising nanoscale ZnO layer and exhibiting cell-specific toxicity, J. Phys.: Condens. Matter 19 (2007) 266203 (24pp). [18] A.S. Risbud, N.A. Spaldin, Z.Q. Chen, S. Stemmer, R. Seshadri, Magnetism in polycrystalline cobalt-substituted zinc oxide, Phys. Rev. B 68 (2003) 1–7 (205202). [19] Z.C. Chen, L.J. Zhuge, X.M. Wu, Y.D. Meng, Initial study on the structure and optical properties of Zn1-xFexO films, Thin Solid Films 515 (2007) 5462–5465. [20] L. Xu, X. Li, Influence of Fe-doping on the structural and optical properties of ZnO thin films prepared by sol–gel method, J. Cryst. Growth 312 (2010) 851–855. [21] D. Karmakar, S.K. Mandal, R.M. Kadam, P.L. Paulose, A.K. Rajarajan, T.K. Nath, A.K. Das, I. Dasgupta, G.P. Das, Ferromagnetism in Fe-doped ZnO nanocrystals: experiment and theory, Phys. Rev. B 75 (2007) 1–14 (144404). [22] K.J. Kim, Y.R. Park, J. Optical investigation of Zn1−xFexO films grown on
340