Structural, optical and photocatalytic studies of trimetallic oxides nanostructures prepared via wet chemical approach

Synthetic Metals 259 (2020) 116228

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Structural, optical and photocatalytic studies of trimetallic oxides nanostructures prepared via wet chemical approach

T

Abdur Rahmana, Humera Sabeeha, Sonia Zulfiqarb, Philips Olaleye Agboolac, Imran Shakird, Muhammad Farooq Warsia,* a

Department of Chemistry, Baghdad-ul-Jadeed Campus, The Islamia University of Bahawalpur, Bahawalpur, 63100, Pakistan Department of Chemistry, School of Sciences & Engineering, The American University in Cairo, New Cairo, 11835, Egypt c College of Engineering Al-Muzahmia Branch, King Saud University, PO-BOX 800, Riyadh, 11421, Saudi Arabia d Sustainable Energy Technologies (SET) Center, College of Engineering, King Saud University, PO-BOX 800, Riyadh, 11421, Saudi Arabia b

A R T I C LE I N FO

A B S T R A C T

Keywords: Nanocomposite Photocatalyst Heterojunction Methylene blue Visible light

Multi metal based photocatalyst (CdO-MgO-Fe2O3) was synthesized via wet chemical approach i.e. co-precipitation method. The single transition metal oxides like CdO, MgO and Fe2O3 were also prepared for comparative studies. Structural elucidation of all prepared powder catalysts (CdO, MgO, Fe2O3 and CdO-MgO-Fe2O3) was conducted by X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). XRD showed the presence of the MgO (cubic) – Fe2O3 (rhombohedral)– CdO (cubic) phase with an average crystallite size of MgO (9 nm), Fe2O3 (12 nm), CdO (23 nm). FTIR proved the existence of Mg–O, Fe2-O3 and Cd-O by distinctive vibrational bands at 772 cm−1, 581 cm−1 and 480 cm−1 respectively. UV–vis spectral studies showed that the bandgap for CdO-MgO-Fe2O3 nanocomposite was 1.76 eV which made it an efficient photocatalyst for use both in the UV and visible regions. Two probe I–V measurements confirmed the pure semiconducting behavior and possible formation of heterojunction in the CdO-MgO-Fe2O3 nanocomposite. The photocatalytic performance of the CdO-MgO-Fe2O3 nanocomposite with equal metal ion concentration of Mg, Fe and Cd at different pH values was explored by visible light illumination on methylene blue (MB) dye as a standard organic pollutant. The CdOMgO-Fe2O3 nanocomposite with metal ion ratio 1:1:1 in an alkaline medium showed the maximum photocatalytic performance. The rate constant (kMB) value for the degradation of MB was ∼ 0.0820 min−1. The photocatalytic activity has been improved due to the effective decrease in the recombination rate of charge carriers. The possible mechanism for improvement of the photocatalytic performance using visible light illumination has also been suggested and discussed.

1. Introduction Consumable water is progressively getting to be scarcer because of a large increase in contamination of hazardous compounds and substances. Organic compounds based dyes are commonly present in wastewater effluents of various industries. These organic compounds and dyes, i.e. rhodamine 6 G, methylene blue and methyl orange, are very lethal and non-biodegradable, imposing an extraordinary danger to amphibian life and other living organisms [1–3]. A considerable number of organic dyes alongside their resulting products have cancercausing consequences for people [4,5]. For instance, rhodamine 6 G, phenol and its derivatives are highly cancer-causing and have antagonistic impact on amphibian life. Tainting of water by these mixes is a serious issue to the entire ecosystem [6–8]. Dyes and phenolic mixes are

⁎

produced as effluents from different industries. These pollutants are not treated properly before releasing into nature [9]. Semiconductor photocatalysis, which is a proficient strategy to change over photon energy into chemical energy and deteriorate contaminations in air and water, has pulled in extraordinary research intrigue [10–12]. The procedure depends on the way that photo-generated charges can be captured onto the photocatalyst surface which can result in interfacial electrontransfer response with different substrates. Metal oxides are the best choice for heterogeneous photocatalysis to dispense with different dyes, phenolic mixes, toxins, and so forth. From the most recent couple of decades, TiO2 has been considered as a standout amongst the best photocatalysts because of its strong oxidizing power and comparatively non-poisonous behavior. However, because of the wide band gap (3.2 eV), the TiO2 could not be commercialized as it absorbs in the UV

Corresponding author. E-mail address: [email protected] (M.F. Warsi).

https://doi.org/10.1016/j.synthmet.2019.116228 Received 25 August 2019; Received in revised form 19 October 2019; Accepted 1 November 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.

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MgO-Fe2O3). The photocatalytic activity of the resulting composite was examined against methylene blue dye under visible light irradiation. The function of photoactive species was examined by adopting a radical scavenger approach. Based on bandgap alignment and optical characterization, a systematic photocatalytic model was proposed to illustrate the photocatalytic degradation mechanism. The results recommended that the synthesized nanocomposites with varying cadmium and magnesium contents have a potential suitability for utilizing as a successful photocatalyst under both UV and visible light irradiation.

region [13–15]. This problem can be solved by many approaches. For example, its badgap can be tuned and engineered from UV-region to visible region by doping it with some metal or/and non-metal ions. Secondly, metal oxides other than titanium oxide can be prepared. Researchers are working to address this issue using many approaches simultaneously. Some are tailoring the properties of TiO2, while others are preparing new metal oxides based photocatalysts. Recently, much attention has been paid to synthesize multi metal based semiconducting composites and their hybrid materials for photocatalytic applications in the presence of visible light. Significant consideration is being paid on structuring the mixed metal oxide semiconductor composites in order to enhance their photocatalytic activity under visible light irradiation, especially for drinking water purification [16]. The semi-conducting materials can be connected together and photo-generated electrons can move easily to the mixed metal oxide surface [17–19]. Thus, the performance of photocatalytic materials can be enhanced. The mixed metal oxide framework can get structures in the mix with the properties that are not possible for a single metal oxide. The coupling of few semiconductors with reasonable energetics can notably improve the selectivity and enhance the charge detachment yield. Cadmium oxide (CdO) is an n-type semiconductor material having direct optical band-gap (2.2–2.5 eV) with versatile applications in the field of electronics and catalysis [20,21]. CdO semiconductor is extensively utilized in photocatalytic decomposition of organic pollutants [22–25]. MgO is an insulator with optical bandgap ∼7 eV and has been generally utilized as an adsorbent, catalyst and in superconductors [26,27]. The insulating metal oxides have a high surface area which improves the dye adsorption and in this way enhancing the photocurrent. Fe2O3 is a narrow band gap (1.9–2.2 eV) semiconductor, act as a donor which can be utilized as a sensitizer under visible light because it tends to be exchange electrons to wide band gap semiconductors [28,29]. Because of the lower band gap (≤ 2.2), fast electron-hole pair recombination restricts its application towards photocatalysis. There are following possible advantages of combining the visible light active CdO and Fe2O3 with a semiconductor having large bandgap like MgO. First, the probability of recombination of photo-excited electron-hole pair is reduced. Secondly the MgO may also result an increased absorption of light and thus the and photocatalytic activity of CdO-MgOFe2O3 nanocomposite. The main idea behind the combination of CdOMgO-Fe2O3 nanocomposite is to improve the properties of individual oxides present in the composite. These include the expansion in surface area, minimization of photo-excited electron-hole pair recombination rate. Mixed metal oxides contain high carrier concentration and high mobility and these semiconductors find potential applications in various fields [30]. Recently, many researchers have investigated the multi-metal-oxide (MMO) based nanocomposites to test their performance as luminescent [31], catalysis [32], adsorbent [33] and wideband-gap semiconductors [34]. Past reports propose that various MMO hybrids have been synthesized and characterized for various potential applications including the photocatalytic degradation of organic pollutants. Qi Yang et al. [35] has investigated photo-degradation of methylene blue and tetracycline from their aqueous solution using Zn/Fe mixed metal oxide (Zn/Fe-MMO) nanocomposite. Guoli et al. [36] has explored the Zn-Al-In multi metal oxide for photocatalytic degradation of methylene blue using a visible light source. Duo Pan et al. [37] has prepared a ternary Ni-Co-Fe mixed metal oxide semiconductor and characterized it. They studied their photocatalytic applications for removal of toxic compounds. Literature survey revealed that still there is a large gap to be covered by researchers regarding multi metal oxides to study their photocatalytic applications. In this work, various compositions of CdO-MgO-Fe2O3 nanocomposite were prepared for the first time by varying precursors concentration. A facile chemical route (co-precipitation method) was used, as it is simple, low cost and requires low growth temperature. By combining the three metal oxides, the individual properties of the metal oxides (CdO, MgO and Fe2O3) are added to the nanocomposite (CdO-

2. Experimental work 2.1. Materials used Iron nitrate nonahydrate (97 % Fe(NO3)2.9H2O, Sigma Aldrich), Cadmium nitrate tetrahydrate (99 % Cd(NO3)2.4H2O, BDH), Magnesium nitrate hexahydrate (99 % Mg(NO3)2.6H2O Sigma Aldrich) and Sodium carbonate (99.9 % Na2CO3 Sigma Aldrich) were used without any purification for the synthesis of various nanocrystalline metal oxide powdered photocatalysts. All these chemicals were of analytical grade.

2.2. MgO, CdO, Fe2O3 nanoparticles synthesis For the synthesis of iron oxide (Fe2O3) nanoparticles, 40 mL of a freshly prepared solution of Fe(NO3)2·9H2O (0.25 M) in deionized water was taken in a beaker and stirred for 15 min at 70 °C. To this solution, 100 mL of freshly prepared solution of Na2CO3 (0.5 M) in deionized water was added. This reaction mixture was stirred for 6 h. The precipitates were separated and washed thoroughly with deionised water for five times. Initially, the washed sample was dried in an oven at 90 °C for one hour. Then the same sample was again dried at 120 °C for two hours. Calcination of the dried sample was carried out in a Muffle Furnace for four hours at 650 °C. During calcination, the carbonates of the samples were converted into their corresponding oxides as shown by the following possible reaction mechanism:

M2 (CO3)n → M2 (O )n + nCO2

(1)

Cadmium oxide (CdO) and magnesium oxide (MgO) nanoparticles were also prepared by using the same procedure.

2.3. MgO-CdO-Fe2O3 nanocomposite synthesis The MgO-CdO-Fe2O3 mixed metal oxide nanocomposite was prepared by a simple co-precipitation method (Scheme 1). Briefly, 0.25 M aqueous solutions of Fe(NO3)2.9H2O, Cd(NO3)2.4H2O and Mg (NO3)2.6H2O were prepared in deionized water. A solution of 0.5 M Na2CO3 was also prepared in deionized water. The solutions were mixed in stoichiometric amounts in a beaker in varying ratios of cadmium and iron while keeping magnesium constant, [(1:1:1); (0.75:1:1.25), (0.50:1:1.5), (0.25:1:1.75) and (0.125:1:1.875)]. The reaction mixtures were stirred for 30 min at 70 °C. Later on, add 60 mL of a freshly prepared 0.5 M Na2CO3 solution to all reaction mixtures. The resulting precipitates were separated and washed with deionized water for five times. Initially, these precipitates were dried at 90 °C for one hour in an oven. Later on, the temperature of the oven was raised to 120 °C. The as prepared samples were calcined for six hours at 650 °C using a Muffle Furnace. The purpose of calcination was to convert the metal carbonates into their corresponding metal oxides as shown by the following reaction.

Metal Carbonate(s) → Metal Oxide(s) + Carbon dioxide(g ) 2

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Scheme 1. Synthesis route for CdO-MgO-Fe2O3 nanocomposite.

(wavelength = 1.5405 Å) radiations within 2θ range of 30–80°. FTIR spectra, which are responsible for the study of vibrational and rotational energy bands and molecular arrangement in the material, were recorded by FTIR Analyzer (Nicolet/Avatar 370). DC electrical resistivity and conductivity analyses were performed on KEITHLEY voltage source meter (6487 Pico ammeter). UV–vis spectra were taken on a Carry-60 dual beam UV–Vis spectrophotometer.

2.4. Photocatalytic experiments Wastewater released from textile enterprises normally has a varying pH range. Very often, pH assumes a fundamental job in the quality of dye wastewaters and is a standout amongst the most significant parameters that impact photo-oxidation processes [38]. In this photocatalytic experiment, pH of dye solution was controlled by 0.2 M HCl and 0.2 M NaOH solutions. The photocatalytic activity of the nanocomposite at pH 6.0, pH 7.0, pH 9.0 and pH 11.0 was explored through degradation analysis of methylene blue (MB) dye as model pollutant. The 200 W bulb was used as source of visible light. Ultraviolet light cut off filter was also used. Photocatalytic tests were completed in a dark room and the separation between the light source and the photoreaction vessel was around 20 cm. For the photocatalytic performance estimation of the nanocomposite at 30 °C, 0.005 g of MgO-CdO-Fe2O3 nanocomposite was dispersed in 45 mL of aqueous solution of methylene blue dye with stock solution concentration of 5 ppm. Later on, 5 mL of 10 % H2O2 solution was also added to above reaction mixture and pH was adjusted accordingly. The attainment of equilibrium stage in the adsorption-desorption process is the major initial part for the degradation of organic dyes in the presence of photocatalyst. Therefore, the suspensions were magnetically mixed for 40 min in the absence of light, ensuring the attainment of the equilibrium stage in the adsorption-desorption process among MB and photocatalyst. To make it sure, the consistency of % adsorption of MB dye in the absence of light was observed. While mixing consistently, the resulting suspension was illuminated by visible light. After fixed time durations, 4 mL of the suspension was transferred to a centrifuge tube and separated the residue photocatalyst using centrifuge machine at 6000 rpm for five minutes. The amount (concentration) of methylene blue in the filtrate was then examined by utilizing a Carry-60 UV–vis dual beam spectrophotometer. The absorption peak heights in the UV–vis spectrum of the methylene blue confirmed the activity of photocatalyst. The degradation and percent degradation of methylene blue (MB) in the presence of nanocomposite as photocatalyst under the visible light irradiation were obtained utilizing the following equation:

Dye degradation (%) = (Xo − Xt )/Xo ⨯100

3. Results and discussion 3.1. Phase analysis Crystalline structure and phase identification of the prepared samples were investigated by X-ray diffractometer. X-ray diffraction patterns of pure CdO, MgO, Fe2O3 nanoparticles and their CdO-MgO-Fe2O3 nanocomposite are shown in the Fig. 1(a–d). As it can be seen from Fig. 1, the diffraction peaks attributed to CdO, MgO and Fe2O3 metal oxide nanoparticles are well resolved because of complete decay of the metal carbonates to metal oxides. The nanocomposite is in this way a combination of individual oxide phases coinciding in one material. The structure of CdO was found face centered cubic (FCC). The CdO peaks appeared at the 2θ values of 33.15, 38.45, 55.40, 65.99 and 69.41°. These peaks are due to reflections from the planes of (111), (200), (220), (311) and (222) respectively. They are compatible with ICCD: 03-065-2908. MgO nanoparticles were very much coordinated with the cubic crystalline structure of MgO. The diffracted peaks belonging to MgO appearing at 2θ values of 42.68, 62.01, 74.45 and 78.15° are related to the reflections from (200), (220), (311) and (222) planes, respectively. These peaks are found in agreement with ICCD: 01-0897746. Fe2O3 nanoparticles and were very much coordinated with the rhombohedral crystalline structure of Fe2O3. The diffraction pattern for Fe2O3 nanoparticles appearing at 2θ values of 33.15, 35.53, 40.83, 49.29, 54.33, 62.55, 64.11, 69.94, 72.07 and 76.30° corresponding to (104), (110), (113), (024), (116), (214), (300), (208), (1010) and (220) hkl values, respectively. These peaks were found comparable with ICCD: 01-079-1741. The XRD pattern of CdO-MgO-Fe2O3 nanocomposite confirmed that three types of phases (Fig. 1d). For example, prevalent phases are cubic MgO, rhombohedral Fe2O3 and the small peaks are present indicating cubic CdO. The diffracted peaks are very much coordinated with the standard JCPDS card no. 03-065-2908 (CdO), 01079-1741 (Fe2O3) and 01-089-7746 (MgO). The average crystallite size of the synthesized nanoparticles (CdO, MgO & Fe2O3) and nanocomposite (CdO-MgO-Fe2O3) was calculated by Debye-Scherrer formula as given in the equation 3,

(2)

Here, Xo is the organic dye concentration in the beginning and Xt is the dye concentration at fixed time intervals during irradiation. 2.5. Characterizations The structural, morphological, electrical and photocatalytic studies of CdO, MgO, Fe2O3 nanoparticles and CdO-MgO-Fe2O3 nanocomposite were performed using X-ray diffractometer, Fourier Transform IR Spectroscope (FTIR), KEITHLEY voltage source meter and UV–vis Spectrophotometer. Structural analyses of the as prepared samples were carried out on D8-Advanced Bruker X-ray diffractometer using Cu Kα

D = Kλ / βcosθ

(3)

where D represents the crystallite size, the wavelength of the X-ray beam indicated by λ , K is the shape factor normally having constant value ∼0.99, FWHM (full width at the half maximum) is represented by 3

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Fig. 1. X-ray diffraction patterns of (a) MgO, (b) CdO, (c) Fe2O3 nanoparticles and (d) CdO-MgO-Fe2O3 nanocomposite.

β (related to instrumental peak broadening), and θ is Bragg’s angle. By using the Debye-Scherrer Eq. (3), the average crystallite sizes for individual Fe2O3, CdO and MgO phases were found 21 nm, 18 nm and 9 nm respectively. While for the nanocomposite, the average crystallite sizes of the Fe2O3, CdO and MgO phases were 10 nm, 32 nm and 14 nm respectively. From comparison, it is clear that the crystallite size of CdO and MgO phases in the nanocomposite increased while that of Fe2O3 phase decreased as compare to their individual oxides. Obviously, the development of CdO and MgO crystallites is improved in the nanocomposite while that of Fe2O3 is restrained. In X-ray diffraction, peak broadening is due to the crystallite size and the micro-strain contribution. The contribution by crystallite size is given by the Scherrer equation (Eq. 3) and the micro-strain [39,40],

βs = 4Ɛtanθ

method while a decrement for CdO and MgO nanoparticles due to the tensile and compressive strain respectively [43]. Dislocation density (δ) which is a measure of defects in the sample is determined by the Eq. (7).

δ = 1/ D 2

Here, represents the crystallite size and dislocation density (δ ) values were calculated from the crystallite sizes obtained from the Williamson Hall plot method as shown in the Fig. 2. The increment in the crystallite sizes is credited for the small dislocation density value because of the inverse relation between crystallite size and dislocation density parameters. The values of calculated lattice constants, crystallite sizes (Sherrer & W.H), micro-strain and dislocation density for synthesized CdO, MgO, Fe2O3 nanoparticles and CdO-MgO-Fe2O3 nanocomposite are given in Table 1.

(4)

Here, βs represents the FWHM due to the peak broadening by microstrain and value is given by Ɛ , which is expected to be identical in each of crystallographic directions. The peak broadening by micro-strain and by the instrument are not dependent upon each other and are thus additive given by [41] :

βhkl = βD + βs

3.2. FE-SEM study The surface morphology of the CdO-MgO-Fe2O3 nanocomposite was examined by FE-SEM. The SEM images of CdO-MgO-Fe2O3 nanocomposite at varying magnifications are presented in Fig. 3. It is obvious that the nanocomposite particles exhibited almost spherical morphology with varying particle size distribution in the range of 40–80 nm. Almost all the particles showed the clear grain boundaries and are distributed with a low degree of aggregation. The SEM images of CdO-MgO-Fe2O3 nanocomposite show that the morphology contains voids, caused by the release of hot gases from powdered sample during calcination.

(5)

Thus, Williamson Hall expression is given by combining Eq. (4) and Eq. (5) as given below [42],

βhkl cosθ = K ⨯λ / D + 4Ɛsinθ

(7)

D2

(6)

When plot the βhkl cosθ versus 4Ɛsinθ for CdO, MgO and Fe2O3 nanoparticles and CdO– MgO – Fe2O3 nanocomposite as represented in Fig. 2, the y- intercept gives the value of average crystallite size while slope gives micro-strain value. The crystallite sizes determined by Williamson’s Hall and Scherer methods for CdO, MgO and Fe2O3 nanoparticles are relatively same. However, for CdO-MgO-Fe2O3 nanocomposite the Williamson Hall method presented a somewhat increment in the value for Fe2O3 nanoparticles as compared to the Scherer

3.3. FTIR analysis The chemical composition and molecular structure of prepared samples were studied using FTIR spectroscopy. The FTIR spectra of CdO, MgO, Fe2O3 nanoparticles and CdO-MgO-Fe2O3 nanocomposite 4

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Fig. 2. Williamsons Hall plots for prepared nanoparticles and their composite.

3.4. Current-voltage (I–V) measurements

are shown in Fig. 4(a–d). The vibrational bands in the low wave number region 400– 850 cm-1 relate to the metal-oxygen (MeO) and oxygenmetal-oxygen (Oe MOe) bonds (Fig. 3). The sharp peak observed at 480 cm-1, due stretching and longitudinal optical phonon modes of vibration, is assigned to magnesium oxide (Mg– O). These mode of vibrations are specific for pure MgO nanoparticles [44]. The vibrational band at about 581 cm−1 is because of the presence of Fe2O3 in the sample [45]. The intense peak at 772 cm−1 is specified for the metallic bond vibration in cadmium oxide (CdO) [46,47]. The stretching mode caused by the carbon-oxygen (CeO) single bond in FTIR spectra corresponds to wavenumber 972 cm−1. While annealing at elevated temperature, CO2 released from the as prepared nanocomposite metal carbonates to form their corresponding oxides. Comparatively larger in size than MgO and Fe2O3, CdO nanoparticles may adsorb this released carbon dioxide gas (CO2) and exist as CdO.CO2. The band of vibration positioned at about 1395 cm−1 relates to carbonate ions (CO3 2−) attached to the surface of the grown sample, though the weak band of vibration at 2752 cm−1 is specified for the stretching vibrations of carbon dioxide molecules (CO2) because of adsorption of atmospheric carbon dioxide onto the surface of the grown sample [44,48,49]. The vibrational band appearing at 1481 cm−1 is ascribed to the carbonyl group (C]O) because of its bending mode of vibration. The broad bands positioned at 1636 cm−1 were observed due to the bending vibrations of hydroxyl groups (−OH) while the stretching vibrations of these functional groups were observed at 3396 cm−1. The presence of hydroxyl groups on the sample surface was due to adsorption of water molecules from the atmosphere. The surface hydroxyl groups are strongly held by hydrogen bonding [50–52]. During photocatalytic reaction, the role of the surface hydroxyl groups becomes very crucial because they prevent the recombination of photo-generated electron hole pairs and produce active oxygen species [53].

The electrical behavior of as prepared CdO, MgO, Fe2O3 nanoparticles and CdO-MgO-Fe2O3 nanocomposite was analyzed by currentvoltage (I–V) studies at room temperature. I–V curves recorded at room temperature for CdO, MgO, Fe2O3 nanoparticles and CdO-MgO-Fe2O3 nanocomposite are depicted in (Fig. 5). The curves clearly showed that the semiconducting behavior of CdO, MgO, Fe2O3 semiconductors has been tailored in their CdO-MgO-Fe2O3 composite. The current-voltage curve relating to the CdO-MgO-Fe2O3 nanocomposite illustrates a rectifying behavior, showing the possible development of diode heterojunction in the CdO-MgO-Fe2O3 nanocomposite [54]. The value of DC resistivity for CdO, MgO, Fe2O3 nanoparticles and CdO-MgO-Fe2O3 nanocomposite was determined using Eq. 8 given below:

ρ = R⨯A/ l

(8)

In this equation, “ ρ ” is DC resistivity value, “R ” represents the resistance, the area and thickness of the pellets of CdO, MgO, Fe2O3 nanoparticles and CdO-MgO-Fe2O3 nanocomposite are shown by “ A ” and “l” respectively. The values of DC resistivity for CdO, MgO, Fe2O3 nanoparticles and CdO-MgO-Fe2O3 nanocomposite were found to be 7.18 × 103, 1.44 × 108, 5.59 × 105 and 1.38 × 109 Ω cm respectively. From these results it is clear that the resistivity value has increased to 1.38 × 109 Ω cm in CdO-MgO-Fe2O3 nanocomposite. The increment in the value of resistivity noticed for CdO-MgO-Fe2O3 nanocomposite might be due to the replacement of Cd2+ ions with Mg2+ ions at the interstitial regions of the crystal lattice made by Fe2O3 and CdO, hence generating the structural deformation. The values of DC resistivity for CdO, MgO, Fe2O3 nanoparticles and CdO-MgO-Fe2O3 nanocomposite were then

Table 1 Structural parameters of CdO, MgO, Fe2O3 nanoparticles and CdO-MgO-Fe2O3 nanocomposite. Samples

Phases

Lattice Parameters a(Å) b(Å) c(Å)

Volume (Å)3

Crystallite Size (nm) Scherrer W.H

Dislocation density δx1014(line/ m2)

Strain Ɛ⨯10-4

Fe2O3 CdO MgO MgO-CdOFe2O3

Rhombohedral Cubic Cubic Fe2O3

5.0108 4.6917 4.2229 5.0305

5.0108 – – 5.0305

13.7623 – – 13.6548

299.251 103.274 75.306 299.253

23 20 11 10

25 27.7 100 69.4

−9.67 −3.63 3.46 −6.97

CdO MgO

4.6856 4.2272

– –

– –

102.872 75.537

32 23 14 10

18.9 123.4

2.41 −17.6

5

20 19 10 12

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Fig. 3. FE-SEM images of CdO-MgO-Fe2O3 nanocomposite at various magnification, (A) 7.57 K X, (B) 22.21 K X, (C) 43.65 K X, (D) 56.03 K X.

Fig. 4. FTIR spectra of a) MgO, b) CdO, c) Fe2O3 nanoparticles and d) CdO-MgO-Fe2O3 nanocomposite.

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Fig. 5. Characteristic I–V curves for a) MgO, b) CdO, c) Fe2O3 nanoparticles and d) CdO-MgO-Fe2O3 nanocomposite.

tetrahedral crystal field. Consequently, this wide absorption peak is ascribed to the presence of ferric ions (Fe3+) at octahedral sites. This wide absorption peak was observed due to shifting of ferric ions (Fe3+) to octahedral sites that were present at tetrahedral sites by increased iron and magnesium (Fe & Mg) contents. Moreover, the location of the absorption edge for CdO-MgO-Fe2O3 nanocomposite somewhat moves towards the higher wavelengths with respect to that of individual CdO, MgO, Fe2O3 nanoparticles. The noticed red-shift upon mixing of CdO, MgO, Fe2O3 nanoparticles shows the incorporation of Mg2+, Fe3+ and Cd2+ ions in the crystal lattice. The band-gap energy (Eg) corresponding to the absorption maxima was calculated by using the Taucplot method and given by the following equation [56],

transformed to electrical conductivity values. The resulting electrical conductivity values for CdO, MgO, Fe2O3 nanoparticles and CdO-MgOFe2O3 nanocomposite were found to be 1.38 × 10−4, 7.142 × 10−9, 1.785 × 10−6 and 7.24 × 10−10 S cm−1, respectively. It may be unveiled that the decrease in conductivity value for CdO-MgO-Fe2O3 nanocomposite was an inventive factor because of two reasons, (i) decreased crystallite boundary scattering and (ii) an increment in the electron mobility due to crystallite size development [55].

3.5. UV–vis spectroscopy The UV–vis spectroscopic method gives information about the absorption behavior and band-gap energy (E g) values. Therefore, to study the absorption performance of the prepared samples, UV–Vis. spectra of pure CdO, MgO, Fe2O3 nanoparticles and CdO-MgO-Fe2O3 nanocomposite were recorded in the spectral range of 200–800 nm as shown in Fig. 6. Normally, two broad absorption bands were observed for Fe2O3 nanoparticles at about 300–450 nm in the UV–vis absorption region and a very small absorption band at about 500−600 nm in only visible region. These absorption bands can be attributed to the absorbance for Fe3+ and Fe2+ ions present in the prepared sample. CdO nanoparticles showed a broad absorption band from 500−600 nm indicating that it absorbs light in visible region. This absorption is due to its small band gap. Furthermore, CdO also exhibited one little absorption band at about 200–250 nm that could be ascribed to the inter-band transition from deep level electrons in the valance band. MgO nanoparticles showed an intense absorption band at 200−250 nm, and the absorption edge at 250 nm. The absorption edge at 250 nm indicated the excitation of four-fold coordinated O2 − anions at the edges and corners [44,48]. The spectrum for CdO-MgO-Fe2O3 nanocomposite has a wide absorption band in the UV–vis range of 340−500 nm. Such a broad absorption band has been observed due to d-d crystal- field transitions among the multiplets of 3d5 configuration of high spin ferric ions (Fe3+) replacing for cadmium ions (Cd2+) under the action of a

(αh ʋ)n = D (h ʋ− Eg )

(9)

In this equation the "h ʋ" is the energy of photon, "α" is the absorption coefficient, “D” is a constant value and "Eg " indicates the band-gap energy. The value of “n” is given to the transition which has different values for the different type of transitions, i.e. allowed, forbidden, direct and indirect [50]. By plotting a graph between (αh ʋ)n and "h ʋ" in eV for n = 2 (direct transitions), and then by the extrapolation point of the resulting curve (in the inset of Fig. 6), the band-gap energy (Eg ) values of CdO, MgO, Fe2O3 nanoparticles were estimated about 1.72 eV, 5.8 eV and 2.1 eV, respectively. However, CdO-MgO-Fe2O3 nanocomposite exhibited the band-gap 1.76 eV. In comparison to individual CdO, MgO, Fe2O3 nanoparticles, the CdO-MgO-Fe2O3 nanocomposite exhibited significant increased absorbance in the visible region due to incorporation of three metal oxides. In addition to this, the absorption edge of the CdO-MgO-Fe2O3 nanocomposite exhibited the progressive red shift, indicating the small band gap in the nanocomposite. The enhanced absorption in the visible light region of the CdO-MgO-Fe2O3 nanocomposite suggested that it may have the improved photocatalytic performance for selected pollutants in the visible light illumination.

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Fig. 6. UV–Vis absorption spectra and optical bandgap of a) MgO, b) CdO, c) Fe2O3 nanoparticles and d) CdO-MgO-Fe2O3 nanocomposite.

Fig. 7. Photocatalytic activity of CdO-MgO-Fe2O3 nanocomposite at different pH values.

illumination. For this investigation, the typical absorption peak of methylene blue (∼663 nm) was examined with UV–vis spectrophotometer after fixed time interims. Fig. 7(a–d) showed the photocatalytic removal of methylene blue (MB) dye using the CdO-MgOFe2O3 nanocomposite as photocatalyst at different pH levels via visible light illumination. Fig. 8(a–b) demonstrated the % dye removal with

3.6. Photocatalytic degradation of MB dye using CdO-MgO-Fe2O3 photocatalyst Photocatalytic activity of as synthesized CdO-MgO-Fe2O3 nanocomposite was investigated by analyzing the photo-degradation of methylene blue (MB) dye as a main reactant via visible light 8

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Fig. 8. (a) % degradation of methylene blue at different pH using CdO-MgO-Fe2O3 photocatalyst after 40 min visible light illumination, (b) Bar graph for %MB dye degradation.

Fig. 9. Photocatalytic removal of methylene blue using CdO-MgO-Fe2O3 nanocomposite under visible light, (a) Rate of degradation, (b) Linear kinetic behavior and (c) Their bar graph. Table 2 Represents the pseudo first order degradation rate (k), half-life time period of reaction (t1/2), linear regression coefficient (R2), % MB degradation at various pH levels and time (min) taken for dye degradation using CdO-MgO-Fe2O3 photocatalyst under visible light irradiation. Photocatalyst

pH

% degradation

Degradation time (min)

K (min-1)

T1/2 (min)

R2

CdO-MgO-Fe2O3

6.0 7.0 9.0 11.0

93.8 93.1 96.3 95.9

120 100 70 40

0.0185 0.0243 0.0441 0.0820

37.5 28.5 15.7 8.4

0.065 0.138 0.114 0.028

40 min of visible light illumination (Fig. 8b). After visible light illumination for 40 min, maximum photocatalytic removal of methylene blue was found at pH 11.0 and ascribed to the production of the large number of hydroxyl radicals (·OH) and electrostatic attraction among opposite charges, i.e. positively charged dye ions and the negatively charged surface of photocatalyst. The electrostatic attraction caused an

time. From these plots, it is obvious that the respective photo-degradation of MB in the presence of CdO-MgO-Fe2O3 photocatalyst is higher at more basic pH values than at neutral or acidic ones. The CdOMgO-Fe2O3 photocatalyst efficiencies in the presence of hydrogen per oxide (H2O2) for the methylene blue dye removal were 96 %, 66 %, 32 % and 30 % at pH 11.0, pH 9.0, pH 7.0 and pH 6.0, respectively, after 9

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trapping analyses were performed by different scavengers to clarify the reactive species (radicals) involved in reactions that takes place for methylene blue degradation. The experimental conditions set for these experiments were the same as mentioned above, for degradation experiments of methylene blue. For these tests, Isopropyl alcohol (IPA), Potassium per sulfate (KPS), Potassium Iodide (KI) and Vit-C (Ascorbic Acid) were utilized as the hydroxyl radical (·O H), electron (e−), hole (h+) and superoxide radicals (·O¯ 2 ) scavengers separately in the photodegradation reaction of MB by CdO-MgO-Fe2O3 nanocomposite [44]. The experimental results can be seen in Fig. 10(a–c). Fig. 10b clearly showed that the rate constant (kapp) for the degradation of methylene blue had a minor effect by adding KPS, recommending that electrons (e−) were not the primary dynamic species, although, a significant decrease in the degradation rate constant (kapp) of methylene blue was observed by adding KI, IPA and Vit-C. These experimental results were further supported by the percent dye removal with and without using various scavengers as shown in Fig. 10(d). It is clear from this diagram (Fig. 10d) that by adding KI, Vit-C and IPA, percent (%) photocatalyzed degradation reduced to 29.2, 24 and 41.8 % respectively. This decrease showed that the significant degradation has been carried out by (h+), (·O¯ 2 ) and (·O H). In light of this study, the hydroxyl radicals (·OH) are generated by the reaction of holes (h+) with water (H2O) and electrons (e−) with hydrogen per oxide (H2O2) [65]. Additionally, in the presence of KPS (Fig. 10c) the percent degradation of methylene blue was closer to as that without scavenger (around 72 %) arguing that (e−) are not the main dynamic species in photocatalyzed degradation. In this way from the above trapping experimental results, we finished up h+, ·O¯ 2 and ·O H as real dynamic (active) species in photocatalyzed removal of methylene blue using visible light illumination which correlated well with aforementioned photo-degradation results. In order to clarify the changes occurring in photocatalytic action using visible light illumination, the excitation and exchange procedure of charges among MgO, CdO and Fe2O3 nanoparticles in the CdO-MgOFe2O3 nanocomposite is systematically represented in Scheme 2. Schematic illustration 2 is revealing that when CdO–ZnO–MgO nanocomposite exposed to visible light, the electron-hole pair generated separately in conduction and valance band. In order to understand the mechanism involved in the separation of electrons and holes, the potentials of CdO, MgO and Fe2O3 associated to valence and conduction bands (VB & CB) were estimated by using the equations,

increase in the rate of adsorption that resulted in enhanced photo-degradation. Clearly, pH significantly affected the photocatalytic removal of methylene blue dye. It is notable that the photocatalytic removal of most dyes follow the pseudo first-order kinetics equation. The kinetic linear curves for the photocatalytic removal of methylene blue follow the pseudo first order kinetics as per Langmuir-Hinshelwood model. A plot between Ct/Co and time for CdO-MgO-Fe2O3 nanocomposite at different pH levels is shown in Fig. 9(a–c). For CdO-MgO-Fe2O3 photocatalyst, the degradation rates are observed to be 0.0185, 0.0243, 0.0441, 0.0820 min−1 at pH 11.0, pH 9.0, pH 7.0 and pH 6.0 respectively. The pseudo first order degradation rate (k), half-life time period of reaction (t1/2), linear regression coefficient (R2), for the CdO-MgOFe2O3 photocatalyst under visible light irradiation are given in Table 2. It is clear from Fig. 9(a–c) and Table 2 that the degradation rate of CdOMgO-Fe2O3 photocatalyst was increased as the pH made more basic. The apparent first-order degradation rate for methylene blue photodegradation by CdO-MgO-Fe2O3 photocatalyst at different pH values revealed the photocatalytic activity order as pH 11.0 > pH 9.0 > pH 7.0 > pH 6.0 and relates well with activity studies performed earlier. The reference diagram in Fig. 8b indicated the relative percent methylene blue dye removal by CdO-MgO-Fe2O3 photocatalyst at acidic, neutral and alkaline media just for 40 min of visible light illumination and Fig. 8a demonstrated the time duration taken by as-synthesized photocatalyst at various pH values to accomplish the maximum photocatalytic effectiveness. It is very clear from Fig. 8(a, b) that the nanocomposite CdO-MgO-Fe2O3 has the maximum photocatalytic performance 95.9 % at pH 11.0 and has decayed practically all the dye in only 40 min under visible light illumination. Additionally, the CdOMgO-Fe2O3 nanocomposite at pH 9.0 has accomplished 96.3 % methylene blue dye removal in just 70 min and at pH 7.0 and pH 6.0 achieved 93.1 % and 93.8 % methylene blue dye degradation in just 100 and120 min, respectively. Further, while comparing the CdO-MgOFe2O3 photocatalyst with that of the commercially available P25 standard photocatalyst, the CdO-MgO-Fe2O3 nanocomposite material appeared to be more effective photocatalyst since P25 gives only 22 % of methylene blue dye removal in one hour in comparison to our present system which exhibits about 95.9 % methylene blue degradation in just 40 min of visible light illumination [57]. These results explained the development of current framework for water purification applications. The current framework further exhibited the better photocatalytic activity than numerous different photocatalytic materials as found in the literature [58–60]. A comparison of photocatalytic performance alongside time of removal of the CdO-MgO-Fe2O3 nanocomposite with different materials reported against methylene blue dye is given in the Table 3.

To understand the photocatalytic mechanism involved in the degradation of methylene blue by CdO-MgO-Fe2O3 nanocomposite, Table 3 Comparison of % MB degradation under visible light irradiation along with time taken for degradation of our system (CdO-MgO-Fe2O3) with other materials reported against MB dye. Dye

% degradation

Time taken for degradation (min)

References

CdO-NiO-ZnO Fe-Cd co-modified ZnO Cu-TiO2/ZnO CdSNPs@ZIF-8 Fe3 O4 /CuO/ZnO TiO2/graphene CdO-MgO-Fe2O3

MB MB

89 82

90 140

[61] [4]

MB MB MB MB MB

73 90 89 70 96

120 120 120 300 40

[62] [63] [64] [35] Present work

(10)

EVB = ECB + Eg

(11)

where, X =semiconductor electronegativity, ECB = CB edge potential, EVB = VB edge potential, Eef = energy of free electrons (e-) on the standard hydrogen scale (about 4.5 eV), Eg = semiconductor band gap energy. The electronegativity values for CdO, Fe2O3 and MgO are 6.02, 6.14 and 3.77 eV respectively. The calculated edge potentials corresponding to VB & CB for CdO, Fe2O3 and MgO are given in Schematic illustration 2. It is assumed that CdO is present between MgO and Fe2O3 and three oxides are adjacent together across the heterojunction in CdO/ZnO/MgO nanocomposite. From Scheme 2, it is observed that photo-generated electrons move from CB of MgO to CB of CdO via CB of Fe2O3. It is due to the fact that CB of MgO is more positive than Fe2O3. Contrary to this, photo-generated holes moved from VB of CdO to VB of MgO via VB of Fe2O3, as VB of Fe2O3 is more negative than MgO. As the photo-induced electrons transfer from conduction band to valance band, molecular oxygen reduces to form superoxide radical anion (·O¯ 2 ). The hole present in the valance band oxidizes water molecules or hydroxyl ions to produce hydroxyl radical (·O H). The resulting hydroxyl radical anions (·O H) and superoxide radicals (·O¯ 2 ) anions degrades organic dye into degradation products i.e. CO2 & H2O. In this way the photo-produced superoxide radical anion (·O¯ 2 ) and hydroxyl radical (·O H) are greatly receptive in the removal of dye from the dye solution [66,67]. These processes not just speed up the rate of

3.7. Possible mechanism of MB dye degradation

System

ECB = X − (Eef + 0.5Eg )

10

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Fig. 10. (a–c) The effect of different scavengers on the dye removal rate and rate constant at pH 6.0, pH 7.0, pH 9.0, pH 11.0 in the presence of photocatalyst and H2O2 (d) % MB removal in absence of scavenger and presence of different scavengers.

with pure CdO, MgO, Fe2O3 was successfully synthesized by co-precipitation technique using corresponding metal nitrates and was annealed at 650 °C. The XRD pattern confirmed the formation of nanocomposite having polycrystalline nature. The micro-strain calculated from the Williamson-Hall method revealed that for CdO micro-strain alter from compressive to tensile, for MgO changes from tensile to compressive while for Fe2O3 remains unaltered in nanocomposite. FTIR spectral studies illustrated the characteristics peaks of Mg–O (772 cm−1), Fe2-O3 (581 cm−1) and Cd-O (480 cm−1) bonds which confirmed the formation of nanocomposite. UV–vis spectra showed the bandgap 1.76 eV for nanocomposite which suggested that it is an efficient photocatalyst for UV and visible light. I–V measurements ascribed the semiconducting nature of the grown nanocomposite and formation of heterojunction. The photocatalytic performance of CdO-MgO-Fe2O3 nanocomposite was estimated for MB dye under visible light irradiation. The photocatalyst composition having equal metal ion concentration (1:1:1) along with H2O2 catalyst in an alkaline medium showed the highest photo-degradation efficiency of 66 % and 96 % in 40 min at pH 9.0 and pH 11.0, respectively. This improved photocatalytic activity results from the large surface area of the grown photocatalyst which provides extra reaction sites for photo-induced electron hole pairs. The presence of heterojunction in grown CdO-MgOFe2O3s nanocomposite improved the charge separation/transportation efficiency of the photo-generated electron hole pair and hinder their recombination rate which alternatively enhance photocatalytic activity. The above study shows that grown CdO-MgO-Fe2O3 nanocomposite is a promising candidate for the elimination of contaminants from polluted water under visible-light illumination.

interfacial charge exchange, but also improve the production of highly receptive oxidative species, for example, superoxide and hydroxyl radicals. The whole degradation mechanism can be expressed as:

CdO − MgO − Fe2 O3 + h ʋ → CdO − MgO − Fe2 O3 (e−CB + h+VB ) (12)

CdO − MgO − Fe2 O3 (e−CB ) + O2 → CdO − MgO − Fe2 O3 + ·O¯ 2

H2 O +

O¯ 2 +

h+

H−

→

H+

(14)

+ ·OH

˙ 2 → HO

(15)

˙ 2 + H+ → H2 O CdO − MgO − Fe2 O3 (e−CB ) + HO CdO − MgO −

Fe2 O3 (h+VB )

(13)

(16)

+ ·O¯ 2 + ·O H+ Dye → degradation products (17)

HO−

2

+

H+

→ H2 O2

(18)

3.8. Stability of CdO-MgO-Fe2O3 nanocomposite photocatalyst The reusability of a photocatalyst is an important parameter that describes its suitability for practical applications [68,69]. In order to assess the reusability of CdO-MgO-Fe2O3 nanocomposite photocatalyst, three successive photocatalytic cyclic runs were carried out at pH 9.0 (Fig. 11). After every photocatalytic experimental run, the used CdOMgO-Fe2O3 photocatalyst was washed with distilled water and dried at 80 °C. This dried photocatalyst was reused for the next run without changing the other reaction parameters (5 ppm, catalyst, H2O2, pH 9.0). It was found that after three cycle runs the photocatalyst exhibited no significant decrease in the photo-degradation of MB. These results refer that as synthesized CdO-MgO-Fe2O3 nanocomposite can be employed as reusable photocatalyst for photo-degradation of MB dye.

Acknowledgements Authors from The Islamia University of Bahawalpur (IUB) are thankful to the IUB and HEC-Islamabad. Authors from King Saud University sincerely appreciate the King Saud University for their contribution through Researchers Supporting Project (RSP-2019/49).

4. Conclusion In summary, multi metal oxide photocatalyst CdO-MgO-Fe2O3 along 11

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Scheme 2. Schematic illustration of MB photo-degradation using CdO-MgO-Fe2O3 nanocomposite.

References [1] J.F.S. Fernando, M.P. Shortell, K.L. Firestein, C. Zhang, K.V. Larionov, Z.I. Popov, P.B. Sorokin, L. Bourgeois, E.R. Waclawik, D.V. Golberg, Photocatalysis with Pt–Au–ZnO and Au–ZnO hybrids: effect of charge accumulation and discharge properties of metal nanoparticles, Langmuir 34 (2018) 7334–7345. [2] H. Javed, A. Rehman, S. Mussadiq, M. Shahid, M.A. Khan, I. Shakir, P.O. Agboola, M.F.A. Aboud, M.F. Warsi, Reduced graphene oxide-spinel ferrite nano-hybrids as magnetically separable and recyclable visible light driven photocatalyst, Synth. Met. 254 (2019) 1–9. [3] T. Munawar, F. Iqbal, S. Yasmeen, K. Mahmood, A. Hussain, Multi metal oxide NiOCdO-ZnO nanocomposite–synthesis, structural, optical, electrical properties and enhanced sunlight driven photocatalytic activity, Ceram. Int. (2019). [4] N. D, K.K. Kondamareddy, H. Bin, D. Lu, P. Kumar, R.K. Dwivedi, V.O. Pelenovich, X.-Z. Zhao, W. Gao, D. Fu, Enhanced visible light photodegradation activity of RhB/ MB from aqueous solution using nanosized novel Fe-Cd co-modified ZnO, Sci. Rep. 8 (2018) 10691. [5] S. Yasmeen, F. Iqbal, T. Munawar, M.A. Nawaz, M. Asghar, A. Hussain, Synthesis, structural and optical analysis of surfactant assisted ZnO–NiO nanocomposites prepared by homogeneous precipitation method, Ceram. Int. 45 (2019) 17859–17873. [6] H. Eskandarloo, A. Badiei, M.A. Behnajady, Study of the effect of additives on the photocatalytic degradation of a triphenylmethane dye in the presence of immobilized TiO2/NiO nanoparticles: artificial neural network modeling, Ind. Eng. Chem. Res. 53 (2014) 6881–6895. [7] G.K. Pradhan, S. Martha, K.M. Parida, Synthesis of multifunctional nanostructured zinc–Iron mixed oxide photocatalyst by a simple solution-combustion technique, ACS Appl. Mater. Interfaces 4 (2012) 707–713. [8] O.I. Omotunde, A.E. Okoronkwo, A.F. Aiyesanmi, E. Gurgur, Photocatalytic behavior of mixed oxide NiO/PdO nanoparticles toward degradation of methyl red in water, J. Photochem. Photobiol. A: Chem. 365 (2018) 145–150. [9] J. Ding, J. Ming, D. Lu, W. Wu, M. Liu, X. Zhao, C. Li, M. Yang, P. Fang, Study of the enhanced visible-light-sensitive photocatalytic activity of Cr2O3-loaded titanate nanosheets for Cr(vi) degradation and H2 generation, Catal. Sci. Technol. 7 (2017) 2283–2297. [10] A.V. Abega, H.M. Ngomo, I. Nongwe, H.E. Mukaya, P.-M.A. Kouoh Sone, X. Yangkou Mbianda, Easy and convenient synthesis of CNT/TiO2 nanohybrid by

Fig. 11. Recyclability of CdO-MgO-Fe2O3 nanocomposite for MB photodegradation at pH 9.0.

Dr. Sonia Zulfiqar is highly grateful to American University in Cairo (AUC) for financial support through STRC mini-grant and research project No. SSE-CHEM-S.Z.-FY19-FY20-FY21-RG (1-19)-2018-Oct-0117-53-22.

12

Synthetic Metals 259 (2020) 116228

A. Rahman, et al.

[11]

[12]

[13]

[14] [15] [16]

[17]

[18]

[19] [20]

[21]

[22]

[23]

[24]

[25] [26]

[27]

[28]

[29]

[30]

[31]

[32] [33]

[34] [35]

[36]

[37]

[38]

[39]

[40]

I, Proc. Phys. Soc. 56 (1944) 174–181. [41] A.R. Stokes, A.J.C. Wilson, The diffraction of X rays by distorted crystal aggregatesI, Proc. Phys. Soc. 56 (1944) 174. [42] W.H. Hall, X-ray line broadening in metals, Proc. Phys. Soc. Sect. A 62 (1949) 741–743. [43] P. Bindu, S. Thomas, Estimation of lattice strain in ZnO nanoparticles: X-ray peak profile analysis, J. Theor. Appl. Phys. 8 (2014) 123–134. [44] J. Zhou, S. Yang, J. Yu, Facile fabrication of mesoporous MgO microspheres and their enhanced adsorption performance for phosphate from aqueous solutions, Colloids Surf. A Physicochem. Eng. Asp. 379 (2011) 102–108. [45] Y. Lei, J. Huo, H. Liao, Fabrication and catalytic mechanism study of CeO2-Fe2O3ZnO mixed oxides on double surfaces of polyimide substrate using ion-exchange technique, Mater. Sci. Semicond. Process. 74 (2018) 154–164. [46] K. Karthik, S. Dhanuskodi, C. Gobinath, S. Prabukumar, S. Sivaramakrishnan, Andrographis paniculata extract mediated green synthesis of CdO nanoparticles and its electrochemical and antibacterial studies, J. Mater. Sci. Mater. Electron. 28 (2017) 7991–8001. [47] Z. Guo, M. Li, J. Liu, Highly porous CdO nanowires: preparation based on hydroxyand carbonate-containing cadmium compound precursor nanowires, gas sensing and optical properties, Nanotechnology 19 (2008) 245611. [48] N.C.S. Selvam, R.T. Kumar, L.J. Kennedy, J.J. Vijaya, Comparative study of microwave and conventional methods for the preparation and optical properties of novel MgO-micro and nano-structures, J. Alloys. Compd. 509 (2011) 9809–9815. [49] H. Niu, Q. Yang, K. Tang, Y. Xie, Large-scale synthesis of single-crystalline MgO with bone-like nanostructures, J. Nanoparticle Res. 8 (2006) 881–888. [50] W. Jiang, X. Hua, Q. Han, X. Yang, L. Lu, X. Wang, Preparation of lamellar magnesium hydroxide nanoparticles via precipitation method, Powder Technol. 191 (2009) 227–230. [51] Z. Zhao, H. Dai, Y. Du, J. Deng, L. Zhang, F. Shi, Solvo- or hydrothermal fabrication and excellent carbon dioxide adsorption behaviors of magnesium oxides with multiple morphologies and porous structures, Mater. Chem. Phys. 128 (2011) 348–356. [52] F. Gu, S.F. Wang, M.K. Lü, W.G. Zou, G.J. Zhou, D. Xu, D.R. Yuan, Combustion synthesis and luminescence properties of Dy3+-doped MgO nanocrystals, J. Cryst. Growth 260 (2004) 507–510. [53] J. Xu, Y. Ao, D. Fu, C. Yuan, Synthesis of Bi2O3–TiO2 composite film with highphotocatalytic activity under sunlight irradiation, Appl. Surf. Sci. 255 (2008) 2365–2369. [54] R. Bomila, S. Suresh, S. Srinivasan, Synthesis, characterization and comparative studies of dual doped ZnO nanoparticles for photocatalytic applications, J. Mater. Sci. Mater. Electron. 30 (2019) 582–592. [55] O. Vigil, F. Cruz, A. Morales-Acevedo, G. Contreras-Puente, L. Vaillant, G. Santana, Structural and optical properties of annealed CdO thin films prepared by spray pyrolysis, Mater. Chem. Phys. 68 (2001) 249–252. [56] A.B. Murphy, Band-gap determination from diffuse reflectance measurements of semiconductor films, and application to photoelectrochemical water-splitting, Sol. Energy Mater. Sol. Cells 91 (2007) 1326–1337. [57] N. Shimizu, C. Ogino, M.F. Dadjour, T. Murata, Sonocatalytic degradation of methylene blue with TiO2 pellets in water, Ultrason. Sonochem. 14 (2007) 184–190. [58] R. Bera, S. Kundu, A. Patra, 2D hybrid nanostructure of reduced graphene oxide–CdS nanosheet for enhanced photocatalysis, ACS Appl. Mater. Interfaces 7 (2015) 13251–13259. [59] T. Lv, L. Pan, X. Liu, T. Lu, G. Zhu, Z. Sun, Enhanced photocatalytic degradation of methylene blue by ZnO-reduced graphene oxide composite synthesized via microwave-assisted reaction, J. Alloys. Compd. 509 (2011) 10086–10091. [60] Y. Zhang, Z.-R. Tang, X. Fu, Y.-J. Xu, Engineering the Unique 2D Mat of Graphene to achieve graphene-TiO2 nanocomposite for photocatalytic selective transformation: what advantage does graphene have over its forebear carbon nanotube? ACS Nano 5 (2011) 7426–7435. [61] K. Karthik, S. Dhanuskodi, C. Gobinath, S. Prabukumar, S. Sivaramakrishnan, Multifunctional properties of microwave assisted CdO–NiO–ZnO mixed metal oxide nanocomposite: enhanced photocatalytic and antibacterial activities, J. Mater. Sci. Mater. Electron. 29 (2018) 5459–5471. [62] M.R. Delsouz Khaki, M.S. Shafeeyan, A.A.A. Raman, W.M.A.W. Daud, Evaluating the efficiency of nano-sized Cu doped TiO2/ZnO photocatalyst under visible light irradiation, J. Mol. Liq. 258 (2018) 354–365. [63] A. Malik, M. Nath, S. Mohiyuddin, G. Packirisamy, Multifunctional CdSNPs@ZIF-8: potential antibacterial agent against GFP-expressing escherichia coli and staphylococcus aureus and efficient photocatalyst for degradation of methylene blue, ACS Omega 3 (2018) 8288–8308. [64] A. Taufik, H. Tju, R. Saleh, Comparison of catalytic activities for sonocatalytic, photocatalytic and sonophotocatalytic degradation of methylene blue in the presence of magnetic Fe3O4/CuO/ZnO nanocomposites, J. Phys. Conf. Ser. 710 (2016) 012004. [65] X. Liu, S. An, W. Shi, Q. Yang, L. Zhang, Microwave-induced catalytic oxidation of malachite green under magnetic Cu-ferrites: new insight into the degradation mechanism and pathway, J. Mol. Catal. A Chem. 395 (2014) 243–250. [66] M.M. Ba-Abbad, A.A.H. Kadhum, A.B. Mohamad, M.S. Takriff, K. Sopian, Visible light photocatalytic activity of Fe3+-doped ZnO nanoparticle prepared via sol–gel technique, Chemosphere 91 (2013) 1604–1611. [67] M. Zhou, J. Yu, B. Cheng, H. Yu, Preparation and photocatalytic activity of Fedoped mesoporous titanium dioxide nanocrystalline photocatalysts, Mater. Chem. Phys. 93 (2005) 159–163. [68] Z. Chen, J. Ma, K. Yang, S. Feng, W. Tan, Y. Tao, H. Mao, Y. Kong, Preparation of Sdoped TiO2-three dimensional graphene aerogels as a highly efficient photocatalyst, Synth. Met. 231 (2017) 51–57. [69] C. Ding, Z. Li, W. Tan, H. Li, J. Ma, Z. Chen, Y. Tao, Y. Qin, Y. Kong, 3D graphene aerogels/Sb2WO6 hybrid with enhanced photocatalytic activity under UV- and visible-light irradiation, Synth. Met. 246 (2018) 137–143.

in-surface oxidation of Ti3+ ions and application in the photocatalytic degradation of organic contaminants in water, Synth. Met. 251 (2019) 1–14. H.-L. Wang, D.-Y. Zhao, W.-F. Jiang, Synthesis and photocatalytic activity of poly-ophenylenediamine (PoPD)/TiO2 composite under VIS-light irradiation, Synth. Met. 162 (2012) 296–302. H. Zhou, C. Zhang, X. Wang, H. Li, Z. Du, Fabrication of TiO2-coated magnetic nanoparticles on functionalized multi-walled carbon nanotubes and their photocatalytic activity, Synth. Met. 161 (2011) 2199–2205. M.N.H. Mia, M.F. Pervez, M.K. Hossain, M. Reefaz Rahman, M.J. Uddin, M.A. Al Mashud, H.K. Ghosh, M. Hoq, Influence of Mg content on tailoring optical bandgap of Mg-doped ZnO thin film prepared by sol-gel method, Results Phys. 7 (2017) 2683–2691. K. Hashimoto, H. Irie, A. Fujishima, TiO2Photocatalysis: a historical overview and future prospects, J. Appl. Phys. 44 (2005) 8269–8285. K. Nakata, A. Fujishima, TiO2 photocatalysis: design and applications, J. Photochem. Photobiol. C Photochem. Rev. 13 (2012) 169–189. M.A. Subhan, N. Uddin, P. Sarker, H. Nakata, R. Makioka, Synthesis, characterization, low temperature solid state PL and photocatalytic activities of Ag2O·CeO2·ZnO nanocomposite, Spectrochim. Acta A. Mol. Biomol. Spectrosc. 151 (2015) 56–63. J. Liqiang, Q. Yichun, W. Baiqi, L. Shudan, J. Baojiang, Y. Libin, F. Wei, F. Honggang, S. Jiazhong, Review of photoluminescence performance of nano-sized semiconductor materials and its relationships with photocatalytic activity, Sol. Energy Mater. Sol. Cells 90 (2006) 1773–1787. G. Liu, J.C. Yu, G.Q. Lu, H.-M. Cheng, Crystal facet engineering of semiconductor photocatalysts: motivations, advances and unique properties, Chem. Commun. 47 (2011) 6763–6783. A. Mills, S. Le Hunte, An overview of semiconductor photocatalysis, J. Photochem. Photobiol. A: Chem. 108 (1997) 1–35. P. Senthil Kumar, M. Selvakumar, S. Ganesh Babu, S. Karuthapandian, S. Chattopadhyay, CdO nanospheres: facile synthesis and bandgap modification for the superior photocatalytic activity, Mater. Lett. 151 (2015) 45–48. J. Ahmad, K. Majid, Enhanced visible light driven photocatalytic activity of CdO–graphene oxide heterostructures for the degradation of organic pollutants, New J. Chem. 42 (2018) 3246–3259. K. Karthik, S. Dhanuskodi, C. Gobinath, S. Prabukumar, S. Sivaramakrishnan, Photocatalytic and antibacterial activities of hydrothermally prepared CdO nanoparticles, J. Mater. Sci. Mater. Electron. 28 (2017) 11420–11429. S. Ghosh, M. Saha, S. Paul, S.K. De, Shape controlled plasmonic Sn doped CdO colloidal nanocrystals: a synthetic route to maximize the figure of merit of transparent conducting oxide, Small 13 (2017) 1602469. M.M. Rahman, M.M. Alam, A.M. Asiri, Sensitive 1,2-dichlorobenzene chemi-sensor development based on solvothermally prepared FeO/CdO nanocubes for environmental safety, J. Ind. Eng. Chem. 62 (2018) 392–400. L. Wang, W. Ma, L. Xu, W. Chen, Y. Zhu, C. Xu, N.A. Kotov, Nanoparticle-based environmental sensors, Mater. Sci. Eng. R Rep. 70 (2010) 265–274. H.S. Jung, J.-K. Lee, M. Nastasi, S.-W. Lee, J.-Y. Kim, J.-S. Park, K.S. Hong, H. Shin, Preparation of nanoporous MgO-Coated TiO2 nanoparticles and their application to the electrode of dye-sensitized solar cells, Langmuir 21 (2005) 10332–10335. M.A. Karimi, A. Hatefi-Mehrjardi, A. Askarpour Kabir, M. Zaydabadi, Synthesis, characterization, and application of MgO/ZnO nanocomposite supported on activated carbon for photocatalytic degradation of methylene blue, Res. Chem. Intermed. 41 (2015) 6157–6168. C. Yu, X. Dong, L. Guo, J. Li, F. Qin, L. Zhang, J. Shi, D. Yan, Template-free preparation of mesoporous Fe2O3 and its application as absorbents, J. Phys. Chem. C 112 (2008) 13378–13382. Z. Wei, R. Xing, X. Zhang, S. Liu, H. Yu, P. Li, Facile template-free fabrication of hollow nestlike α-Fe2O3 nanostructures for water treatment, ACS Appl. Mater. Interfaces 5 (2013) 598–604. X. Xie, Y. Li, Y. Yang, C. Chen, Q. Zhang, UV–Vis–IR driven thermocatalytic activity of OMS-2/SnO2 nanocomposite significantly enhanced by novel photoactivation and synergetic photocatalysis-thermocatalysis, Appl. Surf. Sci. 462 (2018) 590–597. H. Guo, Y. Zhu, S. Qiu, J.A. Lercher, H. Zhang, Coordination modulation induced synthesis of nanoscale Eu1-xTbx-Metal-Organic frameworks for luminescent thin films, Adv. Mater. 22 (2010) 4190–4192. I.E. Wachs, Recent conceptual advances in the catalysis science of mixed metal oxide catalytic materials, Catal. Today 100 (2005) 79–94. K. Polychronopoulou, J.L.G. Fierro, A.M. Efstathiou, Novel Zn–Ti-based mixed metal oxides for low-temperature adsorption of H2S from industrial gas streams, Appl. Catal. B 57 (2005) 125–137. M. Dürr, S. Rosselli, A. Yasuda, G. Nelles, Band-gap engineering of metal oxides for dye-sensitized solar cells, J. Phys. Chem. B 110 (2006) 21899–21902. Y. Zhang, C. Pan, TiO2/graphene composite from thermal reaction of graphene oxide and its photocatalytic activity in visible light, J. Mater. Sci. 46 (2011) 2622–2626. G. Fan, W. Sun, H. Wang, F. Li, Visible-light-induced heterostructured Zn–Al–In mixed metal oxide nanocomposite photocatalysts derived from a single precursor, Chem. Eng. J. 174 (2011) 467–474. D. Pan, S. Ge, J. Zhao, Q. Shao, L. Guo, X. Zhang, J. Lin, G. Xu, Z. Guo, Synthesis, characterization and photocatalytic activity of mixed-metal oxides derived from NiCoFe ternary layered double hydroxides, Dalton Trans. 47 (2018) 9765–9778. A. Franco, M.C. Neves, M.M.L.R. Carrott, M.H. Mendonça, M.I. Pereira, O.C. Monteiro, Photocatalytic decolorization of methylene blue in the presence of TiO2/ZnS nanocomposites, J. Hazard. Mater. 161 (2009) 545–550. A. Khorsand Zak, W.H.A. Majid, M. Ebrahimizadeh Abrishami, R. Yousefi, R. Parvizi, Synthesis, magnetic properties and X-ray analysis of Zn0.97X0.03O nanoparticles (X = Mn, Ni, and Co) using Scherrer and size–strain plot methods, Solid State Sci. 14 (2012) 488–494. A.R. Stokes, A.J.C. Wilson, The diffraction of X rays by distorted crystal aggregates -

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