- Email: [email protected]
RGO nanocomposites and their photocatalytic application
Materials Science in Semiconductor Processing 99 (2019) 44–53
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Investigation on the structure and physical properties of Fe3O4/RGO nanocomposites and their photocatalytic application
T
M.A. Majeed Khana,∗, Wasi Khanb, Maqusood Ahameda, Abdulaziz N. Alhazaaa,c a
King Abdullah Institute for Nanotechnology, King Saud University, Riyadh, 11451, Saudi Arabia Department of Physics, Aligarh Muslim University, Aligarh, 202002, India c Physics and Astronomy Department, College of Science, King Saud University, P.O. Box 2455, Riyadh, 11451, Saudi Arabia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnetite nanoparticles Reduced graphene oxide Optical properties Photocatalytic activity VRH model Superparamagnetism
In the present work, we have investigated the structural, optical, photocatalytic activity, electrical transport and magnetic properties of Fe3O4 nanoparticles and its composite with reduced graphene oxide (Fe3O4/RGO) by varying the ratio of reduced graphene oxide (RGO: 2% and 4%). Fe3O4 nanoparticles and RGO were synthesized by the modified sol-gel and Hummers methods respectively. The as-prepared samples were examined by various analytical techniques to investigate their functional properties and possible applications. Rietveld refinement analysis of the XRD data reveals face-centered cubic (fcc) structure of the Fe3O4 nanoparticles and confirms the characteristic planes of magnetite phase. The average crystallite size of the Fe3O4, Fe3O4/RGO (2%) and Fe3O4/ RGO (4%) samples was estimated using Scherrer's equation and found to be ∼10, 15 and 19 nm respectively. TEM/HRTEM micrographs establish spherical shapes of the magnetite nanoparticles with an average diameter of ∼12 nm for Fe3O4, while 17 and 20 nm in the nanocomposites respectively. The low temperature resistivity measurements exhibit semiconducting nature of all the samples and follow variable range hopping (VRH) mechanism. The magnetic measurements demonstrate typical superparamagnetic behavior of the samples at room temperature. The photocatalytic performance of the samples has been investigated for the decolorization of methylene blue (MB) organic dye under visible light irradiation. The Fe3O4/RGO nanocomposites display better adsorption behaviour and excellent photocatalytic activity than RGO and Fe3O4 nanoparticles. This may be due to strong interactions between Fe3O4 nanoparticles and graphene sheets.
1. Introduction The field of nanotechnology mainly depends on tailor-made materials having typical dimensions in the nanometer range. It is well known that the nanoparticles (NPs) exhibit distinct and improved properties based on the specific characteristics like distribution, morphology and particle size in comparison with the bigger particles of the same material [1]. The surface to volume ratio of the nanoparticles increases with the reduction in particle size. Additionally, the scientific and technological attributes of the nanomaterials are well proved in numerous applications, including use in biotechnology, catalysis, energy storage, photonics, etc [2,3]. Among various nanomaterials, magnetite (Fe3O4) is one of the promising magnetic materials in cubic inverse spinel structure with oxygen anions in face-centered cubic (fcc) closed packing and iron cations occupying interstitial tetrahedral and octahedral sites [4]. Moreover, Fe3O4 NPs have raised the interest of the scientific community in the areas of information storage, biomedicine,
∗
magnetic sensing etc. [5,6]. It has been established that the properties of Fe3O4 NPs are highly sensitive and significantly dependent on various factors including synthesis method, experimental conditions, pH value, solvent, type of precursors, post treatment, gas atmosphere, temperature, etc [7]. On the other hand, graphene is identified as a unique two-dimensional (2D) carbon structure and attracted much attention owing to its fascinating properties such as very high carrier mobility, chemical stability, ballistic conduction and electronic conductivity with large surface-to-volume ratio. These unmatched characteristics make it suitable candidate for the great potential applications in a number of interesting fields including nanoelectronics [8], transparent conducting electrodes [9], composites [10], supercapacitors [11,12], gas sensors [13], and hydrogen storage [14,15]. In addition, a number of researchers have investigated the electrochemical sensing application of G-based nanomaterials for determination of heavy metal ions [16,17]. Particularly, nanocomposites of reduced graphene oxide/magnetite
Corresponding author. E-mail address: [email protected] (M.A.M. Khan).
https://doi.org/10.1016/j.mssp.2019.04.005 Received 20 September 2018; Received in revised form 5 March 2019; Accepted 3 April 2019 Available online 17 April 2019 1369-8001/ © 2019 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 99 (2019) 44–53
M.A.M. Khan, et al.
for an hour. In this way, a homogeneous brown colour GO aqueous suspension was obtained and then the pH value of the solution was tuned as 9–10 by adding H6N2O dropwise. After that a hydrazine hydrate was mixed into the above suspension and heated for 24 h at 90 °C in a water bath. The corresponding product with black flocculent substance was gradually precipitated from the solution. The final suspension was collected by vacuum filtration and washed through methanol/ water and dried at 80 °C for 24 h. Finally, the product (RGO) was obtained after dried in vacuum.
(rGO/Fe3O4) have also been extensively studied [18]. Because, it exhibits rich mechanical, electrical and magnetic properties. Furthermore, the combination of Fe3O4 nanoparticles with reduced graphene oxide (RGO) would enhance photocatalytic properties [19]. To date, great efforts have been made to utilize the unique properties of reduced graphene oxide to improve the efficiency of photocatalysts [20–24], which results to increase the efficiency of solar cells [25,26]. Additionally, the higher value of photocatalytic activity is also observed in these nanocomposites ascribed to the photo-generated charge carriers immediately transfers to graphene sheets from metal oxide nanoparticles that leads increase in the separation of electron/hole pairs and hence improve degradation mechanism. In another work, Teok Peik-See et al. [27] have utilized this concept in Fe3O4/rGO nanocomposite for the deterioration of methylene blue organic dye and they achieved 100% degradation efficiency in 60 min. Hence, the nanocomposite Fe3O4/rGO has great functionalized properties that can dissociate environmental pollution with the higher efficiency as compared to Fe3O4 NPs. There are several methods reported for the synthesis of Fe3O4 and RGO/Fe3O4 magnetic nanoparticles such as hydrothermal [28,29], microemulsion [30], co-precipitation [31,32], microwave irradiation [33], and oxidation of Fe(OH)2 by H2O2 [34]. Several other methods have also been used to prepare iron oxide thin films, such as molecular beam epitaxy (MBE) [35], sputtering [36], pulsed laser deposition (PLD) [37], and sol-gel method [38,39]. Among these, sol-gel is one of the most commonly used methods for preparing iron oxide powders/ thin films for many reasons such as low temperature synthesis, cost effective, easy performance, homogeneity and thin film formability. To the best of our knowledge, limited consideration has been devoted to the use of simple and economical wet chemical method especially solgel. In the present study, a simple sol-gel process has been used to prepare Fe3O4 nanoparticles and modified Hummers method for graphene oxide in order to investigate optical, photocatalytic, electrical and magnetic properties of their nanocomposites employable in modern technology.
2.4. Synthesis of Fe3O4 nanoparticles In a typical experiment, FeCl2·4H2O, ascorbic acid, citric acid monohydrate and ethanol chemicals with proper stoichiometric ratio were dissolved in distilled water and stirred at 50 °C for 5 h. The solution was then dried at 50 °C under a constant stirring to add dimethylformamide (DMF). The xerogel was aged for a week and then dried at 120 °C for 6 h to remove the organic substances. After drying, the gel was annealed at 400 °C for 3 h and then ground into a fine powder. 2.5. Synthesis of Fe3O4/RGO nanocomposites In brief, 0.1 M FeCl2·4H2O was slowly mixed into the graphene oxide dispersion (2 mg/ml) and stirred continuously for 60 min. Thereafter, above solution was placed in a Teflon-lined stainless steel autoclave (100 ml). Then, as-prepared hydrogels were taken out with a tweezer and freeze dried under vacuum. In addition to Fe3O4, two composites of different weight ratio of Fe3O4/RGO (1:0.02 and 1:0.04) were prepared. The corresponding as-synthesized products are named as FG0, FG2 and FG4 respectively. Finally, the synthesized Fe3O4 and Fe3O4/RGO powders were stored in a vacuum desiccator for further characterization. For the phase identification and crystal structure, x-ray diffractometer (XRD, PANalytical XPert) were employed with CuKα radiation (λ = 1.54 Å) operated at 45 kV and 40 mA current. XRD data were recorded for 2θ values in the range of 20–80° at 2°/min scanning rate. Particle size, morphology and crystallinity of the synthesized nanoparticles and nanocomposites were examined by a JEOL, transmission electron microscope (TEM) operated at accelerated voltage of 200 kV. For TEM measurements, the samples were dissolved in ethanol by ultrasonication and dropped directly onto the carbon coated copper grid. The chemical composition of the prepared samples was checked by the energy dispersive x-ray (EDX) spectrometer attached with the TEM. Fourier-transform infrared (FTIR) spectra of the samples were obtained with a PerkinElmer spectrometer in the wave numbers 500–4000 cm−1 using ATR technique. Thermogravimetric and differential thermal analysis (TGA/DTA) measurements of the as-prepared samples were recorded using thermal analyzer (Shimadzu, DTG-60) from room temperature to 800 °C. For this, ∼20 mg of powdered sample was heated in an aluminum pan at the rate of 10 °C/min under a continuous nitrogen flow rate of 30 ml/min. The room temperature photoluminescence (PL) measurements of the samples were carried out on a Hitachi F-7000 spectrometer with the HeeCd laser at an excitation wavelength of 290 nm. The optical absorption spectra were recorded with UV–visible spectrophotometer (Shimadzu) in the wavelength range of 300–900 nm. Magnetic properties measurements at room temperature were performed with a vibrating sample magnetometer (VSM) attached with a quantum design physical property measurement system (PPMS) in an external applied magnetic field ranging from −60 to + 60 kOe. The temperature dependent resistivity measurement was carried out in the temperature range of 10–300 K by a standard Janis Cryogenic Micromanipulated Probe Station. The photocatalytic performance of the synthesized samples (FG0, FG2 and FG4) was examined by observing the decolorization of
2. Experimental details 2.1. Materials For the present work, analytical grade (AR) FeCl2·4H2O (> 99.99%) CAS Number 13478-10-9, graphite (> 99.99%) CAS Number 7782-425, ascorbic acid, citric acid monohydrate (99.0%) CAS Number 594929-1, ethanol, CAS Number 64-17-5 and dimethylformamide (99.8%), CAS Number 68-12-2 were procured and used without any further purification. 2.2. Synthesis of graphene oxide Graphene oxide (GO) was prepared through modified Hummers method using highly pure graphite powder [40,41]. In brief, graphite powder (∼3 g) was dispersed in the blend of conc. sulfuric and phosphoric acids at room temperature. Subsequently, potassium permanganate (3.5 g) was mixed with continuous stirring at 35 °C for 2 h followed by the addition of 100 mL deionized water. Thereafter, hydrogen peroxide solution (8 ml, 30 wt% aqueous solution) was mixed until it becomes bright yellow. At last, the obtained product was centrifuged and washed with HCl/deionized water, respectively to get neutral solution. Finally, the powder was achieved after centrifugation (4000 rpm, 10 min) several times. The product of GO powder was dried at 100 °C in vacuum. 2.3. Preparation of RGO In order to prepare the reduced graphene, GO powder (200 mg) was dissolved in 1000 ml deionized water with the help of an ultrasonicator 45
Materials Science in Semiconductor Processing 99 (2019) 44–53
M.A.M. Khan, et al.
methylene blue (MB) dye in visible light radiation produced through a sodium lamp of 400 W (Philips). In a typical experiment, the Fe3O4 and Fe3O4/RGO nanocomposites were added into the aqueous solution of MB. While initial MB concentration, and the amount of Fe3O4 and Fe3O4/RGO were set as 0.5 mg/ml and in 100 ml distilled water separately, where the concentration of MB was kept 1.5 mg/l. The pH value of dye solutions was adjusted to 7. The mixed suspension is immersed in a vessel of cylindrical shape (600 mL) and then the solution was magnetically stirred at room temperature for 30 min in a dark environment to accomplish adsorption/desorption equilibrium of the photocatalyst and the dye. The decolorization performance was monitored by measuring the intensity of absorption peak of the dye appeared at 664 nm wavelength. Further, photo decolorization efficiency (η) of the products was assessed using a relation, η = [1-(C/Co)] × 100%, here Co and C stand for the initial concentration and concentration after light irradiation respectively. 3. Results and discussion 3.1. Microstructure and morphological studies The crystal structure, purity and phase identification of Fe3O4, RGO, and Fe3O4/RGO (2% & 4%) samples were examined through the XRD diffraction patterns presented in Fig. 1(a and b). The XRD profile of RGO shows a most intense reflection plane (002) at 2θ∼26.6° with corresponding interplanar spacing (d) of ∼0.36 nm, which is slightly higher than the bulk form of graphite (∼0.34 nm) [42,43]. An increase in the interlayer spacing could be attributed to the occurrence of residual oxygen containing functional groups intercalated onto the graphite sheets. The prominent diffraction peaks in the XRD patterns of the nanocomposites (FG2 and FG4) are observed at 2θ values of 30.16°, 36.8°, 43.2°, 54.1°, 58.9° and 63.7° that correspond to the (220), (311), (400), (422), (511) and (440) reflection planes respectively. All diffraction peaks in FG0, FG2 and FG4 samples are well matched to the face centered cubic (fcc) crystal structure of the magnetite (JCPDS no. 65–3107). Further, the average crystallite size is calculated with the help of Scherrer's equation using the most intense diffraction peak (311). The estimated values of the average crystallite size for FG0, FG2 and FG4 samples are found to be 10, 15 and 18.5 nm respectively. No obvious typical diffraction peaks attributed to the RGO are observed in the XRD spectra of Fe3O4/RGO composites that signify that the stacking of the RGO sheets in the Fe3O4/RGO nanocomposites remained disordered. The Rietveld refinement analysis of the Fe3O4 XRD data was performed to investigate other structural parameters and phase purity of the NPs as shown in Fig. 1(a). We have obtained the best fit between the observed and calculated patterns with minimum deviation using a cubic crystal structure in the space group Fd-3m that ascribed to the existence of preferential orientation of the crystallites in the Fe3O4 nanoparticles. Various structural parameters like lattice constant, atomic position, occupancy etc. are obtained. Refinement results exhibit that the Fe3+ ions lie at 8a (0.125, 0.125, 0.125), Fe2+ at 16d (0.5, 0.5, 0.5) and O at 32e (0.25565, 0.25565, 0.25565) positions. The values of lattice parameters (a = b = c), unit cell volume (a3), and χ2 are found to be 8.36402 Å, 585.121 (Å)3 and 1.43 respectively. The lower value of χ2 further confirmed goodness of the fit. These results are well consistent with the earlier published work in literature [44]. No other crystalline/amorphous impurity phases are observed. Moreover, the smaller size of the particles also established by the FWHM (broadening) of the diffraction peaks. It is also noticed that the (311) peak shifts towards the higher angle in 4% RGO sample and shown in the inset of Fig. 1(b). This may be due to shrinkage in the lattice parameters, i.e. 8.363 Å for the Fe3O4/RGO (2%) reduces to 8.356 Å for Fe3O4/RGO (4%). In addition, as concentration of RGO increases, intensity of (222) plane diminish ascribed to the short-range order in the stacked graphene like sheets [45]. The morphology, crystallinity and distribution of particle size of the
Fig. 1. Powder x-ray diffraction patterns of (a) Fe3O4 nanoparticles, (b) Fe3O4/ RGO nanocomposites with different mass ratio of RGO and inset shows magnified view of (311) peak of Fe3O4.
as-prepared Fe3O4 and Fe3O4/RGO nanocomposite were supplementary examined through TEM and high resolution TEM (HRTEM). Fig. 2(a and b) presents a narrow size distribution of nearly spherical shaped Fe3O4 nanoparticles in FG0 and FG2 samples with an average particle size 12 and 17 nm respectively which is almost consistent with the XRD results. TEM image of Fe3O4 shows uniform size and quite agglomerated nature of the NPs that may be due to the stronger magnetic coupling between them. High-resolution TEM micrographs presented in Fig. 2(c and d) indicate highly crystalline phase of the NPs with the lattice fringes of 0.231 and 0.64 nm which corresponds to the d spacing of the highest intense (311) and (002) peaks of crystallographic orientation of cubic magnetite Fe3O4 and Fe3O4/RGO(2%) nanocomposite. The chemical composition of the synthesized nanoparticles and nanocomposites was investigated by the EDX spectrometer attached with TEM and displayed in Fig. 3. EDX spectrum indicates existence of iron (Fe) and oxygen (O) elements in the atomic proportion of 3:4 approximately. The signals of carbon (C) and copper (Cu) are appearing because of the copper grid used for TEM measurement is coated with 46
Materials Science in Semiconductor Processing 99 (2019) 44–53
M.A.M. Khan, et al.
Fig. 2. (a,b) TEM images of Fe3O4 and Fe3O4/RGO (2%) samples, (c,d) HRTEM images of the same.
assigned to the FeeO vibrations of Fe3O4 nanoparticles [54], which is consistent with the pure Fe3O4 nanoparticles [55,56].
carbon. 3.2. X-ray photoelectron spectroscopy
3.4. Thermogravimetry analysis The surface chemical information of Fe3O4/RGO nanocomposites was further evaluated through x-ray photoelectron spectroscopy (XPS). The wide scan spectrum of Fe3O4/RGO nanocomposite in Fig. 4(a) indicates three intense peaks at binding energies 285, 529 and 711 eV that correspond to the carbon (C1s), oxygen (O1s), and Fe2p, respectively. However, the high resolution XPS spectrum of C1s shown in Fig. 4(b) contains non-oxygenated aromatic sp2 bonded carbon ring (CeC) at 284.6 eV, whereas those at 285.7 and 288.3 eV binding energies are due to the oxygenated functional group attached to the carbon atoms, such as CeO and C]O groups, respectively [46]. Fig. 4(c) shows O1s spectrum with two highly intense peaks at 529.98 and 531.21 eV associated to the anionic oxygen in Fe3O4 and existence of the residual oxygen functional groups in the graphene sheets. Furthermore, the high-resolution spectrum of Fe2p presented in Fig. 4(d) exhibits two prominent peaks at 710.80 and 724.5 eV, assigned to the Fe2p3/2 and Fe2p1/2 spin orbit peaks of Fe3O4 nanoparticles respectively [47,48]. These results confirmed the formation of mixed oxide of both Fe2+ and Fe3+ ions. However, the satellite peak situated at about ∼719 eV is a characteristic peak of Fe3+ in γ-Fe2O3 phase and suggests that the Fe3O4 nanoparticles are partially oxides [49,50]. The atomic percentages of the C1s, O1s and Fe2p elements were estimated with the help of sensitivity factors in the FG2 sample and found to be 33.8%, 40.3% and 25.9% respectively. These values are quite consistent with the earlier reported results and clearly demonstrate homogeneous dispersion of the Fe3O4 nanoparticles.
In order to establish the thermal stability of the synthesized nanoparticles and nanocomposites, TGA measurement is carried out in the nitrogen environment at a heating rate of 10 °C/min and presented in Fig. 6. These thermograms exhibit three-steps in the weight loss for the temperature range of 20–800 °C. All samples RGO, FG0, FG2 and FG4 showed substantial weight loss in the range of 45–150 °C that may be due to the evaporation of absorbed water and adsorbed organics in the samples. A rapid weight loss observed between 220 and 330 °C is associated with the decomposition of graphene sheet in air. At the end, the weight loss in the temperature range between 340 and 650 °C may be a response to the decomposition of the organic groups supported on the magnetite nanoparticles. Moreover, the mass retained at 700 °C directly translates into the amount of Fe3O4 in the composite. 3.5. Optical studies Optical properties of Fe3O4 nanoparticles and Fe3O4/RGO (2 & 4%) nanocomposites were studied through UV–visible absorption spectroscopy (fig. not shown here). It is observed that the absorption edge shifts towards the higher wavelength for the RGO doped nanocomposite than Fe3O4 nanoparticles, indicating a red shift in the absorption. The absorption coefficient (α) is estimated using the known relation, α = 2.303A/x, here A is the optical absorbance and x is the thickness of the cuvette [57]. Further, using absorption coefficient and frequency (ν) of the incident radiation; the optical band gap (Eg) is measured by employing the Tauc's relation [58], αhν = B(hν-Eg)n, here h stands for the Planck's constant and B is a constant. The exponent n is associated with the nature of the electronic transition, i.e. n = 2 and n = 3 for an indirect allowed transition and indirect forbidden transition respectively, whereas n = 1/2 and n = 3/2 for the direct allowed transition and forbidden transition respectively. For the present case, all the experimental data were fitted to the direct allowed transition by taking n = 1/2 [59]. Fig. 7 presents (αhν)2 as a function of hν (Tauc's plots) for FG0, FG2 and FG4 samples. Hence, the value of band gap is estimated by the extrapolation of the linear region of the graph to the x-axis
3.3. FTIR studies FTIR spectroscopy study was performed to investigate the microstructural properties through the presence of various chemical bonds in the samples. Fig. 5 exhibits the FTIR spectra of FG0, FG2 and FG4 nanocomposites. The broad absorption band in the spectrum at 3434 cm−1 appeared in all the samples are ascribed to the stretching vibration of OeH [51,52]. The bands at 1047, 1644, and 2357 cm−1 can be attributed to the C]C]O and C]C stretching vibration [53]. Additionally, the small characteristic absorption band at 580 cm−1 is 47
Materials Science in Semiconductor Processing 99 (2019) 44–53
M.A.M. Khan, et al.
Fig. 3. EDX spectrum of Fe3O4 and Fe3O4/RGO (2%) samples.
48
Materials Science in Semiconductor Processing 99 (2019) 44–53
M.A.M. Khan, et al.
Fig. 4. XPS spectrum of Fe3O4/RGO (2%) nanocomposite (a) survey scan, (b) high-resolution C 1s, (c) high-resolution O 1s, and (d) high-resolution Fe 2p spectrum.
Fig. 5. FTIR spectra of Fe3O4 and Fe3O4/RGO nanocomposites.
Fig. 6. Thermogravimetric analysis curves of RGO, Fe3O4, and RGO/Fe3O4 nanocomposites.
intercept. In this way, bandgap of all the samples has been estimated and given in Table 1 which is well matched with the results reported in literature [60]. It is known that the decrease in particle size leads an increase in the band gap of nanomaterials, which tends to blue shift in the optical absorption [61]. Moreover, it is noted that the value of energy gap in the present system is reducing with increase in RGO concentration associated with the red shift in the absorption that signify increase in the carrier concentration. We have also interpreted the variation in energy gap using Moss–Burstein shift that suggests enhancement in the free carrier density is responsible for the corresponding downward shift of the Fermi level below the band edge [62].
For the higher RGO content (4%) sample, density of induced defects increases that leads to the decrease in the band gap. This characteristic is helpful for enhancing the light absorption and photocatalytic efficiency of the Fe3O4/RGO nanocomposites. The energy of the conduction band edge (ECB) and the valence band edge (EVB) are also determined using UV–visible absorption data of the samples. Based on the optical band gap, the valence band and the conduction band edge potentials at the point of zero charge can be written by the following expressions [63],
49
Materials Science in Semiconductor Processing 99 (2019) 44–53
M.A.M. Khan, et al.
Fig. 8. PL spectra of Fe3O4 and RGO/Fe3O4 nanocomposites at excitation wavelength of 290 nm.
Fig. 7. Tauc's plots of Fe3O4 and Fe3O4/RGO nanocomposites.
ECB = χ − EC −
1 Eg 2
3.7. Magnetic properties
(1)
EVB = Eg + ECB
The magnetic properties of Fe3O4 NPs, and Fe3O4/RGO nanocomposites were measured through a vibrating sample magnetometer (VSM) in a magnetic field of −60 to +60 kOe at room temperature and shown in Fig. 9. The M − H loops do not show definite value of the coercivity that signify superparamagnetic nature of all the samples. The lower value of the saturation magnetization (MS) is observed for Fe3O4 NPs (∼ 64 emu/g) than that of the bulk Fe3O4 (MS = 90 emu/g at 300 K) [66]. The reduction in this value at a smaller size is ascribed to the noticeable surface effects of these NPs. Further, the surface of the NPs is considering to be composed of some disorder spins that stop the core spins from aligning along the field direction resulted decrease in the saturation magnetization of the small sized NPs [67]. Moreover, the value of saturation magnetization is found to reduce from 42 to 21 emu/g for higher RGO doping but superparamagnetic behavior remains unaltered which can be attributed to the attachment of Fe3O4 NPs with RGO. Using magnetic measurements data, the particle size (Dm) can also be calculated by the following equation [68],
(2)
where χ represent the absolute electro-negativity of the materials (χ is 5.78 eV for Fe3O4) [64]. EC is the scale factor relating the reference electrode redox level to absolute vacuum scale (AVS) (∼ 4.44 ± 0.02 eV for AVS and 0 V for NHE) [65] and Eg ∼ 2.35, 2.29 and 2.18 eV for the Fe3O4 and Fe3O4/RGO (2 & 4%) nanocomposites respectively (calculated from UV–visible Tauc plots). The calculated values of the conduction band and the valence band edge positions of Fe3O4 and Fe3O4/RGO nanocomposites are at −0.170 eV, 0.106 eV, 0.079 eV and 2.51, 2.39, 2.07 eV respectively.
3.6. Photoluminescence (PL) studies PL spectroscopy is a powerful tool to study the electronic and optical properties of the nanoparticles and nanocomposites, including mobility of the charge carriers to the surface and recombination of electron-hole pairs in photo-induced semiconductors. Fig. 8 presents the room temperature photoluminescence spectra of the pristine Fe3O4 and Fe3O4/RGO nanocomposites. It is evident that the intensity of pure Fe3O4 nanoparticles is higher than that of the Fe3O4/RGO nanocomposites which signify high recombination rate of photo-generated e−/h+ pairs and the intensity of PL spectra is directly proportional to the recombination rate of charge carriers as well as charge separation between conduction and valence bands in the Fe3O4/RGO nanocomposites [27]. This may be due to the presence of structural defects created by the oxygen vacancies. Our findings suggest that the graphene lowers the recombination rate of the charge carriers and result in higher photoactivity performance of Fe3O4/RGO nanocomposites.
D3m =
18kBT ⎛ dM ⎞ πμ oMb . Ms ⎝ dH ⎠H→ 0
(3)
here, kB represents the Boltzmann's constant (kB = 1.38 × 10−23 J/K), T stands for the absolute temperature (T = 300 K), μo is the magnetic permeability of vacuum, Mb is the magnetization of bulk magnetite (for spherical Fe3O4 nanoparticles, Mb = 0.48 × 106 A/m). Based on this approach, the calculated particle size of the NPs and Fe3O4/RGO nanocomposites is given in Table 1, which is smaller than the particle size observed through XRD and TEM measurements [69].
Table 1 Estimated physical parameters for Fe3O4, Fe3O4/RGO (2%) and Fe3O4/RGO(4%) samples. Sample
Fe3O4 Fe3O4/RGO (2%) Fe3O4/RGO (4%)
Particle size (nm) Scherrer's eq.
TEM
10.2 ± 3.4 15.7 ± 2.4 18.5 ± 4.5
12 ± 3.2 17 ± 1.4 20 ± 2.2
Eg (eV)
N(EF) (eV−1cm−3)
R (nm)
W (meV)
Dm (nm)
2.35 ± 0.16 2.28 ± 0.11 2.18 ± 0.09
3.04 × 1022 8.51 × 1021 5.93 × 1020
27 30 51
388 301 104
3.4 ± 1.5 4.3 ± 1.3 6.6 ± 1.1
50
Materials Science in Semiconductor Processing 99 (2019) 44–53
M.A.M. Khan, et al.
Fig. 9. Magnetic hysteresis (M–H) loops for Fe3O4 and RGO/Fe3O4 nanocomposites at room temperature.
3.8. Transport properties The electrical transport behavior of the FG0, FG2 and FG4 samples as a function of temperature were carried out in the temperature range of 10–300 K and shown in Fig. 10(a). The resistivity is found to decrease with the rise in temperature, demonstrates semiconducting behavior of all the samples. It is also evident from the same figure that the resistivity decreases with the increase in RGO concentration throughout the studied temperature range. The observed value of room temperature resistivity of Fe3O4 NPs is about 25.5 mΩ-cm, which is lower than its corresponding bulk form [70]. The temperature dependent resistivity can be explained on the basis of activated hopping mechanism, including two regions with different slopes for the (1/T) dependence of resistivity (ρ). First one is related to the conduction band at the higher temperatures. However, other one at the lower temperature that measured by the hopping conduction or transition between the impurity centers or localized states inside the impurity band. For all synthesized samples, the temperature dependent electrical resistivity is well described in the low temperature regime (from 40 to 150 K) with the help of variable range hopping (VRH) mechanism given by the expression, γ T ρ = ρo exp ⎛ Mott ⎞ ⎝ T ⎠
Fig. 10. (a) Resistivity as a function of temperature, and (b) VRH fitted resistivity plots for Fe3O4 and Fe3O4/RGO nanocomposites.
The typical hopping distance Rhop(T) between two sites and the average hopping energy Whop at a temperature (T) can be calculated by the following expressions [73].
(4)
9LLoc ⎤ Rhop = ⎡ ⎢ 8πkTN(E F) ⎥ ⎦ ⎣
where ρo is a constant that depends on the phonon density and TMott is the Mott characteristic temperature related to the hopping barrier, electronic structure and energy distribution of the localized states. In the above equation, the exponent γ decides the dimensionality 1 (D) of the system using the relation, γ= 1 +D . The values of γ are 1/2, 1/ 3 and 1/4 for 1D, 2D and 3D case respectively. For the existing system, the expression (3D) for the dc resistivity and Mott's characteristic temperature can be expressed as [71]. 1/4 T ρ = ρo exp ⎛ Mott ⎞ ⎝ T ⎠
TMott =
Co kBN(EF)L3Loc
Whop =
3 4πR3hopN(EF)
(7)
(8)
Using above equations, we have extracted the values of N(EF), Rhop and Whop for Fe3O4 and Fe3O4/RGO nanocomposites from the fitted curves and tabulated in Table 1. For the present systems, Co = 18 and LLoc = 10 Å have been taken. It is found that the value of density of states near the Fermi level increases with the doping concentration of RGO, whereas average hopping distance reduces, accompanied by a decrease in hopping energy. The charge transport mechanism increases the conductivity due to generation of polarons associated with the RGO doping.
(5)
(6)
where N(EF) represents the density of states at Fermi level, LLoc is localization length that varies in the range of 3–30 Å and Co is a constant of typical value 16–310 [72]. The linear fit of the temperature dependent resistivity data are found to obey the Mott's VRH model (Fig. 10(b)).
3.9. Photocatalytic performance The photocatalytic response of the as-prepared samples was evaluated for the deterioration of methylene blue (MB) dye and the 51
Materials Science in Semiconductor Processing 99 (2019) 44–53
M.A.M. Khan, et al.
Fig. 11. Time dependent UV–vis absorbance spectra of photocatalytic decolorization of MB dye under visible light irradiation for (a) Fe3O4, (b) Fe3O4/RGO (2%) and (c) Fe3O4/RGO(4%) nanocomposites, (d) Photocatalytic decolorization of MB as a function of irradiation time under visible light irradiation.
3.9.1. Reusability assay The reusability of the catalytic materials plays a major role in the photodegradation experiments. In this respect, the reusability of the Fe3O4/RGO nanocomposite was carried out seven times, the degradation efficiency reached up to 94.6% which indicates that the synthesized nanocomposites can maintain high stability for reuse. Moreover, it established excellent reusability during the repeated photocatalytic experimental cycles (Fig. 12).
outcomes are presented in Fig. 11(a,b,c). The absorption peak of the dye at 664 nm gradually reduces over the time when irradiated under visible light and almost disappeared in 90 min. This signifies complete decolorization of the MB dye. Moreover, a blank test in the absence of the catalysts (synthesized samples) has been performed that shows that the photo-conversion reaction could not proceed without catalyst. It is clear from Fig. 11(d) that the decolorization efficiency of MB for the Fe3O4 NPs is calculated as 76.7% after 90 min of irradiation time, while decolorization rate enhances by the doping of RGO and found to be 86.8% and 93.3% for FG2 and FG4 nanocomposites respectively. Generally, when graphene doped Fe3O4 sample is irradiated by the light of photons energy equal to or more than the bandgap, resulted excitation of an electrons (e−) from the valence band (VB) to the conduction band (CB) with holes (h+) left in the valence band (VB) simultaneously. The photoelectron generated from Fe3O4 and MB dye transfer to the graphene and in this way Fe3+ can certainly capture the electrons to transform in Fe2+. Thereafter, the Fe2+ can stay react with H2O2 to form the Fe3+ and OH∗ on the reduced graphene oxide surface to degrade the dye molecules. The re-generated Fe3+ ions rapidly reduced to Fe2+ by the electron concentrated on the surface of graphene to keep the cycle of Fe3+/Fe2+. Because, Fe− based materials are used as the catalysts and graphene used as an electron capture agent materials make it an ideal support to enhance the transfer and separation of photogenerated electrons and holes, which are responsible for the enhanced activity of the photocatalysts. It is clearly demonstrated that the photocatalytic activity of Fe3O4 NPs can be effectively enhanced by hybridizing with RGO.
Fig. 12. Reusability assay of Fe3O4/RGO nanocomposite for photodegradation of MB dye under visible light irradiation. 52
Materials Science in Semiconductor Processing 99 (2019) 44–53
M.A.M. Khan, et al.
4. Conclusions
[23] Amr A. Nada, M. Nasr, R. Viter, P. Miele, S. Roualdes, M. Bechelany, J. Phys. Chem. C 121 (2017) 24669–24677. [24] M. Nasr, C. Eid, R. Habchi, P. Miele, M. Bechelany, ChemSusChem 11 (2018) 3023–3047. [25] N.L. Yang, J. Zhai, D. Wang, Y.S. Chen, L. Jiang, ACS Nano 4 (2010) 887–894. [26] S.R. Kim, M.K. Parvez, M. Chhowalla, Chem. Phys. Lett. 483 (2009) 124–127. [27] T. Peik-See, A. Pandikumar, L.H. Ngee, H.N. Ming, C.C. Hua, Catal. Sci. Technol. 4 (2014) 4396–4405. [28] S. Saha, M. Jana, P. Samanta, N.C. Murmu, N.H. Kim, T. Kuila, J. Hee Lee, RSC Adv. 4 (2014) 44777–44785. [29] F.Z. Mohammad, Ijaz Ahmad, J.J. Siddiqui, K.M. Ali, M. Mudsar, H. Arshad, J. Mater. Sci. Mater. Electron. 29 (2018) 19775–19782. [30] Xiaojuan Liang, Xiangchen Jia, Lei Cao, Juncai Sun, Yuxiang Yang, J. Dispersion Sci. Technol. 31 (2010) 1043–1049. [31] D.E. Zhang, Z.W. Tong, S.Z. Li, X.B. Zhang, A.L. Ying, Mater. Lett. 62 (2008) 4053–4055. [32] Zong Meng, Ying Huang, Yang Zhao, Lei Wang, Panbo Liu, Yan Wang, Qiufen Wang, Mater. Lett. 106 (2013) 22–25. [33] Rajesh Kumar, Rajesh K. Singh, Alfredo R. Vaz, Raluca Savu, Stanislav A. Moshkalev, ACS Appl. Mater. Interfaces 9 (2017) 8880–8890. [34] L.Q. Yu, L.J. Zheng, J.X. Yang, Mater. Chem. Phys. 66 (2000) 6–9. [35] J.-B. Moussy, S. Gota, A. Bataille, M.-J. Guittet, M. Gautier-Soyer, F. Delille, B. Dieny, F. Ott, T.D. Doan, P. Warin, P. Bayle-Guillemaud, C. Gatel, E. Snoeck, Phys. Rev. B 70 (2004) 174448. [36] Y. Peng, C. Park, D.E. Laughlin, J. Appl. Phys. 95 (2003) 7957. [37] A. Goikhman, P. Shvetsa, U. Koneva, R. Mantovan, K. Maksimova, Thin Solid Films 652 (2018) 28–33. [38] Eken Ali Erdem, Ozenbas Macit, J. Sol-Gel Sci. Technol. 50 (2009) 321–327. [39] Ping Hu, Tian Chang, Wen-Jing Chen, Jie Deng, Shi-Lei Li, Ye-Gai Zuo, Kang Lu, Yang Fan, Megan Hostetter, Alex A. Volinsky, Journal of Alloys and Compounds 773 (2019) 605–611. [40] W.S. Hummers Jr., R.E. Offeman, J. Am. Chem. Society 80 (1958) 1339–1339. [41] A. Romero, M.P. Lavin-Lopez, L. Sanchez-Silva, J.L. Valverde, A. Paton-Carrero, Mater. Chem. Phys. 203 (2018) 284–292. [42] K.H. Liao, A. Mittal, S. Bose, C. Leighton, K.A. Mkhoyan, C.W. Macosko, ACS Nano 5 (2011) 1253–1258. [43] Zhaoling Ma, Xiaobing Huang, Shuo Dou, Jianghong Wu, Shuangyin Wang, J. Phys. Chem. C 118 (2014) 17231–17239. [44] M.V. Reddy, B.V.R. Chowdari, Jagadese J. Vittal, RSC Adv. 4 (2014) 64142–64150. [45] J. Yan, T. Wei, B. Shao, F. Ma, Z. Fan, M. Zhang, C. Zheng, Y. Shang, W. Qian, F. Wei, Carbon 48 (2010) 1731–1737. [46] J. Zhang, Z. Xiong, X. Zhao, J. Mater. Chem. 21 (2011) 3634–3640. [47] L. Ji, L. Zhou, X. Bai, Y. Shao, G. Zhao, Y. Qu, C. Wang, Y.J. Li, Mater. Chem. 22 (2012) 15853–15862. [48] W. Zhang, X. Li, R. Zou, H. Wu, H. Shi, S. Yu, Y. Liu, Sci. Rep. 5 (2015) 1112. [49] Z. Li, J.F. Godsell, J.P. O'Byrne, N. Petkov, M.A. Morris, S. Roy, J.D. Holmes, J. Am. Chem. Soc. 132 (2010) 12540–12541. [50] H. Wu, G. Gao, X. Zhou, Y. Zhang, S. Guo, Cryst. Eng. Comm. 14 (2012) 499–504. [51] Y. Xu, H. Bai, G. Lu, C. Li, G. Shi, J. Am. Chem. Soc. 130 (2008) 5856–5857. [52] Z. Li, J. Wang, L. Niu, J. Sun, P. Gong, W. Honga, L. Ma, S. Yang, J. Power Sources 245 (2014) 224–231. [53] Z. Song, Y. Zhang, W. Liu, S. Zhang, G. Liu, H. Chen, J. Qiu, Electrochim. Acta 112 (2013) 120–126. [54] Lulu Ren, Shu Huang, Wei Fan, Tianxi Liu, Appl. Sur. Sci. 258 (2011) 1132–1138. [55] S. Bhuvaneswari, P.M. Pratheeksha, S. Anandan, et al., Phys. Chem. Chem. Phys. 16 (2014) 5284–5294. [56] K.Y.A. Lin, H. Yang, C. Petit, W.-D. Lee, J. Colloid Interface Sci. 438 (2015) 296–305. [57] Rayan Khalid, A.N. Alhazaa, M.A.M. Khan, Appl. Phys. A 124 (2018) 536. [58] M.A.M. Khan, Sushil Kumar, Tansir Ahamad, Abdulaziz N. Alhazaa, J. Alloy. Comp. 743 (2018) 485–493. [59] J. Tauc, Optical Properties of Solids, (1970) North-Holland, Amsterdam. [60] H.E. Ghandoor, H.M. Zidan, M.M.H. Khalil, M.I.M. Ismail, Int. J. Electrochem. Sci. 7 (2012) 5734–5745. [61] M.A.M. Khan, Wasi Khan, Maqusood Ahamed, Mansour Alhoshan, Mater. Lett. 79 (2012) 119–121. [62] Manas Saha, Sirshendu Ghosh, Vishal Dev Ashok, S.K. De, Phys. Chem. Chem. Phys. 17 (2015) 16067. [63] M.A.M. Khan, Wasi Khan, Maqusood Ahamed, A.N. Alhazaa, Sci. Rep. 7 (2017) 12560. [64] R.G. Pearson, Absolute electronegativity and hardness: application to inorganic chemistry, Inorg. Chem. 27 (1988) 734–740. [65] S.R. Morrison, Electrochemistry at Semiconductor and Oxidized Metal Electrodes, Plenum Press, New York, NY, USA, 1980. [66] Z. Huang, F. Tang, J. Colloid Interface Sci. 281 (2005) 432–436. [67] K. Maaz, S. Karim, A. Mumtaz, S.K. Hasanain, J. Liu, J.L. Duan, J. Magn. Magn. Mater. 321 (2009) 1838–1842. [68] B.Y. Yu, Seung-Yeop Kwak, J. Mater. Chem. 20 (2010) 8320–8328. [69] B.D. Cullity, C.D. Graham, Introduction to Magnetic Materials, second ed., John Wiley & Sons, 2009, pp. 359–408. [70] Y.H. Cheng, L.Y. Li, W.H. Wang, X.G. Luo, Hui Liu, R.K. Zheng, J. Appl. Phys. 109 (2011) 073905. [71] M.A.M. Khan, W. Khan, M. Ahamed, M.S. Alsalhi, T. Ahmed, Elec. Mat. Lett. 9 (2013) 53–57. [72] M.A.M. Khan, S. Kumar, M. Ahamed, Mater. Sci. Semicond. Process. 30 (2015) 169–173. [73] M.A.M. Khan, Sushil Kumar, M.Wasi Khan, M. Husain, M. Zulfequar, Mater. Res. Bull. 45 (2010) 727–732.
In summary, we have synthesized Fe3O4 nanoparticles and Fe3O4/ RGO (2% and 4%) nanocomposites using sol-gel and modified Hammers methods respectively. A comprehensive study of structural, optical, photocatalytic, electrical transport and magnetic properties of the samples has been explored. The Rietveld refinement of the XRD data reveals the inverse spinel cubic structure of the prepared nanostructures with space group Fd-3m. The average crystallite size of the magnetite nanoparticles is found to be in the range of 10-19 nm. Moreover, TEM micrographs ensure almost spherical shape with an average particle size of 12 and 17 nm for the Fe3O4 and Fe3O4/RGO(2%) samples respectively. The HRTEM micrographs exhibit a high crystal quality and ultrasmooth spherical surface of the Fe3O4 nanoparticles. The photocatalytic activity has been significantly enhanced by hybridizing Fe3O4 with RGO. The magnetic measurements revealed superparamagnetic behavior of the prepared samples. The observed value of saturation magnetization found to be lower for the nanocomposites than the magnetite nanoparticles and its bulk form. This may be due to the surface effects and attachment of RGO. The temperature dependent resistivity displayed semiconducting nature of all the samples and described on the basis of VRH conduction mechanism. Hence, the prepared nanocomposites are suitable for the real potential applications in various areas such as biomaterials and environmental remediation. Acknowledgement The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding this work through research group project no. 1437-023. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.mssp.2019.04.005. References [1] J.Y. Song, B.S. Kim, Bioproc. Biosyst. Eng. 32 (2009) 79–84. [2] W.-H. Lia, X.-P. Yuea, C.-S. Guob, J.-P. Lvb, S.-S. Liub, Y. Zhangb, J. Xub, Appl. Surf. Sci. 335 (2015) 23–28. [3] I. Bilecka, M. Niederberger, Nanoscale 2 (2010) 1358–1374. [4] R.M. Cornell, U. Schwertmann, Reactions, Occurences and Uses, second ed., (2003). [5] S. Sun, Adv. Mater. 18 (2006) 393–403. [6] S. Sabale, P. Kandesar, V. Jadhav, R. Komorek, R.M. Kishan, X.Y. Yu, Biomater. Sci. 5 (2017) 2212–2225. [7] S. Rashdan, M. Bououdina, A. Al-Saie, J. Exp. Nanosci. 8 (2013) 210–221. [8] C. Berger, Z. Song, T. Li, X. Li, A.Y. Ogbazghi, R. Feng, Z. Dai, A.N. Marchenkov, E.H. Conrad, P.N. First, W.A. de Heer, J. Phys. Chem. B 108 (52) (2004) 19912–19916. [9] S. Bae, H. Kim, Y. Lee, X. Xu, J.S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. Ri Kim, Y.I. Song, Y.J. Kim, K.S. Kim, B. Ozyilmaz, J.H. Ahn, B.H. Hong, S. Iijima, Nat. Nanotechnol. 5 (8) (2010) 574–578. [10] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Nature 442 (7100) (2006) 282–286. [11] J.J. Yoo, K. Balakrishnan, J. Huang, V. Meunier, B.G. Sumpter, A. Srivastava, M. Conway, A.L. Mohana Reddy, J. Yu, R. Vajtai, P.M. Ajayan, Nano Lett. 11 (4) (2011) 1423–1427. [12] Sanjoy Mondal, Utpal Rana, Sudip Malik, Phys. Chem. C 121 (14) (2017) 7573–7583. [13] W. Yuan, A. Liu, L. Huang, C. Li, G. Shi, Adv. Mater. 25 (5) (2013) 766–771. [14] G.K. Dimitrakakis, E. Tylianakis, G.E. Froudakis, Nano Lett. 8 (10) (2008) 3166–3170. [15] M. Pumera, Chem. Soc. Rev. 39 (2010) 4146–4157. [16] R.K.L. Tan, S.P. Reeves, N. Hashemi, D.G. Thomas, E. Kavak, R. Montazami, N.N. Hashemi, J. Mater. Chem. 5 (2017) 17777. [17] D. Chen, L.H. Tanga, J.H. Li, Chem. Soc. Rev. 157 (2010) 353–393. [18] L. Zhang, X. Yu, H. Hu, Y. Li, M. Wu, Z. Wang, G. Li, Z.S.C. Chen, Sci. Rep. 5 (2015) 9295–9298. [19] H.L. Xu, H. Bi, R.B. Yang, J. Appl. Phys. 111 (2012) 07A522. [20] J.L. Wu, X.P. Shen, L. Jiang, K. Wang, K.M. Chen, Appl. Surf. Sci. 256 (2010) 2826–2830. [21] G. Williams, P.V. Kamat, Langmuir 25 (2009) 13869–13873. [22] M. Nasr, S. Balme, C. Eid, R. Habchi, P. Miele, M. Bechelany, J. Phys. Chem. C 121 (2017) 261–269.
53
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Investigation on the structure and physical properties of Fe3O4/RGO nanocomposites and their photocatalytic application
T
M.A. Majeed Khana,∗, Wasi Khanb, Maqusood Ahameda, Abdulaziz N. Alhazaaa,c a
King Abdullah Institute for Nanotechnology, King Saud University, Riyadh, 11451, Saudi Arabia Department of Physics, Aligarh Muslim University, Aligarh, 202002, India c Physics and Astronomy Department, College of Science, King Saud University, P.O. Box 2455, Riyadh, 11451, Saudi Arabia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnetite nanoparticles Reduced graphene oxide Optical properties Photocatalytic activity VRH model Superparamagnetism
In the present work, we have investigated the structural, optical, photocatalytic activity, electrical transport and magnetic properties of Fe3O4 nanoparticles and its composite with reduced graphene oxide (Fe3O4/RGO) by varying the ratio of reduced graphene oxide (RGO: 2% and 4%). Fe3O4 nanoparticles and RGO were synthesized by the modified sol-gel and Hummers methods respectively. The as-prepared samples were examined by various analytical techniques to investigate their functional properties and possible applications. Rietveld refinement analysis of the XRD data reveals face-centered cubic (fcc) structure of the Fe3O4 nanoparticles and confirms the characteristic planes of magnetite phase. The average crystallite size of the Fe3O4, Fe3O4/RGO (2%) and Fe3O4/ RGO (4%) samples was estimated using Scherrer's equation and found to be ∼10, 15 and 19 nm respectively. TEM/HRTEM micrographs establish spherical shapes of the magnetite nanoparticles with an average diameter of ∼12 nm for Fe3O4, while 17 and 20 nm in the nanocomposites respectively. The low temperature resistivity measurements exhibit semiconducting nature of all the samples and follow variable range hopping (VRH) mechanism. The magnetic measurements demonstrate typical superparamagnetic behavior of the samples at room temperature. The photocatalytic performance of the samples has been investigated for the decolorization of methylene blue (MB) organic dye under visible light irradiation. The Fe3O4/RGO nanocomposites display better adsorption behaviour and excellent photocatalytic activity than RGO and Fe3O4 nanoparticles. This may be due to strong interactions between Fe3O4 nanoparticles and graphene sheets.
1. Introduction The field of nanotechnology mainly depends on tailor-made materials having typical dimensions in the nanometer range. It is well known that the nanoparticles (NPs) exhibit distinct and improved properties based on the specific characteristics like distribution, morphology and particle size in comparison with the bigger particles of the same material [1]. The surface to volume ratio of the nanoparticles increases with the reduction in particle size. Additionally, the scientific and technological attributes of the nanomaterials are well proved in numerous applications, including use in biotechnology, catalysis, energy storage, photonics, etc [2,3]. Among various nanomaterials, magnetite (Fe3O4) is one of the promising magnetic materials in cubic inverse spinel structure with oxygen anions in face-centered cubic (fcc) closed packing and iron cations occupying interstitial tetrahedral and octahedral sites [4]. Moreover, Fe3O4 NPs have raised the interest of the scientific community in the areas of information storage, biomedicine,
∗
magnetic sensing etc. [5,6]. It has been established that the properties of Fe3O4 NPs are highly sensitive and significantly dependent on various factors including synthesis method, experimental conditions, pH value, solvent, type of precursors, post treatment, gas atmosphere, temperature, etc [7]. On the other hand, graphene is identified as a unique two-dimensional (2D) carbon structure and attracted much attention owing to its fascinating properties such as very high carrier mobility, chemical stability, ballistic conduction and electronic conductivity with large surface-to-volume ratio. These unmatched characteristics make it suitable candidate for the great potential applications in a number of interesting fields including nanoelectronics [8], transparent conducting electrodes [9], composites [10], supercapacitors [11,12], gas sensors [13], and hydrogen storage [14,15]. In addition, a number of researchers have investigated the electrochemical sensing application of G-based nanomaterials for determination of heavy metal ions [16,17]. Particularly, nanocomposites of reduced graphene oxide/magnetite
Corresponding author. E-mail address: [email protected] (M.A.M. Khan).
https://doi.org/10.1016/j.mssp.2019.04.005 Received 20 September 2018; Received in revised form 5 March 2019; Accepted 3 April 2019 Available online 17 April 2019 1369-8001/ © 2019 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 99 (2019) 44–53
M.A.M. Khan, et al.
for an hour. In this way, a homogeneous brown colour GO aqueous suspension was obtained and then the pH value of the solution was tuned as 9–10 by adding H6N2O dropwise. After that a hydrazine hydrate was mixed into the above suspension and heated for 24 h at 90 °C in a water bath. The corresponding product with black flocculent substance was gradually precipitated from the solution. The final suspension was collected by vacuum filtration and washed through methanol/ water and dried at 80 °C for 24 h. Finally, the product (RGO) was obtained after dried in vacuum.
(rGO/Fe3O4) have also been extensively studied [18]. Because, it exhibits rich mechanical, electrical and magnetic properties. Furthermore, the combination of Fe3O4 nanoparticles with reduced graphene oxide (RGO) would enhance photocatalytic properties [19]. To date, great efforts have been made to utilize the unique properties of reduced graphene oxide to improve the efficiency of photocatalysts [20–24], which results to increase the efficiency of solar cells [25,26]. Additionally, the higher value of photocatalytic activity is also observed in these nanocomposites ascribed to the photo-generated charge carriers immediately transfers to graphene sheets from metal oxide nanoparticles that leads increase in the separation of electron/hole pairs and hence improve degradation mechanism. In another work, Teok Peik-See et al. [27] have utilized this concept in Fe3O4/rGO nanocomposite for the deterioration of methylene blue organic dye and they achieved 100% degradation efficiency in 60 min. Hence, the nanocomposite Fe3O4/rGO has great functionalized properties that can dissociate environmental pollution with the higher efficiency as compared to Fe3O4 NPs. There are several methods reported for the synthesis of Fe3O4 and RGO/Fe3O4 magnetic nanoparticles such as hydrothermal [28,29], microemulsion [30], co-precipitation [31,32], microwave irradiation [33], and oxidation of Fe(OH)2 by H2O2 [34]. Several other methods have also been used to prepare iron oxide thin films, such as molecular beam epitaxy (MBE) [35], sputtering [36], pulsed laser deposition (PLD) [37], and sol-gel method [38,39]. Among these, sol-gel is one of the most commonly used methods for preparing iron oxide powders/ thin films for many reasons such as low temperature synthesis, cost effective, easy performance, homogeneity and thin film formability. To the best of our knowledge, limited consideration has been devoted to the use of simple and economical wet chemical method especially solgel. In the present study, a simple sol-gel process has been used to prepare Fe3O4 nanoparticles and modified Hummers method for graphene oxide in order to investigate optical, photocatalytic, electrical and magnetic properties of their nanocomposites employable in modern technology.
2.4. Synthesis of Fe3O4 nanoparticles In a typical experiment, FeCl2·4H2O, ascorbic acid, citric acid monohydrate and ethanol chemicals with proper stoichiometric ratio were dissolved in distilled water and stirred at 50 °C for 5 h. The solution was then dried at 50 °C under a constant stirring to add dimethylformamide (DMF). The xerogel was aged for a week and then dried at 120 °C for 6 h to remove the organic substances. After drying, the gel was annealed at 400 °C for 3 h and then ground into a fine powder. 2.5. Synthesis of Fe3O4/RGO nanocomposites In brief, 0.1 M FeCl2·4H2O was slowly mixed into the graphene oxide dispersion (2 mg/ml) and stirred continuously for 60 min. Thereafter, above solution was placed in a Teflon-lined stainless steel autoclave (100 ml). Then, as-prepared hydrogels were taken out with a tweezer and freeze dried under vacuum. In addition to Fe3O4, two composites of different weight ratio of Fe3O4/RGO (1:0.02 and 1:0.04) were prepared. The corresponding as-synthesized products are named as FG0, FG2 and FG4 respectively. Finally, the synthesized Fe3O4 and Fe3O4/RGO powders were stored in a vacuum desiccator for further characterization. For the phase identification and crystal structure, x-ray diffractometer (XRD, PANalytical XPert) were employed with CuKα radiation (λ = 1.54 Å) operated at 45 kV and 40 mA current. XRD data were recorded for 2θ values in the range of 20–80° at 2°/min scanning rate. Particle size, morphology and crystallinity of the synthesized nanoparticles and nanocomposites were examined by a JEOL, transmission electron microscope (TEM) operated at accelerated voltage of 200 kV. For TEM measurements, the samples were dissolved in ethanol by ultrasonication and dropped directly onto the carbon coated copper grid. The chemical composition of the prepared samples was checked by the energy dispersive x-ray (EDX) spectrometer attached with the TEM. Fourier-transform infrared (FTIR) spectra of the samples were obtained with a PerkinElmer spectrometer in the wave numbers 500–4000 cm−1 using ATR technique. Thermogravimetric and differential thermal analysis (TGA/DTA) measurements of the as-prepared samples were recorded using thermal analyzer (Shimadzu, DTG-60) from room temperature to 800 °C. For this, ∼20 mg of powdered sample was heated in an aluminum pan at the rate of 10 °C/min under a continuous nitrogen flow rate of 30 ml/min. The room temperature photoluminescence (PL) measurements of the samples were carried out on a Hitachi F-7000 spectrometer with the HeeCd laser at an excitation wavelength of 290 nm. The optical absorption spectra were recorded with UV–visible spectrophotometer (Shimadzu) in the wavelength range of 300–900 nm. Magnetic properties measurements at room temperature were performed with a vibrating sample magnetometer (VSM) attached with a quantum design physical property measurement system (PPMS) in an external applied magnetic field ranging from −60 to + 60 kOe. The temperature dependent resistivity measurement was carried out in the temperature range of 10–300 K by a standard Janis Cryogenic Micromanipulated Probe Station. The photocatalytic performance of the synthesized samples (FG0, FG2 and FG4) was examined by observing the decolorization of
2. Experimental details 2.1. Materials For the present work, analytical grade (AR) FeCl2·4H2O (> 99.99%) CAS Number 13478-10-9, graphite (> 99.99%) CAS Number 7782-425, ascorbic acid, citric acid monohydrate (99.0%) CAS Number 594929-1, ethanol, CAS Number 64-17-5 and dimethylformamide (99.8%), CAS Number 68-12-2 were procured and used without any further purification. 2.2. Synthesis of graphene oxide Graphene oxide (GO) was prepared through modified Hummers method using highly pure graphite powder [40,41]. In brief, graphite powder (∼3 g) was dispersed in the blend of conc. sulfuric and phosphoric acids at room temperature. Subsequently, potassium permanganate (3.5 g) was mixed with continuous stirring at 35 °C for 2 h followed by the addition of 100 mL deionized water. Thereafter, hydrogen peroxide solution (8 ml, 30 wt% aqueous solution) was mixed until it becomes bright yellow. At last, the obtained product was centrifuged and washed with HCl/deionized water, respectively to get neutral solution. Finally, the powder was achieved after centrifugation (4000 rpm, 10 min) several times. The product of GO powder was dried at 100 °C in vacuum. 2.3. Preparation of RGO In order to prepare the reduced graphene, GO powder (200 mg) was dissolved in 1000 ml deionized water with the help of an ultrasonicator 45
Materials Science in Semiconductor Processing 99 (2019) 44–53
M.A.M. Khan, et al.
methylene blue (MB) dye in visible light radiation produced through a sodium lamp of 400 W (Philips). In a typical experiment, the Fe3O4 and Fe3O4/RGO nanocomposites were added into the aqueous solution of MB. While initial MB concentration, and the amount of Fe3O4 and Fe3O4/RGO were set as 0.5 mg/ml and in 100 ml distilled water separately, where the concentration of MB was kept 1.5 mg/l. The pH value of dye solutions was adjusted to 7. The mixed suspension is immersed in a vessel of cylindrical shape (600 mL) and then the solution was magnetically stirred at room temperature for 30 min in a dark environment to accomplish adsorption/desorption equilibrium of the photocatalyst and the dye. The decolorization performance was monitored by measuring the intensity of absorption peak of the dye appeared at 664 nm wavelength. Further, photo decolorization efficiency (η) of the products was assessed using a relation, η = [1-(C/Co)] × 100%, here Co and C stand for the initial concentration and concentration after light irradiation respectively. 3. Results and discussion 3.1. Microstructure and morphological studies The crystal structure, purity and phase identification of Fe3O4, RGO, and Fe3O4/RGO (2% & 4%) samples were examined through the XRD diffraction patterns presented in Fig. 1(a and b). The XRD profile of RGO shows a most intense reflection plane (002) at 2θ∼26.6° with corresponding interplanar spacing (d) of ∼0.36 nm, which is slightly higher than the bulk form of graphite (∼0.34 nm) [42,43]. An increase in the interlayer spacing could be attributed to the occurrence of residual oxygen containing functional groups intercalated onto the graphite sheets. The prominent diffraction peaks in the XRD patterns of the nanocomposites (FG2 and FG4) are observed at 2θ values of 30.16°, 36.8°, 43.2°, 54.1°, 58.9° and 63.7° that correspond to the (220), (311), (400), (422), (511) and (440) reflection planes respectively. All diffraction peaks in FG0, FG2 and FG4 samples are well matched to the face centered cubic (fcc) crystal structure of the magnetite (JCPDS no. 65–3107). Further, the average crystallite size is calculated with the help of Scherrer's equation using the most intense diffraction peak (311). The estimated values of the average crystallite size for FG0, FG2 and FG4 samples are found to be 10, 15 and 18.5 nm respectively. No obvious typical diffraction peaks attributed to the RGO are observed in the XRD spectra of Fe3O4/RGO composites that signify that the stacking of the RGO sheets in the Fe3O4/RGO nanocomposites remained disordered. The Rietveld refinement analysis of the Fe3O4 XRD data was performed to investigate other structural parameters and phase purity of the NPs as shown in Fig. 1(a). We have obtained the best fit between the observed and calculated patterns with minimum deviation using a cubic crystal structure in the space group Fd-3m that ascribed to the existence of preferential orientation of the crystallites in the Fe3O4 nanoparticles. Various structural parameters like lattice constant, atomic position, occupancy etc. are obtained. Refinement results exhibit that the Fe3+ ions lie at 8a (0.125, 0.125, 0.125), Fe2+ at 16d (0.5, 0.5, 0.5) and O at 32e (0.25565, 0.25565, 0.25565) positions. The values of lattice parameters (a = b = c), unit cell volume (a3), and χ2 are found to be 8.36402 Å, 585.121 (Å)3 and 1.43 respectively. The lower value of χ2 further confirmed goodness of the fit. These results are well consistent with the earlier published work in literature [44]. No other crystalline/amorphous impurity phases are observed. Moreover, the smaller size of the particles also established by the FWHM (broadening) of the diffraction peaks. It is also noticed that the (311) peak shifts towards the higher angle in 4% RGO sample and shown in the inset of Fig. 1(b). This may be due to shrinkage in the lattice parameters, i.e. 8.363 Å for the Fe3O4/RGO (2%) reduces to 8.356 Å for Fe3O4/RGO (4%). In addition, as concentration of RGO increases, intensity of (222) plane diminish ascribed to the short-range order in the stacked graphene like sheets [45]. The morphology, crystallinity and distribution of particle size of the
Fig. 1. Powder x-ray diffraction patterns of (a) Fe3O4 nanoparticles, (b) Fe3O4/ RGO nanocomposites with different mass ratio of RGO and inset shows magnified view of (311) peak of Fe3O4.
as-prepared Fe3O4 and Fe3O4/RGO nanocomposite were supplementary examined through TEM and high resolution TEM (HRTEM). Fig. 2(a and b) presents a narrow size distribution of nearly spherical shaped Fe3O4 nanoparticles in FG0 and FG2 samples with an average particle size 12 and 17 nm respectively which is almost consistent with the XRD results. TEM image of Fe3O4 shows uniform size and quite agglomerated nature of the NPs that may be due to the stronger magnetic coupling between them. High-resolution TEM micrographs presented in Fig. 2(c and d) indicate highly crystalline phase of the NPs with the lattice fringes of 0.231 and 0.64 nm which corresponds to the d spacing of the highest intense (311) and (002) peaks of crystallographic orientation of cubic magnetite Fe3O4 and Fe3O4/RGO(2%) nanocomposite. The chemical composition of the synthesized nanoparticles and nanocomposites was investigated by the EDX spectrometer attached with TEM and displayed in Fig. 3. EDX spectrum indicates existence of iron (Fe) and oxygen (O) elements in the atomic proportion of 3:4 approximately. The signals of carbon (C) and copper (Cu) are appearing because of the copper grid used for TEM measurement is coated with 46
Materials Science in Semiconductor Processing 99 (2019) 44–53
M.A.M. Khan, et al.
Fig. 2. (a,b) TEM images of Fe3O4 and Fe3O4/RGO (2%) samples, (c,d) HRTEM images of the same.
assigned to the FeeO vibrations of Fe3O4 nanoparticles [54], which is consistent with the pure Fe3O4 nanoparticles [55,56].
carbon. 3.2. X-ray photoelectron spectroscopy
3.4. Thermogravimetry analysis The surface chemical information of Fe3O4/RGO nanocomposites was further evaluated through x-ray photoelectron spectroscopy (XPS). The wide scan spectrum of Fe3O4/RGO nanocomposite in Fig. 4(a) indicates three intense peaks at binding energies 285, 529 and 711 eV that correspond to the carbon (C1s), oxygen (O1s), and Fe2p, respectively. However, the high resolution XPS spectrum of C1s shown in Fig. 4(b) contains non-oxygenated aromatic sp2 bonded carbon ring (CeC) at 284.6 eV, whereas those at 285.7 and 288.3 eV binding energies are due to the oxygenated functional group attached to the carbon atoms, such as CeO and C]O groups, respectively [46]. Fig. 4(c) shows O1s spectrum with two highly intense peaks at 529.98 and 531.21 eV associated to the anionic oxygen in Fe3O4 and existence of the residual oxygen functional groups in the graphene sheets. Furthermore, the high-resolution spectrum of Fe2p presented in Fig. 4(d) exhibits two prominent peaks at 710.80 and 724.5 eV, assigned to the Fe2p3/2 and Fe2p1/2 spin orbit peaks of Fe3O4 nanoparticles respectively [47,48]. These results confirmed the formation of mixed oxide of both Fe2+ and Fe3+ ions. However, the satellite peak situated at about ∼719 eV is a characteristic peak of Fe3+ in γ-Fe2O3 phase and suggests that the Fe3O4 nanoparticles are partially oxides [49,50]. The atomic percentages of the C1s, O1s and Fe2p elements were estimated with the help of sensitivity factors in the FG2 sample and found to be 33.8%, 40.3% and 25.9% respectively. These values are quite consistent with the earlier reported results and clearly demonstrate homogeneous dispersion of the Fe3O4 nanoparticles.
In order to establish the thermal stability of the synthesized nanoparticles and nanocomposites, TGA measurement is carried out in the nitrogen environment at a heating rate of 10 °C/min and presented in Fig. 6. These thermograms exhibit three-steps in the weight loss for the temperature range of 20–800 °C. All samples RGO, FG0, FG2 and FG4 showed substantial weight loss in the range of 45–150 °C that may be due to the evaporation of absorbed water and adsorbed organics in the samples. A rapid weight loss observed between 220 and 330 °C is associated with the decomposition of graphene sheet in air. At the end, the weight loss in the temperature range between 340 and 650 °C may be a response to the decomposition of the organic groups supported on the magnetite nanoparticles. Moreover, the mass retained at 700 °C directly translates into the amount of Fe3O4 in the composite. 3.5. Optical studies Optical properties of Fe3O4 nanoparticles and Fe3O4/RGO (2 & 4%) nanocomposites were studied through UV–visible absorption spectroscopy (fig. not shown here). It is observed that the absorption edge shifts towards the higher wavelength for the RGO doped nanocomposite than Fe3O4 nanoparticles, indicating a red shift in the absorption. The absorption coefficient (α) is estimated using the known relation, α = 2.303A/x, here A is the optical absorbance and x is the thickness of the cuvette [57]. Further, using absorption coefficient and frequency (ν) of the incident radiation; the optical band gap (Eg) is measured by employing the Tauc's relation [58], αhν = B(hν-Eg)n, here h stands for the Planck's constant and B is a constant. The exponent n is associated with the nature of the electronic transition, i.e. n = 2 and n = 3 for an indirect allowed transition and indirect forbidden transition respectively, whereas n = 1/2 and n = 3/2 for the direct allowed transition and forbidden transition respectively. For the present case, all the experimental data were fitted to the direct allowed transition by taking n = 1/2 [59]. Fig. 7 presents (αhν)2 as a function of hν (Tauc's plots) for FG0, FG2 and FG4 samples. Hence, the value of band gap is estimated by the extrapolation of the linear region of the graph to the x-axis
3.3. FTIR studies FTIR spectroscopy study was performed to investigate the microstructural properties through the presence of various chemical bonds in the samples. Fig. 5 exhibits the FTIR spectra of FG0, FG2 and FG4 nanocomposites. The broad absorption band in the spectrum at 3434 cm−1 appeared in all the samples are ascribed to the stretching vibration of OeH [51,52]. The bands at 1047, 1644, and 2357 cm−1 can be attributed to the C]C]O and C]C stretching vibration [53]. Additionally, the small characteristic absorption band at 580 cm−1 is 47
Materials Science in Semiconductor Processing 99 (2019) 44–53
M.A.M. Khan, et al.
Fig. 3. EDX spectrum of Fe3O4 and Fe3O4/RGO (2%) samples.
48
Materials Science in Semiconductor Processing 99 (2019) 44–53
M.A.M. Khan, et al.
Fig. 4. XPS spectrum of Fe3O4/RGO (2%) nanocomposite (a) survey scan, (b) high-resolution C 1s, (c) high-resolution O 1s, and (d) high-resolution Fe 2p spectrum.
Fig. 5. FTIR spectra of Fe3O4 and Fe3O4/RGO nanocomposites.
Fig. 6. Thermogravimetric analysis curves of RGO, Fe3O4, and RGO/Fe3O4 nanocomposites.
intercept. In this way, bandgap of all the samples has been estimated and given in Table 1 which is well matched with the results reported in literature [60]. It is known that the decrease in particle size leads an increase in the band gap of nanomaterials, which tends to blue shift in the optical absorption [61]. Moreover, it is noted that the value of energy gap in the present system is reducing with increase in RGO concentration associated with the red shift in the absorption that signify increase in the carrier concentration. We have also interpreted the variation in energy gap using Moss–Burstein shift that suggests enhancement in the free carrier density is responsible for the corresponding downward shift of the Fermi level below the band edge [62].
For the higher RGO content (4%) sample, density of induced defects increases that leads to the decrease in the band gap. This characteristic is helpful for enhancing the light absorption and photocatalytic efficiency of the Fe3O4/RGO nanocomposites. The energy of the conduction band edge (ECB) and the valence band edge (EVB) are also determined using UV–visible absorption data of the samples. Based on the optical band gap, the valence band and the conduction band edge potentials at the point of zero charge can be written by the following expressions [63],
49
Materials Science in Semiconductor Processing 99 (2019) 44–53
M.A.M. Khan, et al.
Fig. 8. PL spectra of Fe3O4 and RGO/Fe3O4 nanocomposites at excitation wavelength of 290 nm.
Fig. 7. Tauc's plots of Fe3O4 and Fe3O4/RGO nanocomposites.
ECB = χ − EC −
1 Eg 2
3.7. Magnetic properties
(1)
EVB = Eg + ECB
The magnetic properties of Fe3O4 NPs, and Fe3O4/RGO nanocomposites were measured through a vibrating sample magnetometer (VSM) in a magnetic field of −60 to +60 kOe at room temperature and shown in Fig. 9. The M − H loops do not show definite value of the coercivity that signify superparamagnetic nature of all the samples. The lower value of the saturation magnetization (MS) is observed for Fe3O4 NPs (∼ 64 emu/g) than that of the bulk Fe3O4 (MS = 90 emu/g at 300 K) [66]. The reduction in this value at a smaller size is ascribed to the noticeable surface effects of these NPs. Further, the surface of the NPs is considering to be composed of some disorder spins that stop the core spins from aligning along the field direction resulted decrease in the saturation magnetization of the small sized NPs [67]. Moreover, the value of saturation magnetization is found to reduce from 42 to 21 emu/g for higher RGO doping but superparamagnetic behavior remains unaltered which can be attributed to the attachment of Fe3O4 NPs with RGO. Using magnetic measurements data, the particle size (Dm) can also be calculated by the following equation [68],
(2)
where χ represent the absolute electro-negativity of the materials (χ is 5.78 eV for Fe3O4) [64]. EC is the scale factor relating the reference electrode redox level to absolute vacuum scale (AVS) (∼ 4.44 ± 0.02 eV for AVS and 0 V for NHE) [65] and Eg ∼ 2.35, 2.29 and 2.18 eV for the Fe3O4 and Fe3O4/RGO (2 & 4%) nanocomposites respectively (calculated from UV–visible Tauc plots). The calculated values of the conduction band and the valence band edge positions of Fe3O4 and Fe3O4/RGO nanocomposites are at −0.170 eV, 0.106 eV, 0.079 eV and 2.51, 2.39, 2.07 eV respectively.
3.6. Photoluminescence (PL) studies PL spectroscopy is a powerful tool to study the electronic and optical properties of the nanoparticles and nanocomposites, including mobility of the charge carriers to the surface and recombination of electron-hole pairs in photo-induced semiconductors. Fig. 8 presents the room temperature photoluminescence spectra of the pristine Fe3O4 and Fe3O4/RGO nanocomposites. It is evident that the intensity of pure Fe3O4 nanoparticles is higher than that of the Fe3O4/RGO nanocomposites which signify high recombination rate of photo-generated e−/h+ pairs and the intensity of PL spectra is directly proportional to the recombination rate of charge carriers as well as charge separation between conduction and valence bands in the Fe3O4/RGO nanocomposites [27]. This may be due to the presence of structural defects created by the oxygen vacancies. Our findings suggest that the graphene lowers the recombination rate of the charge carriers and result in higher photoactivity performance of Fe3O4/RGO nanocomposites.
D3m =
18kBT ⎛ dM ⎞ πμ oMb . Ms ⎝ dH ⎠H→ 0
(3)
here, kB represents the Boltzmann's constant (kB = 1.38 × 10−23 J/K), T stands for the absolute temperature (T = 300 K), μo is the magnetic permeability of vacuum, Mb is the magnetization of bulk magnetite (for spherical Fe3O4 nanoparticles, Mb = 0.48 × 106 A/m). Based on this approach, the calculated particle size of the NPs and Fe3O4/RGO nanocomposites is given in Table 1, which is smaller than the particle size observed through XRD and TEM measurements [69].
Table 1 Estimated physical parameters for Fe3O4, Fe3O4/RGO (2%) and Fe3O4/RGO(4%) samples. Sample
Fe3O4 Fe3O4/RGO (2%) Fe3O4/RGO (4%)
Particle size (nm) Scherrer's eq.
TEM
10.2 ± 3.4 15.7 ± 2.4 18.5 ± 4.5
12 ± 3.2 17 ± 1.4 20 ± 2.2
Eg (eV)
N(EF) (eV−1cm−3)
R (nm)
W (meV)
Dm (nm)
2.35 ± 0.16 2.28 ± 0.11 2.18 ± 0.09
3.04 × 1022 8.51 × 1021 5.93 × 1020
27 30 51
388 301 104
3.4 ± 1.5 4.3 ± 1.3 6.6 ± 1.1
50
Materials Science in Semiconductor Processing 99 (2019) 44–53
M.A.M. Khan, et al.
Fig. 9. Magnetic hysteresis (M–H) loops for Fe3O4 and RGO/Fe3O4 nanocomposites at room temperature.
3.8. Transport properties The electrical transport behavior of the FG0, FG2 and FG4 samples as a function of temperature were carried out in the temperature range of 10–300 K and shown in Fig. 10(a). The resistivity is found to decrease with the rise in temperature, demonstrates semiconducting behavior of all the samples. It is also evident from the same figure that the resistivity decreases with the increase in RGO concentration throughout the studied temperature range. The observed value of room temperature resistivity of Fe3O4 NPs is about 25.5 mΩ-cm, which is lower than its corresponding bulk form [70]. The temperature dependent resistivity can be explained on the basis of activated hopping mechanism, including two regions with different slopes for the (1/T) dependence of resistivity (ρ). First one is related to the conduction band at the higher temperatures. However, other one at the lower temperature that measured by the hopping conduction or transition between the impurity centers or localized states inside the impurity band. For all synthesized samples, the temperature dependent electrical resistivity is well described in the low temperature regime (from 40 to 150 K) with the help of variable range hopping (VRH) mechanism given by the expression, γ T ρ = ρo exp ⎛ Mott ⎞ ⎝ T ⎠
Fig. 10. (a) Resistivity as a function of temperature, and (b) VRH fitted resistivity plots for Fe3O4 and Fe3O4/RGO nanocomposites.
The typical hopping distance Rhop(T) between two sites and the average hopping energy Whop at a temperature (T) can be calculated by the following expressions [73].
(4)
9LLoc ⎤ Rhop = ⎡ ⎢ 8πkTN(E F) ⎥ ⎦ ⎣
where ρo is a constant that depends on the phonon density and TMott is the Mott characteristic temperature related to the hopping barrier, electronic structure and energy distribution of the localized states. In the above equation, the exponent γ decides the dimensionality 1 (D) of the system using the relation, γ= 1 +D . The values of γ are 1/2, 1/ 3 and 1/4 for 1D, 2D and 3D case respectively. For the existing system, the expression (3D) for the dc resistivity and Mott's characteristic temperature can be expressed as [71]. 1/4 T ρ = ρo exp ⎛ Mott ⎞ ⎝ T ⎠
TMott =
Co kBN(EF)L3Loc
Whop =
3 4πR3hopN(EF)
(7)
(8)
Using above equations, we have extracted the values of N(EF), Rhop and Whop for Fe3O4 and Fe3O4/RGO nanocomposites from the fitted curves and tabulated in Table 1. For the present systems, Co = 18 and LLoc = 10 Å have been taken. It is found that the value of density of states near the Fermi level increases with the doping concentration of RGO, whereas average hopping distance reduces, accompanied by a decrease in hopping energy. The charge transport mechanism increases the conductivity due to generation of polarons associated with the RGO doping.
(5)
(6)
where N(EF) represents the density of states at Fermi level, LLoc is localization length that varies in the range of 3–30 Å and Co is a constant of typical value 16–310 [72]. The linear fit of the temperature dependent resistivity data are found to obey the Mott's VRH model (Fig. 10(b)).
3.9. Photocatalytic performance The photocatalytic response of the as-prepared samples was evaluated for the deterioration of methylene blue (MB) dye and the 51
Materials Science in Semiconductor Processing 99 (2019) 44–53
M.A.M. Khan, et al.
Fig. 11. Time dependent UV–vis absorbance spectra of photocatalytic decolorization of MB dye under visible light irradiation for (a) Fe3O4, (b) Fe3O4/RGO (2%) and (c) Fe3O4/RGO(4%) nanocomposites, (d) Photocatalytic decolorization of MB as a function of irradiation time under visible light irradiation.
3.9.1. Reusability assay The reusability of the catalytic materials plays a major role in the photodegradation experiments. In this respect, the reusability of the Fe3O4/RGO nanocomposite was carried out seven times, the degradation efficiency reached up to 94.6% which indicates that the synthesized nanocomposites can maintain high stability for reuse. Moreover, it established excellent reusability during the repeated photocatalytic experimental cycles (Fig. 12).
outcomes are presented in Fig. 11(a,b,c). The absorption peak of the dye at 664 nm gradually reduces over the time when irradiated under visible light and almost disappeared in 90 min. This signifies complete decolorization of the MB dye. Moreover, a blank test in the absence of the catalysts (synthesized samples) has been performed that shows that the photo-conversion reaction could not proceed without catalyst. It is clear from Fig. 11(d) that the decolorization efficiency of MB for the Fe3O4 NPs is calculated as 76.7% after 90 min of irradiation time, while decolorization rate enhances by the doping of RGO and found to be 86.8% and 93.3% for FG2 and FG4 nanocomposites respectively. Generally, when graphene doped Fe3O4 sample is irradiated by the light of photons energy equal to or more than the bandgap, resulted excitation of an electrons (e−) from the valence band (VB) to the conduction band (CB) with holes (h+) left in the valence band (VB) simultaneously. The photoelectron generated from Fe3O4 and MB dye transfer to the graphene and in this way Fe3+ can certainly capture the electrons to transform in Fe2+. Thereafter, the Fe2+ can stay react with H2O2 to form the Fe3+ and OH∗ on the reduced graphene oxide surface to degrade the dye molecules. The re-generated Fe3+ ions rapidly reduced to Fe2+ by the electron concentrated on the surface of graphene to keep the cycle of Fe3+/Fe2+. Because, Fe− based materials are used as the catalysts and graphene used as an electron capture agent materials make it an ideal support to enhance the transfer and separation of photogenerated electrons and holes, which are responsible for the enhanced activity of the photocatalysts. It is clearly demonstrated that the photocatalytic activity of Fe3O4 NPs can be effectively enhanced by hybridizing with RGO.
Fig. 12. Reusability assay of Fe3O4/RGO nanocomposite for photodegradation of MB dye under visible light irradiation. 52
Materials Science in Semiconductor Processing 99 (2019) 44–53
M.A.M. Khan, et al.
4. Conclusions
[23] Amr A. Nada, M. Nasr, R. Viter, P. Miele, S. Roualdes, M. Bechelany, J. Phys. Chem. C 121 (2017) 24669–24677. [24] M. Nasr, C. Eid, R. Habchi, P. Miele, M. Bechelany, ChemSusChem 11 (2018) 3023–3047. [25] N.L. Yang, J. Zhai, D. Wang, Y.S. Chen, L. Jiang, ACS Nano 4 (2010) 887–894. [26] S.R. Kim, M.K. Parvez, M. Chhowalla, Chem. Phys. Lett. 483 (2009) 124–127. [27] T. Peik-See, A. Pandikumar, L.H. Ngee, H.N. Ming, C.C. Hua, Catal. Sci. Technol. 4 (2014) 4396–4405. [28] S. Saha, M. Jana, P. Samanta, N.C. Murmu, N.H. Kim, T. Kuila, J. Hee Lee, RSC Adv. 4 (2014) 44777–44785. [29] F.Z. Mohammad, Ijaz Ahmad, J.J. Siddiqui, K.M. Ali, M. Mudsar, H. Arshad, J. Mater. Sci. Mater. Electron. 29 (2018) 19775–19782. [30] Xiaojuan Liang, Xiangchen Jia, Lei Cao, Juncai Sun, Yuxiang Yang, J. Dispersion Sci. Technol. 31 (2010) 1043–1049. [31] D.E. Zhang, Z.W. Tong, S.Z. Li, X.B. Zhang, A.L. Ying, Mater. Lett. 62 (2008) 4053–4055. [32] Zong Meng, Ying Huang, Yang Zhao, Lei Wang, Panbo Liu, Yan Wang, Qiufen Wang, Mater. Lett. 106 (2013) 22–25. [33] Rajesh Kumar, Rajesh K. Singh, Alfredo R. Vaz, Raluca Savu, Stanislav A. Moshkalev, ACS Appl. Mater. Interfaces 9 (2017) 8880–8890. [34] L.Q. Yu, L.J. Zheng, J.X. Yang, Mater. Chem. Phys. 66 (2000) 6–9. [35] J.-B. Moussy, S. Gota, A. Bataille, M.-J. Guittet, M. Gautier-Soyer, F. Delille, B. Dieny, F. Ott, T.D. Doan, P. Warin, P. Bayle-Guillemaud, C. Gatel, E. Snoeck, Phys. Rev. B 70 (2004) 174448. [36] Y. Peng, C. Park, D.E. Laughlin, J. Appl. Phys. 95 (2003) 7957. [37] A. Goikhman, P. Shvetsa, U. Koneva, R. Mantovan, K. Maksimova, Thin Solid Films 652 (2018) 28–33. [38] Eken Ali Erdem, Ozenbas Macit, J. Sol-Gel Sci. Technol. 50 (2009) 321–327. [39] Ping Hu, Tian Chang, Wen-Jing Chen, Jie Deng, Shi-Lei Li, Ye-Gai Zuo, Kang Lu, Yang Fan, Megan Hostetter, Alex A. Volinsky, Journal of Alloys and Compounds 773 (2019) 605–611. [40] W.S. Hummers Jr., R.E. Offeman, J. Am. Chem. Society 80 (1958) 1339–1339. [41] A. Romero, M.P. Lavin-Lopez, L. Sanchez-Silva, J.L. Valverde, A. Paton-Carrero, Mater. Chem. Phys. 203 (2018) 284–292. [42] K.H. Liao, A. Mittal, S. Bose, C. Leighton, K.A. Mkhoyan, C.W. Macosko, ACS Nano 5 (2011) 1253–1258. [43] Zhaoling Ma, Xiaobing Huang, Shuo Dou, Jianghong Wu, Shuangyin Wang, J. Phys. Chem. C 118 (2014) 17231–17239. [44] M.V. Reddy, B.V.R. Chowdari, Jagadese J. Vittal, RSC Adv. 4 (2014) 64142–64150. [45] J. Yan, T. Wei, B. Shao, F. Ma, Z. Fan, M. Zhang, C. Zheng, Y. Shang, W. Qian, F. Wei, Carbon 48 (2010) 1731–1737. [46] J. Zhang, Z. Xiong, X. Zhao, J. Mater. Chem. 21 (2011) 3634–3640. [47] L. Ji, L. Zhou, X. Bai, Y. Shao, G. Zhao, Y. Qu, C. Wang, Y.J. Li, Mater. Chem. 22 (2012) 15853–15862. [48] W. Zhang, X. Li, R. Zou, H. Wu, H. Shi, S. Yu, Y. Liu, Sci. Rep. 5 (2015) 1112. [49] Z. Li, J.F. Godsell, J.P. O'Byrne, N. Petkov, M.A. Morris, S. Roy, J.D. Holmes, J. Am. Chem. Soc. 132 (2010) 12540–12541. [50] H. Wu, G. Gao, X. Zhou, Y. Zhang, S. Guo, Cryst. Eng. Comm. 14 (2012) 499–504. [51] Y. Xu, H. Bai, G. Lu, C. Li, G. Shi, J. Am. Chem. Soc. 130 (2008) 5856–5857. [52] Z. Li, J. Wang, L. Niu, J. Sun, P. Gong, W. Honga, L. Ma, S. Yang, J. Power Sources 245 (2014) 224–231. [53] Z. Song, Y. Zhang, W. Liu, S. Zhang, G. Liu, H. Chen, J. Qiu, Electrochim. Acta 112 (2013) 120–126. [54] Lulu Ren, Shu Huang, Wei Fan, Tianxi Liu, Appl. Sur. Sci. 258 (2011) 1132–1138. [55] S. Bhuvaneswari, P.M. Pratheeksha, S. Anandan, et al., Phys. Chem. Chem. Phys. 16 (2014) 5284–5294. [56] K.Y.A. Lin, H. Yang, C. Petit, W.-D. Lee, J. Colloid Interface Sci. 438 (2015) 296–305. [57] Rayan Khalid, A.N. Alhazaa, M.A.M. Khan, Appl. Phys. A 124 (2018) 536. [58] M.A.M. Khan, Sushil Kumar, Tansir Ahamad, Abdulaziz N. Alhazaa, J. Alloy. Comp. 743 (2018) 485–493. [59] J. Tauc, Optical Properties of Solids, (1970) North-Holland, Amsterdam. [60] H.E. Ghandoor, H.M. Zidan, M.M.H. Khalil, M.I.M. Ismail, Int. J. Electrochem. Sci. 7 (2012) 5734–5745. [61] M.A.M. Khan, Wasi Khan, Maqusood Ahamed, Mansour Alhoshan, Mater. Lett. 79 (2012) 119–121. [62] Manas Saha, Sirshendu Ghosh, Vishal Dev Ashok, S.K. De, Phys. Chem. Chem. Phys. 17 (2015) 16067. [63] M.A.M. Khan, Wasi Khan, Maqusood Ahamed, A.N. Alhazaa, Sci. Rep. 7 (2017) 12560. [64] R.G. Pearson, Absolute electronegativity and hardness: application to inorganic chemistry, Inorg. Chem. 27 (1988) 734–740. [65] S.R. Morrison, Electrochemistry at Semiconductor and Oxidized Metal Electrodes, Plenum Press, New York, NY, USA, 1980. [66] Z. Huang, F. Tang, J. Colloid Interface Sci. 281 (2005) 432–436. [67] K. Maaz, S. Karim, A. Mumtaz, S.K. Hasanain, J. Liu, J.L. Duan, J. Magn. Magn. Mater. 321 (2009) 1838–1842. [68] B.Y. Yu, Seung-Yeop Kwak, J. Mater. Chem. 20 (2010) 8320–8328. [69] B.D. Cullity, C.D. Graham, Introduction to Magnetic Materials, second ed., John Wiley & Sons, 2009, pp. 359–408. [70] Y.H. Cheng, L.Y. Li, W.H. Wang, X.G. Luo, Hui Liu, R.K. Zheng, J. Appl. Phys. 109 (2011) 073905. [71] M.A.M. Khan, W. Khan, M. Ahamed, M.S. Alsalhi, T. Ahmed, Elec. Mat. Lett. 9 (2013) 53–57. [72] M.A.M. Khan, S. Kumar, M. Ahamed, Mater. Sci. Semicond. Process. 30 (2015) 169–173. [73] M.A.M. Khan, Sushil Kumar, M.Wasi Khan, M. Husain, M. Zulfequar, Mater. Res. Bull. 45 (2010) 727–732.
In summary, we have synthesized Fe3O4 nanoparticles and Fe3O4/ RGO (2% and 4%) nanocomposites using sol-gel and modified Hammers methods respectively. A comprehensive study of structural, optical, photocatalytic, electrical transport and magnetic properties of the samples has been explored. The Rietveld refinement of the XRD data reveals the inverse spinel cubic structure of the prepared nanostructures with space group Fd-3m. The average crystallite size of the magnetite nanoparticles is found to be in the range of 10-19 nm. Moreover, TEM micrographs ensure almost spherical shape with an average particle size of 12 and 17 nm for the Fe3O4 and Fe3O4/RGO(2%) samples respectively. The HRTEM micrographs exhibit a high crystal quality and ultrasmooth spherical surface of the Fe3O4 nanoparticles. The photocatalytic activity has been significantly enhanced by hybridizing Fe3O4 with RGO. The magnetic measurements revealed superparamagnetic behavior of the prepared samples. The observed value of saturation magnetization found to be lower for the nanocomposites than the magnetite nanoparticles and its bulk form. This may be due to the surface effects and attachment of RGO. The temperature dependent resistivity displayed semiconducting nature of all the samples and described on the basis of VRH conduction mechanism. Hence, the prepared nanocomposites are suitable for the real potential applications in various areas such as biomaterials and environmental remediation. Acknowledgement The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding this work through research group project no. 1437-023. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.mssp.2019.04.005. References [1] J.Y. Song, B.S. Kim, Bioproc. Biosyst. Eng. 32 (2009) 79–84. [2] W.-H. Lia, X.-P. Yuea, C.-S. Guob, J.-P. Lvb, S.-S. Liub, Y. Zhangb, J. Xub, Appl. Surf. Sci. 335 (2015) 23–28. [3] I. Bilecka, M. Niederberger, Nanoscale 2 (2010) 1358–1374. [4] R.M. Cornell, U. Schwertmann, Reactions, Occurences and Uses, second ed., (2003). [5] S. Sun, Adv. Mater. 18 (2006) 393–403. [6] S. Sabale, P. Kandesar, V. Jadhav, R. Komorek, R.M. Kishan, X.Y. Yu, Biomater. Sci. 5 (2017) 2212–2225. [7] S. Rashdan, M. Bououdina, A. Al-Saie, J. Exp. Nanosci. 8 (2013) 210–221. [8] C. Berger, Z. Song, T. Li, X. Li, A.Y. Ogbazghi, R. Feng, Z. Dai, A.N. Marchenkov, E.H. Conrad, P.N. First, W.A. de Heer, J. Phys. Chem. B 108 (52) (2004) 19912–19916. [9] S. Bae, H. Kim, Y. Lee, X. Xu, J.S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. Ri Kim, Y.I. Song, Y.J. Kim, K.S. Kim, B. Ozyilmaz, J.H. Ahn, B.H. Hong, S. Iijima, Nat. Nanotechnol. 5 (8) (2010) 574–578. [10] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Nature 442 (7100) (2006) 282–286. [11] J.J. Yoo, K. Balakrishnan, J. Huang, V. Meunier, B.G. Sumpter, A. Srivastava, M. Conway, A.L. Mohana Reddy, J. Yu, R. Vajtai, P.M. Ajayan, Nano Lett. 11 (4) (2011) 1423–1427. [12] Sanjoy Mondal, Utpal Rana, Sudip Malik, Phys. Chem. C 121 (14) (2017) 7573–7583. [13] W. Yuan, A. Liu, L. Huang, C. Li, G. Shi, Adv. Mater. 25 (5) (2013) 766–771. [14] G.K. Dimitrakakis, E. Tylianakis, G.E. Froudakis, Nano Lett. 8 (10) (2008) 3166–3170. [15] M. Pumera, Chem. Soc. Rev. 39 (2010) 4146–4157. [16] R.K.L. Tan, S.P. Reeves, N. Hashemi, D.G. Thomas, E. Kavak, R. Montazami, N.N. Hashemi, J. Mater. Chem. 5 (2017) 17777. [17] D. Chen, L.H. Tanga, J.H. Li, Chem. Soc. Rev. 157 (2010) 353–393. [18] L. Zhang, X. Yu, H. Hu, Y. Li, M. Wu, Z. Wang, G. Li, Z.S.C. Chen, Sci. Rep. 5 (2015) 9295–9298. [19] H.L. Xu, H. Bi, R.B. Yang, J. Appl. Phys. 111 (2012) 07A522. [20] J.L. Wu, X.P. Shen, L. Jiang, K. Wang, K.M. Chen, Appl. Surf. Sci. 256 (2010) 2826–2830. [21] G. Williams, P.V. Kamat, Langmuir 25 (2009) 13869–13873. [22] M. Nasr, S. Balme, C. Eid, R. Habchi, P. Miele, M. Bechelany, J. Phys. Chem. C 121 (2017) 261–269.
53