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Facile synthesis of Gd and Sn co-doped BiFeO3 supported on nitrogen doped graphene for enhanced photocatalytic activity
Journal of Physics and Chemistry of Solids 130 (2019) 222–229
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Facile synthesis of Gd and Sn co-doped BiFeO3 supported on nitrogen doped graphene for enhanced photocatalytic activity
T
Maryam Kiania,b, Abdul Basit Kianic, Syed Ali Khana, Shafiq ur Rehmanaa, Qudrat Ullah Khana, Ikhtesham Mahmoode, Awais Sadique Saleemia, Abdul Jalila, Muhammad Sohaild, Ling Zhua,∗ a
Shenzhen Key Laboratory of Flexible Memory Materials and Devices, College of Physics and Optoelectronic Engineering, Shenzhen University, Nanhai Ave. 3688, Guangdong, 518060, PR China b Department of Physics, School of Natural Sciences, National University of Science & Technology, Islamabad, 44000, Pakistan C Department of Information and Communication Engineering, Beijing University of Technology, Beijing, PR China d Institute of Advanced Study, Shenzhen University, PR China d Key laboratory of Optoelectronic Devices and Systems of Ministry Education, Shenzhen University, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nitrogen doped graphene Spinal BiFeO3 Sol-gel synthesis Photocatalytic activity
Gd and Sn co-doped BiFeO3 nanoparticles were synthesized by sol-gel route and Bi0.95Gd0.05Fe0.95Sn0.05O3/ Nitrogen doped graphene nanohybrid (BGFSO/NG, where NG: 1%, 2%, 3%, 4%, 5%, 6%) were synthesized by sprinkling BGFSO nanoparticles and NG into ethanol solution followed by thermal drying. Structural and morphological properties of BiFeO3 (BFO) nanoparticles, BGFSO nanoparticles, NG and BGFSO/NG, were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The crystallite size BGFSO nanoparticle is about 20 nm. The photocatalytic activities of the as-prepared BiFeO3 (BFO) nanoparticles, BGFSO, NG and BGFSO/NG nanohybrid were measured by the degradation of methyl orange (MO) under simulated sunlight irradiation. BGFSO/N-G nanohybrid shows enhanced photocatalytic activity. The improved photocatalytic activity is attributed to the effective transfer of photogenerated electrons from nanoparticles NG nanosheets, consequently leads to an increased availability of h + for the photocatalytic reaction. Additionally, hydroxyl (⋅OH) radicals were detected by the photoluminescence method through terephthalic acid (TPA) as a probe molecule and are found to be generated on the irradiated BGFSO and BGFSO/NG.
1. Introduction With the incredible rise in industrial development, photocatalytic degradation of harmful materials has been a very significant and essential method for the production of purified water [1]. In the past several decades, the main research is emphasis on the purification of water. Hence a lot of effort has been devoted for the progress of efficient photocatalysts to obtain purified water [2]. The photo-excited nanoparticles can produce electron-hole pairs by the sun light absorption. These electrons and holes are proficient to activate the redox reactions between toxins. In recent times, the employment of ferroelectric nanomaterials to transform light into electrical or chemical energy has produced enormous attention to understand the mechanisms and procedure as well as for applications in photovoltaic, photocatalytic, and photo-transducer devices [3–7]. Multiferroic materials refer to the multifunctional materials that show simultaneous effects of ferroic properties [8] such as ferroelectricity, ferromagnetism, etc. A potential
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way is to use the multiferroics instead of the single-ferroic materials to meet this challenge. Multiferroic materials are rare in nature because of the condition of being ferroelectric and ferro-antiferromagnetic at the same time [9,10]. Among multiferroic materials, perovskite BiFeO3 (BFO) is one of them that show intrinsic spontaneous ferroelectricity (TC 1100 K) and G-type antiferromagnetism (TN 643 K) at room temperature [11,12]. The incorporation of BFO in practical devices has been limited because of the low magnetic moment, large leakage current induced by defects, and weak magneto-electric coupling [13–16]. BFO has the ferroelectric property that originates from the BieO hybridization owing to the stereo-chemical activity of Bi 6s2 lone pair electrons and the magnetic property originates by the reason of the partially filled 3d orbital electrons of Fe3+ ions [17,18]. Several research groups have attempted to dope the A site of BFO (ABO3) with +3 valence lanthanide ions [La3+, Nd3+ or Sm3+] to improve the electrical properties of the bismuth ferrite [19]. The doping of lanthanides leads to the decrease of leakage current density and the
Corresponding author. E-mail address: [email protected] (L. Zhu).
https://doi.org/10.1016/j.jpcs.2019.01.032 Received 23 September 2018; Received in revised form 1 January 2019; Accepted 28 January 2019 Available online 11 February 2019 0022-3697/ © 2019 Published by Elsevier Ltd.
Journal of Physics and Chemistry of Solids 130 (2019) 222–229
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Furthermore we exhibit that BGFSO nanoparticles can be reinforced onto NG by mingling BGFSO nanoparticles and NG into absolute ethanol solution and monitored by thermal drying. The photocatalytic activities of as-prepared BGFSO/NG nanocomposites were measured by the degradation of methylene orange (MO) under irradiation of simulated sunlight. The production of ∙OH radicals on the irradiated BGFSO/ NG nanocomposites was observed by the photoluminescence (PL) technique. By virtue of this, we are able to achieve improved photocatalytic activities.
enhancement in ferroelectric properties of BFO has been observed to some extent. The main criteria to substitute rare-earth elements in to the BFO, was the involvement of their existing magnetic moments. Although, in 2003 Wang [20] reported considerable improvement of multiferroic properties, BFO thin film, compared with bulk samples, epitaxial strained BiFeO3 thin films prepared by the pulsed laser deposition technique. Subsequently, serious efforts has been made to the improvement in properties of BFO thin films by substituting Bi3+ or Fe3+ with other ions such as Nd3+ [21–23], Ba2+ [24], La3+ [25], Ti4+ [26], Gd3+ [27] and Sm3+ [28]. It was found that the incorporation of rare earth elements significantly changes the chemistry of BFO and drastic improvement in its physical properties. BFO has been prepared by changing its structural parameters such as size, morphology, and dimensions and by changing its compositional parameters such as composites, substitution, and plasmon sensitizations. As Gd-doped BFO sample has been reported, it can exhibit both the orthorhombic and rhombohedral structure [29]. Though, the effect of Gd substitution at Bi site revealed a gradual phase transition from rhombohedral to pseudo-tetragonal structure [30]. Hereafter, it is obvious that Gd doping is important for structural phase transformation as well as improving the multiferroic properties in BFO. Several controversial issues are rising from the literature which stresses further works in this area. Furthermore, Sn doped BFO nanoparticles reported for sensing and catalytic application [31]. Moreover, latest studies have revealed that BFO also displays visible light responsive photocatalytic activity for the degradation of organic contaminants [32–34]. BFO is considered as a potential visible-light photo-catalyst for degradation of organic pollutants and water splitting due to a narrow band-gap (2.2 eV) [35,36]. Generally, the photocatalytic activity of a photocatalyst depends on various factors, among all of them the effective split-up of photogenerated electron-hole e−-h+ pairs is most authoritative in improving the photocatalytic activity. Recently researchers have reported Sm-doped BFO nanostructures for improved visible light photocatalytic activity [37]. Furthermore, substitution of rare earth ions with Bi3+ cation which has smaller ionic radii than Bi3+ (1.03 Å), such as Gd3+ (0.938 Å) enhances photocatalytic properties due to significant structural distortion in the BiFeO3 lattice [38,39]. Graphene have possession of remarkable properties comprising of mechanical strength, chemical stability, electrical conductivity, thermal conductivity and electron mobility [40–42]. Recently, graphene has been used as an ideal support owing to its remarkable properties, to form exclusive nanocomposites with improved efficiency in the photocatalysts [43], micro-supercapacitors [44], fuel cells [45–47], and field-emission emitters [48]. Moreover, Carbon based nanomaterials doped with heteroatoms, such as B, N or S, can improve the pseudo capacitance by monitoring its electronic properties and chemical reactivity which leads to enhanced performance [49,50]. So the abovementioned evidences permitting the use of N doped carbon based materials as electrode material for photo-catalysis. Furthermore, photocatalytic activity influenced by the shape and size of nanostructures demonstrated by several nanocomposites such as LaFeO3 [51], GdFeO3 [52], LuFeO3 [53]. Based on the above facts, preparation of nanocomposite system with simplistic synthesis technique, and low cost is highly preferred and effective 2 dimensional (2D) carbon based nanomaterial is desired to enhance photocatalytic performance. In this research work, novel nanocomposite system Gd and Sn codoped BFO (Bi0.95Gd0.05Fe0.95Sn0.05O3 nanoparticles abbreviated as BGFSO) with nitrogen doped graphene (NG) was synthesized by sol–gel synthesis route and has been investigated for photocatalytic properties. According to our best of knowledge BGFSO/NG nanocomposite never studied before. The rare earth element Gd was chosen as a substitution element because of its smaller ionic radius of 0.938 Å, which is smaller than that of Bi3+ ions at 1.17 Å [54,55]. Improvements in its chemical as well as physical properties are expected by the replacement of smaller ionic radii elements to create a larger lattice distortion in BFO.
2. Experimental 2.1. Synthesis of BGFSO nanocomposite BGFSO nanoparticles were synthesized by the improved sol-gel method [39]. Gadolinium nitrate hexahydrate and bismuth nitrate pentahydrate dissolved in acetic acid, ethylene glycol, stirred for 1 h at room temperature. Tin nitrate solution and iron nitrate nonahydrate powder dissolved in acetic acid while constantly stirring for 1 h. Here after both solutions mixed and stirring for 2 h and consequently, a uniform homogeneous solution was produced. Both solutions were synthesized with excess 3% Bi to compensate the Bi loss throughout annealing process. In the process of preparation, acetic acid act as catalyst in the sol system and the hydrolysis speed can be control and solution concentration was adjusted by it during synthesis, although ethylene glycol used as solvent during hydrolysis, can keep the different electronegativity of Bi and Fe and a stable solution form by its linearly structured molecule [19]. After the stirring of the solution, it was dried at 80 °C and then calcined at 600 °C for 12 h. 2.2. Synthesis of BGFSO/NG nanocomposite Fig. 1 represents synthesis schematics of BGFSO nanoparticles anchored on NG. NG used in this research was purchased from XF Nano, INC Advance Materials Tech Co. Ltd. To assemble BGFSO nanoparticles on NG, BGFSO nanoparticles prepared by sol-gel method [39] and NG were dispersed into ethanol solution and ultrasonically treated for 10 min in the ultrasonic bath. The acquired mixture was dried at 60 °C for 10 h in a thermostat drier, to vaporize ethanol and leaving behind BGFSO nanoparticles well anchored on NG By changing NG content, several BGFSO/NG nanocomposite samples with NG weight fractions of 1%, 2%, 3%, 4%, 5% and 6% were synthesized. Fig. 1 shows synthesis schematics of BGFSO/NG nanocomposite. 3. Physical characterizations The structural analysis of the BGFSO/NG nanocomposite has been analyzed by X-ray diffractometer (XRD) in the range of 20–60 °C with Cu-Kɑ radiation working at 40 kV. The morphological analysis of the BGFSO nanocomposites was done by Scanning electron microscope (SEM model JEOL JSM-6490A). A fluorescence spectrophotometer was used to observe the photoluminescence (PL) spectra of the as-synthesized BFO, BGFSO and BGFSO/NG samples. X-ray photoelectron spectroscopy (XPS) was accomplished with monochromatic Al Kα (1486.71 eV) X-ray radiation (15 kV and 10 mA). Raman spectra of the as-synthesized sample were measured on Horiba Jobin Yvon LabRam instrument with a HeNe laser excitation at 633 nm (1.96 eV) with a power of 3.7 mW. 4. Results and discussion 4.1. Structural and morphological analysis Fig. 2 displays XRD patterns of pure BFO, BGFSO nanocomposite, NG and BGFSO/NG. Diffraction peaks of substituted BFO were identified as rhombohedral-distorted perovskite structure with R3C space 223
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Fig. 1. Synthesis schematics of BGFSO/NG nanocomposite.
4.2. X-ray photo electron spectroscopy analysis BGFSO/NG nanocomposite was further investigated by x-ray photoelectron spectroscopy (XPS) analysis. The surface chemical composition and cation oxidation states of the BGFSO/NG (5%) nanocomposite system were characterized by XPS with Al Kα radiation. As estimated, the XPS spectra in Fig. 5a displays C 1s, N 1s, O 1s, Fe 2p, Sn 3d, Gd 4d, Bi 4d, Bi 4p, Bi 5d, and the peak of the BGFSO/NG nanocomposite. The Fe 2p spectrum was shifted into two peaks (Fig. 5b). The peaks at the binding energy of around 710.8 and 725.1 eV are allocated to Fe 2p3/2 and Fe 2p1/2 respectively, representing the presence of Fe3+ cations in the as-prepared BGFSO/NG nanocomposite. The binding energies of Bi 4f5/2, Bi 4f7/2 are detected at 164.2 and 158.6 respectively, displayed in Fig. 5c. Which are reliable with the literature standards of Fe3+ and Bi3+ [56,57] 4.3. Mechanism of photocatalytic activity enhancement
Fig. 2. XRD pattern of pure BFO, BGFSO, NG and BGFSO/NG nanocomposite.
Photocatalytic degradation of methyl orange (MO) is shown in Fig. 6 over BGFSO/NG nanocomposites as a function of irradiation time (t). The degradation percentage is defined as (Co-Ct)/Co × 100%. Where, Co and Ct are the concentrations of MO before and after irradiation time (t) respectively. The blank experimental result is also shown in Fig. 6, it can be seen that the MO is barely degraded under simulated sunlight irradiation without photocatalysts, and its degradation percentage is less than 5% after 6 h exposure in simulated sunlight. After 6 h irradiation, MO is observed to be degraded, about 38% and 17% in the existence of BGFSO nanoparticles and nitrogen doped graphene, respectively. When BGFSO nanoparticles assembled on NG, all samples of BGFSO/NG nanocomposites show enhanced photocatalytic activity than pure BFO nanoparticles and BGFSO nanocomposite. Furthermore, the photocatalytic activity of as-prepared nanocomposites increases gradually with the increase in NG content from 1% to 5%. Further increase in NG content leads to the decrease in photocatalytic activity of BGFSO/NG nanocomposite.
group. The consequence of Gd and Sn substitution on the BFO nanostructure can be seen as (104) and (110) doublet diffraction peaks of Gd and Sn substituted bismuth ferrite nanocomposite is near 2Θ = 32.0 and it became a single sharp peak with a peak shift to lower diffraction angle by increase of Sn concentration [39]. These results show that expansion of unit cell due to the substitution of Fe ions by Sn with larger ionic radius. The co-doping also influences the particle size of BFO nanostructures. The co-doping also influences the particle size of BFO nanostructures. The size of BGFSO nanocomposites was calculated from Scherrer equation and the values of primitive cell volume fitted by JADE 5 software. The crystalline size of BFO and BGFSO was 16.5 nm and 20 nm respectively. Fig. 3 represents the SEM analyses of pure BFO nanoparticles, NG, BGFSO nanoparticles prepared by sol-gel and BGFSO/NG nanocomposite (NG 5%). SEM results revealed that the shape of particles is spherical and size of the BGFSO nanocomposite is 20 nm. Fig. 3 (a) shows low magnification and (b) displays high magnification of BGFSO nanocomposite. Fig. 3c shows SEM of NG and SEM of BGFSO/NG represented in Fig. 3d, respectively. Transmission electron microscopy (TEM) image of BGFSO/NG (5%) nanocomposite represent in Fig. 4. It can be seen that BGFSO nanoparticles are accumulated onto the nitrogen doped graphene sheet. The BGFSO nanoparticles primarily revealed spherical shape and have an average particle size is around 20 nm.
4.4. Photo-catalytic activities Fig. 7 elaborates the schematics of the photocatalytic mechanism of BGFSO/NG nanocomposite photocatalyst under the simulated sunlight irradiation, towards the degradation of MO. The valence band (VB) electrons of BGFSO are stimulated to the conduction band (CB), prompting the generation of e−-h+ pairs. The photogenerated electrons 224
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Fig. 3. SEM image of (a) BFO nanoparticles, (b) BGFSO at high magnification, (c) nitrogen doped graphene, (d) BGFSO/NG nanocomposite.
reaction of the photogenerated h+ with OH− are the main active species accountable for the degradation of MO over simulated sunlight irradiated BGFSO nanocomposite. Therefore, the effective separation of e−-h+ pairs and increased availability of h+ are the main points to increase the photocatalytic activity of the BGFSO. When BGFSO hybrid nanoparticles spread on NG, which is a superb electron acceptor and conductor, the photogenerated electrons eagerly transfer from the BGFSO conduction band to NG, which could defeat the recombination of photoexcited e−-h+ pairs, which leads to the increase in the number of holes that contribute in the photocatalytic reaction. The consequences approve the enhanced yield of ∙OH radicals on the irradiated BGFSO/NG nanocomposite. Thus, with the addition of an appropriate amount of NG, the resulted BGFSO/NG nanocomposite displays an improved photocatalytic activity then BGFSO nanoparticles.
Fig. 4. TEM of BGFSO/NG nanocomposite.
4.5. Photoluminescence (PL) spectra
and holes then contribute in a series of redox reactions to form a number of active species. However, the redox reaction processes are tremendously associated with the CB and VB edge potentials of BGFSO. The VB potential of the BGFSO nanoparticles can be assessed using the following relation [58]: EVB = X-Ee +0.5Eg
PL spectra of the terephthalic acid (TPA) solution after reacting 6 h over the simulated sunlight exposed on pure BFO nanoparticles, BGFSO and BGFSO/NG nanocomposite photocatalysts, shown in Fig. 8. The blank experimental result reveals that no PL signal is detected at 429 nm after irradiation without photocatalyst. The PL signal centered about 429 nm is detected with BGFSO as photocatalyst, indicating that ∙OH radicals are produced on the irradiated BGFSO. Furthermore, when BGFSO spherical shape nanoparticles supported on NG, a fabulous electron acceptor and conductor, the photogenerated electrons keenly relocate from the BGFSO conduction band to NG, which could defeat the recombination of photoexcited e−-h+ pairs. Hence, leads to the increase in the number of holes which play key role in the photo-catalysis reaction mechanism. The PL signal intensity is increased by increasing concentration of NG up to 5% in BGFSO/NG nanocomposite which is used as the photocatalyst, which suggests that the yield of ∙OH radicals is improved on the irradiated BGFSO/NG (5%) nanocomposite. The results are shown in Fig. 6 confirm the improved yield of ∙OH radicals on the irradiated BGFSO/NG nanocomposite. Consequently, with the introduction of an amount of NG, the resulted BGFSO/NG
(1) e
Where the absolute electronegativity is represented by X, E is the energy of free electrons on the hydrogen scale (∼4.5 eV), and Eg is the bandgap energy of the semiconductor the value of X for BGFSO is attained, by the arithmetic mean of the electron affinity and the first ionization of the constituent atoms is 5.93 eV [59,60]. Therefore, the CB and VB potentials of BGFSO are evaluated to be 0.4 and 2.46 V contrasted with normal hydrogen electrode (NHE), respectively. Thus, the VB potential of the as-synthesized nanocomposite is more positive than the redox potential of OH−/∙OH (1.89 V/NHE), signifying that the photogenerated holes have very strong oxidative capability and they can oxide OH− into ∙OH. However, the CB potential of the nanocomposite is not negative enough to reduce O2 to O2∙- (−0.13 V/NHE) via e−. So, it is thoughtful to determine that ∙OH radicals derived by the 225
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Fig. 5. (a) Full XPS spectra of BGFSO/NG nanocomposite, (b) Fe 2p XPS spectra of BGFSO/NG nanocomposite, (c) Bi 4f spectra of BGFSO/NG.
number of photon absorption on BGFSO nanoparticles; (ii) the amount of accessible surface active sites tends to be decreased owing to enlarge the coverage of NG onto the surface of BGFSO nanostructures.
4.6. Raman spectroscopy analysis Raman spectroscopy analysis gives further evidence about the structural properties of the as-prepared BGFSO/NG nanocomposite. Fig. 9 displays the two visible peaks restricted about 1346 and 1583 cm−1 which resemble D and G bands of carbon materials as well as a 2D band is also existed around 2682 cm−1. The D band is the measure of structural disorder ascribed to the amorphous carbon and defects [61]. The G band is related to the E2g phonon vibrations mode in sp2 -bonded carbon–carbon bonds [62]. The 2D band of BGFSO/NG nanocomposite at 2682 cm−1 is related to the amorphous nature of NG. The wide and weak 2D bands of BGFSO/NG nanocomposite intend to the formation of few layers of NG [63]. For the BGFSO/NG nanocomposite, all of the D, G and 2D bands exhibit a red-shift. It could identify the existence of the coupling between BGFSO and the NG, in a good correspondence with the SEM, and XPS consequences. Also, the disorder density of carbon materials can be investigated by the ratio of the intensities ID and IG of the D and G bands, R = ID/IG, [64]. The ID/IG ratio for the BGFSO/NG nanocomposite is calculated to be 1.02 which exposes that defects and the disorder density of the NG sheets rise subsequently. Electrochemical impedance spectra (EIS) acquired for BGFSO/NG and BGFSO under illumination with visible light (λ ≥ 420 nm)
Fig. 6. Photocatalytic degradation of MO as a function of irradiation time over pure bismuth ferrite nanoparticles, BGFSO nanocomposite and BGFSO/NG nanocomposites NG weight fractions of 1%, 2%, 3%, 4%, 5% and 6%.
nanocomposites display an improved photocatalytic activity compared to pure BFO nanoparticles and BGFSO nanocomposite. Although, when the NG content is increased further above its optimal value, the photocatalytic efficiency begins to decline. This is ascribed to the following reasons: (i) the large amounts NG may shield the light and decrease the 226
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Fig. 7. Schematic description of the photocatalytic mechanism of BGFSO/NG nanocomposite towards the degradation of MO.
Fig. 8. PL spectra of the TPA solution after reacting 6 h over the irradiated pure BFO nanoparticles, BGFSO nanocomposite and BGFSO/NG nanocomposite.
Fig. 10. EIS spectra obtained for BGFSO and BGFSO/NG under illumination with visible light.
represented in Fig. 10. Generally, a smaller arc radius in the EIS Nyquist plot specifies proficient charge transfer. Predominantly, the arc radius of the Nyquist plot for BGFSO/NG was smaller than BGFSO, thus demonstrating the efficient separation of photo-excited electron–hole pairs and instantaneous interfacial charge transfer above BGFSO/NG with improved photocatalytic activity. The UV–visible absorption spectra obtained for BGFSO and BGFSO/ NG are presented in Fig. 11. BGFSO is a direct bandgap material, so its optical absorption coefficient follows the equation: (αhν)2 = B(hν – Eg), where α is the absorption coefficient, hν is the photon energy, and Eg is the optical band gap. The Eg value for BGFSO was evaluated based on the linear part of the (αhν)2 vs hν plot at the point where a = 0, which indicated a band gap of around 2.06 eV, as shown in the inset in Fig. 9. NG is half-metallic and the BGFSO/NG nanocomposite exhibited enhanced absorption of visible light compared with the pure BGFSO nanoparticles. Thus, the estimated photocatalytic performance of BGFSO/ NG (5%) was higher compare with that of the pure BGFSO nanoparticles. Fig. 9. Raman spectroscopy of the BGFSO/NG nanocomposite.
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Fig. 11. UV–visible absorption spectra obtained for BGFSO nanoparticles and the BGFSO/NG nanocomposite. The inset of Fig. 11 represents the band gap estimated for BGFSO.
5. Conclusion BGFSO nanoparticles ware synthesized by sol-gel synthesis route and BGFSO/NG nanocomposites were prepared by mixing BGFSO nanoparticles with different concentration of NG into the ethanol solution by thermal drying at 60 °C. XRD, SEM and TEM analysis confirmed the formation of BGFSO and BGFSO/NG (5%) nanocomposite. The size of BGFSO nanoparticle is 20 nm. The photocatalytic investigations reveal that the BGFSO/NG (5%) nanocomposite exhibits enhanced photocatalytic activity for the degradation of MO under simulated sunlight irradiation as compared to BGFSO nanocomposites. The enhanced photocatalytic activity of the BGFSO/NG nanocomposite is mainly due to the strong coupling and synergistic effect between BGFSO and NG, owing to more active sites, which is due to fact that electrons are captured by NG which leads to a large amount of availability of h+ for the photocatalytic reaction. Acknowledgments This work was financially supported by the grants from the Science and Technology Innovation Commission of Shenzhen (Grant no. ZDSYS201707271554071). Startup funding of Shenzhen University (Grant No. 2017007), and Science and Technology Plane Project of Shenzhen of 2019. References [1] E. M. Saggioro, A. S. Oliveira, T. Pavesi, C. G. Maia, L. F. V. Ferreira and J. C. Moreira: 16 (2011) 10370–10386. [2] M.T. Amin, A.A. Alazba, U. Manzoor, Chem. Soc. Rev. (2014) 1–24. [3] S.Y. Yang, et al., Above band-gap voltages from ferroelectric photovoltaic devices, Nat. Nanotechnol. 5 (2010) 143–147. [4] M. Alexe, Local mapping of generation and recombination lifetime in BiFeO3 single crystals by scanning probe photo-induced transient spectroscopy, Nano Lett. 12 (2012) 2193–2198. [5] F. Gao, et al., Visible-light photocatalytic properties of weak magnetic BiFeO3 nanoparticles, Adv. Mater. 19 (2007) 2889–2892. [6] J. Deng, S. Banerjee, S.K. Mohapatra, Y.R. Smith, M. Misra, Bismuth iron oxide nanoparticles as photocatalyst for solar hydrogen generation from Water, J. Fund. Renew. Energy Appl. 1 (2011) 1–10. [7] J. Kreisel, M. Alexe, P.A. Tomas, A photoferroelectric material is more than the sum of its parts, Nat. Mater. 11 (2012) 260. [8] G.A. Smolenski, I.E. Chupis, Ferroelectromagnets, Sov. Phys. Usp. 25 (1982) 475. [9] J. Ryu, S. Priya, K. Uchino, H.E. Kim, Magnetoelectric effect in composites of
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Facile synthesis of Gd and Sn co-doped BiFeO3 supported on nitrogen doped graphene for enhanced photocatalytic activity
T
Maryam Kiania,b, Abdul Basit Kianic, Syed Ali Khana, Shafiq ur Rehmanaa, Qudrat Ullah Khana, Ikhtesham Mahmoode, Awais Sadique Saleemia, Abdul Jalila, Muhammad Sohaild, Ling Zhua,∗ a
Shenzhen Key Laboratory of Flexible Memory Materials and Devices, College of Physics and Optoelectronic Engineering, Shenzhen University, Nanhai Ave. 3688, Guangdong, 518060, PR China b Department of Physics, School of Natural Sciences, National University of Science & Technology, Islamabad, 44000, Pakistan C Department of Information and Communication Engineering, Beijing University of Technology, Beijing, PR China d Institute of Advanced Study, Shenzhen University, PR China d Key laboratory of Optoelectronic Devices and Systems of Ministry Education, Shenzhen University, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nitrogen doped graphene Spinal BiFeO3 Sol-gel synthesis Photocatalytic activity
Gd and Sn co-doped BiFeO3 nanoparticles were synthesized by sol-gel route and Bi0.95Gd0.05Fe0.95Sn0.05O3/ Nitrogen doped graphene nanohybrid (BGFSO/NG, where NG: 1%, 2%, 3%, 4%, 5%, 6%) were synthesized by sprinkling BGFSO nanoparticles and NG into ethanol solution followed by thermal drying. Structural and morphological properties of BiFeO3 (BFO) nanoparticles, BGFSO nanoparticles, NG and BGFSO/NG, were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The crystallite size BGFSO nanoparticle is about 20 nm. The photocatalytic activities of the as-prepared BiFeO3 (BFO) nanoparticles, BGFSO, NG and BGFSO/NG nanohybrid were measured by the degradation of methyl orange (MO) under simulated sunlight irradiation. BGFSO/N-G nanohybrid shows enhanced photocatalytic activity. The improved photocatalytic activity is attributed to the effective transfer of photogenerated electrons from nanoparticles NG nanosheets, consequently leads to an increased availability of h + for the photocatalytic reaction. Additionally, hydroxyl (⋅OH) radicals were detected by the photoluminescence method through terephthalic acid (TPA) as a probe molecule and are found to be generated on the irradiated BGFSO and BGFSO/NG.
1. Introduction With the incredible rise in industrial development, photocatalytic degradation of harmful materials has been a very significant and essential method for the production of purified water [1]. In the past several decades, the main research is emphasis on the purification of water. Hence a lot of effort has been devoted for the progress of efficient photocatalysts to obtain purified water [2]. The photo-excited nanoparticles can produce electron-hole pairs by the sun light absorption. These electrons and holes are proficient to activate the redox reactions between toxins. In recent times, the employment of ferroelectric nanomaterials to transform light into electrical or chemical energy has produced enormous attention to understand the mechanisms and procedure as well as for applications in photovoltaic, photocatalytic, and photo-transducer devices [3–7]. Multiferroic materials refer to the multifunctional materials that show simultaneous effects of ferroic properties [8] such as ferroelectricity, ferromagnetism, etc. A potential
∗
way is to use the multiferroics instead of the single-ferroic materials to meet this challenge. Multiferroic materials are rare in nature because of the condition of being ferroelectric and ferro-antiferromagnetic at the same time [9,10]. Among multiferroic materials, perovskite BiFeO3 (BFO) is one of them that show intrinsic spontaneous ferroelectricity (TC 1100 K) and G-type antiferromagnetism (TN 643 K) at room temperature [11,12]. The incorporation of BFO in practical devices has been limited because of the low magnetic moment, large leakage current induced by defects, and weak magneto-electric coupling [13–16]. BFO has the ferroelectric property that originates from the BieO hybridization owing to the stereo-chemical activity of Bi 6s2 lone pair electrons and the magnetic property originates by the reason of the partially filled 3d orbital electrons of Fe3+ ions [17,18]. Several research groups have attempted to dope the A site of BFO (ABO3) with +3 valence lanthanide ions [La3+, Nd3+ or Sm3+] to improve the electrical properties of the bismuth ferrite [19]. The doping of lanthanides leads to the decrease of leakage current density and the
Corresponding author. E-mail address: [email protected] (L. Zhu).
https://doi.org/10.1016/j.jpcs.2019.01.032 Received 23 September 2018; Received in revised form 1 January 2019; Accepted 28 January 2019 Available online 11 February 2019 0022-3697/ © 2019 Published by Elsevier Ltd.
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Furthermore we exhibit that BGFSO nanoparticles can be reinforced onto NG by mingling BGFSO nanoparticles and NG into absolute ethanol solution and monitored by thermal drying. The photocatalytic activities of as-prepared BGFSO/NG nanocomposites were measured by the degradation of methylene orange (MO) under irradiation of simulated sunlight. The production of ∙OH radicals on the irradiated BGFSO/ NG nanocomposites was observed by the photoluminescence (PL) technique. By virtue of this, we are able to achieve improved photocatalytic activities.
enhancement in ferroelectric properties of BFO has been observed to some extent. The main criteria to substitute rare-earth elements in to the BFO, was the involvement of their existing magnetic moments. Although, in 2003 Wang [20] reported considerable improvement of multiferroic properties, BFO thin film, compared with bulk samples, epitaxial strained BiFeO3 thin films prepared by the pulsed laser deposition technique. Subsequently, serious efforts has been made to the improvement in properties of BFO thin films by substituting Bi3+ or Fe3+ with other ions such as Nd3+ [21–23], Ba2+ [24], La3+ [25], Ti4+ [26], Gd3+ [27] and Sm3+ [28]. It was found that the incorporation of rare earth elements significantly changes the chemistry of BFO and drastic improvement in its physical properties. BFO has been prepared by changing its structural parameters such as size, morphology, and dimensions and by changing its compositional parameters such as composites, substitution, and plasmon sensitizations. As Gd-doped BFO sample has been reported, it can exhibit both the orthorhombic and rhombohedral structure [29]. Though, the effect of Gd substitution at Bi site revealed a gradual phase transition from rhombohedral to pseudo-tetragonal structure [30]. Hereafter, it is obvious that Gd doping is important for structural phase transformation as well as improving the multiferroic properties in BFO. Several controversial issues are rising from the literature which stresses further works in this area. Furthermore, Sn doped BFO nanoparticles reported for sensing and catalytic application [31]. Moreover, latest studies have revealed that BFO also displays visible light responsive photocatalytic activity for the degradation of organic contaminants [32–34]. BFO is considered as a potential visible-light photo-catalyst for degradation of organic pollutants and water splitting due to a narrow band-gap (2.2 eV) [35,36]. Generally, the photocatalytic activity of a photocatalyst depends on various factors, among all of them the effective split-up of photogenerated electron-hole e−-h+ pairs is most authoritative in improving the photocatalytic activity. Recently researchers have reported Sm-doped BFO nanostructures for improved visible light photocatalytic activity [37]. Furthermore, substitution of rare earth ions with Bi3+ cation which has smaller ionic radii than Bi3+ (1.03 Å), such as Gd3+ (0.938 Å) enhances photocatalytic properties due to significant structural distortion in the BiFeO3 lattice [38,39]. Graphene have possession of remarkable properties comprising of mechanical strength, chemical stability, electrical conductivity, thermal conductivity and electron mobility [40–42]. Recently, graphene has been used as an ideal support owing to its remarkable properties, to form exclusive nanocomposites with improved efficiency in the photocatalysts [43], micro-supercapacitors [44], fuel cells [45–47], and field-emission emitters [48]. Moreover, Carbon based nanomaterials doped with heteroatoms, such as B, N or S, can improve the pseudo capacitance by monitoring its electronic properties and chemical reactivity which leads to enhanced performance [49,50]. So the abovementioned evidences permitting the use of N doped carbon based materials as electrode material for photo-catalysis. Furthermore, photocatalytic activity influenced by the shape and size of nanostructures demonstrated by several nanocomposites such as LaFeO3 [51], GdFeO3 [52], LuFeO3 [53]. Based on the above facts, preparation of nanocomposite system with simplistic synthesis technique, and low cost is highly preferred and effective 2 dimensional (2D) carbon based nanomaterial is desired to enhance photocatalytic performance. In this research work, novel nanocomposite system Gd and Sn codoped BFO (Bi0.95Gd0.05Fe0.95Sn0.05O3 nanoparticles abbreviated as BGFSO) with nitrogen doped graphene (NG) was synthesized by sol–gel synthesis route and has been investigated for photocatalytic properties. According to our best of knowledge BGFSO/NG nanocomposite never studied before. The rare earth element Gd was chosen as a substitution element because of its smaller ionic radius of 0.938 Å, which is smaller than that of Bi3+ ions at 1.17 Å [54,55]. Improvements in its chemical as well as physical properties are expected by the replacement of smaller ionic radii elements to create a larger lattice distortion in BFO.
2. Experimental 2.1. Synthesis of BGFSO nanocomposite BGFSO nanoparticles were synthesized by the improved sol-gel method [39]. Gadolinium nitrate hexahydrate and bismuth nitrate pentahydrate dissolved in acetic acid, ethylene glycol, stirred for 1 h at room temperature. Tin nitrate solution and iron nitrate nonahydrate powder dissolved in acetic acid while constantly stirring for 1 h. Here after both solutions mixed and stirring for 2 h and consequently, a uniform homogeneous solution was produced. Both solutions were synthesized with excess 3% Bi to compensate the Bi loss throughout annealing process. In the process of preparation, acetic acid act as catalyst in the sol system and the hydrolysis speed can be control and solution concentration was adjusted by it during synthesis, although ethylene glycol used as solvent during hydrolysis, can keep the different electronegativity of Bi and Fe and a stable solution form by its linearly structured molecule [19]. After the stirring of the solution, it was dried at 80 °C and then calcined at 600 °C for 12 h. 2.2. Synthesis of BGFSO/NG nanocomposite Fig. 1 represents synthesis schematics of BGFSO nanoparticles anchored on NG. NG used in this research was purchased from XF Nano, INC Advance Materials Tech Co. Ltd. To assemble BGFSO nanoparticles on NG, BGFSO nanoparticles prepared by sol-gel method [39] and NG were dispersed into ethanol solution and ultrasonically treated for 10 min in the ultrasonic bath. The acquired mixture was dried at 60 °C for 10 h in a thermostat drier, to vaporize ethanol and leaving behind BGFSO nanoparticles well anchored on NG By changing NG content, several BGFSO/NG nanocomposite samples with NG weight fractions of 1%, 2%, 3%, 4%, 5% and 6% were synthesized. Fig. 1 shows synthesis schematics of BGFSO/NG nanocomposite. 3. Physical characterizations The structural analysis of the BGFSO/NG nanocomposite has been analyzed by X-ray diffractometer (XRD) in the range of 20–60 °C with Cu-Kɑ radiation working at 40 kV. The morphological analysis of the BGFSO nanocomposites was done by Scanning electron microscope (SEM model JEOL JSM-6490A). A fluorescence spectrophotometer was used to observe the photoluminescence (PL) spectra of the as-synthesized BFO, BGFSO and BGFSO/NG samples. X-ray photoelectron spectroscopy (XPS) was accomplished with monochromatic Al Kα (1486.71 eV) X-ray radiation (15 kV and 10 mA). Raman spectra of the as-synthesized sample were measured on Horiba Jobin Yvon LabRam instrument with a HeNe laser excitation at 633 nm (1.96 eV) with a power of 3.7 mW. 4. Results and discussion 4.1. Structural and morphological analysis Fig. 2 displays XRD patterns of pure BFO, BGFSO nanocomposite, NG and BGFSO/NG. Diffraction peaks of substituted BFO were identified as rhombohedral-distorted perovskite structure with R3C space 223
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Fig. 1. Synthesis schematics of BGFSO/NG nanocomposite.
4.2. X-ray photo electron spectroscopy analysis BGFSO/NG nanocomposite was further investigated by x-ray photoelectron spectroscopy (XPS) analysis. The surface chemical composition and cation oxidation states of the BGFSO/NG (5%) nanocomposite system were characterized by XPS with Al Kα radiation. As estimated, the XPS spectra in Fig. 5a displays C 1s, N 1s, O 1s, Fe 2p, Sn 3d, Gd 4d, Bi 4d, Bi 4p, Bi 5d, and the peak of the BGFSO/NG nanocomposite. The Fe 2p spectrum was shifted into two peaks (Fig. 5b). The peaks at the binding energy of around 710.8 and 725.1 eV are allocated to Fe 2p3/2 and Fe 2p1/2 respectively, representing the presence of Fe3+ cations in the as-prepared BGFSO/NG nanocomposite. The binding energies of Bi 4f5/2, Bi 4f7/2 are detected at 164.2 and 158.6 respectively, displayed in Fig. 5c. Which are reliable with the literature standards of Fe3+ and Bi3+ [56,57] 4.3. Mechanism of photocatalytic activity enhancement
Fig. 2. XRD pattern of pure BFO, BGFSO, NG and BGFSO/NG nanocomposite.
Photocatalytic degradation of methyl orange (MO) is shown in Fig. 6 over BGFSO/NG nanocomposites as a function of irradiation time (t). The degradation percentage is defined as (Co-Ct)/Co × 100%. Where, Co and Ct are the concentrations of MO before and after irradiation time (t) respectively. The blank experimental result is also shown in Fig. 6, it can be seen that the MO is barely degraded under simulated sunlight irradiation without photocatalysts, and its degradation percentage is less than 5% after 6 h exposure in simulated sunlight. After 6 h irradiation, MO is observed to be degraded, about 38% and 17% in the existence of BGFSO nanoparticles and nitrogen doped graphene, respectively. When BGFSO nanoparticles assembled on NG, all samples of BGFSO/NG nanocomposites show enhanced photocatalytic activity than pure BFO nanoparticles and BGFSO nanocomposite. Furthermore, the photocatalytic activity of as-prepared nanocomposites increases gradually with the increase in NG content from 1% to 5%. Further increase in NG content leads to the decrease in photocatalytic activity of BGFSO/NG nanocomposite.
group. The consequence of Gd and Sn substitution on the BFO nanostructure can be seen as (104) and (110) doublet diffraction peaks of Gd and Sn substituted bismuth ferrite nanocomposite is near 2Θ = 32.0 and it became a single sharp peak with a peak shift to lower diffraction angle by increase of Sn concentration [39]. These results show that expansion of unit cell due to the substitution of Fe ions by Sn with larger ionic radius. The co-doping also influences the particle size of BFO nanostructures. The co-doping also influences the particle size of BFO nanostructures. The size of BGFSO nanocomposites was calculated from Scherrer equation and the values of primitive cell volume fitted by JADE 5 software. The crystalline size of BFO and BGFSO was 16.5 nm and 20 nm respectively. Fig. 3 represents the SEM analyses of pure BFO nanoparticles, NG, BGFSO nanoparticles prepared by sol-gel and BGFSO/NG nanocomposite (NG 5%). SEM results revealed that the shape of particles is spherical and size of the BGFSO nanocomposite is 20 nm. Fig. 3 (a) shows low magnification and (b) displays high magnification of BGFSO nanocomposite. Fig. 3c shows SEM of NG and SEM of BGFSO/NG represented in Fig. 3d, respectively. Transmission electron microscopy (TEM) image of BGFSO/NG (5%) nanocomposite represent in Fig. 4. It can be seen that BGFSO nanoparticles are accumulated onto the nitrogen doped graphene sheet. The BGFSO nanoparticles primarily revealed spherical shape and have an average particle size is around 20 nm.
4.4. Photo-catalytic activities Fig. 7 elaborates the schematics of the photocatalytic mechanism of BGFSO/NG nanocomposite photocatalyst under the simulated sunlight irradiation, towards the degradation of MO. The valence band (VB) electrons of BGFSO are stimulated to the conduction band (CB), prompting the generation of e−-h+ pairs. The photogenerated electrons 224
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Fig. 3. SEM image of (a) BFO nanoparticles, (b) BGFSO at high magnification, (c) nitrogen doped graphene, (d) BGFSO/NG nanocomposite.
reaction of the photogenerated h+ with OH− are the main active species accountable for the degradation of MO over simulated sunlight irradiated BGFSO nanocomposite. Therefore, the effective separation of e−-h+ pairs and increased availability of h+ are the main points to increase the photocatalytic activity of the BGFSO. When BGFSO hybrid nanoparticles spread on NG, which is a superb electron acceptor and conductor, the photogenerated electrons eagerly transfer from the BGFSO conduction band to NG, which could defeat the recombination of photoexcited e−-h+ pairs, which leads to the increase in the number of holes that contribute in the photocatalytic reaction. The consequences approve the enhanced yield of ∙OH radicals on the irradiated BGFSO/NG nanocomposite. Thus, with the addition of an appropriate amount of NG, the resulted BGFSO/NG nanocomposite displays an improved photocatalytic activity then BGFSO nanoparticles.
Fig. 4. TEM of BGFSO/NG nanocomposite.
4.5. Photoluminescence (PL) spectra
and holes then contribute in a series of redox reactions to form a number of active species. However, the redox reaction processes are tremendously associated with the CB and VB edge potentials of BGFSO. The VB potential of the BGFSO nanoparticles can be assessed using the following relation [58]: EVB = X-Ee +0.5Eg
PL spectra of the terephthalic acid (TPA) solution after reacting 6 h over the simulated sunlight exposed on pure BFO nanoparticles, BGFSO and BGFSO/NG nanocomposite photocatalysts, shown in Fig. 8. The blank experimental result reveals that no PL signal is detected at 429 nm after irradiation without photocatalyst. The PL signal centered about 429 nm is detected with BGFSO as photocatalyst, indicating that ∙OH radicals are produced on the irradiated BGFSO. Furthermore, when BGFSO spherical shape nanoparticles supported on NG, a fabulous electron acceptor and conductor, the photogenerated electrons keenly relocate from the BGFSO conduction band to NG, which could defeat the recombination of photoexcited e−-h+ pairs. Hence, leads to the increase in the number of holes which play key role in the photo-catalysis reaction mechanism. The PL signal intensity is increased by increasing concentration of NG up to 5% in BGFSO/NG nanocomposite which is used as the photocatalyst, which suggests that the yield of ∙OH radicals is improved on the irradiated BGFSO/NG (5%) nanocomposite. The results are shown in Fig. 6 confirm the improved yield of ∙OH radicals on the irradiated BGFSO/NG nanocomposite. Consequently, with the introduction of an amount of NG, the resulted BGFSO/NG
(1) e
Where the absolute electronegativity is represented by X, E is the energy of free electrons on the hydrogen scale (∼4.5 eV), and Eg is the bandgap energy of the semiconductor the value of X for BGFSO is attained, by the arithmetic mean of the electron affinity and the first ionization of the constituent atoms is 5.93 eV [59,60]. Therefore, the CB and VB potentials of BGFSO are evaluated to be 0.4 and 2.46 V contrasted with normal hydrogen electrode (NHE), respectively. Thus, the VB potential of the as-synthesized nanocomposite is more positive than the redox potential of OH−/∙OH (1.89 V/NHE), signifying that the photogenerated holes have very strong oxidative capability and they can oxide OH− into ∙OH. However, the CB potential of the nanocomposite is not negative enough to reduce O2 to O2∙- (−0.13 V/NHE) via e−. So, it is thoughtful to determine that ∙OH radicals derived by the 225
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Fig. 5. (a) Full XPS spectra of BGFSO/NG nanocomposite, (b) Fe 2p XPS spectra of BGFSO/NG nanocomposite, (c) Bi 4f spectra of BGFSO/NG.
number of photon absorption on BGFSO nanoparticles; (ii) the amount of accessible surface active sites tends to be decreased owing to enlarge the coverage of NG onto the surface of BGFSO nanostructures.
4.6. Raman spectroscopy analysis Raman spectroscopy analysis gives further evidence about the structural properties of the as-prepared BGFSO/NG nanocomposite. Fig. 9 displays the two visible peaks restricted about 1346 and 1583 cm−1 which resemble D and G bands of carbon materials as well as a 2D band is also existed around 2682 cm−1. The D band is the measure of structural disorder ascribed to the amorphous carbon and defects [61]. The G band is related to the E2g phonon vibrations mode in sp2 -bonded carbon–carbon bonds [62]. The 2D band of BGFSO/NG nanocomposite at 2682 cm−1 is related to the amorphous nature of NG. The wide and weak 2D bands of BGFSO/NG nanocomposite intend to the formation of few layers of NG [63]. For the BGFSO/NG nanocomposite, all of the D, G and 2D bands exhibit a red-shift. It could identify the existence of the coupling between BGFSO and the NG, in a good correspondence with the SEM, and XPS consequences. Also, the disorder density of carbon materials can be investigated by the ratio of the intensities ID and IG of the D and G bands, R = ID/IG, [64]. The ID/IG ratio for the BGFSO/NG nanocomposite is calculated to be 1.02 which exposes that defects and the disorder density of the NG sheets rise subsequently. Electrochemical impedance spectra (EIS) acquired for BGFSO/NG and BGFSO under illumination with visible light (λ ≥ 420 nm)
Fig. 6. Photocatalytic degradation of MO as a function of irradiation time over pure bismuth ferrite nanoparticles, BGFSO nanocomposite and BGFSO/NG nanocomposites NG weight fractions of 1%, 2%, 3%, 4%, 5% and 6%.
nanocomposites display an improved photocatalytic activity compared to pure BFO nanoparticles and BGFSO nanocomposite. Although, when the NG content is increased further above its optimal value, the photocatalytic efficiency begins to decline. This is ascribed to the following reasons: (i) the large amounts NG may shield the light and decrease the 226
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Fig. 7. Schematic description of the photocatalytic mechanism of BGFSO/NG nanocomposite towards the degradation of MO.
Fig. 8. PL spectra of the TPA solution after reacting 6 h over the irradiated pure BFO nanoparticles, BGFSO nanocomposite and BGFSO/NG nanocomposite.
Fig. 10. EIS spectra obtained for BGFSO and BGFSO/NG under illumination with visible light.
represented in Fig. 10. Generally, a smaller arc radius in the EIS Nyquist plot specifies proficient charge transfer. Predominantly, the arc radius of the Nyquist plot for BGFSO/NG was smaller than BGFSO, thus demonstrating the efficient separation of photo-excited electron–hole pairs and instantaneous interfacial charge transfer above BGFSO/NG with improved photocatalytic activity. The UV–visible absorption spectra obtained for BGFSO and BGFSO/ NG are presented in Fig. 11. BGFSO is a direct bandgap material, so its optical absorption coefficient follows the equation: (αhν)2 = B(hν – Eg), where α is the absorption coefficient, hν is the photon energy, and Eg is the optical band gap. The Eg value for BGFSO was evaluated based on the linear part of the (αhν)2 vs hν plot at the point where a = 0, which indicated a band gap of around 2.06 eV, as shown in the inset in Fig. 9. NG is half-metallic and the BGFSO/NG nanocomposite exhibited enhanced absorption of visible light compared with the pure BGFSO nanoparticles. Thus, the estimated photocatalytic performance of BGFSO/ NG (5%) was higher compare with that of the pure BGFSO nanoparticles. Fig. 9. Raman spectroscopy of the BGFSO/NG nanocomposite.
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Fig. 11. UV–visible absorption spectra obtained for BGFSO nanoparticles and the BGFSO/NG nanocomposite. The inset of Fig. 11 represents the band gap estimated for BGFSO.
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