Lanthanum oxide nanoparticles immobilized reduced graphene oxide polymer brush nanohybrid for environmental vitiation of organic dyes

1 Introduction

Treatment and handling of waste water has become a worldwide concern in the last few decades. Textile dyeing and finishing industries are the major pollutant of clean water and dyes are the key hazardous wastes of such industries (Bhatt and Rani, 2013; Ulson de Souza et al., 2007). These organic dyes are used as colorant in in dyeing industries and also in biological tagging. These dyes can cause serious toxicity to human organs viz. liver, kidneys and nervous system (Kadirvelu et al., 2005; Malik, 2004), furthermore, their precursors and degradation by-products have shown to be carcinogenic and mutagenic (Alves de Lima et al., 2007). Their very bright colors causes a serious impediment to the penetration of oxygen and sunlight into natural water bodies (Spadaro et al., 1994) affecting the aquatic flora and fauna. Their adverse effects on the environment are alarming hence it is necessary to degrade them before discharging them to aquatic environments. In general, decolorization of water polluted with dye can be accomplished through absorption by carbon materials e.g. activated carbon fibers (Shen et al., 2001), biodegradation (Sleiman et al., 2007), enzymatic decomposition (Hao et al., 2000) ozonation (Slokar and Majcen Le Marechal, 1998) and oxidation processes e.g. Fenton catalytic reactions (Devi et al., 2010). However, these methods usually transfer organic compounds from water phase to another phase and thereby require further treatment of the solid wastes. Besides with processes involving Fenton reaction, treatment of ferrous slurry and involvement of H₂O₂ enhances the complications in water treatment. Thorough degradation of dyes has been reported using TiO₂/GO (Pradhan et al., 2011), which is still limited due to its unsatisfactory quantum yield like some other photocatalytic methods.

Graphene the much-investigated allotrope of carbon (Tang and Yamauchi, 2016), in recent years has proved to be a very promising candidate for the realization of new materials. It serves wide range of applications in catalysts (Dreyer et al., 2010; Hu et al., 2017; Iqbal et al., 2017; Zhang et al., 2017, 2016), sensors (Chen et al., 2010), drug delivery (Liu et al., 2008), energy storage (Pumera, 2011), biomedical devices (Shen et al., 2012), hydrogen storage (Tozzini and Pellegrini, 2013), optoelectronic devices (Bonaccorso et al., 2010), super-capacitors (Liu et al., 2010; Salunkhe et al., 2014), etc. owing to its outstanding electrical, mechanical, transport and thermal properties (Geim and Novoselov, 2007; Khan et al., 2015a; Sharma et al., 2016). Graphene and graphene based composites have been the area of interest among researchers and considerable amount of work has been focused on graphene based nanocomposites (Al-Marri et al., 2015b; Hao et al., 2019; Mogha et al., 2016; Tiwari et al., 2017). The structural nanohybrids arising from graphene – polymers and graphene – metal oxides coupling result in rapid flow of electrolytic ions together with high electrical integrity through the conducting matrix (Sahu et al., 2015). Furthermore, incorporation of DNA (Tang et al., 2010), polymer (Lv et al., 2019), nanoparticles (Al-Marri et al., 2015a; Khan et al., 2015b; Mogha et al., 2017b), and quantum dots (Cao et al., 2010; C.I. Wang et al., 2013) over the graphene oxide surface restrains agglomeration of graphene sheet. Different polymers have been attached to graphene sheets to give electrically conductive, fluorescent, transparent and flexible materials (Batrakov et al., 2014; He et al., 2012; Li et al., 2014). Various interesting structures of graphene–polymer as brushes (Lee et al., 2010), micelle (Choi et al., 2013), sponge (Vickery et al., 2009), etc. have been realized in this regard.

Ruoff et al. (Lee et al., 2010) practiced a simple method for the chemical modification of graphene oxide (GO). In this method Atom Transfer Radicle Polymerization (ATRP) has been used to synthesize polymer brushes over GO nanosheets, without altering the properties of GO.

Metal nanoparticles show excellent catalytic properties considering their very high surface to volume ratio and easy electron transfer capabilities through adsorbed molecules (Astruc, 2008; Astruc et al., 2005; Dhakshinamoorthy and Garcia, 2012). Proper immobilization and stabilization of metal nanoparticles is necessity, as these can decompose under extreme conditions. It can be attained by use of polymer matrix for their uniform distribution and stability (Barkoula et al., 2008; Mangal et al., 2015). Different approaches to form polymer nanoparticle composites have been reported, for instance polystyrene capped Fe nanoparticles (Srikanth et al., 2001), polymerized styrene monomer around commercially available TiO₂ nanoparticles using free radical polymerization technique (Rong et al., 2005). In-situ site specific synthesis of nanoparticles by the reduction of a suitable precursor in the polymer matric itself is preferred over other methods of nanoparticle synthesis. In this regard Mogha et al. prepared gold nanoworms on polymeric brushes synthesized over graphene oxide (Mogha et al., 2017a). Similarly, T. P. Radhakrishnan et al. embedded polygonal shaped silver nanoparticles in poly-vinyl-alcholol (Porel et al., 2005).

In the present work as illustrated in Scheme 1, we report ATRP of Tetrahydrofurfuryl methacrylate (THF) on GO sheets to synthesize poly(tetrahydrofurfuryl methacrylate) (PTHF) brushes. Subsequently La₂O₃ nanoparticles are immobilized on the GO supported PTHF brushes by a simple chemical reduction method to result La₂O₃/PTHF/rGO nanohybrid. We also report utilization of the nanohybrid as a reusable catalyst towards the degradation of standard organic dyes namely RhB, MO, ESY.

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Scheme 1 Schematic illustration of the La₂O₃/PTHF/rGO nanohybrid synthesis and its catalytic application.

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2 Experimental

3 Result and discussion

Fig. 1 portrays the FTIR spectra of GO, PTHF/GO and La₂O₃/PTHF/rGO nanohybrids. Some visible differences are seen in the spectra of PTHF/GO and La₂O₃/PTHF/rGO as compared to GO. For example, peak at 617 cm⁻¹ and 623 cm⁻¹ in PTHF/GO and La₂O₃/PTHF/rGO spectra respectively, is attributed to C—Br stretching which is found absent in GO spectrum. Furthermore, characteristic peak for C⚌O stretch can be seen at 1740 cm⁻¹ in GO and 1717 cm⁻¹ in PTHF/GO (acrylatic carbonyl) spectra but shifted to 1726 in La₂O₃/PTHF/rGO spectrum indicating interaction of La₂O₃ nanoparticles with free carbonyl groups in nanohybrids. Modification of GO surface is indicated by loss of intensity in peak representing —OH stretch at 3100–3400 cm⁻¹ in GO with no significant peaks PTHF/GO suggesting polymer brush are grown using ATRP by modifying hydroxyl groups. Whereas, a small broad peak at 3393 cm⁻¹ in La₂O₃/PTHF/rGO spectra is resulted due to opening of tetrahydrofuran ring to free hydroxyl groups which remained uncoordinated with La₂O₃ nanoparticles. Also, a shift in peak position was noted in peaks at 1619 and 1415 cm⁻¹ in GO to 1570 and 1449 cm⁻¹ in PTHF/GO and to 1562 cm⁻¹ and 1456 cm⁻¹ in La₂O₃/PTHF/rGO respectively. These peaks are attributed to carboxy asymmetrical and symmetrical telescopic vibration of GO. Proton NMR of GO and PTHF/GO has also been shown in Fig. S1 in supplementary material.

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Fig. 1 FTIR Spectra of (a) GO, (b) PTHF/GO, (c) La₂O₃/PTHF/rGO.

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Raman spectra of GO, PTHF/GO and La₂O₃/PTHF/rGO are shown in Fig. 2(a). The characteristic D and G peaks of graphene are due to the breathing mode j-point phonons of A_1g symmetry associated with sp³ – bonded carbon atoms in disordered graphene and first order scattering of E_2g phonons in sp² – bonded carbon atoms. Raman spectra of GO shows sp² bonded carbon atom stretching, G band at 1604 cm⁻¹ and disordered D band at 1357 cm⁻¹. Same bands for PTHF/GO are observed at 1602 cm⁻¹ and 1352 cm⁻¹ for La₂O₃/PTHF/rGO at 1594 cm⁻¹ and 1349 cm⁻¹ respectively. Intensity ratio of D and G band i.e. I_D/I_G ratio reflects disorderness in the material. I_D/I_G ratio for GO, PTHF/GO, and La₂O₃/PTHF/rGO is observed as 0.86, 0.99 and 0.99 respectively. Increase in the I_D/I_G ratio from GO to PTHF/GO is due to increased functionalization because of polymer growth on the GO surface. PTHF/GO and La₂O₃/PTHF/rGO have same I_D/I_G ratio, which indicates that La₂O₃ nanoparticle are attached to the surface of polymer rather than GO.

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Fig. 2 (a) Raman spectra and (b) XRD patterns of (i) GO, (ii) PTHF/GO, (iii) La₂O₃/PTHF/rGO showing D and G bands and their characteristic crystal planes.

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Fig. 2(b) depicts XRD patterns of GO, PTHF/GO and La₂O₃/PTHF/rGO. Characteristic peak for GO at 2ϴ = 10.6°, with an interlayer spacing of 8.25 Å represents plane 〈0 0 1〉 and at 2ϴ = 42.4° plane 〈1 0 0〉. In the XRD pattern of the La₂O₃/PTHF/rGO peak at 2ϴ = 29.6° is attributed to the 〈1 0 1〉 crystal plane of the La₂O₃, with interlayer spacing of 3.01 Å (ICDD data: 00-002-0688). Broad merged peaks inside the dotted circle represents the XRD pattern of PTHF in the nanomaterial along with the 〈0 0 2〉 plane of the reduced graphene oxide.

TEM images in Fig. 3 illustrate the morphological characters of GO, PTHF/GO and La₂O₃/PTHF/rGO. A characteristic wrinkled sheet like morphology of GO is observed in Fig. 3(a). Fig. 3(b) shows the morphology of PTHF/GO where dark colored spherical and irregular shape structures are due to the presence of PTHF on the surface of GO. Fig. 3(c) and (d) presents the magnified TEM images of La₂O₃/PTHF/rGO. Uniformly distributed La₂O₃ nanoparticles are clearly depicted on wrinkled surface of rGO. La₂O₃/rGO TEM micrograph is also portrayed in Fig. S2, showing a very large size La₂O₃ nanoparticles, indicating PTHF polymeric brush helps in reducing the size of La₂O₃ nanoparticles drastically. Inset in Fig. 3(c) shows the particle size distribution of La₂O₃ nanoparticles having average particle size in the range of 20–30 nm. HRTEM image of a single La₂O₃ nanoparticle with attachment to PTHF on GO surface is portrayed in inset of Fig. 3(d). Selected Area Electron Diffraction (SAED) pattern as inset in Fig. 3(d) further demonstrate the formation of La₂O₃ nanoparticles indicated by presence of its different crystal planes. Also, Energy-dispersive X-ray (EDX) pattern of La₂O₃/PTHF/rGO is shown in Fig. S3 and tabulated in Table S1 in supplementary material. Also for the stability studies, HRTEM of reused catalyst is also obtained and represented in Fig. S4. Fig. S4 showing no significant change as compared to pre-reaction catalyst, suggesting uniformity in La₂O₃ nanoparticles distribution remains the same after many cycles of reuse. Along with that, post-reaction catalyst studies in Fig. S5 dipicts the Selected Area Electron Diffraction (SAED) pattern of reused catalyst, prominently showing 〈2 0 1〉 and 〈1 0 1〉 planes of La₂O₃, showing La valency of three. Fig. S6 showing XRD pattern of reused catalyst, indicating the presence of 〈1 0 1〉 plane at 2ϴ = 29.58°, with a hump like peak for PTHF.

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Fig. 3 TEM images of (a) GO, (b) PTHF/GO, (c, d) La₂O₃/PTHF/rGO at different magnifications. Inset images showing La₂O₃ nanoparticle size distribution, and HRTEM image of single La₂O₃ nanoparticle showing attachment with PTHF (red dotted lines).

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Catalytic efficacy of La₂O₃/PTHF/rGO nanohybrid towards the degradation of organic dyes RhB, MO, and ESY in the presence of NaBH₄ is investigated as a model reaction. All the degradation experiments were performed at room temperature and optical absorption was measured against time in UV–Visible spectrometer to monitor reaction kinetics.

Fig. 4(a) illustrates the degradation spectra of RhB by La₂O₃/PTHF/rGO in the presence of NaBH₄. RhB shows its maximum absorbance at 554 nm, which disappeared within 50 s in the presence of NaBH₄ using La₂O₃/PTHF/rGO as catalyst. Kinetics studies of the catalytic reaction with respect to the concentration of the dye are also performed. As the ratios of the RhB concentrations i.e. C_t at time t and C_o at time 0 can be determined by their respective absorbance A_t/A_o, using the following equation $$\frac{\mathit{dCt}}{\mathit{dt}}=\frac{\mathit{dAt}}{\mathit{dt}}=-k\text{app}t\text{or}\ln \left(\frac{\mathit{Ct}}{\mathit{Co}}\right)=\ln \left(\frac{\mathit{At}}{\mathit{Ao}}\right)=-k\text{app}t$$

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Fig. 4 UV–Visible kinetics spectra for the degradation of (a) Rhodamine B (b) Methyl orange (c) Eosine Y. Their corresponding ln(At/Ao) vs. Time plot (d), (e), and (f) respectively in the presence of (i) La₂O₃/PTHF/rGO (ii) NaBH₄ (iii) GO (iv) PTHF/GO (v) La₂O₃/rGO.

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Apparent rate constant was calculated as 26.8 × 10⁻³ s⁻¹, which is better than earlier reports (Ai et al., 2011; Kurtan et al., 2015) by plotting ln(At/Ao) against time as shown as Fig. 4(b). Fig. 4(b) also depicts the ln(At/Ao) plots for our control experiments undertaken using NaBH₄ without catalyst and GO, PTHF/GO, and La₂O₃/rGO as catalyst in the presence of NaBH₄. In all these experiments La₂O₃/PTHF/rGO emerged out as most effective catalyst as not much degradation of RhB is observed in the presence of other catalysts.

Degradation spectra of dye MO is shown in Fig. 4(c), MO displays characteristic absorption peak at 464 nm, which disappears in 40 s due to degradation of MO by La₂O₃/PTHF/rGO in the presence of NaBH₄. When the experiment is conducted in the absence of catalyst no degradation takes place even after long time. Comparative plots of ln(At/Ao) against time for the degradation kinetics study of MO by La₂O₃/PTHF/rGO, GO, PTHF/GO, and La₂O₃/rGO as catalyst in the presence of NaBH₄ and NaBH₄ alone is shown in Fig. 4(d). These ln(At/Ao) plots confirm the high activity of La₂O₃/PTHF/rGO as relative to other control catalysts. Apparent rate constant observed for the MO degradation by La₂O₃/PTHF/rGO is 28.6 × 10⁻³ s⁻¹, more than found in literature (Gupta et al., 2011; Rajesh et al., 2014).

Fig. 4(e) represents the degradation spectra of ESY, where characteristic absorption peak of ESY [517 nm] vanishes after 50 s. This degradation of dye ESY take place by La₂O₃/PTHF/rGO in the presence of NaBH₄. Similar to the degradation kinetic study of previous dyes apparent rate constant was found to be 20.5 × 10⁻³ s⁻¹, which is higher than reported elsewhere (Vidhu and Philip, 2014; Wang et al., 2009). Also, same comparative experiments were performed to check the effects of other factors in the degradation of dye and confirm that La₂O₃/PTHF/rGO has the highest activity among them.

The influence of the amount of the catalyst over the degradation of RhB was also investigated. As depicted in Fig. 5(a), an increase in the catalytic degradation of RhB was observed when amount of increased from 0.01 mg to 0.1 mg. A slow increase in the catalytic degradation of RhB was also observed when catalyst amount is further increased to 0.2 mg. Increase in the catalytic degradation of RhB can be attributed to the increased in number of active sites on catalyst as its amount increases. On further increasing the amount no significant increase in degradation is noted as most of the RhB is degraded in the presence of lesser amount of catalyst; hence no RhB is available for catalyst to degrade.

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Fig. 5 (a) Effect of catalyst concentration on the degradation %, (b) Effect of amount of the NaBH₄ used for the degradation studies of the RhB, (c) Percentage degradation of dyes using 0.1 mg catalyst in the presence of 100 µl NaBH₄ (0.1 M), (d) Recyclability of the catalyst up to 5 cycles.

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Similarly Fig. 5(b) illustrates the effect of NaBH₄ amount on the catalytic degradation of RhB. As previously noted, the results coincide with those obtained in the absence of NaBH₄, with no significant degradation of RhB. Higher amount of NaBH₄ (50–100 µl) lead to faster catalytic degradation of RhB. These results suggest that highly nucleophilic BH₄⁻ anions adsorb on the catalyst surface and hence increase the local electron concentration leading to increase in the degradation of RhB. Higher amount i.e. more than 100 µl is found to be insignificant in increasing the degradation rate of the RhB, as no more substrate remains to be degraded with excess NaBH₄. Thus, about 90% of the RhB degradation is noted with the help of 0.1 mg La₂O₃/PTHF/rGO, in the presence of 100 µl 0.1 M NaBH₄ within 50 s.

Percentage degradation of the dyes was also calculated after a set time. A shown in Fig. 5(c) RhB and Eosine Y get degraded up to 90%, meanwhile MO is degraded to more than 99% with the help of catalyst showing a very high activity of La₂O₃/PTHF/rGO nanohybrid as a catalyst. Fig. 5(d) describes the recyclability of catalyst. La₂O₃/PTHF/rGO catalyst was very easily separated from reaction mixture by centrifugation. Five replicates combined together before centrifuge. It was recovered after each degradation reaction, washed with water and ethanol, dried in vacuum and dispersed in water before reuse for next cycle. Stability of the catalyst was confirmed by hot filtration method as reported previously (Yu et al., 2015). ICP results indicated that no significant amount (58 ppb) of La found in filtrate, correlating with our recyclability results. As presented in Fig. 5(d), La₂O₃/PTHF/rGO was reused in five cycles without any significant change in its degradation efficiency.

High catalytic activity of La₂O₃/PTHF/rGO can be attributed to its very large surface area due to GO and very small size La₂O₃ nanoparticles due to presence of PTHF brushes over its surface for adsorption of dye molecules. NaBH₄ molecules increase the rate of reaction by enhancing electron transfer between the adsorbed molecules and the catalyst. La³⁺ ions can bind to the largely available O- atoms of the PTHF brushes to form uniformly distributed nanosize La₂O₃ nanoparticles which act as catalytic site for degradation. Electrons are donated by BH₄⁻ ions to La₂O₃ nanoparticles through PTHF/rGO composite matrix and lastly taken up by acceptor molecules i.e. organic dyes, which reduce them to their corresponding reduced forms as displayed in Table 1 (Šimšíková et al., 2016; W. Wang et al., 2013). Hydrophilic interactions between these large numbers of O atoms of PTHF along with the π–π interaction of GO with dyes bring them closer to the catalytic centers and hence further enhance the catalytic activity. Schematic in Fig. 6 depicts the proposed mechanism for the catalytic degradation of dyes through La₂O₃/PTHF/rGO.

Table 1 Showing reduced forms of organic dyes resulted from catalytic degradation by La₂O₃/PTHF/rGO.

Organic dyes	Reduced forms
Rhodamine B
Methyl Orange
Eosin Y

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Fig. 6 Schematic representing the proposed mechanism for catalytic degradation of various organic dyes by La₂O₃/PTHF/rGO.

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4 Conclusions

In summary, we have synthesized the La₂O₃/PTHF/rGO nanohybrids having uniform distribution of 20–30 nm sized La₂O₃ nanoparticles on polymeric brushes PTHF modified GO surface. Presence of polymer i.e. PTHF acted as the linker agent, stabilizing agent so that La₂O₃ nanoparticles can have small size for better catalysis. La₂O₃/PTHF/rGO nanohybrids have been employed as the catalyst for the degradation studies of RhB, MO and ESY in water in the presence of NaBH₄, giving excellent catalytic performance. More importantly, as compared to other catalysts for the same reactions La₂O₃/PTHF/rGO is found to be highly active, with excellent reusability. Therefore, an excellent catalytic activity, easy recovery and reusability make this catalyst a good candidate for the waste water remediation.