Graphene oxide with zinc partially substituted magnetite (GO–Fe_1−xZn_xO_y) for the UV-assisted heterogeneous Fenton-like reaction

Abstract

A series of graphene oxide (GO) and zinc partially substituted magnetite GO–Fe_1−xZn_xO_y (0 ≤ x ≤ 0.285) catalysts were synthesised through a precipitation-oxidation method. The rate constants for the degradation of acid orange seven (AO7) proceeded at a significant faster rate under UV-irradiation (up to 670%) than the conventional heterogeneous Fenton-like reaction. The resultant catalysts were mesoporous, so there was no mass transfer limitation for AO7 to access active sites in the catalysts. Further, maximum increases of rate constant up to 220% occurred as the zinc molar concentration increased from x = 0 to x = 0.159. GO enhanced to incorporation of zinc into the combined metal oxide, whilst zinc limited crystal growth, thus forming smaller crystallite sizes. These features proved to be essential for the improved catalytic activity of the resultant catalysts. The optimised zinc molar value at x = 0.159 delivered the best catalytic activity.

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1. Introduction

The heterogeneous Fenton-like reaction is of great interest for the degradation of persistent organic compounds using GO supported iron oxide nanocomposites, mainly due to their high surface area and decent catalytic properties.^1–5 Compared to other types of iron oxide, magnetite (Fe₃O₄) offers higher catalytic activity with several interesting structural features.^6,7 First, the presence of Fe²⁺ occupying on the octahedral site of Fe₃O₄ plays a key role as electron donor to initiate decomposition of hydrogen peroxide (H₂O₂) into hydroxyl radicals (HO˙) following the Haber–Weiss mechanism.^7,8 Second, magnetite accommodates both Fe²⁺ and Fe³⁺ on the octahedral sites, allowing the Fe species to be reversibly oxidised and reduced while keeping the structure unchanged. Third, the Fe cations in the Fe₃O₄ structure can be substituted with different types of transition metal cations which can significantly affect the microstructure, physicochemical properties and catalytic activity of the resulting materials.^9–11

Moreover, recent studies have revealed that the isomorphic substitution of Fe₃O₄ with Co,^12,13 Mn,^9,14 Ti,^15,16 Cr,¹⁷ and V^15,18 have significantly enhanced its catalytic activity in various reaction. The exception is the substitution of Fe₃O₄ with Ni,¹⁹ which led to a decrease in catalytic activity as Ni²⁺ were mainly substituted with Fe²⁺ within the structure of Fe₃O₄. These variations were greatly dependent on the type of dopants, their concentration and occupancy of substitution sites which stimulates an effective generation of HO˙ radicals during catalysis. Nevertheless, the role played by GO in the UV-assisted Fenton-like reaction, when coupled with the heterogeneous Fenton-like catalyst (e.g. magnetite) and the photoluminescence catalyst (zinc oxide), is unknown. Therefore, this work investigates the influence of GO in the coupling with Fe₃O₄ and zinc oxide for the oxidative degradation of persistence organic compounds which remains unaddressed in the open literature.

This work shows for the first time the investigation of a series of zinc partially substituted magnetite with different concentrations being immobilised onto graphene oxide sheets as GO–Fe_1−xZn_xO_y. Of particular interest, the effect of zinc on the physicochemical and catalytic properties of GO–Fe_1−xZn_xO_y was systematically investigated by varying the zinc molar value 0 ≤ x ≤ 0.285. The catalytic performance of GO–Fe_1−xZn_xO_y was evaluated using a model reaction of AO7 oxidative degradation in the UV-assisted and conventional heterogeneous Fenton-like reactions.

2. Experimental

FeCl₃·6H₂O (97%), FeCl₂·4H₂O (99%), Zn(NO₃)₂·6H₂O (98%), NH₄OH (30 wt% NH₃), H₂O₂ (30%, w/w) and AO7 (orange II; 85%) were purchased from Sigma-Aldrich. All chemicals were of analytical grade and used as received except for graphite oxide which was prepared by modified Hummers's method.^20,21 A series of composite GO/iron/zinc oxide catalysts with a zinc molar ratio (z) ranging from 0 to 0.4 were synthesised through a precipitation-oxidation method in the presence of graphene oxide (GO). Briefly, 62 mL of graphite oxide suspension (0.85 mg mL⁻¹) was exfoliated under ultrasonication for 1 h to obtain an aqueous dispersion of GO. Typically for a sample z = 0.2, 8.64 mmol of FeCl₃·6H₂O, 4.32 mmol of FeCl₂·4H₂O and 0.86 mmol of Zn(NO₃)₂·6H₂O were dissolved in 200 mL deionised water. After stirring for 30 min, this solution was heated to 90 °C and NH₄OH solution was added dropwise into the heated solution until pH 4 was reached under constant stirring. This was followed by the gradual addition of GO suspension into the heated mixture and continuously stirred for another 30 min until homogeneously mixed. The iron and zinc precursor molar fractions were slightly changed for the preparation of samples with other z values.

Subsequently, an appropriate amount of NH₄OH solution was continuously added dropwise into the mixture until the pH reached a value of 11. The mixture was cooled to room temperature after being aged for 1 h at 90 °C under constant stirring. The resulting black precipitate was magnetically separated and washed three times with deionised water and ethanol and then dried at 60 °C for 48 h. For comparison, the same procedure was employed to synthesise zinc partially substituted magnetite with different zinc molar ratio (z = 0, 0.1, 0.2, 0.4) in the absence of GO solution.

The resultant materials were characterised by nitrogen sorption using a Tristar II 3020 (Micromeritics). The specific surface area and pore volume were determined using Brunauer–Emmett–Teller (BET) equation. The pore size distribution curves were calculated using non-local density functional theory (NLDFT), from the desorption branch of the isotherms. The XRD patterns were obtained using a Rigaku Smartlab X-ray diffractometer at 45 kV, 200 mA with a step size of 0.02° and speed of 4° min⁻¹ using a filtered Cu Kα radiation (λ = 1.5418 Å). Surface analysis was performed on a Kratos Axis ULTRA X-ray photoelectron spectrometer (XPS) equipped with monochromatic Al Kα (hν = 1486.6 eV) radiation. The curve fitting was carried out using a Gaussian–Lorentz peak shape and Shirley background function. The binding energy was calibrated versus the carbon signal at 284.6 eV. The high resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscope-energy dispersive X-ray spectrometer (STEM-EDS) elemental mappings were performed on JOEL 2010 operating at 200 kV, equipped with an energy dispersive X-ray (EDS) detector. Samples were prepared by placing a drop of diluted sample dispersion in ethanol onto a carbon-coated copper grid and air-dried prior to examination.

The catalytic performance of the catalysts was tested in the UV assisted heterogeneous Fenton-like reaction for the oxidative degradation of AO7. The experiments were carried out in a custom-made photo reactor equipped with 8 W UV-A lamps (Sylvania Blacklite F8 W/BL350, 330< λ < 370 nm) as described elsewhere.²² A 200 mL quartz tube was placed in the centre of the reactor, which was surrounded by the 4 UV-A lamps fitted to the wall of a cylindrical lead-line chamber in concentric arrangement. The distance of quartz tube was fixed at 10 cm from the UV lamps. In a typical experiment, 20 mg of the catalysts were added into the quartz tube consisting of 100 mL of 0.1 mM AO7 at initial pH solution of 3. The mixed suspension was kept under constant air bubbling for 30 min of dark adsorption. The reaction was initiated by turning on the UV lamps after the addition of H₂O₂ (22 mM) into the suspension. Sampling was carried out periodically at selected time interval. The collected suspension was then filtered through 0.2 μm Milipore syringe filters and immediately analysed. The concentration of AO7 was analysed by an Evolution 220 UV-Vis spectrophotometer (Thermo Fisher Sci.) at 484 nm. In order to compare the performance of the catalysts in the UV-assisted reactor, the control reaction was carried out in a conventional heterogenous Fenton-like reaction under same reaction conditions except of UV exposure.

3. Results and discussion

The synthesis of the starting catalysts was based on the variation of the zinc molar ratio (z). Table 1 displays the calculated values from XPS spectra to demonstrate the zinc molar value (x) actually incorporated into the mixed oxide. In this work, the general formula Fe_1−xZn_xO_y for mixing oxides is used. In the case of oxygen, the notation (y) is used as the oxygen molar value is generally below 4 by mixing iron and zinc oxides. Table S1 (ESI†) shows that the Zn/Fe fractions for the GO–Fe_1−xZn_xO_y samples are very close to the starting materials synthesis z values based on XPS results. Contrary to this, the Fe_1−xZn_xO_y samples Zn/Fe fractions were much lower under the synthesis conditions used in this work. These results clearly indicate that GO played an important role to enhance Zn incorporation into the mixed oxides.

Table 1 Zinc molar values (x) based on XPS elemental composition values

(z) starting Zn^2+	GO-Fe1−xZnxOy		Fe1−xZnxOy
	(x) XPS for Zn^2+	(1 − x) XPS for Fe(^2+,3+)	(x) XPS for Zn^2+	(1 − x) XPS for Fe(^2+,3+)
0	0	1	0	1
0.1	0.090	0.910	0.001	0.009
0.2	0.159	0.841	0.020	0.980
0.4	0.285	0.715	0.167	0.833

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Fig. 1a shows that the addition of GO was beneficial in terms of improving surface area, an important parameter in reaction engineering. For instance, GO–Fe_1−xZn_xO_y surface areas (270–310 m² g⁻¹) were 68–130% larger than those of Fe_1−xZn_xO_y (117–185 m² g⁻¹). The pore volumes for both samples (Fig. 1b) slightly decreased from 0.32 to 0.23 cm³ g⁻¹ and 0.29 to 0.21 cm³ g⁻¹ for Fe_1−xZn_xO_y and GO–Fe_1−xZn_xO_y as z was raised towards 0.4. As the actual amount of zinc x incorporated into the compounded increased concomitantly, there is a clear indication that increasing the amount of zinc led to denser structures. These results suggest the formation of different structures with and without GO. Fig. 1c and d show that both materials have pore size distribution (PSD) in the mesoporous region, though Fe_1−xZn_xO_y has a much broader PSD aspect which is mainly associated with inter-particle space. In both cases, the PSD was narrowed as the zinc molar ration increased from 0 to 0.2.

Fig. 1 (a) BET surface area, (b) pore volume, and pore size distributions of (c) Fe_1−xZn_xO_y and (d) GO–Fe_1−xZn_xO_y.

Further, GO–Fe_1−xZn_xO_y consisted of more uniform and narrower PSD within the range of 1–10 nm (Fig. 1d). The addition of GO and in situ growth of Fe_1−xZn_xO_y particles onto the GO sheets during the synthesis also induced the formation of micropores (<2 nm). This can be explained by the integration of a high aspect ratio two-dimensional (GO sheets) and zero-dimensional particles into a single material.^2,23 By coupling the high surface areas and narrower PSD, the structural features of GO–Fe_1−xZn_xO_y are therefore attributed to the intercalation of GO within the Fe_1−xZn_xO_y particles.

The XRD patterns of GO–Fe_1−xZn_xO_y are displayed in Fig. 2a. The diffraction peaks at the 2θ values of 30.2°, 35.6°, 43.3°, 53.6°, 57.4° and 63.1° were assigned to the (220), (311), (400), (422), (511) and (440) crystal planes of Fe₃O₄ with spinel structure (JCPDS no. 19-0629). A slight peak shift of (311) planes towards lower angle was observed with increasing the x values to 0.285. This suggests variations in the resultant crystal structure possibly due to the effect of zinc partial substitution into the Fe₃O₄ spinel structure. There was no visible secondary phase or impurity peaks, thus clearly confirming that zinc was isomorphically substituted into the Fe₃O₄ crystal structure, in good agreement with reports elsewhere.^24,25

Fig. 2 (a) XRD patterns of GO–Fe_3−xZn_xO_y, and (b) crystallite sizes of Fe_1−xZn_xO_y () NPs and GO–Fe_1−xZn_xO_y () samples at actual zinc molar values x.

Fig. 2b shows the crystallite sizes of the main peak at (311) planes of both samples calculated according to Scherrer's equation. Apart from a scatter for x = 0.1, the crystallite size of both samples reduced with increasing x. In principle, these results are indicating that zinc is limiting crystal growth, as the crystallite size reduced by 30% (10.08 to 6.99 nm) for Fe_1−xZn_xO_y and by 40% (7.39 to 4.43 nm) for GO–Fe_1−xZn_xO_y. In addition, the GO containing samples also resulted in smaller crystallite sizes ∼32% than non-GO samples, excluding the scatter variation of 56% for x = 0.1. These results therefore suggest that GO has further inhibited crystal growth. This could be attributed to the GO intercalation between particles, thus providing a hindrance effect of the particles to agglomerate and coalesce further.

The XPS analyses of GO–Fe_1−xZn_xO_y in Fig. 3a and b confirm the spin–orbit doublet centred at 1021.7 and 1044.8 eV were corresponded to the Zn 2p_3/2 and Zn 2p_1/2 (except x = 0), respectively. The observed spin–orbit splitting between these two peaks was about 23 eV in line with those reported values of Zn²⁺ state.^26–29 Fig. 3c displays the peaks centre at 711.1 and 724.6 eV in the high resolution Fe 2p scan which were assigned to Fe 2p_3/2 and Fe 2p_1/2 of Fe₃O₄.^2,30–32 The peak of Fe 2p_3/2 shift towards higher binding energy from 711.1 to 711.9 eV and became less intense as x increased. These differences can be explained by the changes in the electronic state of Fe and the Fe–O bond after zinc was partially substituted into Fe₃O₄.

Fig. 3 Wide scan XPS spectra of (a) high resolution spectra of (b) Zn 2p and (c) Fe 2p of GO–Fe_1−xZn_xO_y at different zinc molar value x.

Fig. 4 shows the HRTEM images of the GO–Fe_1−xZn_xO_y. There were no significant differences in the morphology in both nanocomposites materials with x = 0 and x = 0.159. It was found that large amounts of particles were dispersed throughout the surface of GO sheets (Fig. 4a and c). From the magnified images in Fig. 4b and d, it was identified spherical particles with diameters of about 10–15 nm anchored on GO sheets. Furthermore, the inset HRTEM images revealed that the interplanar spacing of the lattice fringes of d = 0.25 nm, which corresponds well to the (311) planes of both Fe₃O₄ and Fe_1−xZn_xO_y as measured at 2θ = 35.69° by XRD (Fig. 2). However, the latter slightly overlaps with the peak (101) at 2θ = 36.14° assigned to ZnO with interplanar spacing of d = 0.264 nm. The presence of zinc was examined by STEM-EDS elemental mapping. As displayed in Fig. 5a, b and d, e, the elements of Fe and O were detected and dispersed throughout the surface of both materials, while the zinc was only observed as white dots in Fig. 5f (GO–Fe_1−xZn_xO_y, x = 0.159) but not in the sample without zinc as shown in Fig. 5c (GO–Fe_1−xZn_xO_y, x = 0). Thus, these results further confirm the presence of zinc in GO–Fe_1−xZn_xO_y.

Fig. 4 HRTEM images of GO–Fe_1−xZn_xO_y at (a, b) x = 0 and (c, d) x = 0.159.

Fig. 5 STEM images of GO–Fe_1−xZn_xO_y with their corresponding EDS elemental mapping at x = 0 (a) Fe, (b) O and (c) Zn; and x = 0.159 (d) Fe, (e) O and (f) Zn.

Oxidative degradation of AO7 in the presence of H₂O₂ was used as a model system to evaluate the catalytic activity of the synthesised samples. Initially, the samples were analysed for the conventional heterogeneous Fenton-like reaction in order to investigate whether the partial substitution of zinc was beneficial or detrimental towards the oxidative degradation of AO7 as displayed in Fig. 6. These results clearly indicate that altering the amount of zinc in the Fe_1−xZn_xO_y did not affect their catalytic activity, which remained constant at ∼30% (±2%) as the x values varied from 0 to 0.167. However, a pronounced increase in the catalytic performance occurred when GO was incorporated as a catalyst support, forming GO–Fe_1−xZn_xO_y. The removal efficiency of AO7 increased from 50% at x = 0 and peaked at 72% for x = 0.159, followed by a decline in removal efficiency to ∼52% for x = 0.285. The enhancement of the catalytic performance is clearly attributed to GO.

Fig. 6 The AO7 removal efficiency by (a) GO–Fe_1−xZn_xO_y and (b) Fe_1−xZn_xO_y at different zinc molar value (x) in the conventional heterogeneous Fenton-like. Experimental conditions: AO7 35 g L⁻¹, H₂O₂ 22 mM, catalyst 0.2 g L⁻¹, T = 25 °C, pH 3 and 180 min of reaction.

Further evaluation of the catalytic performance of the materials was carried out in the UV-assisted heterogeneous Fenton-like reaction. Fig. 7a displays representative UV-Vis spectra collected from an initial (0 min) to a final (120 min) testing condition. The disappearance of all peaks at 120 min of reaction strongly suggest the destruction of azo bond (–N=N–) in the chromophoric structure of AO7.³³ It was found a relative good improvement from ∼30 to 52% of the catalytic activity for the Fe_1−xZn_xO_y (x = 0.02) catalyst as the reaction was switched from the conventional to the UV-assisted. These results show that zinc has improved the UV-assisted catalytic activity of the compound without GO, though the AO7 degradation efficiency is relatively low. However, a complete new set of results was observed for GO–Fe_1−xZn_xO_y. Fig. 7b shows a significant enhancement in efficiency within the first 60 min UV-irradiation as the AO7 degradation increased to 65% (x = 0) and peaked at 80% (x = 0.159). This clearly confirms the photoluminescence effect of zinc, leading to a faster catalytic activity for the partially substituted zinc catalysts. Further, the AO7 degradation went almost to completion at ∼98% at 120 min for all samples tested under UV-assisted conditions.

Fig. 7 The AO7 removal efficiency by GO–Fe_1−xZn_xO_y in (a) UV-Vis spectra and (b) the UV-assisted heterogeneous Fenton-like reaction.

Further analyses of results suggest that the oxidative degradation of AO7 using GO–Fe_1−xZn_xO_y fitted well the pseudo-first-order reaction kinetics (R² > 0.93). The rate constants in Fig. 8 clearly show that the AO7 degradation proceeded at a faster rate under UV-irradiation than the conventional heterogeneous Fenton-like reaction. The rate constants in Fig. 8 clearly show that the AO7 degradation proceeded at a faster rate under UV-irradiation than the conventional heterogeneous Fenton-like reaction, with significant increases in the order of 378 to 670%. Further maximum increases of rate constant 220% occurred as the zinc molar concentration increased from x = 0 to x = 0.159. These results are significant and clearly indicate that the photo-Fenton degradation of AO7 occurs at a much faster pace by combining zinc partially substituted magnetite with GO. Further, it is also noteworthy the improved photo-response of GO–Fe₃O₄ (GO–Fe_1−xZn_xO_y, x = 0) under UV-irradiation, which is mainly attributed to GO.

Fig. 8 Rate constant (k) for AO7 removal efficiency by GO–Fe_1−xZn_xO_y in the conventional and UV-assisted heterogeneous Fenton-like reaction.

The resultant catalysts delivered major improvements under the UV-assisted over the conventional heterogeneous Fenton-like reactions as evidenced by the results in Fig. 7 and 8. In principle, this is attributed to the larger surface areas ∼270–310 m² g⁻¹ of GO–Fe_1−xZn_xO_y as compared to 117–185 m² g⁻¹ for Fe_1−xZn_xO_y. Further, the crystallite growth was limited mainly by GO and marginally by zinc, thus forming smaller crystallite sizes which proved to be essential for the improved catalytic activity of the resultant-catalyst. It is noteworthy that the highest catalytic activity was found to be at zinc molar value of x = 0.159 for both non-UV and UV assisted heterogeneous Fenton-like reaction (Fig. 8). There are no significant variations in surface areas (Fig. 1a) as a function of zinc molar value (x) whilst there is a small reduction in pore volume (Fig. 1b). However, the resultant samples are mesoporous (Fig. 1c and d) in nature, so there is no mass transfer limitation for AO7 to access active sites in the catalysts. Therefore, the catalytic activity peaking at x = 0.159 may be associated with the morphological change in the reduction of the crystallite size (Fig. 2b) coupled with the optimised zinc and magnetite molar ratio.

In order to explain the superior performance of the GO–Fe_1−xZn_xO_y under the UV-assisted heterogeneous Fenton-like reaction, we propose a schematic of the reaction pathway as presented in Fig. 9. When the semiconductor catalyst is irradiated by UV light, a pair of photo-generated electrons holes are generated.^34,35 In the presence of GO as the catalyst support, the holes are likely to be transferred from the valence band (VB) of the catalyst (Fe_1−xZn_xO_y) to the highest occupied molecular orbital (HOMO) of graphene oxide attributed to the lower catalyst's VB position as compared to GO's HOMO.³⁶ Meanwhile, the photo-generated electrons can only stay at the conduction band (CB) of the catalyst and participates in the surface catalysis to form radicals because of the CB position is also lower than the lowest unoccupied molecular orbital (LUMO) of GO.³⁷ Owing to these reasons, GO played an important role in the UV-assisted Fenton-like reaction by inducing an effective charge separation during catalysis, supported by improved catalytic activity.

Fig. 9 Schematic of the UV-assisted heterogeneous Fenton-like reaction.

4. Conclusions

A series of GO–Fe_1−xZn_xO_y catalysts with varying zinc concentration were prepared through a precipitation-oxidation method in the presence of GO. The zinc partial substitution into the Fe₃O₄ was confirmed by the combined characterisation using XRD, XPS, and STEM-EDS analysis. There was no visible formation of secondary phase or impurity peaks, indicating the partial substitution of zinc into the Fe₃O₄ crystal structure with a good dispersion within the Fe₃O₄ matrix. The reaction kinetics peaked at zinc molar value (x) of 0.159 for GO–Fe_1−xZn_xO_y towards the AO7's oxidative degradation in both conventional and UV-assisted heterogeneous and Fenton-like reactions. This improvement was associated with GO enhancing the incorporation of zinc into the mixed oxide matrix, coupled with the morphological features such as the reduction of the crystallite size coupled with the optimised zinc to magnetite molar ratio. Nevertheless, it was found that GO was a major player as it significantly increased the oxidative degradation of AO7 and also the reaction kinetics, thus conferring faster regeneration of the Fenton active species in producing more HO˙ radicals for the degradation of AO7 under both heterogeneous and UV-assisted Fenton-like reactions.

Acknowledgements

The authors acknowledge the facilities and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy and Microanalysis, The University of Queensland. N. A. Zubir gratefully acknowledges the generous financial support from Ministry of Higher Education Malaysia (MOHE) and Universiti Teknologi MARA (UiTM) for her study leave. J. C. Diniz da Costa acknowledges support via the Australian Research Council Future Fellowship Program (FT130100405).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04068c

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