Photocatalytic activity of Gd2O2CO3·ZnO·CuO nanocomposite used for the degradation of phenanthrene
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Multimetal oxides nanocomposite photocatalysts based on Gd2O2CO3·ZnO·CuO were prepared by a co-precipitation method and carefully characterized using a range of analytical techniques. More specifically, analysis by X-ray diffraction and electron microscopies confirmed the identity and quality of the as-synthesized powders. The photocatalytic degradation activities of these nanocomposites towards phenanthrene were then investigated by measuring the effects of catalyst dosage, irradiation time, and oxidant addition. In addition, the pseudo first-order kinetic model was used to determine the rate constant of the degradation reaction. Optimum dosages of 0.6, 0.6, and 0.4 gL−1 were recorded when using CuO, Cu–CuO/ZnO, and Gd2O2CO3·ZnO·CuO, respectively. In addition, the Gd2O2CO3·ZnO·CuO composite exhibited a higher removal efficiency than both Cu–CuO/ZnO and the pure CuO nanoparticles. Furthermore, the addition of oxidants influenced the removal of phenanthrene from solution. Finally, the photocatalytic degradation data followed pseudo first-order kinetics as defined by the Langmuir–Hinshelwood model, which allowed prediction of the faster degradation rate by the Gd2O2CO3·ZnO·CuO nanocomposite. The newly synthesized nanocomposite could therefore be considered for the removal of phenanthrene and related polycyclic aromatic hydrocarbons from contaminated water.
KeywordsMetal oxide Nanocomposite Photocatalytic degradation Kinetic model Polycyclic aromatic hydrocarbons
Water pollution is currently a major problem worldwide partly due to the discharge of various contaminants into rivers, examples of such contaminants include the polycyclic aromatic hydrocarbons (PAHs), dyes, and toxic inorganic ions [1, 2, 3, 4]. PAHs are common components in the coal, petroleum, and oil industries [5, 6]. One such PAHs is phenanthrene, which is also found in cigarettes and has been reported to lead to cardiovascular disease [7, 8]. Although several techniques have been employed for the removal of organic pollutants from wastewater, complete removal has yet to be achieved [9, 10, 11, 12]. In addition, a number of these techniques are undesirable, as they create sludge by-products and require a significant energy input to maintain high pressures [13, 14, 15]. However, photocatalytic degradation is a promising technique for the removal of PAHs because of its low cost, fast degradation rate, and environmental friendliness . As such, the potential of photocatalytic degradation to remove phenanthrene from aqueous solution will be investigated in this study. For this purpose, ZnO will be considered as the parental photocatalyst, due to its high chemical stability, low cost, and its relatively high quantum efficiency . However, metal oxides such as ZnO are active only in the ultraviolet region and exhibit a moderate performance. As such, the development of novel materials with reduced band gap energies has been investigated to increase the response to the abundant visible light photons [18, 19]. For example, CuO is a chemically stable p-type metal oxide with a band energy gap of 1.2–1.8 eV [20, 21]. As such, metallic Cu, CuO, and the corresponding complexes are well-established catalysts for the transformation of various chemicals into valuable products [22, 23, 24]. Due to its low energy band gap, CuO is often employed as a co-catalyst in combination with large band gap energy catalysts such as TiO2 and ZnO to increase the photocatalytic rate under visible light . Indeed, ZnO/CuO composites have been reported to exhibit improved charge carrier separation and a decreased rate of recombination, which in turn improves the photodegradation efficiency . It has been reported that the formation of p–n heterocoupling at the CuO/ZnO interface can lead to superior charge separation and thus an increased photocatalytic activity . Furthermore, the photocatalytic activity of CuO/ZnO can be increased by the introduction of Cu nanoparticles (NPs) on the surface. To the best of our knowledge, the photocatalytic activity of the Cu–CuO/ZnO composite for the degradation of phenanthrene has not yet been investigated. The amendment of the physicochemical properties of semiconductors through doping and heterostructures with rare earth metal/metal oxide for the improvement of their photocatalytic activities have been previously reported [28, 29]; it is suggested that the increased photocatalytic activities of gadolinium incorporated semiconductor is mainly due to the interfacial charge transfer of its 4f shells and the elimination of electron–hole recombination. Hence, the nanocomposite (NC) of CuO/ZnO with gadolinium was considered for improved photodegradation of phenanthrene in water.
Thus, we herein report the synthesis of CuO, Cu–CuO/ZnO nanocomposite (NC), and Gd2O2CO3·ZnO·CuO NC, followed by comparison of their photocatalytic activities towards phenanthrene photodegradation. The obtained products will then be characterized using a range of techniques, including Fourier transform infrared (FTIR) spectroscopy, ultraviolet–visible (UV–vis) spectroscopy, scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and X-ray diffraction (XRD). The kinetics of the photocatalytic activity will be studied by measuring the removal rates of different photocatalysts. Therefore, the objectives of this work are to synthesize multimetal oxide heterocatalysts to improve the excitability of ZnO and CuO under visible light, and enhance the charge carrier mobility for the degradation of phenanthrene.
2 Experimental Section
Copper nitrate trihydrate (> 99%), zinc acetate dihydrate (≥ 98%), copper acetate (98%), gadolinium nitrate hexahydrate (99.9% trace metals basis), and phenanthrene (98%) were obtained from Sigma-Aldrich, South Africa and used directly without any further purification.
2.2 Preparation of CuO, ZnO/CuO, and Gd2O2CO3·ZnO·CuO
Copper oxide nanoparticles (CuO NPs) were prepared using an environmentally friendly synthetic method based on the use of banana peel as a natural source serving as structure-controlling agent. More specifically, a sample of ripe banana peel (20 g) was weighed and washed with ethanol. The banana peel was then cut into pieces of 2 mm2, added to distilled water (40 mL), and heated at 80 °C for 15 min. Following filtration, a portion of the resulting extract (30 mL) was added to a reaction vessel and heated at 80 °C with constant stirring. Copper nitrate trihydrate (1 g) was added to this hot banana peel extract. This mixture was refluxed and the resulting precipitate was transferred to a crucible and heated in a furnace at 400 °C for 3 h to yield a black powder. For synthesis of the Cu–CuO/ZnO NC, the desired quantities of zinc acetate dihydrate and copper acetate (1:1 ratio) were mixed and ground using mortar pestle. The obtained powder was transferred to an alumina crucible and annealed at 350 °C for 3 h under air in a muffle furnace. The multimetal oxide (Gd2O2CO3·ZnO·CuO) composite was prepared via a co-precipitation method similar to that reported by Subhan et al. , with the exception that gadolinium nitrate hexahydrate was used instead of lanthanum nitrate.
The morphology of the photocatalysts was examined by SEM (VEGA SEM, TESCAN), and the chemical compositions were determined by X-ray energy dispersive spectroscopy (EDS) coupled with SEM. The interior characterization of composite was performed using a high resolution transmission electron microscope (HRTEM, JEOL JEM-2100, 200 kV). XRD data for the prepared samples were recorded using a Philips PANalytical X’Pert PRO PW 3040/60 X-ray diffractometer with Cu-Kα radiation (λ = 0.15418 nm). UV–vis absorbance spectra were recorded using a Shimadzu UV-2401PC spectrophotometer. FTIR spectra were recorded on a Perkin Elmer Spectrum 100 FTIR spectrophotometer.
2.4 Evaluation of the photocatalytic activities of the prepared photocatalysts
The photocatalytic degradation of phenanthrene was investigated in a photocatalytic chamber using the prepared photocatalysts (i.e., CuO, ZnO/CuO, and Gd2O2CO3·ZnO·CuO). A UV filter was utilized to cut off wavelengths < 400 nm. All experiments were performed by suspending the photocatalysts in the reactor containing a phenanthrene solution (20 ppm), and the reactions were carried out at 25 °C in the photocatalytic chamber. After the desired time interval (20 min), the concentration of residual phenanthrene in each solution was measured by recording the absorbance intensity of the solution at a maximum absorbance–wavelength of 271 nm. The phenanthrene photodegradation efficiency of each photocatalyst was then calculated using Eq. (1) :
3 Results and discussion
3.1 Structural and morphological characterization
3.2 Photocatalytic degradation of phenanthrene
3.2.1 Effect of photocatalyst loading
3.2.2 Effect of illumination time
3.2.3 Effect of oxidant addition
The thiosulfate ion (S2O82−) reacts with available electron from conduction band (CB) and produces highly active SO 4 ·− ·SO 4 ∙− is a very strong oxidizing agent (E° = 2.6 eV). Later, these radicals might react with water molecules and generate hydroxyl radicals. As such, these generated active radicals might participate in photodegradation of phenanthrene into less harmful products.
3.2.4 Kinetic analysis
Upon irradiation of light, all metal oxides can be excited to produce the photoinduced electrons and holes. Due to establishment of force of electric field, the photogenerated electrons are shifted to conduction band (CB) of p-type CuO NPs to the CB of n-type ZnO. This leads to efficient separation of the photo-generated charge carriers (electrons and holes) at the CuO/ZnO heterojunction interface, resulting to lower recombination rate . The produced charge carriers are moved to nanocomposite surface where the holes participate to convert water molecules to hydroxyl radicals (·OH) and electrons are used by dissolved oxygen to form superoxide anion radicals (·O2−). These reactive oxygen species (ROS- ·O2−, ·OH, OH−) might facilitate the photodegradation of phenanthrene to generate less harmful degraded products [11, 36, 43, 44]. The ROS can attack the reactive positions (i.e., the 9 and/or 10 positions) of phenanthrene and disturb the electron arrangement of the phenanthrene aromatic rings, which results in the formation of intermediate products such as alcohol, ketone, and aldehyde derivatives . These intermediate products can then be converted into stable and less harmful products, including carbon dioxide and water. Thus, the enhanced photocatalytic properties of the Gd2O2CO3·ZnO·CuO NC can possibly be attributed to reduced electron–hole recombination rates due to the photogenerated electrons from CuO being easily transported on the multidirectional Gd2O2CO3 and ZnO and high light absorption.
We herein reported the successful synthesis of multimetal Gd2O2CO3·ZnO·CuO nanocomposite (NC) based photocatalyst via a simple co-precipitation method. Pure CuO nanoparticles (NPs) were synthesized via an environmentally friendly method based on the use of banana peel as structure-controlling agent. In addition, electron microscopies (i.e. SEM, TEM) and XRD study confirmed that the Gd2O2CO3·ZnO·CuO NC contained ZnO and CuO particles anchored onto the composite surface. In terms of the photocatalytic activities of the various prepared photocatalysts, the Gd2O2CO3·ZnO·CuO NC exhibited a superior photocatalytic activity in the degradation of phenanthrene, likely due to reduced electron–hole recombination rates and the production of large amount of reactive oxygen species. The developed multimetal oxides Gd2O2CO3·ZnO·CuO NC photocatalyst gave almost 100% photodegradation of a 20 (mg/L) phenanthrene solution over 180 min. It was also observed that the degradation of phenanthrene by Gd2O2CO3·ZnO·CuO NC was adequately predicted by the Langmuir–Hinshelwood kinetic model. We therefore expect that the developed multimetal oxide NCs will be applicable for the treatment of other emerging pollutants to produce stable and less harmful products. Further studies will focus on this potential application, and on detailed mechanistic studies regarding the degradation of phenanthrene using the system described herein.
The authors are grateful to the sponsor from the North-West University and the National Research Foundation (NRF, Grant 94152) in South Africa. Any opinion, findings and conclusions or recommendations expressed in this material are those of the authors and therefore the NRF does not accept any liability in regard thereto. The authors appreciate the assistance of Mr Nico Lemmer from the North-West University.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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