Visible light functioning photocatalyst based on Al2O3 doped Mn3O4 nanomaterial for the degradation of organic toxin
© Asif et al. 2015
Received: 17 February 2015
Accepted: 24 June 2015
Published: 9 September 2015
Al2O3 doped Mn3O4 nanomaterial was synthesized by low-temperature stirring method and applied as a catalyst for the degradation of organic pollutants under solar light for prospective environmental applications. The morphological and physiochemical structure of the synthesized solar photocatalyst was investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). FESEM showed a mixture of nanowires and aggregated nanoparticles. This Al2O3 doped Mn3O4 nanomaterial exhibited high solar photocatalytic degradation in a short time when applied to brilliant cresyl blue (BCB). Thus, the synthesized nanoparticles can be used as an efficient solar photocatalyst for the degradation of BCB.
Recent industrial development has improved living standards but due to insufficient environmental monitoring, the continuous discharge of massive industrial pollutants (organic compounds, trace metals, etc.) is creating serious environmental problems. Since these pollutants are carcinogenic at trace levels for aquatic and non-aquatic organisms, a wide range of techniques including physical, chemical, and biological methods have been developed for water treatment [1–5]. Environmental degradation due to various types of pollutants has led to global interest in the vital and robust technology known as “nanotechnology.”
In particular, the photocatalytic oxidation process using heterogeneous photocatalysis is regarded as a promising technology to decompose harmful pollutants into final non-toxic products [6–8]. However, most photocatalysts can only be activated under UV-light irradiation because of their large band gap, resulting in low photo-electronic transition efficiency since the ultraviolet light represents only 4 % of the solar spectrum . Therefore, it is necessary and desirable to develop visible light-driven photocatalysts with high efficiency for the degradation of environmental pollutants .
Nanostructured transition metal oxides have been considered important materials because of their electronic, optical, photonic, and catalytic properties. Their size reduction to nanoscale can effectively change their physical and chemical properties and specifically improve their potential . Since they have efficient EMR absorption in the visible region, many metal oxides and doped metal oxides have been recommended for the photocatalytic degradation of organic pollutants. Photocatalytic degradation of azo dye in water was effectively carried out with ZnO. Similarly, acid red B dye was degraded by using TiO2. Further studies reported the photocatalytic degradation of methyl orange by zinc ferrite-doped titania. Semiconductor iron (II) oxide has also been researched in photocatalytic bleaching of dyes. Furthermore, a large number of other systems such as transition metal-doped TiO2 and nitrogen-doped TiO2 have been utilized for the photosensitization of dyes .
Manganese oxides have been extensively studied as a well-known transition metal oxide because of their unique chemical and physical properties. Manganese oxides have potential application in ion sieves, molecular sieves, catalysis, cathode materials for secondary rechargeable batteries, super capacitors, and new magnetic materials. Research has been reported into manganese oxide nanomaterials with different morphologies, such as nanorods, nanowires, nanotubes, and urchin-like nanostructure .
Manganese oxides have been used as one of the most promising electrode materials for super capacitor applications with respect to their natural abundance, low cost, environmentally friendly nature, wide voltage window, and high specific capacitance . More specifically, the performance of manganese oxides dispersed in silica and alumina has been explored in catalytic ozonation of acetone by Oyama and colleagues .
In the research reported in this article, we synthesized Al2O3 doped Mn3O4 nanomaterial (NM) and characterized it by FESEM, EDS, XRD, XPS, FT-IR, and UV-visible. Further, we evaluated the photocatalytic performance of ΝΜ at different pH under solar light, using brilliant cresyl blue (BCB) as organic pollutant.
Analytical grade chemicals including aluminum nitrate nonahydrate (Al(NO3)3 · 9H2O), manganese nitrate tetrahydrate (Mn(NO3)2 · 4H2O) (used as precursors of Al2O3 doped Mn3O4 multi-metal oxide nanoparticles), BCB, sodium hydroxide NaOH, and 99 % pure ethanol were purchased from Sigma-Aldrich.
Synthesis of Al2O3 doped Mn3O4 nanomaterial
Aluminum nitrate nonahydrate (3.7526 g) and manganese nitrate tetrahydrate (7.5634 g) were completely dissolved in 100.0 mL of distilled water, and a homogeneous solution at ambient temperature was obtained. The pH of the solution was adjusted to 10.50 with 0.2 M NaOH solution by drop-wise addition and constant vigorous stirring. Overnight, the solution was heated at 60–70 °C with constant stirring. The solution was then cooled to ambient temperature and the precipitate centrifuged at 2000 rpm. The supernatant solution was discarded and the precipitate preserved. The precipitate was washed with ethanol 1–2 times then allowed to dry at ambient temperature or in oven at 50–60 °C. The precipitate was ground and stored in clean, dry, and inert plastic vials.
Proposed mechanism of nanomaterial growth
The surface morphology of the nanoparticles was studied using a EOL scanning electron microscope (JSM-7600F, Japan). Elemental analysis was carried out using EDS (Oxford). X-ray diffraction patterns (XRD) were taken with a computer-controlled X’Pert Explorer, PANalytical diffractometer. FT-IR spectra were recorded in the range of 400–4000 cm−1 on a Perkin Elmer (spectrum 100) FT-IR spectrometer. UV spectrum was recorded from 200 to 900 nm using UV-visible spectrophotometer (UV-2960, LABOMED INC.). An XPS survey scan was made by Thermo Scientific K-Alpha KA1066 spectrometer (Germany) in the range of 0–1350 eV.
Photocatalytic degradation of dye
The photocatalytic activity of Al2O3 doped Mn3O4 multi-metal oxide nanoparticles was evaluated through degradation of BCB under visible light irradiation. The dye is stable under visible light irradiation in the absence of photocatalyst.
In photocatalytic degradation, two different 100.0 mL, 1 × 10−4 M of BCB dye solutions were taken in different beakers and adjusted to pH 5 and 10, respectively, by drop-wise addition of 0.2 M NaOH solution under vigorous stirring. Then, 0.1193 and 0.1132 g of catalyst was added to the reaction solutions and kept in the dark, for physical adsorption of dye on catalyst surface. It has been reported in previous photocatalytic studies on TiO2 that besides the light absorption capability and charge transportation, the adsorption of reactant is also a critical factor . The sample solution was then irradiated under sunlight with constant stirring.
The dye solution of about 4–5 mL was pipetted out at regular intervals and the absorbance measured at λ max = 595.0 nm by spectrophotometer (LABOMED INC.). Absorbances were measured at time intervals 0, 10, 20, 40, 60, 90, 120, 140, 160, 180, 200, 220, 240, 260, 280, and 300 min. The controlled experiments were also performed under visible light without catalyst to measure any possible direct photocatalysis of dye.
Results and discussion
Physiochemical characterization of Al2O3 doped Mn3O4 nanomaterial
Morphology study (FESEM)
Phase and compositional study (XRD)
X-ray photoelectron spectroscopy (XPS)
Photo-absorption properties and band gap energy
Effect of pH
The effect of pH on the visible light photocatalytic degradation of basic dye BCB was studied in pH range 5–10 at metal oxide nanomaterial and 1 × 10−4 M dye concentration. The results showed that the rate of decolorization increased with the increase in pH from 5 to 10 (Fig. 8b). The photocatalytic performance of the metal oxide was attributed to the surface electrical properties, which facilitate the dye adsorption. It is beneficial for the promotion of visible light generated charge carrier, i.e., electron, to the surface, which leads to the formation of hydroxide radical. Moreover, the pH of the dye solution has a substantial influence on the photocatalytic degradation process, so pH 10 is recommended for basic dyes.
Control experiments and photocatalysis
In this study, three sets of photocatalytic reaction were performed using ΝΜ. First, the experiment without catalyst under visible light irradiation resulted in a small amount of degradation, indicating photolysis reaction. Second, a controlled experiment was performed under dark conditions for 30 min, indicating the physical adsorption of dye on the surface of the photocatalyst. This indicates that the equilibrium time for the dye is reached within 30 min. Finally, photocatalytic degradation of well-stirred dye (BCB) solution was carried out in the presence of the photocatalyst under visible light irradiation. Metal oxide showed efficient activity for degradation of BCB dye at different pH under solar light irradiation (Fig. 8c). Furthermore, the absorption spectrum of BCB dye, presented in Fig. 8a, reflected 50–65 % decolorization after 5 h (300 min).
Kinetic study of dye
C o was the initial concentration of dye, and C was the concentration at time “t”. Using Eq. 6, we calculated the apparent rate constant from the gradient of the graph of ln(C/C o ), corresponding to ln(A/A o ) vs. irradiation time.
Pseudo first-order kinetic study for BCB dye with and without Al2O3 doped Mn3O4 nanomaterial
K app (min−1)
Rate of decolorization
t 1/2 (min)
t 1/2 (years)
Proposed mechanism and discussion of visible light photocatalytic reaction using oxides
Continued attacks of O2 •− and •OH radicals on pollutant species (organic dye) lead to the degradation (oxidation) of the dye molecule (Eq. 9). We can also supply external hydrogen peroxide to the reaction system, which have a significant potential to produce the hydroxyl radicals. It is essential for an efficient photocatalytic process to have the charge carriers separated as far as possible. Moreover, the dye molecule, BCB, adsorbed on nanomaterial may be stimulated to excited state under visible light irradiation. Subsequently, photo-excited dye electrons may be inoculated into the conduction band of nanomaterial via photosensitization. Additionally, dye molecules are degraded (oxidation) by the photo-generated holes in the valence band of nanomaterials [21, 19].
A simple low-temperature and low-cost solar photocatalyst based on Al2O3 doped Mn3O4 nanomaterial has been developed for the degradation of BCB. Singlet oxygen (O2 •−) and hydroxyl (•OH) radicals generated by oxidation and reduction reaction of O2 and H2O, respectively, are primarily responsible for photocatalyzing the degradation of organic pollutant BCB under the irradiation of solar light. The decay of BCB follows the pseudo first-order kinetics which satisfied the Langmuir-Hinshelwood (L-H) kinetic model. This research represents an important advance in the synthesis of novel co-doped oxides for the photocatalytic degradation of organic pollutant under visible light irradiation for industrial effluent.
This work was funded by the King Abdulaziz University, under grant no. T-001/431. The authors therefore acknowledge the technical and financial support of KAU.
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