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Cu hybrid nanorods as superior photocatalyst
Nano-Structures & Nano-Objects 16 (2018) 396–402
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Nano-Structures & Nano-Objects journal homepage: www.elsevier.com/locate/nanoso
Constructing Mn3 O4 /Cu hybrid nanorods as superior photocatalyst ∗
Samaneh Ramezanpour a,b , Iran Sheikhshoaie a , , Massumeh Khatamian c a b c
Department of Chemistry, Shahid Bahonar University of Kerman, Kerman, 76175, Iran Young Research Society, Shahid Bahonar University of Kerman, Kerman, Iran Physical Inorganic Chemistry Research Laboratory, Department of Inorganic Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, 51664, Iran
highlights
graphical abstract
• Synthesis
of Mn3 O4 /Cu hybrid nanorod by combined hydrothermal and sol-gel. • Characterization of the nanomaterials by XRD, TEM, SEM, UV-vis and EDX. • Mn3 O4 /Cu nanorod hybrids exhibit excellent photocatalytic properties towards degradation of MO and MB dyes.
article
info
Article history: Received 17 April 2018 Received in revised form 11 August 2018 Accepted 1 September 2018 Keywords: Hybrid nanorods Photocatalyst Degradation Heterojunction Water treatment
a b s t r a c t Simple and cost-efficient hydrothermal and sol–gel method were applied to synthesize Cu nanorods decorated with Mn3 O4 nanoparticles (Mn3 O4 /Cu hybrid nanorods). This approach employed to improve the photocatalytic properties of each Cu or Mn3 O4 semiconductors. In the synthesis procedure, Cu nanorods were first synthesized by hydrothermal method and Mn3 O4 were then obtained through the sol–gel method on the surface of synthesized Cu nanorods. The samples’ crystal structure, composition, and morphology was characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–Vis spectroscopy, and energy dispersive spectroscopy (EDS) measurements. All the measurements proved that the as-synthesized products were synthesized by the synthetic procedure. The as-synthesized products were used as photocatalyst for the photodegradation of methylene blue (MB) and methyl orange (MO). Under visible light, as-prepared Mn3 O4 /Cu nanorods represents superior photocatalytic performance; more than 85% of the MB and 98% of MO dye have been degraded in 6 min. This superior photocatalytic performance may be because of nanorods morphology of the products which provide high surface for the dye absorption. Moreover, formation of heterojunction on the surface of semiconductors leads to affective charge separation of the Mn3 O4 /Cu hybrid nanorods. © 2018 Published by Elsevier B.V.
1. Introduction It is well-known that water is an essential requirement for life. The accessibility of fresh water is crucial for life sustaining activities [1,2]. During the past decades, water pollution has become a critical global issue. Currently, one of the main sources of water ∗ Corresponding author. E-mail address: [email protected] (I. Sheikhshoaie). https://doi.org/10.1016/j.nanoso.2018.09.004 2352-507X/© 2018 Published by Elsevier B.V.
pollution are organic dyes and their effluents. Dyeing wastewater contains various water-soluble chemical products, which are highly stable, resistant to reactions with chemical agents and low biodegradable. Thus it is very hard to separate them by the usual wastewater treatment methods such as flocculation, filtration, adsorption, or sedimentation [3–7]. Consequently, developing environmentally friendly methods for degrading these pollutants has become a mandatory task, an interesting option is the use of advanced oxidation processes (AOPs) [8].
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AOPs include the combination of oxidants (e.g. H2 O2 ; ozone), light irradiation, catalysts (e.g., Fe, TiO2 ) and ultrasonic waves [9]. Complete classification of AOPs is difficult, but all of them based on physicochemical processes generating in situ hydroxyl radicals which are the strongest oxidants and causes drastic changes in the chemical structure of the pollutants to achieve mineralization of organic matter [10]. Two main class of AOPs are Fenton process (FP) and photocatalyst process. The redox reaction between ferrous ions (Fe2+ ) and H2 O2 is the basis of FP and the photo Fenton process (PFP) is the same besides UV−vis irradiation [11]. While photo-Fenton oxidation is an effective method, it suffer from major drawbacks such as limited pH range (2.5–3.5), large iron sludge, difficult recovery and low activity [12–15]. To overcome these limitations, some alternative transition metal heterogeneous catalysts for degradation of organic pollutants have been developed [16]. The metals with multiple redox states, such as chromium, cerium, copper, cobalt, and manganese, can directly activate H2 O2 into • OH through the Fenton-like pathways [4,17,18]. Heterogeneous semiconductor photocatalysis causes oxidation of organic pollutants via hydroxyl radicals [19–22]. Some researches show that manganese oxide resulted in a remarkable increase in the degradation of the organic pollutants [23]. Recently heterojunction photocatalyst is attracted more attention because of improved photocatalytic performances and better stability [24–26]. In addition, the shape and size of nanoparticles play a key role in the characteristic and performance of the nanostructures. It is proved that by using capping agent one could control shape, size and photocatalytic behavior [27–33]. In this study, we decided to synthesize Mn3 O4 /Cu hybrid nanorods as superior novel catalytic system by using triethylamine as capping agent. To the best of our knowledge, no such material has been synthesized previously. Herein, by coupling of Mn3 O4 with Cu nanorods, the catalytic performance is improved. The present study represents a step forward comparing photocatalytic degradation of MB as cationic dye and MO as an anionic one by using new synthesized Mn3 O4 /Cu hybrid nanorods. The experimental results showed that the as-prepared Mn3 O4 /Cu Nanorods exhibit excellent photocatalytic activity. Under visible light irradiation, more than 85% of the MB and 98% of MO dye has been decomposed in 6 min. In addition, the kinetic of the MB and MO degradation were investigated and the kinetic rate constants were calculated and compared. In summary, the results prove that Mn3 O4 /Cu nanorod hybrids are promising material with excellent photocatalytic properties and can be effectively used for industrial textile dye degradation processes. 2. Experimental 2.1. Materials The materials and reagents used are: manganese(II)chloride tetrahydrate (MnCl2 . 4H2 O), copper(II)sulfate pentahydrate (CuSO4 . 5H2 O), hexamethylenetetramine (HMTA), ethylenediamine (NH2 CH2 CH2 NH2 ), hydrogen peroxide (H2 O2 , 30 wt. %), hydrazinium hydroxide (N2 H4 . H2 O), hydrochloric acid (HCl), sodium hydroxide (NaOH), methylene blue, and methyl orange; all provided from Sigma-Aldrich Co Ltd. 2.2. Synthesis of Cu nanorods Firstly, CuSO4 .5H2 O solution (0.6 g in 20 mL of water) was prepared. To this 400 mL of an NaOH solution (15 M), 4 mL of ethylenediamine, and 1 mL of hydrazinium hydroxide (30%) were added. Then the mixture was transferred to the Teflon reaction autoclave and was placed at 160 ◦ C for 1 h. After hydrothermal reaction, the synthesized Cu nanorods were collected, rinsed, and then interspersed in water for subsequent use [34].
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2.3. Synthesis of Mn3 O4 /Cu hybrid nanorod Mn3 O4 were synthesized on the surface of Cu nanorods using a procedure described previously [35]. In a typical procedure, the 0.4 g of as-synthesized Cu nanorods were fully dispersed in 100 mL by sonication. Next, 0.7 g of Manganese (II) chloride tetra hydrate dissolved in 30 mL of water was added to that. Then, 0.5 g of HMTA in 10 mL of water was added. The mixture was set under magnetic stirring at 120 ± 2 ◦ C for 10 h. Finally, the products were collected, rinsed several times, and dried at 90 o C for 1 h. Finally the assynthesized material was calcinated at 500 o C for 6 h. 2.4. Photocatalytic evaluation In order that the photocatalytic evaluation, 5 mg of as-synthesized Mn3 O4 /Cu hybrid nanorods was dispersed in dye (MO or MB) aqueous solution (50 mL, 5 ppm), the solution was stirred at 500 RPM in a glass beaker (100 mL). The solution was held in the dark position within 30 min to ensure adsorption– desorption equilibrium was attained. After that, 6 ml H2 O2 added to the solution. To adjust pH of mixture for MO, adequate amount of HCl was added to solution until the pH adjusted on 3. For adjusting pH of MB solution, desired amount of NaOH was added until the pH adjusted on 10. Finally, 3 mL of the above mixture was brought off at desired time intervals and the dispersed species were separated by centrifuge. The absorption spectrum of the centrifuged solution was then measured in the range of 300–800 nm by UV–Vis spectrometer. By evaluating the absorption of MO in the filtrate at 514 nm and MO at 664 nm, the photocatalytic performance was assessed. The percentage degradation efficiency of MB was assessed using the relation: A0 − At C0 − Ct × 100 = × 100 R% = C0 A0 Where C0 is the initial concentration of dye, Ct is the concentration of dye after light irradiation at time intervals and is achieved by measuring the absorption in 664 nm for MB and 514 nm for MO using UV–Vis spectrophotometer, which A0 is initial absorption of solution and At is the adsorption of solution after light irradiation. 3. Results and discussion 3.1. Materials characterization The SEM and TEM images of the as-prepared Cu nanorods are depicted in Fig. 1(a, b). As can be seen, the morphology of as the synthesized samples is nanorod which their average diameter is about 86 nm and their length reach about several of micrometers. Fig. 1(c, d) depict the SEM and TEM images of the prepared Mn3 O4 /Cu hybrid nanorod. It is crystal clear that Mn3 O4 were formed on the surface of Cu nanorods. The average diameter of Mn3 O4 is about 19 nm. The energy-dispersive X-ray analysis (EDX) of the samples was measured to investigate their composition. For the Cu nanorods sample (Fig. 2(a)), there are strong signals from Cu and relatively weak signal for O; and no peak from manganese was observed. The possible reason for the presence of oxygen in the structure of the sample should be CuO existed on the surface of nanorods which is formed by oxidation of Cu nanorods. For Mn3 O4 /Cu hybrid nanorods all the signals for Cu, O and also Mn was observed suggesting the formation of Mn3 O4 /Cu hybrid nanorods. This result was proved that Cu nanorods and Mn3 O4 /Cu hybrid nanorod were properly synthesized. XRD was applied to characterize the crystal structure of Cu nanorods and Mn3 O4 /Cu hybrid nanorod. Fig. 3 demonstrate the XRD patterns of as synthesized samples. Three diffraction peaks
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Fig. 1. (a) SEM of the as-prepared Cu nanorods, (b) TEM images of Cu nanorods, (c) SEM of the prepared Mn3 O4 /Cu hybrid nanorod and (d) TEM image of Mn3 O4 /Cu hybrid nanorod.
Fig. 2. EDX of (a) Cu nanorods sample and (b) Mn3 O4 /Cu hybrid nanorod.
at 2θ = 43.22, 50.28, and 74.04◦ are presented in pattern a, which respectively relate to the (111), (200), and (220) (JCPDS 85-1326) crystal planes of the cubic copper. Moreover the very weak peak presented at 2θ = 36.31 corresponds to CuO crystal (JCPDS=65-2309) which is formed by oxidation of Cu nanorods (indicated by O on the pattern). In the XRD pattern of Mn3 O4 /Cu hybrid nanorod, the diffraction peaks at 18.53, 30.34, 35.71 38.83, 53.96, 57.52 and 63.20◦ correspond to the planes of (101), (112), (211), (400), (312), (321) and (224) (JCPDS 80-0382) of the Mn3 O4 tetragonal structure, respectively. The characteristic peaks of Cu nanorod present at 43.47 and 74.33◦ correspond to (111) and (220) of Cu nanorod (marked by stars), indicative of the proper synthesis of hybrid nanorod. These results consent well with the mentioned
EDX results (Fig. 2(a, b)). The mean crystallite diameter of the assynthesized Cu nanorods sample and Mn3 O4 /Cu hybrid nanorod determined by the Deby–Scherrer equation [36] was about 54 nm and 78 nm, respectively. The optical absorption properties of as-synthesized samples was also studied by UV–visible spectroscopy because of its importance for the application in the photocatalysis [37]. The UV– visible spectrum of Cu nanorods and Mn3 O4 /Cu hybrid nanorod is demonstrated in Fig. 4. Using this spectrum and drawing (α hν )2 vs. hν plot (Tauc plot), it is possible to estimate band gap energy (Eg ) of photocatalyst [38]. Here, the band gap of Cu nanorods and Mn3 O4 /Cu hybrid nanorod are 2.3 and 3.1 eV, respectively. The enhancement in the Eg for Mn3 O4 /Cu hybrid nanorod may
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Fig. 3. XRD pattern of the synthesized Cu nanorods and Mn3 O4 /Cu hybrid nanorod.
be attributed to the heterojunction formation. It should be mentions that, it is impossible Cu has band gap because it is an element instead of semiconductor. The attributed band gap of the Cu nanorods belongs to CuO on the surface of Cu nanorod which is detected in EDS spectra and XRD pattern. 3.2. Photocatalytic degradation of MO and MB over Mn3 O4 /Cu hybrid nanorod To investigate the photocatalytic performances of the as-synthesized Cu nanorods and Mn3 O4 /Cu hybrid nanorods, photodegradation of MO and MB were tested in aqueous solution. Fig. 5(a) and (b) shows the evolution of the absorption of MB and MO with light irradiation time on the synthesized photocatalysts. Fig. 5(c) and (d) depicts the visible light irradiation time-dependent degradation of MB and MO on the studied photocatalysts. Both Cu nanorods and Mn3 O4 /Cu hybrid nanorod, exhibited high photocatalytic performance for the degradation of MB molecules. However, the photodegradation of MO over Mn3 O4 /Cu hybrid nanorod are strongly better than Cu nanorods. After 6 min the photodegradation rate of MO over Cu nanorods and Mn3 O4 /Cu hybrid nanorod are 49.09 and 85.45% respectively. At the same, these values for MB are 86.02 and 98.76%. Based on the above results, it is proved that Mn3 O4 /Cu hybrid nanorods could apply as an efficient photocatalyst for the degradation of dye pollutants. This enhanced photodegradation performance observed by the Mn3 O4 /Cu hybrid nanorods compared to the Cu nanorods is because of the synergetic effect, heterojunction formation, higher specific surface area that results in more accessible surface for dye to be degraded, and more efficient charge transfer. As mentioned above, the attributed photocatalytic ability to the Cu nanorods is impossible because Cu is a metal element, instead of semiconductor with energy band gap. However, the possible derivation of photocatalytic performance of this material should be trace CuO existed on the surface of Cu nanorods, which is detected in EDS spectra and XRD pattern. It is the first report about the use of Mn3 O4 /Cu hybrid nanorods photocatalyst for the degradation of MB and MO dye under the sunlight irradiation. However, the previous reports by different photocatalysts were compared with present work for degradation of MB and MO dyes (Table 1).
In comparison to the other photocatalysts used, Mn3 O4 /Cu hybrid nanorods has the highest photocatalytic activity in less time for degradation of MB. In the case of MO degradation, the photocatalytic activity is moderate in 6 min. Moreover, as can be seen in Table 1, the photocatalytic performance is significantly improved in comparison to pure Mn3 O4 nanoparticles. Overall, since the time of photodegradation is less than the other works, Mn3 O4 /Cu hybrid nanorods can be a good choice for wastewater treatment. 3.3. Kinetic study The degradation kinetics of dyes were also studied under visible light irradiation. Fig. 6(a) and (b) shows a linear relationship between ln (C0 /Ct ) and reaction time, which indicate that the degradation kinetics of MO and MB follow pseudo-first-order rate law. The kinetic constants of photocatalytic degradation process of MO and MB by as-prepared photocatalysts were acquired by the pseudo-first order model. The related results are summarized in Fig. 6. As can be seen in Fig. 6(a) kinetic constant for degradation of MO by Cu nanorods and Mn3 O4 /Cu hybrid nanorod are 0.314 and 0.121. Fig. 5(b) indicates the kinetics of MB degradation. Kinetic constant for degradation of MB by Cu nanorods and Mn3 O4 /Cu hybrid nanorod are 0.263 and 0.539 respectively. Findings show that the kinetic constant for Mn3 O4 /Cu hybrid nanorod are much higher than Cu nanorods. The higher constant rate and better photocatalytic performance of the Mn3 O4 /Cu nanorod hybrids mainly attributed to heterojunction formation, which allowed the efficient charge separation of the hybrid nanostructures [45]. 3.4. Mechanism The possible mechanism of photodegradation is depicted in Fig. 7. electrons (e−) and holes (h+) can be generated on the trace CuO which exist on the surface of Cu nanorods. Under irradiation of sunlight, these pairs will separate because electrons migrate to the conduction band of Mn3 O4 , impressing the hole (h+ VB ) in the valence band. Migrated electron can be carry over CuO to Mn3 O4 . Then Mn3+ is reduced to Mn2+ by electrons. Mn3 O4 , trap the electrons results in reducing the recombination rate of pairs. These process cause the lifetime of pairs be enhanced and they can easily move to the surface of the nanoparticles. As moving electrons to the surface,
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Fig. 4. UV–visible spectrum of as synthesized photocatalysts and inset band gap calculation (Tauc plot) of as-synthesized photocatalysts.
Fig. 5. Evolution of the absorption with light irradiation time of (a) MO and (b) MB (c) visible light irradiation time-dependent degradation of MO and (d) MB. Table 1 Comparison the results with literature. Serial no.
Catalyst
Dye
Initial dye conc. (mg/L)
Time (min)
Degradation (%)
Ref.
1 2 3 4 5 6 7 8 9
Mn3 O4 –MnO2 nanorods SnO2 doped ZnO TiO2 Ag2 ZrO3 SnO2 /Zn2 SnO4 CdSe/ZnS core–shell QDs Mn3 O4 Mn3 O4 /Cu nanorods Mn3 O4 /Cu nanorods
MB MB MB MB MO MO MB MB MO
10 6 10 3 20 10 10 5 5
50 120 360 160 30 120 40 6 6
99.5 99 90 99 99 70 82 85 50
[39] [40] [41] [42] [43] [44] [35] This study This study
the adsorbed O2 on Mn3 O4 captures photogenerated electrons and prevent recombination of photogenerated pairs because the generated e− would interact with O2 and reduce into • O2 [46–48]. Meanwhile, h+ which exist in valence band can trap the water, H2 O2 and hydroxyl groups (OH− ) to generate hydroxyl radicals. These OH• and O2 • are strong oxidizing agents with the ability to decompose MB and MO to CO2 and H2 O [49–52].
3.5. Reusability of photocatalyst Cycles of MB decomposition process was investigated to evaluate the stability of our Mn3 O4 /Cu nanorod hybrid (Fig. 8). After each cycle, the photocatalyst were collected and rinsed to be used in the further use (at least 5 cycle). As can be seen in Fig. 8, the recycling studies of Mn3 O4 /Cu nanorod hybrid show that more
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Fig. 8. The reusability diagrams of the Mn3 O4 /Cu nanorod hybrid for 5 runs.
photocatalytic properties and can be impressively applied for dye degradation in textile industry and water treatment. Moreover, the results depicted that the kinetics of this process can be assigned in term of pseudo-first-order kinetic model. 5. Novelty Fig. 6. (a) The degradation kinetics of MO and (b) MB.
Cu nanorods decorated with Mn3 O4 nanoparticles (Mn3 O4 /Cu hybrid nanorod) were prepared by using simple and cost-efficient hydrothermal and sol–gel method. To the best of our knowledge, no such material has been synthesized previously. The experimental results showed that the as-prepared Mn3 O4 / Cu Nanorods exhibit excellent photocatalytic activity. Under visible light irradiation, more than 85% of the MB and 98% of MO dye has been decomposed in 6 min. The results prove that Mn3 O4 /Cu nanorod hybrids are promising material with excellent photocatalytic properties and can be effectively used for industrial textile dye degradation processes. Fig. 7. Mechanism of photodegradation of dye over Mn3 O4 /Cu nanorod hybrid.
Acknowledgment than 90% of MB and 79% of MO were degraded even after a fivecycle for the photocatalytic process. This proves the high stability of as-synthesized nanorods. The reason for the decreasing of performance might be the lose or aggregation of photocatalyst during the cycle runs, or adsorption of dyes on the surface of photocatalyst [53].
The authors are grateful to Research affairs of Shahid Bahonar University of Kerman for the financial support. Conflict of interest The authors declare that they have no any kind of conflict of interest regarding publishing of present manuscript.
4. Conclusion References In summary, two-step simple process based on hydrothermal and sol–gel method was applied to synthesize Mn3 O4 /Cu nanorod hybrids. SEM, TEM, XRD, and EDS analyses proved that, with the present synthetic procedure, Mn3 O4 /Cu nanorod hybrids can be produced properly. The average diameter of Cu nanorods was measured about 86 nm, and the average diameter of Mn3 O4 on the surface of Cu nanorod is about 19 nm. It was found that Mn3 O4 /Cu hybrids nanorod represent superior photocatalytic performance for the degradation of MO and MB dyes under solar irradiations. Due to the interfacial charge transfer process, the photocatalytic process was found to be enhanced for the Mn3 O4 /Cu nanorod hybrids than Cu nanorods. Mn3 O4 coated on the surface of Cu nanorods operate as the sinks which trap electrons, prolonging the pairs lifetimes, which results in enhancing the photocatalytic efficiency. The results demonstrate that Mn3 O4 /Cu nanorod hybrids (with 3.1 eV band gap energy) is promising material with excellent
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Contents lists available at ScienceDirect
Nano-Structures & Nano-Objects journal homepage: www.elsevier.com/locate/nanoso
Constructing Mn3 O4 /Cu hybrid nanorods as superior photocatalyst ∗
Samaneh Ramezanpour a,b , Iran Sheikhshoaie a , , Massumeh Khatamian c a b c
Department of Chemistry, Shahid Bahonar University of Kerman, Kerman, 76175, Iran Young Research Society, Shahid Bahonar University of Kerman, Kerman, Iran Physical Inorganic Chemistry Research Laboratory, Department of Inorganic Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, 51664, Iran
highlights
graphical abstract
• Synthesis
of Mn3 O4 /Cu hybrid nanorod by combined hydrothermal and sol-gel. • Characterization of the nanomaterials by XRD, TEM, SEM, UV-vis and EDX. • Mn3 O4 /Cu nanorod hybrids exhibit excellent photocatalytic properties towards degradation of MO and MB dyes.
article
info
Article history: Received 17 April 2018 Received in revised form 11 August 2018 Accepted 1 September 2018 Keywords: Hybrid nanorods Photocatalyst Degradation Heterojunction Water treatment
a b s t r a c t Simple and cost-efficient hydrothermal and sol–gel method were applied to synthesize Cu nanorods decorated with Mn3 O4 nanoparticles (Mn3 O4 /Cu hybrid nanorods). This approach employed to improve the photocatalytic properties of each Cu or Mn3 O4 semiconductors. In the synthesis procedure, Cu nanorods were first synthesized by hydrothermal method and Mn3 O4 were then obtained through the sol–gel method on the surface of synthesized Cu nanorods. The samples’ crystal structure, composition, and morphology was characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–Vis spectroscopy, and energy dispersive spectroscopy (EDS) measurements. All the measurements proved that the as-synthesized products were synthesized by the synthetic procedure. The as-synthesized products were used as photocatalyst for the photodegradation of methylene blue (MB) and methyl orange (MO). Under visible light, as-prepared Mn3 O4 /Cu nanorods represents superior photocatalytic performance; more than 85% of the MB and 98% of MO dye have been degraded in 6 min. This superior photocatalytic performance may be because of nanorods morphology of the products which provide high surface for the dye absorption. Moreover, formation of heterojunction on the surface of semiconductors leads to affective charge separation of the Mn3 O4 /Cu hybrid nanorods. © 2018 Published by Elsevier B.V.
1. Introduction It is well-known that water is an essential requirement for life. The accessibility of fresh water is crucial for life sustaining activities [1,2]. During the past decades, water pollution has become a critical global issue. Currently, one of the main sources of water ∗ Corresponding author. E-mail address: [email protected] (I. Sheikhshoaie). https://doi.org/10.1016/j.nanoso.2018.09.004 2352-507X/© 2018 Published by Elsevier B.V.
pollution are organic dyes and their effluents. Dyeing wastewater contains various water-soluble chemical products, which are highly stable, resistant to reactions with chemical agents and low biodegradable. Thus it is very hard to separate them by the usual wastewater treatment methods such as flocculation, filtration, adsorption, or sedimentation [3–7]. Consequently, developing environmentally friendly methods for degrading these pollutants has become a mandatory task, an interesting option is the use of advanced oxidation processes (AOPs) [8].
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AOPs include the combination of oxidants (e.g. H2 O2 ; ozone), light irradiation, catalysts (e.g., Fe, TiO2 ) and ultrasonic waves [9]. Complete classification of AOPs is difficult, but all of them based on physicochemical processes generating in situ hydroxyl radicals which are the strongest oxidants and causes drastic changes in the chemical structure of the pollutants to achieve mineralization of organic matter [10]. Two main class of AOPs are Fenton process (FP) and photocatalyst process. The redox reaction between ferrous ions (Fe2+ ) and H2 O2 is the basis of FP and the photo Fenton process (PFP) is the same besides UV−vis irradiation [11]. While photo-Fenton oxidation is an effective method, it suffer from major drawbacks such as limited pH range (2.5–3.5), large iron sludge, difficult recovery and low activity [12–15]. To overcome these limitations, some alternative transition metal heterogeneous catalysts for degradation of organic pollutants have been developed [16]. The metals with multiple redox states, such as chromium, cerium, copper, cobalt, and manganese, can directly activate H2 O2 into • OH through the Fenton-like pathways [4,17,18]. Heterogeneous semiconductor photocatalysis causes oxidation of organic pollutants via hydroxyl radicals [19–22]. Some researches show that manganese oxide resulted in a remarkable increase in the degradation of the organic pollutants [23]. Recently heterojunction photocatalyst is attracted more attention because of improved photocatalytic performances and better stability [24–26]. In addition, the shape and size of nanoparticles play a key role in the characteristic and performance of the nanostructures. It is proved that by using capping agent one could control shape, size and photocatalytic behavior [27–33]. In this study, we decided to synthesize Mn3 O4 /Cu hybrid nanorods as superior novel catalytic system by using triethylamine as capping agent. To the best of our knowledge, no such material has been synthesized previously. Herein, by coupling of Mn3 O4 with Cu nanorods, the catalytic performance is improved. The present study represents a step forward comparing photocatalytic degradation of MB as cationic dye and MO as an anionic one by using new synthesized Mn3 O4 /Cu hybrid nanorods. The experimental results showed that the as-prepared Mn3 O4 /Cu Nanorods exhibit excellent photocatalytic activity. Under visible light irradiation, more than 85% of the MB and 98% of MO dye has been decomposed in 6 min. In addition, the kinetic of the MB and MO degradation were investigated and the kinetic rate constants were calculated and compared. In summary, the results prove that Mn3 O4 /Cu nanorod hybrids are promising material with excellent photocatalytic properties and can be effectively used for industrial textile dye degradation processes. 2. Experimental 2.1. Materials The materials and reagents used are: manganese(II)chloride tetrahydrate (MnCl2 . 4H2 O), copper(II)sulfate pentahydrate (CuSO4 . 5H2 O), hexamethylenetetramine (HMTA), ethylenediamine (NH2 CH2 CH2 NH2 ), hydrogen peroxide (H2 O2 , 30 wt. %), hydrazinium hydroxide (N2 H4 . H2 O), hydrochloric acid (HCl), sodium hydroxide (NaOH), methylene blue, and methyl orange; all provided from Sigma-Aldrich Co Ltd. 2.2. Synthesis of Cu nanorods Firstly, CuSO4 .5H2 O solution (0.6 g in 20 mL of water) was prepared. To this 400 mL of an NaOH solution (15 M), 4 mL of ethylenediamine, and 1 mL of hydrazinium hydroxide (30%) were added. Then the mixture was transferred to the Teflon reaction autoclave and was placed at 160 ◦ C for 1 h. After hydrothermal reaction, the synthesized Cu nanorods were collected, rinsed, and then interspersed in water for subsequent use [34].
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2.3. Synthesis of Mn3 O4 /Cu hybrid nanorod Mn3 O4 were synthesized on the surface of Cu nanorods using a procedure described previously [35]. In a typical procedure, the 0.4 g of as-synthesized Cu nanorods were fully dispersed in 100 mL by sonication. Next, 0.7 g of Manganese (II) chloride tetra hydrate dissolved in 30 mL of water was added to that. Then, 0.5 g of HMTA in 10 mL of water was added. The mixture was set under magnetic stirring at 120 ± 2 ◦ C for 10 h. Finally, the products were collected, rinsed several times, and dried at 90 o C for 1 h. Finally the assynthesized material was calcinated at 500 o C for 6 h. 2.4. Photocatalytic evaluation In order that the photocatalytic evaluation, 5 mg of as-synthesized Mn3 O4 /Cu hybrid nanorods was dispersed in dye (MO or MB) aqueous solution (50 mL, 5 ppm), the solution was stirred at 500 RPM in a glass beaker (100 mL). The solution was held in the dark position within 30 min to ensure adsorption– desorption equilibrium was attained. After that, 6 ml H2 O2 added to the solution. To adjust pH of mixture for MO, adequate amount of HCl was added to solution until the pH adjusted on 3. For adjusting pH of MB solution, desired amount of NaOH was added until the pH adjusted on 10. Finally, 3 mL of the above mixture was brought off at desired time intervals and the dispersed species were separated by centrifuge. The absorption spectrum of the centrifuged solution was then measured in the range of 300–800 nm by UV–Vis spectrometer. By evaluating the absorption of MO in the filtrate at 514 nm and MO at 664 nm, the photocatalytic performance was assessed. The percentage degradation efficiency of MB was assessed using the relation: A0 − At C0 − Ct × 100 = × 100 R% = C0 A0 Where C0 is the initial concentration of dye, Ct is the concentration of dye after light irradiation at time intervals and is achieved by measuring the absorption in 664 nm for MB and 514 nm for MO using UV–Vis spectrophotometer, which A0 is initial absorption of solution and At is the adsorption of solution after light irradiation. 3. Results and discussion 3.1. Materials characterization The SEM and TEM images of the as-prepared Cu nanorods are depicted in Fig. 1(a, b). As can be seen, the morphology of as the synthesized samples is nanorod which their average diameter is about 86 nm and their length reach about several of micrometers. Fig. 1(c, d) depict the SEM and TEM images of the prepared Mn3 O4 /Cu hybrid nanorod. It is crystal clear that Mn3 O4 were formed on the surface of Cu nanorods. The average diameter of Mn3 O4 is about 19 nm. The energy-dispersive X-ray analysis (EDX) of the samples was measured to investigate their composition. For the Cu nanorods sample (Fig. 2(a)), there are strong signals from Cu and relatively weak signal for O; and no peak from manganese was observed. The possible reason for the presence of oxygen in the structure of the sample should be CuO existed on the surface of nanorods which is formed by oxidation of Cu nanorods. For Mn3 O4 /Cu hybrid nanorods all the signals for Cu, O and also Mn was observed suggesting the formation of Mn3 O4 /Cu hybrid nanorods. This result was proved that Cu nanorods and Mn3 O4 /Cu hybrid nanorod were properly synthesized. XRD was applied to characterize the crystal structure of Cu nanorods and Mn3 O4 /Cu hybrid nanorod. Fig. 3 demonstrate the XRD patterns of as synthesized samples. Three diffraction peaks
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Fig. 1. (a) SEM of the as-prepared Cu nanorods, (b) TEM images of Cu nanorods, (c) SEM of the prepared Mn3 O4 /Cu hybrid nanorod and (d) TEM image of Mn3 O4 /Cu hybrid nanorod.
Fig. 2. EDX of (a) Cu nanorods sample and (b) Mn3 O4 /Cu hybrid nanorod.
at 2θ = 43.22, 50.28, and 74.04◦ are presented in pattern a, which respectively relate to the (111), (200), and (220) (JCPDS 85-1326) crystal planes of the cubic copper. Moreover the very weak peak presented at 2θ = 36.31 corresponds to CuO crystal (JCPDS=65-2309) which is formed by oxidation of Cu nanorods (indicated by O on the pattern). In the XRD pattern of Mn3 O4 /Cu hybrid nanorod, the diffraction peaks at 18.53, 30.34, 35.71 38.83, 53.96, 57.52 and 63.20◦ correspond to the planes of (101), (112), (211), (400), (312), (321) and (224) (JCPDS 80-0382) of the Mn3 O4 tetragonal structure, respectively. The characteristic peaks of Cu nanorod present at 43.47 and 74.33◦ correspond to (111) and (220) of Cu nanorod (marked by stars), indicative of the proper synthesis of hybrid nanorod. These results consent well with the mentioned
EDX results (Fig. 2(a, b)). The mean crystallite diameter of the assynthesized Cu nanorods sample and Mn3 O4 /Cu hybrid nanorod determined by the Deby–Scherrer equation [36] was about 54 nm and 78 nm, respectively. The optical absorption properties of as-synthesized samples was also studied by UV–visible spectroscopy because of its importance for the application in the photocatalysis [37]. The UV– visible spectrum of Cu nanorods and Mn3 O4 /Cu hybrid nanorod is demonstrated in Fig. 4. Using this spectrum and drawing (α hν )2 vs. hν plot (Tauc plot), it is possible to estimate band gap energy (Eg ) of photocatalyst [38]. Here, the band gap of Cu nanorods and Mn3 O4 /Cu hybrid nanorod are 2.3 and 3.1 eV, respectively. The enhancement in the Eg for Mn3 O4 /Cu hybrid nanorod may
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Fig. 3. XRD pattern of the synthesized Cu nanorods and Mn3 O4 /Cu hybrid nanorod.
be attributed to the heterojunction formation. It should be mentions that, it is impossible Cu has band gap because it is an element instead of semiconductor. The attributed band gap of the Cu nanorods belongs to CuO on the surface of Cu nanorod which is detected in EDS spectra and XRD pattern. 3.2. Photocatalytic degradation of MO and MB over Mn3 O4 /Cu hybrid nanorod To investigate the photocatalytic performances of the as-synthesized Cu nanorods and Mn3 O4 /Cu hybrid nanorods, photodegradation of MO and MB were tested in aqueous solution. Fig. 5(a) and (b) shows the evolution of the absorption of MB and MO with light irradiation time on the synthesized photocatalysts. Fig. 5(c) and (d) depicts the visible light irradiation time-dependent degradation of MB and MO on the studied photocatalysts. Both Cu nanorods and Mn3 O4 /Cu hybrid nanorod, exhibited high photocatalytic performance for the degradation of MB molecules. However, the photodegradation of MO over Mn3 O4 /Cu hybrid nanorod are strongly better than Cu nanorods. After 6 min the photodegradation rate of MO over Cu nanorods and Mn3 O4 /Cu hybrid nanorod are 49.09 and 85.45% respectively. At the same, these values for MB are 86.02 and 98.76%. Based on the above results, it is proved that Mn3 O4 /Cu hybrid nanorods could apply as an efficient photocatalyst for the degradation of dye pollutants. This enhanced photodegradation performance observed by the Mn3 O4 /Cu hybrid nanorods compared to the Cu nanorods is because of the synergetic effect, heterojunction formation, higher specific surface area that results in more accessible surface for dye to be degraded, and more efficient charge transfer. As mentioned above, the attributed photocatalytic ability to the Cu nanorods is impossible because Cu is a metal element, instead of semiconductor with energy band gap. However, the possible derivation of photocatalytic performance of this material should be trace CuO existed on the surface of Cu nanorods, which is detected in EDS spectra and XRD pattern. It is the first report about the use of Mn3 O4 /Cu hybrid nanorods photocatalyst for the degradation of MB and MO dye under the sunlight irradiation. However, the previous reports by different photocatalysts were compared with present work for degradation of MB and MO dyes (Table 1).
In comparison to the other photocatalysts used, Mn3 O4 /Cu hybrid nanorods has the highest photocatalytic activity in less time for degradation of MB. In the case of MO degradation, the photocatalytic activity is moderate in 6 min. Moreover, as can be seen in Table 1, the photocatalytic performance is significantly improved in comparison to pure Mn3 O4 nanoparticles. Overall, since the time of photodegradation is less than the other works, Mn3 O4 /Cu hybrid nanorods can be a good choice for wastewater treatment. 3.3. Kinetic study The degradation kinetics of dyes were also studied under visible light irradiation. Fig. 6(a) and (b) shows a linear relationship between ln (C0 /Ct ) and reaction time, which indicate that the degradation kinetics of MO and MB follow pseudo-first-order rate law. The kinetic constants of photocatalytic degradation process of MO and MB by as-prepared photocatalysts were acquired by the pseudo-first order model. The related results are summarized in Fig. 6. As can be seen in Fig. 6(a) kinetic constant for degradation of MO by Cu nanorods and Mn3 O4 /Cu hybrid nanorod are 0.314 and 0.121. Fig. 5(b) indicates the kinetics of MB degradation. Kinetic constant for degradation of MB by Cu nanorods and Mn3 O4 /Cu hybrid nanorod are 0.263 and 0.539 respectively. Findings show that the kinetic constant for Mn3 O4 /Cu hybrid nanorod are much higher than Cu nanorods. The higher constant rate and better photocatalytic performance of the Mn3 O4 /Cu nanorod hybrids mainly attributed to heterojunction formation, which allowed the efficient charge separation of the hybrid nanostructures [45]. 3.4. Mechanism The possible mechanism of photodegradation is depicted in Fig. 7. electrons (e−) and holes (h+) can be generated on the trace CuO which exist on the surface of Cu nanorods. Under irradiation of sunlight, these pairs will separate because electrons migrate to the conduction band of Mn3 O4 , impressing the hole (h+ VB ) in the valence band. Migrated electron can be carry over CuO to Mn3 O4 . Then Mn3+ is reduced to Mn2+ by electrons. Mn3 O4 , trap the electrons results in reducing the recombination rate of pairs. These process cause the lifetime of pairs be enhanced and they can easily move to the surface of the nanoparticles. As moving electrons to the surface,
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Fig. 4. UV–visible spectrum of as synthesized photocatalysts and inset band gap calculation (Tauc plot) of as-synthesized photocatalysts.
Fig. 5. Evolution of the absorption with light irradiation time of (a) MO and (b) MB (c) visible light irradiation time-dependent degradation of MO and (d) MB. Table 1 Comparison the results with literature. Serial no.
Catalyst
Dye
Initial dye conc. (mg/L)
Time (min)
Degradation (%)
Ref.
1 2 3 4 5 6 7 8 9
Mn3 O4 –MnO2 nanorods SnO2 doped ZnO TiO2 Ag2 ZrO3 SnO2 /Zn2 SnO4 CdSe/ZnS core–shell QDs Mn3 O4 Mn3 O4 /Cu nanorods Mn3 O4 /Cu nanorods
MB MB MB MB MO MO MB MB MO
10 6 10 3 20 10 10 5 5
50 120 360 160 30 120 40 6 6
99.5 99 90 99 99 70 82 85 50
[39] [40] [41] [42] [43] [44] [35] This study This study
the adsorbed O2 on Mn3 O4 captures photogenerated electrons and prevent recombination of photogenerated pairs because the generated e− would interact with O2 and reduce into • O2 [46–48]. Meanwhile, h+ which exist in valence band can trap the water, H2 O2 and hydroxyl groups (OH− ) to generate hydroxyl radicals. These OH• and O2 • are strong oxidizing agents with the ability to decompose MB and MO to CO2 and H2 O [49–52].
3.5. Reusability of photocatalyst Cycles of MB decomposition process was investigated to evaluate the stability of our Mn3 O4 /Cu nanorod hybrid (Fig. 8). After each cycle, the photocatalyst were collected and rinsed to be used in the further use (at least 5 cycle). As can be seen in Fig. 8, the recycling studies of Mn3 O4 /Cu nanorod hybrid show that more
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Fig. 8. The reusability diagrams of the Mn3 O4 /Cu nanorod hybrid for 5 runs.
photocatalytic properties and can be impressively applied for dye degradation in textile industry and water treatment. Moreover, the results depicted that the kinetics of this process can be assigned in term of pseudo-first-order kinetic model. 5. Novelty Fig. 6. (a) The degradation kinetics of MO and (b) MB.
Cu nanorods decorated with Mn3 O4 nanoparticles (Mn3 O4 /Cu hybrid nanorod) were prepared by using simple and cost-efficient hydrothermal and sol–gel method. To the best of our knowledge, no such material has been synthesized previously. The experimental results showed that the as-prepared Mn3 O4 / Cu Nanorods exhibit excellent photocatalytic activity. Under visible light irradiation, more than 85% of the MB and 98% of MO dye has been decomposed in 6 min. The results prove that Mn3 O4 /Cu nanorod hybrids are promising material with excellent photocatalytic properties and can be effectively used for industrial textile dye degradation processes. Fig. 7. Mechanism of photodegradation of dye over Mn3 O4 /Cu nanorod hybrid.
Acknowledgment than 90% of MB and 79% of MO were degraded even after a fivecycle for the photocatalytic process. This proves the high stability of as-synthesized nanorods. The reason for the decreasing of performance might be the lose or aggregation of photocatalyst during the cycle runs, or adsorption of dyes on the surface of photocatalyst [53].
The authors are grateful to Research affairs of Shahid Bahonar University of Kerman for the financial support. Conflict of interest The authors declare that they have no any kind of conflict of interest regarding publishing of present manuscript.
4. Conclusion References In summary, two-step simple process based on hydrothermal and sol–gel method was applied to synthesize Mn3 O4 /Cu nanorod hybrids. SEM, TEM, XRD, and EDS analyses proved that, with the present synthetic procedure, Mn3 O4 /Cu nanorod hybrids can be produced properly. The average diameter of Cu nanorods was measured about 86 nm, and the average diameter of Mn3 O4 on the surface of Cu nanorod is about 19 nm. It was found that Mn3 O4 /Cu hybrids nanorod represent superior photocatalytic performance for the degradation of MO and MB dyes under solar irradiations. Due to the interfacial charge transfer process, the photocatalytic process was found to be enhanced for the Mn3 O4 /Cu nanorod hybrids than Cu nanorods. Mn3 O4 coated on the surface of Cu nanorods operate as the sinks which trap electrons, prolonging the pairs lifetimes, which results in enhancing the photocatalytic efficiency. The results demonstrate that Mn3 O4 /Cu nanorod hybrids (with 3.1 eV band gap energy) is promising material with excellent
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