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PdO nanoparticles toward degradation of methyl red in water
Journal of Photochemistry & Photobiology A: Chemistry 365 (2018) 145–150
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Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Photocatalytic behavior of mixed oxide NiO/PdO nanoparticles toward degradation of methyl red in water
T
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O.I. Omotunde , A.E. Okoronkwo, A.F. Aiyesanmi, E. Gurgur Department of Chemistry, The Federal University of Technology Akure, Nigeria
A R T I C LE I N FO
A B S T R A C T
Keywords: Mixed oxide catalyst Methyl red Photodegradation Degradation efficiency
Mixed oxide NiO/PdO nanoparticles were successfully prepared by a facile chemical co-precipitation route and were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy with energy dispersive x-ray analysis (SEM-EDX), UV–vis diffuse reflectance spectrophotometry (UVDRS), Brunauer-Emmett-Teller (BET) method and thermal analysis (TGA) techniques. The photocatalytic activities were assessed using methyl red as a model azo dye and the efficiency of the system was evaluated with respect to irradiation time, pH, catalyst load and initial concentration. The pseudo-first-order equation was revealed to fit degradation rate. The results show that the mixed oxide NiO/PdO catalyst has a high photocatalytic performance of 98% toward methyl red degradation and show good photostability.
1. Introduction The degradation of organic pollutants is an increasingly important phenomenon in universal research and semiconductor catalysts have been identified as a promising approach for effective treatment of wastewater [1,2]. This is largely because the use of semiconductor catalysts such as CdS, ZnO, Fe2O3 and TiO2 as photocatalysts does not generate any secondary pollution. The use of these semiconductors is hinged on their low cost, non-toxicity, chemical stability, high photocatalytic activity, optical and electrical properties [3,4]. While TiO2 has been largely used for complete degradation of recalcitrant organic pollutants [5], its wide bandgap limits its functionality because the catalytic activity can only be activated by the use of UV light [6], further, the easy recombination of photogenerated electron-hole pairs in TiO2 results in low quantum efficiency [7,8]. Therefore, the need to find alternatives to TiO2 for more effective excitation of electrons to activate photocatalytic processes. The use of transition metal oxides in the photocatalytic oxidation of organic molecules signifies a promising remediation approach for wastewater systems due to their unique magnetic, optical, electronic, thermal and mechanical properties and probable application in catalyst, gas-sensors and photo-electronic devices [9,10]. In this study, NiO which is a transition metal oxide broadly used as a catalyst with exceeding catalytic, electrical, redox and thermal properties have been identified as the preferred semiconductor making it appropriate for photocatalytic processes [11,12].
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Photocatalytic oxidation reactions deal with photoactivated metal oxides as semiconductors to eliminate contaminants in an aqueous environment [13]. The photocatalytic mechanism is initiated when a photon with energy ‘hv’ equals or surpasses the band gap energy of the semiconductors, the conduction electrons are excited from the valence band into the conduction band (CB) leaving a hole behind. The hole can either oxidize a compound instantly or react with electron donors like water to form OH radicals leading to reaction with the pollutants [14,15]. Recombination of the photogenerated hole (h+) and electron (e−) has proven to be a shortcoming of this approach. This recombination step drops the quantum yield and brings about energy wasting. Therefore, the e−/h+ recombination process should be controlled to guarantee efficient photocatalysis. Coupling of the semiconductor has been confirmed to improve the charge separation of the electron-hole pair which increases the duration of the charge carriers and leads to a decrease in the recombination of electron-hole [16,17]. Investigations have shown that nanosized noble metals such as silver, platinum, gold and palladium when loaded on the semiconductor surface can behave as an electron sink solving the issues of electron-hole recombination process and also act as an efficient charge separator during photocatalysis [18]. This nanosized noble metals exhibit unique physical, chemical, optical and thermodynamical properties at the nano regime leading them into many applications in catalysis [19,20]. Particularly, palladium nanoparticles have a wideranging application in heterogeneous and homogeneous catalysis because of their high surface to volume ratio [21]. Surface plasma
Corresponding author. E-mail address: [email protected] (O.I. Omotunde).
https://doi.org/10.1016/j.jphotochem.2018.08.005 Received 1 June 2018; Received in revised form 2 August 2018; Accepted 3 August 2018 Available online 04 August 2018 1010-6030/ © 2018 Elsevier B.V. All rights reserved.
Journal of Photochemistry & Photobiology A: Chemistry 365 (2018) 145–150
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studies. A diffuse reflectance spectrum was recorded using Shimadzu UV-2450, the surface area was determined by Nitrogen adsorption in a Quantachrome Autosorb using Brunauer-Emmett-Teller (BET) method and thermogravimetric analysis (TGA) was performed on a Perkin Elmer Pyris thermal analyzer.
resonance (SPR) is another important feature in palladium nanoparticles which is useful in sensing, chemo optical transducers, plasmonic waveguiding [22,23]. In this work, to increase the efficiency of photocatalytic reaction through the extension of the absorption band of NiO and restriction of the recombination of photogenerated carriers palladium was chosen due to its ability to drastically enhance the absorption of visible light through localized surface plasmon resonance effects [24]. Azo dyes comprising of one or more azo bonds are the most commonly used synthetic dyes and largely the major pollutants in dye wastewater. Due to their noxiousness and slow degradation, these dyes are categorized as environmentally hazardous materials [25]. The resulting wastewater is very complex and presents difficulties when it comes to treatment due to the existence of non-biodegradable compounds. Numerous approaches have been developed for the removal of synthetic dyes from such wastewaters with the aim of improving their impact on the aquatic environment. Conventional water treatment procedures such as filtration, chemical, sedimentation, and membrane technologies are not viable due to high operating costs. Moreover, these procedures result in the production of toxic secondary pollutants that also have to be treated such as aromatic amines, for examples, aniline and substituted anilines [26]. Photocatalytic technology offers a simple and low-cost method for removing inorganic and organic contaminants from wastewater [27] since most organic pollutants may possibly be degraded or mineralized by use of photocatalytic degradation technology [28]. The most studied semiconductor oxides with photocatalytic activity other than TiO2 are ZnO, WO3, Ta2O5, Nb2O5, Bi2O3, CdO and SnO2, therefore, the use of NiO in this work is to open the bridge for researchers to take advantage of its potent catalytic activity under photocatalytic reaction other than the several chemical reactions nickel catalyst is known for. A limited study has been reported in synthesizing mixed oxides of NiO/PdO for the degradation of azo dyes. In the present work, NiO/PdO nanoparticles were obtained via co-precipitation method. The characteristics of the resulting catalyst and its efficiency in the degradation of methyl red were also examined.
2.4. Photocatalytic degradation studies The photocatalytic degradation of Methyl red (MR) dye by NiO/PdO photocatalyst was carried out using UV and visible light. A photoreactor with UV-C lamp (120 W) emitting 254 nm wavelength was used as the source of UV light. All experiments were performed under identical conditions. The control experiment was performed without photocatalyst to ensure the degradation was solely due to the presence of the photocatalyst. In each experiment 0.15 g of NiO/PdO was added to 50 ml of 1 × 10−4 M MR and kept in the dark for 30 min to establish adsorption-desorption equilibrium before subjecting to irradiation under UV light the mixture was agitated, filtered using 0.2 μm filter and the filtrate was monitored spectrally at λmax of MR = 437 nm. The MR degradation efficiency (DE) was calculated using the equation (1)
DE % =
(Ci−Cf ) Ci
x 100
(1)
where Ci is the initial MR concentration and Cf is the final MR concentration after photocatalytic degradation. The influence of the solution pH ranging between pH 2 and 12 was evaluated on the photocatalytic degradation reaction, the effect of irradiation timeconcentration profiles was studied at various time intervals and various concentrations. The optimum photocatalyst load on the photocatalytic degradation reaction was determined by varying the catalyst load between 0.05 g – 0.2 g. 3. Results and discussion 3.1. Characterization XRD measurement was performed on the NiO/PdO catalyst to investigate the crystal structure and phase purity and the resulting XRD pattern is shown in Fig. 1. The spectrum exhibits the characteristic peaks at 2θ values of 37.25, 43.35 and 62.95°. They were assigned to the (111), (200) and (220) crystal plane spacing of Face Central Cubic (FCC) NiO respectively and are in good agreement with the standard JCPDS data (No 04-0835). No clear diffraction peaks attributed to PdO could be observed due to the low concentration of the palladium, however, a PdO peak (110) at the value of 43° (JCPDS No 41-1107) could have overlapped with a peak of NiO at 2θ = 43.35° suggesting that the PdO ions were partially substituted in the NiO lattice site. Using the Debye Scherrer equation (2), the average crystallite size (D) was calculated to be 22.7 nm.
2. Experimental 2.1. Materials Nickel chloride (NiCl2∙6H2O), Palladium chloride (PdCl2), Sodium hydroxide (NaOH), Hydrochloric acid (HCl) and Methyl red were purchased from Sigma-Aldrich and used without additional purification. All solutions were prepared with deionized water. 2.2. Synthesis of the catalyst NiO/PdO catalyst was synthesized by co-precipitation method [29] from NiCl2∙6H2O and PdCl2 using NaOH. A 10 ml of 0.0047 M PdCl2 solution was added to 10 ml 0.17 M NiCl2∙6H2O solution and stirred for 2 h. Freshly prepared 0.1 M of NaOH was slowly added under continuous stirring till pH value reached 12. The precipitates were recollected by centrifugation and washed with deionized water and ethanol severally. The product was dried at 110 °C and later calcined at 550 °C for 3 h. 2.3. Characterization of the catalyst The synthesized NiO/PdO catalyst was characterized thus; crystallinity was determined using X-ray diffractometer (XRD) (GBC eMMA), surface microstructure and elemental composition were determined using a scanning electron microscope (SEM) equipped with energy dispersive analysis of x-ray equipment (EDX) (Quanta 200 – FEI coupled with EDS probe) and transmission electron microscope (TEM) (TECNAI G2 SPIRIT-FEI) was used for particle size and morphological
Fig. 1. XRD pattern of NiO/PdO catalyst. 146
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Fig. 2. SEM image of NiO/PdO catalyst. Fig. 4. TEM image of NiO/PdO catalyst.
0.9λ D= βCosθ
(2)
compared to NiO/PdO, it indicates a red shift in the absorption edge of the mixed oxide catalyst. Such a shift may be attributed to the surface plasmon absorption of palladium particles. The bandgap energy (Eg) value for the mixed oxide PdO/NiO catalyst is 2.95 eV using the formula in equation (3).
where β is the FWHM, λ is the X-ray wavelength and θ is the diffraction angle. Fig. 2 represents the SEM micrograph of NiO/PdO nanoparticles. It can be observed that the particles are formed in various sizes and the surface of the photocatalyst is irregular, the enlarged image shows that the smaller size PdO particles are distributed on the surface of the NiO particles. The NiO/PdO particles showed some degree of aggregation. Fig. 3 shows the corresponding EDX confirming the presence of Ni, Pd, and O in the synthesized NiO/PdO photocatalyst. The obtained catalyst was further characterized by TEM as shown in Fig. 4. The image shows the presence of highly crystalline NiO/PdO nanoparticles with the coexistence of polyhedral shaped particles. The PdO particles are observed on the surface of the NiO particles as shown with the higher contrast particles in the image. The close contact between PdO and NiO will make a strong electronic interaction which will improve the charge separation leading to an increase in photocatalytic activity [30]. The average particle size of the photocatalyst < 50 nm is also in agreement with the XRD analysis. The UV–vis diffuse reflectance spectrum (DRS) of the synthesized NiO/PdO nanocatalyst is shown in Fig. 5. The spectrum showed that the minimum reflectance was obtained λ = 419 nm and extended into the visible region. This makes the NiO/PdO nanocatalyst suitable for both the UV and visible region. As reported in the literature [31,32], the absorption peak of bulk NiO appeared between 362–368 nm when
Eg =
1239.8 λ
(3)
where λ is the wavelength Nitrogen adsorption-desorption isotherm was measured to determine the surface area of NiO/PdO nanocatalyst using BET method. As shown in Fig. 6, NiO/PdO exhibited a type IV isotherm pattern with H3 hysteresis around P/Po range of 0.6-1.0. The values of the BET specific surface area and pore diameter were 15.931 m2/g and 3.19 nm respectively indicating the presence of mesoporous material. Finally, the TGA graph (Fig. 7) was used to evaluate the thermal stability of NiO/PdO nanocatalyst. The curve demonstrates one stage in the weight loss profile around 800–1000 °C. The weight loss can be attributed to the loss of bonded oxygen present in the catalyst. The total weight loss of 12% shows the high thermal stability of the nanocatalyst which can lead to a longer catalyst lifetime and reduction in decomposition with a change in temperature. 3.2. Photocatalytic degradation reaction Photocatalytic activities of the NiO/PdO were evaluated by measuring efficiency for degradation of MR in an aqueous solution. Fig. 8 shows the results of MR degradation studies of the catalyst upon UV light, visible light (sunlight) and without any source of illumination (in the dark). The degradation efficiency (DE) was given by equation 1. The DE values achieved at 90 min were 97.8%, 92.5% and 6.06% respectively. From the experiment, the UV and sunlight degradation both showed superior photocatalytic efficiency towards MR indicating better hole/electron pair generation efficiency and the generation of hydroxyl and superoxide radicals at the illuminated wavelengths range. Also, the role of photocatalytic activities and surface interaction (adsorption) between the NiO/PdO nanoparticles and the dye solution was ascertained. The low value obtained signifies that the process was initiated by the photocatalytic reaction and not by adsorption and the reaction was induced by the action of the irradiation on the photocatalyst. The effect of catalyst load on degradation efficiency can be observed in Fig. 9. A control experiment was performed with the absence of the photocatalyst before varying its amount from 0.05 to 0.2 g, as expected the control experiment showed almost no degradation of MR. This proved the poor photolysis of MR in the absence of photocatalyst. It was
Fig. 3. EDX spectrum of NiO/PdO catalyst. 147
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Fig. 5. UV–vis diffuse reflectance spectrum of NiO/PdO catalyst.
Fig. 6. BET isotherm of NiO/PdO catalyst.
Fig. 8. Effect of irradiation source on the photocatalytic degradation of MR.
Fig. 7. TGA graph of NiO/PdO catalyst.
observed that as NiO/PdO load increased from 0.05 to 0.15 g the DE increased from 71.3 to 97.8% after which it decreased. This indicates an increase in the exposed total active surface area of the NiO/PdO catalyst with high production of OH radicals and more effective interaction of the substrate. But as the catalyst loading becomes larger, an increase in turbidity of the solution reduces the light transmission through the solution and possible aggregation of the catalyst (particle-particle interaction) at high solid concentration causing a decrease in DE [33]. The interpretation of the pH effect is an important parameter in photocatalytic reactions since it alters the substrate and catalyst surface charge, the interfacial electron transfer and the mechanism of hydroxyl radical generation [34]. This experiment was conducted at various pH
Fig. 9. Effect of catalyst load on the photocatalytic degradation of MR.
(ranging from 2 to 12). The result in Fig. 10 showed that pH significantly affected the DE of MR. The degradation efficiency of MR increased from 96.8% to 99.7% when the pH was increased from 2 to 4 and then decreased to 25.7% at pH 12. The maximum DE was attained at pH 4. The degradation is more effective in the acidic region due to the presence of more hydroxyl radicals produced by the interaction of hydroxide ions and positive holes of the catalyst and the efficient electron transfer due to rich surface complex bond formation [35]. At 148
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Fig. 11. The kinetic plot of the photocatalytic degradation of MR.
Fig. 10. Effect of pH on the photocatalytic degradation of MR.
high pH values, there is a decrease in DE probably caused by the coulombic repulsion between the hydroxide anions and the highly negative charged oxide surface resulting from the diffusion of more generated hydroxide ions on the surface of the photocatalyst. The increase in pH might also cause cathodic displacement of the valence band position of NiO and PdO in the NiO/PdO catalyst resulting in the weakening of the oxidation ability of the holes [36]. It has been shown that the photocatalytic degradation of azo dyes follows pseudo-first-order kinetics according to Langmuir-Hinshelwood model [37], that exhibits a linear relationship between In(C₀/C) and the reaction time (t). The photocatalytic reaction can be described as
−In
C₀ = kt C
(4) Fig. 12. Catalyst reusability on photocatalytic degradation of MR.
where C₀ and C are the initial concentration of MR and concentration of MR at reaction time t respectively, k is the degradation rate constant. The pseudo-first-order reaction constant k and the linear regression coefficient for MR with different initial concentrations are listed in Table 1. The rate constant k was evaluated from the slope of the plots between –In(C₀/C) and time (Fig. 11) and the plots found to be linear in accordance with the Langmuir-Hinshelwood model. The reaction rate constant k values decreased with an increase in the initial concentration of dye. This can be interpreted by considering the Langmuir-Hinshelwood model, according to the model the reactant is first absorbed onto the surface of the catalyst thereafter degradation occurs under irradiation. As the initial concentration of reactants increases the molecules aggregate on the surface of the catalyst resulting in quenching of the excited molecules [38]. Also, as the initial concentration increases the high adsorption of the incident photons for the generation of electronholes decreases, leading to a decrease in the rate constant of the reaction [39]. The R2 values clearly suggest that the photocatalytic degradation of MR seems to fit pseudo-first-order kinetics. To elucidate the stability of the photoactivity for NiO/PdO catalyst, the recycling efficiency of the catalyst was performed under the same experimental conditions. Over the four consecutive cycles, the catalyst did not exhibit any loss of activity and its degradation efficiency was slightly decreased as shown in Fig. 12, this slight reduction in the
efficiency may be attributed to the deposition of photoinsensitive hydroxide (Fouling) on the photocatalyst surface blocking its active sites. This result suggests that the catalyst displayed excellent stability and can be recycled over a period of time before exhaustion. The mechanism of photocatalytic degradation of MR by the surface of NiO/PdO is governed by the generation of conduction band electron and valence band hole pairs into the catalyst and its transportation to the organic pollutants. When the photocatalytic surface is exposed by a radiation of energy (photon) that equals or surpasses the band gap energy of the photocatalyst, it creates a positively charged hole and negatively charged electron in the valence band and the conduction band respectively by exciting the electrons in the valence band to the conduction band [40], the photogenerated holes and electrons react with the adsorbed surface-bound water and dye to form hydroxyl radicals and other reactive species. When PdO comes in contact with NiO core to form a heterojunction, the difference in chemical potential cause a band bending at the interface of the junction [41], this drives the photogenerated electron to transfer from NiO to PdO and photogenerated hole to migrate in the opposite direction until Fermi levels of NiO and PdO reaches equilibrium, this is due to PdO having a bandgap (1.55–2.25 eV) [42] smaller than that of NiO (3.4–3.8 eV) [43] and a likely type I band alignment. Upon UV irradiation, electrons in the valence band could be excited to the conduction band of both oxides but PdO particles having the ability to acquire electrons during photocatalytic process [44] and also a p-type semiconductor [45] has a larger hole concentration than electron concentration, therefore the photogenerated electrons were collected by the PdO particles and the holes by the NiO, that is, electrons transferred from NiO to PdO and holes migrated from PdO to NiO. The presence of Pd prevents recombination by trapping the electrons and generating holes that act as
Table 1 Kinetic parameters for photocatalytic degradation of MR. Initial Conc (M)
K (min−1)
R2
5 × 10−5 1 × 10−4 5 × 10−4
0.0195 0.0122 0.0051
0.976 0.971 0.931
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Fig. 13. Schematic representation of the photocatalytic degradation.
electron scavengers [46]. The retardation of the electron-hole recombination will increase the photocatalytic efficiency of the NiO/PdO catalyst and consequently, accelerate hydroxyl radical formation which will enhance the rate of MR degradation [47]. The photocatalytic mechanism is illustrated in Fig. 13.
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4. Conclusion Mixed oxide NiO/PdO nanoparticles exhibiting a high photocatalytic performance was successfully synthesized by the co-precipitation method. The morphologies of the nanoparticles were observed by SEM and TEM clearly show the formation of polyhedral shaped particles. XRD result shows the synthesized mixed oxide NiO/ PdO nanoparticles has a high crystallinity and average crystallite size of 22.7 nm, which was also in accordance with TEM result. Optical characterization revealed that the bandgap energy of the synthesized mixed oxide NiO/PdO photocatalyst was 2.95 eV indicating functionality within the visible region. Analysis from the EDX confirms the clear peaks of palladium, nickel and oxygen on the photocatalyst. Also, the BET method reveals the mesoporosity of the catalyst with the values for the specific surface area and the pore diameter at 15.931 m2/g and 3.19 nm respectively, the TG analysis confirms the thermal stability up to 800 °C for the catalyst. The results obtained from the studies of different parameters affecting photocatalytic degradation suggest that the mixed oxide NiO/PdO nanoparticles possess high photocatalytic activity in the degradation of methyl red in wastewater under both UV and visible irradiation. It was assumed that the high photocatalytic activity of the mixed oxide NiO/PdO nanoparticles was due to the presence of Pd which facilitated an increased charge separation efficiency of electron-hole pairs, resulting to increased photocatalytic degradation efficiency. Also, the results proved that the pseudo-first-order kinetic model is in good agreement with the experimental data. Furthermore, the synthesized mixed oxide NiO/PdO nanoparticles exhibit good stability and reusability. It is proved that the mixed oxide NiO/PdO can be used as a photocatalystfor environmental remediation application. References [1] [2] [3] [4]
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Photocatalytic behavior of mixed oxide NiO/PdO nanoparticles toward degradation of methyl red in water
T
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O.I. Omotunde , A.E. Okoronkwo, A.F. Aiyesanmi, E. Gurgur Department of Chemistry, The Federal University of Technology Akure, Nigeria
A R T I C LE I N FO
A B S T R A C T
Keywords: Mixed oxide catalyst Methyl red Photodegradation Degradation efficiency
Mixed oxide NiO/PdO nanoparticles were successfully prepared by a facile chemical co-precipitation route and were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy with energy dispersive x-ray analysis (SEM-EDX), UV–vis diffuse reflectance spectrophotometry (UVDRS), Brunauer-Emmett-Teller (BET) method and thermal analysis (TGA) techniques. The photocatalytic activities were assessed using methyl red as a model azo dye and the efficiency of the system was evaluated with respect to irradiation time, pH, catalyst load and initial concentration. The pseudo-first-order equation was revealed to fit degradation rate. The results show that the mixed oxide NiO/PdO catalyst has a high photocatalytic performance of 98% toward methyl red degradation and show good photostability.
1. Introduction The degradation of organic pollutants is an increasingly important phenomenon in universal research and semiconductor catalysts have been identified as a promising approach for effective treatment of wastewater [1,2]. This is largely because the use of semiconductor catalysts such as CdS, ZnO, Fe2O3 and TiO2 as photocatalysts does not generate any secondary pollution. The use of these semiconductors is hinged on their low cost, non-toxicity, chemical stability, high photocatalytic activity, optical and electrical properties [3,4]. While TiO2 has been largely used for complete degradation of recalcitrant organic pollutants [5], its wide bandgap limits its functionality because the catalytic activity can only be activated by the use of UV light [6], further, the easy recombination of photogenerated electron-hole pairs in TiO2 results in low quantum efficiency [7,8]. Therefore, the need to find alternatives to TiO2 for more effective excitation of electrons to activate photocatalytic processes. The use of transition metal oxides in the photocatalytic oxidation of organic molecules signifies a promising remediation approach for wastewater systems due to their unique magnetic, optical, electronic, thermal and mechanical properties and probable application in catalyst, gas-sensors and photo-electronic devices [9,10]. In this study, NiO which is a transition metal oxide broadly used as a catalyst with exceeding catalytic, electrical, redox and thermal properties have been identified as the preferred semiconductor making it appropriate for photocatalytic processes [11,12].
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Photocatalytic oxidation reactions deal with photoactivated metal oxides as semiconductors to eliminate contaminants in an aqueous environment [13]. The photocatalytic mechanism is initiated when a photon with energy ‘hv’ equals or surpasses the band gap energy of the semiconductors, the conduction electrons are excited from the valence band into the conduction band (CB) leaving a hole behind. The hole can either oxidize a compound instantly or react with electron donors like water to form OH radicals leading to reaction with the pollutants [14,15]. Recombination of the photogenerated hole (h+) and electron (e−) has proven to be a shortcoming of this approach. This recombination step drops the quantum yield and brings about energy wasting. Therefore, the e−/h+ recombination process should be controlled to guarantee efficient photocatalysis. Coupling of the semiconductor has been confirmed to improve the charge separation of the electron-hole pair which increases the duration of the charge carriers and leads to a decrease in the recombination of electron-hole [16,17]. Investigations have shown that nanosized noble metals such as silver, platinum, gold and palladium when loaded on the semiconductor surface can behave as an electron sink solving the issues of electron-hole recombination process and also act as an efficient charge separator during photocatalysis [18]. This nanosized noble metals exhibit unique physical, chemical, optical and thermodynamical properties at the nano regime leading them into many applications in catalysis [19,20]. Particularly, palladium nanoparticles have a wideranging application in heterogeneous and homogeneous catalysis because of their high surface to volume ratio [21]. Surface plasma
Corresponding author. E-mail address: [email protected] (O.I. Omotunde).
https://doi.org/10.1016/j.jphotochem.2018.08.005 Received 1 June 2018; Received in revised form 2 August 2018; Accepted 3 August 2018 Available online 04 August 2018 1010-6030/ © 2018 Elsevier B.V. All rights reserved.
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studies. A diffuse reflectance spectrum was recorded using Shimadzu UV-2450, the surface area was determined by Nitrogen adsorption in a Quantachrome Autosorb using Brunauer-Emmett-Teller (BET) method and thermogravimetric analysis (TGA) was performed on a Perkin Elmer Pyris thermal analyzer.
resonance (SPR) is another important feature in palladium nanoparticles which is useful in sensing, chemo optical transducers, plasmonic waveguiding [22,23]. In this work, to increase the efficiency of photocatalytic reaction through the extension of the absorption band of NiO and restriction of the recombination of photogenerated carriers palladium was chosen due to its ability to drastically enhance the absorption of visible light through localized surface plasmon resonance effects [24]. Azo dyes comprising of one or more azo bonds are the most commonly used synthetic dyes and largely the major pollutants in dye wastewater. Due to their noxiousness and slow degradation, these dyes are categorized as environmentally hazardous materials [25]. The resulting wastewater is very complex and presents difficulties when it comes to treatment due to the existence of non-biodegradable compounds. Numerous approaches have been developed for the removal of synthetic dyes from such wastewaters with the aim of improving their impact on the aquatic environment. Conventional water treatment procedures such as filtration, chemical, sedimentation, and membrane technologies are not viable due to high operating costs. Moreover, these procedures result in the production of toxic secondary pollutants that also have to be treated such as aromatic amines, for examples, aniline and substituted anilines [26]. Photocatalytic technology offers a simple and low-cost method for removing inorganic and organic contaminants from wastewater [27] since most organic pollutants may possibly be degraded or mineralized by use of photocatalytic degradation technology [28]. The most studied semiconductor oxides with photocatalytic activity other than TiO2 are ZnO, WO3, Ta2O5, Nb2O5, Bi2O3, CdO and SnO2, therefore, the use of NiO in this work is to open the bridge for researchers to take advantage of its potent catalytic activity under photocatalytic reaction other than the several chemical reactions nickel catalyst is known for. A limited study has been reported in synthesizing mixed oxides of NiO/PdO for the degradation of azo dyes. In the present work, NiO/PdO nanoparticles were obtained via co-precipitation method. The characteristics of the resulting catalyst and its efficiency in the degradation of methyl red were also examined.
2.4. Photocatalytic degradation studies The photocatalytic degradation of Methyl red (MR) dye by NiO/PdO photocatalyst was carried out using UV and visible light. A photoreactor with UV-C lamp (120 W) emitting 254 nm wavelength was used as the source of UV light. All experiments were performed under identical conditions. The control experiment was performed without photocatalyst to ensure the degradation was solely due to the presence of the photocatalyst. In each experiment 0.15 g of NiO/PdO was added to 50 ml of 1 × 10−4 M MR and kept in the dark for 30 min to establish adsorption-desorption equilibrium before subjecting to irradiation under UV light the mixture was agitated, filtered using 0.2 μm filter and the filtrate was monitored spectrally at λmax of MR = 437 nm. The MR degradation efficiency (DE) was calculated using the equation (1)
DE % =
(Ci−Cf ) Ci
x 100
(1)
where Ci is the initial MR concentration and Cf is the final MR concentration after photocatalytic degradation. The influence of the solution pH ranging between pH 2 and 12 was evaluated on the photocatalytic degradation reaction, the effect of irradiation timeconcentration profiles was studied at various time intervals and various concentrations. The optimum photocatalyst load on the photocatalytic degradation reaction was determined by varying the catalyst load between 0.05 g – 0.2 g. 3. Results and discussion 3.1. Characterization XRD measurement was performed on the NiO/PdO catalyst to investigate the crystal structure and phase purity and the resulting XRD pattern is shown in Fig. 1. The spectrum exhibits the characteristic peaks at 2θ values of 37.25, 43.35 and 62.95°. They were assigned to the (111), (200) and (220) crystal plane spacing of Face Central Cubic (FCC) NiO respectively and are in good agreement with the standard JCPDS data (No 04-0835). No clear diffraction peaks attributed to PdO could be observed due to the low concentration of the palladium, however, a PdO peak (110) at the value of 43° (JCPDS No 41-1107) could have overlapped with a peak of NiO at 2θ = 43.35° suggesting that the PdO ions were partially substituted in the NiO lattice site. Using the Debye Scherrer equation (2), the average crystallite size (D) was calculated to be 22.7 nm.
2. Experimental 2.1. Materials Nickel chloride (NiCl2∙6H2O), Palladium chloride (PdCl2), Sodium hydroxide (NaOH), Hydrochloric acid (HCl) and Methyl red were purchased from Sigma-Aldrich and used without additional purification. All solutions were prepared with deionized water. 2.2. Synthesis of the catalyst NiO/PdO catalyst was synthesized by co-precipitation method [29] from NiCl2∙6H2O and PdCl2 using NaOH. A 10 ml of 0.0047 M PdCl2 solution was added to 10 ml 0.17 M NiCl2∙6H2O solution and stirred for 2 h. Freshly prepared 0.1 M of NaOH was slowly added under continuous stirring till pH value reached 12. The precipitates were recollected by centrifugation and washed with deionized water and ethanol severally. The product was dried at 110 °C and later calcined at 550 °C for 3 h. 2.3. Characterization of the catalyst The synthesized NiO/PdO catalyst was characterized thus; crystallinity was determined using X-ray diffractometer (XRD) (GBC eMMA), surface microstructure and elemental composition were determined using a scanning electron microscope (SEM) equipped with energy dispersive analysis of x-ray equipment (EDX) (Quanta 200 – FEI coupled with EDS probe) and transmission electron microscope (TEM) (TECNAI G2 SPIRIT-FEI) was used for particle size and morphological
Fig. 1. XRD pattern of NiO/PdO catalyst. 146
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Fig. 2. SEM image of NiO/PdO catalyst. Fig. 4. TEM image of NiO/PdO catalyst.
0.9λ D= βCosθ
(2)
compared to NiO/PdO, it indicates a red shift in the absorption edge of the mixed oxide catalyst. Such a shift may be attributed to the surface plasmon absorption of palladium particles. The bandgap energy (Eg) value for the mixed oxide PdO/NiO catalyst is 2.95 eV using the formula in equation (3).
where β is the FWHM, λ is the X-ray wavelength and θ is the diffraction angle. Fig. 2 represents the SEM micrograph of NiO/PdO nanoparticles. It can be observed that the particles are formed in various sizes and the surface of the photocatalyst is irregular, the enlarged image shows that the smaller size PdO particles are distributed on the surface of the NiO particles. The NiO/PdO particles showed some degree of aggregation. Fig. 3 shows the corresponding EDX confirming the presence of Ni, Pd, and O in the synthesized NiO/PdO photocatalyst. The obtained catalyst was further characterized by TEM as shown in Fig. 4. The image shows the presence of highly crystalline NiO/PdO nanoparticles with the coexistence of polyhedral shaped particles. The PdO particles are observed on the surface of the NiO particles as shown with the higher contrast particles in the image. The close contact between PdO and NiO will make a strong electronic interaction which will improve the charge separation leading to an increase in photocatalytic activity [30]. The average particle size of the photocatalyst < 50 nm is also in agreement with the XRD analysis. The UV–vis diffuse reflectance spectrum (DRS) of the synthesized NiO/PdO nanocatalyst is shown in Fig. 5. The spectrum showed that the minimum reflectance was obtained λ = 419 nm and extended into the visible region. This makes the NiO/PdO nanocatalyst suitable for both the UV and visible region. As reported in the literature [31,32], the absorption peak of bulk NiO appeared between 362–368 nm when
Eg =
1239.8 λ
(3)
where λ is the wavelength Nitrogen adsorption-desorption isotherm was measured to determine the surface area of NiO/PdO nanocatalyst using BET method. As shown in Fig. 6, NiO/PdO exhibited a type IV isotherm pattern with H3 hysteresis around P/Po range of 0.6-1.0. The values of the BET specific surface area and pore diameter were 15.931 m2/g and 3.19 nm respectively indicating the presence of mesoporous material. Finally, the TGA graph (Fig. 7) was used to evaluate the thermal stability of NiO/PdO nanocatalyst. The curve demonstrates one stage in the weight loss profile around 800–1000 °C. The weight loss can be attributed to the loss of bonded oxygen present in the catalyst. The total weight loss of 12% shows the high thermal stability of the nanocatalyst which can lead to a longer catalyst lifetime and reduction in decomposition with a change in temperature. 3.2. Photocatalytic degradation reaction Photocatalytic activities of the NiO/PdO were evaluated by measuring efficiency for degradation of MR in an aqueous solution. Fig. 8 shows the results of MR degradation studies of the catalyst upon UV light, visible light (sunlight) and without any source of illumination (in the dark). The degradation efficiency (DE) was given by equation 1. The DE values achieved at 90 min were 97.8%, 92.5% and 6.06% respectively. From the experiment, the UV and sunlight degradation both showed superior photocatalytic efficiency towards MR indicating better hole/electron pair generation efficiency and the generation of hydroxyl and superoxide radicals at the illuminated wavelengths range. Also, the role of photocatalytic activities and surface interaction (adsorption) between the NiO/PdO nanoparticles and the dye solution was ascertained. The low value obtained signifies that the process was initiated by the photocatalytic reaction and not by adsorption and the reaction was induced by the action of the irradiation on the photocatalyst. The effect of catalyst load on degradation efficiency can be observed in Fig. 9. A control experiment was performed with the absence of the photocatalyst before varying its amount from 0.05 to 0.2 g, as expected the control experiment showed almost no degradation of MR. This proved the poor photolysis of MR in the absence of photocatalyst. It was
Fig. 3. EDX spectrum of NiO/PdO catalyst. 147
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Fig. 5. UV–vis diffuse reflectance spectrum of NiO/PdO catalyst.
Fig. 6. BET isotherm of NiO/PdO catalyst.
Fig. 8. Effect of irradiation source on the photocatalytic degradation of MR.
Fig. 7. TGA graph of NiO/PdO catalyst.
observed that as NiO/PdO load increased from 0.05 to 0.15 g the DE increased from 71.3 to 97.8% after which it decreased. This indicates an increase in the exposed total active surface area of the NiO/PdO catalyst with high production of OH radicals and more effective interaction of the substrate. But as the catalyst loading becomes larger, an increase in turbidity of the solution reduces the light transmission through the solution and possible aggregation of the catalyst (particle-particle interaction) at high solid concentration causing a decrease in DE [33]. The interpretation of the pH effect is an important parameter in photocatalytic reactions since it alters the substrate and catalyst surface charge, the interfacial electron transfer and the mechanism of hydroxyl radical generation [34]. This experiment was conducted at various pH
Fig. 9. Effect of catalyst load on the photocatalytic degradation of MR.
(ranging from 2 to 12). The result in Fig. 10 showed that pH significantly affected the DE of MR. The degradation efficiency of MR increased from 96.8% to 99.7% when the pH was increased from 2 to 4 and then decreased to 25.7% at pH 12. The maximum DE was attained at pH 4. The degradation is more effective in the acidic region due to the presence of more hydroxyl radicals produced by the interaction of hydroxide ions and positive holes of the catalyst and the efficient electron transfer due to rich surface complex bond formation [35]. At 148
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Fig. 11. The kinetic plot of the photocatalytic degradation of MR.
Fig. 10. Effect of pH on the photocatalytic degradation of MR.
high pH values, there is a decrease in DE probably caused by the coulombic repulsion between the hydroxide anions and the highly negative charged oxide surface resulting from the diffusion of more generated hydroxide ions on the surface of the photocatalyst. The increase in pH might also cause cathodic displacement of the valence band position of NiO and PdO in the NiO/PdO catalyst resulting in the weakening of the oxidation ability of the holes [36]. It has been shown that the photocatalytic degradation of azo dyes follows pseudo-first-order kinetics according to Langmuir-Hinshelwood model [37], that exhibits a linear relationship between In(C₀/C) and the reaction time (t). The photocatalytic reaction can be described as
−In
C₀ = kt C
(4) Fig. 12. Catalyst reusability on photocatalytic degradation of MR.
where C₀ and C are the initial concentration of MR and concentration of MR at reaction time t respectively, k is the degradation rate constant. The pseudo-first-order reaction constant k and the linear regression coefficient for MR with different initial concentrations are listed in Table 1. The rate constant k was evaluated from the slope of the plots between –In(C₀/C) and time (Fig. 11) and the plots found to be linear in accordance with the Langmuir-Hinshelwood model. The reaction rate constant k values decreased with an increase in the initial concentration of dye. This can be interpreted by considering the Langmuir-Hinshelwood model, according to the model the reactant is first absorbed onto the surface of the catalyst thereafter degradation occurs under irradiation. As the initial concentration of reactants increases the molecules aggregate on the surface of the catalyst resulting in quenching of the excited molecules [38]. Also, as the initial concentration increases the high adsorption of the incident photons for the generation of electronholes decreases, leading to a decrease in the rate constant of the reaction [39]. The R2 values clearly suggest that the photocatalytic degradation of MR seems to fit pseudo-first-order kinetics. To elucidate the stability of the photoactivity for NiO/PdO catalyst, the recycling efficiency of the catalyst was performed under the same experimental conditions. Over the four consecutive cycles, the catalyst did not exhibit any loss of activity and its degradation efficiency was slightly decreased as shown in Fig. 12, this slight reduction in the
efficiency may be attributed to the deposition of photoinsensitive hydroxide (Fouling) on the photocatalyst surface blocking its active sites. This result suggests that the catalyst displayed excellent stability and can be recycled over a period of time before exhaustion. The mechanism of photocatalytic degradation of MR by the surface of NiO/PdO is governed by the generation of conduction band electron and valence band hole pairs into the catalyst and its transportation to the organic pollutants. When the photocatalytic surface is exposed by a radiation of energy (photon) that equals or surpasses the band gap energy of the photocatalyst, it creates a positively charged hole and negatively charged electron in the valence band and the conduction band respectively by exciting the electrons in the valence band to the conduction band [40], the photogenerated holes and electrons react with the adsorbed surface-bound water and dye to form hydroxyl radicals and other reactive species. When PdO comes in contact with NiO core to form a heterojunction, the difference in chemical potential cause a band bending at the interface of the junction [41], this drives the photogenerated electron to transfer from NiO to PdO and photogenerated hole to migrate in the opposite direction until Fermi levels of NiO and PdO reaches equilibrium, this is due to PdO having a bandgap (1.55–2.25 eV) [42] smaller than that of NiO (3.4–3.8 eV) [43] and a likely type I band alignment. Upon UV irradiation, electrons in the valence band could be excited to the conduction band of both oxides but PdO particles having the ability to acquire electrons during photocatalytic process [44] and also a p-type semiconductor [45] has a larger hole concentration than electron concentration, therefore the photogenerated electrons were collected by the PdO particles and the holes by the NiO, that is, electrons transferred from NiO to PdO and holes migrated from PdO to NiO. The presence of Pd prevents recombination by trapping the electrons and generating holes that act as
Table 1 Kinetic parameters for photocatalytic degradation of MR. Initial Conc (M)
K (min−1)
R2
5 × 10−5 1 × 10−4 5 × 10−4
0.0195 0.0122 0.0051
0.976 0.971 0.931
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Fig. 13. Schematic representation of the photocatalytic degradation.
electron scavengers [46]. The retardation of the electron-hole recombination will increase the photocatalytic efficiency of the NiO/PdO catalyst and consequently, accelerate hydroxyl radical formation which will enhance the rate of MR degradation [47]. The photocatalytic mechanism is illustrated in Fig. 13.
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4. Conclusion Mixed oxide NiO/PdO nanoparticles exhibiting a high photocatalytic performance was successfully synthesized by the co-precipitation method. The morphologies of the nanoparticles were observed by SEM and TEM clearly show the formation of polyhedral shaped particles. XRD result shows the synthesized mixed oxide NiO/ PdO nanoparticles has a high crystallinity and average crystallite size of 22.7 nm, which was also in accordance with TEM result. Optical characterization revealed that the bandgap energy of the synthesized mixed oxide NiO/PdO photocatalyst was 2.95 eV indicating functionality within the visible region. Analysis from the EDX confirms the clear peaks of palladium, nickel and oxygen on the photocatalyst. Also, the BET method reveals the mesoporosity of the catalyst with the values for the specific surface area and the pore diameter at 15.931 m2/g and 3.19 nm respectively, the TG analysis confirms the thermal stability up to 800 °C for the catalyst. The results obtained from the studies of different parameters affecting photocatalytic degradation suggest that the mixed oxide NiO/PdO nanoparticles possess high photocatalytic activity in the degradation of methyl red in wastewater under both UV and visible irradiation. It was assumed that the high photocatalytic activity of the mixed oxide NiO/PdO nanoparticles was due to the presence of Pd which facilitated an increased charge separation efficiency of electron-hole pairs, resulting to increased photocatalytic degradation efficiency. Also, the results proved that the pseudo-first-order kinetic model is in good agreement with the experimental data. Furthermore, the synthesized mixed oxide NiO/PdO nanoparticles exhibit good stability and reusability. It is proved that the mixed oxide NiO/PdO can be used as a photocatalystfor environmental remediation application. References [1] [2] [3] [4]
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