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Facile fabrication of Zr2Ni1Cu7 trimetallic nano-alloy and its composite with Si3N4 for visible light assisted photodegradation of methylene blue
Journal of Molecular Liquids 272 (2018) 170–179
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Facile fabrication of Zr2Ni1Cu7 trimetallic nano-alloy and its composite with Si3N4 for visible light assisted photodegradation of methylene blue Gaurav Sharma a,b,c,⁎, Amit Kumar a,b,c, Shweta Sharma c, Mu. Naushad d, Tansir Ahamad d, Sameerah I. Al-Saeedi e, Ghadah M. Al-Senani e, Nada S. Al-kadhi e, Florian J. Stadler a,⁎ a College of Materials Science and Engineering, Shenzhen Key Laboratory of Polymer Science and Technology, Guangdong Research Center for Interfacial Engineering of Functional Materials, Nanshan District Key Lab. for Biopolymers and Safety Evaluation, Shenzhen University, Shenzhen 518060, PR China b Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, PR China c School of Chemistry, Shoolini University, Solan 173212, Himachal Pradesh, India d Department of Chemistry, College of Science, King Saud University, Bld.#5, Riyadh, Saudi Arabia e Department of Chemistry, College of Science, Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 18 July 2018 Received in revised form 6 September 2018 Accepted 11 September 2018 Available online 13 September 2018 Keywords: Trimetallic nano-alloy Composite Photocatalytic degradation Methylene blue
a b s t r a c t Novel Zr2Ni1Cu7 trimetallic nano-alloy (TNA) and Zr2Ni1Cu7/Si3N4 trimetallic nano-alloy composite (TNAC) have been successfully prepared by facile microwave reduction method. The synthesis of TNA and TNAC was ascertained by characterizing them by using various techniques such as SEM, TEM, XRD, EDX and FTIR etc. The EDX shows the presence of all constituents in TNA and TNAC. The XRD study demonstrated the crystalline and semi-crystalline nature of TNA and TNAC. The optical band gap study revealed that TNAC (2.47 eV) has lower band gap then TNA (2.54 eV). The visible light photocatalytic activity of Zr2Ni1Cu7TNA and Zr2Ni1Cu7/Si3N4 TNAC has been successfully utilized for the photodegradation of methylene blue (MB). The presence of H2O2 has been found to influence the photodegradation rate. Photodegradation results presented that maximum degradation was observed in the presence of H2O2 and Zr2Ni1Cu7TNAC (92%). Scavenging activity has revealed that the hydroxyl radicals were the major reacting species. These species successfully degraded the MB into various intermediates that has successfully been determined by the LC-MS. Kinetic studies indicated that the degradation of MB by Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC followed pseudo-first-order kinetics. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Nanocomposite materials, composed of more than two phases, are the extensively explored materials having lavishing properties such as high mechanical, electrical, thermal, elastic and optical properties, etc. Number of techniques has been put forward for designing nanocomposites which can be categorized into approaches; bottom up and topdown. Combination at the nanoscale provides them high surface areato- mass ratio that helps in enhancing their adsorbing ability which in turn helps in removing noxious pollutants from environment. Nanocomposite materials can also be designed in such a way that it reduces the electron-hole recombination and provide synergistic effect between the constituents that helps in degradation of pollutants. These can be categorized into various forms [1,2] and one such form is nanoalloys. In material chemistry, mixing number of metals helps in extending the properties of the obtained system, generally called alloys or ⁎ Corresponding authors. E-mail addresses: [email protected] (G. Sharma), [email protected] (F.J. Stadler).
https://doi.org/10.1016/j.molliq.2018.09.063 0167-7322/© 2018 Elsevier B.V. All rights reserved.
intermediate compounds [3]. Alloying helps in enriching the properties because of optimized composition, structure diversity and synergistic effect between the constituents that increases their utilization in engineering, catalysis and electronics etc. [4]. Emergence of nanotechnology has led to the urge to synthesize materials with controllable properties at the nanoscale geometry that has led to the fabrication of nanoalloys [5]. Number of techniques has been suggested for their designing such as electrochemical, radiolysis, sonochemical, biosynthesis, ion implantation, co-precipitation and chemical reduction etc. [6,7]. Nanoalloys are of great interest due to their ability of monitoring the physical and chemical properties by varying the composition, atomic ordering and size [8,9]. The engineering properties like tensile and shear strength differ from constituent units whereas physical properties like electrical and thermal conductivity, reactivity, density and Young's modulus, of nanoalloys differ from its individual elemental properties. Pure metals have single melting point but nanoalloys doesn't, inspite they have a range of melting points which consist of blend of solid and liquid phases. The temperature at which the process of melting starts is called solidus and where melting completes is known as liquidus. Number of bimetallic and trimetallic nanoalloys have been designed in the recent decades
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[10]. Trimetallic nanoalloys have been extensively employed for various applications due to better synergistic behavior of its constituents leading to their enhanced optical, mechanical and electronic effects [11,12]. Rapid increase in the discharge of effluents from various industries consisting of organic dyes has resulted in water pollutions challenge all across the world. The organic dyes structurally consist of varying number of benzene rings which are not easily degraded by the available conventional methods such as physical and biological methods [13,14]. These dyes contain various toxic chemical groups which have diverse harmful effects on aquatic and public life. Approximately 15% of the total dyes are acquired as the textile effluents which are added to the water system during dyeing process [15]. These are colored compounds and thus result into various devastating effects such as non-aesthetic pollution, eutrophication and perturbations in the aquatic life [16]. So their removal from the water bodies is necessary. Since the international standards of environment are becoming more rigorous (ISO 14001, October 1996), various technical systems for the exclusion of these organic dyes have been established. One such organic dye is methylene blue (MB), which is a heterocyclic compound having potential applications in various industries such as plastic, pharmaceuticals, cosmetic and food, etc. [17]. MB was first synthesized in 1876 for use in textile industries. In initial times of its origin, it was even utilized for curing malaria and it was also the first synthetic compound ever used as an antiseptic in clinical therapy [18]. However, its complete studies have generalized that it has few side effects also which includes diarrhea, vomiting, jaundice, tissue necrosis and nausea, etc. [19]. So number of approaches have been reported in the literature for their removal such as osmosis [20], biosorption [21], adsorption [22–26], ultrafiltration [27], chlorination, ion exchange [28], membrane filtration [29], liquid-liquid extraction [30] and heterogeneous photocatalysis [31–34] etc. Among these techniques, photocatalytic degradation is getting extensively explored due to its easy operation, less toxic after effects and high removal efficiency [35–37]. Recent decades have marked the use of various photocatalysts such as TiO2, Fe2O3 and ZnS, etc. for the degradation of noxious dyes due to their high solar light absorption ability. In addition, not much explored area of photocatalysts; bi and trimetallic nanomaterials are also emerging as proficient photocatalyst due to their synergistic properties and high catalytic activity [12,38,39]. Silicon nitride (Si3N4) is a newly explored material having exceptional physical and chemical properties such as improved chemical stability, high melting point and boiling point, high resistance to thermal shock and mechanical strength [40,41]. Its covalent bonding made it hard to sinter and thus require prolonged time and temperature for sintering [42]. Various methods utilized for its fabrication are reduction of silica carbothermically, direct nitration of silicon [43] and high temperature pyrolysis [44,45]. Nickel nanoparticles have high ability of being used in catalysis but due to high agglomeration their potential applications decreases. In order to overcome this problem, trimetallic nanoclusters or nanoalloy; Zr2Ni1Cu7 has been fabricated that helped in diminishing its drawbacks. Combination provided high catalytic and better optical properties to the fabricated nanoalloy [46]. Herein, we have presented the synthesis and characterization of trimetallic nano-alloy (Zr2Ni1Cu7 TNA) and its nano-alloy composite (Zr2Ni1Cu7/Si3N4 TNAC) that have been successfully utilized for the remediation of highly toxic organic dye; methylene blue (MB). Importance of presented paper lies in the fact that we have combined the properties of three metals and silicon nitride so to provide better nano-alloy; having photocatalytic properties that can be efficiently utilized for the remediation of MB from the aqueous solution.
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nitrate (99.99%, Chemical Drug House, India), cupric nitrate (N98%, Qualikem, India), nickel nitrate (N99%, LobaChemie, India) and trisodium citrate (N97%, LobaChemie, India). Double distilled water was used throughout the experiments. Muffle furnace (MSW-275), UV–visible spectrophotometer (Systronics 2202), X-ray diffractrometer (Phillips Holland), Fourier transform infrared (FTIR) spectrometer (Perkin Helmer), Scanning electron microscope (SEM QANT-250, Model 9393), Transmission electron microscope (TEM Technai G2 20 S-Twin) and Liquid chromatography mass spectrometer (Q-ToF Micro Water LCMS spectrometer). 2.2. Synthesis of silicon nitride Synthesis of silicon nitride was done by simple pyrolysis process. In typical procedure, finely powdered silica and urea were mixed in the ratio of 1:0.9 in a silica crucible. Nitrogen gas was purged into the crucible so to provide it an oxygen free environment. The crucibles were then subjected to 600 °C with a ramping rate of 10 °C/min for initial 60 min and then allowed to withstand at this temperature for another 120 min. After time completion, greyish white colored powder of silicon nitride was obtained which was utilized for further experimental procedures. 2.3. Synthesis of Zr2Ni1Cu7 trimetallic nano-alloy and Zr2Ni1Cu7/Si3N4 trimetallic nano-alloy composite Zr2Ni1Cu7 trimetallic nano-alloy was synthesized by stepwise microwave reduction method. To the 20 mL of 20 mM zirconium oxy-nitrate solution, 2% trisodium citrate (3 mL) was added which acted as the reducing agent. The reaction mixture was then heated for 2 min in microwave oven at 800 W in a cyclic mode (20 s. ON, 10 s. OFF). In the same manner, 20 mM nickel nitrate and cupric nitrate solutions with 2% tri sodium citrate solution as a reducing agent were simultaneously added and the solution then again placed in microwave oven at 800 W in cyclic mode. Finally obtained nano-alloy Zr2/Ni1/Cu7 was washed with double distilled water and dried in hot air oven at 50 °C for 60 min [47,48]. Zr2Ni1Cu7/Si3N4 trimetallic nano-alloy composite was synthesized by similar stepwise microwave reduction method. Firstly, 0.02 g above prepared Si3N4 was dispersed in 50 mL double distilled water. In presence of this suspension, whole above process for synthesis of Zr2Ni1Cu7 was repeated to obtain trimetallic nano-alloy composite. 2.4. Characterization Structural elucidations of TNA and TNAC were carried out by Perkin Helmer FTIR spectrophotometer in the range of 400–4000 cm−1. The X-ray diffraction patterns were obtained by Phillips Holland diffractometer. The morphological and surface analysis of Zr/Ni/Cu TNA and Zr2Ni1Cu7/Si3N4 TNAC were done by QANT-250, Model 9393 scanning electron microscope and Technai G2 20 S-Twin transmission electron microscope. Optical and photocatalytic examinations were carried out by Systronics 2202 double beam UV–Vis spectrophotometer. 2.5. Optical studies
2.1. Chemicals and instruments
The optical properties of Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC were generalized by dissolving 5 mg materials in 10 mL ethanol solution. The suspensions were ultrasonicated for 30 min and then analyzed at various wavelengths using double beam UV–Vis spectrophotometer. Tauc relation was used for determining the optical band gap values of Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC [49]:
All the chemicals were of analytical grade and used without any treatment process. The reagents used in the study were zirconium oxy
αhν ¼ A hν−Eg n
2. Material and methodology
ð1Þ
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where ɑ is the adsorption coefficient, hν represents the photon energy, Eg is optical band gap of Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC, A = band tailing parameter and n = 2 or 1/2 for direct or indirect band gap, respectively. The band edge values of Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC were examined using following equations [50,51]: EVB ¼ X−Ee þ 0:5 Eg
ð2Þ
ECB ¼ EVB −Eg
ð3Þ
where EVB is the valence band edge potential; ECB is the conduction band edge potential; X is the Pearson absolute electronegativity of photocatalyst, Eg represents the band gap of the photocatalyst and Ee is the energy of electrons on hydrogen scale (4.5 eV). 2.6. Photocatalytic experiments The photocatalytic activity of fabricated Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC were estimated by the photodegradation of methylene blue (MB) as test dye. In typical experiments, 0.020 g of TNA and TNAC were dispersed into 20 ppm MB solution and solar light was used as the visible-light source. Before subjecting the suspension to visible light irradiations, it was stirred on a magnetic stirrer to achieve adsorption-desorption equilibrium. Pre-defined 3 mL aliquots of the suspensions were taken out at various time intervals, centrifuged at 2500 rpm for 5 min to remove suspended TNA and TNAC particles and then analyzed by double beam UV–Vis spectrophotometer at 664 nm. Percent degradation of MB by synthesized TNA and TNAC were calculated using formula [52]: Percent degradation ¼
C 0 −C t 100 Ct
Fig. 1. FTIR spectra of Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC.
respectively [55]. Fig. 2(a) shows the XRD pattern of synthesized Zr2Ni1Cu7TNC. The peaks at 2θ value of 34.12°, 39.01° and 49.21° corresponds to (002), (111) and (202) diffraction plane of copper metal which is in accordance with the JCPDS No.05–661 [56]. Presence of nickel has been confirmed by the peaks at 36.96°, 43.14° and 32.06° which corresponds to (111), (200) and (331) diffraction planes (JCPDS No.01–073-1523) [57]. The peak at 29.820 and 50.37° corresponds to the (101) and (112) diffraction plane in accordance with the JCPDS No. 81-1544 for zirconium [58]. However, Fig. 2(b) shows the change in 2θ values, generalizing the successful formation of the Zr2Ni1Cu7/Si3N4
ð4Þ
The removal rate of methylene blue was examined by pseudo-firstorder kinetics model using following equation: r¼−
dc ¼ kapp t dt
ð5Þ
On integrating the above equation ln
C0 ¼ −kapp t Ct
ð6Þ
where kapp is the apparent rate constant, C0 is the initial concentration of MB before visible light irradiation and Ct is the concentration of MB after visible light irradiation at particular time t (minute). 3. Results and discussion 3.1. Characterization FTIR spectra of Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC have been presented in Fig. 1. Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC shows prominent peak at 3221 cm−1and 3236 cm−1 which can be ascribed to the O\\H stretching mode of hydroxyl groups [53]. Intense peaks at 1568 cm−1, 846 cm−1 and 461 cm−1 specifies the strong absorption band for Zr\\O, Ni\\O and Cu\\O stretching [12,38,54]. Peak around 2352 cm−1 and 1374 cm−1 corresponds to the atmospheric \\CO2 groups present in Zr2Ni1Cu7TNA (Fig. 1(b)). Change in intensities of characteristic peaks signifies the imprinting of TNA onto Si3N4 (Fig. 1 (a)). The additional peak at 1075 cm−1 for Zr2Ni1Cu7/Si3N4 TNAC corresponds to stretching mode of Si\\N [45]. XRD results show that there was change in 2θ values of Zr2Ni1Cu7/Si3N4 TNAC as compared to Zr2Ni1Cu7 TNC. The XRD study of Zr2Ni1Cu7 TNC and Zr2Ni1Cu7/Si3N4 TNAC confirms the crystalline and semi-crystalline nature of materials
Fig. 2. XRD patterns of (a) Zr2Ni1Cu7 TNA and (b) Zr2Ni1Cu7/Si3N4 TNAC.
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TNAC. Also, some additional peaks at 2θ values of 32.05° and 39.05° corresponding to (200) and (101) has also been observed confirming the presence the Si3N4. For analyzing the surface features of Zr2Ni1Cu7 TNC and Zr2Ni1Cu7/Si3N4 TNAC, SEM study was performed which generalize the change in morphology from TNA to TNAC formation (Fig. 3 (a-d)). The Fig. 3 (a-b) shows Zr2Ni1Cu7 TNA highly stacked on each other with irregular shape, whereas, Fig. 3 (c-d) represents the attachment of Zr2Ni1Cu7 TNA on to the surface of Si3N4. The Fig. 3 (e-f) demonstrate the EDX patterns of Zr2Ni1Cu7TNC and Zr2Ni1Cu7/Si3N4TNAC reveling the presence on zirconium, copper, nickel, silicon and nitrogen in materials. Transmission electron microscopy was done to elucidate the structural features of TNA and TNAC. Fig. 4(a,b,c) shows the TNA at various magnifications. Fig. 4(a) shows the highly agglomerated structure of TNA and Fig. 4(b and c) shows the magnified features of TNA. Fig. 4 (c) shows HRTEM with lattice fringes arrangement showing d-spacing of the constituting metals which confirms the successful fabrication of the TNA. However, Fig. 4(d,e,f) shows the morphological changes that arises due to the imprinting of TNA onto Si3N4. Sheet- like structure of
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the Si3N4 is well elucidated and the highly agglomerated TNA onto its surface is clearly visible.
3.2. Optical studies The tauc plot for Zr2Ni1Cu7 TNA, Zr2Ni1Cu7/Si3N4 TNAC and Si3N4 has been shown in Fig. 5(a-c). The optical band gap values for Zr2Ni1Cu7 TNA, Zr2Ni1Cu7/Si3N4 TNAC and Si3N4 has been calculated to be 2.54, 2.47 and 2.19 eV respectively which lies in the visible range region. Valence and conduction band energy value for Zr2Ni1Cu7 TNA has been found to be 0.654 and − 1.886 eV respectively. In case of Si3N4, valence band energy value is 2.805 eV and conduction band energy value is 0.615 eV which shows the movement of electrons from the conduction band of Zr2Ni1Cu7 TNA to conduction band of Si3N4 and holes movement from the valence band of Si3N4 to valence band of Zr/Ni/Cu TNA. This electron- hole movement in opposite direction lowers the electron-hole pair recombination, thus generalizing its photocatalytic
Fig. 3. SEM morphologies of (a,b) Zr2Ni1Cu7 TNA, (c,d) Zr2Ni1Cu7/Si3N4 TNAC and EDX of (e) Zr2Ni1Cu7 TNA (f) Zr2Ni1Cu7/Si3N4 TNAC.
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Fig. 4. TEM images of (a,b,c) Zr2Ni1Cu7 TNA and (d,e,f) Zr2Ni1Cu7/Si3N4 TNAC.
degradation ability. Fig. 5(d) shows the probable heterojunction for the fabricated Zr2Ni1Cu7/Si3N4 TNAC. 3.3. Photocatalytic activity As the targeted pollutant, methylene blue (MB) was chosen for evaluating the photocatalytic degradation ability of synthesized TNA and TNAC. Firstly, plain photolysis of MB was experimented for 90 min under visible light irradiation using solar light without the addition of TNA and TNAC. Results presented in Fig. 6(a) predicted the stability of MB in water as only 4% degradation was recorded. Fig. 6(b) depicts the adsorption ability of TNA and TNAC for MB molecules. Results generalized that only 18 and 27% adsorption was obtained for TNA and TNAC respectively. High adsorption rate of TNAC can be attributed to the presence of Si3N4 that provide surface area for the adsorption of MB molecules. The effect of solution pH was studied in the range of 2–10 and the results presented in Fig. 6(c) demonstrate that maximum degradation was observed at pH 8 for both Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC. MB is one of the dyes that do not have any pKa value between 0 and 14 pH and exists only in the form of MB+ [59]. So at pH 8, the MB dye is positively charged and the surface of both the TNA and TNAC is negatively charged as determined from pHpzc which thus offers maximum attraction between photocatalyst surface and MB molecules [60].In addition to pH, how the TNA and TNAC amount is affecting the
degradation rate was also studied. For this, various amounts of TNA and TNAC ranging from 5 to 30 mg was added into the 20 ppm solution of MB and irradiated with visible light for 90 min (Fig. 6(d)). Results indicated that maximum degradation rate was observed with 20 mg of TNAC and 25 mg of TNA. Reason can be that increase in the amount of TNA and TNAC increased the number of photons absorbed and also the number of MB molecules adsorbed onto their surface. This increased the density of TNA and TNAC in the area of illumination as a result of which the rate was enhanced. However, above a certain level, the increase in TNA and TNAC concentration do not effect degradation rate because at high concentration the exposer to light get hindered due to scattering also known as photon dispersion not. Fig. 7 (a,b) presents the time- dependent change in absorbance of MB degradation at various wavelengths under visible light irradiation for 90 min by Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC. Maximum absorption peak for MB has been observed at 664 nm that gradually decreased with increased time of visible light irradiation and maximum decrease has been observed in presence of Zr2Ni1Cu7/Si3N4 TNAC. Decrease in absorption peak intensity confirmed the high photocatalytic activity of fabricated Zr2Ni1Cu7/Si3N4 TNAC as compared to Zr2Ni1Cu7 TNA. Final photocatalytic degradation experimentation of the MB by Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC was done under two respective conditions; one was the photodegradation in presence of H2O2 and other was photodegradation in absence of H2O2. Results presented
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Fig. 5. Tauc plots for (a) Zr2Ni1Cu7 TNA (b) Zr2Ni1Cu7/Si3N4 TNAC and (c) Si3N4 (d) heterojunction ofZr2Ni1Cu7/Si3N4.
that H2O2 was enhancing the degradation rate. H2O2 is generally considered as the degradation rate enhancing agent. Actually, it helps in generating high reactive hydroxyl radicals that on interaction with MB molecules carry out its degradation and helps in its complete removal.
The degradation observed without H2O2 is 67% for TNA and 79% for TNAC. The degradation was significantly high in presence of H2O2 i.e., 81% for TNA and 92% for TNAC. Fig. 7 (c, d) demonstrate that high degradation rate of MB has been observed in the presence of H2O2
Fig. 6. (a) Plain photolysis of MB and (b) Percent adsorption of MB onto Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC; pH effect on photocatalytic degradation of MB by Zr2Ni1Cu7 TNAandZr2Ni1Cu7/Si3N4 TNAC; Effect of photocatalyst amount on degradation of MB by Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC.
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Fig. 7. Absorabnce plots for photodegradation of MB by (a) Zr2Ni1Cu7 TNAand (b) Zr2Ni1Cu7/Si3N4 TNAC; Percent degradation of MB by (c) Zr2Ni1Cu7 TNA (d) Zr2Ni1Cu7/Si3N4 TNAC; Pseudo- first order kinetics for the photocatalytic degradation of MB by (e) Zr2Ni1Cu7 TNA and (f) Zr2Ni1Cu7/Si3N4 TNAC; Scavenger effect on the photocatalytic degradation of MB by (g) Zr2Ni1Cu7 TNA (h) Zr2Ni1Cu7/Si3N4 TNAC (MB concentration = 20 ppm, photocatalyst amount = 0.020 g and time = 90 min).
with both Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC. However, out of the two, maximum rate has been observed with Zr2Ni1Cu7/Si3N4 TNAC in the presence of H2O2 which can be attributed to reasons such as high absorption in the visible region, lower band gap, increased charge Table 1 Comparison of photocatalytic activity of various materials for methylene blue along with their degradation percentage. Sr. No
Material
Percent degradation
Reference
1. 2. 3. 4. 5.
ZrC nanopowder TiO2 nanoparticles V2O3/CNT/TiO2 TiO2/carbon Zr2Ni1Cu7/Si3N4 TNAC
80.21% 90% 70% 84.7% 92%
[61] [62] [63] [64] Present study
separation efficiency and reduced electron-hole recombination. Enhancement in migration of charge carriers increased the synergistic effect between Zr2Ni1Cu7 TNA and Si3N4. Table 1 presents the Table 2 First order kinetic rate constant (k) and regression coefficient (R2) of MB degradation using photocatalyst Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC under visible light illumination.
Photocatalyst
Experimental condition
Rate constant (min−1)
Regression coefficient (R2)
Zr2Ni1Cu7 TNA
Without H2O2 With H2O2 Without H2O2 With H2O2
0.002 0.006 0.002 0.004
0.894 0.979 0.895 0.991
Zr2Ni1Cu7/Si3N4 TNAC
G. Sharma et al. / Journal of Molecular Liquids 272 (2018) 170–179
comparative study of photocatalytic activity of various materials for methylene blue along with their degradation percentage. Kinetics studies further revealed that degradation process followed pseudofirst order kinetics (Fig. 7(e,f)). Corresponding rate constant values and regression coefficient (R2) values have been presented in Table 2. Illumination of TNA and TNAC surfaces with visible light generates highly reactive species such as hydroxyl radicals (•OH), superoxide radicals (•O2ˉ), holes (h+) and electron (eˉ) which helps in the degradation of MB. In the reaction mixture, generation and reactivity of the radicals can be determined easily by the scavenger quenching behavior. Scavengers used for •OH, •O2ˉ, h+ and eˉ were dimethyl sulphoxide (DMSO), pbenzoquinone (BQ), tri- ethanolamine (TEOA) and potassium dichromate (PD). In typical procedure, 10 mM solutions of above mentioned scavengers were added to the MB solutions containing optimized TNA and TNAC photocatalyst amount. The reaction mixture was afterwards exposed to the visible light illumination and the corresponding scavenger effect on the degradation was studied. Results presented in Fig. 7 (g, h) showed that in case of BQ, TEOA and PD not much significant decrease was observed. However, the degradation percent decreased to 38% and 27% in case of TNA and TNAC for DMSO generalizing that the major reacting species out of the considered during the degradation of MB were •OH radicals. Based upon the results, we can design a pattern for the reactive species participating in the degradation of MB as: þ
˙OHN˙O2 ˉ Nh Neˉ
177
All possible degradation intermediates of MB were analyzed by LCMS, represented in m/z ratio and determined by comparison with commercially available standards (Fig. 8). Intermediates have been recorded in Scheme 1 where they are arranged in order of their decreasing m/z ratio. Initially substitution of two methyl groups by formyl groups have been obtained with m/z = 312. Attack by •OH radicals on C\\S+C group of MB resulted in cleavage of its bonds and formed a product with m/z = 303. This cleavage prompts the opening of the middle aromatic ring. Attack of hydroxyl radicals onto it resulted in the definitive dissociation of the two benzene rings with oxidation state of sulphur changes from 0 to +5 and yielded metabolites at m/z = 218, 174, 158 and 160 [65]. Further attack by the hydroxyl radicals yielded phenol with m/z = 94 which have been mineralized into CO2 and H2O very easily [66,67]. 4. Conclusions In summary, we have efficaciously synthesized Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC by microwave reduction method. It has been efficiently used for the photodegradation of MB from aqueous solution due to visible light absorption capability, low charge recombination capability and high electron-hole charge separation. Photodegradation results presented that maximum degradation was observed in the presence of H2O2 and Zr2Ni1Cu7/Si3N4 TNAC. Scavenger studies revealed that hydroxyl radicals were the major reacting species and various
Fig. 8. Probable degradation intermediates of MB determined by LC-MS.
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Scheme 1. Photocatalytic degradation mechanism of MB.
degradation intermediates were proposed by the LC-MS. Present study provides an insight into the utilization of Zr2Ni1Cu7TNA based nanocomposites for the wastewater purification. 92% MB was degraded by TNAC in presence of H2O2.
Acknowledgment The work was funded by the Deanship of Scientific Research at Princess Nourah bint Abdulrahman University through the Research Groups program, Grant no (RGP-1438-0005). References [1] T.R. Tatarchuk, N.D. Paliychuk, M. Bououdina, B. Al-Najar, M. Pacia, W. Macyk, A. Shyichuk, Effect of cobalt substitution on structural, elastic, magnetic and optical properties of zinc ferrite nanoparticles, J. Alloys Compd. 731 (2018) 1256–1266. [2] M. Satalkar, S.N. Kane, T. Tatarchuk, J.O.P. Araãjo, Ni addition induced changes in structural, magnetic, and cationic distribution of Zn0.75−xNixMg0.15Cu0.1Fe2O4 nano-ferrite, International Conference on Nanotechnology and Nanomaterials, Springer 2017, pp. 357–375.
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Facile fabrication of Zr2Ni1Cu7 trimetallic nano-alloy and its composite with Si3N4 for visible light assisted photodegradation of methylene blue Gaurav Sharma a,b,c,⁎, Amit Kumar a,b,c, Shweta Sharma c, Mu. Naushad d, Tansir Ahamad d, Sameerah I. Al-Saeedi e, Ghadah M. Al-Senani e, Nada S. Al-kadhi e, Florian J. Stadler a,⁎ a College of Materials Science and Engineering, Shenzhen Key Laboratory of Polymer Science and Technology, Guangdong Research Center for Interfacial Engineering of Functional Materials, Nanshan District Key Lab. for Biopolymers and Safety Evaluation, Shenzhen University, Shenzhen 518060, PR China b Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, PR China c School of Chemistry, Shoolini University, Solan 173212, Himachal Pradesh, India d Department of Chemistry, College of Science, King Saud University, Bld.#5, Riyadh, Saudi Arabia e Department of Chemistry, College of Science, Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 18 July 2018 Received in revised form 6 September 2018 Accepted 11 September 2018 Available online 13 September 2018 Keywords: Trimetallic nano-alloy Composite Photocatalytic degradation Methylene blue
a b s t r a c t Novel Zr2Ni1Cu7 trimetallic nano-alloy (TNA) and Zr2Ni1Cu7/Si3N4 trimetallic nano-alloy composite (TNAC) have been successfully prepared by facile microwave reduction method. The synthesis of TNA and TNAC was ascertained by characterizing them by using various techniques such as SEM, TEM, XRD, EDX and FTIR etc. The EDX shows the presence of all constituents in TNA and TNAC. The XRD study demonstrated the crystalline and semi-crystalline nature of TNA and TNAC. The optical band gap study revealed that TNAC (2.47 eV) has lower band gap then TNA (2.54 eV). The visible light photocatalytic activity of Zr2Ni1Cu7TNA and Zr2Ni1Cu7/Si3N4 TNAC has been successfully utilized for the photodegradation of methylene blue (MB). The presence of H2O2 has been found to influence the photodegradation rate. Photodegradation results presented that maximum degradation was observed in the presence of H2O2 and Zr2Ni1Cu7TNAC (92%). Scavenging activity has revealed that the hydroxyl radicals were the major reacting species. These species successfully degraded the MB into various intermediates that has successfully been determined by the LC-MS. Kinetic studies indicated that the degradation of MB by Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC followed pseudo-first-order kinetics. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Nanocomposite materials, composed of more than two phases, are the extensively explored materials having lavishing properties such as high mechanical, electrical, thermal, elastic and optical properties, etc. Number of techniques has been put forward for designing nanocomposites which can be categorized into approaches; bottom up and topdown. Combination at the nanoscale provides them high surface areato- mass ratio that helps in enhancing their adsorbing ability which in turn helps in removing noxious pollutants from environment. Nanocomposite materials can also be designed in such a way that it reduces the electron-hole recombination and provide synergistic effect between the constituents that helps in degradation of pollutants. These can be categorized into various forms [1,2] and one such form is nanoalloys. In material chemistry, mixing number of metals helps in extending the properties of the obtained system, generally called alloys or ⁎ Corresponding authors. E-mail addresses: [email protected] (G. Sharma), [email protected] (F.J. Stadler).
https://doi.org/10.1016/j.molliq.2018.09.063 0167-7322/© 2018 Elsevier B.V. All rights reserved.
intermediate compounds [3]. Alloying helps in enriching the properties because of optimized composition, structure diversity and synergistic effect between the constituents that increases their utilization in engineering, catalysis and electronics etc. [4]. Emergence of nanotechnology has led to the urge to synthesize materials with controllable properties at the nanoscale geometry that has led to the fabrication of nanoalloys [5]. Number of techniques has been suggested for their designing such as electrochemical, radiolysis, sonochemical, biosynthesis, ion implantation, co-precipitation and chemical reduction etc. [6,7]. Nanoalloys are of great interest due to their ability of monitoring the physical and chemical properties by varying the composition, atomic ordering and size [8,9]. The engineering properties like tensile and shear strength differ from constituent units whereas physical properties like electrical and thermal conductivity, reactivity, density and Young's modulus, of nanoalloys differ from its individual elemental properties. Pure metals have single melting point but nanoalloys doesn't, inspite they have a range of melting points which consist of blend of solid and liquid phases. The temperature at which the process of melting starts is called solidus and where melting completes is known as liquidus. Number of bimetallic and trimetallic nanoalloys have been designed in the recent decades
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[10]. Trimetallic nanoalloys have been extensively employed for various applications due to better synergistic behavior of its constituents leading to their enhanced optical, mechanical and electronic effects [11,12]. Rapid increase in the discharge of effluents from various industries consisting of organic dyes has resulted in water pollutions challenge all across the world. The organic dyes structurally consist of varying number of benzene rings which are not easily degraded by the available conventional methods such as physical and biological methods [13,14]. These dyes contain various toxic chemical groups which have diverse harmful effects on aquatic and public life. Approximately 15% of the total dyes are acquired as the textile effluents which are added to the water system during dyeing process [15]. These are colored compounds and thus result into various devastating effects such as non-aesthetic pollution, eutrophication and perturbations in the aquatic life [16]. So their removal from the water bodies is necessary. Since the international standards of environment are becoming more rigorous (ISO 14001, October 1996), various technical systems for the exclusion of these organic dyes have been established. One such organic dye is methylene blue (MB), which is a heterocyclic compound having potential applications in various industries such as plastic, pharmaceuticals, cosmetic and food, etc. [17]. MB was first synthesized in 1876 for use in textile industries. In initial times of its origin, it was even utilized for curing malaria and it was also the first synthetic compound ever used as an antiseptic in clinical therapy [18]. However, its complete studies have generalized that it has few side effects also which includes diarrhea, vomiting, jaundice, tissue necrosis and nausea, etc. [19]. So number of approaches have been reported in the literature for their removal such as osmosis [20], biosorption [21], adsorption [22–26], ultrafiltration [27], chlorination, ion exchange [28], membrane filtration [29], liquid-liquid extraction [30] and heterogeneous photocatalysis [31–34] etc. Among these techniques, photocatalytic degradation is getting extensively explored due to its easy operation, less toxic after effects and high removal efficiency [35–37]. Recent decades have marked the use of various photocatalysts such as TiO2, Fe2O3 and ZnS, etc. for the degradation of noxious dyes due to their high solar light absorption ability. In addition, not much explored area of photocatalysts; bi and trimetallic nanomaterials are also emerging as proficient photocatalyst due to their synergistic properties and high catalytic activity [12,38,39]. Silicon nitride (Si3N4) is a newly explored material having exceptional physical and chemical properties such as improved chemical stability, high melting point and boiling point, high resistance to thermal shock and mechanical strength [40,41]. Its covalent bonding made it hard to sinter and thus require prolonged time and temperature for sintering [42]. Various methods utilized for its fabrication are reduction of silica carbothermically, direct nitration of silicon [43] and high temperature pyrolysis [44,45]. Nickel nanoparticles have high ability of being used in catalysis but due to high agglomeration their potential applications decreases. In order to overcome this problem, trimetallic nanoclusters or nanoalloy; Zr2Ni1Cu7 has been fabricated that helped in diminishing its drawbacks. Combination provided high catalytic and better optical properties to the fabricated nanoalloy [46]. Herein, we have presented the synthesis and characterization of trimetallic nano-alloy (Zr2Ni1Cu7 TNA) and its nano-alloy composite (Zr2Ni1Cu7/Si3N4 TNAC) that have been successfully utilized for the remediation of highly toxic organic dye; methylene blue (MB). Importance of presented paper lies in the fact that we have combined the properties of three metals and silicon nitride so to provide better nano-alloy; having photocatalytic properties that can be efficiently utilized for the remediation of MB from the aqueous solution.
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nitrate (99.99%, Chemical Drug House, India), cupric nitrate (N98%, Qualikem, India), nickel nitrate (N99%, LobaChemie, India) and trisodium citrate (N97%, LobaChemie, India). Double distilled water was used throughout the experiments. Muffle furnace (MSW-275), UV–visible spectrophotometer (Systronics 2202), X-ray diffractrometer (Phillips Holland), Fourier transform infrared (FTIR) spectrometer (Perkin Helmer), Scanning electron microscope (SEM QANT-250, Model 9393), Transmission electron microscope (TEM Technai G2 20 S-Twin) and Liquid chromatography mass spectrometer (Q-ToF Micro Water LCMS spectrometer). 2.2. Synthesis of silicon nitride Synthesis of silicon nitride was done by simple pyrolysis process. In typical procedure, finely powdered silica and urea were mixed in the ratio of 1:0.9 in a silica crucible. Nitrogen gas was purged into the crucible so to provide it an oxygen free environment. The crucibles were then subjected to 600 °C with a ramping rate of 10 °C/min for initial 60 min and then allowed to withstand at this temperature for another 120 min. After time completion, greyish white colored powder of silicon nitride was obtained which was utilized for further experimental procedures. 2.3. Synthesis of Zr2Ni1Cu7 trimetallic nano-alloy and Zr2Ni1Cu7/Si3N4 trimetallic nano-alloy composite Zr2Ni1Cu7 trimetallic nano-alloy was synthesized by stepwise microwave reduction method. To the 20 mL of 20 mM zirconium oxy-nitrate solution, 2% trisodium citrate (3 mL) was added which acted as the reducing agent. The reaction mixture was then heated for 2 min in microwave oven at 800 W in a cyclic mode (20 s. ON, 10 s. OFF). In the same manner, 20 mM nickel nitrate and cupric nitrate solutions with 2% tri sodium citrate solution as a reducing agent were simultaneously added and the solution then again placed in microwave oven at 800 W in cyclic mode. Finally obtained nano-alloy Zr2/Ni1/Cu7 was washed with double distilled water and dried in hot air oven at 50 °C for 60 min [47,48]. Zr2Ni1Cu7/Si3N4 trimetallic nano-alloy composite was synthesized by similar stepwise microwave reduction method. Firstly, 0.02 g above prepared Si3N4 was dispersed in 50 mL double distilled water. In presence of this suspension, whole above process for synthesis of Zr2Ni1Cu7 was repeated to obtain trimetallic nano-alloy composite. 2.4. Characterization Structural elucidations of TNA and TNAC were carried out by Perkin Helmer FTIR spectrophotometer in the range of 400–4000 cm−1. The X-ray diffraction patterns were obtained by Phillips Holland diffractometer. The morphological and surface analysis of Zr/Ni/Cu TNA and Zr2Ni1Cu7/Si3N4 TNAC were done by QANT-250, Model 9393 scanning electron microscope and Technai G2 20 S-Twin transmission electron microscope. Optical and photocatalytic examinations were carried out by Systronics 2202 double beam UV–Vis spectrophotometer. 2.5. Optical studies
2.1. Chemicals and instruments
The optical properties of Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC were generalized by dissolving 5 mg materials in 10 mL ethanol solution. The suspensions were ultrasonicated for 30 min and then analyzed at various wavelengths using double beam UV–Vis spectrophotometer. Tauc relation was used for determining the optical band gap values of Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC [49]:
All the chemicals were of analytical grade and used without any treatment process. The reagents used in the study were zirconium oxy
αhν ¼ A hν−Eg n
2. Material and methodology
ð1Þ
172
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where ɑ is the adsorption coefficient, hν represents the photon energy, Eg is optical band gap of Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC, A = band tailing parameter and n = 2 or 1/2 for direct or indirect band gap, respectively. The band edge values of Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC were examined using following equations [50,51]: EVB ¼ X−Ee þ 0:5 Eg
ð2Þ
ECB ¼ EVB −Eg
ð3Þ
where EVB is the valence band edge potential; ECB is the conduction band edge potential; X is the Pearson absolute electronegativity of photocatalyst, Eg represents the band gap of the photocatalyst and Ee is the energy of electrons on hydrogen scale (4.5 eV). 2.6. Photocatalytic experiments The photocatalytic activity of fabricated Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC were estimated by the photodegradation of methylene blue (MB) as test dye. In typical experiments, 0.020 g of TNA and TNAC were dispersed into 20 ppm MB solution and solar light was used as the visible-light source. Before subjecting the suspension to visible light irradiations, it was stirred on a magnetic stirrer to achieve adsorption-desorption equilibrium. Pre-defined 3 mL aliquots of the suspensions were taken out at various time intervals, centrifuged at 2500 rpm for 5 min to remove suspended TNA and TNAC particles and then analyzed by double beam UV–Vis spectrophotometer at 664 nm. Percent degradation of MB by synthesized TNA and TNAC were calculated using formula [52]: Percent degradation ¼
C 0 −C t 100 Ct
Fig. 1. FTIR spectra of Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC.
respectively [55]. Fig. 2(a) shows the XRD pattern of synthesized Zr2Ni1Cu7TNC. The peaks at 2θ value of 34.12°, 39.01° and 49.21° corresponds to (002), (111) and (202) diffraction plane of copper metal which is in accordance with the JCPDS No.05–661 [56]. Presence of nickel has been confirmed by the peaks at 36.96°, 43.14° and 32.06° which corresponds to (111), (200) and (331) diffraction planes (JCPDS No.01–073-1523) [57]. The peak at 29.820 and 50.37° corresponds to the (101) and (112) diffraction plane in accordance with the JCPDS No. 81-1544 for zirconium [58]. However, Fig. 2(b) shows the change in 2θ values, generalizing the successful formation of the Zr2Ni1Cu7/Si3N4
ð4Þ
The removal rate of methylene blue was examined by pseudo-firstorder kinetics model using following equation: r¼−
dc ¼ kapp t dt
ð5Þ
On integrating the above equation ln
C0 ¼ −kapp t Ct
ð6Þ
where kapp is the apparent rate constant, C0 is the initial concentration of MB before visible light irradiation and Ct is the concentration of MB after visible light irradiation at particular time t (minute). 3. Results and discussion 3.1. Characterization FTIR spectra of Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC have been presented in Fig. 1. Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC shows prominent peak at 3221 cm−1and 3236 cm−1 which can be ascribed to the O\\H stretching mode of hydroxyl groups [53]. Intense peaks at 1568 cm−1, 846 cm−1 and 461 cm−1 specifies the strong absorption band for Zr\\O, Ni\\O and Cu\\O stretching [12,38,54]. Peak around 2352 cm−1 and 1374 cm−1 corresponds to the atmospheric \\CO2 groups present in Zr2Ni1Cu7TNA (Fig. 1(b)). Change in intensities of characteristic peaks signifies the imprinting of TNA onto Si3N4 (Fig. 1 (a)). The additional peak at 1075 cm−1 for Zr2Ni1Cu7/Si3N4 TNAC corresponds to stretching mode of Si\\N [45]. XRD results show that there was change in 2θ values of Zr2Ni1Cu7/Si3N4 TNAC as compared to Zr2Ni1Cu7 TNC. The XRD study of Zr2Ni1Cu7 TNC and Zr2Ni1Cu7/Si3N4 TNAC confirms the crystalline and semi-crystalline nature of materials
Fig. 2. XRD patterns of (a) Zr2Ni1Cu7 TNA and (b) Zr2Ni1Cu7/Si3N4 TNAC.
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TNAC. Also, some additional peaks at 2θ values of 32.05° and 39.05° corresponding to (200) and (101) has also been observed confirming the presence the Si3N4. For analyzing the surface features of Zr2Ni1Cu7 TNC and Zr2Ni1Cu7/Si3N4 TNAC, SEM study was performed which generalize the change in morphology from TNA to TNAC formation (Fig. 3 (a-d)). The Fig. 3 (a-b) shows Zr2Ni1Cu7 TNA highly stacked on each other with irregular shape, whereas, Fig. 3 (c-d) represents the attachment of Zr2Ni1Cu7 TNA on to the surface of Si3N4. The Fig. 3 (e-f) demonstrate the EDX patterns of Zr2Ni1Cu7TNC and Zr2Ni1Cu7/Si3N4TNAC reveling the presence on zirconium, copper, nickel, silicon and nitrogen in materials. Transmission electron microscopy was done to elucidate the structural features of TNA and TNAC. Fig. 4(a,b,c) shows the TNA at various magnifications. Fig. 4(a) shows the highly agglomerated structure of TNA and Fig. 4(b and c) shows the magnified features of TNA. Fig. 4 (c) shows HRTEM with lattice fringes arrangement showing d-spacing of the constituting metals which confirms the successful fabrication of the TNA. However, Fig. 4(d,e,f) shows the morphological changes that arises due to the imprinting of TNA onto Si3N4. Sheet- like structure of
173
the Si3N4 is well elucidated and the highly agglomerated TNA onto its surface is clearly visible.
3.2. Optical studies The tauc plot for Zr2Ni1Cu7 TNA, Zr2Ni1Cu7/Si3N4 TNAC and Si3N4 has been shown in Fig. 5(a-c). The optical band gap values for Zr2Ni1Cu7 TNA, Zr2Ni1Cu7/Si3N4 TNAC and Si3N4 has been calculated to be 2.54, 2.47 and 2.19 eV respectively which lies in the visible range region. Valence and conduction band energy value for Zr2Ni1Cu7 TNA has been found to be 0.654 and − 1.886 eV respectively. In case of Si3N4, valence band energy value is 2.805 eV and conduction band energy value is 0.615 eV which shows the movement of electrons from the conduction band of Zr2Ni1Cu7 TNA to conduction band of Si3N4 and holes movement from the valence band of Si3N4 to valence band of Zr/Ni/Cu TNA. This electron- hole movement in opposite direction lowers the electron-hole pair recombination, thus generalizing its photocatalytic
Fig. 3. SEM morphologies of (a,b) Zr2Ni1Cu7 TNA, (c,d) Zr2Ni1Cu7/Si3N4 TNAC and EDX of (e) Zr2Ni1Cu7 TNA (f) Zr2Ni1Cu7/Si3N4 TNAC.
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Fig. 4. TEM images of (a,b,c) Zr2Ni1Cu7 TNA and (d,e,f) Zr2Ni1Cu7/Si3N4 TNAC.
degradation ability. Fig. 5(d) shows the probable heterojunction for the fabricated Zr2Ni1Cu7/Si3N4 TNAC. 3.3. Photocatalytic activity As the targeted pollutant, methylene blue (MB) was chosen for evaluating the photocatalytic degradation ability of synthesized TNA and TNAC. Firstly, plain photolysis of MB was experimented for 90 min under visible light irradiation using solar light without the addition of TNA and TNAC. Results presented in Fig. 6(a) predicted the stability of MB in water as only 4% degradation was recorded. Fig. 6(b) depicts the adsorption ability of TNA and TNAC for MB molecules. Results generalized that only 18 and 27% adsorption was obtained for TNA and TNAC respectively. High adsorption rate of TNAC can be attributed to the presence of Si3N4 that provide surface area for the adsorption of MB molecules. The effect of solution pH was studied in the range of 2–10 and the results presented in Fig. 6(c) demonstrate that maximum degradation was observed at pH 8 for both Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC. MB is one of the dyes that do not have any pKa value between 0 and 14 pH and exists only in the form of MB+ [59]. So at pH 8, the MB dye is positively charged and the surface of both the TNA and TNAC is negatively charged as determined from pHpzc which thus offers maximum attraction between photocatalyst surface and MB molecules [60].In addition to pH, how the TNA and TNAC amount is affecting the
degradation rate was also studied. For this, various amounts of TNA and TNAC ranging from 5 to 30 mg was added into the 20 ppm solution of MB and irradiated with visible light for 90 min (Fig. 6(d)). Results indicated that maximum degradation rate was observed with 20 mg of TNAC and 25 mg of TNA. Reason can be that increase in the amount of TNA and TNAC increased the number of photons absorbed and also the number of MB molecules adsorbed onto their surface. This increased the density of TNA and TNAC in the area of illumination as a result of which the rate was enhanced. However, above a certain level, the increase in TNA and TNAC concentration do not effect degradation rate because at high concentration the exposer to light get hindered due to scattering also known as photon dispersion not. Fig. 7 (a,b) presents the time- dependent change in absorbance of MB degradation at various wavelengths under visible light irradiation for 90 min by Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC. Maximum absorption peak for MB has been observed at 664 nm that gradually decreased with increased time of visible light irradiation and maximum decrease has been observed in presence of Zr2Ni1Cu7/Si3N4 TNAC. Decrease in absorption peak intensity confirmed the high photocatalytic activity of fabricated Zr2Ni1Cu7/Si3N4 TNAC as compared to Zr2Ni1Cu7 TNA. Final photocatalytic degradation experimentation of the MB by Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC was done under two respective conditions; one was the photodegradation in presence of H2O2 and other was photodegradation in absence of H2O2. Results presented
G. Sharma et al. / Journal of Molecular Liquids 272 (2018) 170–179
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Fig. 5. Tauc plots for (a) Zr2Ni1Cu7 TNA (b) Zr2Ni1Cu7/Si3N4 TNAC and (c) Si3N4 (d) heterojunction ofZr2Ni1Cu7/Si3N4.
that H2O2 was enhancing the degradation rate. H2O2 is generally considered as the degradation rate enhancing agent. Actually, it helps in generating high reactive hydroxyl radicals that on interaction with MB molecules carry out its degradation and helps in its complete removal.
The degradation observed without H2O2 is 67% for TNA and 79% for TNAC. The degradation was significantly high in presence of H2O2 i.e., 81% for TNA and 92% for TNAC. Fig. 7 (c, d) demonstrate that high degradation rate of MB has been observed in the presence of H2O2
Fig. 6. (a) Plain photolysis of MB and (b) Percent adsorption of MB onto Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC; pH effect on photocatalytic degradation of MB by Zr2Ni1Cu7 TNAandZr2Ni1Cu7/Si3N4 TNAC; Effect of photocatalyst amount on degradation of MB by Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC.
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Fig. 7. Absorabnce plots for photodegradation of MB by (a) Zr2Ni1Cu7 TNAand (b) Zr2Ni1Cu7/Si3N4 TNAC; Percent degradation of MB by (c) Zr2Ni1Cu7 TNA (d) Zr2Ni1Cu7/Si3N4 TNAC; Pseudo- first order kinetics for the photocatalytic degradation of MB by (e) Zr2Ni1Cu7 TNA and (f) Zr2Ni1Cu7/Si3N4 TNAC; Scavenger effect on the photocatalytic degradation of MB by (g) Zr2Ni1Cu7 TNA (h) Zr2Ni1Cu7/Si3N4 TNAC (MB concentration = 20 ppm, photocatalyst amount = 0.020 g and time = 90 min).
with both Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC. However, out of the two, maximum rate has been observed with Zr2Ni1Cu7/Si3N4 TNAC in the presence of H2O2 which can be attributed to reasons such as high absorption in the visible region, lower band gap, increased charge Table 1 Comparison of photocatalytic activity of various materials for methylene blue along with their degradation percentage. Sr. No
Material
Percent degradation
Reference
1. 2. 3. 4. 5.
ZrC nanopowder TiO2 nanoparticles V2O3/CNT/TiO2 TiO2/carbon Zr2Ni1Cu7/Si3N4 TNAC
80.21% 90% 70% 84.7% 92%
[61] [62] [63] [64] Present study
separation efficiency and reduced electron-hole recombination. Enhancement in migration of charge carriers increased the synergistic effect between Zr2Ni1Cu7 TNA and Si3N4. Table 1 presents the Table 2 First order kinetic rate constant (k) and regression coefficient (R2) of MB degradation using photocatalyst Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC under visible light illumination.
Photocatalyst
Experimental condition
Rate constant (min−1)
Regression coefficient (R2)
Zr2Ni1Cu7 TNA
Without H2O2 With H2O2 Without H2O2 With H2O2
0.002 0.006 0.002 0.004
0.894 0.979 0.895 0.991
Zr2Ni1Cu7/Si3N4 TNAC
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comparative study of photocatalytic activity of various materials for methylene blue along with their degradation percentage. Kinetics studies further revealed that degradation process followed pseudofirst order kinetics (Fig. 7(e,f)). Corresponding rate constant values and regression coefficient (R2) values have been presented in Table 2. Illumination of TNA and TNAC surfaces with visible light generates highly reactive species such as hydroxyl radicals (•OH), superoxide radicals (•O2ˉ), holes (h+) and electron (eˉ) which helps in the degradation of MB. In the reaction mixture, generation and reactivity of the radicals can be determined easily by the scavenger quenching behavior. Scavengers used for •OH, •O2ˉ, h+ and eˉ were dimethyl sulphoxide (DMSO), pbenzoquinone (BQ), tri- ethanolamine (TEOA) and potassium dichromate (PD). In typical procedure, 10 mM solutions of above mentioned scavengers were added to the MB solutions containing optimized TNA and TNAC photocatalyst amount. The reaction mixture was afterwards exposed to the visible light illumination and the corresponding scavenger effect on the degradation was studied. Results presented in Fig. 7 (g, h) showed that in case of BQ, TEOA and PD not much significant decrease was observed. However, the degradation percent decreased to 38% and 27% in case of TNA and TNAC for DMSO generalizing that the major reacting species out of the considered during the degradation of MB were •OH radicals. Based upon the results, we can design a pattern for the reactive species participating in the degradation of MB as: þ
˙OHN˙O2 ˉ Nh Neˉ
177
All possible degradation intermediates of MB were analyzed by LCMS, represented in m/z ratio and determined by comparison with commercially available standards (Fig. 8). Intermediates have been recorded in Scheme 1 where they are arranged in order of their decreasing m/z ratio. Initially substitution of two methyl groups by formyl groups have been obtained with m/z = 312. Attack by •OH radicals on C\\S+C group of MB resulted in cleavage of its bonds and formed a product with m/z = 303. This cleavage prompts the opening of the middle aromatic ring. Attack of hydroxyl radicals onto it resulted in the definitive dissociation of the two benzene rings with oxidation state of sulphur changes from 0 to +5 and yielded metabolites at m/z = 218, 174, 158 and 160 [65]. Further attack by the hydroxyl radicals yielded phenol with m/z = 94 which have been mineralized into CO2 and H2O very easily [66,67]. 4. Conclusions In summary, we have efficaciously synthesized Zr2Ni1Cu7 TNA and Zr2Ni1Cu7/Si3N4 TNAC by microwave reduction method. It has been efficiently used for the photodegradation of MB from aqueous solution due to visible light absorption capability, low charge recombination capability and high electron-hole charge separation. Photodegradation results presented that maximum degradation was observed in the presence of H2O2 and Zr2Ni1Cu7/Si3N4 TNAC. Scavenger studies revealed that hydroxyl radicals were the major reacting species and various
Fig. 8. Probable degradation intermediates of MB determined by LC-MS.
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Scheme 1. Photocatalytic degradation mechanism of MB.
degradation intermediates were proposed by the LC-MS. Present study provides an insight into the utilization of Zr2Ni1Cu7TNA based nanocomposites for the wastewater purification. 92% MB was degraded by TNAC in presence of H2O2.
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