- Email: [email protected]
Evaluation of the photocatalytic efficiency of cobalt oxide nanoparticles towards the degradation of crystal violet and methylene violet dyes
Journal Pre-proof Evaluation of the photocatalytic efficiency of cobalt oxide nanoparticles towards the degradation of crystal violet and methylene violet dyes R. Sukhin Saravan, M. Muthukumaran, M. Mubashera, M. Abinaya, P. Varun Prasath, R. Parthiban, Faruq Mohammad, Won Chun Oh, Suresh Sagadevan
PII:
S0030-4026(20)30262-X
DOI:
https://doi.org/10.1016/j.ijleo.2020.164428
Reference:
IJLEO 164428
To appear in:
Optik
Received Date:
27 December 2019
Revised Date:
12 February 2020
Accepted Date:
16 February 2020
Please cite this article as: Saravan RS, Muthukumaran M, Mubashera M, Abinaya M, Prasath PV, Parthiban R, Mohammad F, Oh WC, Sagadevan S, Evaluation of the photocatalytic efficiency of cobalt oxide nanoparticles towards the degradation of crystal violet and methylene violet dyes, Optik (2020), doi: https://doi.org/10.1016/j.ijleo.2020.164428
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier.
Evaluation of the photocatalytic efficiency of cobalt oxide nanoparticles towards the degradation of crystal violet and methylene violet dyes
R. Sukhin Saravan1, M. Muthukumaran2, M. Mubashera3, M. Abinaya2, P. Varun Prasath2*, R. Parthiban1, Faruq Mohammad4, Won Chun Oh5 and Suresh Sagadevan6*
Department of Chemical Engineering, SSN College of Engineering, Kalavakkam – 603110, India 1
Department of Analytical Chemistry, University of Madras, Chennai -25, India
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2 3
Department of Chemical Engineering, Alagappa College of Technology, Anna University, Chennai – 25, India 4
Surfactants Research Chair, Department of Chemistry, College of Science, King Saud
5
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University, Riyadh, Kingdom of Saudi Arabia 11451
Department of Advanced Materials Science and Engineering, Hanseo University, Seosan-si,
6*
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Chungnam 356-706, Korea
Nanotechnology & Catalysis Research Centre, University of Malaya, Kuala Lumpur 50603,
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Malaysia
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Abstract
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Corresponding author mail Ids: [email protected]; [email protected]
The present work deals with the synthesis, characterization, and testing towards the photochemical degradation efficiency of cobalt oxide (Co3O4) nanoparticles (NPs). Following the synthesis of Co3O4 NPs by the hydrothermal method, the NPs are confirmed to be formed in their cubic lattice structure with the spherical shape morphology. The testing studies revealed that the Co3O4 NPs degraded the Methylene violet (MV) and Crystal violet (CV) dyes very 1
efficiently under the UV light irradiation. A Langmuir–Hinshelwood (L–H) model has been effectively used to display the adsorption of CV and MV dye molecules taking place using surface monolayer exposure to the UV light. The kinetics and reaction mechanism behind the elimination of CV and MV dyes shows for a pseudo-second-order kinetics model. With these Co3O4 NPs as photocatalysts, the maximum photodegradation rate was observed to be 92% and 64% for the MV and CV (respectively) within the same period of 45 min and this enhanced photocatalytic performance is due to the effective reduction of electron-hole recombination
ro of
process by the Co3O4 NPs.
Keywords: Cobalt oxide nanoparticles, Photocatalysis, Dye degradation, Methylene violet,
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Crystal violet, Dye adsorption.
1. Introduction
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In recent years, the increased interest with the nanostructured metal oxides due to their high surface areas with supportive thermal, mechanical, electrical, and optical properties made them
na
to include in the development of optical, electronic, and sensing devices [1-2]. With that view, the transition metal oxide nanoparticles (NPs) like Fe3O4, ZnO, TiO2, ZnO, CoFe2O4 etc are
ur
found to be very efficient and among many different kinds, the cobalt oxide (Co3O4) NPs exhibit interesting properties that can be suitable for the lithium storage, gas sensing, and
Jo
electro-catalysis. The most stable phase of Co3O4 with a direct bandgap of 1.48 - 2.19 eV is used as a p-type semiconductor and received considerable attention [3-7]. The formation of Co3O4 NPs by the hydrothermal approach generally requires the reduction and precipitation of chemical agents and this particular method is considered to be the green synthesis and is less expensive [6-7] as against the other methods like microwave-assisted [8] and reverse micelles [9]. Therefore, taking into the consideration of unique band energy gaps associated with the 2
nanostructured Co3O4, the present study deals with the photocatalytic dye degradation efficiency of Co3O4 NPs operated by the UV light. Following the synthesis of Co3O4 NPs by the hydrothermal approach, they were thoroughly characterized for the crystal type and nature, surface morphology, functionality, optical properties etc. Further, the Co3O4 NPs photocatalytic performance towards the degradative removal of organic contaminants like
ro of
methylene violet (MV) and crystal violet (CV) dyes are being investigated.
2. Materials and Methods
For the formation of Co3O4 NPs, about 50 mL of 1 M aqueous NaOH solution was mixed
-p
dropwise with 0.5 M of cobalt nitrate and on stirring, we see the turning of a pink coloured
re
solution to dark green first and then to black precipitate. This black precipitate is then transferred to an autoclave and kept in a furnace maintained at 180ºC so as to undergo the
lP
hydrothermal process. The black precipitate is filtered off and dried in an air oven set at 110ºC. The dried filtered flakes are crushed and powdered using mortar and pestle, and the black
Co3O4 NPs.
na
powder is subjected to heating again at 550ºC for the sintering process which finally forms the
ur
The powdered X-ray diffraction (XRD) analysis was performed on Model Smart lab SE X-Ray instrument (Make-Rigaku, Japan) that uses CuKα radiation (1.5418 Å), 2θ scanning
Jo
range of 20-80º with 0.02º step size. The Fourier transform infrared (FTIR) spectrometer, Spectrum Two FTIR/ATR instrument in the wavenumber range of 500-4000 cm-1 with a resolution of 0.5 cm-1 was used. The UV-vis spectroscopic analyses were performed using a double beam spectrophotometer (UV-1800, Shimadzu) in the wavelength range of 200-800 nm. For the field emission scanning electron microscopy (FESEM), Carl Zeiss microscopy
3
Ltd., (UK & SIGMA) connected to an energy dispersive analysis by X-rays (EDAX) detector was used. The photocatalytic performance of Co3O4 NPs was studied as against the degradation of CV and MV dyes in an aqueous solution and is investigated under the sunlight. For the testing, about 1.0 g/L of Co3O4 nanocatalyst was added to the 10 ppm concentration of dye with stirring and by keeping the reaction mixture under sunlight. A magnetic stirrer was used continuously for the proper mixing of the solution. After dissolving the catalyst in aqueous
ro of
solution and by placing the reaction mixture in the sunlight for up to 45 min, the reactants were shifted to the dark environment and kept there for another 30 min in order to attain the adsorption equilibrium and then the absorbance was measured using the UV-vis
-p
spectrophotometer. Now, the UV light was turned on and the absorbance of the sample was
using the formula (1) [10]:
(1)
lP
Degradation % = (C0-C)/C0 x 100 %
re
measured for every 30 min. The absorption percentage of individual sample was calculated
where C0 and C are the initial and final concentrations. The experiments are performed to
na
evaluate the effect of the catalyst with UV radiation for the degradation of CV and MV dye. 3. Results and discussion
ur
The FTIR spectrum of Co3O4 NPs showed in Fig.1a indicates for the two strong bands at 557 cm-1 and 626 cm-1 belonging to the spinel structure of Co3O4. The peak at 626 cm-1 can be
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attributed to the Co-O stretching frequency at which Co is Co+2 and is tetrahedrally coordinated, while the other peak at 557 cm-1 linked to the Co-O of octahedrally coordinated Co+3 ion [7]. The band appeared at 1656 cm-1 can be assigned to the stretching and bending vibrations of absorbed water molecules onto the Co3O4 NPs [6]. Similarly, from the powdered XRD study of Co3O4 NPs shown in Fig. 1b, the peaks are reflecting at (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) revealing the polycrystalline cubic structure of Co3O4 NPs. 4
The sharp and high intense peak at (3 1 1) indicating for the high crystallinity of the product and based on this peak, the average crystallite size was calculated to be 22 nm by making use of the Scherer formula. Also from the diffraction pattern, no other impurity phases are detected and thus confirming for the successful formation of Co3O4 NPs in their purest form by this
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hydrothermal method.
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Fig. 1. (a) FTIR spectrum, (b) powdered XRD pattern, (c) FESEM image, and (d) EDAX of Co3O4 NPs. The morphological analysis of Co3O4 NPs examined by FESEM (Fig. 1c) indicates for the
ur
formation of spherical-shaped particles with visible clumped distributions, in addition to the Co3O4 NPs agglomeration. Also, the FESEM image clearly shows for the formation of particles
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in their crystalline nanosizes, spherical/granular in shape, uniform, and with porous morphology, where the nanoparticles agglomeration results from their exceedingly small sizes and high surface energies. Further, the EDAX analysis of Co3O4 NPs illustrated in Fig. 1d shows respective peaks for the elements of Co and O and in addition, this analysis clearly confirms for the formation of Co3O4 NPs in their purest form (appropriate elemental composition) without any unwanted impurities. 5
The UV-vis diffuse reflectance spectra (DRS) of the Co3O4 NPs (Fig. 2a) indicate for the strong absorption ability especially in the absorption range of 200-800 nm and implies that the introduction of Co3O4 NPs can promote for the absorption of visible light and produce more photogenerated charge carries which further responsible for an enhancement in the photocatalytic performance. Also, from the UV-vis DRS of the ageing sample indicated that no significant absorption peaks have appeared and this can probably be due to the energy gaps generated by the quantum confinement effects. Since the particles sizes are very narrow and in
ro of
the nanosize range which made the absorption peak to be slightly broader and in addition, the stability of Co-O bond in the Co3O4 NPs can be attributed to the polar-symmetrical structure and is strongly influenced by the weak Vander Waals interactions within particle regime. The
-p
bandgap of the sample was calculated using the Kubelka-Munk (KM) method and based on this, the Tauc plot (αhυ)1/2 versus (hυ), the bandgap of 2.44 eV was obtained for the Co3O4 NPs
n/2
(hv) A(hv E g )
(2)
(3)
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n/2
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(hv) C (hv E g )
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as showed in Fig. 2(b).
Where A, α, υ, h, n, and Eg corresponds to the proportionality constant, absorption coefficient,
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ur
frequency of light, Planck’s constant, and the bandgap, respectively.
Fig. 2. (a) UV-vis spectrum and (b) Tauc plot of Co3O4 NPs. 6
In order to test the Co3O4 NPs photocatalytic efficiency, two different dyes, CV and MV were selected and studied their degradation properties under visible light illumination. Figure 3(a-b) shows the comparison of UV–vis absorption spectrums of 10-5 M concentration of MV and CV dyes (respectively) as a function of irradiation time. From the figure, we observed that the increase of irradiation time (from 0 min to 45 min) caused a gradual increase in the degradation as provided by the individual absorption band and thereby suggesting for the Co3O4-mediated photodegradation of CV and MV dyes on regular basis. From the figure, the decrease in the
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absorbance intensity with time for both dyes is due to the change in colour of dye solutions from blue to colourless with respect to an increase in the adsorption capacity for every 30 mins and also indicating for the strong catalytic activity of Co3O4 NPs. On the further increase of
-p
time, the absorbance intensities of CV and MV dyes got decreased with time (results not shown), i.e. after 2 h, the absorbance is very minimal and the dye solutions became colourless,
re
indicating the complete degradation of pollutants under UV radiation. In addition, we observed
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that the catalyst showing a linear increase in the adsorption of CV and MV dyes under UV
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ur
na
irradiation and this absorbance is due to the smaller crystallite size and enhanced surface area.
Fig. 3. Comparison of the photocatalytic degradation of changes in concentrations of (a) CV, and (b) MV dyes. The photocatalytic degradation of CV and MV dye solutions in presence of Co3O4 NPs were quantified by the ratio, Ct/C0, where C0 and Ct correspond to the initial and final concentrations 7
of CV and MB dye solutions. Fig. 4a shows the comparison of changes in the Ct/C0 ratio as against the irradiation time over a period of 45 min and from the graph, we observed that the ratio is getting decreased with regards to an increase in the irradiation time. Also, the observation of the same optical absorbance for both dyes at initial stages (after allowing the reactants to equilibrium in dark) provides a proof that the dyes are not self-degrading. Nevertheless, it was detected that the capacity for adsorption in Co3O4 NPs was significantly increased. From Fig. 4a, we observed that the Co3O4 NPs caused a total decrease of 92% of the
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-p
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MV dye and 64% of CV dye within the first 45 min period under identical conditions.
Fig. 4. Comparison of degradation rates (a), and first-order kinetic models (b) for the Co3O4
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NPs-induced decomposition of CV and MV dyes.
The kinetic rate constant (k) from the -ln(C/C0) curve (Fig. 4b) can be used to calculate the
ur
photocatalytic activity of Co3O4 NPs under the visible light using the following expression (4): dc / dt kC
n
(4)
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Where k is the rate constant (mM min-1), t is the reaction time (min), C is the dye concentration (mM) at reaction time t, and n is the order of the reaction. According to the above equation (4), the particle having the smallest size shows the maximum photocatalytic activity as they acquire the largest bandgap and highest surface to volume ratio. The Langmuir–Hinshelwood (L–H) model for the evaluation of photocatalytic kinetic processes assumes that if the initial concentration of the dye under investigation is low then the 8
photocatalytic degradation process follows pseudo-first-order kinetics and is represented as (5): dc / dt kK C
/ 1
KC
(5)
Where K is the absorption coefficient of reactants (mM-1k). If the concentration “C” is minute, then KC would be equivalent to the negligible value of unity, so that the above equation (5) can be simplified to pseudo-first-order kinetics represented in Fig. 4b. Hence, the equation becomes: d c / d t k K C K app C
(6)
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Where kapp is the apparent pseudo-first-order rate constant (min-1). However, the rate constant can be estimated by the plot of the natural logarithm of dye concentration with respect to
ln [ C t / C 0 ] K a p p t
(7)
-p
irradiation time, i.e.:
re
Where, C0 and Ct correspond to the initial and final dye concentrations (mM), t is the reaction time, and kapp is the apparent pseudo-first-order reaction rate constant (min−1). A linear plot of
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ln(C0/Ct) versus t gives the slope of kapp and further, the rate constant ‘k’ for Co3O4 NPs were determined. The kinetic studies of the CV and MV degradation by Co3O4 NPs, apparent rate
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constant were shown in Fig. 4b. From the figure, the rate constants were determined for the CV degradation, R2= 0.9659 and for MV degradation is R2= 0.9994.
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Figure 5 shows the photocatalytic degradation efficiency of Co3O4 NPs with the change of
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irradiation time (over a 45 min time period) following the exposure to the CV and MV dye solutions. From the figure, it indicates that the degradation efficiency of Co3O4 NPs is very high for the MV dye as against the CV dye. The high photocatalytic activity of the nanoparticles is due to the resent of the electron that gets moved from the valance band to the conduction band to create a hole in the conduction band and further the electron participates in the free radical oxidation reaction to degrade its colour.
9
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Fig. 5. The degradation efficiency of CV and MV dye solutions by the Co3O4 NPs. 4.Conclusion
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In conclusion, we indicate the efficacy of Co3O4 NPs towards the photocatalytic degradation of organic dyes and for that, the particles formed by the effective combustion
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method were thoroughly analysed for their crystal lattice, surface morphology, functionality,
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size and shape, optical absorption etc. The XRD analysis confirmed for the polycrystalline cubic structure of Co3O4 NPs and the FESEM images clearly indicated for the spherical size
na
with agglomeration and in a uniform size. The FTIR analysis supported for the presence of CoO bonds and the UV-vis spectra for the enhanced optical properties that can influence the
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photocatalytic activity. On testing, a degradation efficiency of 92% for the MV dye and 64% for the CV dye was observed after 0-45 min irradiation. Based on the photocatalytic studies
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which reveal the Co3O4 NPs activity for the effective degradation of CV and MV dyes, the Co3O4 NPs can be used as excellent adsorbents for degradation of effluents in the industries. Competing interests: The authors declare no conflict of interest.
10
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements The King Saud University author is grateful to the Deanship of Scientific Research, King Saud
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University for funding through Vice Deanship of Scientific Research Chairs.
11
References
[1] S. Sagadevan, S. Venilla, A. R. Marlinda1, M. R. Johan, Y. A. Wahab, R. Zakaria, A. Umar, H. H. Hegazy, H. Algarni, N. Ahmad, Effect of synthesis temperature on the morphologies, optical and electrical properties of MgO nanostructures, J. Nanosci. Nanotechnol. 20: 2488–2494 (2020). [2] P. Singh, M. M. Abdullah, S. Sagadevan, C. Kaur, S. Ikram, Highly sensitive
ro of
ethanol sensor based on TiO2 nanoparticles and its photocatalyst activity, Optik, 182: 512-518 (2019).
[3] W. Yuan, D. Xie, Z. Dong, Q. Su, J. Zhang, G. Du, B. Xu, Preparation of porous
battery. Mater. Lett. 97: 129-132 (2013).
-p
Co3O4 polyhedral architectures and its application as anode material in lithium-ion
re
[4] C. Sun, X. Su, F. Xiao, C. Niu, J. Wang, Synthesis of nearly monodisperse Co3O4
lP
nanocubes via a microwave-assisted solvothermal process and their gas sensing properties. Sens. Actuat. Chem. B157: 681-685 (2011). [5] R. Manigandan, K. Giribabu, R. Suresh, L. Vijayalakshmi, A. Stephen, V.
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Narayanan, Cobalt oxide nanoparticles: Characterization and its electrocatalytic activity towards nitrobenzene. Chem. Sci. Trans. 2: S47-S50 (2013).
ur
[6] M. Ren, S. Yuan, L. Su, Z. Zhou, Chrysanthemum-like Co3O4 architectures:
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Hydrothermal synthesis and lithium storage performance. Solid State Sci. 14: 451-455 (2012).
[7] E. Lester, G. Aksomaityte, J. Li, S. Gomez, J. Gonzalez-Gonzalez, M. Poliakoff, Controlled continuous hydrothermal synthesis of cobalt oxide (Co3O4) nanoparticles. Prog. Cryst. Growth Charact. Mater. 58: 3- 13 (2012).
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[8] S. Vijayakumar, A. K. Ponnalagi, S. Nagamuthu, G. Muralidharan, Microwave assisted synthesis of Co3O4 nanoparticles for high-performance supercapacitors. Electrochim. Acta. 106: 500-505 (2013). [9] C. Vidal-Abarca, P. Lavela, J. L. Tirado, Cobalt oxide nanoparticles prepared from reverse micelles as high- capacity electrode materials for Li-ion cells. Electrochem. Solid State Lett. 11: A198-A201 (2008).
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na
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[10] M. Muthukumaran, G. Dhinagaran, V. Narayanan, T. Raju et al., Enhanced photocatalytic behavior of (GO/Cu2O) composite with Cu2O being synthesized through green route, J. Nanosci. Nanotechnol. 19: 7215–7220 (2019)
13
PII:
S0030-4026(20)30262-X
DOI:
https://doi.org/10.1016/j.ijleo.2020.164428
Reference:
IJLEO 164428
To appear in:
Optik
Received Date:
27 December 2019
Revised Date:
12 February 2020
Accepted Date:
16 February 2020
Please cite this article as: Saravan RS, Muthukumaran M, Mubashera M, Abinaya M, Prasath PV, Parthiban R, Mohammad F, Oh WC, Sagadevan S, Evaluation of the photocatalytic efficiency of cobalt oxide nanoparticles towards the degradation of crystal violet and methylene violet dyes, Optik (2020), doi: https://doi.org/10.1016/j.ijleo.2020.164428
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier.
Evaluation of the photocatalytic efficiency of cobalt oxide nanoparticles towards the degradation of crystal violet and methylene violet dyes
R. Sukhin Saravan1, M. Muthukumaran2, M. Mubashera3, M. Abinaya2, P. Varun Prasath2*, R. Parthiban1, Faruq Mohammad4, Won Chun Oh5 and Suresh Sagadevan6*
Department of Chemical Engineering, SSN College of Engineering, Kalavakkam – 603110, India 1
Department of Analytical Chemistry, University of Madras, Chennai -25, India
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2 3
Department of Chemical Engineering, Alagappa College of Technology, Anna University, Chennai – 25, India 4
Surfactants Research Chair, Department of Chemistry, College of Science, King Saud
5
-p
University, Riyadh, Kingdom of Saudi Arabia 11451
Department of Advanced Materials Science and Engineering, Hanseo University, Seosan-si,
6*
re
Chungnam 356-706, Korea
Nanotechnology & Catalysis Research Centre, University of Malaya, Kuala Lumpur 50603,
lP
Malaysia
Jo
Abstract
ur
na
Corresponding author mail Ids: [email protected]; [email protected]
The present work deals with the synthesis, characterization, and testing towards the photochemical degradation efficiency of cobalt oxide (Co3O4) nanoparticles (NPs). Following the synthesis of Co3O4 NPs by the hydrothermal method, the NPs are confirmed to be formed in their cubic lattice structure with the spherical shape morphology. The testing studies revealed that the Co3O4 NPs degraded the Methylene violet (MV) and Crystal violet (CV) dyes very 1
efficiently under the UV light irradiation. A Langmuir–Hinshelwood (L–H) model has been effectively used to display the adsorption of CV and MV dye molecules taking place using surface monolayer exposure to the UV light. The kinetics and reaction mechanism behind the elimination of CV and MV dyes shows for a pseudo-second-order kinetics model. With these Co3O4 NPs as photocatalysts, the maximum photodegradation rate was observed to be 92% and 64% for the MV and CV (respectively) within the same period of 45 min and this enhanced photocatalytic performance is due to the effective reduction of electron-hole recombination
ro of
process by the Co3O4 NPs.
Keywords: Cobalt oxide nanoparticles, Photocatalysis, Dye degradation, Methylene violet,
re
-p
Crystal violet, Dye adsorption.
1. Introduction
lP
In recent years, the increased interest with the nanostructured metal oxides due to their high surface areas with supportive thermal, mechanical, electrical, and optical properties made them
na
to include in the development of optical, electronic, and sensing devices [1-2]. With that view, the transition metal oxide nanoparticles (NPs) like Fe3O4, ZnO, TiO2, ZnO, CoFe2O4 etc are
ur
found to be very efficient and among many different kinds, the cobalt oxide (Co3O4) NPs exhibit interesting properties that can be suitable for the lithium storage, gas sensing, and
Jo
electro-catalysis. The most stable phase of Co3O4 with a direct bandgap of 1.48 - 2.19 eV is used as a p-type semiconductor and received considerable attention [3-7]. The formation of Co3O4 NPs by the hydrothermal approach generally requires the reduction and precipitation of chemical agents and this particular method is considered to be the green synthesis and is less expensive [6-7] as against the other methods like microwave-assisted [8] and reverse micelles [9]. Therefore, taking into the consideration of unique band energy gaps associated with the 2
nanostructured Co3O4, the present study deals with the photocatalytic dye degradation efficiency of Co3O4 NPs operated by the UV light. Following the synthesis of Co3O4 NPs by the hydrothermal approach, they were thoroughly characterized for the crystal type and nature, surface morphology, functionality, optical properties etc. Further, the Co3O4 NPs photocatalytic performance towards the degradative removal of organic contaminants like
ro of
methylene violet (MV) and crystal violet (CV) dyes are being investigated.
2. Materials and Methods
For the formation of Co3O4 NPs, about 50 mL of 1 M aqueous NaOH solution was mixed
-p
dropwise with 0.5 M of cobalt nitrate and on stirring, we see the turning of a pink coloured
re
solution to dark green first and then to black precipitate. This black precipitate is then transferred to an autoclave and kept in a furnace maintained at 180ºC so as to undergo the
lP
hydrothermal process. The black precipitate is filtered off and dried in an air oven set at 110ºC. The dried filtered flakes are crushed and powdered using mortar and pestle, and the black
Co3O4 NPs.
na
powder is subjected to heating again at 550ºC for the sintering process which finally forms the
ur
The powdered X-ray diffraction (XRD) analysis was performed on Model Smart lab SE X-Ray instrument (Make-Rigaku, Japan) that uses CuKα radiation (1.5418 Å), 2θ scanning
Jo
range of 20-80º with 0.02º step size. The Fourier transform infrared (FTIR) spectrometer, Spectrum Two FTIR/ATR instrument in the wavenumber range of 500-4000 cm-1 with a resolution of 0.5 cm-1 was used. The UV-vis spectroscopic analyses were performed using a double beam spectrophotometer (UV-1800, Shimadzu) in the wavelength range of 200-800 nm. For the field emission scanning electron microscopy (FESEM), Carl Zeiss microscopy
3
Ltd., (UK & SIGMA) connected to an energy dispersive analysis by X-rays (EDAX) detector was used. The photocatalytic performance of Co3O4 NPs was studied as against the degradation of CV and MV dyes in an aqueous solution and is investigated under the sunlight. For the testing, about 1.0 g/L of Co3O4 nanocatalyst was added to the 10 ppm concentration of dye with stirring and by keeping the reaction mixture under sunlight. A magnetic stirrer was used continuously for the proper mixing of the solution. After dissolving the catalyst in aqueous
ro of
solution and by placing the reaction mixture in the sunlight for up to 45 min, the reactants were shifted to the dark environment and kept there for another 30 min in order to attain the adsorption equilibrium and then the absorbance was measured using the UV-vis
-p
spectrophotometer. Now, the UV light was turned on and the absorbance of the sample was
using the formula (1) [10]:
(1)
lP
Degradation % = (C0-C)/C0 x 100 %
re
measured for every 30 min. The absorption percentage of individual sample was calculated
where C0 and C are the initial and final concentrations. The experiments are performed to
na
evaluate the effect of the catalyst with UV radiation for the degradation of CV and MV dye. 3. Results and discussion
ur
The FTIR spectrum of Co3O4 NPs showed in Fig.1a indicates for the two strong bands at 557 cm-1 and 626 cm-1 belonging to the spinel structure of Co3O4. The peak at 626 cm-1 can be
Jo
attributed to the Co-O stretching frequency at which Co is Co+2 and is tetrahedrally coordinated, while the other peak at 557 cm-1 linked to the Co-O of octahedrally coordinated Co+3 ion [7]. The band appeared at 1656 cm-1 can be assigned to the stretching and bending vibrations of absorbed water molecules onto the Co3O4 NPs [6]. Similarly, from the powdered XRD study of Co3O4 NPs shown in Fig. 1b, the peaks are reflecting at (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) revealing the polycrystalline cubic structure of Co3O4 NPs. 4
The sharp and high intense peak at (3 1 1) indicating for the high crystallinity of the product and based on this peak, the average crystallite size was calculated to be 22 nm by making use of the Scherer formula. Also from the diffraction pattern, no other impurity phases are detected and thus confirming for the successful formation of Co3O4 NPs in their purest form by this
lP
re
-p
ro of
hydrothermal method.
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Fig. 1. (a) FTIR spectrum, (b) powdered XRD pattern, (c) FESEM image, and (d) EDAX of Co3O4 NPs. The morphological analysis of Co3O4 NPs examined by FESEM (Fig. 1c) indicates for the
ur
formation of spherical-shaped particles with visible clumped distributions, in addition to the Co3O4 NPs agglomeration. Also, the FESEM image clearly shows for the formation of particles
Jo
in their crystalline nanosizes, spherical/granular in shape, uniform, and with porous morphology, where the nanoparticles agglomeration results from their exceedingly small sizes and high surface energies. Further, the EDAX analysis of Co3O4 NPs illustrated in Fig. 1d shows respective peaks for the elements of Co and O and in addition, this analysis clearly confirms for the formation of Co3O4 NPs in their purest form (appropriate elemental composition) without any unwanted impurities. 5
The UV-vis diffuse reflectance spectra (DRS) of the Co3O4 NPs (Fig. 2a) indicate for the strong absorption ability especially in the absorption range of 200-800 nm and implies that the introduction of Co3O4 NPs can promote for the absorption of visible light and produce more photogenerated charge carries which further responsible for an enhancement in the photocatalytic performance. Also, from the UV-vis DRS of the ageing sample indicated that no significant absorption peaks have appeared and this can probably be due to the energy gaps generated by the quantum confinement effects. Since the particles sizes are very narrow and in
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the nanosize range which made the absorption peak to be slightly broader and in addition, the stability of Co-O bond in the Co3O4 NPs can be attributed to the polar-symmetrical structure and is strongly influenced by the weak Vander Waals interactions within particle regime. The
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bandgap of the sample was calculated using the Kubelka-Munk (KM) method and based on this, the Tauc plot (αhυ)1/2 versus (hυ), the bandgap of 2.44 eV was obtained for the Co3O4 NPs
n/2
(hv) A(hv E g )
(2)
(3)
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n/2
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(hv) C (hv E g )
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as showed in Fig. 2(b).
Where A, α, υ, h, n, and Eg corresponds to the proportionality constant, absorption coefficient,
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frequency of light, Planck’s constant, and the bandgap, respectively.
Fig. 2. (a) UV-vis spectrum and (b) Tauc plot of Co3O4 NPs. 6
In order to test the Co3O4 NPs photocatalytic efficiency, two different dyes, CV and MV were selected and studied their degradation properties under visible light illumination. Figure 3(a-b) shows the comparison of UV–vis absorption spectrums of 10-5 M concentration of MV and CV dyes (respectively) as a function of irradiation time. From the figure, we observed that the increase of irradiation time (from 0 min to 45 min) caused a gradual increase in the degradation as provided by the individual absorption band and thereby suggesting for the Co3O4-mediated photodegradation of CV and MV dyes on regular basis. From the figure, the decrease in the
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absorbance intensity with time for both dyes is due to the change in colour of dye solutions from blue to colourless with respect to an increase in the adsorption capacity for every 30 mins and also indicating for the strong catalytic activity of Co3O4 NPs. On the further increase of
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time, the absorbance intensities of CV and MV dyes got decreased with time (results not shown), i.e. after 2 h, the absorbance is very minimal and the dye solutions became colourless,
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indicating the complete degradation of pollutants under UV radiation. In addition, we observed
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that the catalyst showing a linear increase in the adsorption of CV and MV dyes under UV
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irradiation and this absorbance is due to the smaller crystallite size and enhanced surface area.
Fig. 3. Comparison of the photocatalytic degradation of changes in concentrations of (a) CV, and (b) MV dyes. The photocatalytic degradation of CV and MV dye solutions in presence of Co3O4 NPs were quantified by the ratio, Ct/C0, where C0 and Ct correspond to the initial and final concentrations 7
of CV and MB dye solutions. Fig. 4a shows the comparison of changes in the Ct/C0 ratio as against the irradiation time over a period of 45 min and from the graph, we observed that the ratio is getting decreased with regards to an increase in the irradiation time. Also, the observation of the same optical absorbance for both dyes at initial stages (after allowing the reactants to equilibrium in dark) provides a proof that the dyes are not self-degrading. Nevertheless, it was detected that the capacity for adsorption in Co3O4 NPs was significantly increased. From Fig. 4a, we observed that the Co3O4 NPs caused a total decrease of 92% of the
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MV dye and 64% of CV dye within the first 45 min period under identical conditions.
Fig. 4. Comparison of degradation rates (a), and first-order kinetic models (b) for the Co3O4
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NPs-induced decomposition of CV and MV dyes.
The kinetic rate constant (k) from the -ln(C/C0) curve (Fig. 4b) can be used to calculate the
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photocatalytic activity of Co3O4 NPs under the visible light using the following expression (4): dc / dt kC
n
(4)
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Where k is the rate constant (mM min-1), t is the reaction time (min), C is the dye concentration (mM) at reaction time t, and n is the order of the reaction. According to the above equation (4), the particle having the smallest size shows the maximum photocatalytic activity as they acquire the largest bandgap and highest surface to volume ratio. The Langmuir–Hinshelwood (L–H) model for the evaluation of photocatalytic kinetic processes assumes that if the initial concentration of the dye under investigation is low then the 8
photocatalytic degradation process follows pseudo-first-order kinetics and is represented as (5): dc / dt kK C
/ 1
KC
(5)
Where K is the absorption coefficient of reactants (mM-1k). If the concentration “C” is minute, then KC would be equivalent to the negligible value of unity, so that the above equation (5) can be simplified to pseudo-first-order kinetics represented in Fig. 4b. Hence, the equation becomes: d c / d t k K C K app C
(6)
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Where kapp is the apparent pseudo-first-order rate constant (min-1). However, the rate constant can be estimated by the plot of the natural logarithm of dye concentration with respect to
ln [ C t / C 0 ] K a p p t
(7)
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irradiation time, i.e.:
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Where, C0 and Ct correspond to the initial and final dye concentrations (mM), t is the reaction time, and kapp is the apparent pseudo-first-order reaction rate constant (min−1). A linear plot of
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ln(C0/Ct) versus t gives the slope of kapp and further, the rate constant ‘k’ for Co3O4 NPs were determined. The kinetic studies of the CV and MV degradation by Co3O4 NPs, apparent rate
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constant were shown in Fig. 4b. From the figure, the rate constants were determined for the CV degradation, R2= 0.9659 and for MV degradation is R2= 0.9994.
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Figure 5 shows the photocatalytic degradation efficiency of Co3O4 NPs with the change of
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irradiation time (over a 45 min time period) following the exposure to the CV and MV dye solutions. From the figure, it indicates that the degradation efficiency of Co3O4 NPs is very high for the MV dye as against the CV dye. The high photocatalytic activity of the nanoparticles is due to the resent of the electron that gets moved from the valance band to the conduction band to create a hole in the conduction band and further the electron participates in the free radical oxidation reaction to degrade its colour.
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Fig. 5. The degradation efficiency of CV and MV dye solutions by the Co3O4 NPs. 4.Conclusion
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In conclusion, we indicate the efficacy of Co3O4 NPs towards the photocatalytic degradation of organic dyes and for that, the particles formed by the effective combustion
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method were thoroughly analysed for their crystal lattice, surface morphology, functionality,
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size and shape, optical absorption etc. The XRD analysis confirmed for the polycrystalline cubic structure of Co3O4 NPs and the FESEM images clearly indicated for the spherical size
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with agglomeration and in a uniform size. The FTIR analysis supported for the presence of CoO bonds and the UV-vis spectra for the enhanced optical properties that can influence the
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photocatalytic activity. On testing, a degradation efficiency of 92% for the MV dye and 64% for the CV dye was observed after 0-45 min irradiation. Based on the photocatalytic studies
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which reveal the Co3O4 NPs activity for the effective degradation of CV and MV dyes, the Co3O4 NPs can be used as excellent adsorbents for degradation of effluents in the industries. Competing interests: The authors declare no conflict of interest.
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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements The King Saud University author is grateful to the Deanship of Scientific Research, King Saud
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University for funding through Vice Deanship of Scientific Research Chairs.
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