DOI: 10.1002/slct.201800797 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57
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Photocatalytic Performance of Indigo-Dye-Doped Graphene-Supported BiVO4 Nanocomposite under Visible Light Ahsan Shameem,[a] Ahmed Ali Kashmeri,[a] Faisal Nawaz,[b] Muhammad Shabir Mahr,*[c] Amjad Pervaiz,[d] and Javed Iqbal*[a, e] In this work, novel graphene supported BiVO4 based nanocomposites were derived successfully by employing hydrothermal method. Pure bismuth vanadate limits a number of applications because of its insufficient photo catalytic activity in ordinary sun light. So, Indigo-Reduced graphene oxide (RGO)/BiVO4 nanocomposites were designed as a next step to achieve efficient photo-catalytic activity in visible light in relation to pure BiVO4 or its other nanocomposites. Character-
Introduction Rapid development in industry and huge increase in the consumption of fossil fuels has raised some serious environmental issues. Different approaches have been used to address such issues. One of these approaches is to degrade the pollutant materials present in atmosphere as well as to find new and environmental friendly energy resources. Photo catalysts are among the potential candidates that consume light as energy resource and are used extensively for degradation of organic pollutants as well as for producing hydrogen from water. Different nano-materials such as ZnO, CdS, WO3 and TiO2 have been used as photo-catalyst for water splitting and for the degradation of organic pollutants.[1–2] Semiconducting materials are conventionally used as photo-catalytic [a] A. Shameem, A. A. Kashmeri, Dr. J. Iqbal Department of chemistry University of Agriculture, Faisalabad 38000, Faisalabad, Pakistan E-mail:
[email protected] [email protected] [b] Dr. F. Nawaz University of Engineering and Technology, Lahore Faisalabad, Campus, Pakistan [c] Dr. M. S. Mahr Department of Physics University of Agriculture, Faisalabad 38000, Faisalabad, Pakistan E-mail:
[email protected] [d] Dr. A. Pervaiz Quaid-e-Azam University Islamabad, Pakistan [e] Dr. J. Iqbal Punjab Bio-energy Institute University of Agriculture, Faisalabad Pakistan Supporting information for this article is available on the WWW under https://doi.org/10.1002/slct.201800797
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ization of nanocomposites was carried out by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and[1] UV–Vis spectroscopy (UV). The enhancement of the photo-catalytic activities of the prepared nanocomposite were contributed to the effective charge transfer of photo-generated electron from BiVO4 to RGO and pi to pi* stacking improved absorption performance.
materials. But large energy bandgap of natural semiconductors disturbs the mobility of electron hole pairs and hence limit their applications as photo-catalysts. It has been seen that adsorbing powers decrease with the increase in band gap as in case of ZnO and TiO2 which have large band gaps and are only able to absorb UV radiation. UV is a very little fraction of electromagnetic spectrum which has smaller capability of absorption as compared to visible fraction.[3] Bismuth vanadate is among the promising semiconductor materials with efficient ferro-elasticity and pyro-optical properties. Its band gap is 2.4 eV which promises better photocatalytic performance in the visible range due to its effective stability.[4] However, pure bismuth vanadate shows less photocatalytic activity due to its limited absorptive capacity in the visible region. Such problem is resolved for instance by using combination of nano based bismuth vanadate with other advanced materials such as graphene.[5] Graphene is among the most appealing materials these days especially due to its extraordinary optical and electrical properties.[6] It comprises of single layer atomic sheet of sp2 hybridized carbon. Graphene provides a high surface area due to 2D structures. It shows a zero band gap[7] as electronic band structure overlaps due to less carbon-carbon distance of the electron and hole so it acts as a mass free charge.[8] Graphene and reduced graphene oxide both exhibit photo-catalytic activity sufficient in the UV light.[9] Electronic properties of graphene can be further extended by the introduction of hetero-atoms/functionalization of graphene.[10] Graphene oxide based composites provide profound photo-catalytic activities. Interaction of the semiconductor and the graphene oxide results in creation of electrons holes and consequently provides efficient photo-catalytic activity. Graphene and bismuth vanadate nano-composites have been involved explicitly in the degradation of organic pollutant and the water splitting 6701
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Figure 1. Schematic synthesis of Indigo-RGO/BiVO4 Nano-composite photo-catalyst.
process.[11] Different organic molecules have been used for the coupling reaction and for the covalent or non-covalent staking with the interaction of graphene and reduced graphene oxide.[12] BiVO4 and RGO based nano-composite photo catalyst showed 10 fold enhancements in photolytic and electrochemical performances as compared to pure BiVO4.[13] One of the strategies to enhance the photo-catalytic performance of the conventional catalytic system in visible region is achieved by incorporation of the dye molecule into it.[14] Adsorption of dye molecule into a semiconductor acts as energy carrier so electrons shift from the conduction band of the semiconductor to the valance band easily. In current study novel indigo-BiVO4/RGO nanocomposite were synthesized hydrothermally as potential efficient sunlight driven photo-catalyst to reduce organic pollution. Hydrogen bonding lead to the strong interaction between the indigo and the RGO in the presence of bismuth vanadate under Visible light irradiation. It is anticipated that nanocomposite of BiVO4 with RGO in the presence of indigo will increase the reaction sites for interaction and charge efficiency resulting remarkable enhancement in its photo-catalytic activity as compared to its pure BiVO4 counterpart.
Result and Discussion XRD To compare with the XRD pattern of pure BiVO4 (JCPDS No.65) and the newly synthesized nano-composites (Figure 1), all Bragg diffraction peaks[15] were related to indigo-BiVO4/RGO in the range of 2q from 17–668 in. Three sharp peaks appeared at 18.88, 34.98, and 44.58 belonging to the monoclinic structure’s ionic peak-scan be clearly observed in the patterns of BiVO4/ RGO composite.[16] ˚ Reported value of GO appear at 108 with spacing 8.32 A ˚ and a graphite at (002) 26.538 corresponding to 3.36 A. The sharp half width peaks at 29.08, 38.18, 40.08, 48.88 and 63.98 corresponds to (112), (111), (211), (024) and (220) planes of tetragonal BiVO4 correspondingly. No peak of graphene oxide is observed so it might be converted into reduced graphene oxide so peak of RGO after hydrothermal treatment is shifted ChemistrySelect 2018, 3, 6701 – 6706
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to (310) 27.28 2 h (Figure 2). The VO3 is transferred to VO4 and graphite oxide to RGO during hydrothermal process verifying that composites preparation process is highly efficient, simple and green. This conclusion is further supported by SEM and EDX results. SEM/EDX The morphological analysis of the prepared samples was carried out by Scanning electron microscopy. SEM result shows that in the graphene based nano-composite in Figure 3 (a, b), the bismuth vanadate particles exhibit well-defined morphologies, with an average size of 1 mm. RGO acts as a support with bread like structure along with plate-like BiVO4 particles.[17] Because of the relatively large particles size and rough morphology of aggregated Bismuth vanadate particles, it forms sufficient interfacial contact between Bismuth vanadate particles and reduced graphene oxide nano-sheets which act as photo-generated charge carrier within the composite[18] that leads to the efficient charge separation during coupling of RGO. Indigo, organic based dye generally used in dye sensitizer solar cells, contains indoxyl group which acts as an electron donor or an acceptor. The dye has conjugated system attached to the substrate surface that absorbed light. We have also performed SEM analysis of the samples after the catalytic activity was performed at pH 3 and pH 11 to have an idea about the degradation of the catalytic surface. We came up with conclusion that the catalytic surface is not completely damaged which shows that the catalysts can be reused (Supporting information, S1, S2). As at pH 11 the catalytic activity is much better as compared to pH 3 so due to more adsorption of the dye on the catalytic surface its surface is relatively more damaged as compared to at pH 3. Energy dispersive X-rays (EDX) spectra helps in the determination of elemental analysis and structural determination (S1, S2, S3 and S4) Figure 4. An EDX spectrum normally displays peaks corresponding to the energy levels for which the most Xrays had been received. Each of these peaks corresponds to an atom, and therefore corresponds to a single element. Peak intensity depends upon the concentration of the element present in sample. Bi exhibits the highest intensity peak along 6702
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Figure 2. X-ray diffraction pattern of Indigo-BiVO4/RGO composite.
Figure 3. SEM images of Indigo-BiVO4/RGO (a and b).
with carbon and oxygen which shows that BiVO4 and carbon concentration is high between the samples. Photo-catalytic activity When ultra sonicated, the photo-catalyst showed excellent dispersibility in recation medium which made more reaction surfaces exposed to solution. After addition of the photocatalyst to methylene blue solution, the water dispensability of nano-composite was further improved due to interaction of dye with water molecules.
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As displayed in Figure 5, comparative analyses of catalytic performance at different pH were done and it was helpful in depicting catalytic behavior of the photo-catalyst. (a) At pH 3 decrease in catalytic performance (32%) is thought to be due to lower electron donor effect of indigo dye which is protonated in acidic solution by the involvement of functional groups of COOH and OH. (b) Moderate degradation effect is shown at pH 7 due to stability of graphene sheet considerably at pH 7 as compared with the acidic media. (c) In basic solution maximum degradation (91%) is observed at pH 11 which is considered due to the strong interaction between the indigo
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Figure 4. The EDX of the prepared Indigo-BiVO4/RGO (a) S1 (b) S2 (c) S3 and (d) S4.
Figure 5. Photo-catalytic performance of under investigated systems under direct sunlight at different pH values (a) pH 3 (b) pH 7 (c) pH 11 (d) degradation efficiency with time exposed for 100 minutes as studied by UVVis spectroscopy.
dye and the RGO, de-protonation to the RGO creates negative charge there by increasing the photo-degradation ability. Indigo-RGO/BiVO4 shows maximum degradation performance of 91% degradation in Figure 6 while pure BiVO4 shows only 49% results. It is proven that Indigo-RGO/BiVO4 show efficient performance under sunlight/visible light in comparison with RGO/BiVO4 and BiVO4 catalysts. It is depicted that RGO act ChemistrySelect 2018, 3, 6701 – 6706
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Figure 6. Photo catalytic performance of investigated systems under direct sunlight (a) BiVO4 (b) RGO/BiVO4 (c) Indigo-RGO/BiVO4 (d) Indirect sun light for 100 minutes as studied by UV-Vis spectroscopy.
as electron acceptor in photo-reduction activity so it provides more surface area for reaction sites for electron shifting. RGO makes sure of quick transfer of photo-excited electrons from the conduction band of semiconductor to RGO. Indigo-RGO/BiVO4 performs effectively in sunlight light because the energy band gap between the Indigo-RGO develops high electron affinity with RGO and excitation take place by the transfer of electrons from excited dye to RGO to 6704
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Figure 7. Degree of degradation of 10 ppm MB (d) BiVO4, BiVO4/RGO and Indigo-RGO/BiVO4 exposed under direct sunlight (b) at pH 11 irradiated under direct sunlight and shadow.
accomplish equilibrium among the charges (Figure 7, (a). Conjugation of p-p*network that arise double layer provides a pathway to trapped electron and facilitates the charge stabilization from indigo to the RGO. (b) Indirect effect established due to less degradation effect and its band gap which is not sufficient for shadow light create less excitation and less charge transfer for catalytic activity. Raman Spectrum Raman spectra is among the useful techniques for carbon materials analysis, so, characterization of catalyst by raman spectral analysis was also performed. A major peak was observed at 2935 cm 1 which was attributed to-CH2 groups (symmetric and symmetric vibration while D and G peaks at 1352 cm 1 and 1588 cm 1 confirm the presence of Graphene oxide in the catalyst, while small peak at 916 cm 1 is attributed to the presence of C O-H bond and such peak are observed in BiVO4 samples[19] and hence confirmed bismuth vandate presence (Figure 8).
Conclusions Newly designed nanocomposites of Indigo-BiVO4/RGO were synthesized by using hydrothermal routes. The photo-catalytic activities of nanocomposite and individual components such as BiVO4, BiVO4/RGO and Indigo-BiVO4/RGO system were investigated and compared under sunlight. This work demonstrated that the (Indigo-RGO/BiVO4) is an excellent candidate for photo-catalytic activity than other reported nanocomposite such as RGO/BiVO4 or BiVO4. This study opens good possibilities to design new class of modified photo-catalysts with more environmental applications under ambient conditions of light.
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Figure 8. Raman spectrum of Indigo-RGO/BiVO4.
Supporting information summary SEM images of samples after catalytic activity at pH 3 and pH 11 are described in supporting information.
Acknowledgement The authors acknowledge the financial and technical support from Punjab Bio-energy Institute located at University of Agriculture Faisalabad (UAF), Pakistan funded by Govt. of the Punjab, Pakistan. The authors also acknowledge the technical support from Department of Chemistry, University of Agriculture, Faisalabad, Pakistan.
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Submitted: March 17, 2018 Accepted: June 12, 2018
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