Synthesis, characterization and daylight active photocatalyst with antiphotocorrosive property for detoxification of azo dyes
Graphical abstract
Introduction
The global energy crisis and related ecological concerns are among the prime technological challenges being confronted by chemists and technologists in the 21st century. The rate of total energy used by all of human civilization reached 15 TW in 2008 and is unsurprising to virtually twice by 2050 due to the emergent global creation and inhabitants [1]. On the contrary, our most important energy resources still instigate from restricted and non-renewable fossil fuels, such as firewood, oil and natural gas. Besides, the combustion of these fossil fuels has caused a succession of critical environmental tribulations, ranging from air and water contamination to worldwide humidity. For this reason, looking for renewable, clean as well as carbon–neutral unusual energy resources is very quickly needed to replace our dependence on fossil fuels. Solar energy is the primary source of energy for the life on our planet. It’s very safe, abundant and carbon neutral. Next to nuclear energy and a combine of other renewables, it is amongst the best options to substitute the fossil fuels, causing environmental problems [2]. On the other hand, solar energy is diffuse, sporadic and its collection, concentration and storage hinder the full utilization of its potential. Plants and organisms have learnt how to use it to exchange plentiful compounds such as water and CO2 into constructive chemicals for their intensification.
Semiconductor photocatalysis like TiO2, ZnO and WO3 have attracted more extensive awareness in current years owing to their immense potential in environmental contaminant degradation and water splitting [3], [4], [5], [6], [7], [8], [9], [10], [11]. Among them, ZnO is almost considered to be an excellent sun light receptive photocatalyst material due to its relatively narrow band gap (3.2 eV). Conversely, concerning its application in environmental remediation, there are a number of critical negative aspects to be noted: (i) the separation efficiency of photogenerated electrons and holes is very low; (ii) it is prone to photocorrosion in aqueous media containing oxygen during photochemical reaction [12], (iii) it is usually present as fine or ultrafine particles, and it is a challenging and expensive task to separate the catalyst particles from the reaction systems. To overcome these problems, modifying ZnO photocatalysts to enhance light absorption and photocatalytic activity under sun light has been a main research direction in recent years. One of the efficient methods to modify semiconductor surface is by doping noble metals such as Cu, Ag, Ce, Au, and Mg [13], [14], [15], [16], [17], [18]. Additionally, the formation of a coupled semiconductor structure can efficiently improve the optical absorption capacity and at the same time reduce the charge recombination under sun light irradiation because they can reimburse for the disadvantages of the individual component, and persuade a synergistic effect [6], [7], [19], [20], [21], [22], [23], [24], [25]. Chalcogenides such as Ag2S, CdS, CoS2, ZrS2 and ZnS have been studied extensively, since they have ideal edge positions of the valence and conduction bands for the redox reactions [26], [27], [28], [29], [30], [31]. CdS is an important II–VI semiconductor with direct band-gap energy of 2.42 eV [32], which could be excited by visible light to produce photogenerated electrons and holes. As the conduction band of ZnO is about 0.5 eV more positive than that of CdS, the band position between CdS and ZnO favors the transfer of photogenerated electrons from the conduction band of CdS into that of ZnO efficiently. Furthermore, the improvement in the stability of CdS based photocatalysts utilizing visible range of the spectrum is a great challenge. In the present work, CdS loaded Ag–ZnO composite catalyst was synthesized by direct loading of CdS on Ag–zinc oxalate substrate with simple precipitation – thermal deposition method under mild condition. The photocatalytic activities of CdS–Ag–ZnO were evaluated with azo dyes degradation under solar light.
Section snippets
Materials
The commercial azo dyes Reactive Red 120 (RR 120) (Fig. S1, see Supplementary data), Reactive Orange 4 (RO 4), (Fig. S2, see Supplementary data) and Reactive Yellow 84 (RY 84) (Fig. S3, see Supplementary data), from Balaji Colour Company, Dyes and Auxiliaries (Chennai) were used as received. Oxalic acid dihydrate (99%) and zinc nitrate hexahydrate (99%) were obtained from Himedia chemicals. AgNO3− and CdS from sigma Aldrich, ZnO (Himedia), TiO2 (Merck) were used as received. A gift sample of
Characterization of catalyst
To find out the optimum amount of CdS loading, initially, we had carried out the degradation of three azo dyes RR 120, RO4 and RY 84 with different wt% of CdS in Ag–ZnO. The percentages of degradation with 1, 2, 3, 4, and 5 wt% were found to be 70, 93, 85, 81, and 75 for RR 120 (64%, 82%, 75%, 76% and 68% for RO 4, 69, 80, 78, 74 and 65 for RY 84), respectively at 20 min irradiation. When we raise the amount of CdS from 1 to 2 wt% the percentage of degradation also increases, further raising the
Conclusion
In summary, we have demonstrated cost-effective precipitation-thermal decomposition method for the production of CdS loaded Ag–ZnO photocatalyst. This synthesis is simple and cost efficient. TEM and XRD reveal the presence of hexagonal and wurtzite structure of ZnO, respectively. EDS shows the presence of Ag, S and Cd in the catalyst. Presence of Ag and CdS increase the absorption of ZnO to entire visible region. DRS spectra indicate the reduction of band gap of the CdS–Ag–ZnO, when compared to
Acknowledgements
M. Shanthi is highly thankful to UGC, New Delhi, India for financial support through research project F.No 41-288/2012 (SR). This work was supported by FCT/QREN-COMPETE through the project PTDC/AAC-CLI/118092/2010 and grant SFRH/BPD/86971/2012 (Balu Krishnakumar).
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