High performance visible-light responsive Chl-Cu/ZnO catalysts for photodegradation of rhodamine B
Graphical abstract
Introduction
Organic dyes are one of the main pollutants in wastewater released from food, textile, plastic and cosmetic industries. Even though the residual dyes in wastewater are present in low concentrations, they are still harmful to plants, aquatic animals and humans due to their non-biodegradable properties and toxicity [[1], [2], [3]]. Therefore, removal of dyes from wastewater is an essential process to lessen this environmental concern. To find out an appropriate solution, various techniques, including photocatalytic degradation [4], Fenton process [5], bioremediation [6] and ozonation [7], have been proposed. Among these techniques, catalytic photodegradation is considered as a potential method as it can be operated without expensive oxidants at low pressure and temperature using stable and low-cost catalysts [[8], [9], [10]]. In order to examine the degradation on efficiencies of photocatalysts, rhodamine B (RhB) is generally used as a representative dye since it is important in reference to its harmful impacts for human health, such as cancer, skin irritation, and allergic dermatitis [[11], [12], [13]]
Among various kinds of photocatalysts, zinc oxide (ZnO) and titanium dioxide (TiO2) have been widely used for photodegradation [[14], [15], [16]] due to their non-toxicity, excellent thermal and chemical stability and high photocatalytic efficiency [17,18]. In comparison to TiO2, ZnO has received much attention because of its strong oxidation activity, low cost and high quantum efficiency [19,20]. However, due to the fact that the band-gap energy for visible-light activation is approximately 1.8–3.1 eV, ZnO, with its wide band-gap energy (3.3 eV), can be activated only under UV irradiation [[21], [22], [23]]. Taking inspiration from natural photosynthesis, chlorophyll, which plays a potential role as a photopigment, has been applied in the catalytic field to reduce the band-gap energy for photocatalytic applications under visible-light irradiation. Phongamwong et al. [24] used chlorophyll in Spirulina with N–doped TiO2 for CO2 reduction and found that chlorophyll could improve the photocatalytic activity under visible-light irradiation by acting as a sensitizer and electron donor, effectively giving an electron for promoting photoreaction. In addition, chlorophyll has also been found to promote the degradation of adsorbed dye molecules by facilitating the absorption of dye and thus leading to better photocatalytic ability [25,26]
In order to improve ZnO based catalysts in terms of the vital problems that are the recombination of electron-hole (e−–h+) pairs and backward reaction [27,28], metals such as Fe, Ag, Ni and Cu have been doped onto ZnO [[29], [30], [31]]. Among these, Cu is promising in the prevention of the formation of recombination centers [[32], [33], [34]] as it can potentially substitute zinc atoms in ZnO lattice because of the similar size of ionic radius of Cu and Zn. Moreover, copper can improve photocatalytic activity of ZnO by providing electron capture to separate charges [35,36]. In addition, the strong coupling interaction between O 2p and Cu 3d plays an important role in narrowing the band-gap energy in Cu/ZnO system, resulting in visible-light activation ability [37].
In this work, a series of chlorophyll-Cu co-modified ZnO (Chl-Cu/ZnO) catalysts were synthesized and investigated for photocatalytic degradation of rhodamine B under visible-light irradiation. Outstanding performances of Chl-Cu/ZnO catalysts, particularly in terms of high catalytic activity under visible-light irradiation, were clearly observed due to the high capacity for dye adsorption and the inhibition potential of electron-hole pairs recombination.
Section snippets
Synthesis of ZnO nanoparticles
ZnO nanoparticles were synthesized through a low-temperature precipitation technique modified from the work reported by Akir et al. [38]. In this work, zinc acetate dihydrate (Zn(CH3COO)2·2H2O: Loba) was used as a ZnO precursor, and deionized water was used as a solvent. A certain amount of sodium hydroxide (NaOH: PanReac AppliChem) was dissolved in deionized water and added dropwise into zinc acetate dehydrate solution at 60 °C; then the mixture was vigorously stirred for 2 h. After that, the
Performance of chlorophyll-modified ZnO catalysts
As shown in Fig. 1, dye adsorption capacities over Chl/ZnO catalysts were significantly higher than that on the ZnO catalyst. The degradation of RhB was found to increase with increased loading of chlorophyll, such that the optimum chlorophyll loading, giving the best degradation efficiency among all Chl/ZnO catalysts was found to be 0.5 wt.% Chl a. At loading amounts higher than 0.5 wt.% Chl a, the degradation activity tended to decrease due to significant decreases in the specific surface
Conclusion
Chlorophyll-Cu co-modified ZnO (Chl-Cu/ZnO) photocatalyst was successfully prepared and played a significant role in photodegradation of RhB under visible-light irradiation. Among all catalysts studied, chlorophyll-Cu co-modified ZnO catalyst gave the most outstanding degradation of RhB, attaining almost 99% within 2 h. The photocatalytic activities over ZnO catalysts were significantly improved by loadings of copper and the extracted chlorophyll on the ZnO surface. As an electron acceptor,
Acknowledgments
This work was financially supported by the Kasetsart University Research and Development Institute (KURDI), the Center of Excellence on Petrochemical and Materials Technology (PETROMAT), and the Institutional Research Grant (grant no. IRG5980004). The authors would like to thank the Synchrotron Light Research Institute (BL8: XAS and BL5.3: Nanotec-XPS) for their support regarding XAS and XPS measurements.
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