Synthesis, characterization and daylight active photocatalyst with antiphotocorrosive property for detoxification of azo dyes

https://doi.org/10.1016/j.seppur.2016.03.029Get rights and content

Highlights

  • CdS–Ag–ZnO is more antiphotocorrosive than bare ZnO.

  • The catalyst is found to be stable and reusable.

  • A dual mechanism has been proposed for efficient removal of RR 120 dye under solar light.

Abstract

Utilization of solar energy is an energy efficient process for dye wastewater treatment. Photocatalytic degradation of toxic azo dyes was carried out using modified semiconductors under direct sun light. Concerning this, the different wt% of CdS loaded Ag–ZnO catalysts were prepared by the simple precipitation – thermal decomposition method and used for degradation studies. Cadmium sulfide (CdS) is a kind of semiconductor with fine band gap of 2.4 eV, and its valence electron can be effortlessly evoked to conduction band under solar or visible light illuminations. Among the different CdS prepared catalysts, highly efficient 2 wt% of CdS loaded Ag–ZnO was characterized by different characterization techniques. Metal sulfide loading increases the absorbance of ZnO into the entire visible region. XPS reveals that the presence of metallic silver in the catalyst. The photocatalytic activity of 2 wt% CdS loaded Ag–ZnO was compared with single metal doped, undoped, and other commercial catalysts, especially Degussa P25, a standard bench mark photocatalyst. The photodegradation of RR 120, RO 4, and RY 84 had been analyzed in detail. Mineralization of these dyes has been confirmed by chemical oxygen demand (COD) measurements. A dual mechanism has been proposed for the higher efficiency of CdS–Ag–ZnO at neutral pH under solar light. Antiphotocorrosive study reveals that bare ZnO suffers more dissolution by photocorrosion than our prepared photocatalyst CdS–Ag–ZnO. This catalyst is found to be more stable and reusable.

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).

References (46)

  • B. Krishnakumar et al.

    Influence of operational parameters on photocatalytic degradation of a genotoxic azo dye Acid Violet 7 in aqueous ZnO suspensions

    Spectrochim. Acta A

    (2011)
  • L. Wu et al.

    Characterization and photocatalytic mechanism of nanosized CdS coupled TiO2 nanocrystals under visible light irradiation

    J. Mol. Catal. A: Chem.

    (2006)
  • A.B. Murphy

    Band-gap determination from diffuse reflectance measurements of semiconductor films, and application to photo electrochemical water-splitting

    Sol. Energy Mater. Sol. Cells

    (2007)
  • A. Deshpande et al.

    Interfacial and physico-chemical properties of polymer-supported CdS⋅ZnS nanocomposites and their role in the visible-light mediated photocatalytic splitting of water

    J. Colloid Interface Sci.

    (2009)
  • C. Ren et al.

    Synthesis of Ag/ZnO nanorods array with enhanced photocatalytic performance

    J. Hazard. Mater.

    (2010)
  • T. Vdovenkova et al.

    ZnS wide band gap semiconductor thin film electronic structure sensitivity to Mn impurity

    Thin Solid Films

    (1999)
  • P.W. Du et al.

    Catalysts made of earth-abundant elements (Co, Ni, Fe) for water splitting: recent progress and future challenges

    Energy Environ. Sci.

    (2012)
  • V. Balzani et al.

    Photochemical conversion of solar energy

    ChemSusChem.

    (2008)
  • M.R. Hoffmann et al.

    Environmental applications of semiconductor photocatalysis

    Chem. Rev.

    (1995)
  • A. Mills et al.

    Water purification by semiconductor photocatalysis

    Chem. Soc. Rev.

    (1993)
  • N. Lakshminarasimhan et al.

    Enhanced photocatalytic production of H2 on mesoporous TiO2 prepared by template-free method: role of interparticle charge transfer

    J. Phys. Chem. C

    (2007)
  • Yunqi Li et al.

    Synthesis of mesoporous TiO2/SiO2 hybrid films as an efficient photocatalyst by polymeric micelle assembly

    Chem. Eur. J.

    (2014)
  • N. Suzuki et al.

    Hybridization of photoactive Titania nanoparticles with mesoporous silica nanoparticles and investigation of their photocatalytic activity

    Bull. Chem. Soc. Jpn.

    (2011)
  • Cited by (19)

    • Microwave assisted green synthesis Ce<inf>0.2</inf>Ni<inf>0.8</inf>Fe<inf>2</inf>O<inf>4</inf> nanoflakes using calotropis gigantean plant extract and its photocatalytic activity

      2019, Ceramics International
      Citation Excerpt :

      From the results we infer that the removal of dye was declined 18.37% by the addition of t-butyl alcohol (TBA), which indicate hydroxyl radical act as a main oxidizing agent [51]. TOC measurements were carried out to evaluate the mineralization of pollutant with CNF catalyst [52]. The complete removal of pollutant is reported by calculating the TOC values by various times of illumination under optimized conditions in visible light.

    View all citing articles on Scopus
    View full text