Photocatalytic reduction of Cr(VI) by using stacked ZnS layers of ZnS(en)x complex

https://doi.org/10.1016/j.jece.2015.02.028Get rights and content

Highlights

  • ZnS(en)x hybrid semiconductor with low number of layer structural was obtained.

  • ZnS(en)x complex of flakes morphology was obtained with CS2 source.

  • Cr(VI) photoreduction was achieved with flakes hybrid photocatalyst.

  • The protonation of ZnS(en)x surface enhanced the adsorption of Cr(VI) species.

Abstract

We investigated the photoreduction of hexavalent chromium in the absence or presence of citric acid using ZnS(en)x complex photocatalyst of different intercalated layers of ZnS and of different morphology; wires, sheets and flakes. The hybrid semiconductors were prepared by precipitation method with a mixture of ethylenediamine-H2O solution varying the zinc precursor and the sulfur source. The flake-like of ZnS(en)x hybrid with low layer-by-layers structure prepared from zinc acetate and carbon sulfide provides of enhanced photocatalytic properties that allows an efficient electron transfer process in the photoreduction of Cr(VI) in the presence of citric acid in acid media. The highest photoactivity of the flake hybrid was considered in function of the number of layers of ZnS stacked and of its high degree of protonation. The possible mechanism of photoreduction of chromic acid (HCrO4) species absorbed on protonated surface was discussed.

Graphical abstract

Photoreduction of Cr(VI) over wire, sheet or flakes of ZnS(en)0.5 hybrid semiconductor in the presence of hole scavenger (citric acid).

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Introduction

Chromium can be present in aqueous solution as different species, for example, chromic acid (HCrO4), dichromate anion (Cr2O72−), hydrogen chromate anion (H2CrO4) and/or chromate anion (CrO42−), depending on the redox potential of the couple Cr(VI)/Cr(III) and the pH solution [1], [2], [3]. Heterogeneous photocatalytic reduction of Cr(VI) to Cr(III) ions in the presence of organic molecules and using semiconductors like CaSb2O5(OH)2 [4], N-doped TiO2 [5], WO3-doped TiO2 nanotubes [6], SO4-TiO2 modified [7] and C, N doped ZnS [8] have demonstrated to produce an efficient photocatalytic Cr(VI) reduction when are irradiated under UV light. However, the reduction rate of aqueous Cr(VI) practically stops at pH close to neutral media (pH 7). Increasing the acidity by adding H2SO4 until reaching the pH 1, can increase the reduction rates [7]. Furthermore, the addition of formic acid, citric acid or ammonia oxalate used as hole scavenger, increases the reduction efficiency because they accept holes from the bottom valence band [9], [10]. These scavenger molecules are subsequently oxidized, thus suppressing the electron–hole recombination process and preventing the re-oxidation of chromium species by the holes or hydroxyl radicals [11]. One alternative to improve the electron–hole separation process is by using an adequate semiconductor with a larger band gap energy that endows the photogenerated holes and electrons over photocatalysts with strong redox ability, but larger surface area, adequate morphology with enhanced adsorption capacity should be favorable for the adsorption of Cr(VI) species on the photocatalyst surface to enhance the photocatalytic activity. Taking into account these factors, our research group reported that ZnS(en)0.5 hybrid material exhibit photocatalytic property for the decontamination of waste water containing heavy metals [12], [13]. Its photocatalytic activity was attributed to the strong quantum confinement effect, which provided the thermodynamic conditions to perform photocatalytic redox reactions and to be stable to the photocorrosion process. Several authors have also reported the photodegradation of organic molecules, dyes and phenolic, using this hybrid semiconductor of high crystallinity, of unique sheet morphology with high degree of intercalation of ZnS layers (superstructures), but with the low specific surface area [14], [15], [16]. Nevertheless, the effect of the morphology, including surface area property on the photocatalytic activity, with a controlled degree of ZnS layers intercalation has not been studied. In this work, ZnS(en)x hybrid semiconductors with low intercalation of layers of ZnS in diverse shapes (wire, sheets and flakes) were synthesized at relatively low temperature and short aging time by the precipitation method using nitrate or acetate zinc precursor and thiourea or CS2 as sulfur source. The formation of hybrid semiconductor was confirmed by different techniques such as XRD, FTIR, DRS-UV–vis spectroscopies and TG analysis, whereas the structural morphology was determined by scanning electron microscopy. The degree of intercalation of ZnS layers was proposed from the reflection peak at a low angle by using the X-ray diffraction analysis. The influence of the photocatalytic property of ZnS(en)x complex was investigated in the Cr(VI) photoreduction process in the presence or absence of citric acid at relatively soft acid media (pH 4.8). The enhanced photocatalytic reduction of the adsorbed Cr(VI) species on hybrid semiconductor was considered in function of the morphological property, of the ZnS layers stacked, and of the protonation degree over semiconductor surface.

Section snippets

Synthesis of ZnS(en)x hybrid semiconductor

The ZnS(en)x hybrid semiconductors were prepared by the conventional precipitation method in aqueous solution, using ethylenediamine (en) at moderate synthesis conditions (relatively low temperature and short aging time), as shown in [12], [13]. In a typical procedure appropriate amounts of Zn(NO3)2H2O (Reasol, Mexico) were first dissolved in an ethylenediamine–water solution (at 60 or 70% vol. of en) and either thiourea (Reasol, Mexico) or CS2 (Sigma–Aldrich, México) was immediately added

Morphological property

SEM images of the ZnS(en)x hybrid semiconductor synthesized with thiourea at short aging time (2 h) shows a wire-like morphology with average diameter size of 29 nm and a length of 373–556 nm (Fig. 1A), but when the hybrid semiconductor is prepared with a long aging time (6 h), exhibited typical sheet-like morphology [17], [18], [19], [20], with the dimensions enlarger to D = 652 nm and L = 1.28 μm (Fig. 1B). In contrast, a large amount of flakes with an average diameter between 60 and 200 nm are obtained

Conclusion

ZnS(en)x hybrid semiconductors with a low number of stacked layers of ZnS were synthesized in a mixture of ethylenediamine–H2O solution at moderate conditions (short aging time). The morphology, textural and surface properties of the hybrid ZnS(en)x semiconductors were affected by the confinement of ethylenediamine in different number of stacked layers of ZnS, depending of the zinc precursor and sulfur source. Hybrid ZnS(en)x semiconductor in flake forms with a low number of stacked ZnS layers

Acknowledgments

This research was made with the support of CONACYT-SEP CB-2010-01 157156 and FOINS/75/2012c project Artificial Photosynthesis. Agileo Hernández-Gordillo thanks to CONACYT for financial support through the Cátedras-Conacyt/1169 project.

References (25)

Cited by (18)

  • Enhancing the photocatalytic activity of Cd–ZnS(EN)<inf>0.5</inf> hybrid sheets for the H<inf>2</inf> production

    2020, International Journal of Hydrogen Energy
    Citation Excerpt :

    When the hybrid is prepared by solvothermal or chemical precipitation using EN-water volume ratios between 40 and 70% vol., the protonated ethylenediamine (ENH+) is linked to Zn2+ ions at the surface creating a great number of sulfur vacancies (Vs+), which generates localized states in the conduction band. Such states act as electron-traps leading to a high capacity of carrier separation, which inhibits electron-hole (e− − h+) recombination and therefore, improves the photocatalytic activity in processes like dye photodegradation, heavy metal photoreduction, and H2 production from water splitting [31,36–38]. However, due to the multilayer stacked structure of the ZnS(EN)0.5 hybrids, low specific surface areas (5–30 m2/g) have been obtained [31,33,39], and the major disadvantage is the structure instability when the material is submitted to the irradiation process in aqueous solution.

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These papers were selected from the LACP 2014 conference.

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