Mechanism of visible-light-driven photocatalytic degradation of endosulfan pesticide by gold nanoparticles

Spherical shaped gold nanoparticles (GNPs) of size around ∼9 nm were successfully used for photocatalytic degradation of hazardous endosulfan (ES) pesticide. GNPs absorb sunlight at ambient condition in aqueous medium to enhance localized surface plasmon resonance (LSPR) which initiate hydrolysis, oxidation and reduction chemical reactions for the mineralization of ES molecules. Infrared transmittance spectra of GNPs treated ES solutions revealed formation of amorphous carbon and hydrocarbon as final reaction products indicating the nearly complete mineralization of ES as evidenced from their TEM images also. The disappearance of chloro carbon and sulphite functional groups peaks of ES in IR transmittance spectra confirmed their degradation. TEM images also support the optical absorption spectra consisting of inter-band and LSPR bands pertaining to individual and clustered GNPs ensembles. The redshift in LSPR absorption peak positions along minor and major axis of clustered GNPs ensembles is attributed to the change in GNPs polarizability on interaction with the electric component of visible light. The chemical reaction pathway for ES molecule degradation by the photocatalytic GNPs has been proposed in the light of inferences drawn from TEM images, IR transmittance and optical absorption spectra.


Introduction
Photocatalysts as clean and green sources play a vital role in meeting the demand of pollution free environment by the decontamination of life saving natural resources like soil, water, air and also in energy related applications. Extensive research is going on the development of new economically efficient and advanced photocatalytic materials suitable for maximum energy absorption from visible region of solar spectrum [1][2][3][4][5][6][7][8][9][10]. Studies were reported on both homogeneous and heterogeneous photocatalytic materials for the oxidation reactions. Hydrogen peroxide (H 2 O 2 ), ozone (O 3 ), Fenton reagent, sodium hypochlorite (NaOCl) are categorized under homogeneous photocatalytic materials [1] and used either individually or in combination with light to undergo oxidation reaction for the degradation of pollutants. Semiconducting II-VI metal oxides/sulphides e.g. TiO 2 [2], ZnO [3], ZnS [4]; CdS [4], WO 3 [5], Bi 2 O 3 [6], Bi 2 S 3 [7], SnO 2 [8], MnO 2 [9] and Au/Ag/Cu metal nanoparticles supported on different metal oxides [10] etc are characterized under the heterogeneous photocatalysts. They utilizes UV and/or visible solar radiation to generate variety of reactive oxygen species for photocatalytic oxidation. These metal oxides/sulphides exhibit best efficiencies under UV-light irradiation because of their wide energy band gap. In addition to these photocatalysts, work on new nanocomposite materials suitable for visible region absorption is also reported [11][12][13][14][15]. Some of those are Bi/BiVO 4 , g-C 3 N 4 /SmVO 4 [11], Sr 0. 25 Bi 0.75 O 1.36 [12], CeO 2 /SnO 2 /rGO [13], Ag 3 PO 4 [14] and Fe 2 (MoO 4 ) 3 [15]. However, these composites have disadvantages such as limited visible-light absorption, swift charge recombination and poor stabilities which restricts their usage in different applications. Recently, the study of optical and catalytic properties of GNPs as photocatalyst has attracted researchers owing to its capability of simultaneously absorbing visible and ultraviolet light from solar spectrum. The LSPR signal of noble metals of size more than 5 nm absorbs light in the visible region [16][17][18][19][20] and also acts as an electron trap for photoinduced electrons [21]. The intensity of plasmonic band is sensitive to the morphology, dielectric constant of dispersing medium and electronic interactions involved in stabilizing ligands and nanoparticles [19]. In nobel metals, photocatalytic reaction, light absorption and activation of reactants take place simultaneously which enhances their light harvesting and thermal energy absorption capacity i.e. photocatalytic quantum efficiency to initiate chemical reactions [20]. In GNPs, LSPR effect couples with sunlight to generate high energy conduction electrons at the surface of the nanoparticle to initiate chemical reaction with molecules adsorbed on their surface [19,20,[22][23][24][25][26][27]. The excited electrons and enhanced electric field of GNPs convert solar energy into chemical energy by the photon-driven photocatalytic reaction. It implies that during photocatalysis, light harvesting from sunlight and chemical reaction with adsorbed species/molecules on noble metal NPs surface is going on sideby-side. The strong visible light absorption, recyclability and large surface area to volume ratio of GNPs have proved them to be a suitable candidate for photocatalytic reaction. Current investigation aims to exploit the photocatalytic activity of GNPs for mineralization of hazardous organochlorine pesticide ES. Long environmental life span and deadly health effects of ES and its metabolites have encouraged the researchers for pursuing easy and economic detoxification pathways from the environment. Different bulk and nanomaterials have been used for their mineralization via hydrolysis, oxidation and reduction reactions e.g. carbon based nanomaterials, semiconducting oxides, magnetic oxides and nobel metals nanoparticles [22-25, 28, 29]. In the present work, different concentrations of ES are treated with GNPs solution. The final reaction products were examined by TEM, UV-Vis and FTIR techniques to confirm the mineralization of ES molecules. The mechanism of ES degradation is discussed in the light of photocatalytic activity of GNPs.

Synthesis of GNPs
The colloidal solution of GNPs were synthesized by the reduction of Tetra-Chloro Auric Acid (TCAA) (CDH Make, AR grade) via Turkevich method [30]. The details of synthesis process have already been described in P. Goel and M. Arora [26]. 150 ml of 0.5 mM TCAA (2% w/v) and 15 ml of 38.8 mM sodium citrate (3% w/v ) solutions were prepared in double distilled water in separate volumetric flasks. The magnetically stirred TCAA solution was heated at 180°C on hot plate then 15 ml sodium citrate solution was swiftly transferred to it. Resulting solution after 5 min turned slightly bluish and by the end of the reduction process i.e. within 20 min, colloidal gold nanoparticle solution of brilliant wine red color was obtained. The schematic of the synthesis procedure is shown in figure 1.
To explore the degradation of organochlorine ES via photocatalytic GNPs, different concentration of ES solutions ranging from 1 ppm to 50 ppm (i.e. 1, 5, 10, 20, 50 ppm) were prepared in 2-Propanol (HPLC and Spectroscopy grade, CDH Make) by dissolving technical grade endosulfan flakes. Successively, 5 ml of each from 1 ppm to 50 ppm ES solutions were reacted with 0.5 ml GNPs suspension. The morphological details of as    resonance in visible region and photocatalytically produced hot electrons and holes initiate hydrolysis, oxidation and reduction chemical reaction for the degradation of ES pesticide into non-hazardous products [19,20,27].

UV-Vis spectroscopy studies
Optical absorption spectrum of as-prepared GNPs of ∼9 nm size derived by citrate route of ruby red wine colour solution (figure 4(a)) exhibit absorption peaks in UV and visible region at 324 nm and 529 nm due to the interband electrons transition from 5d (valence) to 6sp (conduction) levels whereas the strong absorption at 529 nm in as prepared GNPs is due to the LSPR transitions arising from 6sp electrons transition to conduction band [16,18,31]. LSPR peak appears due to the absorption of visible light because in small nanoparticles momentum conservation is not required. This fact is further supported by TEM image of GNPs in which uniform dispersion of spherical shaped nanoparticles without any agglomeration is clearly visible. Absorption Spectra of ES with absorption peaks at ∼242 and 290 nm is shown in figure 4(b). In figure 5, the absorption spectra of 0.  bigger ensembles have a large number of GNPs which are arranged in cylindrical/rod, floral petals and feather shaped structures. The systematic variation in solution colour change is observed i.e. from wine red to pink to violet to purple and finally to light blue as the reaction proceeds between GNPs and different concentration ES solutions. The colour gradient with ES concentrations is clearly apparent with naked eyes [26].

FTIR spectroscopy
The non-destructive infrared spectroscopy has been widely used for the analysis of structure by tentatively assigning observed peaks pertaining to different bonding groups present in materials [32][33][34][35][36][37][38][39][40]. In this work, GNPs, endosulfan (ES) and GNPs treated ES solutions in varied concentrations were characterized by IR transmittance spectra recorded in 4000-500 cm −1 region at ambient temperature. This inferred information has been used for proposing the reaction mechanism for the degradation of ES by photocatalytic GNPs. Technical grade ES used in the present investigations, is a mixture of 70% alpha and 30% beta isomers having chemical structures as presented below in figure 6.
IR transmittance spectra of pure ES and GNPs exhibit characteristic vibrational peaks pertaining to adsorbed moisture and different functional groups present as discussed in our earlier work [26]. IR transmittance spectra of GNPs reacted with 1 ppm, 5 ppm, 10 ppm, 20 ppm and 50 ppm ES concentration solutions recorded in 4000-500 cm −1 region are presented in figure 7. The variations in peak positions and intensity of different functional groups are discussed in detail in the forthcoming sections: C-H stretching region ν C-H stretching vibrations of hydrocarbon and hydrogenated amorphous carbon appear in 3000-2800 cm −1 region. In ES, the asymmetric (ν as ) and symmetric (ν s ) stretching modes of -CH 3 group are obtained at 2971 and  2886 cm −1 respectively and ν as CH 2 stretching vibration at 2933 cm −1 . The -CH 2 stretching modes are stronger than -CH 3 stretching modes. However, in ES/GNPs samples, reverse trend is observed, i.e. -CH 2 stretching mode intensity is weaker than -CH 3 stretching modes. This suggests the conformational sensitivity of -CH 2 groups with some of them in gauche configuration [41]. This indicates the formation of amorphous phase in the ES:GNPs solution as seen in the TEM micrographs also.

CH 3 bending region
In ES, medium intensity δ (CH 2 ) peak is observed in 1500-1450 cm −1 region as broad band with three components at 1482, 1464 and 1452 cm −1 which shows the weak side-by-side interaction between chains to ease charge transfer and chain motion in ES solution [42][43][44]. The component at 1482 cm −1 arises from C-C skeletal vibrations of transconformational chains. While, 1464 cm −1 and 1452 cm −1 components are tentatively assigned as of CH 3 bending asymmetry mode of partially ordered chain structure and combination mode of gauche defects with asymmetric deformation of the terminal CH 3 group of ES molecules [45][46][47] respectively. In GNPs treated solutions, this peak appears at 1468 cm −1 with loss in intensity due to ES degradation and aggregation of GNPs into different size ensembles. These results are supported by TEM images showing small clusters, chain and petal or floral shaped GNPs ensembles and visible change in solution colour initially from wine red to finally light sky blue.

CH 2 wagging region
The characteristic CH 2 wagging modes of gauche conformation [34] is obtained as medium intensity band at 1373 cm −1 in ES and as doublet band with maxima at 1379 and 1364 cm −1 in GNPs:ES solution. It means methyl groups exist in gem-or iso-dimethyl form in ES degraded final hydrocarbon product. The band observed at 1364 cm −1 corresponds to gauche conformation with methyl groups in gem-form. While the 1379 cm −1 peak component arises from -CH 3 iso-form umbrella mode. The medium intensity for this mode confirms both crystalline and amorphous phases in ES-GNP solution.
-SO 3 Vibrational modes A-component and E-component of ν -SO 3 asymmetric (ν asym ) and symmetric (ν sym ) stretching modes of -SO 3 functional group are observed in the ES spectrum at 1269 and 1245 cm −1 respectively. The intensity of 1269 cm −1 band is more than 1245 cm −1 , due to strong in-plane hydrogen bond interaction between sulphite oxygen and propanol -OH functional group hydrogen atom as compared to their lateral interactions between ES molecules. ν -SO 2 asymmetrical stretching mode is obtained as strong peak at 1095 cm −1 along with medium intensity peak at 1072 cm −1 and very weak intensity component at 1043 cm −1 . The symmetrical stretching modes of -SO 2 group appear as strong bands at 1006 and 981 cm −1 [41]. While its bending modes are observed as medium intensity peaks at 538 and 508 cm −1 in ES. These peaks disappear in GNPs treated ES solutions IR transmittance spectra which confirms the degradation of ES sulphite group with GNPs treatment.

Proposed mechanism
The use of noble metal nanoparticles (NPs) as a photocatalyst has many advantages like high optical absorption in the visible region of solar spectrum, no band gap and the availability of large number of high energy electrons at their surface to initiate chemical reaction with adsorbed molecules [19,20,22,[48][49][50][51][52][53][54][55]. In GNPs, most of the conduction electrons (6sp states) are present at the surface of NPs and very small number of them are distributed above Fermi levels. At ambient condition, GNPs absorb both visible and UV components from normal sunlight simultaneously due to LSPR and inter-band transitions between 5d and 6sp states. The conduction electrons gain the energy of light irradiation and distribute large number of conduction electrons to high energy levels.
The availability of such high energy electrons (hot electrons) at the surface of GNPs are suitable for the degradation of ES molecules adsorbed on their surface by the oxidation, reduction and hydrolysis chemical reactions. The schematic of the proposed mechanism is shown in the following figure 8.
In the present investigations, the degradation of commercial grade ES solutions in varied concentrations using GNPs is discussed in the light of phototcatalytic activity of GNPs. Based on the earlier reported ES degradation pathways, we exploited the probable photochemical process in the current studies. ES sulphite functional group hydrolyses to form less toxic endosulfan diol [16] which oxidises to endosulfan ether to endosulfan hydroxyl carboxylic acid/endosulfan lactone and endosulfan hydroxyether. The intermediate reaction products finally mineralize to sulphur dioxide and carbon dioxide. The mineralization of carcinogenic ES chlorinated skeleton ring takes place by reductive dechlorination reaction [56][57][58][59][60]. Very fast reductive reaction removes all six chlorine atoms labelled as 1a/1b, 2a/2b, 3a/3b simultaneously as Cl 2 gas. The proposed mechanism is supported by TEM, UV-Vis and FTIR studies. The mineralization chemical reaction pathway is also given below (figure 9).

Conclusion
The results presented in this study provide definitive evidence for the mineralization of ES pesticide by photocatalytic GNPs. TEM, optical absorption and FTIR analytical techniques were used to confirm the nearly complete degradation of ES pesticide. Based upon the results derived from morphological, optical absorption and IR transmittance spectra of GNPs colloidal solution treated ES (1, 5, 10, 20 and 50 ppm) solutions, ES degradation via intermediate hydrolysis, oxidation and reduction reaction mechanism has been proposed. The agglomeration of GNPs into small and big ensembles bring visible change in colloidal solution coloration. TEM images confirm the agglomeration of GNPs into different patterns whose size varies from few micrometer to millimetre. Optical absorption spectra of GNPs treated ES solutions show peaks pertaining to the inter-band, LSPR of individual and clustered GNPs in UV and visible regions with the variation in peaks intensity. IR transmittance spectra also reveal the nearly complete disintegration of ES pesticide by the formation of amorphous carbon and hydrocarbons as final reaction products. Hence, novel GNPs are found to be a suitable and efficient photocatalyst for providing an easy and swift way to eradicate highly persistent toxic ES pesticide from the environment with a zero waste.