Dipyrrin complex assisted in situ synthesis of ultra-small gold nanoparticles decorated on a partially reduced graphene oxide nanocomposite for efficient catalytic reduction of Cr(VI) to Cr(III)

Abstract

Rapid and convenient syntheses of ultra-small gold nanoparticles (AuNPs) utilizing a novel heteroleptic dipyrrin complex with dual functionality have been described. The AuNPs immobilized on GO led to partial reduction of GO to form AuNPs/prGO and reduce Cr(VI) to Cr(III) in the presence of HCOOH with the best performance achieved from AuNPs@prGO500.

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Noble metal nanoparticles (NPs) have attracted a great deal of attention due to their excellent electrical, optical, biological and catalytic activities.¹ Usually, their properties strongly depend on the size, shape and composition of the nanoparticles.² During the synthesis of the gold nanocomposites, designing capping and reducing agents, particularly with the metal complexes, needs utmost care because byproducts generated by the reducing agent may hamper their intrinsic properties.³ Several methodologies developed for synthesis of AuNPs are based on direct functionalization and post functionalization of metal complexes.³ Recent studies have shown that appropriately selected microorganisms, plant extracts, inorganic reagents, organic molecules, could serve as both reducing and capping agents.⁴ It has been also demonstrated that piperazine ring in 4-(2-hydroxy-ethyl)piperazine-1-ethane-sulfonic acid (HEPES, goods buffer) and its analogues are capable of generating nitrogen-centered free radicals and can reduce gold ions (AuCl₄⁻) even at very low concentration (∼6 ppm).⁵ Also, it serves as capping agent and directs subsequent growth of the AuNPs.^5,6

Furthermore, due to occurrence of ample oxygen functionalities such as hydroxyl, epoxide, carboxyl, carbonyl, and its high surface area, graphene oxide (GO) has proved to be outstanding matrix for immobilizing metal complexes and NPs.⁷ Simultaneously, reduction of GO is achieved by chemical or thermal reduction of GO, flash reduction under N₂ atmosphere, laser writing etc.⁷ However, these reductions are associated with major drawbacks like use of toxic chemicals viz. hydrazine, rapid heating, high reaction temperature, high pH, special instruments, or expensive high-energy light sources, etc., which limit their practical applications.⁸ It is well established that noble metal–semiconductor nanocomposites can be used as highly active photocatalysts in the reduction of GO due to the synergistic effect of basic material and surface plasmon resonance (SPR) effect of the noble metals.⁹ Catalytic activity of the AuNPs are greatly enhanced by direct in situ deposition on graphene material due to high thermal/chemical stability and large surface area.¹⁰ Despite several advancements, development of more efficient and facile routes for rational design and fabrication of well-defined super-structures eliminating additional functionalization still remains challenging.¹¹

Further, due to its excessive use in industries chromium (Cr) has emerged as a pervasive environmental contaminant and poses serious health problems.¹² The oxidation states of Cr range from +6 to −2, among these Cr(VI) is supposed to be the third most common pollutant, a proven mutagen and carcinogen. In contrast, Cr(III) is relatively insoluble and essential dietary requirement in trace amounts.¹³ Therefore, reductive transformation of Cr(VI) to Cr(III) is a promising approach to remediate Cr(VI) contamination. Palladium nanoparticles are well known for reduction of chromium, but they are active at elevated temperatures.¹⁴ Recently, Mayer et al. have reported the first metallic complex that serves both as reducing and capping agent.³ However, reduction of HAuCl₄ occurs only under basic conditions. To best of our knowledge, reports dealing with the metallic complexes exhibiting dual functionality i.e. both as a reducing and capping agent in the synthesis of AuNPs at room temperature without involvement of any external agents are still challenging.

Through this article, we report applicability of a piperazine containing heteroleptic dipyrrin complex in rapid and convenient synthesis of small-sized AuNPs at room temperature without using any external agent for the first time. The as-synthesized complex capped AuNPs have also been used to fabricate AuNPs/prGO nanocomposites and in reductive transformation of Cr(VI) to Cr(III) in presence of formic acid at room temperature.

Complex (1) was synthesized by reaction of [Ni(dedtc)₂] (dedtc = diethyldithiocarbamate) with pypzdpm [5-(2-pyridyl-piperazine)-phenyldipyrromethene] obtained in situ by oxidation of pypdpmH [5-(2-pyridyl-piperazine)phenyldipyrromethane] with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in presence of triethylamine in CH₂Cl₂/C₆H₆ at room temperature (Scheme S1†).¹⁵ This complex has been thoroughly characterized by ESI-MS, NMR, UV-vis and single crystal X-ray analysis (Fig. S1–S5 and Table S1 and S2, ESI†). Synthesis of AuNPs has been achieved by treatment of a solution of 1 (c, 1.0 × 10⁻⁵, THF : H₂O, 10 : 90, 5 mL) with HAuCl₄ (c, 1.0 × 10⁻³, H₂O, 5 μL) at room temperature. It was noticed that colour of the solution of 1 turned violet red from brown red (Fig. S6, ESI†) instantly upon addition of HAuCl₄ and only (1.0 equiv.) of it was sufficient for the formation of AuNPs (monitored by UV-vis titration studies). To exclusivity explore applicability of 1 as a reducing and capping agent, analogous studies were performed using 1-(2-pyridyl)piperazine (pypz), Ni(dedtc)₂, and pypzdpmH, also. It was observed that pyp and Ni(dedtc)₂ could not only reduce HAuCl₄ to give AuNPs, while pypzdpmH afforded AuNPs with average particle size of 10–15 nm after 15 min as confirmed by transmission electron microscopy (TEM) (Fig. S7 and S8, ESI†). However, 1 afforded AuNPs instantly with an average particle size of 2–4 nm, without flocculation (stable for over six months) in tetrahydrofuran (THF). It also gave stable AuNPs in other solvents like methanol, ethanol, acetonitrile, dimethylformamide (DMF) and dimethyl sulfoxide (Fig. S9, ESI†). From the above experiment we surmise that presence of Ni(II)dedtc moiety of the complex 1 is prerequisite to accelerate AuNPs synthesis. It is well documented that Good's Buffers (HEPES, MES, PIPES, etc.) having piperazine ring generates nitrogen-centered cationic free radicals in the presence of Au(III) and reduces Au(III) to Au(II)/Au(I), and finally to Au(0), consequently formation of gold nanoparticles.^5,16 Thus, on this basis we assumed that the piperazine ring present in dipyrrin core forms nitrogen-centered cationic free radicals and are able to reduce Au(III) to Au(0). The resulting gold nanoclusters Au(0) were stabilized by capping of the pyridyl nitrogen present in the metal complexes under ambient conditions.

Resulting AuNPs have been utilized in fabrication of graphene oxide (GO) in different ratios (100, AuNPs@GO100; 300 AuNPs@GO300 and 500 mg mL⁻¹, AuNPs@GO500). The GO used for fabrication was prepared using purified natural graphite following Hummer's method¹⁷ with slight modifications. It readily disperses in water and forms a suspension under mild ultrasonic treatment. The AuNPs were immobilized on GO sheets by simply mixing as-prepared AuNPs and a suspension of GO at room temperature for 5 min which led to aggregation of GO suggesting reduction of GO (confirmed by UV-vis).¹⁸ Aggregation of AuNPs@GO is extremely useful as it provides an apparent purification method of nanocomposites by separating excess ungrafted AuNPS on GO sheets (Fig. S10, ESI†). After purification by centrifugation, flocculated nanocomposites were dispersed in DMF and triple distilled water (5%, v/v) (Fig. S10, ESI†). Synthesized nanocomposites were characterized by UV-vis absorption spectroscopy, scanning electron (SEM) and transmission electron microscopy (TEM) analyses.

As shown in Fig. 1, UV-vis spectrum of 1 exhibits intense low energy absorptions at ∼456 nm along with a broad hump at ∼517 nm assignable to π–π* charge transfer transitions associated with conjugated dipyrrinato core and metal to ligand charge transfer (MLCT) transition, respectively.¹⁵ In addition, high energy absorptions observed at ∼280 nm may be attributed to intra-ligand π–π* transitions. After gradual addition of HAuCl₄ (1.0 equiv.) the band at 456 nm exhibited hyperchromism. Simultaneously, the band at 517 nm displayed red shift with an increase in the absorption intensity and appeared at 542 nm (characteristic of SPR band) indicating formation of AuNPs.¹⁹ As expected, GO exhibited characteristic absorptions at 228 and a hump at 300 nm.²⁰ Absorption spectra of the nanocomposite showed bands at 232, 267 and 312 nm suggesting partial reduction of GO to prGO along with characteristic SPR band at 540 nm.²¹ Presence of respective absorptions due to prGO and AuNPs suggested dispersion of the AuNPs on prGO through electrostatic interaction. Partial reduction of GO may be explained by considering the attachment of AuNPs on GO and creation of a quasi-Fermi level in the AuNPs–GO composites.⁹ The AuNPs intensely absorb visible light due to SPR effect leading to a large enhancement of the local electromagnetic fields near the rough Au surface by photoexcited metallic “electrons” and “holes”. Consequently, these photo-excited electrons are injected into the conduction band leading to reduction of GO.²²

Fig. 1 UV-vis titration spectrum of 1 with increasing amounts of HAuCl₄ (a) UV-vis spectra of AuNPs, GO and AuNPs@prGO (b).

Further to confirm partial reduction of GO to prGO, FT-IR and Raman spectroscopic studies were carried out. The comparative FT-IR spectra GO and AuNPs@prGO500 is shown in Fig. S11, ESI.† In GO (red curve), bands at 1070, 2919 and 3440 cm⁻¹ are due to epoxy symmetrical ring deformation, C–O stretching mixed with C–OH bending, and out of the plane wagging of O–H, respectively. In addition, observed broad absorption at 1680 cm⁻¹ is due to C=O vibration.²³ However, in the case of AuNPs@prGO500, a dramatic decrease in the intensity of these peaks were observed (black curve) which further affirmed partial reduction of GO. Raman spectrum (Fig. S12, ESI†) of GO demonstrated well documented D band peak at 1350 cm⁻¹, due to the sp³ defects and an another peak of G band at 1597 cm⁻¹, which can be ascribed to the in plane vibrations of sp² carbon atoms and a doubly degenerated phonon mode (E_2g symmetry) at the Brillouin zone centre.²⁴ Upon partial reduction into prGO, peak position of both D band and G remains the same with change in peak intensity. The intensity ratio of D to the G band (I_D/I_G) for GO was measured to be 0.68 which is lower than prGO i.e. 1.50.²⁵ The increased I_D/I_G ratio in prGO is due to combined effect of reduction (restoration of sp² domains upon)²⁶ and surface plasmon resonance of gold nanoparticles. Moreover, intensity of the 2D peak at 2680 cm⁻¹ and S3 peak at 2910 cm⁻¹ increased after reduction showing better graphitization.²⁶

The morphology and surface structure of AuNPs and AuNPs@prGO were characterized by SEM and TEM studies. SEM, image shows a uniform decoration of spherical AuNP over and in between nano sheets of prGO (Fig. S13–16, ESI†). The TEM images of AuNPs showed a uniform, mono dispersed nanoparticles with an average particle size of 2–4 nm (Fig. 2a, and S17, ESI†). The energy dispersive X-ray spectroscopy (EDX) spectrum of nanoparticles exhibited strong peaks for elemental gold along with weak signal for Ni indicating that these nanoparticles are composed of gold via capping with 1 (Fig. S18, ESI†). Further, TEM analysis of the AuNP@prGO composite was carried out with great care keeping in mind the damaging effect of high voltage of electrons towards graphene sheets.²⁷ TEM images of AuNPs@prGO100 exhibited clumsy aggregated nanoparticles on prGO sheets (Fig. 2b). Additionally, TEM images of AuNPs@prGO300 exhibited spherical particles of uniform size with lower aggregation relative to AuNPs@prGO100 (Fig. 2c). Furthermore, for AuNPs@prGO500, the average size of AuNPs is slightly greater than AuNPs (∼2–6 nm) as shown in Fig. 2d. Thus optimization of the nanocomposite synthesis was confirmed by TEM. It may be accredited to the fact that on increasing ratio of GO, more surface area will be available for AuNPs to disperse and anchor on it.

Fig. 2 TEM images of AuNPs (a) and AuNPs@prGO100 (b) AuNPs@prGO300 (c) AuNPs@GO500 (d).

After careful inspection of as-synthesized AuNPs and AuNPs@prGO these were utilized for catalytic reduction of Cr(VI) using HCOOH as a reducing agent at room temperature. Potassium dichromate (K₂Cr₂O₇) was used as representative source for Cr(VI). K₂Cr₂O₇, exhibits a strong absorption maximum (λ_max) at ∼348 nm due to ligand-to-metal charge transfer (LMCT) transition. The progress of catalytic reaction was monitored by UV-vis spectroscopy as a function of time. A control reaction for Cr(VI) reduction was performed in dark for 5 h and representative data is shown in Fig. S19 in ESI.† We did not observe any change except very small reduction in the peak intensity. Reduction of Cr(VI) did not occur only in presence of HCOOH even after 50 min, indicating that reductive conversion do not occur in absence of a catalyst (Fig. S20, ESI†). Similar behavior has been observed by using only the composite catalyst without HCOOH, which attested that presence of both HCOOH as well as catalyst in the reaction medium is a must (Fig. S21, ESI†). In presence AuNPs or AuNPs@prGO the absorption intensity of Cr₂O₇²⁻ at 348 nm decreased sequentially, accompanied by a change in color from yellow to colorless with an increase in reaction time. Addition of AuNPs to the reaction mixture resulted in 60% reduction of Cr(VI) in ∼100 min, suggesting lower efficacy of AuNPs. Similarly, GO also exhibited very low efficiency, almost 50% reduction in ∼100 min (Fig. 3). In contrast, addition of highly dispersed multilayered AuNPs immobilized on prGO in varying ratio exhibited superior catalytic activities with almost 100% conversion (AuNPs@prGO100, 25; AuNPs@prGO300, 20; AuNPs@prGO500, 15 min, Fig. 3).

Fig. 3 UV-vis spectra showing reductive conversion of Cr(VI) to Cr(III) AuNPs (a) AuNPs@prGO500 (b) the plot of conversion vs. time as monitored by UV-vis spectroscopy (c) schematic presentation for Cr(VI) reduction on AuNPs@prGO500 (d). Reaction conditions: HCOOH = 0.2 mL, [Cr₂O₇²⁻] = 0.2 mM and catalytic amount = 1 mg mL⁻¹ (25 μL).

Low catalytic activity of AuNPs@prGO100 and AuNPs@prGO300 may be credited to the fact that low amount of prGO induce aggregation of AuNPs and hamper the electron transfer. Whereas, highest catalytic activity of AuNPs@prGO500 may be attributed to well dispersed AuNPs decorated on prGO with high surface area and large size of gold nanoparticles inducing stronger SPR effect and light absorption, boosting the photocatalytic performance. To validate the existence of Cr(III) in solution an excess of NaOH was added to it. The colourless solution turned green suggesting presence of Cr(III) due to formation of hexahydroxychromate(III).²⁸ The reduction mechanism can be explained by assuming adsorption of formic acid on the surface of AuNPs and decomposition of HCOOH to H₂ and CO₂ by AuNPs@prGO. Nascent hydrogen generated during this reaction accelerates the rate of reaction. Recyclability of the catalyst AuNPs@prGO500 has also been tested for successive reactions. It can be collected by centrifugation, filtration followed by repeated washing with distilled water and regenerated after heat treatment at 150 °C. The revived material exhibited activity similar to that of the starting material under analogous conditions. It has been observed that revived material can reduce ≥95% of Cr(VI) within 15 min of the reaction even after 5 cycle (Fig. S22, ESI†). To understand the structural changes of the catalyst during their catalytic activity, we performed TEM studies of the catalyst after 1^st cycle. As shown in the TEM image (Fig. S23, ESI†), there was no noticeable change in their morphology.

In summary, through this work we have developed a heteroleptic dipyrrin complex capable of exhibiting dual functionality (reducing and capping agent) and its potential application in the synthesis of small sized AuNPs. The combination of beneficial features of small gold nanoparticles and prGO sheets makes it excellent candidate for catalytic reduction of Cr(VI) to Cr(III) in presence of formic acid at room temperature with best activity from AuNPs@prGO500. We anticipate that this kind of catalyst with unique features would find wide applications in diverse areas in future.

Acknowledgements

We acknowledge financial support from the Department of Science and Technology (DST), New Delhi through the Scheme SR/S1/IC-25/2011. RKG gratefully acknowledges the Council of Scientific and Industrial Research (CSIR) New Delhi, India for the award of a Junior Research Fellowship (No. 09/013(0210)/2009-EMR-I). We are also grateful to Prof. Q. Xu, AIST, Osaka, Japan for providing single crystal X-ray data on complex 1.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details UV-vis spectra, SEM and TEM images of nanoparticles. CCDC 1057645 For ESI and crystallographic data in CIF or other electronic format. See DOI: 10.1039/c6ra03835b

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