Electrostatically enabled dye reduction using laser synthesized gold nanoparticles

Highlights

• Gold nanoparticles were synthesized via laser ablation and subsequently coated with different surfactants.

• The catalytic activities of these nanoparticles against the cationic dye methylene blue and anionic dye methyl orange were tested.

• Bare nanoparticles have shown many folds higher catalytic activity compared to surfactant-coated nanoparticles.

• For coated nanoparticles, the activity is much faster if the surface coating of nanoparticles and dyes have opposite polarities.

Abstract

The size and shape-dependent catalytic activities of noble metal nanoparticles (NPs) have been well studied. However, the effect of the surface coating on their catalytic performance is relatively less explored. Herein, we demonstrate that the catalytic activity of NPs has a marked dependence on the type of their surfactant coating. Gold NPs were synthesized by pulsed laser ablation and subsequently coated with different surfactants such as cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and polyvinylpyrrolidone (PVP). This enabled us to keep the size, size distribution, and concentration of Au NP the same for the comparative catalysis study. The bare and coated NPs were employed as a catalyst to carry out the reduction of methylene blue (MB) and methyl orange (MO). The results showed remarkable dependence of the reaction rates on the type of surfactant coating of the catalyst NPs. Bare NPs were found to be far superior to the coated NPs for both reactions. In the case of bare NPs both the dyes were completely reduced in less than 5 mins. Whereas, for coated NPs, the zeta potential values determined their effectiveness for a certain reaction. For instance, the reduction of the cationic dye MB was relatively more effective with NPs with a negative zeta potential value, whereas the reduction of anionic dye MO was better for NPs possessing a positive zeta potential. The apparent rate constant of SDS-coated NPs was 40 times higher than that of the CTAB-coated NPs for the reduction of MB. Conversely, the rate constant is 27-fold higher for CTAB-coated NPs compared to SDS-coated NPs for the reduction of MO. These results imply that there is a strong dependence of the catalytic reaction rates on the relative charge of reactants and NPs. Moreover, the better catalytic performance of bare NPs in both cases highlights the significance of the availability of the unhindered surface for catalysis. These results signify the importance of engineering the right surface coating for NPs for their best catalytic performance.

Introduction

Heterogeneous catalysis is at the forefront of the global chemical and petrochemical industries. About 85% of all chemical products are made with at least one catalysis step [1]. It has also been frequently employed for the degradation of toxic organic dyes [2], [3]. Environmental contamination of industrial waste is a growing global concern. Organic dyes are widely used in the food, textile, and pharmaceutical industries. These dyes are hazardous, carcinogenic, and can cause great damage to the environment and ecosystem [4], [5]. It is, therefore, crucial to degrade these dyes to protect the environment [6], [7].

Noble metal NPs have been widely used as a catalyst [8], [9], [10]. Among various noble metals, Au is especially interesting owing to its stability and superior catalytic properties at the nanoscale [11], [12], [13]. The Au catalyst has been employed for various reactions such as oxidation of CO at low temperatures, hydrochlorination of ethyne, and degradation of various organic pollutants such as methylene blue, methyl orange, Congo red, etc. [6], [14], [15], [16], [17].

It has been shown that the catalytic activity of the NPs depends strongly on their size, shape, and surface morphology [18], [19], [20]. Xu et. al. demonstrated that Ag nanocubes having {1 0 0} surfaces showed 14 times higher catalytic activity for the oxidation of styrene compared to the Ag nanoplates having {1 1 1} surfaces [21]. This was associated with the higher surface energy of the {1 0 0} surface. The catalytic activity has also been shown to be strongly dependent on the size of NPs. Fenger et. al. showed that CTAB-coated Au NPs of intermediate size (13 nm) were better suited for the catalytic conversion of 4-nitrophenol to 4-aminophenol, compared to the smaller and larger-sized NPs [22]. Similarly, anisotropic Au NPs have been shown to have much higher catalytic performance as compared to their spherical counterparts. Tao et. al. showed that star-shaped Au nanostars have 3–4 times higher catalytic activity compared to Au nanospheres [23]. Kundus et. al. showed that Au nanospheres were the most catalytically active among the Au nanoprisims, nanorods, and nanospheres, all capped with CTAB [20]. They also suggested that the difference in CTAB coating on different particle shapes does not change their relative catalytic capabilities. However, in a conflicting report, nanorods have been reported to be better catalysts compared to nanospheres [24].

Another important and relatively less explored aspect of NP-based heterogeneous catalysis is the type of surfactant present on NP’s surface. Nanoparticles synthesized in solution are typically coated with a surfactant that controls their growth and prevents agglomeration. The surfactants can be anionic, cationic, or nonionic [22], [25], [26], [27], [28], [29]. The presence of the surfactant on NP’s surface can limit the effective surface area available for the dye adsorption. Moreover, the relative surface charge of the molecules and NPs is expected to affect the affinity of molecules on NPs. The electrostatic interaction can enhance the adsorption of molecules on NPs when the relative charges are opposite, and vice versa. This can markedly affect the rate of reaction [30]. The effect of NP’s surface coverage on the adsorption of molecules and their catalytic reduction has been studied previously. Narband et. al. showed that the absorbance of cationic thiazine dyes on citrate-capped anionic Au NPs was much better compared to the anionic thiazine dyes [31]. Dai et. al. synthesized the Au NPs using diblock and triblock polymer stabilizers and showed that triblock polymer stabilized Au NPs showed better stability and catalytic activity compared to the diblock polymer stabilized Au NPs [32]. Ansar et. al. studied the catalytic reduction of 4-NP using Au NPs stabilized with different molecular weights thiolated polyethylene glycol legends (HS-PEG). They showed that lower molecular weight HS-PEG coating completely inhibited the catalytic activity of Au NPs due to its higher surface coverage on NP’s surface, while the long chain HS-PEG coating provided a relatively higher catalytic activity due to its lower surface coverage resulting into the availability of active sites for catalysis [33]. Despite the previous work, a detailed study of the effect of the presence of surfactants of different polarities on the catalytic reduction of cationic and anionic dyes is still missing. Such a study could help to choose NPs with right surface coating for a specific catalytic reaction. To explore the effect of different surfactants on catalysis, all other parameters such as size, size distribution, and concentration of NPs must be kept the same. This is difficult to achieve as NPs coated with different surfactants are usually synthesized with different recipes and it is almost impossible to attain the same NPs in two different syntheses. Herein, we circumvent this by first synthesizing surfactant-free NPs using a nanosecond pulsed Nd-YAG laser, and subsequently coating them with different surfactants. This way the particle size, size distribution, and concentration can be kept the same. The NPs coated with different surfactants were used to study the catalytic reduction of MB and MO dyes.