Simultaneous electropolymerization/Au nanoparticle generation at an electrified liquid/liquid micro-interface
Graphic abstract
Introduction
Electrosynthesis of conductive polymer films has been of considerable interest for decades for a host of catalytic, senor, and biomedical applications [1,2]. Electropolymerization of conductive polymer films has been studied extensively at solid electrode/electrolyte interfaces using aniline [3,4], pyrrole [5,6], or thiophenes [7], [8], [9] as electroactive monomers. Recently, growth of 2D and 3D structures at the interface between two immiscible electrolyte solutions (ITIES) has gained attention, e.g., between water|oil (w|o) [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. The liquid|liquid interface is advantageous as it is molecularly sharp/defect free [22]; moreover, the generated film is not covalently bonded to a solid electrode substrate making its removal much less complicated to create a free-standing film [10,11]. Additionally, surface defects in the solid electrode surface can be transcribed on to the polymer film and play a role in the final morphology, which can be avoided via polymerization at an ITIES.
In the 2000′s, Cunnane's group transposed electropolymerization to the macro-ITIES (mm scale), by dissolving the monomer species in 1,2-dichloroethane (DCE) and an oxidizing agent in the aqueous phases [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. For example, they showed that Fe2(SO4)3/FeSO4(aq) could be employed as electron acceptors for the electrogeneration of 1-methyl- and 1-phenyl-pyrrole oligomers [21]. The interfacial electron transfer reaction can be summarized as follows,Ox(aq) + D(org) → Red(aq) + D+(org)Where D is the monomer/electron donor. Cunnane and Konturri's groups [20] also investigated 2,2′:5′,2″-terthiophene (TT) electropolymerization at a large aqueous|1,2-dichloroethane (w|DCE) interface using the Ce(IV)/Ce(III) redox couple in the aqueous phase; however, Vignali et al. [14,15] reported the first free-standing film that could be extracted from the ITIES. Simultaneously, Johans et al. [23], Trojánek's group [24], and Su et al. [25] were investigating metal nanoparticle (NP) nucleation and assembly/electrocatalysis at the ITIES as well as developing analytical solutions to the complex dynamic problem of NP nucleation at an interface which has no surface defects to act as nucleation sites. At solid surfaces, defect sites lower the overall thermodynamic driving force for NP formation/initiation. In the following decade, Uehara and Dryfe's groups would extensively investigate Au NP nucleation at the liquid|liquid interface [26], [27], [28], [29], [30] using different reducing agents and go on to characterize the Brust-Schiffrin mechanism for Au NP preparation [28], first demonstrated by Brust et al. [31] in the mid 1990s. While recently our group [32] and others [33] have shown electrogeneration of metal NPs at a micro-ITIES using metallocenes as electron donors.
Combining these two efforts Johans et al. [19] and Lepková et al. [12,13] evidenced simultaneous electropolymerization and NP nucleation at a macro-ITIES, which in the latter reports resulted in polymer coated NPs whose size could be controlled by the applied Galvani potential difference across the ITIES ; however, no free-standing, nanocomposite thin-films where evidenced, only polymer coated NPs. Although, by employing tetraoctylammonium tetrachloroaurate (TOAAuCl4), this time dissolved in DCE and acting as the oxidizing agent, coupled with tyramine as the monomer dissolved in the aqueous phase, Cunnane's group were able to electrogenerate a Au nanoparticle (NP) incorporated conductive polymer film at the macro-ITIES [16]. They analyzed their nanocomposite material with x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and transmission electron microscopy (TEM). Investigation of electropolymerization at the ITIES has remained relatively dormant since the work of Cunnane's group in the mid-2000′s.
Herein, we demonstrate the affect of pH on the simultaneous electrogeneration of Au NPs and electropolymerization of TT or 2,2′-bithiophene (BT, see Fig. 1). Our approach employs KAuCl4 dissolved in the aqueous phase as an electron acceptor and TT dissolved in the organic phase as electron donor and monomer. In this way, small, <2 nm in diameter, Au NPs are generated in situ and incorporated into the growing TT thin film. By performing these experiments at a micro-ITIES (25 μm in diameter) positioned at the tip of a pulled borosilicate glass capillary, sensitive cyclic voltammetric curves were able to record the interfacial electron transfer wave at relatively low overpotentials. This is critical to avoid overoxidation of the monomer and growing polymer film [15,34]. Our micro-ITIES platform can also be exploited as a mechanical delivery system for the free-standing film that, after formation, was deposited on a solid substrate for AFM imaging. AFM and SEM images indicate that the polymer/NP growth phase likely proceeds through several stages. Initially, when the interface is pristine, Au NPs are small; however, as the ITIES becomes occluded by the film, TT can no longer access the aqueous side and act as a capping agent but since the film is conductive, it can still mediate electron transfer. Therefore, later in the growth cycle Au NPs likely become large, >10 nm in diameter.
Electrochemical impedance spectroscopy (EIS) combined with TEM images provide semi-quantitative evidence that a Brønsted base is necessary to capture the protons from the intermediate TT radical cation (TT+•) and facilitate electropolymerization. Thus, electropolymerization is enhanced at high pH (∼8.5) and inhibited at low pH. While TT electropolymerization mechanisms have been investigated extensively [7], the influence of biphasic w|o systems on the reaction pathway is shown herein for the first time.
Section snippets
Chemicals and materials
Potassium tetrachloroaurate (KAuCl4 >98%), hydrochloric acid (HCl >37%), sodium hydroxide (NaOH, ≥98%), trioctylphosphine (97%), bromooctane (99%), ferrocene (Fc, >99%), dichloromethane (CH2Cl2, >99%), terthiophene (TT, 99%), bithiophene (BT, 99%), and 1,2-dichloroethane (DCE, ≥99.0%) were acquired from Sigma-Aldrich and used without additional purification. Tetrakis(pentafluorophenyl)borate lithium etherate (Li(Et2O)nB(C6F5)4) (>99%) was sourced from Boulder Scientific Inc. Ultrapure water
Results and discussion
Fig. 2 shows cyclic voltammograms (CVs) obtained at a w|DCE micro-ITIES at 0.020 V s–1 using Cells 1, 2, 3, and 4 at pH 2, ∼5.5, 8.5, and ∼5.5, respectively, with no electron donor added to the DCE phase (i.e., x = 0). At pH 2 and ∼5.5, the polarizable potential window (PPW) is limited at positive and negative ends by the respective transfer of K+/H+ and Cl– [39], [40], [41], [42], [43]. At pH 2 (red curve in Fig. 2) and when scanning from positive to negative potentials, the lone negative
Conclusions
Au NP/TT nanocomposite thin-films have been electrogenerated at a micro-ITIES formed at the tip of a pulled glass capillary. In this approach, AuCl4−(aq) accepts electrons through an interfacial electron transfer reaction from TT dissolved in the DCE phase. A combination of TEM and AFM images evidence a growth mechanism in which early Au NPs are small, < 2 nm in diameter, and TT acts as a capping agent, that then transitions into a period where the ITIES is covered in the nanocomposite material
CRediT authorship contribution statement
Reza Moshrefi: Formal analysis, Investigation, Writing – review & editing, Validation. Evan P. Connors: Investigation. Erika Merschrod: Writing – review & editing, Supervision, Funding acquisition. T. Jane Stockmann: Conceptualization, Methodology, Writing – original draft, Validation, Supervision, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Talia Jane Stockmann reports financial support was provided by Memorial University of Newfoundland.
Acknowledgment
TJS is grateful to the Natural Sciences and Engineering Research Council (NSERC) of Canada for funding through an NSERC Discovery Grant (#006074-2019) and Memorial University for startup funding. EM is grateful to the Canada Foundation for Innovation (CFI) for instrument funding, Memorial University's School of Graduate Studies and Vice President of Research for personnel funding.
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