Stability of microstructure at high temperatures in silver nanoparticles coated with an in situ grown thin graphitic carbon layer
Graphical abstract
Introduction
In recent times, metal nanoparticles are in research attention thanks to their exceptional optical, electronic, chemical and other properties that have unlocked diverse pathway for pertinent technological applications in broad areas of photonics, sensing and imaging, catalysis and biotechnology [[1], [2], [3], [4], [5], [6], [7], [8]]. Functionalization of metal nanoparticles with an assortment of chemical groups allow them to bind with various antibodies/ligands for high-tech biological applications including targeted gene and drug delivery, probing of DNA structure, and protein detection [1,[9], [10], [11]].
Among the various metals, AgNPs have gained special attention because of multiple striking properties such as surface-enhanced Raman scattering, high plasmon excitation efficiency, excellent structural and chemical stability, catalytic activity, antimicrobial properties, etc. Surface functionalized AgNPs find applications in electronics industry (e.g., conductive inks, high performance capacitors, etc.) [12,13], electrochemical and biological sensing [14,15], photocatalysis for degrading organic pollutants [16,17], optical receptors [18], textiles [19], biological labeling [20] and medical applications [[3], [4], [5],7]. The morphological features and chemical/physical environment near the surface of the AgNPs can be explicitly controlled in an appropriate chemical process by controlling the reaction conditions [3,[21], [22], [23], [24]]. A convenient and highly reproducible process for commercial production of AgNPs with fine control over particle morphology, size distribution, and purity is in high demand.
Several methods were developed for deriving AgNPs; each having pros and cons related to various issues including uniformity in particle shape and size, size distribution, suitability for bulk scale production, and cost effectiveness [7,[25], [26], [27], [28], [29], [30]]. For example, in few physical deposition processes the AgNPs are synthesized by reducing Ag+ by laser ablation or radiation of UV light or gamma rays [31,32]. Such processes have the drawbacks of low production rate, high cost, and large energy consumption. AgNPs are also produced by quite convenient photochemical methods based on the reduction of metal cations, either by direct or indirect photolysis [26,33]. Recently, biological processes have also drawn attention, where natural reducing agents such as biological microorganism are used for deriving AgNPs [7,20,34,35]. AgNPs decorated reduced graphene oxide were also fabricated by convenient process for antibacterial and catalytic applications [36,37]. Chemical methods are also widely approached due to their simple and low cost prescriptions that execute reduction of metal precursors using reducing agents followed by encapsulation [24,29,38,39]. The chemical synthesis approach exploits a number of techniques including microemulsion, polyol, electrochemical, reverse micelles, and sol-gel methods [35,[40], [41], [42]]. These routes also support precise control over the morphology of the AgNPs [24,[41], [42], [43], [44]].
Among the various reactants used in the chemical synthesis processes of AgNPs, poly-vinyl alcohol (PVA) is considered to be one of the most convenient, both as a reducing agent of the reagents as well as a capping agent and/or host matrix of the nascent AgNPs. PVA possess several striking properties such as biocompatibility, water solubility, dopant dependent electrical conductivity, excellent film forming capacity, thermostability, chemical resistance, and high mechanical strength [43,[45], [46], [47]]. Chandran et al. [48] synthesized AgNPs using Ocimum sanctum leaf extract at room temperature with PVA encapsulating the nanoparticles. Díaz-Cruz et al. [49] synthesized AgNPs using PVA as a reducing agent. One-step synthesis of AgNPs was followed to form PVA-AgNPs composite nanofibers via electrospinning using a mixture of PVA and hydroxypropyl-beta-cyclodextrin as both reducing and stabilizing agent to control the size and uniform dispersion of AgNPs [50]. Xu et al. [51] synthesized nanocomposites composed of cellulose nanocrystals and AgNPs incorporated into PVA as nanofillers. The activity of PVA stabilized AgNPs were studied by Pencheva et al. [52], where AgNPs in PVA matrix were synthesized by heating AgNO3 and PVA mixture at 100 °C for 1 h in dark. Chitte et al. [53] derived AgNPs-PVA composite films by casting process using trisodium citrate as a reducing agent. Ananth et al. [54] used sodium borohydride as reducing agent and prepared AgNPs stabilized with PVA and functionalized the composite with Bovin Serum Albumin (BSA) for biosensing applications. Bogle et al. [55] synthesized AgNPs by irradiating solutions of AgNO3 and PVA with electrons. Filippo et al. [38] reported on the synthesis of AgNPs by thermal reduction of an aqueous AgNO3 precursor for hydrogen peroxide sensor applications.
In such processes, the controlling parameters to initialize nucleation and the subsequent growth of the nuclei to form colloids of Ag, are the reaction temperature and time, pH, concentrations of the metal salts and reducing agent [26,42,56,57]. Huang and Paul [58] observed that a low pH value (<4) in the PVA/AgNO3 aqueous solution resulted in slow reduction of Ag+ ions. Dong et al. [59] reported that in the synthesis of AgNPs following citrate reduction method, a pH level higher than neutral results in fast nucleation and growth, while it decreased with the decrease in pH below neutral. Zielińska et al. [41] derived AgNPs by reducing Ag+ ions in a solution of pH 7.6 using hydrazine as reducing agent and PVA as stabilizer. Soukupová et al. [60] obtained AgNPs in basic medium (pH 11.5) and reported the effect of surfactant in controlling the size and polydispersity of AgNPs.
As reported in recent literature, graphene oxide or graphitic carbon coated AgNPs (with or without a host of other functional groups and nanoscale materials) could also be a highly efficient candidate for novel applications in catalysis, diagnostics and therapeutics, bioimaging, etc. [36,37,61] Thermal stability of such type of surface functionalized AgNPs at high temperatures shall be an added advantage. Here, we report a novel and facile synthesis route of monodispersed AgNPs stabilized with an in situ grown thin layer of graphitic carbon on the surface by a simple sol-gel type chemical precursor method using PVA as a reducing as well as stabilizing agent in presence of ammonia (to maintain the reaction solution pH between 8 and 9). Recrystallized AgNPs were derived after heating the composite precursor at selected temperatures in the 400–500 °C range. Structural, optical, and thermal properties of the AgNPs were evaluated and studied in correlation with their typical core-shell microstructure. The novelty of this work is the development of highly monodispersed AgNPs coated with an in situ formed thin graphitic carbon layer by a simple process which ensures thermal stability of the AgNPs at least upto 900 °C in ambient air. Such type of stable AgNPs is suitable for high temperature catalysis, e.g., fuel oxidation in solid oxide fuel cells. Furthermore, the surface layer of graphitic carbon can be conveniently utilized as a buffer layer for functionalization of the AgNPs for several pertinent applications.
Section snippets
Chemicals
The list of the chemicals used in this work is as follows, silver nitrate (AgNO3, mol. wt. = 169.87 g/mol, 99.90% pure, Sigma-Aldrich), PVA (mol. wt. ∼96800 g/mol, degree of polymerization ∼2000, Fischer Scientific), ammonia solution (NH4OH, 25%, Merck), and sucrose (C12H22O11, mol. wt. = 342.30 g/mol, 99.95% pure, Merck). All the chemicals were of analytical grade (AR) and used as procured with no further purification.
Synthesis
In the outset, a fresh transparent solution of 4 wt% (100 ml) PVA in
Results and discussion
Under continuous magnetic stirring, the PVA and sucrose molecules dispersed in the solution create tiny micelles in the form of extended laminates or closed rings containing abundance of –OH groups (free from H-bonding), which are readily available to carry out a well-controlled surface-activated reaction [[62], [63], [64]]. These micelles act as adsorption sites for the Ag+ cations in the solution and readily form Ag+-PVA-sucrose chelate complex. The chelate structures gain enough thermal
Conclusion
Stable AgNPs encapsulated with graphitic carbon were produced by a simple and highly reproducible chemical method. The synthesis steps provide a precise control over the morphology and stability of this state-of-the-art material for several pertinent applications. XRD analysis confirmed the fcc crystal structure of the AgNPs. HRTEM analysis revealed the monodispersity of the AgNPs with a narrow size distribution, viz, 7–9 nm in the AgNPs synthesized at 400 °C. The graphitic nature of the
Conflicts of interest
The authors declare that they have no conflict of interest.
Acknowledgments
The authors gratefully acknowledge the financial support from the Department of Science and Technology, Government of India (DST-SERB sanction order # EMR/2017/001271). The authors also sincerely express thanks to (i) Materials Research Centre, Malaviya National Institute of Technology, Jaipur and (ii) UGC-DAE Consortium for Scientific Research, Kalpakkam for the technical support.
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