Shape-directing Agents Research Articles

Environmental analysis of nitrobenzene using newly synthesized anisotropic gold nanostructures: Reaction kinetics and electrocatalytic activity

In this article, we established to produce anisotropic, colloidally stable gold nanostructures (AuNS) by rapidly mixing aqueous solutions of gold(III) chloride trihydrate (HAuCl4) and citric acid (CA) as dual reducing and capping agents at room temperature without the aid of a templating agent. Furthermore, the impact of various anions and shape-directing agents on the morphology and reaction kinetics of citric acid-capped gold nanostructures (CA-AuNS) was investigated. Additionally, the electrocatalytic activity of AuNS towards nitrobenzene (NB) was examined using a CA-AuNS modified glassy carbon (GC) electrode. The rate of reaction for the formation of CA-AuNS increased rapidly with higher concentrations of CA, indicating a strong dependence on the reductant’s concentration. This process followed first-order kinetics, suggesting that the reaction rate is directly proportional to the citric acid concentration. Consequently, optimizing CA levels can effectively control the synthesis rate and yield of CA-AuNS. The presence of nitrite ions led to rapid aggregation of gold nanostructures (AuNS) and a quick reaction time. Chloride ions increased the size of the AuNS. As bromide ion concentration increased, the reaction time slowed significantly, resulting in spherical gold nanoparticles. Both acetate ions and polyvinylpyrrolidone (PVP) inhibited the growth of AuNS. High-resolution transmission electron microscopy (HR-TEM) images showed the formation of various shapes, including spherical, hexagonal, pentagonal, and boat-like structures. In the presence of cetyltrimethylammonium bromide (CTAB), both complex formation between CTAB and HAuCl4 and the formation of spherical gold nanoparticles occurred. Upon addition of silver nitrate (AgNO3) to the colloidal solution, the color shifted to pale violet, and the morphology transitioned from anisotropic to irregular. However, when ascorbic acid (AA) was utilized, it proved ideal for maintaining the morphology of gold nanostructures (AuNS). The analysis of X-ray diffraction spectroscopy (XRD), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Fourier transform infrared spectroscopy (FT-IR) further confirmed the synthesized CA-AuNS. The FT-IR spectra of CA before and after the formation of AuNS indicated the participation of carboxylic acid (–COOH) and hydroxyl (–OH) groups in the reduction process of Au3+ metal ions. The GC/CA-AuNS electrode exhibits a larger electroactive surface area due to the coating of AuNS onto the electrode surface, enhancing the electrode’s electrical conductivity by providing additional active sites for electron transfer. In addition, the electrocatalytic activity of NB was investigated in this study. The GC/CA-AuNS electrode had better electrocatalytic activity than the unmodified GC electrode. As a result, with a LOD of 28.2 nM (S/N = 3), this electrode was chosen for sensitive NB determination.

Nanoparticle Superlattices with Nonequilibrium Crystal Shapes.

Nanoparticle assembly is a material synthesis strategy that enables precise control of nanoscale structural features. Concepts from traditional crystal growth research have been tremendously useful in predicting and programming the unit cell symmetries of these assemblies, as their thermodynamically favored structures are often identical to atomic crystal analogues. However, these analogies have not yielded similar levels of influence in programming crystallite shapes, which are a consequence of both the thermodynamics and kinetics of crystal growth. Here, we demonstrate kinetic control of the colloidal crystal shape using nanoparticle building blocks that rapidly assemble over a broad range of concentrations, thereby producing well-defined crystal habits with symmetrically oriented dendritic protrusions and providing insight into the crystals' morphological evolution. Counterintuitively, these nonequilibrium crystal shapes actually become more common for colloidal crystals synthesized closer to equilibrium growth conditions. This deviation from typical crystal growth processes observed in atomic or molecular crystals is shown to be a function of the drastically different time scales of atomic and colloidal mass transport. Moreover, the particles are spherical with isotropic ligand grafts, and these kinetic crystal habits are achieved without the need for specifically shaped particle building blocks or external templating or shape-directing agents. Thus, this work provides generalizable design principles to expand the morphological diversity of nanoparticle superlattice crystal habits beyond the anhedral or equilibrium polyhedral shapes synthesized to date. Finally, we use this insight to synthesize crystallite shapes that have never before been observed, demonstrating the ability to both predict and program kinetically controlled superlattice morphologies.

Silver(I)-induced anisotropy in the plasmon-driven growth of nanoscale gold

ABSTRACT The plasmon-mediated synthesis of anisotropic gold nanoparticles (AuNPs) has only recently been investigated and achieved. This process typically utilizes the surface plasmon resonances (SPRs) of the nanoparticles themselves to drive metal deposition and growth. Many of the traditional methods for shape control, including use of functional surfactants and etchants, have yet to be utilized within the context of light-driven synthesis. Here, the underpotential deposition of Ag(I) is utilized as a shape-controlling strategy in a hot electron-driven photochemical synthesis to grow anisotropic gold nanoparticles. The strategy also relies on L-pyroglutamic acid, a natural amino acid metabolite, as a photochemical relay aiding in the transport and localization of plasmon-generated hot electrons. By carefully adjusting reagent concentrations, the morphologies of the resulting nanoparticles can be facilely tuned from spheres to nanostars and nanoflowers. The high extinction in the NIR region of the ‘ideal’ gold nanostars synthesized advocates their use as photothermal therapy agents, supported by a sufficient photothermal conversion efficiency (η = 30.8%) under 808 nm CW laser irradiation. This contribution expands the repertoire of gold nanostructures accessible to visible-light-driven plasmonic photochemistry. Different gold nanoparticle shapes have been accessed using a plasmon-mediated growth technique. Ag(I) ions induce symmetry breaking, yielding nano-sized gold spheres, stars, and flowers. A specialized photoreactor, equipped with 505 nm LED illumination, facilitates the transfer of hot electrons from a pre-made gold nanocube seed surface. Gold reduction and deposition occur, and anisotropic structures grow with the aid of L-pyroglutamic acid and Triton X-100 as coordinating and shape-directing agents.

Chitosan-assisted hydrogen adsorption and reversibility of Ni-doped hierarchical carbon scaffolds.

We investigated the effects of chitosan (CS) on the hydrogen adsorption and reversibility of hierarchical carbon scaffold (HCS) loaded with Ni nanoparticles. As size-controllable, stabilizing, and shape-directing agents for the green synthesis of metal nanoparticles of CS, Ni nanoparticles with uniform distribution and shape are deposited onto HCS. The latter results in the superior specific surface area of Ni nanoparticles for hydrogen chemisorption. The best hydrogen adsorption capacities at room temperature under 20-70 bar H2 of 0.5-1.70 wt% H2 were obtained from 10 wt% Ni-doped HCS-CS. Although macropores of the HCS collapsed upon cycling due to hydrogen pressure applied during adsorption, average hydrogen capacities of 1.17 ± 0.05 wt% H2 (T = 25 °C and p(H2) = 50 bar) were maintained for 14 cycles. This is because not only uniform distribution and shape of Ni nanoparticles and microporous structures of the HCS were preserved upon cycling but also the interactions between Ni and heteroatoms (N and O) of the HCS and CS prevented particle agglomeration.

Investigating Reaction Intermediates during the Seedless Growth of Gold Nanostars Using Electron Tomography.

Good's buffers can act both as nucleating and shape-directing agents during the synthesis of anisotropic gold nanostars (AuNS). Although different Good's buffers can produce AuNS shapes with branches that are oriented along specific crystallographic directions, the mechanism is not fully understood. This paper reports how an analysis of the intermediate structures during AuNS synthesis from HEPES, EPPS, and MOPS Good's buffers can provide insight into the formation of seedless AuNS. Electron tomography of AuNS structures quenched at early times (minutes) was used to characterize the morphology of the incipient seeds, and later times were used to construct the growth maps. Through this approach, we identified how the crystallinity and shape of the first structures synthesized with different Good's buffers determine the final AuNS morphologies.

Photochemical synthesis of Au nanostars on PMMA films by ethanol action as flexible SERS substrates for in-situ detection of antibiotics on curved surfaces

• Photochemical fabrication of Au NSs by the action of ethanol has been demonstrated. • Au NSs are employed directly on the flexible PMMA substrate for SERS application. • High enhancement factors about 10 9 are achieved for the antibiotics. • Effective multiplex SERS detection towards CPX and CAP is presented. • In-situ detection of antibiotics in chicken-wing samples was performed. Enormous Raman signal enhancement using gold nanostars (Au NSs) has attracted great attention due to the excitation of strong electric field on the nanostar tips. The synthetic methods of Au NSs mainly utilize specialized shape-directing agents, such as surfactants or polymers, to assist the tip formation. However, these agents passivate nanostar surfaces and weaken the interaction between Au NSs and analyte molecules. Herein, we report the synthesis of Au NSs with clean surface by the action of ethanol through a facile photoreduction process. The growth mechanism of Au NSs is elucidated and discussed with the action of ethanol as a polar protic solvent. The synthesis process of Au NSs is employed directly on poly(methyl methacrylate) (PMMA) films for the application of surface-enhanced Raman spectroscopy (SERS). The proposed flexible Au-NSs/PMMA SERS substrate is characterized by the SERS detection of two antibiotics, ciprofloxacin (CPX) and chloramphenicol (CAP). The superior detection performance includes high sensitivity, high enhancement factor (2.03 × 10 9 ), low limit of detection (3.41 × 10 -11 M), superior multiplex detection ability, good mechanical stability, excellent uniformity and reproducibility (<7.32 %). These superior sensing properties can be attributed to a large number of hotspots formed by diverse nanostructures at different positions of Au NSs on the Au-NSs/PMMA SERS substrate. The multiplex SERS detection ability is also explored for the simultaneous detection of multiple analytes. The transparent and flexible nature of the Au-NSs/PMMA SERS substrate enables the in-situ, real-time detection of CPX and CAP in chicken-wing samples by using the fiber-coupled Raman probe with the low LODs. The reported flexible SERS substrate opens the way to improve the practical detection of various antibiotics for the worldwide food safety issues.

Seed-assembly-mediated fabrication and application of highly branched gold nanoshells having hollow and porous morphologies

Highly branched gold nanoshells (BAuNSs) having hollow and porous morphologies have been fabricated via a seed-assembly-mediated strategy. Gold seed assemblies can be prepared by removal of SiO2 nanotemplates with help of polyvinylpyrrolidone (PVP) molecules, which weakly link gold nanoparticles together even after SiO2 etching. L-3,4-dihydroxy phenylalanine (L-DOPA) and AgNO3 are employed as shape-directing agents to induce the anisotropic growth of gold. BAuNSs exhibit 7.4 and 4.4 times stronger activities than SiO2@Au nanoparticles in catalysis and surface-enhanced Raman scattering (SERS) applications, respectively, due to their large surface areas and numerous hot spots. It is necessary to find the optimal amount of gold deposition in fabrication to effectively utilize the hollow and porous morpologies of BAuNSs for catalysis and SERS applications. Overgrown nanobranches can fill the nanopores and nanogaps of BAuNSs, resulting in decrease of activities in applications. Overall, the seed-assembly-mediated fabrciation can be employed to produce plasmonic nanostructures having unique morphologies and high application activities.

Influence of Shape-Directing Agents on the Formation of Anisotropic Gold Nanoparticles

Understanding the role of shape-directing substances is crucial for the controllable preparation of anisotropic nanostructures through solution growths. Herein, we report a comparative investigation about the morphologies and structures of Au nanoparticles (Au NPs) synthesized using two different capping agents (hexadecyltrimethylammonium chloride (CTAC) and polyvinylpyrrolidone (PVP)) with/without the addition of iodide salts. The study manifests a synergistic effect of shape-directing agents on the growth pathways and shape formation. Depending on the specific use of surfactants/polymers, the iodide additives can promote the growth of prismatic anisotropic nanoparticles or lead to the shape evolution from irregular to quasi-spherical configurations.

Single-parameter-tuned synthesis for shape-controlled gold nanocrystals stimulated by iron carbonyl

Shape-controlled synthesis is essential for functional nanomaterials, allowing deeper insights intothe relationship between the structures and the catalytic properties. Synthesis of nanocrystals with particular morphologies are usually studied independently among various synthetic methods, those underline that different surface capping ligands or shape-directing agents bring about disparate shapes. However, a single quantitative parameter method is still lacking to realize precise control of well-defined morphology nanocrystals, especially anisotropic structures, which is essential to understanding the growth process of nanocrystals. Herein, we proposed a single-parameter-tuned synthesis strategy for preparation of shape-controlled gold nanocrystals by regulating the amount of iron carbonyl, by which we produced highly monodisperse Au nanocrystals with various shapes in organic phase including nanoplates (diameter of 16.02 ± 1.13 nm and thickness of 5.35 ± 0.58 nm), nanorods (length of 37.53 ± 3.73 nm and width of 5.26 ± 0.37 nm) and nanospheres (diameter of 8.26 ± 0.38 nm). The single-parameter-tuned method reveals the dual roles of iron carbonyl for controlling the shapes of gold nanocrystals including reductant and oxidative etchant and empowers versatility in synthetic methodology for other noble metals. Moreover, catalytic activity shifting in shapes of nanocrystals was revealed based on the reduction of 4-nitrophenol, showing that the as-synthesized Au nanoplates displayed the enhanced catalytic performance with the lowest activation energy. Our work provides a brand-new pathway for shape-controlled synthesis of noble-metal nanocrystals and has a strong practical value in application fields.

Dual-Functional Template-Induced In Situ Polymerization Process Enables the Hierarchical Carbonaceous Nanotubes with Simultaneous Sn Cluster Incorporation and Nitrogen-Doping for Superior Potassium-Ion Storage.

Potassium ion-based energy storage devices have received extensive attention for grid-level applications due to their abundant natural resources and low cost. However, the large ionic radius of K+ leads to inferior capacities and cyclic stability, which hinders their practical application. Herein, hierarchical carbonaceous nanotubes with simultaneous ultrasmall Sn cluster incorporation and nitrogen doping (denoted as u-Sn@NCNTs) are fabricated using MnO2 nanowires as a dual-functional template (in situ polymerization and shape-directing agents) and subsequent carbonization treatment. The u-Sn@NCNTs exhibit a superior K+ storage capability with a high reversible capacity (220.1 mA h g-1 at 0.1 A g-1) and long cycling stability (149.9 mA h g-1 at 1 A g-1 after 4000 cycles). Besides, the u-Sn@NCNTs exhibit superior cycling stability up to 10000 cycles at 5 A g-1 for Na+ storage. The potassium storage mechanism and kinetics are investigated based on ex situ X-ray photoelectron spectroscopy, in situ Raman spectrum, and galvanostatic intermittent titration technique. More importantly, u-Sn@NCNTs can be used as the anode for potassium ion hybrid capacitors, achieving a superior energy density of 181.4 W h kg-1 at a power density of 185 W kg-1 with excellent cycling capability. This work could push forward the development and application of carbonaceous-based anode materials for next-generation high-performance rechargeable batteries.

Computational assessment of the efficacy of halides as shape-directing agents in nanoparticle growth

We report a comprehensive study of aqueous halide adsorption on nanoparticles of gold and palladium that addresses several limitations hampering the use of atomistic modeling as a tool for understanding and improving wet-chemical synthesis and related applications. A combination of thermodynamic modeling with density functional theory (DFT) calculations and experimental data is used to predict equilibrium shapes of halide-covered nanoparticles as a function of the chemical environment. To ensure realistic and experimentally relevant results, we account for solvent effects and include a large set of vicinal surfaces, several adsorbate coverages as well as decahedral particles. While the observed stabilization is not significant enough to result in thermodynamic stability of anisotropic shapes such as nanocubes, non-uniformity in the halide coverage indicates the possibility of obtaining such shapes as kinetic products. With regard to technical challenges, we show that inclusion of surface-solvent interactions lead to qualitative changes in the predicted shape. Furthermore, accounting for non-local interactions on the functional level yields a more accurate description of surface systems.

On the Effect of Modified Carbohydrates on the Size and Shape of Gold and Silver Nanostructures.

Gold (Au) and silver (Ag) nanostructures have widespread utilization from biomedicine to materials science. Therefore, their synthesis with control of their morphology and surface chemistry have been among the hot topics over the last decades. Here, we introduce a new approach relying on sugar derivatives that work as reducing, stabilizing, and capping agents in the synthesis of Au and Ag nanostructures. These sugar derivatives are utilized alone and as mixture, resulting in spherical, spheroid, trigonal, polygonic, and star-like morphologies. The synthesis approach was further tested in the presence of acetate and dimethylamine as size- and shape-directing agents. With the use of transmission electron microscopy (TEM), selected area electron diffraction (SAED), x-ray diffraction (XRD), scanning electron microscopy (SEM), and ultraviolet-visible (UV-vis) absorption spectroscopy techniques, the particle size, shape, assembly, aggregation, and film formation characteristics were evaluated. NPs’ attributes were shown to be tunable by manipulating the sugar ligand selection and sugar ligand/metal-ion ratio. For instance, with an imine side group and changing the sugar moiety from cellobiose to lactose, the morphology of the Ag nanoparticles (NPs) transformed from well dispersed cubic to rough and aggregated. The introduction of acetate and dimethylamine further extended the growth pattern and morphological properties of these NPs. As examples, L5 AS, G5AS, and S5AS ligands formed spherical or sheet-like structures when used alone, which upon the use of these additives transformed into larger multicore and rough NPs, revealing their significant effect on the NP morphology. Selected samples were tested for their stability against protein corona formation and ionic strength, where a high chemical stability and resistance to protein coating were observed. The findings show a promising, benign approach for the synthesis of shape- and size-directed Au and Ag nanostructures, along with a selection of the chemistry of carbohydrate-derivatives that can open new windows for their applications.

One-pot synthesis of corolla-shaped gold nanostructures with (110) planes.

In this work, corolla-shaped gold nanostructures with (110) planes were successfully synthesized using ethylenediaminetetraacetic acid (EDTA) and polyvinylpyrrolidone (PVP) as the co-reductants and shape-directing agents. The structure and the mechanism of the nanostructures were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical characterization. On the basis that surface energies of different gold crystallographic planes are in the order γ(111) < γ(100) < γ(110), the (110) plane has the highest surface energy among the low-index planes. The gold nanocrystals with exposed high-energy planes are important in facilitating their potential applications such as highly efficient catalysts. This research is of great significance for the subsequent work on the synthesis of nanocrystals dominated by high-energy crystal planes.

Single-Crystal Electrochemistry Explains Why Cu Nanoplates Form in the Presence of I-

Shape-directing agents such as halide ions and organic adsorbates play an important role on the growth of metallic nanocrystals. Halide ions, in particular, have been of interest due to their importance in determining the shape of metallic nanocrystals. A systematic study showed that halide ions determine the final shape of Au nanocrystals such as cubes, plates, and various polyhedrons.1 Additionally, it has been demonstrated that the diameter of Ag nanowires can be tuned by modulating the concentration of Br- in a polyol synthesis.2 Finally, it was reported that varying concentrations of I- impurities found in different brands of CTAB affect the formation of gold nanorods.3 However, although we know that halide ions can dictate the shape of nanocrystals, there is no direct evidence clarifying the role of halide ions and their interaction with other organic shape-directing agents on metal surfaces. In a recently published article, Kim et al demonstrated the power of single-crystal electrochemistry in elucidating the previously-misunderstood mechanism of Cu nanowire growth.4 Until recently, it was believed that capping agents selectively bind onto {100} facets—which make up the sides of penta-twinned Cu nanowires—and inhibit atomic addition, thereby directing anisotropic growth on the {111} facets that make up the ends of the nanowires.5,6 However, there were limited experimental results that supported this hypothesis. By using Cu(111) and Cu(100) single-crystal electrodes to simulate the crystal facets of Cu nanowires, Kim et al demonstrated that HDA (a shape-directing agent) passivated both the Cu(111) and Cu(100) surfaces equally, unlike previously hypothesized.4 In fact, they found that it was Cl- that selectively facilitated atomic addition on Cu(111) by interrupting the passivating HDA self-assembled monolayer (SAM), leading to anisotropic growth. Using a similar method, this presentation aims to demonstrate how and why Cu nanoplates grow when I- is introduced to a Cu nanowire growth solution using single-crystal electrochemistry. Synthetic results have shown that when a minimum concentration of NaI (75 µM) is introduced to an HDA-Cl--based synthesis for Cu nanowires, Cu nanoplates can be obtained (Fig. 1). Preliminary electrochemical measurements with a polycrystalline Cu electrode showed that I- was more effective in disrupting the HDA SAM compared to Cl-. In this presentation, we will introduce the interactions between I-, Cl-, and HDA on Cu single-crystal electrodes to understand why the addition of I- ions causes a change in nanocrystal structure from nanowires to nanoplates.

(Electrodeposition Division Early Career Investigator Award) Additives in Cu Electrodeposition for Through-Silicon Via Filling and the Growth of Cu Nanocrystals

Additives play a key role in modulating the kinetics of electrochemical reactions. They adsorb onto metal surfaces and their surface coverage changes the charge transfer resistance of electrochemical reactions. This phenomenon is critical in a variety of electrochemical processes such as corrosion, electrochemical deposition, electrocatalysis, and colloidal synthesis of metal nanocrystals. The use of additives in Cu electrodeposition is key to achieving defect-free interconnections for semiconductor devices. Controlling the surface coverage of additives enables Cu to selectively grow from the bottom of filling features, while Cu deposition outside of the features is effectively inhibited.1 This filling process is referred to as superfilling, superconformal deposition, or bottom-up filling. As a result of tremendous efforts to advance electrodeposition processes, nanometer-scaled Damascene features can be easily filled with Cu in the presence of polyether suppressors and accelerators.1,2 Furthermore, micrometer-scaled features including through-silicon vias (TSVs), microvias, and through-holes can also be successfully filled by adopting additional levelers.3,4 To further advance the metallization process, various experimental and theoretical studies are still on-going to develop new additives,5,6 to improve properties of Cu interconnections,7 and to gain a better understanding of the additives.8,9 Inorganic/organic additives, referred to as shape-directing agents or capping agents, are also essential in the colloidal syntheses of various nanocrystals.10 With the aid of additives, the shape of nanocrystals can be modulated from spherical nanoparticles to one-dimensional nanowires or to two-dimensional nanoplates. The growth of nanocrystals in colloidal syntheses is similar to metal electroless deposition because both processes are based on spontaneous redox reactions, i.e. the reduction of metal ions and the oxidation of reducing agents. However, in colloidal synthesis, the behavior of shape-directing agents is crystal facet-dependent, determining the final shape of nanocrystals by facilitating or inhibiting metal growth on certain crystal surfaces. Recently, electrochemical techniques using single-crystal electrodes have proven to be a powerful analytical tool that can clarify the growth mechanism of anisotropic nanocrystals by providing the metal growth rates on each single-crystal electrode.11,12 Successive research is still on-going to fully understand the facet-selective chemistry that occurs during nanocrystal growth. This presentation summarizes my recent progress in TSV filling and the mechanistic study of Cu nanowire growth. The first part of this presentation will mainly focus on bromide and iodide ions that act as a leveler for TSV filling,13,14 and the second part will introduce how electrochemical analyses using single-crystal electrodes reveal the growth mechanism of Cu nanowires and how halide ions change the shape of Cu nanocrystals.11,12,15 References T.P. Moffat, J.E. Bonevich, W.H. Huber, A. Stanishevsky, D.R. Kelly, G.R. Stafford, and D. Josell, J. Electrochem. Soc., 147, 4524–4535 (2000).P. Broekmann, A. Fluegel, C. Emnet, M. Arnold, C. Roeger-Goepfert, A. Wagner, N.T.M. Hai, and D. Mayer, Electrochim. Acta, 56, 4724–4734 (2011).V.-H. Hoang and K. Kondo, J. Electrochem. Soc., 164, D795–D797 (2017).J. Luo, Z. Li, M. Shi, J. Chen, Z. Hao, and J. He, J. Electrochem. Soc., 166, D104–D112 (2019).M.J. Kim, Y. Seo, H.C. Kim, Y. Lee, S. Choe, Y.G. Kim, S.K. Cho, and J.J. Kim, Electrochim. Acta, 163, 174–181 (2015).H.C. Kim, S. Choe, J.Y. Cho, D. Lee, I. Jung, W.-S. Cho, M.J. Kim, and J.J. Kim, J. Electrochem. Soc., 162, D109–D114 (2015).Q. Zhu, X. Zhang, S. Li, C. Liu, and C.-F. Li, J. Electrochem. Soc., 166, D3097–D3099 (2019).G.-K. Liu, S. Zou, D. Josell, L.J. Richter, and T.P. Moffat, J. Phys. Chem. C, 122, 21933–21951 (2018).T. Akita, M. Tomie, R. Ikuta, H. Egoshi, and M. Hayase, J. Electrochem. Soc., 166, D3058–D3065 (2019).D. Huo, M.J. Kim, Z. Lyu, Y. Shi, B.J. Wiley, and Y. Xia, Chem. Rev., in press, doi:10.1021/acs.chemrev.8b00745 (2019).M.J. Kim, P.F. Flowers, I.E. Stewart, S. Ye, S. Baek, J.J. Kim, and B.J. Wiley, J. Am. Chem. Soc., 139, 277–284 (2017).M.J. Kim, S. Alvarez, Z. Chen, K.A. Fichthorn, and B.J. Wiley, J. Am. Chem. Soc., 140, 14740–14746 (2018).M.J. Kim, H.C. Kim, and J.J. Kim, J. Electrochem. Soc., 163, D434–D441 (2016).H.C. Kim, M.J. Kim, and J.J. Kim, J. Electrochem. Soc., 165, D91–D93 (2018).M. J. Kim, S. Alvarez, T. Yan, V. Tadepalli, K.A. Fichthorn, and B.J. Wiley, Chem. Mater., 30, 2809–2818 (2018).

Counter Anion-Directed Growth of Iron Oxide Nanorods in a Polyol Medium with Efficient Peroxidase-Mimicking Activity for Degradation of Dyes in Contaminated Water.

Development of nanozymes, which are nanomaterials with intrinsic enzymatic properties, has emerged as an appealing alternative to the natural enzymes with tremendous application potential from the chemical industry to biomedicine. The self-assembled growth of micrometer-sized oxide materials with controlled nonspherical shapes can be an important tool for enhancing activity as artificial enzymes, as the formation of these superstructures often results in high surface area with favorable impact on catalytic activity. Herein, the growth of rod-shaped Fe3O4 microstructures via a one-pot microwave-based method and using a water–poly(ethylene glycol) mixture as a solvent is reported, without the involvement of external shape-directing agents. The precursor metal salt played a key role in the size, shape, and phase selective evolution of iron oxide micro/nanomaterials. Whereas self-assembled microrod superstructures were obtained using Fe(NO3)3 as the metal salt precursor, use of FeCl3 or Fe-acetate as precursors afforded hollow Fe2O3 microparticles and Fe3O4 nanoparticles, respectively. A graphitic layer was deposited on the Fe3O4 surface, imparting a negative surface charge as a result of a high-temperature treatment of poly(ethylene glycol). The rod-shaped Fe3O4 microcrystals show efficient peroxidase-mimicking activity toward 3,3,5,5′-tetramethylbenzidine and pyrogallol as peroxidase substrates with a Michaelis–Menten rate constant (Km) value of 0.05 and 0.52 mM, respectively. The proficient enzyme mimicking behavior of these magnetic superstructures was further explored for the degradation of organic dyes that includes rhodamine B, methylene blue, and methyl orange with a rate constant (k) of 0.038, 0.011, and 0.007 min–1 respectively, using H2O2. This fast and simple method could help to develop a new pathway for differently shaped oxide nanoparticles in a sustainable and economical manner that can be harnessed as nanozymes for industrial as well as biological applications.

Chitosan-mediated facile green synthesis of size-controllable gold nanostars for effective photothermal therapy and photoacoustic imaging

With the large near-infrared (NIR) absorption and the high NIR photothermal effect, gold nanostar (AuNS) has potential application in the biomedical field. However, the complicated synthesis process with toxic precursors is a serious issue for its practice. Herein, we reported a novel green synthesis procedure using chitosan polymer from marine and vitamin C for the preparation of AuNS. For the first time, chitosan was used as a stabilizer, shape-directing, and size-controllable agents for the preparation of AuNS. The size of the obtained AuNS ranged from 111 to 225 nm with different λmax. The AuNS exhibited near-infrared absorption with the excellent biocompatibility toward non-cancerous cell line (MG63) and cancerous cell line (MDA-MB-231). The in vitro photothermal therapy and photoacoustic imaging with assisting of obtained AuNS were also proved the high efficiency of these nanoparticles.

Potential of graphene for shape-directing agent free growth of highly oriented silver particles and their application in surface enhanced Raman scattering

Controlling the shape of metal nanoparticles is generally accomplished using shape-directing agents. Particle synthesis without shape-directing agents and shape control have attracted strong interest given the side effects of the shape-directing agents. Here, the potential of graphene in the deposition of metallic nanoparticles with different morphologies without using shape-directing agents was investigated. Doping graphene with nitrogen via plasma treatment enhanced the metal reduction. The nitrogen content was varied by changing the plasma treatment time. Nitrogen doping mainly occurred at boundary-like defects and involved pyrrolic nitrogen as the major chemical configuration. Graphene grown on copper substrate grown via chemical vapor deposition was used to deposit spherical silver nanoparticles throughout the graphene along with microparticles that were randomly distributed on the surface. Transition of microparticles of silver from dendrites to well-defined platelets morphologies was observed with nitrogen-doped graphene after 10 min of nitrogen plasma exposure. The number density of silver particles was controlled by varying the plasma treatment time. Surface-enhanced Raman scattering activity of these silver particles grown on graphene using copper substrates was assessed using a Rhodamine 6G Raman probe in the concentration range of 10 nM to 1 mM.

Surfactant‐Free Shape Control of Gold Nanoparticles Enabled by Unified Theoretical Framework of Nanocrystal Synthesis

Gold nanoparticles have unique properties that are highly dependent on their shape and size. Synthetic methods that enable precise control over nanoparticle morphology currently require shape-directing agents such as surfactants or polymers that force growth in a particular direction by adsorbing to specific crystal facets. These auxiliary reagents passivate the nanoparticles' surface, and thus decrease their performance in applications like catalysis and surface-enhanced Raman scattering. Here, a surfactant- and polymer-free approach to achieving high-performance gold nanoparticles is reported. A theoretical framework to elucidate the growth mechanism of nanoparticles in surfactant-free media is developed and it is applied to identify strategies for shape-controlled syntheses. Using the results of the analyses, a simple, green-chemistry synthesis of the four most commonly used morphologies: nanostars, nanospheres, nanorods, and nanoplates is designed. The nanoparticles synthesized by this method outperform analogous particles with surfactant and polymer coatings in both catalysis and surface-enhanced Raman scattering.

Control of the crystal habit and magnetic properties of Co nanoparticles through the stirring rate

Ferromagnetic hcp cobalt nanoparticles (NPs) are prepared by the polyol process using long-chain carboxylates as shape-directing agents, applying different stirring rates during the synthesis. Particles prepared with a stirring rate lower than ca. 150 rpm are very well-defined anisotropic crystals with smooth lateral facets and low stacking fault densities. The growth proceeds along the c axis of the hcp structure which corresponds to the long axis of the anisotropic particles. Increasing the stirring speed leads to a lowering of the mean aspect ratio, an increase in structural disorder as well as to a strong modification of the particle habit. The resulting Co NPs then display a marked surface roughness. We propose here a mechanism to explain the different morphologies observed. A high stacking fault density triggers the local disorganisation of the adsorbed capping agent layer which in turn favours the diffusion of Co monomers through the organic layer, resulting in the formation of excrescences perpendicular to the long axis of the particles. The magnetic properties are strongly related to the morphological and micro-structural properties of the particles. The stirring rate appears, so far, as the driving force to control the magnetic properties of the NPs from hard magnetic behaviour at low stirring rates to an almost soft magnetic one under high stirring conditions.