Peina Zhang

Revitalizing the Frens Method To Synthesize Uniform, Quasi-Spherical Gold Nanoparticles with Deliberately Regulated Sizes from 2 to 330 nm

In this work, we have successfully developed a new and consistent model to describe the growth of gold nanoparticles (Au NPs) via citrate reduction of auric acid (HAuCl4) by carefully assessing the temporal evolution of the NP sizes and surface charges by means of dynamic light scattering (DLS) and zeta-potential measurements. The new model demonstrates that the nucleation and growth of the Au NPs occur exclusively in the particles of the complexes of Au(+) ions and sodium acetone dicarboxylate (SAD) derived from the citrate/HAuCl4 redox reaction, which proceeds as described by the classic LaMer model. Concomitant with the Au NP growing therein, the Au(+)/SAD complex particles undergo reversible agglomeration with the reaction time, which may result in an abnormal color change of the reaction media but have little impact on the Au NP growth. Built on the new model, we have successfully produced monodisperse quasi-spherical Au NPs with sizes precisely regulated from 2 to 330 nm via simple citrate reduction in a one-pot manner. To date, highly uniform Au NPs with sizes spanning such a large size range could not be formed otherwise even via deliberately controlled seeded growth methods.

Effect of Latent Heat in Boiling Water on the Synthesis of Gold Nanoparticles of Different Sizes by using the Turkevich Method

The Turkevich method, involving the reduction of HAuCl4 with citrate in boiling water, allows the facile production of monodisperse, quasispherical gold nanoparticles (AuNPs). Although, it is well-known that the size of the AuNPs obtained with the same recipe vary slightly (as little as approximately 4 nm), but noticeably, from one report to another, it has rarely been studied. The present work demonstrates that this size variation can be reconciled by the small, but noticeable, effect that the latent heat in boiling water has on the size of the AuNPs obtained by using the Turkevich method. The increase in latent heat during water boiling caused an approximately 3 nm reduction in the size of the as-prepared AuNPs; this reduction in size is mainly a result of accelerated nucleation driven by the extra heat. It was further demonstrated that, the heating temperature can be utilized as an additional measure to adjust the growth rate of AuNPs during the reduction of HAuCl4 with citrate in boiling water. Therefore, the latent heat of boiling solvents may provide one way to control nucleation and growth in the synthesis of monodisperse nanoparticles.

Template-directed synthesis of nitrogen- and sulfur-codoped carbon nanowire aerogels with enhanced electrocatalytic performance for oxygen reduction

Heteroatom doping, precise composition control, and rational morphology design are efficient strategies for producing novel nanocatalysts for the oxygen reduction reaction (ORR) in fuel cells. Herein, a cost-effective approach to synthesize nitrogen- and sulfur-codoped carbon nanowire aerogels using a hard templating method is proposed. The aerogels prepared using a combination of hydrothermal treatment and carbonization exhibit good catalytic activity for the ORR in alkaline solution. At the optimal annealing temperature and mass ratio between the nitrogen and sulfur precursors, the resultant aerogels show comparable electrocatalytic activity to that of a commercial Pt/C catalyst for the ORR. Importantly, the optimized catalyst shows much better long-term stability and satisfactory tolerance for the methanol crossover effect. These codoped aerogels are expected to have potential applications in fuel cells.

Empirical structural design of core@shell Au@Ag nanoparticles for SERS applications

In this work, we synthesized a series of core@shell Au2r@Agt nanoparticles (CS Au2r@Agt NPs) with defined but varied Au core diameter (2r) and Ag shell thickness (t) via overgrowth of Ag on preformed Au NPs at room temperature. We demonstrate that the surface enhanced Raman scattering (SERS) activity of as-prepared Au2r@Agt NPs is dependent on the Ag shell thickness (t). The critical t value (tc), above which Au2r@Agt NPs reach the maximal SERS activity, can be empirically correlated with the r values as tc = 0.301·r + 0.695 when the diameters of Au cores (2r) are smaller than 42 nm, the mean free path of bulk gold, while the tc is fixed at about 3 nm when 2r > 42 nm. The simple empirical rule should be very useful for the design of Au@Ag NPs for SERS applications, which is hardly discussed in the literature.

Transition metal ion-assisted synthesis of monodisperse, quasi-spherical gold nanocrystals via citrate reduction

We demonstrate that monodisperse, quasi-spherical gold nanocrystals (Au NCs) can be produced via quick addition of an aqueous solution of sodium citrate containing Ag+ and Fe2+ or Cu+ ions into a boiling aqueous solution of HAuCl4. The size of the resulting Au NCs increases from 12 nm to 60 nm with the decrease of citrate concentration. Compared with those obtained via a conventional Frens method, the NCs obtained via our approach have a fairly narrow distributed size and spherical shape. The significant improvement in the size polydispersity and shape sphericity is a result of the synergic effect of Ag+ and Fe2+ (or Cu+) ions. During the formation of Au NCs, the Ag+ ions are utilized to accelerate the nucleation via catalysis of the oxidation of citrate and regulate the NC isotropic growth, while the Fe2+ (or Cu+) ions significantly accelerate the reduction of Au3+ ions to Au0 atoms to speed up the nucleation when the concentration of citrate and silver ions was reduced to below 6.0 × 10−3 wt% and 8.5 × 10−4 wt%, respectively.

Synthesis of composition and size controlled AuAg alloy nanocrystals via Fe2+-assisted citrate reduction

We report one new method for direct synthesis of monodisperse, quasi-spherical AuAg alloyed nanocrystals (ANCs) with sizes of 11 to 25 nm and core–shell (CS) AuAg@AuAg NCs (CS AuAg ANCs) with sizes of 30 to 80 nm via the Fe2+-assisted citrate coreduction of auric ions and silver ions in a one-pot reaction. It is found that at fixed concentration of auric ions, the concentrations of ferrous ions and silver ions play key roles in tuning the size and the types of NCs (AuAg ANCs and CS AuAg ANCs), respectively. Moreover, AuAg ANCs and CS AuAg ANCs exhibit excellent and poor chemical stability in nonideal chemical environments, respectively. The potential application of these AuAg ANCs for SERS detection of industrial pollutants (such as benzidine and p-cresol) in artificial industrial wastewater has been successfully demonstrated.

Revitalizing spherical Au@Pd nanoparticles with controlled surface-defect density as high performance electrocatalysts

In this work, core–shell (CS) Au@Pd nanoparticles (NPs) with about one Pd atomic layer (named CmS Aux@Pd1−x NPs, in which m is the size of the core, and is 6, 12, 19, 30, and 57 nm and x is the molar content of Au in the whole NP) were first synthesized by ascorbic acid reduction of Na2PdCl4 solution on Au-NP seeds of various amounts of surface defects in water at room temperature. To the best of our knowledge, the concept of “surface-defect density” and the calculation method are proposed for the first time in NP-based nanocatalysts. On the basis of electrocatalytic results, it is found that the specific activities of the as-prepared CmS Aux@Pd1−x NPs are increased with increase in their surface-defect densities while their mass activities and electrochemically active surface areas (ECSAs) both exhibit “volcano-type” dependence with respect to their size in the absence of carbon supports. Moreover, C19S Au0.91@Pd0.09 NPs with Vulcan carbon supports show an optimal electrocatalytic performance and a long-term high electrocatalytic activity for ethanol oxidation. Their ECSA value, mass activity and specific activity are 119.8 m2 g−1, 11.0 A mgPd−1, and 9.3 mA cm−2, respectively, which are about 4.8-fold, 36.7-fold, and 7.8-fold better than those (24.9 m2 g−1, 0.3 A mgPd−1, and 1.2 mA cm−2) of commercial Pd/C catalysts, respectively. This work provides not only a direct correlation between defect-density and catalytic activity, but also design rules for metal NP electrocatalysts with superior electrocatalytic performance.

Simple Synthesis of Au–Pd Alloy Nanowire Networks as Macroscopic, Flexible Electrocatalysts with Excellent Performance

The present work introduces a new way to prepare Au-Pd alloy nanowire networks (NWNs) via deposition of Pd atoms onto Au nanowires in reaction media at room temperature without the aid of additional reducing agents. Thanks to their excellent colloidal stability in water as well as in ethanol, the resulting NWNs can be utilized to produce composite thin films with Nafion (perfluorinated sulfonic acid) with dimensions above dozens of square centimeters by means of solution casting on the glass substrate. Most importantly, these films can be easily transferred onto different solid substrates by lift-off technology. Moreover, the resulting Au-Pd alloy NWNs can also be easily and thoroughly loaded into macroscopic carbon fiber cloth (CFC). Both the Au-Pd alloy NWN/Nafion composite film and the Au-Pd alloy NWN-loaded CFC can be used as flexible electrodes for electrocatalysis of ethanol oxidation, with electrocatalytic performance at different distorted states superior by 2 orders of magnitude to those reported in the literature (e.g., commercial Pd/C catalysts and Pd-based nanostructured catalysts). This work opens new possibilities for the large-scale manufacturing of electrodes for fuel cells.

Simple synthesis of uniformly small gold nanoparticles for sensitivity enhancement in colorimetric detection of Pb2+ by improving nanoparticle reactivity and stability

We first synthesized high-quality gold nanoparticles (Au NPs) with size ranging from 5.5 to 17.8 nm in high yield via a robust, one-step seeded growth method using Tris-base (TB). It was found that the narrow distribution in size and shape of the as-prepared Au NPs was affected by reaction temperature and pH value of the reaction media. The sizes of these small Au NPs could be finely controlled by varying the gold precursor and/or seed amount according to an established relationship between the NP diameter and the particle number of the seeds (or the mass of the gold precursor in some cases). Due to their highly colloidal stability, TB-stabilized Au NPs (TB-Au NPs) could be directly used for the detection of Pb2+. Pb2+ can be determined in a large linear range of 0.1 nM to 5 μM with a high determination coefficient, good repeatability (10 times at least) and high selectivity (1 : 200) by our 2-mercaptoethanol (2-ME)/S2O32−–TB-Aum NP sensors (m represents the size of Au NPs, m = 5.5, 10, and 15 nm). Moreover, the limit of detection (LOD) of the 2-ME/S2O32−–TB-Au5.5 NP sensor for Pb2+ could be as low as 0.1 nM, which, to the best of our knowledge, is the lowest one obtained by a colorimetric method thus far. Furthermore, our 2-ME/S2O32−–TB-Au5.5 NP sensor could also detect Pb2+ in real soil extracts with satisfactory recovery. Thus, the 2-ME/S2O32−–TB-Au5.5 NP sensor could be applied for the determination of Pb2+ in real samples from complicated natural environments.

Regulating Surface Facets of Metallic Aerogel Electrocatalysts by Size-dependent Localized Ostwald Ripening

It is well known that the activity and stability of electrocatalysts are largely dependent on their surface facets. In this work, we have successfully regulated surface facets of three-dimensional (3D) metallic Au m- n aerogels by salt-induced assembly of citrate-stabilized gold nanoparticles (Au NPs) of two different sizes and further size-dependent localized Ostwald ripening at controlled particle number ratios, where m and n represent the size of Au NPs. In addition, 3D Au m- n-Pd aerogels were further synthesized on the basis of Au m- n aerogels and also bear controlled surface facets because of the formation of ultrathin Pd layers on Au m- n aerogels. Taking the electrooxidation of small organic molecules (such as methanol and ethanol) by the resulting Au m- n and Au m- n-Pd aerogels as examples, it is found that surface facets of metallic aerogels with excellent performance can be regulated to realize preferential surface facets for methanol oxidation and ethanol oxidation, respectively. Moreover, they also indeed simultaneously bear high activity and excellent stability. Furthermore, their activities and stability are also highly dependent on the area ratio of active facets and inactive facets on their surfaces, respectively, and these ratios are varied via the mismatch of sizes of adjacent NPs. Thus, this work not only demonstrates the realization of the regulation of the surface facets of metallic aerogels by size-dependent localized Ostwald ripening but also will open up a new way to improve electrocatalytic performance of 3D metallic aerogels by surface regulation.