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A Novel Ultraviolet Irradiation PhotoreductionTechnique for the Preparation of Single-CrystalAg Nanorods and Ag Dendrites**

By Yong Zhou, Shu H. Yu, Cui Y. Wang, Xiao G. Li,Yu R. Zhu, and Zu Y. Chen*

Nanoparticles of noble metals have been the subject ofmuch intensive research due to their potential applications

in microelectronics,[1±5] and their optical, electronic, andcatalytic properties.[6±10] There have been several recent re-ports on promising attempts to create such nanoparticles,e.g., producing one-dimensional (1D) nanostructured metalmaterials,[11±18] controlling the shapes and particle sizes ofnoble metal nanoparticles,[19±23] and fabricating self-orga-nized nanostructures for gold particles by polymeric andcolloidochemical processes.[24±26]

Several approaches have been reported to inserting met-al into the nanotubes prepared by the arc-discharge evapo-ration technique.[11±15] Kyotani et al. have reported thepreparation of platinum nanorods and nanoparticles in uni-form carbon nanotubes obtained by a template carboniza-tion method.[16] The scanning tunneling microscopy (STM)technique has been applied to fabricated metallic nano-wires and a single gold atom wire.[17,18] Exploration of novelmethods for synthesis of the 1D nanostructured metal ma-terials is a challenging research area.

It is well known that catalytic reactivity depends on thesize and shape of the metal nanoparticles, and thereforethe synthesis of well-controlled shapes and sizes of colloid-al particles could be critical for their application.[19] Severalmethods have been used in recent years for the preparationof colloidal metal sols. The method generally involves thereduction of the relevant metal salt in the presence of asuitable surfactant, which is useful in the control of thegrowth of the metal particles. Previous studies on colloidalparticles have focused on the control of particle sizes andtheir growth kinetics, and have related particle sizes andcatalytic activity. The research has demonstrated that thedegree of polymerization[20] and the concentration of thestabilizing polymer[21,22] influence the size distribution, sta-bility, and catalytic activity of colloidal particles. A recentstudy has shown that a higher ratio of capping material tometal produces smaller Au particles. El-Sayed and co-workers[19] demonstrated a beautiful example of controllingthe shape and size of Pt nanoparticles by changing the ratioof the concentration of the platinum cations used in the re-ductive synthesis of colloidal particles in solution at roomtemperature. The produced Pt nanoparticles display tetra-hedral, cubic, irregular-prismatic, icosahedral, and cubo-oc-tahedral particle shapes. Another interesting class of col-loids generated by very slow UV-reduction are large butthin, platelet-like Au nanocrystals with triangular or trun-cated hexagonal shape, the size and homogeneity of whichalso crucially depend on the type of protective polymer.[23]

However, shape control has been much more difficult toachieve. A challenge in colloid chemistry is to control notonly the particle sizes but also the particle shapes andmorphologies. Shape control is an alternative tool for ad-justing the optical or catalytic properties.

Rao and co-workers[25] reported a novel method of pre-paring thiol-derivatized nanoparticles of Au, Pd, and Agforming superstructures. Thio-derivatized nanoparticles ofAu, Pd, and Ag forming superstructures have recently beenprepared by the acid-facilitated transfer of well-character-

±

[*] Prof. Z. Y. Chen, Dr. Y. Zhou, Dr. S. H. Yu, Dr. C. Y. Wang,Prof. Y. R. ZhuDepartment of ChemistryUniversity of Science and Technology of ChinaHefei 230026, Anhui (PR of China)

Prof. X. G. LiDepartment of Materials Science and EngineeringUniversity of Science and Technology of ChinaHefei 230026, Anhui (PR of China)

[**] This work was supported by the Chinese National Foundation of Natu-ral Science Research (Nos. 59572031 and 19772049).

ized particles in hydrosol to a toluene layer containing thethiol. Recently, Selvan[26] reported novel nanostructures ofgold±polypyrrole composites by using PY (pyrrole) to treata micellar solution of PS-P2VP [polystyrene-block-poly(2-vinylpyridine)] treated with tetrachloroauric acid.

In this communication, we report a novel ultraviolet irra-diation photoreduction technique for preparation of single-crystal Ag nanorods and dendritic supramolecular nano-structures at room temperature using polyvinylalcohol(PVA) as a protecting agent. It was found that the concen-tration of both AgNO3 and PVA has a significant effect onthe formation and growth of these novel nanostructures.

A 30 W column-like low pressure mercury lamp (l =253.7 nm) was used as an ultraviolet light source. Solutionscontaining 3 wt.-% PVA (average molecular weight ca. 8 ´104) and AgNO3 of various concentrations were irradiatedunder this ultraviolet source for 48 h at 15 �C. The productsobtained were washed with distilled water and absoluteethanol, then dried at 60 �C for 5 h.

The transmission electron microscopy (TEM) images ofthe products were taken with a Hitachi model H-800 TEM,using an accelerating voltage of 200 kV. The X-ray powderdiffraction (XRD) patterns for the products were deter-mined at a scanning rate of 0.02� s±1 in 2y ranging from 10�to 70�, using a Rigaku (Japan) Dmax gA-ray diffractometerwith high-intensity Cu Ka radiation (l = 0.151478 nm) anda graphite monochromator.

The concentration of AgNO3 was found to play a signifi-cant role in the formation and growth of the Ag nanopar-ticles. Figure 1a presents a typical TEM image of the prod-uct obtained by irradiating the solution containing 3 wt.-%PVA and 10±4 M AgNO3 for 48 h. It shows that the Agnanorods are about 15±20 nm in diameter, and up to350 nm in length. The corresponding electron diffraction(ED) pattern in Figure 1b reveals that only a hexagonaldiffraction spot pattern was observed, indicating that theAg nanorods prepared by the present ultraviolet irradia-tion method are single crystals and have a preferentialgrowth direction along the Ag[111] axis. The very slow ul-traviolet irradiation photoreduction process may favor theformation of the Ag single crystals, which is consistent withprevious results.[21] The corresponding lattice constant is a= 0.4056 nm, which is close to the reported data (JointCommittee on Powder Diffraction Studies File No. 4-0783).The corresponding XRD diffraction pattern also confirmedthe preferential growth of single crystals.

The results indicated that the Ag nanorods grow thickerand longer with increasing concentration of AgNO3 in thesolution. Figure 2a shows a TEM image of the product ob-tained by irradiating the solution containing 3 wt.-% PVAand 10±3 M AgNO3 for 48 h. The nanorods can reach up to1 mm in length and 40 nm in width. Further observationshows that the produced Ag nanorods display dendriticgrowth. We believe that the excess of silver in the solutionmay be favorable for the aggregation and growth into thedendritic structures of the Ag clusters. A high magnifica-

tion TEM image (Fig. 2b) shows that many convex areasappeared on the surface of the Ag nanorods. These ªcon-vex areasº may further develop into dendrites. This propo-sition was confirmed by further increasing the concentra-tion of AgNO3 in the system.

Fig. 1. a) A typical TEM image of the product obtained by irradiating thesolution containing 3 wt.-% PVA and 10±4 M AgNO3 for 48 h. b) The corre-sponding electron diffraction pattern.

Fig. 2. a) The TEM image of the product obtained by irradiating the solu-tion containing 3 wt.-% PVA and 10±3 M AgNO3 for 48 h. b) A high magni-fication TEM image of the sample in (a).

Figure 3 shows the TEM image of the product obtainedby irradiating the solution containing 3 wt.-% PVA and10±2 M AgNO3. Elegant Ag dendrites were observed. Thislarge, new, random supramolecular structure of Ag nano-particles, abbreviated as DLA (diffusion-limited aggre-gate),[27,28] represents a very wide variety of growth, inwhich one particle after the other is formed and thendiffuses, sticking to the growing structure. The Au±polypyr-role �dendritic' nanostructures were also observed by Sel-van[26] using vapor phase polymerization of pyrrole ontosolution-cast films of block copolymer ionomers.

It was found that the concentration of PVA also plays akey role in the formation of Ag nanorods. The results con-firmed that almost no Ag nanorods were present in theproducts if the concentration of PVA was lower than1.0 wt.-% in the presence of 10±4 M AgNO3, for the same

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period of irradiation, and that the Ag particles display ir-regular shapes. Increasing the concentration of PVA in thesystem is found to be favorable for the formation of theshaped Ag particles. This result of the influence of the con-centration of PVA on the shape of the Ag nanoparticles isvery similar to that obtained by El-Sayed and co-work-ers,[19] who reported that the ratio of the concentration ofthe capping polymer material to the concentration of theplatinum cations can influence the shapes and sizes of plati-num nanoparticles. In the present study, the protectingagent PVA may also be a kind of capping polymer materi-al, which usually acts as a molecularly dissolved surfacemodifier or steric stabilizer. Its presence in the system playsan important role in the formation of the Ag nanostruc-tures. However, the mechanism of the shape- or morphol-ogy-dependent synthesis of colloidal nanoparticles is notyet known and needs to be investigated further.

In summary, single-crystal Ag nanorods and elegant,highly ordered dendritic supramolecular nanostructures ofAg nanoparticles have been prepared via a novel ultravio-let irradiation photoreduction technique at room tempera-ture using PVA as a protecting agent. It was found that theconcentrations of both AgNO3 and PVA play a significantrole in the formation and growth of the Ag nanorods anddendrites. These Ag nanoparticles with unusual nanostruc-tures may have important applications in catalysis. Thismethod may be extended to prepare novel nanostructuresof other noble metals.

Received: February 17, 1999Final version: April 14, 1999

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High Luminescence Gold(I) and Copper(I)Complexes with a Triplet Excited State for Usein Light-Emitting Diodes**

By Yuguang Ma,* Chi-Ming Che, Hsiu-Yi Chao,Xuemei Zhou, Wing-Han Chan, and Jiaocong Shen

Many luminescent organic and polymer materials havebeen used for the fabrication of light-emitting diodes(LEDs).[1±3] Generally, electroluminescence (EL) was con-sidered to originate from the singlet excited state[4] becausefor the majority of organic molecules, the triplet excitedstate exhibits a low emission quantum yield, thus does notcontribute to EL emission. In EL, the existence of a boundtriplet excited state can severely limit the quantum effi-ciency. If the triplet binding energy and correspondingcross section for forming a triplet from a pair of injected

Fig. 3. The TEM image of the product obtained by irradiating the solutioncontaining 3 wt.-% PVA and 10±2 M AgNO3.

±

[*] Prof. Y. Ma, Prof. J. Shen, Dr. X. ZhouKey Lab for Supramolecular Structure and SpectraJilin UniversityChangchun 130023 (China)

Prof. C.-M. Che, Dr. H.-Y. Chao, Dr. W.-H. ChanDepartment of ChemistryHong Kong UniversityPokfulam Road, Hong Kong (China)

[**] Yuguang Ma thanks the National Science Foundation of China for fi-nancial support (No. 597905006) and Prof. B. F. Li and Prof.H. X. Zhang for their valuable advice. C.-M. Che is grateful to theUniversity of Hong Kong, the Hong Kong Research Grants Council,and the Croucher Foundation for funding for this project.