High Resolution TEM Studies of Small Gold Particles Prepared by the Reduction of HAuCl4 with Trisodium Citric Acid

We found that gold particles, prepared by reduction of HAuCl4 with trisodium citric acid in the solutions of weak acidity, with a diameter of about 17 nm have different ratios between unsmooth surfaces and smooth surfaces. Gold particles with a diameter of about 7 nm have the structure in which amorphous coexist with crystalline. This paper put forward that study on synthesis of the gold particles of partial amorphous structure and growth increase of unsmooth surfaces and growth control of smooth surfaces should be the future developing direction. [DOI: 10.1380/ejssnt.2009.134]


I. INTRODUCTION
Small gold particles have a lot of wonderful physical and chemical properties that are often very different from bulk material.Over the last decades, the small gold particles have been extensively researched.The lattice imaging in a high resolution electron microscope (HREM) is the most straightforward and powerful method for examining the small gold particles.The multiply twinned particle model for gold nanoparticles formed by vacuum evaporation and the structure with five-fold symmetry have been extensively discussed, which are considered as existing widely in metal particles or nanowires [1][2][3][4][5][6].In the 1960s, Ino reported a multiply twinned particle model (having an icosahedron and decahedron structure) for gold nanoparticles prepared by deposition of gold on a NaCl surface in ultrahigh vacuum.He pointed out that with increasing thickness gold nanoparticles with the (111) orientation, and without the multiple twinning exclusively grow up, while multiply twinned gold nanoparticles and those with the (001) orientation seem to stop their growth, forming continuous gold film with a thickness of 50nm, discussed the stability of multiply twinned gold particles, and calculated critical sizes of gold particles for stable and quasistable states [1,2].HREM studies of icosahedral and decahedral gold particles prepared by vacuum evaporation were showed by Marks and Smith [3] and a doubling of the lattice spacing of the (100) face of gold nanopartiles was observed by Nepijko and Bondarenko [7].Penisson, et al. noted that the different gold grain boundaries existing in the bicrystal represented a dimensional quasi-crystal [8].Lu and Tanaka showed the formation of decahedral and icosahedral particles of Cu-Au alloy by codeposition of Au and Cu on an amorphous carbon electrode in the underpotential deposition region of Cu 2+ in acid solutions [9].Takami, et al. found that there was truncated icosahedron structure in Au MTP-micelles which composed of Au particles formed by evaporation and thiols adsorbed onto the surface of the Au multiply twinned particles [10].Mohr, et al. reported that Au particle catalysts supported on ZrO 2 were prepared by reduction of Au(OH) 3 (calcined at 573 K for 3 h) with H 2 at 573 K.A higher number of the multiply twinned particles in the Au catalysts re-sulted in a lowering of selectivity as well as a lowering of the turnover frequency [11].
However, Gold nanoparticles prepared by different methods may have different structures.In this paper we present the HREM studies of gold nanoparticles prepared by reduction of HAuCl 4 with trisodium citric acid in the solutions of weak acidity.We will report the gold particles of various kinds have different ratios between unsmooth surfaces and smooth surfaces.We will also present the gold particle structure in which amorphous coexists with crystalline, and put forward study on synthesis of the particles in which amorphous coexist with crystalline and growth increase of unsmooth surfaces and growth control of smooth surfaces should the future developing direction.

II. EXPERIMENTAL
The method for preparation of gold nanoparticles with a diameter of about 17 nm: the 2.5 ml of 1% aqueous HAuCl 4 solution was added into a 250 ml graduated flask.Purity water was added to the calibration graduation of the flask.After boiling for 3 min, 6 ml of freshly prepared trisodium citric acid was rapidly added to the stirring HAuCl 4 solution.The color of the mixture changed rapidly from yellow to purplish red, indicating formation of gold nanoparticles.Gold particles with a diameter of about 7 nm were prepared by changing the amount of trisodium citric acid.Trisodium citric acid used in the experiment was analytical reagent grade (AR) that has ideal quality for the experiment.Samples for transmission electron microscope (TEM) analyses were prepared by dip-coating a solution of the samples on carbon-film Cu grids.TEM analyses were performed on a JEM-2010HR instrument (JEOL LTD, Japan) equipped with an energy dispersive spectroscope (Energy TEM 200, OXFORD-INCA) at an accelerating voltage of 200 kV in the Instrument Analysis Center of the Sun Yat-sen University.Pictures were digitized using a CCD camera.crystal.Their some surfaces, which are parallel to lattice plane, are smooth.But, other surfaces, have a certain angle to lattice plane, are unsmooth.The particle on the upper half of Figure 1 have a shorter smooth surface (5-5.8 nm).We calculated ratios between unsmooth surfaces and smooth surfaces using the line length occurred by unsmooth surfaces and smooth surfaces on also found a number of gold nanoparticles oriented with a three-fold axis parallel to the electron beam, indicating that it is an icosahedral multiply-twinned particle consisting of twenty tetrahedra.Figure 3 shows an icosahedral multiply-twinned particle with the diameter of 17 nm.Its peripheral surfaces on Fig. 3 are unsmooth.Figure 4 shows a gold nanoparticle with a diameter of 7 nm, which may be an octahedral form.It is partially crystalline, but, other part is noncrystalline.Moreover, the shape, size, and distribution of crystalline part were irregular.This is a common structure for gold nanoparticles with a diameter of 3∼8 nm in the samples (Fig. 5).This is different with gold nanoparticles with a diameter of about 17 nm.The latter are commonly long range ordered structure nano-crystals, or multiply-twinned particles.

III. RESULTS AND DISCUSSION
Gold nanoparticles prepared by reduction of HAuCl 4 with trisodium citric acid have wide applications in the physical, chemical, and biomedical field.Their structures should be taken into account before it can be used.
Note that the gold particles have different ratios between unsmooth surfaces and smooth surfaces.The atoms of unsmooth surfaces have less coordination number and more dangling bonds compared to the atoms of smooth surfaces.The atoms have higher chemical activity and are of extreme strong absorbability.Unsmooth surface have http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/)

nm nm
FIG. 4: An octahedral gold particle with a diameter of 7 nm and its partial amorphous structure.

nm nm
FIG. 5: A gold particle with a diameter of 7 nm and its partial amorphous structure.
higher chemical activity than smooth surfaces.The partial amorphous structure of gold nanoparticles with size of < 10 nm diameter may be one of the reasons that leads to low melting point [12].
Over the past decade, scientists have made tremendous advances in synthesizing nanoparticles of different sizes and shapes [5].Tang and Ouyang reported on the protocol they have developed for synthesizing spherical particles that have either a single-or a multiply twinned crystal structure [5,13].Gold nanorods synthesized in the presence of silver ions are single crystals with a [100] growth direction [14].With the nitrate anion, the nuclei grow to form multiply twinned particles.In the case of the chloride anion, the chloride coordinates at the high-energy sites provided by the twinning defects, thereby blocking further growth of the multiply twinned particles [15].The result is that only the single-crystal particles grow, providing a pure product consisting of 100% single-crystal nanoparticles [5].But, how to impove the ratio between unsmooth surfaces and smooth surfaces is lack of study.However, it is an important work, which will result in further improvements in the properties of gold particles such as adsorption etc., which have wide applications in the physical, chemical, and biomedical field.Moreover, study on gold particles that have partially crystalline is also lack.

IV. CONCLUSIONS
At last the conclusion is drawn, which shows that gold nanoparticles, prepared by reduction of HAuCl 4 with trisodium citric acid, with a diameter of about 17 nm are the single-crystal particles or the multiply twinned particles.Their unsmooth surfaces have superior properties compared with smooth surfaces.Gold nanoparticles with diameters of about 7 have the partially amorphous structure.Study on growth increase of unsmooth surfaces and growth control of smooth surfaces and synthesis of particles with the structure in which amorphous coexists with crystalline should be the future developing direction.

Figure 1 FIG. 1 :FIG. 2 :
Figure1shows the long-range-ordered structure of two gold nanoparticles.The d-spacing of the two gold nanoparticles was measured to be 0.235 nm.It can be seen that the two gold nanoparticles appear in form of single