The high specific surface area is one of important characteristics of the nanostructures. The population of the atoms at the surfaces increases extremely as the dimension decreases. In order to compare to the influence of the morphology on surface areas, we estimated surface area ratios of NB, NW and nanoparticle (NP) to a bulk cube (1 mm3) as functions of corresponding geometric variables. Total amount of each nanostructure is equal to that of the bulk cube. As shown in Figure 1.1, the surface area of each nanostructure with comparable dimensions under 70 nm is at least 10,000 times larger than that of bulk cube. Interestingly, calculations show that NBs possess surface areas higher than NPs and NWs with comparable dimensions over 40 nm.
Although the surface area of NPs increases obviously as the diameter reduces from 40 nm, serious aggregation could happen and cause passivation of active sites in the process of crystal growth. On the other hand, the growth of 1D nanostructures and the formation of three-dimensional architectures on flat substrates increase the surface areas. This may boost the charge transfer happening between the interface of electrodes and species in solution.
Figure 1.1 Comparison of estimated surface area ratios of NB, NW and NP to a bulk cube (1 mm3) as functions of corresponding geometric variables: WNB (NB width, ▲), dNW (NW diameter, ■), and DNP (NP diameter, ○). Total amount of each nanostructure is equal to that of the bulk cube.
1.2.2 Adsorption and Catalytic Property
As mentioned above, nanomaterials display high surface-area-to-volume ratios. It means that most atoms expose to uncoordinated sites and stand under unstable states. It is required to passivate these active sites by adsorbing atoms or molecules at the surface. Hence, strong adsorption behavior and high active sites are important features of nanomaterials.
Heterogeneous catalysis is a common technique in the chemical industry. It can synthesize various chemicals and reduce production cost. Metal nanoclusters have been long used as heterogeneous catalyst in synthetic organic compounds. Solid supported palladium nanoclusters are a typical example.11 They can be employed in carbon-carbon coupling reaction, such as Suzuki Miyaura Cross-Coupling, Heck reaction and Stille coupling. Many
studies also showed Pd NPs displayed high catalytic efficiency for hydrogenation of alkene and oxidation of carbon monoxide. These properties were contributed to their excellent adsorption ability and high surface area.
Cu is a common catalyst because its abundance and chemical activity. In the syntheses of organic compounds, Cu NPs can catalyze the cyclization of Schiffs’ bases and condensation of iodo-benzene to biphenyl.2, 3 Besides, it can be used as a catalytic electrode for speeding up electro-reduction of oxygen and carbon dioxygen.4, 5 These interesting properties involving energy and environment issues have attracted intense attention on the possible usage of Cu.
1.2.3 Electrochemical Property and Sensing
Electrochemical reactions taking place at the interface between flat electrodes and electrolyte solutions are often impeded by diffusion process. Recent development of nanotechnology has promoted the fabrication of nanostructured electrodes with the high roughness (ratio of real surface area to geometric area). These electrodes display high electrochemical active surface areas and accelerate the electrochemical reaction.
NP-modified electrodes, such as copper, gold, silicon and platinum NP electrodes, used as excellent electron transfer mediators, were known examples.12-15 In recent years, 1D nanostructures have become interesting building blocks for constructing highly sensitive electrodes. Carbon nanotube (CNT), Pt NT, and Cu2O NW electrodes have been demonstrated to be able to increase electro-oxidation ability of glucose and enhance the sensitivity for glucose detection.16-18 Besides, mesoporous and macroporous Pt electrodes also displayed the same electrocatalytic ability.19, 20 These designs of electrodes with high roughness increased undoubtedly high electro-catalytic active sites and boosted the kinetic-control reaction.
The morphologic effect of nanostructures on electrocatalytic activity is an important subject in material science. Each shape is correlated with which crystallographic facets exposed in the crystal. For example, three Pt electrodes, including (100), (110) and (111) single crystal,
displayed the different oxidation ability of hydrogen.21 Wang et al. synthesized the monodispersed Pt nanocubes, which are consistent of six {100} family planes. Compare to Pt NPs, they enhanced catalysis for oxygen reduciton.22 The morphology dependent electrochemical properties have started attracting more and more attention.
Cu NP electrodes could be used as catalytic electrodes for speeding up electro-reduction of oxygen and carbon dioxygen.4, 5 They could be employed as sensing electrodes for glucose, diphenol and amino acid.12, 23, 24
1.2.4 Surface Plasmon Resonance (SPR) Absorption
The study of the colors of metal NPs can be traced back to 19th century when Michael Faraday synthesize colloidal solutions of gold exhibiting colours ranging from ruby red to amethyst.25, 26 The various colors resulted from the surface plasmon band (SPB), which is a phenomenon observed in transmission, due to the presence of NPs, in solution or in the solid phase. For a special domain of frequency, NPs interact with incident light, resulting in a global scattering of it. This macroscopic feature can be explained by the collective resonance of the conduction electrons of the NP. A NP can be seen as an immobile and periodical cationic network in which a cloud of conducting electrons move. The latter are usually considered as free electrons.
Mie presented an analytical solution to Maxwell’s equations which describe a isolated spherical particle in 1908.27 Over the last three decades several numerical methods based on finite elements have been developed for overcoming the limitation in calculations of particles with arbitrary shape and multicomposition.28 Among those the discrete dipole approximation (DDA) has been proven to be an effective method for estimating the optical properties of metal particles in nanoscale.29 In most cases the extinction spectra of metal NPs under different conditions were simulated by the DDA method and compared with the experimental results.30 The investigation of the influence of particle shapes on surface SPR wavelengths
was also performed
The particle shape and size are an important factor to the SPR wavelength, and many papers concerning the size effect of the spherical nanoparticles have been published.31 Naturally, geometrical parameters also have strong influences on the SPR peaks, for example, triangle nanoplates. Schatz and co-workers have demonstrated that the increase of the side length of triangle nanoplates could lead their SPR peak to red shift by several hundred nanometers.32
This controllable optical property in terms of wavelength is quite exciting and interesting, and it enables the particles to be applied in biological sensing and drug delivery.33 Because the photons with near-IR wavelengths can harmlessly pass through biological tissues, nanoplates with high extinction coefficients at the target tissues will convert the near-IR photons to heat at high yields. So they can be used in photothermal cancer therapy and photothermally triggered drug release.
1.2.5 Surface-enhanced Raman Scattering (SERS)
The change in wavelength that is observed when a photon undergoes Raman scattering is attributed to the excitation (or relaxation) of vibrational modes of a molecule. Because different functional groups have different characteristic vibrational energies, every molecule has a unique Raman spectrum. In accordance with the Raman selection rule, the molecular polarizability changes as the molecular vibrations displace the constituent atoms from their equilibrium positions. The intensity of Raman scattering is proportional to the magnitude of the change in molecular polarizability. Thus, aromatic molecules exhibit more intense Raman scattering than aliphatic molecules.
Even so, Raman scattering cross sections are typically 14 orders of magnitude smaller than those of fluorescence; therefore, the Raman signal is still several orders of magnitude weaker than the fluorescence emission in most cases. Because of the inherently small intensity of the Raman signal, the sensitivity limits of available detectors, and the intensity of the excitation
sources, the applicability of Raman scattering was restricted for many years.
In 1977, Jeanmaire and Van Duyne demonstrated that the magnitude of the Raman scattering signal can be greatly enhanced when the scatterer is placed on or near a roughened noble-metal substrate.34 Strong electromagnetic fields are generated when the localized surface plasmon resonance (LSPR) of nanoscale roughness features on a silver, gold, or copper substrate is excited by visible light. When the Raman scatterer is subjected to these intensified electromagnetic fields, the magnitude of the induced dipole increases, and accordingly, the intensity of the inelastic scattering increases. This enhanced scattering process is known as surface-enhanced Raman (SER) scattering—a term that emphasizes the key role of the noble metal substrate in this phenomenon.
SER spectroscopy (SERS) can be exploited for sensitive and selective molecular identification. Recently, SERS has been used extensively as a signal transduction mechanism in biological and chemical sensing. Examples are trace analysis of pesticides, anthrax35, prostate-specific antigen36, glucose37, 38, and nuclear waste39. SERS has also been implemented for identification of bacteria40, genetic diagnostics41, and immunoassay labeling42-44. A miniaturized, inexpensive, and portable SERS instrument makes the technique practical for trace analysis in clinics, the field, and urban settings45.
1.2.6 Electron Field Emission (FE) Property
1D nanomaterials have a potential application as electron emitters in flat panel displays.46 The FE of electrons under applied electric fields is a quantum-mechanical phenomenon which can be understood as a tunneling process across the energy barrier between the vacuum and the emitter. In general, Fowler–Nordheim (F-N) equation is used to describe field emission characteristics of metals,47 which is expressed as J = A(β2E2/Φ)exp(-BΦ3/2/βE), where J is the current density, E the applied field, Φ the work function of the metal, β the field enhancement factor, indicating the degree of field emission enhancement by the tip shape of the
electron-emitting edge, and A and B are constants. According to the equation, the F-N plot, ln(J/E2) versus 1/E, is expected to be a straight line.
In recent years, there are some researches demonstrating that metal NW arrays display FE properties with low threshold voltages and high enhancement factors.48, 49 It could be contributed to their high aspect ratio and good electrical conductance. Kim et al. further fabricated a FE display device of Cu NW arrays grown by chemical vapor deposition (CVD).50 These revealed that metal NWs are promising candidates as FE electron sources.