## 1 Introduction  
Recently, control over silver nanoparticle morphologies has received considerable attention due to their potential applications in catalysis [1; 2], biological and chemical sensors [3; 4; 5; 6; 7; 8] and surface-enhanced Raman spectroscopy [9; 10; 11]. Actually, for over a thousand years ago, people have used silver as an antibacterial and disinfectant as recorded in a book on Chinese herbal medicine called _Compendium of Materia Medica_. In decades past, synthesis of silver nanostructures has been an active research area because of their excellent optical properties such as surface-enhanced Raman scattering (SERS) [12] and plasmonic resonance, which strongly depend on size, shape and composition [13; 14; 15]. Particularly, the shape control is vital to improve the optical properties of the resulting nanostructures. Therefore, many groups have devoted their efforts to exploring ways to prepare well-defined silver nanostructures in high yields. Among the numerous methods, the chemical method is thought to be the most popular. [PERSON]'s group [16; 17; 18; 19; 20] successfully synthesized various well shape-controlled silver nanostructures by the polyol process in ethylene glycol (EG) through varying the precursor concentration, molar ratio of the stabilizer and silver ions, reaction temperture and addition of helper agents. However, during these processes, reaction conditions are harsh, and are complex or difficult to control. For example, the reactant injection rate is critical for the shape of the final products, which makes the procedure difficult to operate. In addition, the reaction atmosphere is very important for the synthesis of the desired silver nanostructures [21], because in the presence of oxygen, twinned particles may be etched preferentially because of higher reactivity. Conversely, without oxygen, there was no oxidation etching to dissolve twinned particles leading to the formation of silver nanowires. Helper agents such as stabilizers and some ionic species also play a great role in the shape and size control of silver nanostructures. Polyvinyl pyrrolone (PVP) is one of common stabilizers used in the synthesis of silver nanostructures. [PERSON]'s group [22; 23; 24] produced uniform silver nanowires by taking advantage of the selectively adsorption on the (100) facets of PVP. If PVP is absent or added in low amounts, the main products are mainly nanospheres. Meanwhile, they found that silver nanoparticles with irregular morphology were formed in the presence of lower molecular weight PVP. In our previous work [25], we also demonstrated the shape of silver nanostructures can be controlled by varying the molecular weight. [PERSON] _et al_. [26] explored the role of different ions in the shape-controlled synthesis process. Results indicated that the addition of Cl\({}^{-}\), Br\({}^{-}\) and Fe\({}^{3+}\) promote the formation of silver nanocubes, right bipyramids and nanowires, respectively. Without fine control of reactant conditions and growth process, the obtained silver nanostructures are always obtained in low yield accompanied by large amounts of by-products. In these cases, the post processing, such as low rotation-rate centrifugation or special separation technique to purify products, is usually indispensable. Therefore, it is highly desirable to develop a reliable and facile method for the synthesis of silver nanostructures in high yield with well shape and size control. Besides the chemical reduction method, photochemical synthesis is also applied in the preparation of silver nanostructures. Compared with chemical reduction methods, the size distribution of the nanoparticles obtained is uniform. Stabilized intermediates can be obtained, which is difficult to carry out by other methods. Monodisperse silver decahedrons with finely tuned sizes were produced by [PERSON] and [PERSON] [15] using a novel photochemical technique. Moreover, the high symmetry and uniform size distribution causes narrow plasmon peaks with a tuned range and significant enhancement of Raman signature.