Understanding of the major reactions in solution synthesis of functional nanomaterials

REVIEW SCIENCE CHINA Materials mater.scichina.com link.springer.com Published online 14 November 2016 | doi: 10.1007/s40843-016-5112-0 Sci China Mater 2016, 59(11): 938–996 Understanding of the major reactions in solution synthesis of functional nanomaterials Yuen Wu1,2, Dingsheng Wang1 and Yadong Li1* ABSTRACT This review covers the major reactions involved in the solution synthesis of nanomaterials. It was designed to classify the traditional strategies such as precipitation, reduction, seed growth, etching, and so on into two basic processes which are termed as bottom-up and top-down routines. The discussion is focused on the basic mechanism and principles during the nucleation and growth of nanocrystals, especially in the solution system. This review also presents a prediction for how to utilize these intrinsic processes to artificially construct the desired specific and functional nanostructures. We try to describe the most directive and effective way to control the structures of nanocrystals for researchers who can master the major reaction mechanism and grasp the basic technologies in synthetic nanoscience. Keywords: nanomaterials, solution synthesis, bottom-up, topdown, nanocrystals INTRODUCTION Since unique chemical and physical properties emerge when the size scale is down to nanoscale, inorganic nanomaterials such as metal, oxide and semiconductor, have gradually drawn increased interests in a variety of research fields such as electronics, catalysis and optics. The properties of nanomaterials are not only related to their intrinsic matter themselves but also their existence form such as sizes, compositions, crystal phases, surface facets, and so on. The synthesis of nanocrystals (NCs) is a prerequisite topic for their further performance tests and applications. During recent decades, abundant efforts have been paid to develop the general strategy to synthesize and control the nanostructure. For instance in physical methods, chemical vapour deposition (CVD) has been widely utilized to prepare two-dimensional (2D) materials such as graphene-based composites, and metal dichalcogenides. Owning to the merits such as high yield, versatile manipulation, systematical control, low energy consumption and air pollution, well-defined structure and so on, solution-based methods have gained great successes in preparation of nanomaterials among all the developed synthetic methods. Although manipulating every single atom in solution is difficult at present, researchers have contributed great efforts in the controllable synthesis of various nanomaterials with homogeneous size, uniform morphology, and well-defined structure. In 2007, our group reviewed the synthesis of monodispersed NCs by carefully designing the interface between air, solid and solution [1]. In 2009, Xia’s group [2] summarized the basic chemistry and physical rules in controlling the shape of metal NCs. Further, the progresses in branched [3], concave [4], and high-index facet [5–7] metallic structures were also summarized and reviewed by various research groups. Chen et al. [8] discussed the roles of small adsorbates in tailoring the surface facets and surface atomic configuration of Pd and Pt based NCs. Liu et al. [9] concluded how to utilize the template to regulate the structure of nanomaterials. Recently, Huang’s group [10] demonstrated and reviewed the biomimetic molecules that can also be adopted as the capping agent to modulate the surface structures of NCs. Solvothermal/hydrothermal methods are also effective tools, which usually occur in a sealed heated solution above ambient temperature and pressure, in synthesis of nanomaterials such as semiconducting nanostructures [11] and oxides [12]. Actually, the energy activating the reactions in solution can also be introduced from the exterior reaction environments such as the microwave and ultrasonic. Nadagouda et al. [13] provided an overview of microwave-assisted synthetic approach in 1 Department of Chemistry and Collaborative Innovation Center for Nanomaterial Science and Engineering, Tsinghua University, Beijing 100084, China Department of Chemistry and Center of Advanced Nanocatalysis, University of Science and Technology of China, Hefei 230026, China * Corresponding author (email: ) 2 938 November 2016 | Vol.59 No.11 © Science China Press and Springer-Verlag Berlin Heidelberg 2016 REVIEW SCIENCE CHINA Materials synthesizing the Ag nanostructures, which can be further extended to the oxides and chalcogenides. Afterwards, Suslick’s group summarized the synthesis of nanomaterials by sonochemical procedures [14]. This review is focused on the major reactions in the synthetic approaches during the solution-based synthesis of colloidal nanoparticles (NPs). In tradition, strategies for fabrication of functional nanostructures can be defined as either “bottom-up” or “top-down” processes. Starting with atoms, molecules, clusters or even NPs, the “bottom-up” approach enables the synthesis of multitudinous nanostructures by assembly from these “building blocks”. On the contrary, “top-down” methods involving the carving of pre-formed architectures allow to alter the size/geometry-dependent electronic structures considerably, and assist the sophisticated design of specific properties. Understanding the inherent features of reactions occurring in the solvent allows for better controlling the size, composition, and structure of nanomaterials. We discuss representative reactions for both the bottom-up and top-down synthesis, namely precipitation, reduction, and decomposition/hydrolysis, for bottom-up strategy, replacement, chemical etching, cation exchange reactions, and exfoliation for top-down strategy. So far as the present research, the development of bottom-up process mainly focuses on the goal of versatile and generalized molecular-level control of nanomaterials under rational design and predictable structures (Scheme 1). Moreover, a brief overview combining the bottom-up and top-down strategies in the aim of fabricating more complex and elaborate nanostructures and sequential bottom-up strategy in the purpose for hierarchical structure are talked about. In general, the solution-based synthesis mainly involves the nucleation and growth of NCs and the solubility changes of substances in hydrophilic or hydrophobic solution. There are several parameters that should be considered during the preparation of nanomaterials in solution. For intrinsic reaction system conditions, the concentration of precursors, pH value and viscosity of the solution, reaction temperature and time, pressure of the reaction system, capping agents, and so on play a vital role in controlling the reaction rate and equilibrium. Although the exterior reaction environment conditions such as air bubble [15], microwave, ultrasonic, and magnetic field [16–19] can also bring significant influence to the reaction system, we mainly select the interior factors as the emphasis to dis (...truncated)