Temporal growth studies of Cu2(M)SnS4 (M=Zn, Mn, Fe, Co) nanoparticles formation
Tan, Joel Ming Rui
Date of Issue2016-06-06
Interdisciplinary Graduate School (IGS)
Energetics Research Institute
In the last few decades, there has been much emphasis placed on the development of solar harvesting materials. Multi-component (Cu-(M)-Sn-S) materials which is made up of naturally abundant components are highly promising as solar harvesting materials. However to realize an efficient solar harvesting device from these materials, several challenges such as: producing single phase material, controlling appropriate carrier concentration with good carrier transport properties, suitable bandgap, matching band energy level, doping control and minimal intrinsic defect states need to be overcome. These fundamental properties of the materials can be controlled if there is understanding of the material formation mechanism, kinetics and thermodynamics. The objective of this thesis is to investigate the synthesis of pure CMTS; Cu2-M-Sn-S4 (M=Zn, Fe, Mn, Co), based semiconductors as potential solar harvesting materials for both solar cell and solar fuel applications. From these studies, crucial growth steps have been pinpointed and conflicting growth kinetics have been solved via a proposed reaction pathway. In the initial stage of the study, the synthesis of Cu2ZnSnS4 (CZTS) nanoparticles was used as a model system to establish general reaction conditions for the synthesis of CMTS nanoparticles. Three key research foundations were laid. Firstly, Surface-enhanced Raman scattering (SERS) spectroscopy is demonstrated for the first time to be a sensitive characterization tool for unambiguous characterization of phases present in as-synthesized nanoparticles. This resolves the inability to differentiate mixed compositional phase (e.g., CZTS, CTS and ZnS) by powder X-ray diffraction (XRD) and conventional Raman spectroscopy. Secondly, the growth of CZTS is established to proceed strictly via formation of Cu2-xS nuclei, followed by diffusion of Sn4+ to form Cu3SnS4 and lastly diffusion of Zn2+ to form Cu2ZnSnS4 phase. Lastly, the reaction kinetics of Sn4+ ion were identified to be the limiting factor for CZTS nanoparticles’ purity. It is demonstrated that by carefully controlling the reactivity of organometallic ligand complex, it is possible to achieve balance in the reaction kinetics allowing one to synthesize CZTS phase without any observable binary/ternary phase from XRD and SERS. Following on, using the knowledge gained from the first study, a model study is presented to highlight the complexity of phase transition via cation exchange to form multi-component sulfide nanomaterials (Cu29S16 to Cu4SnS4 to Cu2ZnSnS4). Using transmission electron microscopy (TEM) and XRD, direct evidence indicates that the phase transformation of binary Cu29S16 to ternary Cu4SnS4 proceeds via sequential three steps reaction processes involving two intermediate phases (P121n1 Cu31S16 and P121c1 Cu32S16) while preserving the anionic framework of Cu29S16. In addition, the ex-situ growth study of CZTS from CTS reveals a negative domino cationic exchange growth effect resulting in precipitation of parasitic phases. Furthermore, the use of HR-HAADF imaging with Z-contrast capability enables the visualization of Sn fixed occupancy sites in our as-synthesized quaternary CZTS nanoparticles. This confirms that the crystal does not follow the commonly accepted P63mc. Lastly, using the knowledge gained from the first two studies, Cu2-M-Sn-S4 (M= Fe, Mn, Co) nanoparticles synthesis were carried out based on reaction conditions obtained from the initial study to identify conflicting growth mechanism. It is revealed that Cu2(M)SnS4 nanoparticles proceed via two different reaction pathways. Growth of Cu2(Mn)SnS4 nanoparticles is similar to Cu2(Zn)SnS4 nanoparticles where ternary Cu-Sn-S phase is the intermediate phase. On the other hand, the growth of Cu2(Co)SnS4 and Cu2(Fe)SnS4 proceeds via an intermediate ternary Cu-(M)-S phase. XRD characterization of as-synthesized Cu2(M)SnS4 nanoparticles revealed CZTS and CMTS to crystallize in hexagonal crystal structure while CFTS and CCTS crystallize in zinc blende crystal structure. In addition, as a proof of concept, the as-synthesized chemically treated CZTS and CMTS nanoparticles are drip-casted to form thin film solar cells. We demonstrated an encouraging power conversion efficiency of 1.16% (CZTS) and 0.078% (CMTS). In the three hypothesis proposed for this thesis, the first hypothesis on SERS having the capability to characterize nanomaterials unambiguously is proven to be true. For the second hypothesis that the growth of CZTS nanoparticle proceeds via nucleation of Cu-S phase, followed by formation of ternary Cu-Sn-S phase is proven to be accurate. Lastly, for the third hypothesis that the growth of Cu2(M)SnS4 (M= Mn, Co and Fe) nanoparticles follows the growth of CZTS nanoparticles, Cu2MnSnS4 is proven to follow the growth of CZTS nanoparticles while Cu2(M)SnS4 (M= Co and Fe) show interesting growth phenomena which prompt the need for further investigations.