Crystal engineering of composite materials

Scientists and engineers have painstakingly amassed a great amount of experimental data over the few past decades in order to achieve a fundamental understanding of nanocrystal formation and growth. The areas of inorganic nanocrystals, surface chemistry and electrochemistry have been particularly important in nanocrystal growth. Reaction kinetics and thermodynamic aspects have also been investigated to further our understanding of nanocrystal growth. Most of the research, however, has been confined to single component or single phase materials, and there still remains a lot of work to do in order to understand the crystal growth of complex systems such as ternary or quaternary alloys or heteronanostructures. Also, organic nanocrystals have been relatively little studied, while it is clear that our vast collective knowledge on the art of conventional macroscopic organic crystal growth can shed light on the growth of organic crystals in much smaller versions.

Phase, composition, and morphology-controlled nanocrystals of inorganic or organic nature are becoming increasingly important in energy-related applications. Recent studies indicate that nanoscale alloy phase formation and segregation are closely related to their catalytic performance. Combination of multiple components, sometimes completely disparate in nature, in one system often yields synergistic cooperation between the individual constituents. Therefore it is very important to shift the research focus to the crystal growth of composite materials to impact future energy technologies. In this themed issue, we present 2 highlight articles, 10 articles, and 5 communications, featuring cutting edge research results and insights, pointing to an emerging research direction of crystal growth of multiple component systems.

The two highlight articles cover completely different areas, namely, crystal engineering of metal oxide solid solution and crystal growth of perovskite compounds. Bowen et al. recognized the effective parameters of relaxor-ferroelectric single crystals with the composition of (1 − x)Pb(Mg_1/3Nb_2/3)O₃ − xPbTiO₃ and (1 − y)Pb(Zn_1/3Nb_2/3)O₃ − yPbTiO₃ in determination of their electrochemical properties for piezoelectric transducer and energy-harvesting applications (DOI: 10.1039/c6ce00825a). Park provided a tutorial review on the perovskite solar cell, namely, the emergence of this specific type of solar cell and the optoelectronic characteristics of perovskite materials. Most importantly, Park discussed recently developed methodologies for the growth of perovskite crystals without traps and grain boundaries, which is critical in the solar cell performance (DOI: 10.1039/c6ce00813e).

Combination of carbon materials and inorganic materials have been intensively studied because the high conductivity of carbon materials can greatly boost the utility of inorganic materials. Park et al. reported that the combination of SnO₂ nanoparticles with 3-dimensional reduced graphene oxide (RGO) results in a novel Li ion battery anode with excellent performance, which does not suffer from rapid capacity decay (DOI: 10.1039/c6ce00362a). Rolison et al. demonstrated an interesting phase conversion of lamella structured manganese oxide material affixed to carbon nanofoam (CNF) paper into the spinel LiMn₂O₄@CNF (DOI: 10.1039/c6ce00861e). Notably, they could precisely follow the detailed phase transformation of the lamella manganese oxide to tetragonal Mn₃O₄ and finally to LiMn₂O₄ by using synchrotron X-ray total scattering methods. Kim et al. fabricated peptide-carbon nanotube composites and anchored AuPt nanoparticles onto the exposed peptide surface of the peptide-CNT composites (DOI: 10.1039/C6ce00841k). AuPt nanoparticles anchored on the peptide surface are very stable during oxygen reduction reaction and are not lost during catalysis, demonstrating the utility of peptide-CNT as a robust catalyst support. McCarthy et al. prepared an interesting nanocomposite adhesive composed of epoxy resin and hollow iron oxide nanospheres (DOI: 10.3039/c6ce01359g). They showed that the hollow iron oxide nanospheres act as an electromagnetic receptor to convert microwave energy to thermal energy, which in turn can degrade the epoxy resin adhesive. This re-workable adhesive might be advantageously used in unbinding glued objects without causing physical damage to them, potentially useful in the plastic recycling industry.

Stoichiometric balancing of multiple metal species in a nanoparticle has been a consistent goal among synthetic nanoscientists. Cation exchange in metal chalcogenide systems has been particularly useful in generating otherwise difficult to obtain material phases. Song et al. demonstrated that CuSe_2−x nanoparticles can react with Ir³⁺ to form stiochiometrically precise CuInSe₂ nanoparticles, which would be superior to the mixture of Cu_2−xSe and InSe nanoparticles in forming CISe thin films (DOI: 10.1039/c6ce00950f). Chen et al. showed that Cu nanowires can undergo cation exchange reaction by PdCl₄²⁻ to form PdCu nanotubes (DOI: 10.1039/c6ce00688d). Interestingly, the PdCu nanotubes further react with the sulfur atom of dimethyl sulfoxide (DMSO) to form Pd₁₃Cu₃S₇ nanotubes, identifying DMSO as a useful S source in the fabrication of metal sulfide materials. Lee et al. showed that Pt nanowires can react with surface deposited Ni phase to form PtNi@Ni core–shell nanostructures (DOI: 10.1039/c6ce00830e). Upon acid treatment, the Ni component in the shell and in the PtNi core is selectively removed to give bare Pt₃Ni nanowires, which show excellent catalytic efficiency toward oxygen reduction reaction. They also showed that dendritic Pt nanostructures can react with surface deposited Sn to form hollow PtSn nanostructures (DOI: 10.1039/c6ce00831c). Notably, a hollow alloy structure can be formed in the absence of the Kirkendall effect, thereby significantly broadening the metal pair combinations for hollow structures.

Inorganic nanocomposites can be fabricated by combining pre-made nanoparticles or by growing sequentially a heterophase on a seed. Qian et al. fabricated nanofibers of upconversion nanoparticles (UCNPs)/CdS/PVP/tetrabutyl titanate by electrospinning and converted the titanium precursor into TiO₂ nanoparticles via hydrothermal treatment to form UCNPs/CdS/TiO₂ nanofibers, which showed weak upconversion fluorescence upon excitation of near-infrared light (980 nm) (DOI: 10.1039/c6ce00987e). Polyoxometallates can be used as both reducing and surface-stabilizing agents in the nanoparticle synthesis. Han et al. investigated the role of s-based polyoxometallates on the morphology controlled growth of Pt phase on Pd seed (DOI: 10.1039/c6ce00816j). The detailed experimental data and discussions might greatly benefit synthetic chemists attempting heteronanoparticle growth. Chen et al. reported interesting metal impurity-doping methodology in the morphology of cobalt oxide nanoparticles (DOI: 10.1039/c6ce00796a). While Zn-doped Co₃O₄ nanoparticles exhibit better performance among investigated systems in the oxygen evolution reaction, further studies would elucidate whether the catalytic performance stems from the morphology control or electronic structure control of mixed metal oxides. Joo et al. investigated the morphology-dependent catalytic performance of nickel phosphides toward hydrogen evolution reaction (DOI: 10.1039/c6ce00985a). Their study suggests the dual role of TOP as both a surface binding agent and P source in determining nickel phosphide phases.

Crystallization of organic or organometallic compounds is also of great interest in energy-related applications. Kim et al. describes a facile method to increase the single crystalline domain size of CH₃NH₃PbI₃ perovskite solar cells by the addition of 3% Pb(SCN)₂ impurity; a large domain size is beneficial to overall solar cell performance. Ciszek et al. described a surface hydrophilicity control of tetracene or pentacene thin films by reacting the surface exposed aromatic compounds with small adsorbates via Diels–Alder reaction (DOI: 10.1039/c6ce00728g). This surface wettability control method might be applied to the surface property tailoring of numerous polymer-based thin films for organic photovoltaic devices. Finally, Poizot et al. described a unique intercalation property of disodium 2,5-(dianilino)terephthalate crystals (DOI: 10.1039/c6ce01112h). The described compound could chemically exchange cations, solvents, as well as anions under electrochemical charging conditions.

As glimpsed in this themed issue, crystal engineering of composite materials based upon an understanding of crystal growth can greatly impact applications in energy conversion and storage. Despite the limited sample size, there appears to be a strong interest in nanoscale-to-nanoscale inorganic material transformation in terms of morphology, composition, and phase. Understanding fundamental issues such as diffusion of elements at the interface of heterostructures and thermodynamic/kinetic aspects of binary or ternary alloy formation would be critical in the advance of this research field. Also, the nanoscale crystal growth of organic and organometallic compounds deserves a stronger research interest in view of their importance in fabricating flexible or printable energy conversion or storage devices. I hope that this themed issue will act as a pointer to these important research areas. Finally, I sincerely thank the editorial staff of CrystEngComm, Dr Helen Lunn, Dr Debora Giovanelli, development editor Alice Jensen, Dr Simon Neil, and Prof. Georg Garnweitner, to name but a few, for their excellent reviewing and editing processes, ensuring the successful and timely publication of this themed issue.

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