Creating ground truth for nanocrystal morphology: a fully automated pipeline for unbiased transmission electron microscopy analysis.

Control over colloidal nanocrystal morphology (size, size distribution, and shape) is important for tailoring the functionality of individual nanocrystals and their ensemble behavior. Despite this, traditional methods to quantify nanocrystal morphology are laborious. New developments in automated morphology classification will accelerate these analyses but the assessment of machine learning models is limited by human accuracy for ground truth, causing even unsupervised machine learning models to have inherent bias. Herein, we introduce synthetic image rendering to solve the ground truth problem of nanocrystal morphology classification. By simulating 2D images of nanocrystal shapes via a function of high-dimensional parameter space, we trained a convolutional neural network to link unique morphologies to their simulated parameters, defining nanocrystal morphology quantitatively rather than qualitatively. An automated pipeline then processes, quantitatively defines, and classifies nanocrystal morphology from experimental transmission electron microscopy (TEM) images. Using improved computer vision techniques, 42 650 nanocrystals were identified, assessed, and labeled with quantitative parameters, offering a 600-fold improvement in efficiency over best-practice manual measurements. A classification algorithm was trained with a prediction accuracy of 99.5%, which can successfully analyze a range of concave, convex, and irregular nanocrystal shapes. The resulting pipeline was applied to differentiating two syntheses of nominally cuboidal CsPbBr3 nanocrystals and uniquely classifying binary nickel sulfide nanocrystal phase based on morphology. This pipeline provides a simple, efficient, and unbiased method to quantify nanocrystal morphology and represents a practical route to construct large datasets with an absolute ground truth for training unbiased morphology-based machine learning algorithms.

Statistical Multiobjective Optimization of Thiospinel CoNi2S4 Nanocrystal Synthesis via Design of Experiments.

Thiospinels, such as CoNi2S4, are showing promise for numerous applications, including as catalysts for the hydrogen evolution reaction, hydrodesulfurization, and oxygen evolution and reduction reactions; however, CoNi2S4 has not been synthesized as small, colloidal nanocrystals with high surface-area-to-volume ratios. Traditional optimization methods to control nanocrystal attributes such as size typically rely upon one variable at a time (OVAT) methods that are not only time and labor intensive but also lack the ability to identify higher-order interactions between experimental variables that affect target outcomes. Herein, we demonstrate that a statistical design of experiments (DoE) approach can optimize the synthesis of CoNi2S4 nanocrystals, allowing for control over the responses of nanocrystal size, size distribution, and isolated yield. After implementing a 25-2 fractional factorial design, the statistical screening of five different experimental variables identified temperature, Co:Ni precursor ratio, Co:thiol ratio, and their higher-order interactions as the most critical factors in influencing the aforementioned responses. Second-order design with a Doehlert matrix yielded polynomial functions used to predict the reaction parameters needed to individually optimize all three responses. A multiobjective optimization, allowing for the simultaneous optimization of size, size distribution, and isolated yield, predicted the synthetic conditions needed to achieve a minimum nanocrystal size of 6.1 nm, a minimum polydispersity (σ/d̅) of 10%, and a maximum isolated yield of 99%, with a desirability of 96%. The resulting model was experimentally verified by performing reactions under the specified conditions. Our work illustrates the advantage of multivariate experimental design as a powerful tool for accelerating control and optimization in nanocrystal syntheses.

Phase control in the colloidal synthesis of well-defined nickel sulfide nanocrystals.

Morphologically well-defined colloidal nanocrystals of Ni3S4, NiS, Ni9S8, and Ni3S2 were independently prepared through a solution-phase synthesis using N,N'-disubstituted thioureas as the sulfur precursor. Synthetic control over phase and composition of the resulting colloidal nickel sulfide nanocrystals was achieved by primarily adjusting the reactivity of substituted thioureas as well as tuning the key reaction parameters of temperature and precursor ratio. In general, the more reactive N,N'-diphenyl thiourea yields more sulfur-rich phases (Ni3S4 and NiS) while less reactive N,N'-dibutyl thiourea yields sulfur-poor phases (Ni9S8 and Ni3S2). This phase control can be further tuned through the use of 1-dodecanethiol as an important secondary reactivity-directing agent. In the presence of 1-dodecanethiol, nanocrystals of more sulfur-deficient phases are prepared, while in the absence of 1-dodecanethiol, more sulfur-rich phases are prepared. Under the most sulfur-rich synthetic conditions (i.e., with N,N'-diphenyl thiourea and no thiol) a phase progression from Ni3S4 to the α-NiS and ß-NiS phases was observed upon an increase in reaction temperature and sulfur-to-nickel precursor ratio. This study establishes, for the first time, a systematic evaluation of factors that simultaneously control the phase and yield well-defined nickel sulfide nanocrystals.

Investigating the Mechanism of Reversible Lithium Insertion into Anti-NASICON Fe2(WO4)3.

The gram-scale preparation of Fe2(WO4)3 by a new solution-based route and detailed characterization of the material are presented. The resulting Fe2(WO4)3 undergoes a reversible electrochemical reaction against lithium centered around 3.0 V with capacities near 93% of the theoretical maximum. Evolution of the Fe2(WO4)3 structure upon lithium insertion and deinsertion is probed using a battery of characterization techniques, including in situ X-ray diffraction, neutron total scattering, and X-ray absorption spectroscopy (XAS). A structural transformation from monoclinic to orthorhombic phases is confirmed during lithium intercalation. XAS and neutron total scattering measurements verify that Fe2(WO4)3 consists of trivalent iron and hexavalent tungsten ions. As lithium ions are inserted into the framework, iron ions are reduced to the divalent state, while the tungsten ions are electrochemically inactive and remain in the hexavalent state. Lithium insertion occurs via a concerted rotation of the rigid polyhedra in the host lattice driven by electrostatic interactions with the Li+ ions; the magnitude of these polyhedral rotations was found to be slightly larger for Fe2(WO4)3 than for the Fe2(MoO4)3 analog.

Effect of hygroscopicity of the metal salt on the formation and air stability of lyotropic liquid crystalline mesophases in hydrated salt-surfactant systems.

It is known that alkali, transition metal and lanthanide salts can form lyotropic liquid crystalline (LLC) mesophases with non-ionic surfactants (such as CiH2i+1(OCH2CH2)jOH, denoted as CiEj). Here we combine several salt systems and show that the percent deliquescence relative humidity (%DRH) value of a salt is the determining parameter in the formation and stability of the mesophases and that the other parameters are secondary and less significant. Accordingly, salts can be divided into 3 categories: Type I salts (such as LiCl, LiBr, LiI, LiNO3, LiClO4, CaCl2, Ca(NO3)2, MgCl2, and some transition metal nitrates) have low %DRH and form stable salt-surfactant LLC mesophases in the presence of a small amount of water, type II salts (such as some sodium and potassium salts) that are moderately hygroscopic form disordered stable mesophases, and type III salts that have high %DRH values, do not form stable LLC mesophases and leach out salt crystals. To illustrate this effect, a large group of salts from alkali and alkaline earth metals were investigated using XRD, POM, FTIR, and Raman techniques. Among the different salts investigated in this study, the LiX (where X is Cl(-), Br(-), I(-), NO3(-), and ClO4(-)) and CaX2 (X is Cl(-), and NO3(-)) salts were more prone to establish LLC mesophases because of their lower %DRH values. The phase behavior with respect to concentration, stability, and thermal behavior of Li(I) systems were investigated further. It is seen that the phase transitions among different anions in the Li(I) systems follow the Hofmeister series.

Highly conducting lyotropic liquid crystalline mesophases of pluronics (P65, P85, P103, and P123) and hydrated lithium salts (LiCl and LiNO3).

Demand for ionically conducting materials, as membranes and electrodes, is one of the driving forces of current research in chemistry, physics, and engineering. The lithium ion is a key element of these materials, and its assembly into nanostructures and mesophases is important for the membrane and electrode technologies. In this investigation, we show that hydrated lithium salts (such as LiCl·xH2O and LiNO3·xH2O, x is as low as 1.5 and 3.0, respectively) and pluronics (triblock copolymer such as PX where X is 65, 85, 103, and 123) form lyotropic liquid crystalline mesophases (LLCM), denoted as LiY·xH2O-PX-n (Y is Cl(-) or NO3(-), and n is the salt/PX mole ratio). The structure of the mesophase is hexagonal over a broad salt concentration and transforms to a cubic mesophase and then to disordered gel phase with an increasing salt content of the mixtures. The mesophases are unstable at low salt contents and undergo a phase separation into pure pluronics and salt-rich LLCMs. The salt content of the ordered mesophase can be as high as 30 mole ratio for each pluronic, which is a record high for any known salted phases. The mesophases also display high ac ionic conductivities, reaching up to 21 mS/cm at room temperature (RT), and are sensitive to the water content. These mesophases can be useful as ion-conducting membranes and can be used as media for the synthesis of lithium-containing nanoporous materials.

Lyotropic liquid crystal to soft mesocrystal transformation in hydrated salt-surfactant mixtures.

Hydrated CaCl2, LiI, and MgCl2 salts induce self-assembly in nonionic surfactants (such as C12H25(OCH2CH2)10OH) to form lyotropic liquid-crystalline (LLC) mesophases that undergo a phase transition to a new type of soft mesocrystal (SMC) under ambient conditions. The SMC samples can be obtained by aging the LLC samples, which were prepared as thin films by spin-coating, dip-coating, or drop-casting of a clear homogenized solution of water, salt, and surfactant over a substrate surface. The LLC mesophase exists up to a salt/surfactant mole ratio of 8, 10, and 4 (corresponding to 59, 68, and 40 wt% salt/surfactant) in the CaCl2, LiI, and MgCl2 mesophases, respectively. The SMC phase can transform back to a LLC mesophase at a higher relative humidity. The phase transformations have been monitored using powder X-ray diffraction (PXRD), polarized optical microscopy (POM), and FTIR techniques. The LLC mesophases only diffract at small angles, but the SMCs diffract at both small and wide angles. The broad surfactant features in the FTIR spectra of the LLC mesophases become sharp and well resolved upon SMC formation. The unit cell of the mesophases expands upon SMC transformation, in which the expansion is largest in the MgCl2 and smallest in the CaCl2 systems. The POM images of the SMCs display birefringent textures with well-defined edges, similar to crystals. However, the surface of the crystals is highly patterned, like buckling patterns, which indicates that these crystals are quite soft. This unusual phase behavior could be beneficial in designing new soft materials in the fields of phase-changing materials and mesostructured materials, and it demonstrates the richness of the phase behavior in the salt-surfactant mesophases.