Building Materials from Colloidal Nanocrystal Arrays: Evolution of Structure, Composition, and Mechanical Properties upon the Removal of Ligands by O2 Plasma

Colloidal TiO2 nanocrystals with engineered defectivity and optical properties

Partially reduced forms of titanium dioxide (sometimes called "black" titania) have attracted widespread interest as promising photocatalysts of oxidation due to their absorption in the visible region. The main approaches to produce it rely on postprocessing at high temperatures (up to 800 °C) and high pressures (up to 40 bar) or on highly reactive precursors (e.g., TiH2), and yield powders with poorly controlled sizes, shapes, defect concentrations and distributions. We describe an approach for the one-step synthesis of TiO2 colloidal nanocrystals at atmospheric pressure and temperatures as low as 280 °C. The temperature of the reaction allows the density of oxygen vacancies to be controlled by nearly two orders of magnitude independently of their size, shape, or colloidal stability. This synthetic pathway appears to produce vacancies that are homogeneously distributed in the nanocrystals, rather than being concentrated in an amorphous shell. As a result, the defects are protected from oxidation and result in stable optical properties in oxidizing environments.

Plasma-based post-processing of colloidal nanocrystals for applications in heterogeneous catalysis

This review summarizes the work on the use of plasmas to post-process nanostructures, in particular colloidal nanocrystals, as promising candidates for applications of heterogeneous catalysis. Using plasma to clean or modify the surface of nanostructures is a more precisely controlled method compared to other conventional methods, which is preferable when strict requirements for nanostructure morphology or chemical composition are necessary. The ability of plasma post-processing to create mesoporous materials with high surface areas and controlled microstructure, surfaces, and interfaces has transformational potential in catalysis and other applications that leverage surface/interface processes.

Plasma-Enabled Ligand Removal for Improved Catalysis: Furfural Conversion on Pd/SiO2

A nonthermal, atmospheric He/O2 plasma (NTAP) successfully removed polyvinylpyrrolidone (PVP) from Pd cubic nanoparticles supported on SiO2 quickly and controllably. Transmission electron microscopy (TEM) revealed that the shape and size of Pd nanoparticles remain intact during plasma treatment, unlike mild calcination, which causes sintering and polycrystallinity. Using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS), we demonstrate the quantitative estimation of the PVP plasma removal rate and control of the nanoparticle synthesis. First-principles calculations of the XPS and CO FTIR spectra elucidate electron transfer from the ligand to the metal and allow for estimates of ligand coverages. Reactivity testing indicated that PVP surface crowding inhibits furfural conversion but does not alter furfural selectivity. Overall, the data demonstrate NTAP as a more efficient method than traditional calcination for organic ligand removal in nanoparticle synthesis.

Inorganically Connecting Colloidal Nanocrystals Significantly Improves Mechanical Properties

Understanding and characterizing the mechanical behavior of colloidal nanocrystal (NC) assemblies are important for developing nanocrystalline materials with exceptional mechanical properties for robust electronic, thermoelectric, photovoltaic, and optoelectronic devices. However, the limited ranges of Young's modulus, hardness, and fracture toughness (≲1-10 GPa, ≲50-500 MPa, and ≲10-50 kPa m^1/2, respectively) in as-synthesized NC assemblies present challenges for their mechanical stability and therefore their practical applications. In this work, we demonstrate using a combination of nanoindentation measurements and coarse-grained modeling that the mechanical response of assemblies of as-synthesized NCs is governed by the van der Waals interactions of the organic surface ligands. More importantly, we report tremendous ∼60× enhancements in Young's modulus and hardness and an ∼80× enhancement in fracture toughness of CdSe NC assemblies through a simple inorganic Sn₂S₆^4- ligand exchange process. Moreover, our observation of softening in nanocrystalline materials with decreasing CdSe NC diameter is consistent with atomistic simulations.

Mechanistic Insight into the Precursor Chemistry of ZrO2 and HfO2 Nanocrystals; towards Size-Tunable Syntheses

One can nowadays readily generate monodisperse colloidal nanocrystals, but a retrosynthetic analysis is still not possible since the underlying chemistry is often poorly understood. Here, we provide insight into the reaction mechanism of colloidal zirconia and hafnia nanocrystals synthesized from metal chloride and metal isopropoxide. We identify the active precursor species in the reaction mixture through a combination of nuclear magnetic resonance spectroscopy (NMR), density functional theory (DFT) calculations, and pair distribution function (PDF) analysis. We gain insight into the interaction of the surfactant, tri-n-octylphosphine oxide (TOPO), and the different precursors. Interestingly, we identify a peculiar X-type ligand redistribution mechanism that can be steered by the relative amount of Lewis base (L-type). We further monitor how the reaction mixture decomposes using solution NMR and gas chromatography, and we find that ZrCl₄ is formed as a by-product of the reaction, limiting the reaction yield. The reaction proceeds via two competing mechanisms: E1 elimination (dominating) and S_N1 substitution (minor). Using this new mechanistic insight, we adapted the synthesis to optimize the yield and gain control over nanocrystal size. These insights will allow the rational design and synthesis of complex oxide nanocrystals.

Nonaqueous Chemistry of Group 4 Oxo Clusters and Colloidal Metal Oxide Nanocrystals

We review the nonaqueous precursor chemistry of the group 4 metals to gain insight into the formation of their oxo clusters and colloidal oxide nanocrystals. We first describe the properties and structures of titanium, zirconium, and hafnium oxides. Second, we introduce the different precursors that are used in the synthesis of oxo clusters and oxide nanocrystals. We review the structures of group 4 metal halides and alkoxides and their reactivity toward alcohols, carboxylic acids, etc. Third, we discuss fully condensed and atomically precise metal oxo clusters that could serve as nanocrystal models. By comparing the reaction conditions and reagents, we provide insight into the relationship between the cluster structure and the nature of the carboxylate capping ligands. We also briefly discuss the use of oxo clusters. Finally, we review the nonaqueous synthesis of group 4 oxide nanocrystals, including both surfactant-free and surfactant-assisted syntheses. We focus on their precursor chemistry and surface chemistry. By putting these results together, we connect the dots and obtain more insight into the fascinating chemistry of the group 4 metals. At the same time, we also identify gaps in our knowledge and thus areas for future research.

Designing nanoparticle interfaces for inner-sphere catalysis

Interfacial chemistry dramatically impacts the activity (performance) and reactivity (mechanism) of nanoparticle catalysts.

Effect of nanoparticle size on the mechanical properties of nanoparticle assemblies

Nanoparticle assemblies (NPAs) have attracted tremendous interests of various research communities. The particle-size-effect on mechanical properties of NPAs is systematically studied. With decreasing the particle size d from 300 nm to 10 nm, the SiO2 NPAs become drastically harder (∼39×), stiffer (∼15×), and tougher (>3.5×). The results are consistent with the data scattered in the literature for various nanoparticle (NP) systems, indicating a fundamentally universal d-effect for all NPAs. A model is developed to correlate the hardness and the NP junction (NPJ) strength f. Here, f is mainly due to van der Waals and capillary interactions, roughly a constant (140 nN) for d = 100-300 nm, and then f decreases with decreasing d from ∼100 nm. The deformation mechanism of NPAs (for indentation depth ≫d) is shear plasticity involving shear breaking of NPJs. The fundamental mechanism for the d-effect is that, with decreasing d, the NPJ's planar density increases much faster than the decrease of f. Moreover, three deformation mechanisms of NPAs, (1) nanoparticle dislodging, (2) shear-band formation, and (3) cracking are naturally d-dependent. These new findings can provide important insights into the fundamental understanding of the inter-NP interaction, the mechanical behavior of the NPAs, and the design of robust NP-based devices.

Heterobimetallic Single-Source Precursors: A Springboard to the Synthesis of Binary Intermetallics

Intermetallics are atomically ordered crystalline compounds containing two or more main group and transition metals. In addition to their rich crystal chemistry, intermetallics display unique properties of interest for a variety of applications, including superconductivity, hydrogen storage, and catalysis. Because of the presence of metals with a wide range of reduction potentials, the controlled synthesis of intermetallics can be difficult. Recently, soft chemical syntheses such as the modified polyol and ship-in-a-bottle methods have helped advance the preparation of these materials. However, phase-segregated products and complex multistep syntheses remain common. Here, we demonstrate the use of heterobimetallic single-source precursors for the synthesis of 10-15 and 11-15 binary intermetallics. The coordination environment of the precursor, as well as the exact temperature used play a critical role in determining the crystalline intermetallic phase that is produced, highlighting the potential versatility of this approach in the synthesis of a variety of compounds. Furthermore, we show that a recently developed novel plasma-processing technique is successful in removing the surface graphitic carbon observed in some of the prepared compounds. This new single-source precursor approach is a powerful addition to the synthesis of atomically ordered intermetallic compounds and will help facilitate their further study and development for future applications.

On the kinetics of the removal of ligands from films of colloidal nanocrystals by plasmas

This paper describes the kinetic limitations of etching ligands from colloidal nanocrystal assemblies (CNAs) by plasma processing. We measured the etching kinetics of ligands from a CNA model system (spherical ZrO₂ nanocrystals, 2.5-3.5 nm diameter, capped with trioctylphosphine oxide) with inductively coupled plasmas (He and O₂ feed gases, powers ranging from 7 to 30 W, at pressures ranging from 100 to 2000 mTorr and exposure times ranging between 6 and 168 h). The etching rate slows down by about one order of magnitude in the first minutes of etching, after which the rate of carbon removal becomes proportional to the third power of the carbon concentration in the CNA. Pressure oscillations in the plasma chamber significantly accelerate the overall rate of etching. These results indicate that the rate of etching is mostly affected by two main factors: (i) the crosslinking of the ligands in the first stage of plasma exposure, and (ii) the formation of a boundary layer at the surface of the CNA. Optimized conditions of plasma processing allow for a 60-fold improvement in etching rates compared to the previous state of the art and make the timeframes of plasma processing comparable to those of calcination.

Thin film depth profiling by ion beam analysis

The analysis of thin films is of central importance for functional materials, including the very large and active field of nanomaterials. Quantitative elemental depth profiling is basic to analysis, and many techniques exist, but all have limitations and quantitation is always an issue. We here review recent significant advances in ion beam analysis (IBA) which now merit it a standard place in the analyst's toolbox. Rutherford backscattering spectrometry (RBS) has been in use for half a century to obtain elemental depth profiles non-destructively from the first fraction of a micron from the surface of materials: more generally, "IBA" refers to the cluster of methods including elastic scattering (RBS; elastic recoil detection, ERD; and non-Rutherford elastic backscattering, EBS), nuclear reaction analysis (NRA: including particle-induced gamma-ray emission, PIGE), and also particle-induced X-ray emission (PIXE). We have at last demonstrated what was long promised, that RBS can be used as a primary reference technique for the best traceable accuracy available for non-destructive model-free methods in thin films. Also, it has become clear over the last decade that we can effectively combine synergistically the quite different information available from the atomic (PIXE) and nuclear (RBS, EBS, ERD, NRA) methods. Although it is well known that RBS has severe limitations that curtail its usefulness for elemental depth profiling, these limitations are largely overcome when we make proper synergistic use of IBA methods. In this Tutorial Review we aim to briefly explain to analysts what IBA is and why it is now a general quantitative method of great power. Analysts have got used to the availability of the large synchrotron facilities for certain sorts of difficult problems, but there are many much more easily accessible mid-range IBA facilities also able to address (and often more quantitatively) a wide range of otherwise almost intractable thin film questions.

Large-Scale Synthesis of Colloidal Si Nanocrystals and Their Helium Plasma Processing into Spin-On, Carbon-Free Nanocrystalline Si Films

This paper describes a simple approach to the large-scale synthesis of colloidal Si nanocrystals and their processing into spin-on carbon-free nanocrystalline Si films. The synthesized silicon nanoparticles are capped with decene, dispersed in hexane, and deposited on silicon substrates. The deposited films are exposed to nonoxidizing room-temperature He plasma to remove the organic ligands without adversely affecting the silicon nanoparticles to form crack-free thin films. We further show that the reactive ion etching rate in these films is 1.87 times faster than that for single-crystalline Si, consistent with a simple geometric argument that accounts for the nanoscale roughness caused by the nanoparticle shape.

Calcination does not remove all carbon from colloidal nanocrystal assemblies

Removing organics from hybrid nanostructures is a crucial step in many bottom-up materials fabrication approaches. It is usually assumed that calcination is an effective solution to this problem, especially for thin films. This assumption has led to its application in thousands of papers. We here show that this general assumption is incorrect by using a relevant and highly controlled model system consisting of thin films of ligand-capped ZrO2 nanocrystals. After calcination at 800 °C for 12 h, while Raman spectroscopy fails to detect the ligands after calcination, elastic backscattering spectrometry characterization demonstrates that ~18% of the original carbon atoms are still present in the film. By comparison plasma processing successfully removes the ligands. Our growth kinetic analysis shows that the calcined materials have significantly different interfacial properties than the plasma-processed counterparts. Calcination is not a reliable strategy for the production of single-phase all-inorganic materials from colloidal nanoparticles.

Simplicity as a Route to Impact in Materials Research

Materials scientists and engineers desire to have an impact. In this Progress Report we postulate a close correlation between impact - whether academic, technological, or scientific - and simple solutions, here defined as solutions that are inexpensive, reliable, predictable, highly performing, "stackable" (i.e., they can be combined and compounded with little increase in complexity), and "hackable" (i.e., they can be easily modified and optimized). In light of examples and our own experience, we propose how impact can be pursued systematically in materials research through a simplicity-driven approach to discovery-driven or problem-driven research.