Synthesis of Porous Crystalline Doped Titania Photocatalysts Using Modified Precursor Strategy

We propose a new strategy for the synthesis of porous crystalline doped titania materialsdubbed the modified precursor strategy. The modified precursors are prepared by reacting generic titania precursors with organic acids in order to introduce “carbonizable” groups into the precursor’s structure, so that carbon–titania composites can form upon carbonization. The resulting carbon framework serves as a scaffold for TiO₂ and supports the structure during crystallization. Afterward, removal of the carbon scaffold through calcination results in titania with a well-developed structure and high crystallinity. The titanias synthesized according to this strategy, using common organic acids as the modifiers, have specific surface areas reaching 100 m² g^–1 and total pore volumes exceeding 0.20 cm³ g^–1, even after crystallization at temperatures from 500 to 1000 °C. The materials possess high crystallinity and tunable phase composition, and some show visible light absorption and significantly narrowed band gaps (2.3–2.4 eV). Photocatalytic degradation of methylene blue proved that these photocatalysts are active under visible light. All tested titanias show an excellent photocatalytic performance due to the combined effects of the well-developed structure, high crystallinity, and narrow band gap. This strategy can easily be adopted for the preparation of other porous crystalline materials

Iridium-Based Nanowires as Highly Active, Oxygen Evolution Reaction Electrocatalysts

Iridium–nickel (Ir–Ni) and iridium–cobalt (Ir–Co) nanowires have been synthesized by galvanic displacement and studied for their potential to increase the performance and durability of electrolysis systems. Performances of Ir–Ni and Ir–Co nanowires for the oxygen evolution reaction (OER) have been measured in rotating disk electrode half-cells and single-cell electrolyzers and compared with commercial baselines and literature references. The nanowire catalysts showed improved mass activity, by more than an order of magnitude compared with commercial Ir nanoparticles in half-cell tests. The nanowire catalysts also showed greatly improved durability, when acid-leached to remove excess Ni and Co. Both Ni and Co templates were found to have similarly positive impacts, although specific differences between the two systems are revealed. In single-cell electrolysis testing, nanowires exceeded the performance of Ir nanoparticles by 4–5 times, suggesting that significant reductions in catalyst loading are possible without compromising performance

Universal and Versatile Route for Selective Covalent Tethering of Single-Site Catalysts and Functional Groups on the Surface of Ordered Mesoporous Carbons

A universal and benign strategy for the surface functionalization of OMCs through lithium-mediated chemistry has been reported. For this purpose, a hard templating method for the facile synthesis of monodispersed ordered mesoporous carbons (OMCs) with well-defined morphology templated from large pore mesoporous silica nanoparticles (<i>l-</i>MSN) has been used. These OMCs have high surface areas (800–1000 m²g^–1) and large pore sizes (4–6 nm) suitable for anchoring bulky inorganic complexes. It has been demonstrated that the numerous defect sites present in the graphitic structure of OMCs can be effectively utilized for selective and covalent tethering of functional groups and single-site catalysts through lithiation of OMCs. Accordingly, for the first time a copper-based single-site oxidation catalyst has been covalently anchored onto the surface of OMCs. This novel system has been thoroughly characterized with advanced techniques such as electron microscopy, Raman spectroscopy, thermogravimetric analysis, X-ray diffraction, and acid–base titrations along with structural insights regarding the tethered copper catalyst by X-ray photoelectron spectroscopy. As a proof-of-principle, this active catalytic system has been used to demonstrate environmentally benign, room temperature selective oxidation of benzyl alcohol. We envision that this strategy for surface functionalization would be universal and can be applied for tethering a variety of different single-site catalysts onto OMCs with high surface areas. We also believe that it would have a direct impact on the currently available limited syntheses and surface functionalization techniques of mesoporous carbons for catalytic, electrocatalytic, and biological applications

Single-Step Plasma Synthesis of Carbon-Coated Silicon Nanoparticles

We have developed a novel single-step technique based on nonthermal, radio frequency (rf) plasmas to synthesize sub-10 nm, core–shell, carbon-coated crystalline Si (<i>c</i>-Si) nanoparticles (NPs) for potential application in Li⁺ batteries and as fluorescent markers. Hydrogen-terminated <i>c</i>-Si NPs nucleate and grow in a SiH₄-containing, low-temperature plasma in the upstream section of a tubular quartz reactor. The <i>c</i>-Si NPs are then transported downstream by gas flow, and are coated with amorphous carbon (<i>a</i>-C) in a second C₂H₂-containing plasma. X-ray diffraction (XRD), X-ray photoelectron spectroscopy, and in situ attenuated total reflection Fourier transform infrared spectroscopy show that a thin, < 1 nm, 3C-SiC layer forms at the <i>c</i>-Si/<i>a</i>-C interface. By varying the downstream C₂H₂ plasma rf power, we can alter the nature of the <i>a</i>-C coating as well as the thickness of the interfacial 3C-SiC layer. The transmission electron microscopy (TEM) analysis is in agreement with the Si NP core size determined by Raman spectroscopy, photoluminescence spectroscopy, and XRD analysis. The size of the <i>c</i>-Si NP core, and the corresponding light emission from these NPs, was directly controlled by varying the thickness of the interfacial 3C-SiC layer. This size tunable emission thus also demonstrates the versatility of this technique for synthesizing <i>c</i>-Si NPs for potential applications in light emitting diodes, biological markers, and nanocrystal inks

A Viewpoint on X‑ray Tomography Imaging in Electrocatalysis

With the emerging demands for clean energy and an economy with net-zero greenhouse gas emissions, electrocatalysis areas have attracted tremendous interest in recent years. The electrochemical devices that use electrocatalysis, such as fuel cells, electrolyzers, and flow batteries, consist of hierarchical structures, requiring comprehension and rational designs across scales from millimeter and micrometer all the way down to atomic scale. In past decades, electron microscopy techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been extensively utilized for imaging different scales of these devices in both two and three dimensions. However, electron-based techniques for high-resolution imaging require uninterrupted maintenance of a high-vacuum environment, leading to difficulties of sample preparation and lack of integrated observation without intrusion/disassembly. To overcome these disadvantages, more and more efforts have been dedicated to the development of X-ray imaging techniques recently, specifically absorption-based two-dimensional (2D) transmission X-ray microscopy and three-dimensional (3D) X-ray tomography, due to much better transmission behaviors of X-rays than electrons. X-ray tomography imaging mostly focuses on answering questions related to morphology and morphological changes at the microscale or near 1 μm resolution and nanoscale of 30 nm resolution. The method is nondestructive and it allows for the visualization of operando electrochemical devices, such as fuel cells, electrolyzers, and redox flow batteries. Operando X-ray microscopic tomography typically focuses on catalyst layers and morphology changes during degradation, as well as mass transport. Nanoscale tomography still predominantly is used for ex situ studies, as multiple challenges exist for operando studies implementation, including X-ray beam damage, sample holder design, and beamline availability. Both microscale and nanoscale tomography beamlines now couple various spectroscopic techniques, enabling electrocatalysis studies for both morphology and chemical transformations. This viewpoint highlights the recent advances in X-ray tomography for electrocatalysis, compares it to other tomographic techniques, and outlines key complementary techniques that can provide additional information during imaging. Lastly, it provides a perspective of what to anticipate in coming years regarding the method use for electrocatalysis studies

Strong Metal–Support Interactions of TiN– and TiO₂–Nickel Nanocomposite Catalysts

This study investigates the electronic configuration of titanium nitride-nickel (TiN–Ni) nanocomposites, in order to explain the high stability and activity of this hydrogenolysis catalyst. TiN–Ni is compared to a titanium oxide-nickel reference (TiO₂–Ni). Strong metal–support interactions are observed between the TiN and Ni. Scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) illustrate that the Ni distributes more homogeneously on the nitride support. Computational comparison of TiN–Ni and TiO₂–Ni provides evidence of preferential Ni adsorption onto nitrogen sites of the nitride support. DFT calculations also predict a charge polarization between Ti and Ni atoms. X-ray photoelectron spectroscopy (XPS) corroborates computational analysis by revealing a suppression of surface nitride species upon deposition of Ni onto TiN. Shifts in Ti 2p and Ni 2p binding energy positions are also evident, which indicate an electronic perturbation between TiN and Ni. We conclude that the presence of nitrogen in the nitride support influences the electronic and structural properties of TiN–Ni and is partly responsible for the beneficial catalytic properties reported for this nanocomposite

Effects of Graphitic and Pyridinic Nitrogen Defects on Transition Metal Nucleation and Nanoparticle Formation on N‑Doped Carbon Supports: Implications for Catalysis

Functionalization of carbon supports with heteroatom dopants is now widely regarded as a promising route for stabilizing and strengthening the interactions between the support and metal catalysts. Tuning the type and density of heteroatom dopants allows for the tailoring of nanoscale catalyst–support interactions; however, an understanding of these phenomena has not yet been fully realized because of the complexity of the system. In this work, computational modeling, materials synthesis, and advanced nanomaterial characterization are used to systematically investigate the intriguing effect of the two most common nitrogen functionalities in the carbon-based supports on the interactions with selected transition metals toward realizing catalytic applications. Specifically, this study utilized density functional theory to evaluate adsorption energies and modes of adsorption for 12 metals located in groups 8–11 and periods 4–6 with pyridinic and graphitic N defects. Based on these results, further electronic structure investigation of the period 4 metals was conducted to elucidate periodic group trends. Experimental work included synthesis and nanomaterial characterization of a subset of materials featuring three metals each supported on two types of N-doped carbon supports and undoped graphene. Characterization of nanomaterials with scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed that N functionalities enhanced the interactions with the selected transition metals when compared to the undoped support and demonstrated that the nature of the defect influences these interactions. Both computations and experiments agreed that Fe and Co are biased toward the graphitic sites over pyridinic sites, while Ni has an affinity to both defects without a statistically significant preference. This work established a correlation between computational and experimental work and a framework that can be expanded to other metals and alternative dopants beyond nitrogen in tailoring nanoscale catalyst–support interactions for a breadth of catalytic applications

Bandgap Tuning of Silicon Quantum Dots by Surface Functionalization with Conjugated Organic Groups

The quantum confinement and enhanced optical properties of silicon quantum dots (SiQDs) make them attractive as an inexpensive and nontoxic material for a variety of applications such as light emitting technologies (lighting, displays, sensors) and photovoltaics. However, experimental demonstration of these properties and practical application into optoelectronic devices have been limited as SiQDs are generally passivated with covalently bound insulating alkyl chains that limit charge transport. In this work, we show that strategically designed triphenylamine-based surface ligands covalently bonded to the SiQD surface using conjugated vinyl connectivity results in a 70 nm red-shifted photoluminescence relative to their decyl-capped control counterparts. This suggests that electron density from the SiQD is delocalized into the surface ligands to effectively create a larger hybrid QD with possible macroscopic charge transport properties

Platinum-Coated Nickel Nanowires as Oxygen-Reducing Electrocatalysts

Platinum (Pt)-coated nickel (Ni) nanowires (PtNiNWs) are synthesized by the partial spontaneous galvanic displacement of NiNWs, with a diameter of 150–250 nm and a length of 100–200 μm. PtNiNWs are electrochemically characterized for oxygen reduction (ORR) in rotating disk electrode half-cells with an acidic electrolyte and compared to carbon-supported Pt (Pt/HSC) and a polycrystalline Pt electrode. Like other extended surface catalysts, the nanowire morphology yields significant gains in ORR specific activity compared to Pt/HSC. Unlike other extended surface approaches, the resultant materials have yielded exceptionally high surface areas, greater than 90 m² g_Pt^–1. These studies have found that reducing the level of Pt displacement increases Pt surface area and ORR mass activity. PtNiNWs produce a peak mass activity of 917 mA mg_Pt^–1, 3.0 times greater than Pt/HSC and 2.1 times greater than the U.S. Department of Energy target for proton-exchange membrane fuel cell activity