The synthesis of refractory materials usually relies on high-temperature conditions to drive diffusion-limited solid-state reactions. These reactions result in thermodynamically stable products that are rarely amenable to low-temperature topochemical transformations that postsynthetically modify subtle structural features. Here, we show that topochemical deintercalation of Al from MoAlB single crystals, achieved by room-temperature reaction with NaOH, occurs in a stepwise manner to produce several metastable Mo-Al-B intergrowth phases and a two-dimensional MoB (MBene) monolayer, which is a boride analogue to graphene-like MXene carbides and nitrides. A high-resolution microscopic investigation reveals that stacking faults form in MoAlB as Al is deintercalated and that the stacking fault density increases as more Al is removed. Within nanoscale regions containing high densities of stacking faults, four previously unreported Mo-Al-B (MAB) intergrowth phases were identified, including MoAlB, MoAlB, MoAlB, and MoAlB. One of these deintercalation products, MoAlB, is identified as the likely MAB-phase precursor that is needed to achieve a high-yield synthesis of two-dimensional MoB, a highly targeted two-dimensional MBene. Microscopic evidence of an isolated MoB monolayer is shown, demonstrating the feasibility of using room-temperature metastable-phase engineering and deintercalation to access two-dimensional MBenes.

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The rational synthesis of metastable inorganic solids, which is a grand challenge in solid-state chemistry, requires the development of kinetically controlled reaction pathways. Topotactic strategies can achieve this goal by chemically modifying reactive components of a parent structure under mild conditions to produce a closely related analogue that has otherwise inaccessible structures and/or compositions. Refractory materials, such as transition metal borides, are difficult to structurally manipulate at low temperatures because they generally are chemically inert and held together by strong covalent bonds. Here, we report a multistep low-temperature topotactic pathway to bulk-scale Mo2AlB2, which is a metastable phase that has been predicted to be the precursor needed to access a synthetically elusive family of 2-D metal boride (MBene) nanosheets. Room-temperature chemical deintercalation of Al from the stable compound MoAlB (synthesized as a bulk powder at 1400 °C) formed highly strained and destabilized MoAl1–x B, which was size-selectively precipitated to isolate the most reactive submicron grains and then annealed at 600 °C to deintercalate additional Al and crystallize Mo2AlB2. Further heating resulted in topotactic decomposition into bulk-scale Mo2AlB2–AlO x nanolaminates that contain Mo2AlB2 nanosheets with thickness of 1–3 nm interleaved by 1–3 nm of amorphous aluminum oxide. The combination of chemical destabilization, size-selective precipitation, and low-temperature annealing provides a potentially generalizable kinetic pathway to metastable variants of refractory compounds, including bulk Mo2AlB2 and Mo2AlB2–AlO x nanosheet heterostructures, and opens the door to other previously elusive 2-D materials such as 2-D MoB (MBene).

Colloidal hybrid nanoparticles integrate two or more nanocrystal domains into a single architecture that can have properties not found in, or enhanced relative to those of, the individual components. These hybrid nanomaterials are typically constructed using multistep seeded-growth reaction sequences, which are conceptually analogous to the total synthesis approaches used in molecular synthesis. Here, we discuss in detail the synthetic protocols that lead to the formation of three-component Ag−Pt−Fe 3 O 4 and Au−Pt−Fe 3 O 4 heterotrimers. These instructive model systems highlight the important synthetic details that underpin successful hybrid nanoparticle reactions. We provide detailed, step-by-step protocols for generating these materials, focusing on describing and rationalizing the key reaction parameters that need to be rigorously controlled to minimize unwanted nanoparticle byproducts. The importance of comprehensive analysis using a suite of materials characterization tools is highlighted, as such efforts are useful for diagnosing subtle chemical and morphological features that can lead to synthetic bottlenecks throughout the course of the reaction sequences. Finally, we offer strategies for circumventing these commonly encountered problems as well as insights that can lead to increased hybrid nanoparticle yields and improved sample-to-sample reproducibility. Although this work specifically details the synthesis of Ag−Pt−Fe 3 O 4 and Au−Pt−Fe 3 O 4 heterotrimers, these synthetic strategies and protocol guidelines are generally applicable to many other hybrid nanoparticle systems.

Cation exchange reactions modify the composition of a nanocrystal while retaining other features, including the crystal structure and morphology. In many cases, the anion sublattice is considered to be locked in place as cations rapidly shuttle in and out. Here we provide evidence that the anion sublattice can shift significantly during nanocrystal cation exchange reactions. When the Cu + cations of roxbyite Cu 1.8 S nanorods exchange with Zn 2+ to form ZnS nanorods, a high density of stacking faults emerges. During cation exchange, the stacking sequence of the close-packed anion sublattice shifts at many locations to generate a nanorod product containing a mixture of wurtzite, zincblende, and a wurtzite/zincblende polytype that contains an ordered arrangement of stacking faults. The reagent concentration and reaction temperature, which control the cation exchange rate, serve as synthetic levers that can tune the stacking fault density from high to low, which is important because once introduced, the stacking faults could not be modified through thermal annealing. This level of synthetic control through nanocrystal cation exchange is important for controlling properties that depend on the presence and density of stacking faults.

Cooperative catalysis enables synthetic transformations that are not feasible using monocatalytic systems.Such reactions are often diffusion controlled and require multiple catalyst interactions at high dilution. We developed a confined dual-catalytic polymer nanoreactor that enforces catalyst co-localization to enhance reactivity in a fully-homogeneous system. The photocatalyzed-dimerization of substituted styrenes is disclosed using confined-single-chain polymers bearing triarylpyrylium-based pendants, with pyrene as an electron relay catalyst. Enhanced reactivity with low catalyst loadings was observed compared to monocatalytic polymers with small-molecule additives. Our approach realizes a dual-catalytic single-chain polymer that provides enhanced reactivity under confinement, presenting a further approach for diffusion-limited-photoredox catalysis. File list (2) download file view on ChemRxiv Elacqua_dimerization_final.pdf (2.07 MiB) download file view on ChemRxiv Elacqua_Dimerization SI_final.pdf (2.27 MiB)

Single-chain polymer nanoparticles (SCNPs) are emerging as versatile catalytic platforms that provide excellent control over solubility. The confined nature of SCNPs can improve the rate of catalysis. While significant headway has been made in thermally-induced transition-metal catalysis with SCNPs, lightactivated SCNP catalysts have received little attention. We are developing triarylpyrylium tetrafluoroborate (TPT)-functionalized SCNPs as oxidative photocatalysts. Herein, we comprehensively study the impact of light source on both SCNP compaction and TPT absorbance through gel-permeation chromatography and UV/Vis spectroscopy. We observe that compaction is expedited using light sources that excite the photocatalyst (e.g., blue LEDs), which is attributed to the ability of TPT to dimerize sytrenics under similar photoredox conditions. The resultant metal-free SCNP photocatalysts enable the oxidation of benzyl alcohols in good yields. The SCNP is further investigated for the amidation of 4-bromobenzaldehyde, wherein it affords higher yields of the benzamide product compared to both small-molecule and unfolded polymer controls. We attribute the combined results to the colocalization of the TPT photoredox catalyst and pyrene electron relay within the SCNP, which likely aids in single-electron transfer processes. The scope of amidation reactions was also extended to other aryl aldehydes, wherein deactivated substrates afforded the highest yield of the desired amide.

Cation exchange is an increasingly common pathway for nanoparticle synthesis, as it modifies the composition in a controllable way while maintaining other key features, including crystal structure and morphology. However, cation exchange pathways can compete with other nanoparticle formation pathways, depending on the system and reaction conditions. Simple strategies for monitoring such reactions can therefore be informative. Here, we use benchtop light scattering with a laser pointer as a simple tool to monitor putative cation exchange reactions and to help differentiate, in real time, between pathways that involve cation exchange versus pathways that involve dissolution and reprecipitation. We use the transformation of digenite copper sulfide into manganese sulfide as a model system. When a laser pointer shines through the reaction flask as digenite copper sulfide nanoparticles react with Mn 2+ at 100 °C, light scattering is observed continuously, indicating that nanoparticles are present during the entire reaction as would be required for a cation exchange pathway. At higher temperatures, light scattering disappears and then reappears, indicating that nanoparticles are not always present and that a different pathway involving dissolution and reprecipitation is operable. Using this approach, along with additional control experiments, we were able to identify the threshold temperature below which zincblende MnS, a metastable polymorph, forms through a cation exchange pathway. We were also able to establish that at higher temperatures, the thermodynamically favored product, rocksalt MnS, forms through a dissolution/reprecipitation pathway. These results provide useful insights into the temperature dependence of a model cation exchange reaction and suggest that light scattering could provide highlevel insights, in real time on the benchtop, into nanoparticle reaction pathways that involve postsynthetic modifications where multiple competing pathways could be possible.