Synthetic efforts to prepare indium phosphide (InP) quantum dots (QDs) have historically generated emissive materials with lower than unity quantum yields. This property has been attributed to structural and electronic defects associated with the InP core as well as the chemistry of the shell materials used to overcoat and passivate the InP surface. Consequently, the uniformity of the core-shell interface plays a critical role. Using X-ray emission spectroscopy (XES) performed with a recently developed benchtop spectrometer, we studied the evolution of oxidized phosphorus species arising across a series of common, but chemically distinct, synthetic methods for InP QD particle growth and subsequent ZnE (E = S or Se) shell deposition. XES afforded us the ability to measure the speciation of phosphorus reliably, quantitatively, and more efficiently (with respect to both the quantity of material required and the speed of the measurement) than with traditional techniques, i.e., X-ray photoelectron spectroscopy and magic angle spinning solid state nuclear magnetic resonance spectroscopy. Our findings indicate that even with deliberate care to prevent phosphorus oxidation during InP core synthesis, typical shelling approaches unintentionally introduce oxidative defects at the core-shell interface, limiting the attainable photoluminescence quantum yields. Disciplines

We have broadened the scope of the aminophosphine precursor chemistry that has been developed for InP quantum dots to the synthesis of cadmium, zinc, cobalt, and nickel phosphide nanocrystals. The generalized synthetic conditions involve thermolysis of the appropriate MX 2 salt with tris-diethylaminophosphine in a long chain primary amine. The resulting Cd 3 P 2 nanocrystals exhibit size tuning effects based on the metal halide reactivity. 31 P NMR studies show that the II-V materials form via the previously described mechanism observed for InP, demonstrating the invariance of this chemistry to the metal valence. We also demonstrate that electrocatalytically active transition metal phosphides, specifically Co 2 P, CoP, and Ni 2 P, can be produced using this synthetic method at relatively mild temperatures and in high yields.

Cobalt phosphide (CoP) is one of the most promising earthabundant replacements for noble metal catalysts for the hydrogen evolution reaction (HER). Critical to HER is the binding of H atoms. While theoretical studies have computed preferred sites and energetics of hydrogen bound to transition metal phosphide surfaces, direct experimental studies are scarce. Herein, we describe measurements of stoichiometry and thermochemistry for hydrogen bound to CoP. We studied both mesoscale CoP particles, exhibiting phosphide surfaces after an acidic pretreatment, and colloidal CoP nanoparticles. Treatment with H 2 introduced large amounts of reactive hydrogen to CoP, ca. 0.2 H per CoP unit, and on the order of one H per Co or P surface atom. This was quantified using alkyne hydrogenation and Hatom transfer reactions with phenoxy radicals. Reactive H atoms were even present on the as-prepared materials. On the basis of the reactivity of CoP with various molecular hydrogen donating and accepting reagents, the distribution of binding free energies for H atoms on CoP was estimated to be roughly 51−66 kcal mol −1 (ΔG°H ≅ 0 to −0.7 eV vs H 2 ). Operando X-ray absorption spectroscopy gave preliminary indications about the structure of hydrogenated CoP, showing a slight lattice expansion and no significant change of the effective nuclear charge of Co under H 2flow. These results provide a new picture of catalytically active CoP, with a substantial amount of reactive H atoms. This is likely of fundamental relevance for its catalytic and electrocatalytic properties. Additionally, the approach developed here provides a roadmap to examine hydrogen on other materials.

Aminophosphine precursors were used to synthesize copper-doped indium phosphide nanocrystals (InP NCs) via direct doping in a slow-injection bottom-up method and postsynthetic cation exchange. By both methods, the amount of copper incorporated into the NCs could be tuned simply by varying the molar ratio during synthesis. Common postsynthetic surface modifications such as Lewis acid treatment and zinc chalcogenide shelling were performed on these samples, resulting in an enhancement of the copper-based emission from 10% to 40%. For samples with thick shells, the copper-based photoluminescence quantum yield reached over 60%, a record value for doped InP NCs. Time-resolved photoluminescence spectroscopy showed increasing carrier lifetimes after surface treatments concurrent with the disappearance of a 2 ns decay process previously attributed to surface trapping in native InP NCs, showing the broad applicability and consistent impacts of the surface treatments. In this way, we have successfully developed a route to obtain high-quality near-infrared emitters utilizing less toxic alternatives to the popular lead-and cadmium-containing materials.