Colloidal synthesis and photophysical characterization of silicon-compatible Ge1−xSnx alloy quantum dots with composition-tunable near-infrared absorption and photoluminescence is reported.

Tin phosphides make up a class of materials that have received a noteworthy amount of interest in photocatalysis, charge storage, and thermoelectric devices. Dual stable oxidation states of tin (Sn2+ and Sn4+) allow tin phosphides to exhibit different stoichiometries and crystal phases. However, the synthesis of such nanostructures with control over morphology and crystal structure has proven to be a challenging task. Herein, we report the first colloidal synthesis of size-, shape-, and phase-controlled, narrowly disperse rhombohedral Sn4P3, hexagonal SnP, and trigonal Sn3P4 nanoparticles (NPs) displaying tunable morphologies and size-dependent physical properties. The control over NP morphology and crystal phase was achieved by tuning the nucleation/growth temperature, Sn/P molar ratio, and incorporation of additional coordinating solvents (alkylphosphines). The absorption spectra of Sn3P4 NPs (3.0 ± 0.4 to 8.6 ± 1.8 nm) exhibit size-dependent blue shifts in energy gaps (1.38–0.88 eV) compared to the theoretical value of bulk Sn3P4 (0.83 eV), consistent with quantum confinement effects. The trigonal Sn3P4 NPs adopt rhombohedral Sn4P3 and hexagonal SnP crystal structures at 180 and 250 °C, respectively. Structural and surface analysis indicates consistent bond energies for phosphorus across different crystal phases, whereas the rhombohedral Sn4P3 NPs demonstrate Sn oxidation states distinctive from those of the hexagonal and trigonal phases because of the complex chemical structure. All phases exhibit N­(1s) and ν(N–H) energies suggestive of alkylamine surface functionalization and are devoid of tetragonal Sn impurities.

Electrocatalytic water splitting presents an exciting opportunity to produce environmentally benign fuel to power human activities and reduce reliance on fossil fuels. Transition metal nanoparticles (NPs) and their alloys are emerging as promising candidates to replace expensive platinum group metal (PGM) catalysts. Herein, we report the synthesis of distinct crystal phases and compositions of Ni1–x Mo x alloy NPs as low-cost, earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) in alkaline medium. Phase-pure cubic and hexagonal Ni and Ni1–x Mo x alloy NPs, with sizes ranging from 18 to 43 nm and varying Mo composition (∼0–11.4%), were produced by a low-temperature colloidal chemistry method. As-synthesized NPs show spherical to polygonal morphologies and a systematic shifting of Bragg reflections to lower 2θ angles with increasing Mo, suggesting the growth of homogeneous alloys. XPS analysis indicates the dominance of metallic Ni(0) and Mo(0) species in the core of the alloy NPs as well as the presence of higher valent Ni n+ and Mo n+ surface species, stabilized by surfactant ligands. The cubic alloys exhibit significantly higher HER activity in comparison to the hexagonal alloys. For a current density of −10 mA/cm2, the cubic alloys demonstrate overpotentials of −62 to −177 mV compared to −162 to −242 mV for the hexagonal alloys. The overpotentials of cubic alloys are comparable to the commercial Pt-based electrocatalysts for which the overpotentials range from −68 to −129 mV at −10 mA/cm2. In general, a decrease in overpotential and an increase in HER activity were observed with increasing concentration of Mo (up to 6.6%) for the cubic alloys. The cubic Ni0.934Mo0.066 alloy NPs exhibit the highest activity as alkaline HER electrocatalysts.

Using hybrid functional calculations and experimental characterization, we analyze optical properties of 2–3 nm Ge1–x Sn x alloy quantum dots, synthesized by colloidal chemistry methods. Hybrid functional theory, tuned to yield experimental bulk band structure of germanium, reproduces directly measured properties of Ge1–x Sn x quantum dots, such as lattice constants, energy gaps, and absorption spectra. Time-dependent hybrid functional calculations yield optical absorption in good agreement with experiments, and allow probing the nature of the dark excitons in quantum dots. Calculations suggest a spin-forbidden dark exciton ground state, which is supported by the changes in the photoluminescence lifetimes with temperature and tin concentrations. The synthesis and theoretical understanding of Ge1–x Sn x alloy quantum dots will add to the overall toolbox of low to nontoxic, silicon-compatible group IV semiconductors with potential application in visible to near-infrared optoelectronics.

Narrowly disperse rhombohedral Sn4P3 (I), hexagonal SnP (II), and trigonal Sn3P4 (III) nanoparticles (NPs) are prepared by size‐, shape‐, and phase‐controlled colloidal synthesis.

Facile synthesis of NaCl type cubic SnAs with narrow dispersity and tunable nanocrystals using Sn(iv) iodide and aminoarsene as precursors in an alkylamine system at 250 °C.

Correction for ‘Facile synthesis of size-tunable tin arsenide nanocrystals’ by Venkatesham Tallapally et al., Chem. Commun., 2019, DOI: 10.1039/c8cc08101h.

To provide a sustainable carbon free energy system while moving away from fossil fuels and greenhouse gas (CO2) generating energy sources, an inexpensive and reliable process to produce hydrogen has to be implemented. Water splitting by electrolysis has been thought of as the ideal way to produce hydrogen but the current electrolysis of water requires energy or rare materials that make the process neither carbon independent or feasible due to the amount energy gained vs energy input. In this talk there are two methods to perform the electrolysis of water: a) electrocatalytic water-splitting and b) solar photo-electrochemical water-splitting. Recently, transition metal phosphides (TMP) have shown promising catalytic properties for the water splitting half reactions. Two key aspects to studying TMPs for the electrolysis of water are 1) to generate hydrogen with as little energy input as possible; meaning using materials that can harness solar energy or that have significantly low overpotentials for each water splitting half reactions and 2) to find competitive materials to rival the performance and stability of platinum and other noble metals so that industrial production and manufacturing may be achieved. This talk will focus on binary and ternary transition metal phosphides as electocatalysts and photocatalysts for the hydrogen and oxygen evolution reactions; transition metals of focus primarily include Nickel, Molybdenum, and Cobalt with Platinum and Titanium as reference metals. Phosphorus (P) atoms have been determined to stabilize the TMP surface structure and thus the concentration of surface P is a key factor in controlling the surface activity. To control size, shape and crystallinity, a colloidal synthesis method was used to synthesize the nano particles. MoxCo2-xP and Ni2-xMoxP ternary TMPs were synthesized owing to their synergistic effects towards electrocatalysis. These two specific ternary phosphides are being considered for the proposed work influenced by commercial hydrotreating ternary catalyst system (sulfided Co-Mo and Ni-Mo). Nickel phosphide is of primary interest resulting from its consistent performance over other TMPs as an electrocatalyst for water splitting. As electrocatalysts, TMPs show low overpotentials and good stability for the hydrogen-evolution reaction (HER) and oxygen-evolution reaction (OER) under relevant current densities (10 mA/cm2) without any sacrificial agent present. The particles show high stability under most conditions, specifically in acidic electrolytes. In our group, work has been done to synthesize, characterize and test TMP nanoparticles as electrocatalysts and photocatalysts using PEC methods for the HER and OER to understand the material’s electrochemical behavior. Electrode preparation methods may influence the behavior, stability and reproducibility of the electrode. Therefore, multiple preparation methods were conducted to determine the true performance of the material. FTO on glass and Titanium foil were used as substrates/current collectors for the catalytic nano-particles. The electrocatalytic performance of the binary and ternary TMPs have been evaluated as well as the photocatalytic performance. The photocatalytic testing was evaluated under 1 SUN (100mW/cm2) illumination from a 150 W Xe Lamp with and without an AM1.5 filter. The photocatalytic performance was evaluated for the materials tuned to have more semiconductor characteristics and that show signs of photovoltage. Various ratios of metal to phosphide and metal to metal alloys were examined to study the effect of the electrocatalytic and photocatalytic performance and the material’s stability.