Structure, Morphology and Composition-Property Elucidation of Ni–Mo and Ni−Mo−P Nanocrystals for Water Splitting Reactions and Group IV Alloy and Silicate Nanocrystals for Visible to Near IR Optoelectronics

Author

Ebtesam Hassan Abuelenein EladghamFollow

DOI

https://doi.org/10.25772/GWK8-GE73

Author ORCID Identifier

https://orcid.org/0000-0001-9556-4252

Defense Date

2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

Dr. Indika U. Arachchige

Abstract

Abstract

Structure, Morphology and Composition-Property Elucidation of Ni–Mo and Ni−Mo−P Nanocrystals for Water Splitting Reactions and Group IV Alloy and Silicate Nanocrystals for Visible to Near IR Optoelectronics

by

Ebtesam Hassan Abuelenein Eladgham

A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Virginia Commonwealth University.

Advisor: Indika U Arachchige

Associate Professor, Department of Chemistry

Nanomaterials have gained a great attention because of their unique physical properties with size intermediate between molecular materials and extended solids. They have a large surface to volume ratio and display size tunable physical properties that was not observed in bulk materials. Colloidal chemistry synthesis provides an efficient control on composition, size, and morphology along with the crystal structures of the nanomaterials. In this dissertation study, wet-colloidal routes for four different nanostructures (Ni-Mo, Ni-Mo-P, Ge-Si-Sn and Li-Si-O NPs) were developed to investigate their electrocatalytic activity for HER, optical and electronic properties.

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. In chapter 3, we report the synthesis of distinct crystal phases and compositions of Ni_1-xMo_x alloy NPs as low-cost, earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) in alkaline medium. Phase-pure cubic and hexagonal Ni_1-xMo_x alloy NPs, with sizes ranging from 18–25 nm and varying Mo composition (~1–9%), were produced by a low-temperature colloidal chemistry method. As-synthesized NPs show spherical to polyhedral morphologies and a systematic shifting of Bragg reflections to lower 2q 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 Niⁿ⁺ and Moⁿ⁺ 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 over-potentials of -65 to -177 mV compared to -162 to -242 mV for the hexagonal alloys. The over-potentials of cubic alloys are comparable to the commercial Pt-based electrocatalysts for which the over-potentials range from -68 to -129 mV at -10 mA/cm². In general, a decrease in over-potential and an increase in HER activity was observed with increasing concentration of Mo for the cubic alloys. The cubic Ni_0.935Mo_0.065 alloy NPs exhibit the highest activity as alkaline HER electrocatalysts.

Nanostructured binary and ternary transition metal phosphides (TMPs) are emerging as efficient catalysts to replace costly noble metals in water splitting applications. In chapter 4, two colloidal synthesis methods were developed to produce nickel molybdenum phosphide (Ni-Mo-P) nanoparticles (NPs) with two distinct crystal structures, as high-performing electrocatalysts for the HER. The one-pot synthesis produced smaller NPs, ranging from 4–11 nm, with a near spherical and homogeneous morphology. In contrast, the two-pot synthesis resulted in larger NPs, ranging from ~53–86 nm, with a heterogeneous polygonal morphology. Both nanostructures exhibited either hexagonal Ni_2-xMo_xP or tetragonal Ni_12-xMo_xP₅ crystal structures. Furthermore, the ternary NPs showed a shift of diffraction patterns to smaller 2q angles, consistent with an admixture of Mo atoms. The presence of partially charged core species (Ni^d+, Mo^d+, and P^d-), revealed by X-ray photoelectron spectroscopy (XPS), correlates to polarization of the metal-phosphorous bonds, induced by the successful formation of the ternary NPs. In addition, high valent Niⁿ⁺, Moⁿ⁺, and Pⁿ⁺ (where n > 2) charged species were observed and potentially correlate to surface atoms stabilized by residual ligands/advantageous oxides. Among heterogeneous and homogeneous NPs, the hexagonal Ni_2-xMo_xP NPs showed lower over-potentials than the tetragonal Ni_12-xMo_xP₅ NPs. The catalytic activity was determined to follow a Volmer-Heyrovsky mechanism for majority electrocatalysts with Tafel slopes of 54.8–100.6 mV/dec, where the highly active samples with a Mo content of ~4.2% exhibit a near Heyrovsky rate-determining step of ~49.5 mV/dec. A slight increase in over-potential of 4.15 mV was observed after continuously applying a constant current density (-10 mA/cm²) for 10 h. The hexagonal Ni_1.87Mo_0.13P showed low over-potentials of -96 and -101 mV at -10 mA/cm² for the heterogeneous and homogeneous NPs, respectively. Therefore, the hexagonal Ni_1.87Mo_0.13P show the highest activity as alkaline HER electrocatalysts and outperform both the binary Ni₂P NPs (-156 mV at -10 mA/cm²) and tetragonal Ni_12-xMo_xP NPs (-198 mV at -10 mA/cm²).

Low-cost, less-toxic, and abundantly produced Ge_1-x-ySi_xSn_y alloys are interesting class of direct band-gap semiconductors for a broad range of optoelectronic devices. Alloying of α-Sn with Si and Ge induces an indirect-to-direct band-gap crossover and increases the light absorptivity. However, the metallic character of Sn, narrows the band-gap (0.15−0.58 eV) and limits their use to far or mid IR spectral region. In order to shift the bandgaps to visible-to-near IR spectral region, decreasing the particles size to the confinement regime widens and blue shifts the energy gap. In chapter 5, we report the first colloidal synthesis of Ge_1-x-ySi_xSn_y alloy NPs with size in the range of ~20–80 nm and ~1–9 nm and wider range of Si (0–38%) and Sn (0–22.2%) compositions to probe the size and composition-dependent optical properties. The successful incorporation of Si and α-Sn into crystalline Ge have been confirmed by STEM-EDS and Raman spectroscopy, which suggests the homogeneous solid solution behavior of ternary NPs. The quantum confinement effects have shifted the optical properties to visible spectra and showed a tunability with size and composition. With decreasing the alloy QDs size (4.7−1.0 nm), the absorption onsets increase (1.96−2.47 eV) whereas the PL maxima blue shift (2.09−2.39 eV). The PL maxima also blue shift (2.15-2.31 eV) with increasing Si content (16-24 %) at fixed 4% Sn concentration. Time-resolved PL (TRPL) spectroscopy revealed nanosecond timescale decays at 15 K and 295 K, likely due to band-to-band optical transitions. Overall, controlling the elemental concentration and the size of Ge_1-x-ySi_xSn_y alloy QDs, a direct-gap can be potentially tuned across the visible-to-near IR spectrum window, that maximizes their use in optoelectronic applications.

Lithium silicates have received noteworthy interest as a class of materials with significant potential in lithium ion batteries, ionic conductors, optical waveguides and sensors, and efficient sorbents for CO₂ capture. In chapter 6, we report the optical properties, electronic structures, and surface characteristics of two distinct lithium silicate crystal phases using first-principles hybrid density functional theory (DFT) calculations and in-depth experimental characterization studies. Orthorhombic Li₂SiO₃ (space group Cmc21) and Li₂Si₂O₅ (space group Ccc2) nanoparticles (NPs) passivated with alkylamine and alkane surface functionalities were produced by reaction of SiI₄ with n-butyllithium in the presence of 1,2-hexadecanediol. As-synthesized nanostructures exhibit poor crystallinity, which upon annealing at 600 ºC adopt phase-pure orthorhombic crystal structures with spherical (Li₂SiO₃), polyhedral or rod-shaped (Li₂Si₂O₅) morphologies. Surface analysis of Li₂SiO₃ and Li₂Si₂O₅ NPs reveals distinct chemical states for Li⁺, Siⁿ⁺, and O^n-, consistent with their stoichiometry and higher binding energies for constituent elements in Li₂Si₂O₅ NPs. Hybrid functional calculations predict indirect and direct energy gaps of 7.79 and 7.80 eV for Li₂SiO₃ and Li₂Si₂O₅, respectively signifying the insulating nature of extended solids. Nonetheless, as-synthesized Li₂SiO₃ and Li₂Si₂O₅ NPs exhibit high intensity visible photoluminescence (quantum yields = 10−30%) with nanosecond timescale decays at 15 K and 295 K, which we attribute to radiative recombination from surface/interface traps. The facile synthesis reported here provides control over crystal structure and composition of nanostructured lithium silicates, which will potentially widen their applications as visible to IR transparent optical materials, chromophores, waveguides, sensors, and high surface area CO₂ sorbents.

Rights

© Ebtesam Eladgham

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

5-19-2020