Article pubs.acs.org/cm
Oxidant-Activated Reactions of Nucleophiles with Silicon Nanocrystals Mita Dasog,*,†,‡ Jonathan R. Thompson,§ and Nathan S. Lewis*,†,∥,⊥ †
Division of Chemistry and Chemical Engineering, California Institute of Technology, 210 Noyes Laboratory, Pasadena, California 91125, United States ‡ Department of Chemistry, Dalhousie University, Halifax, NS, Canada B3H 4R2 § Division of Engineering and Applied Sciences, California Institute of Technology, Pasadena, California 91125, United States ∥ Kavli Nanoscience Institute, California Institute of Technology, Pasadena, California 91125, United States ⊥ Beckman Institute, California Institute of Technology, Pasadena, California 91125, United States S Supporting Information *
ABSTRACT: The oxidant-activated reactivity of Si toward nucleophiles was evaluated for Si nanocrystals (Si-NCs) of differing diameters, d. In the presence of ferrocenium as a oneelectron, outer-sphere oxidant, d ≥ 8 nm Si-NCs readily reacted with nucleophiles, including methanol, butanol, butylamine, butanoic acid, butylthiol, and diethylphosphine. However, d < 8 nm Si-NCs did not undergo such reactions, and stronger oxidants such as acetylferrocenium or 1,1′-diacetylferrocenium were required. Butylamine-, butylthiol-, and butanol-functionalized d ≥ 8 nm Si-NCs were partially oxidized and exhibited photoluminescence originating from defect states. In contrast, butanoic acid-functionalized Si-NCs were minimally oxidized and displayed core emission resulting from the excitation and relaxation of electrons across the Si-NC bandgap. Diethylphosphine-functionalized Si-NCs were stable only under inert conditions and showed core emission, with the Si−P bonds being highly susceptible to oxidation and rapidly decomposing upon exposure to ambient conditions. The general reactivity is consistent with the redox potential of the one-electron oxidant and the valence band edge position of the Si-NCs. The trends in reactivity thus provide an example of differential chemical reactions of nanoparticles relative to bulk materials, reflecting the differences in electronic structure and the continuum of electronic properties between variously sized Si nanoparticles and bulk Si samples.
1. INTRODUCTION Quantum dots are promising optoelectronic materials, because of their tunable optical emission and absorption properties, multiexciton generation, high emission quantum yields, and possibilities for inexpensive and scalable processing.1−3 Silicon nanocrystals (Si-NCs) are of special interest because of the earth abundance and nontoxicity of Si as well as the compatibility of silicon (Si) with the existing semiconductor industry.4,5 Modification of Si-NCs with organic functional groups minimizes surface oxidation, renders the particles dispersible, and adds desired functionality and solubility.6 The most common method of surface modification involves the formation of Si−C bonds via hydrosilylation reactions in which terminal alkenes and alkynes are added to hydrogen-terminated silicon (Si) surfaces.7 Alternative linkages to Si-NCs, such as SiX (X = O or S),8−11 Si−N,12−15 and Si−Y (Y = Cl, Br, or I)16,17 bonds, have also been developed. The nature of the Si linkage influences the electron transport, charge transfer, electronic structure, and photoluminescence properties of the nanocrystals.12,18−22 © 2017 American Chemical Society
Si-NCs have generally been functionalized with Si−O bonds by reacting halide-terminated nanoparticles with alcohols or silanols.15,23−25 Hydrogen-terminated Si surfaces also react with alcohols, albeit at a slow rate.26,27 Ketones react with hydrogenterminated Si-NCs to yield alkoxy-functionalized surfaces.28 Aldehydes react with bulk or porous Si−H surfaces,29,30 but not with Si-NCs. Surface functionalization of Si/SiO2 core−shell nanoparticles with glutaric acid was achieved in situ during the synthesis of Si-NCs,31 where the carboxylate group was bound to the Si surface in a chelating fashion. Si-NCs that have Si−O linkages exhibit photoluminescence (PL) that is heavily influenced by oxygen defect states.32−37 SiO2 defects typically result in both red and blue emission, whereas Si suboxide defects lead to orange-red PL.12 Si−N linkages can be made either by the direct reaction of Si−H with amines to yield hydrogen as the byproduct or by the Received: June 22, 2017 Revised: June 30, 2017 Published: July 11, 2017 7002
DOI: 10.1021/acs.chemmater.7b02572 Chem. Mater. 2017, 29, 7002−7013
Article
Chemistry of Materials Scheme 1. Oxidant-Activated Reactivity of Nucleophiles with Hydrogen-Terminated Si-NCs
reaction of halide-terminated Si surfaces with amines.12−14,38 The nitrogen-tethered Si-NCs usually exhibit blue PL from oxynitride defect states.39 Thiol-capped Si-NCs have been prepared via high-temperature thiolation of the hydrogenterminated Si surface.9 Phosphine (PH3) and diphosphine (P 2 H 4 ) gas adsorption and decomposition have been investigated on bulk Si surfaces,40−42 whereas reactions of alkylphosphines with Si surfaces or formation of Si−P linkages in Si-NCs has yet to be documented. Many of the reported methodologies for the formation of non-carbon Si linkages involve a halide intermediate that is difficult to produce in a nanocrystalline system given its propensity to etch the nanoparticles.16 Direct reaction of Si−H surfaces with the desired ligands can be slow and/or lead to subsurface oxidation and quenching of the nanocrystal PL.43−45 In contrast, alcohols readily (
