J. Phys. Chem. C 2010, 114, 897–906
897
The Effect of a Common Purification Procedure on the Chemical Composition of the Surfaces of CdSe Quantum Dots Synthesized with Trioctylphosphine Oxide Adam J. Morris-Cohen, Martin D. Donakowski, Kathryn E. Knowles, and Emily A. Weiss* Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208-3113 ReceiVed: October 2, 2009; ReVised Manuscript ReceiVed: NoVember 25, 2009
This paper describes a quantitative analysis of the chemical composition of organic/inorganic interfaces of colloidal 3.1-nm CdSe quantum dots (QDs) synthesized with trioctylphosphine oxide (TOPO) as the coordinating solvent and purified by successive precipitations from a chloroform/methanol solvent/nonsolvent system. A combination of X-ray photoelectron spectroscopy, inductively coupled plasma-atomic emission spectroscopy, and NMR (both 1H and 31P) reveals that the only ligands that form a stable population on the surface of the QDs are X-type alkylphosphonate and carboxylate ligands. n-Octylphosphonate (OPA), a known impurity in technical-grade (90%) TOPO, and P′-P′-(di-n-octyl) pyrophosphonate (PPA), the self-condensation product of OPA, cover ∼84% of the atoms on the surface of the QDs, whereas few of the L-type (datively bound) ligands hexadecylamine (HDA), TOPO, and trioctylphosphine selenide (TOPSe) are present as bound ligands once the excess free surfactant is removed from the reaction mixture. Purified QDs synthesized in 99% TOPO (with no alkylphosphonates present) have no phosphorus-containing ligands on the surface. Despite the approximately constant surface coverage of phosphorus-containing ligands, the photoluminescence quantum yield of the solution of QDs steadily decreases during purification from ∼15% to less than 1%. Proton NMR analysis of the QD samples and photoluminescence spectra of QDs exposed to various concentrations of methanol suggest that this decrease is due to a combination of progressive loss of small amounts of HDA and adsorption of methanol to the surface of the QDs during purification. Introduction This paper describes the identification and quantitative characterization of organic ligands on the surfaces of colloidal CdSe quantum dots (QDs) synthesized with trioctylphosphine oxide (TOPO) as the coordinating solvent and separated from excess ligand by successive precipitations from a chloroform/ methanol solvent/nonsolvent mixture. This purification method is commonly used to isolate QD-organic complexes from excess surfactant present in the synthetic reaction mixture and after ligand exchange.1-3 Knowledge of the distribution of ligands in the QD-organic system after purification is critical for predicting and interpreting the structural, electronic, and optical properties of solutions and films of QDs; this distribution depends on the strength of the respective QD-ligand interactions and the relative solubility of the ligands in the solvent system. Murray and co-workers observed that CdSe QD-ligand systems contain reversibly and irreversibly bound ligands and that precipitation-based purification removes the reversibly bound ligands from the system.4 For CdSe QDs synthesized with technical grade (90%) TOPO, the nominal surfactantssTOPO, hexadecylamine (HDA), and trioctylphosphine (TOP)sare not the only ligands present in the reaction mixture; recent work has shown that 90% TOPO contains up to 10 different phosphorus-containing impurities.5 One of these impurities, n-octylphosphonic acid (OPA), selfcondenses during the high-temperature synthesis of the QDs to form P′-P′-(di-n-octyl)dihydrogen pyrophosphonic acid (PPA).6 Chart 1 shows the chemical structures of the ligands present in the QD-organic system. Phosphorus NMR studies indicate PPA and OPA are present on the surface of CdSe QDs and that * Corresponding author. Phone: 847-491-3095. Fax: 847-491-7713. E-mail:
[email protected].
CHART 1: Chemical Structures of Seven Ligands Present after the Synthesis of CdSe QDs Using TOPO, Either on the Surface of the QDs or in Solutiona
a The neutral L-type ligands are HDA, TOP, TOPSe, and TOPO, and the negatively charged X-type ligands are PPA, OPA, and stearate, shown here in their fully deprotonated form.
these multidentate alkylphosphonate ligands are bound more tightly than are TOPO, HDA, TOP, and TOPSe.6 These conclusions are supported by computational results that indicate that, even in their protonated form, alkylphosphonic acids such as OPA and PPA have higher energies of adsorption to nonpolar and cadmium-enriched surfaces of CdSe than do phosphines,
10.1021/jp909492w 2010 American Chemical Society Published on Web 12/28/2009
898
J. Phys. Chem. C, Vol. 114, No. 2, 2010
phosphine oxides, and amines.7-9 Owen, et al.10 formalized these results by separating the set of native ligands into X-type, negatively charged molecules (alkylphosphonates and carboxylates), and L-type, neutral molecules that coordinate by donating a lone pair of electrons to form a dative bond (TOPO, TOPSe, and HDA).11 The same work10 reports that only X-type ligands are bound to CdSe QDs synthesized with TOPO and that these ligands provide the negative charges necessary to balance Cd2+enriched CdSe surfaces.12 Here, we establish a methodology that uses the complementary techniques of inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray photoelectron spectroscopy (XPS), and NMR spectroscopy to both identify and quantify the organic ligands on the surfaces of Cd2+-enriched CdSe QDs. Using quantitative elemental analysis (ICP-AES) and the ability of XPS and 31P NMR to distinguish among various phosphoruscontaining ligands, we confirm previous assertions that X-type ligands, predominantly the alkylphosphonates formed from deprotonation of OPA and PPA, are the only stably bound ligands for CdSe QDs synthesized with the TOPO/TOP/HDA procedure13 and determine the population of these ligands that remain bound through several purification steps. In contrast, the total population of L-type ligandsspredominantly HDA, but also including TOPO and TOPSeson the surface of the QDs is
