Ligand Adsorption/Desorption on Sterically Stabilized InP Colloidal Nanocrystals: Observation and Thermodynamic Analysis

Abstract

Colloidal semiconductor nanocrystals are widely studied in nanoscience and nanotechnology, since they implement almost perfectly the concept of a quantum dot. Even at room temperature, they have truly discrete energy levels, providing them with unique electrical and optical properties.[1] Typically, colloidal quantum dots come with an organic layer of ligands. These ligands play a key role during nanocrystal synthesis, as they form an adsorbed monolayer (capping) that slows down nanocrystal growth and stabilizes the nanocrystals in suspension.[2] In addition, the capping determines the properties and the processing possibilities of colloidal nanocrystals. For example, the improvement of photoluminescence efficiency, [3][4][5] chemical reactivity, [6] the formation of two-dimensional and three-dimensional aggregates, [7,8] and the attachment of quantum dots to surfaces [6,[9][10][11] all involve the appropriate engineering of the ligands.A number of literature reports demonstrate that solution 1 H NMR spectroscopy is a suitable technique for studying the capping of colloidal metal and semiconductor nanocrystals in situ, providing chemical and structural information on the ligands and on ligand exchange. [12][13][14][15][16][17] Herein, we present the results of a quantitative one-dimensional 1 H NMR study on suspensions of InP nanocrystals with trioctylphosphine oxide (TOPO) ligands. It is shown that the ligands are involved in an adsorption/desorption equilibrium, and that the adsorption isotherm can be obtained from the NMR data. In the case of TOPO adsorption at InP nanocrystals, the adsorption isotherm fits a Fowler isotherm, an extension of the Langmuir isotherm incorporating adsorbate-adsorbate interactions. [18,19] This fitting leads to a set of parameters characterizing the adsorption equilibrium, most notably, the equilibrium constant, the adsorbate density at full coverage, and the free energy of the adsorbate-adsorbate interaction. This work indicates that solution NMR spectroscopy not only provides an identification of the ligands, but may also be used to obtain quantitative, thermodynamic data on ligand adsorption. Figure 1 shows the aliphatic part of the 1 H NMR spectrum of a suspension of trioctylphosphine (TOP)-TOPO capped InP nanocrystals in [D 8 ] toluene (see Methods). As TOP and TOPO cannot be distinguished by 1 H NMR spectroscopy, we will only mention TOPO when discussing the capping molecules. The distinct resonances were attributed previously to free (N) and adsorbed TOPO (*).[17] For free TOPO, the end methyl group is found at 0.92 ppm; for adsorbed TOPO, it yields a broad resonance centered at 1.05 ppm. Both resonances are well resolved, and are integrated reliably by decomposing the spectrum into separate contributions (see Figure 1, inset). In this way, the concentrations of free and adsorbed TOPO can be determined quantitatively (see Methods). Figure 2 shows the con-

Solution NMR techniques for investigating colloidal nanocrystal ligands: A case study on trioctylphosphine oxide at InP quantum dots

“…In that case, the adsorption isotherm can be described by the Fowler-isotherm, which takes both adsorbate-substrate and adsorbate-adsorbate interactions into account [20,21]:…”

Nuclear Magnetic Resonance Spectroscopy Demonstrating Dynamic Stabilization of CdSe Quantum Dots by Alkylamines

“…In the case of ligands forming a dative or dipolar bond, adsorption isotherms and binding energies can be determined under these conditions, as was shown for Q-InP/TOPO. 19 …”

Surface Chemistry of Colloidal Semiconductor Nanocrystals: Organic, Inorganic, and Hybrid

“…7.11b) [76]. As no TOPO is added, this means that dilution leads to a transfer of bound ligands to the pool of free TOPO.…”

“…In the particular case of NC-InP/TOPO, this relation is best described using a Fowler isotherm, which takes into account both the free energy of adsorption of TOPO on the InP surface and the mutual interaction between bound TOPO ligands, yielding an overall adsorption free energy of 31 kJ mol -1 [76]. Alternatively, Donakowski et al determined the adsorption isotherm of para-substituted aniline ligands on CdSe NCs-which exhibit fast exchange behavior-by interpreting experimental chemical shifts as weighted averages of a bound and a free value [75].…”

“…This can been exploited to determine adsorption isotherms [76], yet it can also be used to determine the equilibrium in a true ligand exchange process where, for example, an acidic ligand (AH) could replace an X-type bound moiety (B) by proton transfer:…”