Water molecules bonded to the carboxylate groups at the inorganic–organic interface of an inorganic nanocrystal coated with alkanoate ligands

Abstract High-quality colloidal nanocrystals are commonly synthesized in hydrocarbon solvents with alkanoates as the most common organic ligand. Water molecules with an approximately equal number of surface alkanoate ligands are identified at the inorganic–organic interface for all types of colloidal nanocrystals studied, and investigated quantitatively using CdSe nanocrystals as the model system. Carboxylate ligands are coordinated to the surface metal ions and the first monolayer of water molecules is found to bond to the carboxylate groups of alkanoate ligands through hydrogen bonds. Additional monolayer(s) of water molecules can further be adsorbed through hydrogen bonds to the first monolayer of water molecules. The nearly ideal environment for hydrogen bonding at the inorganic–organic interface of alkanoate-coated nanocrystals helps to rapidly and stably enrich the interface-bonded water molecules, most of which are difficult to remove through vacuum treatment, thermal annealing and chemical drying. The water-enriched structure of the inorganic–organic interface of high-quality colloidal nanocrystals must be taken into account in order to understand the synthesis, processing and properties of these novel materials.

Synthesis of ZnSe nanocrystals. The ZnSe QDs are synthesized according to literature. 3 The synthetic procedures were divided into two steps. Firstly, oleic acid (10 mmol) was neutralized with an equal molar quantity of tetramethylammonium hydroxide in methanol (10 mL). Zinc nitrate hexahydrate (5 mmol) dispersed in 5 mL of methanol was mixed with this solution under vigorous stirring. White waxy zinc oleate precipitate was formed and collected by filtration and dried under vacuum overnight. Secondly, zinc oleate (1 mmol) in 10 mL of ODE were loaded into a 50 mL three-neck flask. The mixture was heated to 290 o C and 1 mL Se-SUS (0.4 M) was injected swiftly into the hot solution. The reaction was allowed to proceed at this temperature for 15 min.
Reaction process was monitored by taking aliquots for UV-vis measurements.
Synthesis of InP QDs. The InP QDs were synthesized according to literature. 4 Synthesis of InP QDs were divided into two steps. In the first step, indium stearate (In(St)3) was prepared by heating 0.3 mmol of indium acetate and 0.9 mmol stearic acid at 150 ºC for 20 min. Argon was blowing continuously to remove acetic acid produced by the replacement reaction. Consequently, 0.4 mL (~0.9 mmol) of TOP was injected into the three-necked flask. Five minutes later, the mixture was allowed to cool down to 30 ºC by removing the heating mantle. Meanwhile, 0.48 mL of (TMS)3P in ODE solution (~0.15 mmol (TMS)3P) was injected rapidly into the flask, and the reaction was allowed to proceed for approximately 1 minute and obtain the InP clusters solution.
In the second step, In(St)3 (1 mmol) was prepared in the way described above in another flask. Next, 3 mL of ODE and 1.4 mL (~3 mmol) of TOP was added into the solution.
After being kept at 150 o C for 5 min, the mixture was heated to 260 o C. InP clusters solution prepared above was rapidly injected into the reaction mixture in the flask. After the first injection, the reaction temperature decreased to 240 o C and was allowed to remain at this temperature. For secondary-injection growth of InP QDs, additional InP clusters in solution was loaded into a syringe and dropped into the mixture at a speed of 0.9 mL/h, which was equivalent to the P precursor 0.135 mmol/h. UV-vis measurements of aliquots talking along the reaction were carried out to monitor the reaction. When a desired size of InP QDs was reached, the reaction mixture was allowed to cool down to room temperature by removing the heating mantle.
Synthesis of CdSe/CdS core/shell nanocrystals. The CdSe/CdS QDs were synthesized according to literature. 5 In a typical synthesis, Cd(Ac)2·2H2O (4 mmol), capric acid (4 mmol), oleic acid (12 mmol), purified CdSe core nanocrystals (0.4 μmol) with absorption peak at 550 nm and ODE (16 mL) were loaded in a 50 mL three-neck flask. The temperature was promoted to 260 o C and sulfur dissolved in ODE (0.2 M) was dropwise added into the solution with the speed of 4 mL/h until a targeted size of CdSe/CdS was obtained. Reaction process was monitored by taking aliquots for UVvis measurements.
Synthesis of iron oxide nanocrystals. The iron oxide (magnetite, Fe3O4) nanocrystals was synthesized according to literature. 6 Firstly, the iron-oleate precursor was prepared by iron chloride reacting with NaOH and oleic acid. 1.35 g FeCl3·6H2O and 4.25 mL OA were dissolved in 25 mL methanol. After the solid dissolved, a NaOH solution with 0.6 g NaOH in 50 mL methanol was dropped under magnetic stirring conditions. The observed precipitate was washed by 100 mL methanol three times and dried under vacuum at room temperature overnight. Then the dried precipitate was dissolved in 5 mL ODE at 80 oC and preserved as a stable stock solution. Next, 1 ml stock solution and 40 μL OA were dissolved in 4 mL ODE. The reaction mixture was heated to 300 oC under argon atmosphere and kept at that temperature for 30 min. The resulting solution containing the nanocrystals was then cooled to room temperature.
Purification of Iron Oxide Nanocrystals. The purification process for iron oxide nanocrystals consists of three steps. For the first step, the original reaction mixture with iron oxide nanocrystals was mixed with 12 mL of acetone. After vortex and centrifugation at 4000 rpm, the supernatant was removed. For the second step, 3 mL toluene was used to dissolve the precipitate, 3 mL of methanol was added and the mixture was centrifuged at 4000 rpm. The supernatant was removed. For the third step, the precipitate in step 2 was re-dissolved in 1 mL hexane and 10 mL acetonitrile was added. After vortex and centrifugation at 4000 rpm, the supernatant was removed. The remaining nanocrystals were dried under vacuum at room temperature for 30 minutes.
Synthesis of In2O3 nanocrystals. The In2O3 nanocrystals were synthesized according to a literature method. 7 Firstly, 0.3 mmol of In(Ac)3 and 0.9 mmol myristic acid were mixed and heated at 150 ºC for 30 min. Argon was blowing continuously to remove the acetic acid produced during reaction. The product indium myristate was dissolved in 1 mL ODE and kept at 120 ºC. Next, 2.4345 g 1-octadecyl alcohol (9 mmol) and 18 mL ODE were loaded in a 50 mL three-necked flask and the mixture was heated to 290 ºC.
1 mL indium myristate in ODE was swiftly injected into the solution of 1-octadecyl alcohol and ODE. The mixture was kept at 290 ºC for 30 min and then allowed to be cooled to room temperature.
Purification of nanocrystals. The purification process for CdSe nanocrystals reported in our previous literature consists of three steps. 8 For the first step, 2 mL of original reaction mixture with CdSe nanocrystals was mixed with 2 mL of ethyl acetate. After vortex and centrifugation at 4000 rpm, the supernatant was removed. Another 2 mL of ethyl acetate was added and repeated the above operation once again. For the second step, 1.1 mL of 10 vol% capric acid in toluene was used to dissolve the precipitate. The mixture was heated at 110 o C until a clear solution was formed. Then, 1 mL of methanol was added and the mixture was centrifuged at 4000 rpm. The supernatant was removed, and the resulting precipitate was re-dissolved in 1.1 mL of 10 vol% capric acid in toluene to repeat this cycle once again. For the third step, the precipitate in step 2 was re-dissolved in 0.5 mL hexane and 8 mL acetonitrile was added. After vortex and centrifugation at 10000 rpm, the supernatant was removed, and the resulting precipitate was re-dissolved in 0.5 mL of hexane to repeat this cycle once again. The remaining nanocrystals were dried under vacuum at room temperature for 10 minutes. The same procedures were applied for purification of CdSe/CdS, CdS, ZnSe, and InP QDs but with no addition of capric acid in step 2.
Purification for iron oxide nanocrystals consists of three steps. For the first step, 3.6 mL of original reaction mixture with iron oxide nanocrystals was mixed with 14 mL of ethanol. After vortex and centrifugation at 4000 rpm, the supernatant was removed. For the second step, 4 mL toluene was used to dissolve the precipitate. Into the toluene solution, 4 mL of methanol was added and the mixture was centrifuged at 4000 rpm.
The supernatant was removed. For the third step, the precipitate in step 2 was redissolved in 1 mL hexane and 10 mL acetonitrile was added. After vortex and centrifugation at 4000 rpm, the supernatant was removed. The remaining nanocrystals were dried under vacuum at room temperature for 30 minutes. Quantifying the number of thiolates on QDs. In a typical procedure, 2 mL of purified QDs in tetrachloromethane after ligand exchange with thiol (50 μM) was loaded in a quartz cell with 1 cm optical path for FTIR measurements. The concentration of thiolates was determined according to the standard curve of lauryl mercaptan in tetrachloromethane ( Figure S4). The concentration of QDs was determined by the UVvis.

Quantifying the amount of hydrogen in the form of interface-bonded water on
QDs. In a typical procedure, 3 mL of purified QDs (0.4 mM) dissolved in tetrachloromethane was loaded into a 10 mL plastic centrifuge tube. Then 0.4 mL of water was added into the solution. The mixture was vibrated for 5 minutes at room temperature to form a two-phase mixture. From the mixture, 2.5 mL of the tetrachloromethane phase was taken out carefully and mixed with 0.4 mL of deuterium oxide in another tube for 5 minutes to form another two-phase mixture. From this new mixture, 2 mL of the tetrachloromethane phase was taken out carefully and mixed with 0.2 mL water for 5 minutes. Finally, the water phase was taken for FTIR measurements.
The concentration of deuterium oxide in water was determined according to the standard curve of deuterium oxide in water ( Figure S6

Computational Details and Discussion
To model water adsorption on the surface of CdSe quantum dots, we considered three types of low-index facets, namely, {100}, {111}, and {110}. As revealed in our recent study, 9 chelating and bridging modes are dominant coordinations of carboxylate ligands on the polar {100} and {111} facets of CdSe nanocrystals, while cadmium carboxylates form weak coordinations on the nonpolar {110} facets. As shown in Figure S9, we use slab models with supercells to simulate ideal facets of CdSe nanocrystals and represent long-chain carboxylates as butyrate ligands to reduce the computational cost. On each kind of facet, one to three water molecules are considered in the supercell (see Figures S10-S12). The structures were optimized by the density functional theory with Quantum Espresso. 10 The vdW-DF 11 exchange-correlation functional was adopted to describe electronic and hydrogen-bond interactions. The plane-wave kinetic energy cutoff was set to be 60 Ry. The optimizations were completed until forces on all atoms were lower than 0.025 eV/ Å.      Table S1. Average distance between a hydrogen atom in the water molecules and the closest cadmium atom on different facets of the CdSe nanocrystals.
We further calculated the binding energies of water molecules on the different facets. For the Nth water molecule adsorbed on a surface of CdSe nanocrystals, the binding energy (ΔEN) is calculated by where EN is the energy of the slab with N adsorbed water molecules, EN-1 is the energy of the slab with N-1 adsorbed water molecules, and EH₂O is the energy of an isolated water molecule. Thereby, E0 denotes the energy of the slab without water molecules (see Figure S9). As shown in Table S2, the binding energies of water molecules all fall in the range of -5 ~ -15 kcal/mol, which are typical values for intermolecular hydrogen bonds. For {100} and {110} facets, the second water molecule binds slightly stronger than the first water molecule due to the higher flexibility for geometry relaxation. In contrast, the binding of water molecules on the {111} facet gets weaker with the adsorption of more water molecules. This is also reasonable because the bridging mode is associated with a lower ligand density and more hydrogen bonds are present between the first layer of water molecules and the carboxylate groups of surface ligands. Due to the higher binding energies of water molecules on the {111} facets, water adsorption seems to favor the {111} facets than the other two facets.