From Gel to Crystal: Mechanism of HfO2 and ZrO2 Nanocrystal Synthesis in Benzyl Alcohol

Nonaqueous sol–gel syntheses have been used to make many types of metal oxide nanocrystals. According to the current paradigm, nonaqueous syntheses have slow kinetics, thus favoring the thermodynamic (crystalline) product. Here we investigate the synthesis of hafnium (and zirconium) oxide nanocrystals from the metal chloride in benzyl alcohol. We follow the transition from precursor to nanocrystal through a combination of rheology, EXAFS, NMR, TEM, and X-ray total scattering (PDF analysis). Upon dissolving the metal chloride precursor, the exchange of chloride ligands for benzylalkoxide liberates HCl. The latter catalyzes the etherification of benzyl alcohol, eliminating water. During the temperature ramp to the reaction temperature (220 °C), sufficient water is produced to turn the reaction mixture into a macroscopic gel. Rheological analysis shows a network consisting of strong interactions with temperature-dependent restructuring. After a few minutes at the reaction temperature, crystalline particles emerge from the gel, and nucleation and growth are complete after 30 min. In contrast, 4 h are required to obtain the highest isolated yield, which we attribute to the slow in situ formation of water (the extraction solvent). We used our mechanistic insights to optimize the synthesis, achieving high isolated yields with a reduced reaction time. Our results oppose the idea that nonaqueous sol–gel syntheses necessarily form crystalline products in one step, without a transient, amorphous gel state.


Main figures
Tables Table S1: Conversion (%) of alcohol (BnOH) in dibenzyl ether (BnOBn) and benzyl chloride (BnCl) as detected via quantitative 1 H NMR at different reaction times in the HfO 2 microwave-assisted synthesis.Each reaction was repeated at least three times to calculate the average and standard deviation.Ratios of the samples 5 min at 80 • C and 4 hours at 220 • C were calculated via peak deconvolution due to overlapping peaks.

Figure
Figure S1: (A) Storage modulus G' and loss modulus G" with increasing temperature of benzyl alcohol in overlay with the measurement for 0.25M HfCl 4 .2THF in benzyl alcohol.(B) Recovery cycles of G' and G" after applying stress to the gel, both at 25 • C and 160 • C. G' and G" with increasing temperature of (C) 0.25M ZrCl 4 .2THF in benzyl alcohol, a sharp viscosity increase between 140 • C and 150 • C is noted, and (C) 0.25M HfCl 4 .2THF in benzyl alcohol with a fast (0.03 • C/sec) and slow (0.01 • C/sec) heating ramp.

Figure
Figure S4: (A) Titration of ZrCl 4 .2THFwith benzyl alcohol in C 6 D 6 , followed via 31 P NMR.The spectra have a relative x-offset of 1 ppm with respect to each other for clarity.(B) Coordination numbers of chloride and oxygen surrounding the zirconium center, calculated from the EXAFS data.

Figure
Figure S5: A control experiment of 38 mmol benzyl alcohol with 2.5 mmol HCl (37 w/w%) shows that both benzyl chloride and dibenzyl ether are present at 160 • C and at 220 • C and therefore are created by the reaction between benzyl alcohol and HCl.No gel phase is observed with the microwave camera.

Figure S6 :
Figure S6: Assignment of benzyl alcohol, benzyl chloride and dibenzyl ether in the region between 4.4 and 4.8 ppm for 1 H NMR (top).The equivalents calculated from 1 H NMR spectra for a solution of 0.20M HfCl 4 .2THFdissolved in benzyl alcohol where the microwave reaction is stopped at different temperature and time points.Each synthesis was done separately and in triplicate, shading indicates the error bars.See tableS1for exact values in %.The water content is calculated based on the conversion of benzyl alcohol to dibenzyl ether and benzyl chloride.

Figure S7 :
Figure S7: Equivalents of benzyl alcohol, water, benzyl chloride and dibenzyl ether in the reaction mixture of 0.25M ZrCl 4 .2THFdissolved in benzyl alcohol at increasing temperature, calculated by 1 H NMR. The water content is calculated based on the conversion of benzyl alcohol to dibenzyl ether and benzyl chloride.

Figure
Figure S8: (A) 1 H NMR of the reaction mixture supernatant after the synthesis of Hf(OiPr) 4 .iPrOH(1) in benzyl alcohol and with additional (2) trifluoroacetic acid (TFA) and (3) acetic acid added.The resonances corresponding to benzylacetate, benzyl alcohol and dibenzylether are assigned.The spectra are shifted with a relative x-offset of 0.05 ppm for clarity.(B) TEM of the particles synthesized by the reaction of Hf(OiPr) 4 .iPrOHand TFA in benzyl alcohol.(C) TEM of the particles synthesized by the reaction of Hf(OtBu) 4 and TFA in benzyl alcohol.

Figure
Figure S10: (A) Contour plot of the F(Q) with highlighted transitions and patterns plotted.(B) Complete PDF of the precursor structure.Sequential refinements showing the (C) lattice parameters and (D) volume as a function of time.

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Figure S11: (A) The refined crystallite (in real space) and the area of the (111) peak (in reciprocal space) as a function of time and (B) The R w value, representing the quality of the fit in reciprocal space, as a function of time.

Figure S12 :
Figure S12: Possible precursor structures upon dissolving ZrCl 4 .2THF in benzyl alcohol.The structures were calculated via DFT with increasing exchange of chlorides to benzyl alcoholates.Relevant bond lengths (Zr-O, Zr-Cl) are indicated.

Figure
Figure S15: (A) 31 P NMR titration of HfCl 4 .2THFwith TOPO and BnOH without the vacuum step shows only the tetrachloride complex.(B) Shift of the free TOPO due to complexation with benzyl alcohol.

Table S4 :
Yield determination of HfO 2 NC syntheses at different reaction times.