Ligand Exchange of Colloidal CdSe Nanocrystals with Stibanates

Oct 14, 2014 - Our group has had much success using small organic ligands such as tert-butylthiol, as well as a variety of other small chalcogenol lig...
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Ligand Exchange of Colloidal CdSe Nanocrystals with Stibanates Derived from Sb2S3 Dissolved in a Thiol-Amine Mixture Jannise J. Buckley, Matthew J. Greaney, and Richard L. Brutchey* Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States S Supporting Information *

ABSTRACT: Molecular stibanates derived from the dissolution of bulk Sb 2S3 in a binary ethylenediamine and mercaptoethanol solvent mixture have been studied as capping ligands for colloidal CdSe nanocrystals. A phase transfer ligand exchange strategy was utilized to effectively install the stibanate ligands onto the CdSe nanocrystals to form stable colloidal suspensions in polar solvents, such as formamide. This methodology was very effective in the removal of insulating native ligands on the as-prepared nanocrystals, with the resulting stibanate-capped CdSe nanocrystals giving low organic content thin films upon spin coating with improved interparticle coupling after heating to temperatures 25-fold increase in photocurrent response relative to asprepared CdSe nanocrystal films.

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INTRODUCTION The performance of colloidal semiconductor nanocrystals in optical and electronic devices is reaching the tipping point of being technologically relevant. Of particular importance, colloidal nanocrystals hold promise for the inexpensive manufacture of semiconducting thin films through solution processing, such as spray coating and roll-to-roll printing.1−3 Focused efforts to make semiconductor nanocrystals attractive for optical and electronic applications have resulted in impressive research progress in nanocrystal-based thin films,4 whereby significant increases in conductivities and carrier mobilities have been realized.5 Many of these improvements can be directly attributed to the development of new ligand exchange strategies that replace the electrically insulating, longchain surface ligands (e.g., oleate, stearate, or alkylphosphonates) from the as-synthesized nanocrystals with smaller ligands that allow for stronger interparticle coupling. Our group has had much success using small organic ligands such as tert-butylthiol, as well as a variety of other small chalcogenol ligands, which very effectively displace the longchain ligands present on as-prepared CdSe nanocrystals and allow for improved interparticle coupling in nanocrystal films.6,7 Of the ligands studied, the tert-butylthiol ligand most notably lead to significantly improved photocurrent, carrier mobility, and power conversion efficiencies in nanocrystal-based devices relative to the as-prepared and pyridine-exchanged CdSe nanocrystals.6,8,9 Another approach that has been employed to enhance the interparticle coupling of nanocrystal-based thin films is to perform ligand exchange reactions with small inorganic ligands. The newest class of inorganic ligands for © XXXX American Chemical Society

semiconducting nanocrystals is made up of various halide, pseudohalide, and halometallate ions.10−13 Use of these ligands has allowed for efficient electronic passivation and significant improvements in the charge transport of nanocrystal-based devices; for example, iodide-capped CdSe nanocrystals give high carrier mobilities of 12 cm2 V−1 s−1.11 Electron mobilities as high as 25 cm2 V−1 s−1 were observed in chloride-capped CdSe nanocrystal-based field-effect transistors (FETs); however, the nanocrystals were sintered, which may contribute to the high carrier mobilities.13 Earlier successes were also achieved with thiocyanate14,15 and sulfide ligands,16,17 both of which were shown to facilitate strong interparticle coupling and charge transport between nanocrystals once assembled in thin films. Lastly, chalcogenidometalates are another early and wellknown class of inorganic ligands that have been utilized for semiconductor nanocrystals.18−21 While originally hydraziniumbased (e.g., (N2H5)4Sn2S6, (N2H4)2(N2H5)2In2Se4), these ligands can now be prepared and utilized as nonhydraziniumbased salts (e.g., (NH4)4Sn2S6, K4SnTe4). Since their first use by Talapin and co-workers, these strongly binding ligands have proven their utility by allowing for improved conductivity in nanocrystal thin films.18−20 Many of the chalcogenidometalates utilized by Talapin and co-workers were originally prepared by Mitzi at IBM for the solution-processing of semiconductor thin films from dissolved semiconductor inks.22−24 Their general synthesis involves the Received: September 8, 2014 Revised: October 11, 2014

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electrospray ionization with negative ion detection. FT-IR spectra were recorded on a Bruker Vertex 80 spectrometer. To obtain quantitative data, we used a weighed internal standard (Fe4[Fe(CN)6]3, Alfa Aesar) according to a known procedure.6 The resulting spectra were then baseline corrected using a rubber band mode with 5 iterations and normalized to the internal standard peaks from 2160 to 2020 cm−1. All other samples were dropcast onto ZnSe windows and dried at 100 °C. Raman spectra of the solutions were recorded under ambient conditions using a Horiba Jobin Yvon spectrometer, equipped with a liquid sample holder. An excitation source of 785 nm from a diode laser was employed at a power level of 50 mW. Thermogravimetric analysis (TGA) measurements were made on a TA Instruments TGA Q50 instrument, using sample sizes of ∼5−10 mg in an alumina crucible under a flowing nitrogen atmosphere. TGA samples were prepared by fully drying the samples under flowing nitrogen at 100 °C for 30 min prior to analysis. Transmission electron microscopy (TEM) images were obtained using a JEOL JEM2100F (JEOL Ltd.) microscope operating at 80 kV. Samples for TEM studies were prepared by drop-casting a stable suspension of nanocrystals in toluene or formamide on a 400 mesh Cu grid coated with a lacey carbon film (Ted Pella, Inc.). Average particle diameters and standard deviations were derived by measuring a minimum of 200 individual particles per experiment and by averaging over multiple images. X-ray photoelectron spectroscopy (XPS) was obtained on a Kratos Axis Ultra X-ray photoelectron spectrometer with the analyzer lens in hybrid mode. High-resolution scans were performed using a monochromatic aluminum anode with an operating current of 5 mA and voltage of 10 kV using a step size of 0.1 eV, a pass energy of 20 eV, and a pressure range between 1 and 3 × 10−8 Torr. The binding energies for all spectra were referenced to the C 1s core level at 284.6 eV. UV−vis spectra were acquired on a Shimadzu UV-1800 spectrophotometer using a quartz cuvette. Photoluminescence (PL) spectra were collected on a Horiba Jobin Yvon Nanolog spectrofluorimeter system equipped with a 450 W Xe lamp as the excitation source and a photomultiplier tube as the detector. The excitation wavelength was 550 nm for all measurements. ζ-potential measurements were obtained using a Zetasizer Nano-ZS (Malvern Instruments, U.K.). Suspensions were measured in formamide after being transferred into disposable capillary cells (DTS1070, Malvern). ζpotential was calculated from electrophoretic mobility using Henry’s equation in the Smoluchowski limit.28 Film Deposition and Photoelectrochemistry. Spin-coating was carried out using a Laurell Technologies Corporation WS400Ez6NPP-LITE Single Wafer Spin Processor, fitted with a continuous nitrogen purge and a heat lamp to assist with film drying. The asprepared CdSe nanocrystals were spun at room temperature from a 20 mg mL−1 suspension in toluene at 1000 rpm for 60 s (acceleration = 770 rpm s−1), while the ligand-exchange nanocrystals were spun at 80−100 °C from a 20 mg mL−1 suspension in formamide at 1000 rpm for 60 s (acceleration =770 rpm s−1). When precleaned microscope slides were employed as a substrate, no preparation was necessary. For ITO-coated glass (Delta Technologies 7 × 50 × 0.7 mm3, Rs = 5−15 Ω), the substrates were sequentially sonicated in isopropyl alcohol for 15 min and acetone for 15 min before being ozone treated for 20 min. Thermal treatment was carried out on a thermostatically controlled Al heating block under flowing nitrogen. Photoelectrochemical response was performed using a BASi Epsilon-EC potentiostat. A quartz cuvette was used with a Pt-wire counter electrode and a Pt-wire pseudoreference electrode. The working electrode was a CdSe nanocrystal film on ITO-coated glass. An aqueous 0.050 M Na2S/ 0.005 M sulfur electrolyte was made from nitrogen-sparged deionized water, and care was taken to avoid exposure of the electrolyte solution to oxygen throughout the course of the experiments. For photoelectrochemical experiments, two 472 nm LEDs mounted ca. 6 cm from either side of the sample were used to illuminate the working electrode. The illumination intensity was measured using a Newport model 818-ST photodiode connected to a Newport 2832-C dualchannel power meter. Measurements were taken for both illumination directions and summed, giving a result of 4.9 mW cm−2. The total illumination area of the CdSe working electrode was 2.3 cm2.

simple dissolution of bulk metal chalcogenides in hydrazine with stoichiometric chalcogen through a process known as dimensional reduction. In an analogous way, we recently developed an alternative route to dissolve semiconductors using a binary solvent mixture comprised of ethylenediamine (en) and a thiol without the need for added chalcogen. This solvent mixture possesses the remarkable solvent power to rapidly dissolve a variety of notoriously insoluble semiconductors at room temperature and ambient pressure.25−27 Intrigued by the parallels between hydrazine and our en/thiol solvent system, we set out to investigate the molecular nature of dissolved Sb2S3 (herein used as a model system) in en/ mercaptoethanol (ME), and subsequently installed these dissolved species as ligands on CdSe nanocrystals. Indeed, these molecular stibanate species can be successfully utilized for the rapid and efficient ligand exchange of native stearate ligands present on as-prepared CdSe nanocrystals to produce colloidally stable, stibanate-capped CdSe nanocrystals. Solution-processed nanocrystal thin films of the stibanate-capped CdSe nanocrystals demonstrate improved interparticle coupling relative to the as-prepared nancorystals, which leads to markedly higher electrochemical photocurrent generation.

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EXPERIMENTAL METHODS

General Considerations. CdCO3 (99.998% metals basis, “Puratronic” grade), selenium (200 mesh powder, 99.999% metals basis), tri-n-octylphosphine oxide (TOPO, 98%), sulfur (precipitated, 99.5%), and formamide (99.5+%) were purchased from Alfa Aesar. Tri-n-octylphosphine (TOP, ≥ 97%) was purchased from Strem. Stearic acid (95%), ethylenediamine (en, ≥ 99%), 2-mercaptoethanol (ME, 99+%), and Sb2S3 (99.995%) were purchased from SigmaAldrich. Na2S hydrate (>60% Na2S) was purchased from Lancaster. All reagents and solvents were used as received without further purification. Dissolution of Bulk Sb2S3. Prior to ligand exchange, a Sb2S3/ ethyelendiamine (en)/mercaptoethanol (ME) stock solution was prepared by mixing Sb2S3 (20 mg) with en (2.00 mL) and ME (0.05 mL) at room temperature under a nitrogen atmosphere. The solution was stirred until complete dissolution of the bulk Sb2S3 occurred (ca. 30 min) to yield a transparent light-yellow solution that was then handled in air. Ligand Exchange. For ligand exchange of the as-prepared CdSe nanocrystals with the dissolved Sb2S3, a phase transfer procedure was utilized. In this procedure, a Sb2S3/en/ME stock solution (0.2 mL) was diluted with formamide (2.0 mL) under a nitrogen atmosphere. A second immiscible suspension of the as-prepared CdSe nanocrystals in toluene (5.5 mL, 2.5 mg mL−1) was layered on top of the polar phase. This resulted in immediate phase transfer of the CdSe nanocrystals from the top toluene phase to the bottom polar phase. After 5 min, quantitative phase transfer was achieved as evidenced by the colorless toluene phase and colored polar phase. The top toluene layer was then fully extracted. The stibanate-capped CdSe nanocrystals suspended in the polar formamide phase were then purified by flocculation with 12 mL of acetone, centrifugation (6000 rpm, 1 min), with the supernatant being discarded. This purification procedure was repeated 2× with final redispersion in 1 mL of formamide. To test for agglomeration, we centrifuged the final product (6000 rpm, 1 min) and no solid was deposited, indicating minimal or no agglomeration. Material Characterization. The chemical composition of the CdSe nanocrystals was determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Galbraith Laboratories, Knoxville, TN). Additionally, carbon, hydrogen, and nitrogen elemental analysis was made using a PerkinElmer2400 Series II CHNS/O Analyzer (Galbraith Laboratories, Knoxville, TN). Samples (25 mg) were prepared for elemental analysis by drying under vacuum at 90 °C for 24 h. Electrospray ionization mass spectroscopy (ESI-MS) was performed using a Waters LCT premier operated using B

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RESULTS AND DISCUSSION Characterization of the Dissolved Sb2S3 Species. Stibanates from dissolved Sb2S3 were chosen as the model chalcogenidosemimetalate ligands in this study because of their promise for electronic and optoelectronic applications.21,29−31 In the bulk form, Sb2S3 possesses a direct band gap (Eg = 1.5− 1.8 eV) with a strong absorption coefficient (7.5 × 104 cm−1 at 550 nm).32 Small chalcogenidosemimetalate clusters of Sb2S3 prepared by the dissolution of the bulk material in hydrazine and excess chalcogen have previously been utilized as inorganic ligands on CdSe nanocrystals.21 Although small chalcogenidometalate ligands are generally known to enhance the performance of nanocrystalline thin film field-effect transistors by increasing their conductivity by several orders of magnitude when compared to organic ligands, not much is known about the properties of stibanate-capped CdSe nanocrystals.21,30 Herein, the stibanate ligands were prepared by dissolution of bulk Sb2S3 in a 1:40 vol/vol mixture of mercaptoethanol (ME) and ethylenediamine (en) (Figure 1). Because of the novelty of

Figure 2. Negative-ion ESI-MS spectrum of Sb2S3 dissolved in 1:40 vol/vol mixture of en/ME at a concentration of 240 μg mL−1.

ion clusters at m/z 416.8 and m/z 460.8 are consistent with the formulas [Sb2S4C2H5O]− and [Sb2S4C4H9O2]−, respectively. The possible identity of the stibanate species present in the en/ME solution can be garnered from these elemental analysis and ESI-MS data. ESI-MS clearly indicates that the dissolved Sb2S3 produces a mixture of compounds that contain ME ligands. ICP-AES elemental analysis suggests the presence of en because of the additional C and N content. From these data a few formulas are possible, including, [SbS][C 2H 4 SO][C2H9N2], [Sb][C4H8S2O2][C2H9N2], [Sb2S3][C2H4SO][C2H9N2] and [Sb2S2][C4H9S2O2][C2H9N2]. Characterization of similar species in the literature may point to the structure of some of the stibanate species present here (see the Supporting Information, Figure S1). The reaction between Sb(III) isopropoxide and ME is known to yield a compound [Sb][C4H8S2O2(H)] similar in formula to the one found in this work (i.e., [Sb][C4H8S2O2][C2H9N2]).33 The structure of [Sb][C4H8S2O2(H)] is 3-coordinate about Sb with approximate pyramidal geometry. One of the two ME ligands forms a five-membered chelate ring with Sb in [Sb][C4H8S2O2(H)], whereas the other ligand is bonded only through the S atom.33 FT-IR spectroscopic analysis on Sb2S3 dissolved in en/ME after it had been evaporated to dryness displayed similar features to that reported for [Sb][C4H8S2O2(H)], while also displaying additional spectral features originating from en (see the Supporting Information, Figure S2). For example, the presence of bands at 1279, 1045, and 543 cm−1 can be matched to the ν(S−C), ν(C−O), and ν(Sb-OC) bands from [Sb][C4H8S2O2(H)], respectively. The broad ν(N−H) band around 3300 cm−1 likely results from protonated en counterions in the species investigated here.34 Finally, the stretching band corresponding to the ν(S−H) thiol group at 2560 cm−1 is not present in the spectrum of the dissolved Sb2S3 in en/ME, thus indicating the likely deprotonation of the thiol with concomitant thiolate coordination to the Sb center. Dimeric species, which may be similar to the [Sb2S2][C4H8S2O2][C2H9N2] and [Sb2S3][C2H4SO][C2H9N2] compounds, are also well-known in the literature. In fact, the majority of molecular species produced by dissolving Sb2S3 in caustic sulfidic solutions are dimeric with two Sb atoms bound together through two bridging sulfur atoms.35,36 Interestingly, the Raman spectrum of the stibanates from dissolved Sb2S3 in en/ME displays two key bands at 300 and 411 cm−1 in the region characteristic for Sb−S bonds, and which closely match with the calculated stretching frequencies of the neutral, dimeric Sb2S2(SH)2 complex (see the Supporting Information, Figure S3).35 Ligand Exchange Using Stibanates from Dissolved Sb2S3. Because of its well-known synthesis and surface chemistry, CdSe was used as the model nanocrystal platform

Figure 1. Photograph before (left) and after (right) dissolution of 20 mg of bulk Sb2S3 powder in 2 mL of en and 0.05 mL of ME. After dissolution, the light-yellow solution is optically transparent and free of visible scattering.

this solvent system, characterization of the dissolved Sb2S3 species was needed in order to gain insights into the possible identity of the resulting chalcogenidosemimetalate ligands. Chemical information on the stibanate species was obtained using a combination of ICP-AES elemental analysis, thermogravimetric analysis (TGA), ESI-MS, Raman and FT-IR spectroscopies. Initial information on the composition of the dissolved Sb2S3 was obtained by analyzing the chemical composition of the dissolved species after the solution had been evaporated to dryness. The chemical composition of the resulting sample was confirmed by ICP-AES and combustion elemental analysis to have the formula C4.6H12.0N2.1SbS2.3 (see the Supporting Information, Table S1). This composition, made up of roughly 47 wt % organics (C, H, and N) and 53 wt % heavier inorganic elements (Sb and S), is corroborated by TGA on the same material, which demonstrates a 45% mass loss up to 500 °C. Further analysis of dissolved Sb2S3 in the binary en/ME solvent mixture implies that rather than a singular species, the stibanates likely exist as multiple molecular species in solution. A negative-ion ESI-MS spectrum acquired from a dilute solution of Sb2S3 in en/ME (240 μg mL−1) indicates the presence of four primary stibanate species that can be classified into two groups containing either one or two Sb atoms (Figure 2). The main ion clusters observed at m/z 228.9 and m/z 272.9 are consistent with the formulas [SbS 2 C 2 H 4 O] − and [SbS2C4H8O2]−, respectively. Assignment of the other two C

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ligand exchange, the stibanate-capped CdSe nanocrystals could be easily filtered through a 0.1 μm filter and were colloidally stable in formamide for periods >1 month. Zeta potential measurements on the stibanate-capped CdSe nanocrystals corroborate the empirically observed colloidal stability and revealed a ζ-potential of −32 mV (Figure 3b), indicating (i) a stable suspension and (ii) that negatively charged surface ligands are responsible for the electrostatic stabilization of these particles in formamide. In an effort to show the general scope of this ligand exchange procedure, additional ligands and nanocrystalline cores were screened. The en/ME solvent system and ligand exchange procedure was successfully applied to other chalcogenidosemimetalate/chalcogenidometalate ligands on CdSe nanocrystals, as well as using the stibanate ligands on other nanocrystalline cores (Figure 4). Successful dissolution and ligand exchange was achieved with the following semiconductors, as evidenced by efficient phase transfer: As2S3, As2Se3, Sb2Se3, SnS, and ZnS on CdSe nanocrystals. Furthermore, ligand exchange with the stibanates was amenable to other inorganic nanocrystals, such as CdS/CdSe core/shell nanocrystals and Pt nanocrystals, which were synthesized according to literature procedures.39,40 Similar to the CdSe nanocrystals, the UV−vis spectra of the CdS/CdSe core−shell nanocrystals before and after ligand exchange with the stibanate ligands show no apparent scattering or blue shift of the first exciton peak, indicating that the nanocrystals are colloidally stable and not etched, respectively. Control reactions were carried out to gain insight into the ligand exchange chemistry and colloidal stability of the CdSe nanocrystals. Formamide proved to be irreplaceable for the phase transfer reaction. When other solvents, such as water or N,N-dimethylformamide (DMF), were used as the second phase during the phase transfer, the CdSe nanocrystals did not fully transfer from the toluene phase or were not colloidally stable, respectively. The importance of formamide is possibly due to its high dielectric constant (ε = 111) and donor number (DN = 39.8), which allow for the electrolytic dissociation of cations, while also facilitating strong electrostatic stabilization of the stibanate-capped CdSe nanocrystals.10,41 Control experiments performed in the absence of dissolved Sb2S3 (i.e., using just en/ME/formamide as the polar phase) underwent complete phase transfer; however, the resulting nanocrystals were not colloidally stable upon purification, indicating the importance of the stibanate ligands for proper stabilization of the particles in formamide. Efficacy of Ligand Exchange. The efficacy of the phasetransfer ligand exchange was probed using a number of complementary techniques. First, the ligand exchange was investigated using TGA. All TGA samples were extensively dried and purified, so as to specifically study the surface bound ligands, before running the analysis under flowing nitrogen (5 °C min−1 to 100 °C, then 10 °C min−1 to 500 °C, Figure 3c). The data showed that the as-prepared CdSe nanocrystals possess a single high temperature mass loss of 28% beginning at 300 °C, which was assigned to the loss of stearate native ligands (vide supra). After ligand exchange with the stibanate ligands, the high temperature mass loss corresponding to the native ligands was no longer observed by TGA, indicating quantitative removal of the native ligands. Additionally, in the stibanatecapped CdSe nanocrystal sample, new mass loss events are observed that are consistent with those observed for dried Sb2S3/en/ME (i.e., the stibanate ligands). Both traces display three mass loss events occurring in the same temperature

to study the ligand exchange chemistry described herein. The CdSe nanocrystals were synthesized using a previously reported procedure whereby tri-n-octylphosphine selenide (TOPSe) is injected into a hot solution of Cd(stearate)2 in TOPO and stearic acid.37 The resulting nanocrystals displayed a first excitonic transition at ∼593 nm in the UV−vis absorption spectrum, indicating a CdSe nanocrystal diameter of ca. 4.4 nm.38 Transmission electron microscopy (TEM) revealed that the CdSe nanocrystals possess a spherical morphology with a diameter of 4.9 ± 0.5 nm, which is in general agreement with empirical sizing by UV−vis spectroscopy. TGA data showed a single mass loss event of 28% up to 500 °C. This is consistent with previous work,6,7 and is indicative of a native ligand shell comprised mainly of anionically bound stearate, which makes the as-synthesized CdSe nanocrystals readily dispersible in nonpolar solvents such as toluene. Facile removal of the stearate native ligands through installation of a stibanate ligand shell was accomplished in a single step using a simple phase transfer procedure. In a typical two-phase system, a solution of as-prepared CdSe nanocrystals in toluene was stirred with a dilute solution of Sb2S3/en/ME in formamide (Figure 3a). Complete transfer of the CdSe nanocrystals from the top toluene layer to the bottom formamide layer was achieved within 5 min, indicating facile removal of the insulating stearate native ligands. Subsequent extraction of the top toluene layer followed by a two-step purification process yielded a stable colloidal suspension of stibanate-capped CdSe nanocrystals in formamide. Following

Figure 3. (a) Photograph before (left) and after (right) phase transfer of CdSe nanocrystals from the nonpolar toluene phase to the polar formamide phase caused by exchange of the native aliphatic ligands with stibanate ligands. (b) ζ-potential of stibanate-capped CdSe nanocrystals in formamide with a photograph of the colloidal suspension given as an inset. (c) TGA traces comparing the mass loss events observed for the stibanate ligands, as-prepared CdSe nanocrystals, and ligand exchanged CdSe nanocrystals. (d) FT-IR spectra of as-prepared and ligand exchanged CdSe nanocrystals after drying. The spectra are offset and normalized in intensity to the 2089 cm−1 ν(CN) stretch of a measured Fe4[Fe(CN)6]3 internal standard (not shown). D

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Figure 4. (a) Generality of this approach is illustrated by performing the ligand exchange using a variety of dissolved bulk semiconductors on CdSe nanocrystals. The same procedure was used in all cases with Sb2S3 being replaced with the semiconductors listed in the figure. (b) Scope of this ligand exchange procedure was further illustrated by performing the ligand exchange on various inorganic nanocrystals using the stibanate ligands. (c) Solution absorption spectra of the CdSe nanocrystals before and after ligand exchange with various dissolved bulk semiconductors. (d) Solution absorption spectra of CdS/CdSe core/shell nanocrystals before and after ligand exchange with stibanate ligands.

ranges (100−200, 200−300, and 300−500 °C). Comparison of the mass% ratios of these three steps relative to the total mass loss gives 66:27:7 and 61:29:10 for the stibanate ligands and stibanate-capped CdSe nanocrystals, respectively. Therefore, it is likely that the new ligand shell on the ligand exchanged CdSe nanocrystals is composed of roughly the same species obtained after drying the Sb2S3/en/ME solution. Finally, it should be noted that the decomposable ligand mass went from 28% to 3.8% after ligand exchange with the stibanate ligands, which is in agreement with the 5 wt % Sb coverage achieved after ligand exchange (vide infra). The low degree of organic content, in addition to the low temperature (