Highly Soluble Ligand Stabilized Tin Oxide Nanocrystals: Gel

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Highly Soluble Ligand Stabilized Tin Oxide Nanocrystals: Gel Formation and Thin Film Production James S. McManus, Patrick D. Cunningham, Laura B. Regan, Alison Smith, Dermot W. McGrath, and Peter W. Dunne*,† School of Chemistry, National University of Ireland, Galway, Ireland ABSTRACT: Highly soluble ligand stabilized tin oxide nanocrystals have been prepared by the postsynthetic modification of hydrous tin oxide with the simple shortchain carboxylic acids, acetic acid and trifluoroacetic acid. The surface modified nanoparticles are soluble in common organic solvents at concentrations of up to 2 g/mL and may be stored indefinitely in the solid-state with no loss of solubility. The nature and role of the surface carboxylate groups has been assessed through a combination of thermogravimetric analysis and infrared spectroscopy. The effect of surface modification on the sintering behavior of the nanomaterials has also been investigated. Tuning the surface chemistry of the hybrid nanocrystals allows the generation of monolithic gels, while their high solubility permits the preparation of high quality thin films using simple solution processing techniques.

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INTRODUCTION Metal oxides are among the most widely studied inorganic materials. Cheap and abundant, they exhibit a wide range of properties making them technologically important in applications as diverse as pigments,1 catalysis,2 batteries,3 gas-sensing,4 and solar cells.5 Many such applications require the oxide to be in the form of a thin film or coating, while others benefit greatly from the use of structured nano- and mesoscale materials. Tin(IV) oxide (SnO2), a high refractive index, wide band gap semiconductor, and prototypical transparent conducting oxide (TCO), is ubiquitous across the many fields in which metal oxides find applications. It has been used since antiquity as a ceramic glaze and pigment,6 and while cheaper alternatives are now available the quality of tin oxide glazes remains unsurpassed. In the 1940s it was used as a coating in the Duraglas process to confer scratch resistance and durability to beer bottles (among others). As a TCO tin oxide was initially used as an antifogging coating on cockpit glass shortly after WWII.7 More recently tin oxide and doped tin oxide have also found use in the plate glass industry as thin films for lowemissivity energy saving architectural windows. Tin oxide based materials are similarly widely used as gas sensing materials.8,9 In catalysis and battery applications tin oxide is used both as a coating or thin film and as free-standing nano- and mesostructures.10−13 Tin oxide, like many metal oxides, is a highly intractable material, which presents a significant disadvantage for processing the materials into the required forms. A wide variety of techniques have been employed to generate tin oxide. The oldest and simplest method for the deposition of SnO2 coatings is spray pyrolysis. Physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques offer a © XXXX American Chemical Society

greater degree of control over the composition and overall quality of the obtained coatings and thin films, though they can require an expensive apparatus and careful precursor selection.14−16 These approaches are also limited to the production of coatings and thin films. The sol−gel technique is generally more versatile, allowing the production of thin-films and coatings, free-standing nanostructures, and, through templating, hierarchically structured materials.12,17,18 Some major drawbacks of the sol−gel method include poor control over crystallinity, particle size, limited reproducibility, and the need for fresh sols each time.19 Over the past several years the synthesis of surface modified hybrid organic−inorganic nanocrystals has emerged as a versatile alternative to these established methods.20−26 Surface modification allows the physical and chemical properties of metal oxide nanocrystals to be tuned, such that the nanocrystals may be processed easily from solution or suspension using simple wet chemical methods. The use of preformed nanocrystals separates crystal formation from device production. This permits the initial generation of nanoparticles with the desired size, shape, or composition, which may then be processed into thin films, self-assembled into mesostructures, or incorporated into pre-existing template structures. We have previously reported the synthesis of highly soluble surface modified anatase phase titanium oxide nanoparticles using a simple short-chain carboxylic acid.27 Here we extend this work to the production of ligand stabilized tin oxide nanocrystals, which are highly soluble in common organic Received: July 3, 2014 Revised: July 25, 2014

A

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solvents. These modified nanocrystals may be stored indefinitely in the solid state without the loss of solubility. The nature of the surface capping ligands has been investigated, and their role in the solubility of the hybrid nanomaterials has been established. The soluble tin oxide can be treated to yield monolithic gels and can be easily processed from solution into high quality thin films.

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EXPERIMENTAL SECTION

Synthesis. Hydrous tin oxide was prepared by the hydrolysis of tin tetrachloride. In a typical reaction 200 mL of tin tetrachloride was added slowly to a large (2.5×) excess of distilled water with vigorous stirring. This can be a violent reaction, with the evolution of large quantities of HCl vapor, and appropriate care should be taken. Ten molar sodium hydroxide was then added dropwise while monitoring the pH. A thick viscous gel was formed at pH 2−3. Increasing the pH to 6 gave hydrous tin oxide as a thick white precipitate. The precipitate was washed repeatedly by centrifugation until the supernatant was free of chloride ions, as determined by the silver nitrate test. The hydrous tin oxide was allowed to dry under ambient conditions until it reached a damp, but friable state. Tin oxide acetate was prepared by refluxing the obtained hydrous tin oxide in acetic acid. Fifty grams of the hydrous tin oxide was loaded into a round-bottomed flask containing 125 mL of glacial acetic acid under stirring. On increasing the temperature of the mixture to 80 °C the lumps of hydrous tin oxide were seen to disperse, yielding a fine milky white suspension. Upon refluxing at 90 °C this suspension forms a colloidal dispersion before becoming completely transparent. This solution was dried by rotary evaporation at 60 °C and ∼50 mbar to yield an off-white powder. In a typical reaction, 50 g of the hydrous starting material yields 30 g of soluble product. Tin oxide trifluoroacetate was obtained in the same manner, refluxing the hydrous tin oxide in trifluoroacetic acid with an oil bath temperature of 80 °C. The product was isolated by rotary evaporation at 60 °C and 50 mbar. Yields were on the order of 35 g per 50 g of hydrous starting material. Characterization. X-ray diffraction (XRD) patterns were recorded using a Siemens Kyrystalloflex diffractometer using CuKα radiation (λ = 1.54054 Å), with an applied potential of 40 kV and a current of 20 mA over a 2θ range of 5 to 75°, with a step size of 0.05° and a scan rate of 2 s/step. Elemental analyses were carried out on a PerkinElmer 2400 elemental analyzer. Thermogravimetric analysis and differential scanning calorimetry (TGA/DSC) were carried out on a Rheometric STA 650 instrument under a flow of nitrogen at a heating rate of 5°/ min. Infrared spectra were recorded on a PerkinElmer Spectrum One FTIR-ATR spectrometer in the range of 4000−650 cm−1. Transmission electron microscopy (TEM) images were acquired using and FEI Titan microscope operating at 300 kV. Samples were prepared by depositing a drop of the oxide solutions of lacey carbon coated 300 mesh copper grids. Scanning electron microscopy (SEM) images were recorded on a Hitachi S-4700 instrument. Atomic force microscopy (AFM) images were obtained on an Agilent 5500 microscope operating in contact mode on a 5 × 5 μm area. XPS spectra were collected using a VG ESCALab Mark II instrument.

Figure 1. XRD patterns of hydrous tin oxide and the acetate and trifluoroacetate modified nanoparticles with corresponding photographs of 10% (w/v) solutions of tin oxide acetate in methanol and tin oxide trifluoroacetate in acetone (a). TEM images of acetate and trifluoroacetate modified nanoparticles (b and c, respectively).

does not alter the cassiterite crystal structure; the reaction is purely a surface modification. The average crystallite diameters of all materials, as calculated by the Scherrer equation, are of the order of 2 to 4 nm. Transmission electron microscopy images (Figure 1b,c) show crystalline domains of 3−5 nm with some amount of bridging between particles. The infrared spectra of the hydrous and modified nanoparticles, Figure 2a, show that the water and surface hydroxyl groups of the hydrous tin oxide are largely replaced by carboxylate groups, with only extremely weak bands observed in the 3600 to 2900 cm−1 region. Adsorption of carboxylic acids to the surface of tin oxide occurs via the reaction of the acids with surface hydroxyl groups and coordination of surface tin atoms by the oxygen atoms of the carboxylate (COO−) group. There are a variety of ways in which the carboxylates can coordinate to oxide surfaces.28,29 Unidentate coordination occurs through one oxygen atom, while bidentate coordination involves both oxygens. This may be symmetric, asymmetric, or by bridging between two tin atoms. Each type of binding gives a different infrared spectrum, characterized by Δν − the separation between the antisymmetric and symmetric stretching modes of the carboxylate groups (between 1700−1500 and 1500−1200 cm−1, respectively).30,31 The spectrum of hydrous tin oxide exhibits a broad envelope at ∼3300 cm−1 due to OH stretches of water and surface hydroxyls and a band at 1640 cm−1 arising from the OH bending mode. The significant tail-off toward 650 cm−1 results from tin oxygen bonds. The infrared spectrum of the tin oxide acetate material still shows the presence of Sn−O bonds, as evidenced by the tail off toward 650 cm−1. The strong OH stretching band, which was observed in the hydrous tin oxide, has significantly weakened, indicative of the removal of hydroxyl groups and their involvement in hydrogen bonding. A small shoulder is observed at 1754 cm−1, which can be assigned to the carbonyl stretch of very loosely associated acetic

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RESULTS AND DISCUSSION The product from the reaction of hydrous tin oxide with acetic acid dissolves readily in acetic acid and methanol at levels of up to 1 g/mL. The trifluoroacetate product is soluble in trifluoroacetic acid, THF, and acetone up to 2 g/mL. No additional dispersants, heat treatment, or sonication are required to obtain transparent solutions. When kept sealed these solutions remain stable for several months. The dried powders may be stored indefinitely (5+ years) without any appreciable loss of solubility. The XRD patterns of the hydrous tin oxide and the soluble products, shown in Figure 1a, reveal that the reaction of the hydrous oxide with the carboxylic acids B

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Figure 2. Infrared spectra (a) and TG/DSC traces (b) of hydrous tin oxide and surface modified nanoparticles and a schematic representation of carboxylate binding to the SnO2 (110) surface (c).

acid. The band at 1711 cm−1 indicates hydrogen bonded acetic acid. The two broad bands centered at 1290 and 1400 cm−1 arise from the symmetric stretching modes of the OCO carboxylate groups. The sharp overlapping band at 1370 cm−1 is attributed to CH3 scissoring vibrations. A large number of overlapping bands are clearly visible in the antisymmetric stretching region, giving a broad envelope, indicative of many different coordination modes from unidentate to asymmetric bidentate, bridging bidentate, and symmetric bidentate. The infrared spectrum of the tin oxide trifluoroacetate similarly shows some loose and hydrogen bonded trifluoroacetic acid, indicated by the bands at 1780 and 1736 cm−1, respectively. The symmetric OCO stretching modes of surface coordinated trifluoroacetate groups generate absorption bands overlapping the CF3 stretching band at 1155 cm−1 and another at 1417 cm−1. The antisymmetric stretching region in this case appears significantly more defined and is centered at 1690 cm−1 with some shouldering on the low frequency side. Together with the clearly defined band at 1417 cm−1, this is indicative of a high degree of bidentate character. Thermogravimmetric analysis and differential scanning calorimetry, Figure 2b, support the conclusions drawn from the infrared spectra. It should be noted that different batches give slightly different results: the acetate content, as determined by elemental analysis, varies between 23% and 26%, while the trifluoroacetate content can vary between 29% and 34% by weight. What follows relates to the typical TG and DSC traces shown in Figure 2. The TG trace of the hydrous tin oxide shows a relatively sharp weight loss between 65 and 150 °C, coupled with an endotherm centered at 110 °C, due to the evaporation of surface water. This is followed by a more gradual weight loss curve, which may be attributed to the condensation of surface hydroxyl groups and the subsequent loss of water. The tin oxide acetate also exhibits a relatively rapid weight loss of 6.8% up to 110 °C, with a broad shallow endotherm, due to the removal of surface waters and loosely associated and hydrogen bound acetic acid. A further 10% weight loss takes place between 110 and 310 °C, ascribed to the loss of unidentate acetate groups, and the conversion of unidentate acetates to more strongly bound bidentate coordination. A final more rapid exothermic weight loss of 10% occurs up to 400 °C due to the decomposition of strongly bound acetate moieties. The tin oxide trifluoroacetate shows an initial endothermic

weight loss of 6.5% up to 91 °C due to the removal of water and very loosely bound acid. An additional 8.5% is lost up to 272 °C from the loss of unidentate groups. This is followed by a precipitous exothermic 17% weight loss from the decomposition of bound trifluoroacetate up to 353 °C.32 This decomposition is significantly less exothermic than that of the acetate due to the difficulty in breaking CF bonds; it has previously been shown that fluorocarbon species remain after the thermal decomposition of trifluoroacetate complexes.33,34 A summary of the surface composition of the nanocrystals, as determined by combined elemental analysis and thermogravimmetry, is given in Table 1. Table 1. Surface Composition of the Tin Oxide Nanocrystals, with the Total Mass Loss Determined by TGA and the Organic Component from Elemental Analysis; Surface Coverage Was Calculated Based on a Spherical Morphology surface coverage (molecules/nm2)

mass loss % sample

crystallite size (nm)

total

hydrous acetate trifluoroacetate

3 3 3

12.5 25.7 35.2

organic

water

organic

water

23.3 32.3

12.5 2.4 2.9

11.1 9.2

16.6 3.8 5.2

The nature and role of the surface acetate and trifluoroacetate species was further investigated by heating powdered samples of the soluble materials in vacuo at temperatures of 100, 150, and 250 °C in an effort to confirm the assignments of loose acid, unidentate, and bidentate species and to determine the role of each species in the solubility of the hybrid nanocrystals. This was achieved by placing samples of the soluble oxide under investigation into a round-bottomed flask attached to a standard vac-line with the pressure maintained at 6.6 × 10−3 mbar and placed in an oil/silica sand bath preheated to the desired temperature, as outlined in Tables 2 and 3. The resulting products were investigated by elemental analysis, thermogravimmetry, and infrared spectroscopy. Solubility was assessed by preparing 10% weight solutions C

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Table 2. Conditions and Results of in Vacuo Heat Treatment of Tin Oxide Acetate sample

temp (°C)

time (h)

solubility

gelation (days)

A0 A1 A2 A3 A4 A5 A6 A7 A8 A9 A10

100 100 100 100 100 100 150 150 250 250

0.5 1 1.5 2 3 24 0.5 1 0.5 1

yes yes yes yes yes yes yes no no no no

2−3 2−3 2−3 2−3 2−3 1−2 N/A N/A N/A N/A

Table 3. Conditions and Results of in Vacuo Heat Treatment of Tin Oxide Trifluoroacetate sample TFA0 TFA1 TFA2 TFA3 TFA4 TFA5 TFA6 TFA7 TFA8 TFA9 TFA10

temp (°C) 100 100 100 100 100 100 150 150 250 250

time (h)

solubility

gelation (days)

0.5 1 1.5 2 3 24 0.5 1 0.5 1

yes yes yes yes yes yes yes yes yes yes yes

2−3 2−3 1 1

in the appropriate solvent. The total organic content was ascertained by elemental analysis and the amount of bound unidentate and bidentate carboxylate groups were calculated based on the second and third weight loss regions in the TG traces. The results of these investigations are shown in Figure 3 and Figure 4. The tin oxide acetate samples after treatment at 100 °C under vacuum all retained their solubility in methanol. The clear solutions prepared from these samples formed opaque colloidal gels on standing for several days. The ratio of acetate to tin, as determined by elemental analysis, decreases significantly from 0.9 to 0.6 on prolonged treatment at 100 °C. A small proportion of unidentate acetate is also removed with any heat treatment. The relative ratios of the unidentate and bidentate acetate species remain largely unchanged on further heating at 100 °C. The infrared spectrum of sample A2, treated at 100 °C for 1 h no longer exhibits the band at 1754 cm−1, confirming the removal of loosely associated acetic acid and the band at 1711 cm−1 has reduced in intensity. The band at 1290 cm−1 is significantly reduced in intensity and shifts to higher frequency, while the envelope between 1700 and 1500 cm−1 has lost several bands and shifted to lower frequencies. This is indicative of the removal of asymmetric acetate species, such as the acid itself and unidentate coordinated groups.35 A broad band develops at 3500 cm−1, which may indicate the reuptake of atmospheric water prior to analysis. This is believed to be due to the presence of pendant carboxylate groups made available for hydrogen bonding with atmospheric water by the removal of the hydrogen bonded acetic acid groups. Samples treated at 150 °C become completely insoluble in methanol. Thermogravimmetry shows that unidentate acetate groups are

Figure 3. TG traces of acetate samples after heat treatment under vacuum (a), the ratio of organic species to tin (b), and IR spectra of select samples (c). Inset photograph shows the gel obtained from sample A2.

removed under these conditions. The infrared spectra show a further decrease in the intensity of the hydrogen bonded acetic acid band at 1711 cm−1 and a sharpening of the symmetric carboxylate stretching band at 1300 cm −1, confirming the removal of unidenate groups. It should also be noted that the reuptake of water is not as evident in this case, further supporting the removal of unidentate acetate species. Treatment at 250 °C also leads to the loss of a similar amount of unidentate acetate, as per the TGA data. The infrared spectrum of the sample treated for 1 h shows the complete removal of hydrogen bonded acetic acid (Figure 3c). Significant differences are also observed in the carboxylate stretching region. The D

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groups have rearranged to a more symmetrical bidentate coordination arrangement. The tin oxide trifluoroacetate material shows a general decrease in organic content when heated at 100 °C. The total organic content calculated by elemental analysis remains higher than the level of bound trifluoroacetate calculated by TGA, except in the case of the sample treated for 24 h. This suggests that loose and hydrogen bonded trifluoroacetic acid is not completely removed at shorter treatment times. There is a small but steady decrease in the relative amount of unidentate trifluoroacetate. The ratio of bidentate trifluoroacetate to tin varies between 0.35 and 0.3. Little change is observed in the infrared spectra of these samples. The band at 1695 cm−1 becomes marginally sharper, and the shouldering on the high frequency side of the band at 1155 cm−1 becomes more resolved. All samples heated at 100 °C retained their solubility in acetone, and gelation was not observed. Treatment at 150 °C effectively removes all loose acid. The amount of unidentate trifluoroacetate decreases in comparison to that of the parent sample; while the ratio of bidentate trifluoroacetate to tin remains at ∼0.35. The infrared spectra confirm the removal of the loose acid and also show a comparative increase in bidentate character with the development of a well resolved band at 1193 cm−1. Treatment at 250 °C yields similar results. Loose acid is completely removed, and the band at 1193 cm−1 resolves further, again indicative of increased bidentate character. All samples of the trifluoracetate material remained soluble. Those treated at 150 °C were observed to gel slowly over the course of several days. Samples treated at 250 °C gelled more rapidly. The gels obtained from the heat treated tin oxide trifluoroacetate were highly transparent, as shown in Figure 4. In the case of the acetate modified tin oxide it is clear that the unidentate acetate groups are key to the solubility of the material. It is reasonable to suggest that the solubility of this material arises through the formation of a hydrogen bonding network between the surface bound unidentate acetate, acetic acid, and methanol. Removal of these unidentate groups prohibits the formation of this network, and solubility is lost. The tin oxide trifluoroacetate retained its solubility, even after treatment at 250 °C, and the removal of unidentate groups. Coupled with the material ssolubility in polar, aprotic solvents, it may be inferred that the solubility of the tin oxide trifluoroacetate is due primarily to electrostatic dipole−dipole interactions between the polar −CF3 groups and the solvent.36 Thus, bidentate coordination is sufficient to promote solubility. In both cases it is apparent that loose and hydrogen bonded acid groups are necessary for the long-term stability of the solutions. It is likely that these groups act as a buffer, preventing the removal of surface bound carboxylate groups through reaction or ligand exchange with the solvent. In order to examine the effect of the acetate and trifluoroacetate surface modification on the thermal behavior of the nanoparticles, samples of each material, including the hydrous SnO2, were heated in air in a standard furnace for 1 h from 200 to 1000 °C at 100 °C intervals. XRD patterns were collected from the same sample after treatment at each temperature and crystallite diameters were calculated using the Scherrer equation. The results of these investigations are shown in Figure 5. The hydrous tin oxide shows little growth at temperatures below 300 °C, with crystallite diameters remaining 2−3 nm. Between 400 and 800 °C, linear growth from 3 to ∼18 nm is observed, which may be attributed to

Figure 4. TG traces of trifluoroacetate samples after heat treatment under vacuum (a), the ratio of organic species to tin (b), and IR spectra of select samples (c). Inset photograph shows the gel obtained from sample TFA7.

envelope between 1700 and 1500 cm−1 has become shallower with a loss of intensity of the band at 1585 cm−1, accompanied by the emergence of an absorption band at 1524 cm−1, which had previously been observed only as a shoulder on the broad envelope. The shoulder at 1420 cm−1 on the 1380 cm−1 band is also significantly more defined, while the band at 1316 cm−1 has narrowed with the loss of absorption on the low-frequency side. Considered with the TG data, this decrease in Δν shows that subtle surface rearrangements have taken place. It may be suggested that asymmetric and bridging bidentate acetate E

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Figure 5. Contour plots showing the thermal evolution of the XRD patterns of hydrous SnO2 (a), SnO2 acetate (b), and SnO2 trifluoroacetate (c), with their calculated crystallite diameters (d) and selected XPS spectra (e).

Figure 6. XRD patterns of drop cast coatings of tin oxide acetate on a silicon wafer calcined for the indicated times at 300 °C (a) and 400 °C (b), AFM image and photograph of dip-coated glass slide (c), and SEM images of a thin-film obtained from three spin-coating applications of a 10% (w/ v) solution of tin oxide acetate in methanol annealed at 500 °C for 30 min (d−f).

growth by condensation reactions. The growth rate increases dramatically on further increasing the temperature, and

crystallite diameters of 70 nm are achieved on sintering at 1000 °C. In contrast the acetate modified tin oxide nanocrystals F

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show very little growth up to 800 °C with calculated diameters below 10 nm. Sintering at 900 and 1000 °C gives crystallite diameters of ∼20 and 40 nm, respectively. This would suggest that crystallite growth is hindered below these temperatures, which may be the result of residual carbonate species on the surfaces of the crystallites. This is supported by infrared spectra of the sintered samples, which show a band at 1420 cm−1. The trifluoroacetate modified nanocrystals retain their small size up to temperatures of 300 °C and thereafter exhibit almost linear growth from 4 nm after sintering at 400 °C up to 23 nm after treatment at 1000 °C. The lack of growth up to 400 °C is attributed to the surface trifluoroacetate groups, which are not removed until 350 °C as revealed by the TG traces of this material. Between 400 and 800 °C its growth behavior closely mirrors that of the hydrous tin oxide. Where both the hydrous oxide and the acetate modified nanoparticles show a rapid increase in crystallite size on sintering above 800 °C, the trifluoroacetate modified material maintains a steady growth rate. It can be suggested that the former materials undergo true sintering, while in the latter case fluorine doping and the resultant oxygen vacancies prevent grain growth by inhibiting the Sn−O−Sn bridge formation necessary for sintering. Fluoride doping has been confirmed by XPS (Figure 5e) in the case of the samples calcined at 500 °C, while for higher temperatures no fluorine signal is observed. This is consistent with previous observations by other researchers.37,38 The exceptional solubility of these tin oxide nanoparticles allows the facile preparation of coatings and thin films using simple wet chemical methods. XRD patterns of coatings prepared by drop-casting onto a silicon wafer from a methanolic solution of tin oxide acetate are shown in Figure 6a,b. These coatings were calcined at 300 and 400 °C for the times as indicated. It can be seen that calcination at 300 °C even for extended periods does not induce crystallite growth, as would be expected on the basis of both thermogravimetric analysis and sintering studies of the powders. Heat treatment at 400 °C does lead to an increase in crystallite size, in keeping with prior observations. Furthermore, the XRD pattern of the calcined coating shows no evidence of preferred orientation effects, which indicates the isotropic growth of the tin oxide nanocrystals. This is in contrast to many vapor deposition techniques where film growth from initial nuclei formed on the substrate surface proceeds in an anisotropic manner.39,40 This difference is thought to be due to the use of pre-existing nanocrystals. AFM analysis (Figure 6b) of a dip coated glass slide gives very low surface roughness values of between 5 and 7 Å. SEM images of a typical tin oxide thin film on glass are shown in Figure 6e,f. The coating was prepared by spin-coating from a 10% (w/v) solution of thin oxide acetate in methanol onto a silica coated glass coupon provided by Pilkington plc. Three coatings were applied with sintering at 500 °C for 1 h between each application. The thickness of the obtained thin films may be finely controlled through solution concentration and the number of depositions.

and electrostatic and steric effects in the latter. In both cases loosely bound carboxylic acid groups act to stabilize the solutions, preventing gelation. Removal of these free acids allows the rapid formation of monolithic gels. The presence of acetate groups hinders crystallite growth below the sintering temperature due to the formation of carbonate species, while calcination of the trifluoroacetate modified nanoparticles results in fluorine doping of the tin oxide, which inhibits sintering at elevated temperatures. The soluble hybrid nanocrystals can be processed from solution by a variety of methods to yield extremely high quality thin films.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

† Department of Chemical and Environmental Engineering, University of Nottingham, University Park Campus, Nottingham, NG7 2RD, United Kingdom.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.W.D. wishes to thank IRCSET for funding under the Embark initiative. L.B.R. thanks Mayo County Council and NUIG for financial support. Mr. Martin Roe of the Mechanical, Materials and Manufacturing Engineering Department at the University of Nottingham is gratefully acknowledged for XPS measurements. CRANN at TCD is thanked for access to HRTEM facilities.

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REFERENCES

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CONCLUSIONS Here we have demonstrated the synthesis of highly soluble ligand stabilized tin oxide nanocrystals using simple, short chain carboxylate ligands. The solubility of the acetate and trifluoroacetate surface modified nanoparticles arises from the formation of a hydrogen-bonding network between pendant surface carboxylate groups and the solvent in the former case, G

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dx.doi.org/10.1021/cg5009957 | Cryst. Growth Des. XXXX, XXX, XXX−XXX