Zinc Oxide/Carbohydrate Hybrid Materials via Mineralization of Starch and Cellulose in the Strongly Hydrated Ionic Liquid Tetrabutylammonium Hydroxide Dragana Mumalo-Djokic,† Willem B. Stern,‡ and Andreas Taubert*,†,§
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 1 330–335
Department of Chemistry, UniVersity of Basel, CH-4056 Basel, Switzerland, Department of EnVironmental Geosciences, UniVersity of Basel, CH-4056 Basel, Institute of Chemistry, UniVersity of Potsdam, D-14476 Golm, Germany, and Max-Planck-Institute of Colloids and Interfaces, D-14476 Golm ReceiVed February 6, 2007; ReVised Manuscript ReceiVed August 23, 2007
ABSTRACT: Zinc oxide/carbohydrate hybrid materials have been fabricated via mineralization of ZnO in the strongly hydrated ionic liquid (IL) tetrabutylammonium hydroxide (TBAH) in the presence of cellulose or starch. TBAH dissolves the ZnO precursor zinc acetate; it also reacts with zinc acetate to form ZnO nanoparticles. Tetrabutylammonium acetate forms as a side product and further acts as a template for nanoparticle formation. X-ray diffraction reveals that the primary building blocks have a rod- or needle-like shape (i.e., the coherence length along the crystallographic c-axis is larger than along the crystallographic a-axis), which is unusual for ZnO grown from solution in the presence of organic additives. In the presence of starch, organic/inorganic hybrid sponges form. The mineralization of cellulose is less effective due to its lower solubility in TBAH, but the results suggest that our process can be used to coat cellulose efficiently. Overall, TBAH acts as a solventreactant (an ionic liquid precursor, ILP), which leads to ZnO/carbohydrate hybrid materials where the architecture of the precipitate is controlled by the carbohydrate additive. Introduction Ionic liquids have recently attracted quite some attention from the inorganic materials community, because they can in some cases lead to inorganic materials with new structures and properties.1–3 Furthermore, ionic liquids (ILs) have also widely been promoted as “green” solvents for a wide variety of chemical reactions and processes.4–10 Recently, also the “green” fabrication of inorganic nanoparticles has attracted quite some interest.11 We have recently put forward the concept of “all-in-one” solvent-reactant(-template)s,2,12 that is, ionic liquids or ionic liquid crystals13 that are at the same time the solvent, the reactant, and the template for the fabrication of an inorganic. These special systems have been termed ionic liquid precursors and ionic liquid crystal precursors (ILPs and ILCPs, respectively).12,14 IL(C)Ps allow for the controlled mineralization of inorganics with complex structure and morphology. For example, we have made CaF2 tubes, CuCl platelets, and Au platelets from IL(C)Ps or their crystalline analogues.12,15–18 Other research groups have extended this concept of reactive IL(C)s to other compounds.14,19–21 Unlike mineralization from ILs, mineralization of inorganics from organic or aqueous solutions has been studied for decades, see for example refs 22–26. For example, Walsh et al. have shown that dextran is an efficient template for the fabrication of many inorganics with complex morphology, including ZnO.27 Starch is another efficient growth modifier for ZnO precipitated from aqueous solution. In the presence of starch, roughly spherical zincite microparticles, which are made up of smaller zincite nanoparticles, form.28 * Corresponding author: Institute of Chemistry, University of Potsdam, KarlLiebknecht-Strasse 24-25, Building 26, D-14476 Golm, Germany. E-mail:
[email protected]. Tel.: ++49 (0)331 977 5773. † Department of Chemistry, University of Basel. ‡ Environmental Geosciences, University of Basel. § University of Potsdam and Max-Planck-Institute.
The current paper shows that starch can also act as a template for the formation of extended zinc oxide/starch hybrid sponges by using the strongly hydrated ionic liquid tetrabutylammonium hydroxide (TBAH). As TBAH is a strongly hydrated compound, it can or cannot be viewed as an ionic liquid, but regardless of how one defines an IL, the presence of water in the system allows for accessing some useful chemistry. In the current process, TBAH dissolves the ZnO precursor zinc acetate, reacts with zinc acetate to form ZnO nanoparticles, and acts as a template for nanoparticle formation. The carbohydrates then act as templates for nanoparticle organization. The useful aspect of this process from a “green chemistry” point of view is that only natural starting materials such as cellulose, starch, and a rather benign hydroxide IL are employed. Experimental Procedures Mineralization. In a typical experiment, 400, 500, or 600 mg of starch (soluble starch from potatoes, Aldrich S2004; Mn approximately 60 000 g/mol) or cellulose (microcrystalline cellulose powder, Aldrich 435236) were dissolved (starch) or suspended (cellulose) in 5 g of tetrabutylammonium hydroxide (TBAH, N(C4H9)4OH · 30H2O, mp 26–28 °C, Aldrich 86866) yielding final concentrations of 80, 100, and 120 mg of carbohydrate/g of TBAH. These mixtures were stirred at 80 °C for 15 min. The starch solutions were clear, whereas the mixtures containing cellulose contained individual suspended particles. The mixtures were heated to the desired reaction temperature (80 or 100 °C), and 500 mg of zinc acetate dihydrate powder (Aldrich 96459) were added under vigorous stirring, yielding a final concentration of 100 mg of zinc acetate/g of TBAH. The solutions were reacted for 1 to 14 h at 80 or 100 °C. After a few minutes, a white precipitate appeared and the solutions remained turbid until the end of the reaction. The white solid was recovered via centrifugation, washing with water and ethanol, and drying at 40–50 °C. Characterization. X-ray diffraction (XRD) was done on a Siemens D5000 with Cu KR1 radiation and graphite monochromator, a Nonius PDS 120 with Cu KR radiation and position sensitive detector, and a Nonius D8 with Cu KR radiation. Estimation of particle sizes from line broadening was done as described previously using OriginLab Origin 6.1.29 In short, after background subtraction, the peaks were fitted individually with Loretzian peak profiles from which the full width
10.1021/cg0701372 CCC: $40.75 2008 American Chemical Society Published on Web 11/27/2007
Zinc Oxide/Carbohydrate Hybrid Materials
Crystal Growth & Design, Vol. 8, No. 1, 2008 331 Table 1. Dhkl (Nanometers, Estimated via the Scherrer Equation) for Samples Precipitated with 100 mg of Carbohydrate (where Applicable) and 100 mg of Zinc Acetate/g of TBAHa Dhkl
Dhkl
Dhkl
hkl
control sample
cellulose/ZnO
starch/ZnO
100 (a-axis) 002 (c-axis) 101
50.4 ( 1.4 117.5 ( 21.5 46.6 ( 2.5
24.3 ( 1.9 40.2 ( 3.9 22.0 ( 0.9
20.9 ( 1.2 50.3 ( 1.1 21.2 ( 0.7
a Values for samples precipitated with 80 and 120 mg carbohydrate/g of TBAH are equal to within 5 nm.
Figure 1. (a) XRD patterns of samples precipitated with (i) 100 mg of starch and 100 mg of zinc acetate/g of TBAH, (ii) 100 mg of cellulose and 100 mg of zinc acetate/g of TBAH, (iii) a control sample precipitated with 100 mg of zinc acetate/g of TBAH, but without carbohydrate. (iv) Pure cellulose exhibits just one reflection at 20.3° 2θ. (b) Magnified view of the three most important zincite reflections (100, 002, and 101) demonstrating the line broadening. at half-maximum (fwhm) was determined. The main peaks of the X-ray patterns were then analyzed using the Scherrer formula, kλ (1) π β cos θ 180 where Dhkl is the coherence length of the crystalline domain perpendicular to the respective hkl plane, k is a constant between 0.8 and 1.4 (here 0.9), λ is the X-ray wavelength (here Cu KR ) 1.5408 Å), β is the background corrected line broadening in degrees, (π/180) is a correction factor to calculate β in radians, and θ is the scattering angle. Scanning electron microscopy (SEM) was done on a Philips XL-30 ESEM operated at 10 kV and on a LEO 1550 Gemini operated at 20 kV. Samples were sputtered with Au or Pt (Philips) or Au/Pt (LEO) prior to imaging. Transmission electron microscopy (TEM) was done on a Zeiss 912 Omega operated at 120 kV. Thermogravimatric analysis was done on a Mettler Toledo TGA/SDTA851e with a heating rate of 10 °C/min in nitrogen. IR spectra were recorded on the neat samples with a Shimadzu FTIR 8300 with a Golden Gate ATR at a resolution of 4 cm-1 and 32 runs per measurement. Dhkl )
( )
Results and Discussion Figure 1 shows representative X-ray diffraction (XRD) patterns of various samples after mineralization. All samples are zincite, and the peaks exhibit a significant line broadening, indicating the presence of small crystallites. Estimations of the crystallite sizes (coherence lengths Dhkl) using the Scherrer equation are shown in Table 1. Table 1 points to an interesting effect of the current approach on zincite mineralization: in most cases, when ZnO is grown from solution in the presence of organic growth modifiers, the
002 reflection shows the strongest line broadening; that is, the particle size is reduced most prominently along the crystallographic c-axis of the zincite lattice.29–31 Here, however, the 100 and 101 reflections show a more significant broadening than the 002 reflection, and D100 and D101 are thus smaller than D002. The Dhkl values for the samples precipitated with 80 and 120 mg of carbohydrate/g of TBAH are identical to within 5 nm and hence confirm this finding. As a result, the ratio of D100/D002 is 0.43 for the control sample, 0.60 ( 0.13 for the samples mineralized with 80, 100, and 120 mg of cellulose/g of TBAH, and 0.42 ( 0.11 for the samples mineralized with 80, 100, and 120 mg of starch/g of TBAH. This implies that the primary building blocks (the individual nanocrystals) of the precipitates presented here are composed of small needle- or rodlike crystallites, whose length is ca. twice their width. Comparison of the current data with an earlier study on polymer-controlled ZnO mineralization shows that ZnO precipitated from aqueous solution in the absence of growth modifiers has a D100/D002 ratio of 0.5.29 This is similar to the control sample shown here. However, the presence of watersoluble diblock copolymers in the earlier study reverses the D100/ D002 ratio, and ratios well over 1 were found, when polymeric additives were present in the reaction solution. This indicates that there the primary crystalline domains of these precipitates are platelike and not (as found here) needle- or rodlike. Scheme 1 illustrates the differences between primary crystallites obtained from TBAH and from aqueous solution. The main difference between the two cases is that the polymers used in the earlier study29 were polyanions (polymethacrylic and polystyrene sulfonic acid, respectively), and here the precipitation is done in a solution containing a large amount of small organic cations along with acetate ions. Several modes of action of the IL can be postulated. (i) Adsorption of the acetate or tetrabutylammonium (TBA) ions onto the charged basal planes of the zincite lattice, which would retard growth along the crystallographic c-axis. As we observe a “rod” rather than a plate, this is unlikely. (ii) The butyl groups of the TBA cations are hydrophobic, so adsorption of the TBA cations on the less polar zincite side faces is therefore also possible. In this case, the formation of long rods could be expected. Finally, (iii) the tetrahydroxozincate anion Zn(OH)42- can also coordinate to organic cations such as cetyltrimethylammonium (CTA). Xiong et al. have shown that hydrothermal treatment of solutions containing CTA-tetrahydroxozincate complexes leads to ZnO wires.32 A similar mechanism can also be postulated here. However, as TBA is, in contrast to CTA, symmetric, the coordination around the tetrahydroxozincate ion is different, which will affect structure formation. A detailed study of the solution behavior of metal ions in TBAH is underway. Regardless of the detailed growth mechanism, the current results suggest that a change from anionic to cationic growth modifiers (or to ILPs with larger cations) could provide a general strategy for the fabrication of
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Scheme 1. Schematic Representation of the Primary Building Blocks Formed in the Presence of Diblock Copolymers from Aqueous Solution29 (left) and from the Ionic Liquid TBAH (this study, right)a
a
Dimensions are approximately to scale.
Figure 2. Scanning electron micrographs of some mineralized samples. (a, b) Control sample. (c, d) Mineralized cellulose. (d) View of the ridges and grooves along the main fiber axis mentioned in the text. Inset is a magnified view showing the individual nanoparticles.
ZnO with rod-, needle-, or even wire-like primary building blocks from solution. The XRD patterns of ZnO samples precipitated with cellulose also exhibit a reflection at 20.3 degrees 2θ, which can be indexed as the 101 reflection of crystalline cellulose II.33 In contrast, no reflection from starch is observed in the ZnO/starch composites. XRD thus suggests that starch must be molecularly dissolved in the hydrated ionic liquid TBAH. Consequently, starch must be intimately associated with ZnO, and the resulting precipitate is a real composite material. In contrast to starch, TBAH cannot dissolve cellulose. Therefore, we still observe an X-ray signal from crystalline cellulose after mineralization. This is not surprising, because cellulose can only dissolve in water-free ionic liquids,34,35 whereas starch is water-soluble and can thus dissolve in a hydrated IL such as TBAH as well. As a result, however, two different composites form with the two carbohydrates. Figure 2 shows SEM images of a control sample and a sample mineralized with cellulose. Control samples contain large blocks of densely aggregated nanoparticles, which are much smaller than control samples obtained from aqueous solution.28–31,36
Also unlike samples grown from aqueous solution, the resulting particles have no distinct hexagonal prismatic morphology. The samples mineralized in the presence of suspended cellulose show fibers that are hundreds of micrometers long and microns to tens of microns wide. They exhibit smaller ridges and grooves consisting of very small primary particles, which are roughly parallel to the main fiber axis. These precipitates are essentially intact cellulose fibers, which have been mineralized during the reaction. As they have (unlike starch) not been dissolved, but only suspended during reaction, they are much larger and remain discrete entities. This is similar to Ghule et al.,37 who have shown that paper can be coated with ZnO from basic solutions containing zinc salts. Figure 3 shows that the presence of starch in the reaction solution leads to large ZnO/starch hybrid “sponges”. The precipitates have pores with a broad size distribution. The walls are rather thick and composed of aggregated particles; some of the structures are reminiscent of corals (although on a different length scale). (Note: This analogy is only morphological. It does not imply that our crystals grow the way corals grow.) High magnification SEM also shows that the particles surrounding the pores are often composed of even smaller particles, which are barely visible in the SEM. TEM shows that the sponges contain denser and less dense regions, the latter of which are surrounded by the inorganic particles. TEM further confirms the nanocrystalline nature of the precipitate as the particles have a size of ca. 10 to 150 nm. TEM however also shows that XRD averages over a rather broad size distribution. Electron diffraction shows that the sponges are composed of zincite nanoparticles with no common orientation but rather a random orientation distribution. This suggests that the hybrid sponges do not form via oriented attachment38 or via mesocrystal transformation,26,39 but most likely via aggregation of individual particles on the carbohydrate chains without much control over the orientation of the individual particles during the growth process. Overall, SEM and TEM confirm the XRD measurements. Electron microscopy and XRD suggest that the starch/ZnO composite is a real hybrid material where the inorganic and the carbohydrate are intimately associated with one another. On the other hand, SEM also suggests that the cellulose/zinc oxide composite is not a “true” hybrid material. Much rather, the cellulose is present as a fiber with micrometer dimensions, and ZnO is
Zinc Oxide/Carbohydrate Hybrid Materials
Crystal Growth & Design, Vol. 8, No. 1, 2008 333
Figure 4. XRD pattern of a sample precipitated with 100 mg/g of starch and 100 mg/g of zinc acetate before and after calcination at 700 °C.
Figure 3. (a) Low and (b) high magnification SEM images of a ZnO/ starch sponge. Inset in (b) shows the tiny particles mentioned in the text. (c) TEM image showing an area with open (circle) and dense regions. (d) Higher magnification view of aggregated nanoparticles. The particles in the circle have roughly the size determined from XRD. (e) Electron diffraction pattern showing that overall, the sponge has no preferred crystallographic orientation.
predominantly growing on the surface of this already existing fiber. This is further supported by high magnification SEM, which shows that the mineralized cellulose is covered with ZnO nanoparticles aligned along the main fiber axis. We interpret this as a feature imposed onto the mineral by the semicrystalline cellulose template, where individual cellulose molecules and smaller fibrous entities are arranged roughly parallel to the fiber axis. Figure 4 shows XRD patterns of a sample precipitated with 100 mg/g of starch and 100 mg/g of zinc acetate before and after calcination at 700 °C. Interestingly, calcination does not affect the size of the crystallites. Even after calcination, we observe Dhkl values that are identical to the values reported in Table 1 within the experimental error. Figure 5 shows SEM images of a sample calcined at 700 °C. SEM clearly shows that the morphology of the precipitate does not change significantly. Intriguingly, even after calcination, the samples exhibit small particles and a sponge-like appearance. This indicates that the materials formed from the IL/starch mixtures are rather stable up to high temperatures, which makes them attractive as potential supports, for example, for catalyst nanoparticles. SEM also shows that the small primary particles do not change significantly upon calcination. This is in line with
Figure 5. SEM image of a sample precipitated in the presence of starch and calcined at 700 °C for 10 h. (a) Low magnification and (b) higher magnification image. Inset in (b) is a high magnification image showing the individual nanoparticles making up the larger precipitates.
the XRD data, which also show that calcination does not lead to a further growth of the ZnO primary particles making up the larger particles. The incorporation of the carbohydrates into the precipitates has been further studied with thermogravimetric analysis (TGA) and attenuated total reflection IR (ATR-IR) spectroscopy. Figure 6 shows that a significant amount of carbohydrate is incorporated into the precipitate: control samples exhibit weight losses from 1–3%, samples mineralized with starch typically lose 10–20%, and samples mineralized with cellulose lose 40–55% of their original weight. Differential thermogravometric analysis (DTG) shows that the decomposition of the carbohydrates does not significantly change upon incorporation into the inorganic. The control sample shows one weight loss at 154 °C, which can be assigned to water loss and the decomposition of some incorporated tetrabutylammonium or acetate ions. Neat starch shows two transitions at 213 and 280 °C, mineralized starch at 204 and 275 °C. Cellulose exhibits one transition at 363 °C, and mineralized cellulose shows two transitions at 219 and 354 °C. The lower transitions can again be assigned to the loss of water
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aggregate into a 3D sponge-like ZnO/starch hybrid network, even though the primary building blocks and the amount of incorporated starch are comparable (10–20% in the current study and 10–32% in the earlier work). Replacing water with the ionic liquid TBAH and the much higher polymer concentrations used here lead to sponge-like architectures, despite that fact that TBAH contains a large fraction of water and might at a first glance be regarded as very similar to an aqueous starch solution. This suggests that the IL plays an important role in the selfassembly of the starch molecules. We suspect that TBAH and the higher starch concentration used here lead to a more extended conformation of the starch chains in solution, which would lead to a bridging effect (and hence to 3D solids rather than individual micrometer-sized particles). Alternatively, the tetrabutylammonium and acetate ions could coordinate to the starch and lead to regions that are more hydrophobic than others, which would again change the self-assembly of the starch template in solution. These questions are currently being addressed. Conclusion This paper shows that carbohydrates can be mineralized using the highly hydrated ionic liquid TBAH as a “solvent-reactant” (starch) or “suspension medium-reactant” (cellulose). The process is a one-pot reaction, and preliminary studies in our laboratory have shown that it is also viable for many other (transition) metal salts that can be dissolved in TBAH. As a result, the current process appears to be more straightforward (although less elegant) than a similar process reported by Zhu et al., who used tailor-made Zn/alkylamine/tetramethylammonium hydroxide-ILPs for the fabrication of ZnO with various morphologies.14 Our approach eliminates the need for a specific (separately made) metal-organic compound with ionic liquid behavior. We have therefore developed a generic approach for the controlled fabrication of carbohydrate/metal (hydr)oxide materials with a defined structure and morphology. The resulting materials could for example be interesting for antibacterial coatings similar to ZnO/paper hybrids reported by Ghule et al.37 Further studies on other metal oxides and the details of their formation are underway. Figure 6. (a) Representative TGA curves of a control sample, a sample mineralized with starch, a sample mineralized with cellulose, neat cellulose, and neat starch. (b) DTG curves of the same samples. (c) IR spectra of a control sample, a sample mineralized with starch, and neat starch. The bands between 2000 and 700 cm-1 are from the carbohydrate and the tetrabutylammonium and the acetate ions. Spectra are too noisy for analysis below 700 cm-1.
trapped in the hybrid materials and the decomposition of some incorporated tetrabutylammonium or acetate ions. The higher transitions are due to decomposition of the carbohydrates. IR spectroscopy confirms the TGA evidence of carbohydrate incorporation, as in all samples bands of the organic compounds are observed before TGA. At this point it is interesting to compare the current samples to ZnO precipitated from aqueous solution in the presence of starch.28 There, discrete spherical particles with a bimodal size distribution form at much lower concentrations of 220 mg/L of starch. At 960 mg/L of starch, reasonably monodisperse particles with a diameter of ca. 2 µm form. They are composed of nanometer-sized primary particles with a diameter of 10–30 nm. In contrast to this earlier study, the current samples contain no discrete particles, but the nanometer-sized primary particles
Acknowledgment. We thank M. Düggelin, D. Mathys, and R. Pitschke for help with electron microscopy, I. Zenke for help with XRD, Dr. Z. Li for preparation of a test sample, Profs. K. M. Fromm and E. C. Constable for access to their TGA and IR, respectively, and the Swiss National Science Foundation, the MPI of Colloids and Interfaces, and the University of Potsdam for funding. A.T. acknowledges the Holcim Foundation for a Habilitation Fellowship.
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