Study on the microheterogeneity of aqueous alcohol solutions: formation mechanism of inner pores of ZnO nanostructures

Abstract

The self-association of alcohols in water has been demonstrated by a number of special experiments and theoretical work. Here, we report a simple and effective method to investigate the presence of these hydrophobic aggregation areas. Porous ZnO nanostructures were prepared using a simple alcohol assisted solution synthetic method at room temperature. The presence of hydrophobic groups of alcohols resulted in the existence of microheterogeneities and organic aggregations in the mixed solution, which act as a soft template and play key roles in the formation of the porous structure of ZnO. A high reaction temperature resulted in decreased hydrogen bonding between the alcohol and water molecules, and increased hydrophobic interactions among the alcohol molecules as well as increased pore sizes of the ZnO nanostructures. Sphere-like and hexagonal pores were observed, which were a result of the in situ enwrapping of hydrophobic aggregation areas in alcohol aqueous solution.

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Introduction

The microheterogeneity is one of the typical features of aqueous solutions of organic substances.^1–7 The hydrophobic groups of alcohol molecules in aqueous solution cluster together and form single-component aggregations.^7–10 These types of aggregation phenomena in aqueous solutions have been utilized for the synthesis of porous nanostructures in solvothermal processes.^11–13 Previous work suggests that, under hydrothermal conditions, polyalcohol is partly polymerized and results in the formation of microheterogeneity and emulsified spheres, which serve as soft templates for the subsequent formation of micro hollow structures.^14–16 On the one hand, the polymerized polyalcohol should be responsible for the hollow structures. On the other hand, the hollow structures also present the shape and size of the emulsified spheres or hydrophobic aggregations. Furthermore, when a simple alcohol was mixed with water, the entropy of the system increased far less than expected for an ideal solution of randomly mixed molecules, which was attributed to the hydrophobic groups, resulting in ice-like or clathrate-like structures in the surrounding water.^8,17,18 Although the microheterogeneity has been proved by more and more special experimental and theoretical works,^19–27 it is still a challenge to describe the shape and size of the hydrophobic aggregations using electron microscopy.

It is well known that, under optimum conditions, ZnO nanomaterials will be formed in pure water when Zn²⁺ is reacted with OH⁻ at room temperature.^28–30 In our previous work, porous ZnO nanoplates were synthesized at room temperature using polyethylene glycol (PEG) as a template.³¹ The presence of hydrophobic groups of PEG resulted in the existence of microheterogeneities and PEG aggregations in the mixed aqueous solution, which play key roles in the formation of the porous structures of ZnO. Herein, the microheterogeneity of a series of mixed simple alcohols and water was investigated. The hydrophobic organic aggregations resulting from the hydrophobic groups of the alcohols were enwrapped by the surrounding water. When ZnO was formed, these hydrophobic organic aggregations were enwrapped in ZnO nanoplates, as shown in Scheme 1. So, porous ZnO nanoplates were obtained, and these pores present the size and the shape of the hydrophobic organic aggregations.

Scheme 1 Schematic illustration of the formation of the nanopores in ZnO nanoplate in aqueous alcohol solution.

Experimental section

Preparation of flower-like ZnO

The zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), sodium hydroxide (NaOH), ethanol, ethylene glycol, n-propanol, isopropanol, 1,2-propylene glycol, glycerol and n-butanol in these experiments, purchased from Shanghai Chemical Reagents Company, were of analytical grade and used without further purification. In a typical synthesis process, 1.5 g Zn(NO₃)₂·6H₂O was dissolved in a mixture of 300 mL deionized water and 10 mL ethanol to obtain solution 1, and 2 g NaOH was dissolved in 10 mL deionized water to obtain solution 2. Under vigorous stirring, solution 2 was poured into solution 1 at room temperature, and the mixture was kept stirring vigorously during the reaction process. After being further stirred for 6 h, the white precipitates were collected by centrifugation and washed with deionized water repeatedly until neutral pH was reached. The precipitates were then dried at 60 °C. The method to synthesize ZnO products using other alcohols was the same as the above process.

Characterization

The morphology and structure of the as-prepared products were characterized using Quanta 200 FEG field-emission scanning electronic microscopy (FESEM) operated at an accelerating voltage of 10.0 kV, JEOL-2010 transmission electron microscopy (TEM) operated at 200 kV, Philips X' Pert Pro X-ray diffractometer (XRD) with Cu Kα radiation (1.5418 Å), Fourier transform infrared spectroscopy using KBr pellets (FTIR, Nicolet Analytical Instruments, NEXUS-870). Nitrogen adsorption–desorption measurements for the products were performed using a Micromeritics ASAP 2020M+C instrument using Barrett–Emmett–Teller calculations for surface area and Barret–Joyner–Halender (BJH) calculations for pore size distribution.

Results and discussion

Structures of the ZnO product

To investigate the key roles of the simple alcohols in the formation of the pores, ZnO products were also synthesized in pure water. The SEM (Fig. S1, ESI†) and TEM images shown in Fig. 1 suggest that flower-like products of 2–4 μm in diameter were obtained. The XRD pattern confirms that the products were wurtzite-type ZnO (JCPDS 36-1451) (Fig. S2, ESI†). It is clear that ZnO products synthesized in pure water were composed of nonporous nanoplates. Our previous work suggested that mesoporous ZnO nanoplates were formed in the presence of organic hydrophobic aggregations of PEG.³¹ Herein, mesoporous ZnO nanoplates were also obtained in the mixed simple alcohol and water. In the presence of ethanol, the obtained products also were wurtzite-type ZnO (Fig. S2, ESI†). Fig. 2a and b show the TEM images of the ZnO nanoplates synthesized in mixed ethanol and water. It is clear that large numbers of sphere-like nanopores of 3–5 nm in diameter were obtained. Some of the large pores were about 8–9 nm.

Fig. 1 Low-magnification (a and b) and high-magnification (c and d) TEM images of the ZnO prepared in pure water.

Fig. 2 TEM images of ZnO prepared in the presence of ethanol (a and b) and ethylene glycol (c and d).

To further confirm the porous structures of the ZnO synthesized in the presence of ethanol, the obtained ZnO products were characterized by HRTEM. Fig. 3 show the HRTEM images and the corresponding SAED pattern. From Fig. 3a and b, a large number of nanopores with a diameter less than 10 nm were clearly observed, which confirms the porous structures of the ZnO products. Fig. 3c presents well-resolved two-dimensional lattice fringes with spacings of 0.52 and 0.28 nm, which are in good agreement with the interspacings of (0001) and (011̄0) planes of wurtzite-type ZnO, respectively.³¹ The SAED shown in Fig. 3d and HRTEM confirm that the as-prepared ZnO nanoplates are single crystalline.

Fig. 3 HRTEM images (a–c) and the corresponding SAED pattern (d) of the ZnO nanoplates prepared in the presence of ethanol.

The formation of the nanopores was definitely attributed to the presence of ethanol. Previous reports suggested that, under hydrothermal conditions, the sphere-like hydrophobic aggregations resulting from the surfactant were approximately tens of microns.³² However, in the mixed solution of water and isopropanol, the sphere-like emulsified areas resulting from water were decreased to 200–300 nm.¹¹ The results suggested that, in a certain concentration range, the aggregations formed in hydrothermal conditions are sphere-like, regardless of their type or formation mechanism.^{11,14–16,32} Apparently, the hydrophobic aggregations resulting from a simple alcohol were far smaller than the ones formed by the surfactant, owing to their short carbon chains. So, the presence of hydrophobic groups of ethanol will result in the formation of microheterogeneities in aqueous solution. Obviously, ZnO cannot nucleate in these hydrophobic aggregations, and as a result these hydrophobic aggregations were preserved and enwrapped in the ZnO plate. Thus, the microheterogeneity of aqueous ethanol solutions should be responsible for the formation of the nanoporous structures of ZnO plates. Accordingly, most of the hydrophobic group aggregations resulting from the ethanol were sphere-like and about 3–5 nm. The results suggest that though ethanol is water-soluble, owing to the strong hydrogen bonding between the hydroxyl groups and water molecules, the microheterogeneity is still formed by the ethyl groups of ethanol.

In ethanol aqueous solution, the hydrophobic aggregations should be related to the hydrogen bonding strength between ethanol and water. Furthermore, it is well known that the hydrogen bonding strength was greatly dependent on the temperature.^33–35 High temperature will result in decreased hydrogen bonding and increased hydrophobic interactions between the macromolecular chains.^36,37 So, the influence of the reaction temperature on the sizes of the pores was studied. TEM images (Fig. S3, ESI†) of the ZnO nanoplates synthesized at 60 °C confirmed the presence of the large pores. The XRD pattern further confirms the products were ZnO (Fig. S2, ESI†). The diameters of some of the pores were increased to about 10 nm. The large pores imply the presence of the large hydrophobic aggregations, which should be a result of the decreased hydrogen bonding strength between ethanol and water at high temperature.

Influence of the alcohol type

The hydrophobic aggregation group of ethanol was ethyl, which has two –CH_n groups and one –OH group. To further investigate the roles of the hydrophobic group, ethanol was replaced by ethylene glycol, which has two –OH and two –CH₂ groups. TEM images of the products (Fig. 2c and d), prepared in the mixed water and ethylene glycol, suggest that a large number of nanopores were also formed. These pores were also sphere-like, and their diameter was similar to the ones of the product prepared in the mixed ethanol and water, suggesting that the number of –OH groups has no obvious influence on the shape and size of the hydrophobic aggregations.

The presence of the nanopores was confirmed by N₂ adsorption–desorption isotherms and BJH adsorption pore size distributions shown in Fig. 4. Considering the TEM results, the mesopores around 10–30 nm were resulted from the interspaces between the ZnO nanoplates. For ZnO synthesized with the presence of ethanol and ethylene glycol, the mesopores about 3–4 nm were obviously observed. The results suggested the existence of the mesopores with a diameter of 3–4 nm, which was consistent with the TEM images. However, for ZnO synthesized in pure water, no peak was observed in this region. So, the results definitely confirmed that the formation of the pores was attributed to the presence of the simple alcohol.

Fig. 4 N₂ adsorption–desorption isotherms (a) and BJH adsorption pore size distributions (b) of the obtained ZnO products.

Furthermore, based on the pore formation process, the organic aggregations should be enwrapped in the nanoplates. To show this, the ZnO products were further characterized by transmittance FTIR and attenuated total reflectance-FTIR (ATR-FTIR), and the results are shown in Fig. 5. From the transmittance FTIR spectrum shown in Fig. 5a, the peak centered at 3387 cm⁻¹ was attributed to the O–H bond stretching vibrations of the physically absorbed water, and the band at 1636 cm⁻¹ was assigned to the H–O–H bending vibrations.³⁸ The peaks at 2926 and 2856 cm⁻¹ can be assigned to ν_C–H of the ethanol or ethylene glycol. The peaks at 1473 and 900 cm⁻¹ were assigned to the Zn–OH deformation mode,^39,40 and the peak at 1384 cm⁻¹ can be assigned to the carbonate-like species or the adsorbed NO₃⁻ ions.^40–42 The peak centered at 572 cm⁻¹ was the characteristic band of ZnO.⁴³ For the ATR-FTIR spectrum shown in Fig. 5b, the peaks at 1473 and 900 cm⁻¹ also resulted from the deformation mode of Zn–OH. The peak at 1384 cm⁻¹ means the presence of carbonate-like species or the adsorbed NO₃⁻ ions. In addition, the strong FTIR peaks around 2200 cm⁻¹ were assigned to adsorbed CO₂. However, in the ATR-FTIR spectrum, the ν_C–H vibrational modes were hardly detected any more. According to the experimental process, the C–H groups can only come from the added ethanol molecules. So, some alcohols were enwrapped in the ZnO plate. The obtained ZnO products prepared in the presence of ethylene glycol were characterized by transmittance FTIR and ATR-FTIR (Fig. S4, ESI†), and the results were similar to the ones synthesized in the presence of ethanol. Furthermore, the ZnO products synthesized in pure water were also characterized by transmittance FTIR and ATR-FTIR, and the results are shown in Fig. 6. Compared with the transmittance FTIR of the ZnO prepared in the presence of ethanol or ethylene glycol, the ν_C–H vibrational modes were no longer detected. The ATR-FTIR spectrum shown in Fig. 6b is similar to the ones synthesized in the presence of alcohol. The results suggested that no organic species were enwrapped in the ZnO nanoplate, which further confirmed that the C–H groups came from the added alcohol molecules. In addition, the thickness of the nanoplate was obviously larger than the diameter of the pores. Thus, the alcohol molecules were enwrapped in the ZnO nanoplates, which further confirmed the proposed mechanism of the porous structure.

Fig. 5 Transmission FTIR (a) and ATR-FTIR (b) spectra of ZnO synthesized in the presence of ethanol.

Fig. 6 Transmission FTIR (a) and ATR-FTIR (b) spectra of ZnO synthesized in pure water.

To further investigate the influence of the types of alcohol on the shape and size of the hydrophobic aggregations, n-propanol, isopropanol, 1,2-propylene glycol, and glycerol mixed aqueous solution were also studied. ZnO nanoplates were prepared using a similar method at room temperature. TEM images suggest that, in the presence of n-propanol, the pores in ZnO plates were obviously larger than those prepared in ethanol and ethylene glycol aqueous solution (Fig. S5, ESI†). It is very interesting that some large hexagonal or quasi-hexagonal pores were formed when using isopropanol as a template, as shown in Fig. 7. For 1,2-propylene glycol and glycerol, large hexagonal pores were also observed (Fig. S7–S9, ESI†). The presence of the enwrapped alcohols was confirmed by FTIR and ATR-FTIR analysis (Fig. S6 and S10, ESI†). The large and hexagonal pores clearly resulted from the large hydrophobic groups because the above four alcohols have three –CH_n groups. Therefore, the long carbon chains were in favor of the formation of the large hydrophobic aggregations and the final large pores. However, the pores enwrapped in the ZnO nanoplates, synthesized in the mixed n-butanol and water, were obviously smaller than the ones prepared in mixed three-carbon alcohols and water (Fig. S11, ESI†). Some nanopores were connected together to form threadlike pores. In addition, no hexagonal or quasi-hexagonal pores were formed, which was similar to those in n-propanol. The results suggested that the shape and size of the hydrophobic aggregations were greatly dependent on the type of carbon chain. More work is needed to reveal the formation mechanism of the hexagonal or quasi-hexagonal pores.

Fig. 7 TEM images of the ZnO prepared in mixed isopropanol and water.

Conclusions

In summary, we demonstrated a simple method to prove the presence of the hydrophobic aggregations in simple alcohol aqueous solution. The hydrophobic groups aggregated and resulted in the formation of sphere-like and hexagonal hydrophobic areas, which were enwrapped in situ by the solid ZnO nanoplate, which was observed by TEM. The enwrapped hydrophobic groups were confirmed by FTIR and ATR-FTIR analysis. The results present an important experimental approach to study the microheterogeneity of alcohol aqueous solutions, which should be of importance for the in-depth understanding of the aqueous solutions.

Acknowledgements

This work was supported by the Natural Science Foundation of Education Committee of Anhui Province (KJ2012A179), the National Key Scientific Program, Nanoscience and Nanotechnology (2011CB933700), the China Postdoctoral Science Foundation (2011M501073), the Natural Science Foundation of Anhui University of Chinese Medicine (2011zr017B), and the National Natural Science Foundation of China (21103198, 21073197, 11205204 and 61273066).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, SEM, TEM, XRD and FTIR data. See DOI: 10.1039/c3ra45477k

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