TiO₂/ZnO nanocomposite, ZnO/ZnO bi-level nanostructure and ZnO nanorod arrays: microstructure and time-affected wettability change in ambient conditions

Abstract

Wettability is an important property of surfaces and interfaces. Understanding the wetting behavior of semiconductors and its relationship with their microstructures has aroused much interest because of the great advantages this gives to various functional applications. Herein, we report the fabrication of ZnO-based nanoarrays, including ZnO nanorods, TiO₂/ZnO nanocomposites and ZnO/ZnO bi-level nanostructures, by hydrothermal growth and physical vapor deposition and also their wettability conversion. The ZnO matrix arrays consist of many hexagonal wurtzite nanorods. The microstructure and morphology of TiO₂/ZnO and ZnO/ZnO nanostructure arrays were studied. The wettability conversion of these ZnO-based nanoarrays in ambient conditions was monitored over a long period of time, and it was found that their surfaces transited from hydrophilic to hydrophobic without any external stimulation. It was also found that morphological features and surface chemical changes, such as replacing the hydroxyl groups and adding organic contaminants, affect their wettability behaving. Our results also demonstrated the possibility of slowing the loss of hydrophilic states by TiO₂ decoration.

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1. Introduction

Wettability is one of the essential properties of a solid material surface and is of great importance to the interface between liquids and solids. Generally, according to the water contact angle (CA) on a solid surface, surfaces can be defined as hydrophobic (CA > 90°) or hydrophilic (CA < 90°).¹ It is believed that wettability is governed by both the inherent chemical composition and the geometrical structure of solid surfaces.^2–4

Controlling surface wettability is highly demanded for a wide range of chemical, electronic devices, biological, and agricultural industries and also for everyday life.^1,5 Additionally, the wetting process of solid material surfaces by liquids is of great interest to researchers in both fundamental studies and practical applications. For instance, in the development of self-cleaning surfaces,^6,7 antifogging coatings,⁸ microfluidics,⁹ sensors¹⁰ and the biocompatibility of solid surfaces,¹¹ are all related to the wetting process.

Recently, there has been enthusiasm in studying the wetting behaviors of metal oxide semiconductors, such as ZnO,^1,2,12–14 SiO₂,¹⁵ and TiO₂.¹⁶ Among these, ZnO has drawn particular attention on its special electrical, optical, and catalytic properties due to its direct wide band gap (3.37 eV), large exciton binding energy (60 meV) at room temperature, and its transparency to visible light.^17–19

Much research has focused on controlling ZnO surfaces to obtain desirable functional properties, such as hydrophobicity or hydrophilicity, or even superhydrophobicity (CA > 150°) and superhydrophilicity (CA > 5°). The special wettability can usually be achieved by applying external stimuli and treatments such as UV light illumination,^13,20 chemical modification,^21,22 electric field,²³ plasma,^12,22 and thermal treatment.²⁴

However, studies on the time-affected wettability change of ZnO nanostructures are quite limited. Mondal et al. reported the reversible switching from a superhydrophobic to a superhydrophilic state of the ZnO nanocolumnar thin film by UV irradiation; and they also observed that on keeping the film under ambient conditions for a certain time, the hydrophilic as-prepared surface became hydrophobic.¹³ Zhu et al. obtained the superhydrophobic ZnO nanorod-arrayed surface by n-octadecanoic acid modification and indicated that the as-prepared fresh ZnO surface showed a hydrophilic behavior without any chemical modification.²¹ They compared this result with the superhydrophobic ZnO surface from Feng et al. and attribute the difference to the time of testing since they measured the wettability immediately after the sample was fabricated, while Feng et al. stored the ZnO nanorod films in the dark for several days.²⁵ Although the wettability conversion of ZnO surfaces in the air was mentioned, there is no explanation for this phenomenon in these studies; and indeed, the spontaneous change of the surface wetting property during long-term periods under ambient conditions has rarely been reported.

In the present study, we prepared the ZnO nanorod (ZNR) arrays by a benign aqueous solution method and used magnetron sputtering of TiO₂ and ZnO to modify the ZNR arrays to obtain more complex arrays composed of “core–shell” nanostructures. While the microstructure of ZnO based nanoarrays was studied, we also monitored CA on their surface during a long period of time when they were placed in the dark at ambient temperature. During the sample storage, no external stimuli or other treatment was conducted on the surface, so the variation of wettability was totally from a “self-managed” process. The mechanism and the reasons for the time-affected wettability alteration and the relationship with their microstructures were investigated. The main objective of this work is to gain a deeper understanding of the surface chemistry variations and the wettability transition of the semiconductor nanostructure, which would enrich our knowledge of design, storage and application of these materials.

2. Experimental

2.1 Sample preparation

2.1.1 Formation of ZnO nanorod arrays. All chemicals were of analytical grade, and deionized (DI) water were used throughout this study. The hydrothermal method for growing immobilized ZnO nanorods has been described in our previous report.²⁶ In brief, clean glass slides were precoated with ZnO seed layers by magnetron sputtering to get the ZNRs substrates. An aqueous solution containing 25 mM zinc nitrate (Zn(NO₃)₂·6H₂O, 98%) and hexamethylenetetramine (HMT) (C₆H₁₂N₄, 99%) in an equal molar ratio was prepared as the reactant resource. The substrates were immersed into a sealable glass jar with the reactant solution and kept face-down at 95 °C for 4 h. Afterwards, the glass slides were rinsed and dried, and kept in petri dishes in the dark.

2.1.2 Fabrication of TiO₂/ZnO and ZnO/ZnO nanostructure arrays. The complex nanoarrays were synthesised by depositing a layer of TiO₂ or ZnO onto the surface of ZNRs using a magnetron sputter. ZNRs were mounted into a working chamber installed with the sputtering target (TiO₂ or ZnO, 99.99%). The unit was evacuated until the background vacuum pressure was lower than 6.7 × 10⁻⁴ Pa. Argon gas with a flow rate of 10 sccm and a pressure of 1.33 Pa was used as the working gas. The target was then activated by a DC current of 0.25 A. The deposition was performed for 2 h and 4 h with TiO₂ and ZnO targets. The ZnO-based nanoarrays were named after their composition and preparation time, e.g. 4TZ for TiO₂-decorated ZNRs by 4 h deposition using a TiO₂ target, 2TZ for TiO₂-decorated ZNRs by 2 h deposition using a TiO₂ target, and 2ZZ for ZnO-decorated ZNRs by 2 h deposition using a ZnO target.

2.2 Sample characterization

The crystal structure of the ZnO-based nanoarrays was identified by X-ray diffraction (XRD, Philips X'Pert MPD) using Cu Kα radiation. Their morphology and microstructure were characterized by a field-emission gun scanning electron microscope (FEG-SEM, FEI XL-30S), transmission electron microscope (TEM, Philips CM12 120 KV), and a high resolution transmission electron microscope (HRTEM, Philips Tecnai G² F20, 200 KV). The chemical composition of 4TZ was analysed with energy dispersive spectroscopy (EDS) equipped on SEM. Raman scattering (Renishaw System 1000) and X-ray photoelectron spectroscopy (XPS, PHI Quantera-II SXM) were employed to study the microstructure and chemical states of the TiO₂ outer layers of 4TZ.

2.3 Wettability measurement

The surface wettability of the ZnO-based nanoarrays was evaluated by the water contact angle, measured on a contact-angle goniometer (KSV CAM 101). A Milli-Q water droplet was gently placed on the surface of the samples using a microsyringe. To obtain a stable contact angle, all measurements were conducted 30 s after positioning the water droplet. Each contact angle was repeated at least five times at different positions, and the mean value was calculated. The wettability of each sample was intermittently measured after different storage times.

3. Results and discussion

3.1 Morphology and composition

The morphology of the ZnO-based nanoarrays are illustrated in Fig. 1. The top section views disclose the distribution of the nanostructures of each sample. The nanorod structures are uniformly spread and are closely packed into large arrays. The size of the tips increases in tandem with the decreasing density from ZNRs to 2ZZ (Fig. 1a–d).

Fig. 1 SEM images of (a–d) top section and (e–h) cross section of ZNRs, 2TZ, 4TZ and 2ZZ.

The ZnO nanorods possess similar diameters throughout their entire bodies, and a majority of them are vertical to the substrates, as shown in the cross-sectional views of Fig. 1e–h. The nanorod-based structures from 2TZ, 4TZ, and 2ZZ are also perpendicularly arrayed, but the rod bodies are enlarged from the bottom to the top, to various extents. Fluffy matter covered the top surface and upper bodies of 2TZ, which also had many tiny cracked blocks. This became more obvious for 4TZ, which looked like a cotton swab array with bigger tops than 2TZ. 2ZZ exhibited a lollipop configuration, in which the lower bodies of the narrow nanorods are supporting the much thicker upper parts compared to ZNRs, 2TZ and 4TZ. Not many cracks can be seen, indicating the good compatibility of the deposition with the original nanorods. The gaps within the arrays became narrower from ZNRs to 2TZ and 4TZ and were even closed in 2ZZ.

The elemental composition along the TiO₂/ZnO nanocomposite bodies was detected by EDS. Three adjacent areas (500 nm × 500 nm) were selected from the top to the bottom of 4TZ, as shown in Fig. 2. In the EDC spectra, the highest Zn peaks indicate the nanoarrays are based on the ZnO nanorod matrixes, and the Ti peaks come from the TiO₂ deposition. Apart from Si of the glass substrates and Pt from the pre-SEM coating, O is the only element left, proving the purity of 4TZ.

Fig. 2 EDS spectra from the adjacent areas along the cross section of 4TZ.

From area #1 to area #2 and to area #3, the intensity of the Zn peaks increases, while the Ti peaks reduce and the atomic ratio of Ti/Zn decreases accordingly from 0.864 to 0.265 and to 0.113, implying that most of TiO₂ is concentrated on the top surfaces and some is attached on the upper parts, but also a little has reached the bottom. This is related to the feature of magnetron sputtering. The target materials arrive on the top and the upper side surfaces first, and gradually, the space for passing through becomes obstructed. Therefore, more TiO₂ accumulates on the top parts of the ZnO nanorods with a prolongation of the deposition time, and they develop into a cotton swab-like structure, as shown in the SEM images.

2ZZ also experienced a similar accumulating process. In the early stage (0.5 h) of magnetron sputtering, the ZnO outer layers on the ZnO nanorods are not as compressed as 2ZZ, and those ZnO on the top parts are in particles shape, as shown in the HRTEM result of 0.5ZZ (0.5 h decorated ZNRs by ZnO) (Fig. S1 in the ESI†). The distance between the regular lattice planes of the ZnO outer layers is 0.26 nm, corresponding to the interspacing of the (002) planes of wurtzite ZnO, suggesting a c-axis texture, which is in good agreement with the XRD result (discussed thereinafter).

3.2 Crystal structure

XRD was applied to reveal the crystal structures of the nanoarrays. Compared with the XRD pattern of ZNRs grown from the hydrothermal method,²⁶ no significant difference can be found for 2TZ, 4TZ, and 2ZZ, as shown in Fig. 3. The dominant (002) peak for all samples is indexed to a typical hexagonal wurtzite ZnO crystal structure, which also indicates that the ZnO nanorods have a preferential growth along the c-axis in the [0001] orientation. No obvious peaks of TiO₂ crystal could be noticed for TiO₂/ZnO nanoarrays, which could be because the total amount of TiO₂ is very small compared with the ZnO matrix. The overwhelming (002) peak and the little peak at 36.2° for 2ZZ imply that the ZnO outer layers deposited by sputtering have a strong c-axis texture and some (101) phases.

Fig. 3 XRD patterns of ZNRs, 2TZ, 4TZ and 2ZZ.

Interestingly, when we used the fixed position-sensitive detectors (PSD) mode to give a fine X-ray scan (Bruker D2 phaser) between 23° to 28° on 2TZ and 4TZ, a peak at 25.3° was detected for 4TZ, but no such peak was captured for 2TZ, due to the very limited number of TiO₂ outer layers (Fig. 4). This peak is indexed to the anatase TiO₂ (101) phase, which means the TiO₂ outer layers were crystallized. The anatase TiO₂ phase of 4TZ can be verified by Raman scattering. The Raman spectrum of 4TZ gives five Raman active fundamental vibrational modes (E_g, E_g, B_1g, B_1g/A_1g, and E_g) of the tetragonal anatase structured TiO₂ ^27,28 and one characteristic E₂ mode of wurtzite ZnO.^29,30 (Fig. S2 in the ESI†).

Fig. 4 XRD patterns between 23° to 28° of 2TZ and 4TZ.

The electronic states of the element Ti of 4TZ were analyzed by XPS. Fig. 5 shows the fine scan spectrum of Ti 2p. The binding energies of the Ti 2p region exhibit two peaks at 464 eV and 458.3 eV, corresponding to Ti 2p_1/2 and Ti 2p_3/2, respectively, indicating the presence of Ti⁴⁺ in the TiO₂ lattice.

Fig. 5 High resolution XPS spectrum of Ti 2p of 4TZ.

3.3 TEM analysis

The detailed morphology and crystal structures of the ZnO-based nanoarrays were investigated by TEM and HRTEM, as shown in Fig. 6. ZnO nanorods are straight with a uniform diameter (60–80 nm) along the entire length, indicating that the anisotropic growth along the c-axis was perfectly preserved during the growth process (Fig. 6a). It also implies that the growth of ZnO nanorods has a great crystal orientation.^31,32

Fig. 6 (a) TEM image of ZNRs; (b and c) TEM images of 4TZ; (d) HRTEM image of TiO₂ outer layers of 4TZ; (e) SAED pattern of the TiO₂ outer layers; (f) TEM image of 2ZZ; (g) HRTEM images of ZnO outer layers of 2ZZ.

The bush-like TiO₂ layers on the surface of ZnO nanorods make the 4TZ into a core–shell cotton swab-like structure (Fig. 6b and c). The distances between the lattice planes of the TiO₂ part were measured to be 0.35 nm and 0.32 nm, corresponding to the interspacing of the (101) planes of anatase TiO₂ and the (110) planes of rutile TiO₂, respectively (Fig. 6d). The selected area diffraction (SAED) pattern of the TiO₂ outer layers displays a polycrystalline diffraction with certain concentric rings, indexing to several phases of anatase and rutile TiO₂ (Fig. 6e). The HRTEM image and SAED pattern of 4TZ signify that the TiO₂ outer layers have polycrystalline phases including both anatase and rutile crystals.

The TEM image of 2ZZ is in accordance with the SEM result and shows that the top part of the ZnO nanorod core was accumulated by ZnO crystals into a lollipop-like bi-level structure (Fig. 6f). The ZnO outer layers show some (101) phases of wurtzite ZnO, with an interplanar space of 0.25 nm (Fig. 6g).

3.4 Wettability study

The variation on the surface wettability of ZnO-based nanoarrays was monitored via water contact angles during their storage times. As displayed in Fig. 7, the surface of ZNRs was hydrophilic initially because the contact angle (CA) was 10° when it was measured on the day they were prepared. The surface of ZNRs became hydrophobic after storing in ambient condition for 60 days, as the CA increased to 113°. CA kept gradually increasing and reached 145° after over 11 months.

Fig. 7 Wettability conversion of ZNRs during the storage in ambient conditions.

Fig. 8 and 9 demonstrate the surface wettability conversion of the TiO₂/ZnO nanocomposite arrays. The as-prepared 2TZ also had a hydrophilic surface, since the CA was 19° in the beginning but increased slowly to 96° after 264 days and was observed at 116° after storage for more than 21 months. From hydrophilicity to hydrophobicity, it took a longer time for 2TZ than for ZNRs, and also the change rate of CA was slower than that of ZNRs. The surface wettability of 4TZ experienced a similar trend as 2TZ, but the variation was even slower. It was only after 339 days that the 4TZ changed to hydrophobic (CA changed from 12° to 96°), and later, after more than 21 months, CA became 106°.

Fig. 8 Wettability conversion of 2TZ during the storage in ambient conditions.

Fig. 9 Wettability conversion of 4TZ during the storage in ambient conditions.

2ZZ, however, exhibited a rather smooth wettability change (Fig. 10). The surface had a quite high CA (72°) at the early stage and increased very slightly up to 105° after over two years. Although it changed from hydrophilic to hydrophobic like the ZNRs, 2TZ, and 4TZ, the extent was much smaller than the other samples.

Fig. 10 Wettability conversion of 2ZZ during the storage in ambient conditions.

During the dark storage at ambient conditions, no intentional modification or chemical treatment was applied to the samples, so their surface geometric microstructure and inherent elemental nature remain unaltered. We therefore could assume that this conversion of the wettability of the ZnO-based nanoarrays is attributable to two kinds of changes on the surface: the reduction of hydroxyl groups and the adsorption of organic contaminants.

Because of the nature of semiconductors (ZnO or TiO₂), the surface of the as-prepared nanoarrays are terminated with hydroxyl groups for the charge neutralization.¹³ The hexagonal wurtzite structure of the ZnO nanorods are composed of polar top surfaces and nonpolar side surfaces. On the nonpolar planes, an equal number of oxygen and zinc ions are terminated in the same face, but the polar surface is dominated by Zn⁺ ions.³³ Since surface oxygen ions are considered to act as reactive sites for attracting OH species, the nonpolar side facets are generally terminated with hydroxyl groups. The hydroxyl groups on the surface are known to have a hydrophilic nature, and they can facilitate the imbibing of water droplets via a 3D capillary effect into the nanostructure network, which shows up as a low CA and a hydrophilic surface.^8,12,33

A surface rich in hydroxyl groups is energetically unstable, whereas oxygen adsorption is thermodynamically preferred to replace the hydroxyl groups when the surface is exposed to an atmosphere containing a higher oxygen partial pressure (in either the air or pure oxygen).^20,25 On losing the hydroxyl groups, the capillary effect will be weakened. When air becomes trapped within the topographical network, it will act like a cushion and prevent water from seeping into the free space, making CA higher than before.

On the other hand, gradually during the storage, the surface would attract amphiphilic contaminants or organic molecules contained in the air,^20,34 which will also contribute to the hydrophobicity since they would lower the surface free energy. The effect of the hydroxyl groups and contaminants on the CA of ZnO nanorods and TiO₂/ZnO nanocomposites is illustrated in Fig. 11.

Fig. 11 Schematic models of the water contact angle on the surface of ZnO nanorod and TiO₂/ZnO nanocomposite arrays when they were produced and after a certain time storage in ambient conditions (–OH:hydroxyl group; –C:contaminant).

The change rates of the wettability of the ZnO-based nanoarrays are in the sequence of ZNRs > 2TZ > 4TZ > 2ZZ, which would be ascribed to their microstructure and morphology. The morphology of the nanoarrays differs in surface area and further affects the surface-absorbed chemicals (dehydroxylation and contaminant adsorption).

For 2TZ, 4TZ, and 2ZZ, the gaps among the nanostructures were obstructed by the outer layers of TiO₂ or ZnO, which decreases the velocity of the replacement of hydroxyl groups by oxygen on the side surfaces of the ZnO nanorods. The bush-like TiO₂ outer layers not only provide more local contact area for water droplets but also contain interstices to support the capillary effect for the liquid wicking. Besides, as a similar semiconductor as ZnO, TiO₂ is naturally hydrophilic and is dominated by hydroxyl groups. Consequently, the OH species on the TiO₂ outer layers and their structural features also delay the hydrophobicizing rates of 2TZ and 4TZ (Fig. 11).

From 2TZ to 4TZ and to 2ZZ, the outermost surface of the nanoarrays becomes more and more flat as a result of the longer time sputtering deposition and better matching of the ZnO layers on the ZnO top surfaces, therefore less reactive sites can be attached by organic molecules and thus become available. In this case, the smoother the outermost surface is, the longer time the wettability conversion is. Moreover, the very even surface of 2ZZ leads to a high initial CA (72°), due to the limited contact areas and the lack of capillary effect.

To prove the effect of oxygen absorption on the surface wettability, we placed the fresh ZNRs in a tube furnace under an O₂ stream at a low flow rate (25 sccm) to eliminate the influence of other gas components and certain contaminants in the ambient air, and the whole set was maintained at 80 °C to accelerate the dehydroxylation process. The surface changed to hydrophobic significantly only in 7 days as the CA increased to 110° and further reached 126° after 14 days (Fig. S3 in the ESI†). This result indicated that the wettability conversion is largely ascribed to the reduction of hydroxyl groups on the surface.

We soaked hydrophobic ZnO-based nanoarrays after long time storage in acetone for 45 h to verify the adsorption of organic molecules onto the surface. They all became hydrophilic when they were dried and tested immediately as their CAs decreased from over 100° to less than 80° (65° for ZNRs, 63° for 4TZ and 77° for 2ZZ; Fig. S4 in the ESI†). It is believed that the acetone can dissolve many organic chemicals, hence, the loss of the adsorbed contaminants can lead to hydrophilicity. The higher resultant CA of 2ZZ results from the flatter surface feature.

When the long-time stored samples were treated by ethanol, the hydrophilic conversion was more dramatic even though they were only soaked for 22 h. The resultant CAs were 22° for ZNRs, 23° for 4TZ, and 73° for 2ZZ when they were dried and tested immediately (Fig. S5 in the ESI†). The ethanol not only dissolved some organic compounds, but also provided a large amount of extra hydroxyl groups to the surface, confirming the critical role of hydroxylation and the contaminants in the wettability transition.

We could imagine that if the as-prepared ZnO-based nanoarrays were stored in circumstances lacking oxygen and organic species, such as in a vacuum, from the beginning, the surface wettability will not be influenced by exterior matters and their initial hydrophilic states would last for a longer time.

4. Conclusion

To conclude, wurtzite ZnO nanorod arrays were synthesized by an aqueous solution method. On the basis of the ZnO array matrixes, the TiO₂/ZnO and ZnO/ZnO nanostructure arrays were realized by magnetron sputtering. The TiO₂/ZnO heterostructures are composed of the ZnO nanorod backbones with fluffy TiO₂ outer layers. The ZnO/ZnO nanoarrays have a bi-level structure with compact ZnO outer layers attached onto the upper part of the ZnO nanorods.

The surface wettability of these ZnO-based nanoarrays transformed spontaneously from hydrophilic to hydrophobic under ambient atmosphere over long term storage. This is attributed to the replacement of the initial surface hydroxyl groups by oxygen atoms and the adsorption of contaminants in the air. The different wettability change rates of the ZnO-based nanoarrays are related to their topographic features and surface chemical components. The abundant open gaps in the ZnO nanorod arrays lead to the most rapid conversion. The flattest ZnO/ZnO nanoarrays experienced the least significant variation, and TiO₂ outer layers delayed the changes of surface hydrophilic property.

This research reveals how ambient conditions can affect the surface wetting behavior over a long time and may shed new light on the design of the storage and package of semiconductor nanodevices.

Acknowledgements

The authors gratefully appreciate the assistance from staff members in the Department of Chemical and Materials Engineering and the Research Centre for Surface and Materials Science at the University of Auckland, especially Ms Catherine Hobbis, Ms Laura Liang and Dr Alec Asadov. The authors would also like to thank the financial support of CSC scholarship.

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Footnote

† Electronic supplementary information (ESI) available: HRTEM images of 0.5ZZ. Raman spectrum of 4TZ. CA of ZNRs after O₂ treatment for (a) 7 days and (b) 14 days. CA of (a) ZNRs, (b) 4TZ and (c) 2ZZ after soaking in acetone for 45 h. CA of (a) ZNRs, (b) 4TZ and (c) 2ZZ after soaking in ethanol for 22 h. See DOI: 10.1039/c4ra04904g

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